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Keywords:

  • subduction zone magmatism;
  • slab melt;
  • sediment melt;
  • mantle wedge

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[1] The magmatic record of the easternmost part of the Trans-Mexican Volcanic Belt elucidates how temporal changes in subduction parameters influence convergent margin volcanism. In the Palma Sola massif, three phases of magmatic rocks with distinct chemical characteristics were emplaced in a relatively short time span (∼17 Ma): Miocene calc-alkaline plutons, latest Miocene-Pleistocene alkaline plateau basalts, and Quaternary calc-alkaline cinder cones. Plutons have arc-like trace element patterns (Ba/Nb = 16–101), and their Sr, Nd, and Pb isotopic compositions become more “depleted” with increasing SiO2 contents. Their Pb isotopes are bracketed by the subducted sediments and Pacific mid-ocean ridge basalts (MORB), requiring the participation of an unradiogenic component that mixes with a sediment contribution. High Sr/Y and Gd/Yb ratios in the least radiogenic pluton might indicate a melt coming from the subducted MORB. Trace element patterns of the plateau basalts show moderate or negligible subduction contributions (Ba/Nb = 6–31). Rocks without subduction signatures are similar to ocean island basalts, indicating melting of an enriched mantle wedge. The plateau basalts form an array in 206Pb/204Pb-207Pb/204Pb space that trends toward the composition of the subducted sediment. The sediment component is also indicated by the inverse correlations between Pb isotopes and subduction signals. This component has high Th/Nd coupled with low 143Nd/144Nd, but lower Pb/Nd and Sr/Nd ratios than the bulk sediment. These suggest melting of a sediment that has lost fluid mobile elements prior to melting. The Quaternary cinder cones have moderate subduction signals (Ba/Nb = 16–41), and their isotopic compositions correlate with differentiation indices. Contamination with the local Paleozoic basement can explain the petrogenesis of the youngest rock suite. The geochemical differences among the suites indicate temporal modifications in the chemical characteristics of the slab input. These variations can be associated with modifications in the Pacific subduction regime. We suggest the Miocene magmatic phase was formed by an essentially flat subduction angle that favored melting of the subducted oceanic crust. Slab rollback in the Pliocene allowed melting of deeper portions of the wedge by the injection of dehydrated sediment melts. In the Quaternary, an even steeper subduction angle provided negligible slab contributions to the Palma Sola region, and upper crustal contamination largely controls the petrogenesis.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[2] Most workers agree that subduction zone magmatism involves the transfer of chemical components from the subducted slab into the mantle wedge, a process that modifies the mantle composition and triggers melting [Gill, 1981]. Nonetheless, it is often difficult to recognize the specific number of components and the physical ways these elements are transported and interact with the subarc mantle. Magmas erupted at the surface can be mixtures among: (1) Melts from a compositionally heterogeneous mantle wedge; (2) fluids coming from the dehydration of the altered subducted slab and/or the overlaying sediments; (3) fluid-rich silicate melts coming from the sediments and/or the oceanic crust. Arcs constructed over thick continental crust provide additional complications because the subducted materials often have very similar characteristics to the continental crust, and thus the processes that take place in the deep mantle could be largely obscured by crustal assimilation. Disentangling the proportions and ways in which all these constituents interact in the arc environment is an important goal because of the implications this system has on processes of crustal growth and the global recycling of the terrestrial materials.

[3] Experimental petrology has shown that the specific transport processes of the subduction agents largely depend on the multiple metamorphic reactions that affect the subducted slab under different P-T gradients [Schmidt and Poli, 1998; Johnson and Plank, 1999]. Since subduction zone magmatism is rarely a stationary phenomenon, and the volcanic front can migrate in time and space in response to variations in the convergence dynamics, it is expected that the chemical characteristics of the slab agents should vary throughout the arc history, affecting the compositions of the volcanic rocks. Therefore comprehensive petrological characterization of stratigraphically controlled magmatic sequences is essential to constrain the tectonic evolution and thermal structure of a subduction zone.

[4] This paper describes the geochemical variations of distinct magmatic sequences that were emplaced on the easternmost part of the Trans-Mexican Volcanic Belt (TMVB), on the Palma Sola massif (Figure 1). Magmatic rocks with arc-like and OIB-like features were emplaced in the area with different eruptive styles and in distinctive stratigraphic levels in a relatively short time span (∼17 Ma). We show that the variations in compositions among the different sequences can be attributed to the injection of different subduction components to a significantly enriched, OIB-like, mantle wedge. The temporal modifications in the geochemical character of these subduction components can be directly linked to gradual changes in the subduction dynamics, and provide a first-hand evidence to constrain the tectonic-petrogenetic evolution of the TMVB.

image

Figure 1. (a) Regional geology of the Trans-Mexican Volcanic Belt modified from Gómez-Tuena and Carrasco-Núñez [2000]. A representative section of the subducted sediments was sampled at DSDP site 487 and analyzed by LaGatta et al. (manuscript in preparation, 2003). Miocene stratovolcanoes: Palo Huérfano, La Joya, Zamorano (PH-LJ-Z) and Cerro Grande (CG). A–B profile will be discussed in Figure 13. (b) Simplified geologic map and stratigraphy of the Palma Sola area modified from Negendank et al. [1985] and López-Infanzón [1991]. Late-Paleozoic rocks that belong to the “Teziutlán Massif” were verified in exploratory wells in the Palma Sola area [López-Infanzón and Torres-Vargas, 1984; Jacobo, 1985; López-Infanzón, 1991], and sampled in this study around Altotonga.

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2. Geologic Setting

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[5] The Trans-Mexican Volcanic Belt (TMVB) is a 1000 km long, middle-Miocene to Holocene continental volcanic arc that traditionally has been related to the subduction of Cocos and Rivera plates beneath the North American plate (Figure 1). Nonetheless, the petrologic origin and evolution of the arc has been intensely debated over the last decade. It has been long recognized that two compositionally contrasting suites of rocks have erupted all across the arc since its inception in the middle-late Miocene: (1) calc-alkaline rocks with high LILE/HFSE ratios, that seem compatible with a subduction environment; and (2) alkaline or transitional rocks with low LILE/HFSE ratios with compositions similar to oceanic island basalts (OIB). While most researchers acknowledge the presence of enriched domains in the subarc mantle wedge, no real consensus has emerged on the origin of the subduction signal. Some workers have suggested a strong relation between the subduction environment and the local tectonic regime to explain both kinds of rock suites [Luhr et al., 1989a; Luhr, 1997; Ferrari et al., 2001]. In these models the high LILE/HFSE ratios are acquired by the injection of fluids coming from the subducted slab into a heterogeneous mantle wedge. The OIB-like basalts, in contrast, are generally attributed to the migration of an enriched asthenospheric mantle into the melting zone of an active extensional regime. For these authors, the enriched mantle is not significantly affected by the subduction environment nor has it been depleted by previous melt extractions. Others suggest that the OIB-like basalts can be formed by decompression melting of an enriched mantle source, with no modifications from the subduction environment, and that the arc-like signatures of the “subduction related” rock suite are inherited by mechanisms of crustal contamination [Márquez et al., 1999; Verma, 1999a, 1999b, 2000; Sheth et al., 2000]. For these investigators there is no evidence for the participation of the subduction regime in the petrology of the TMVB.

[6] The most prominent geomorphologic feature of the eastern TMVB is a NNE trending, composite volcanic chain, limited to the north by the Pleistocene Cofre de Perote volcano, and in the south by the Citlaltépetl (Pico de Orizaba) volcano with historical volcanic activity. Although a regional migration of the volcanic front to the south is apparent, Siebert and Carrasco-Núñez [2002] recently discovered basaltic rocks as young as 900 years in the northern part of the chain. Therefore volcanism in the easternmost part of the TMVB appears to be active not only at the current front, but also in the back-arc region.

[7] To the northeast of the chain of stratovolcanoes, a prominent volcanic province formed by plutons, extensive plateau basalts, and scattered cinder cones dominates the landscape (Figure 1). This sector has been named the Palma Sola massif or the Palma Sola - Chiconquiaco volcanic province [Cantagrel and Robin, 1979; Negendank et al., 1985; López-Infanzón, 1991]. Early studies on the Palma Sola volcanics [Cantagrel and Robin, 1978, 1979; Robin and Nicolas, 1978; Robin and Tournon, 1978; Robin, 1982] proposed that these rocks should not be considered part of the volcanic arc, but instead be grouped as part of an extensional alkaline province that extends along the coast of the Gulf of Mexico from the Sierra de Tamaulipas in northeastern Mexico, to Los Tuxtlas Volcanic field in the southeast (Figure 1). More recent research showed, however, that most of the volcanic rocks from Palma Sola and Los Tuxtlas volcanic fields have prominent arc geochemical signatures and therefore should be related to the Pacific subduction regime [Negendank et al., 1985; Besch et al., 1988; López-Infanzón, 1991; Nelson et al., 1995].

[8] The geologic and stratigraphic framework of the Palma Sola massif has been previously described in several publications and mapping efforts [Cantagrel and Robin, 1979; Negendank et al., 1985; López-Infanzón, 1991]. Figure 1 shows a simplified geologic map and stratigraphy based on these previous studies and our own geologic reconnaissance. The oldest rocks that have been verified to the northwest of Cofre de Perote volcano, and in exploratory wells in the Palma Sola region, are schists, andesitic lava flows, and dioritic to granitic plutons of Paleozoic age (K-Ar dates 269-252 Ma) that belong to the so-called “Tezuitlán Massif” [López-Infanzón and Torres-Vargas, 1984; Jacobo, 1985; López-Infanzón, 1991]. Stratigraphic and petrologic studies associate these rocks to a Permo-Triassic volcanic arc that extends from Chihuahua in northern Mexico to Chiapas in the south [Torres et al., 1999]. Locally, these rocks are covered by Jurassic to early Miocene, marine to continental sedimentary sequences that have been subject to numerous studies by PEMEX, the Mexican national oil company.

[9] The initial magmatic phases related to the TMVB activity are characterized by middle to late Miocene (17–6.5 Ma) intrusive rocks of variable composition (gabbros to granites) that crop out along the coast of the Gulf of Mexico: the so-called “old formation” of Negendank et al. [1985]. They usually form prominent stocks or strongly tectonized dike complexes with sulfur mineralization and chlorite alteration. In several localities, these plutonic rocks are directly cut by dikes that fed widespread alkaline plateau basalts of late Miocene to Pleistocene age (6–0.82 Ma). These plateau basalts are formed by 1 to 10 m thick, stratigraphically continuous sheets, that were clearly not erupted from central volcanic vents. At least one of these lava flows contains abundant spinel lherzolite mantle xenoliths and clinopyroxene megacrysts, indicating that these magmas ascended rapidly through the upper mantle and crust. Although there is not a detailed structural study of the area, these characteristics indicate that the magmatic event that formed the plateau basalts is most likely related to an important phase of tectonic extension. By the late Pleistocene and into the Holocene, the volcanic activity gradually changed its character to form central volcanic structures. In the Palma Sola area, several calc-alkaline basaltic cinder cones, some as young as 900 a [Siebert and Carrasco-Núñez, 2002], were unconformably emplaced over the plateau basalts or the plutonic rocks.

3. Analytical Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[10] All samples were initially crushed using steel plates, steel jaw crushers, or a heavy steel mortar and pestle. Fresh, hand picked chips were cleaned ultrasonically in milli-Q water and then powdered in an alumina mill and shatter box. Powders were used for major and trace element analyses, while 1–0.5 mm, clean, hand-picked chips were directly leached and dissolved in acids for the isotopic measurements.

[11] All samples were analyzed for mayor elements by X-ray fluorescence on a Siemens SRS-3000 instrument at Instituto de Geología, UNAM following procedures described elsewhere [Lozano et al., 1995; Verma et al., 1996]. Trace element data were obtained by ICP-MS at the Lamont-Doherty Earth Observatory (LDEO) using a VG-PQ2 + instrument. Sr, Nd and Pb isotopic ratios were also measured at LDEO using a TIMS VG sector 54 equipped with nine Faraday collectors.

[12] For the trace element determinations 50 mg of sample were initially digested in 2 mL HF plus 1 mL 7N HNO3 in closed Savillex Teflon beakers, and put on a hot plate overnight at 125°C. Samples were evaporated to dryness followed by the addition of 15–20 drops of concentrated (∼16N) HNO3. This step was repeated at least twice in order to break down the fluorides. After evaporation to dryness, 2 mL of quartz distilled H2O (QD-H2O) plus 2 mL of 7N HNO3 were added, and samples were left closed overnight at 125°C. All samples were in complete solution after this step. The samples were diluted to 1:2000 and 1:10000 to provide adequate concentrations within the instrument detection limits. All samples were spiked with an internal standard solution made of 115In (5 ppb), 169Tm (7 ppb) and 209Bi (9 ppb) to accurately monitor instrumental drift. Calibration and data reduction were based on international and in-house rock standards (BCR-1, BHVO-1, MAR and Tp10c) and at least 2 blanks that were run through the same chemical procedure as the samples. Reproducibility within a run was monitored by at least 20 analyses of in-house standard K1919, an equivalent to BHVO-1. Precision and reproducibility of trace element data are given by the values obtained from multiple digestions of BHVO-1 and BCR-1 rock standards, together with 1 sigma standard deviations (Table 2).

[13] For isotopic analyses, between 200 and 300 mg of rock chips were initially leached in 1.0 mL 1N HCl on a hot plate at 100°C for 1 hour. Leaches were saved and the sample was rinsed 3 times in QD-H2O. Samples were then digested with 2 mL HF plus 1 mL 8N HNO3 for at least 2 days on a hot plate at 125°C, followed by at least two steps of drying and fluxing with ∼16N HNO3 to break down the fluorides. Pb was separated using AG1-X8 100-200 anion mesh resin and 100 μl Teflon columns. Sr was separated using Eichrom Sr-spec resin using 30 μl Teflon columns. Nd was separated using two independent column chemistry procedures: the separation of the REE using Eichrom Tru-spec resin and 100 μl columns, followed by the separation of Nd using AG 50W-X4 200-400 mesh cation exchange resin with 0.15M, 4.8 pH alpha-hydroxy isobutyric acid (alpha-HIBA) in 0.8 mL Teflon columns.

[14] The measured 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0.1194 and adjusted to NBS-987 standard ratio of 0.710230. During two separated intervals of analyses, measured values of NBS-987 standard were 87Sr/86Sr = 0.710245 ± 0.000016 (2σ, n = 4) and 0.710271 ± 0.000014 (2σ, n = 6). The measured 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.72190 and corrected to La Jolla Standard value of 143Nd/144Nd = 0.511860. Over the course of this study, the measured 143Nd/144Nd ratio of La Jolla standard was 0.5118359 ± 0.000013 (2σ, n = 15). Common Pb isotopic ratios were corrected for mass fractionation using the LDEO 207Pb-204Pb double spike. The samples were analyzed twice: once mixed with the double spike and once without it. The fractionation corrected values were adjusted to NBS-981 standard values of 206Pb/204Pb = 16.9356, 207Pb/204Pb = 15.4891, and 208Pb/204Pb = 36.7006 [Todt et al., 1996]. Over the course of this study, the double-spiked fractionation corrected Pb isotopic compositions of the NBS-981 standard were 206Pb/204Pb = 16.9356 ± 0.0048 (143 ppm), 207Pb/204Pb = 15.4912 ± 0.0047 (152 ppm), 208Pb/204Pb = 36.7025 ± 0.014 (191 ppm) (2σ, n = 13). Total procedural blanks between 300–500 pg for Pb were negligible compared to the concentrations of this element in the dissolved rock samples.

4. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[15] Miocene to Quaternary magmatic rocks from the eastern TMVB can be divided in terms of their stratigraphic, chemical and geological characteristics into three main groups: (a) Miocene plutons, (b) latest Miocene to Pleistocene plateau basalts, and (c) Quaternary cinder cones. The new mineralogical and geochemical data (Tables 1, 2, and 3), and subsequent figures, use this division.

Table 1. Modal Analyses and CIPW Norms of the Palma Sola Magmatic Suites and Paleozoic Basementa
SampleModal AnalysisbCIPW NormcAlteration
OlPlCpxOpxHnblBiMsQtzEpChlOpGmsQtzCrnOrAbAnNeDiHyOlMagIlmAp
  • a

    Mineral abbreviations: olivine (Ol), plagioclase (Pl), clinopyroxene (Cpx), orthopyroxene (Opx), hornblende (Hnbl), biotite (Bi), muscovite (Ms), quartz (Qtz), epidote (Ep), chlorite (Chl), opaque minerals (Op), groundmass (Gms), corundum (Crn), orthoclase (Or), albite (Ab), anorthite (An), nepheline (Ne), diopside (Di), hypersthene (Hy), magnetite (Mag), ilmenite (Ilm), apatite (Ap), iddingsite (Id).

  • b

    Modal proportions of phenocrysts (>0.3 mm) from a minimum of 1200 points.

  • c

    CIPW norms were calculated with a constant Fe2O3/(FeO + Fe2O3) ratio of 0.15, using the Chemcast software developed by Cliff Ford at the University of Edinburgh http://www.glg.ed.ac.uk/glgsoft).

Plutons
PS-99-2177.814.14.63.37.132.429.58.017.30.51.92.11.2Incipient to Chl
PS-99-30A85.613.80.69.526.429.612.713.63.32.02.10.7Incipient to Chl
PS-99-3185.314.20.52.420.814.531.715.710.51.81.90.7Incipient to Chl
PS-99-3785.514.40.37.823.924.719.40.619.81.91.50.5Fresh
PS-99-3824.23.61.570.612.714.848.412.92.27.00.70.80.5Fresh
 
Plateau Basalts
PS-99-149.84.42.483.26.022.229.10.516.317.62.44.61.4Fresh
PS-99-189.117.30.273.413.735.021.45.911.55.31.93.81.4Fresh
PS-99-193.93.53.21.088.39.523.722.88.013.512.42.35.02.8Sparse Ol to Id
PS-99-258.30.75.60.185.39.622.326.213.12.216.82.75.81.4Fresh
PS-99-3512.017.12.70.567.56.613.026.15.523.016.82.65.41.2Sparse Ol to Id
PS-99-62.015.10.582.34.80.320.245.114.510.21.32.31.4Fresh
PS-99-265.812.62.778.79.628.926.71.512.612.62.34.01.9Sparse Ol to Id
PS-99-275.013.43.01.077.66.026.530.112.13.313.62.44.21.9Fresh
PS-99-1117.30.21.80.879.77.711.527.93.522.820.62.02.91.2Fresh
PS-99-5C5.40.20.194.315.515.423.75.116.416.42.13.81.6Fresh
 
Cinder Cones
PS-99-111.02.00.186.93.623.931.212.211.311.92.33.10.5Fresh
PS-99-35.01.23.090.88.423.927.517.45.110.42.24.01.2Sparse Ol to Id
PS-99-23.36.61.587.94.98.331.525.08.017.61.52.30.9Fresh
PS-99-20B2.88.23.285.63.06.628.129.710.616.91.82.70.7Fresh
PS-99-224.94.20.30.8      89.6 7.129.028.18.420.01.91.92.70.9Fresh
PS-99-93.43.90.192.53.28.329.026.67.620.71.62.30.7Fresh
CP359.416.00.20.373.94.826.830.51.211.919.22.13.5Fresh
CP225.46.887.75.431.125.913.62.315.72.43.7Sparse Ol to Id
CP276.22.80.590.36.029.327.018.60.413.91.82.9Fresh
CP295.311.10.483.08.328.027.10.115.714.52.03.30.9Fresh
 
Paleozoic Basement
FO-99-15.010.058.012.015.029.91.316.117.918.8  13.1 1.21.50.2 
FO-99-313.44.91.80.279.527.12.922.539.90.8  5.2 0.60.80.2 
FO-99-418.02.015.02.063.03.9 0.641.118.5 7.022.7 2.83.10.5 
Table 2. Major and Trace Element Data of the Palma Sola Magmatic Suites and Paleozoic Basementa
 Sample
PS-99-21PS-99-30APS-99-31PS-99-37PS-99-38PS-99-14PS-99-18PS-99-19PS-99-25PS-99-35PS-99-6PS-99-26PS-99-27PS-99-11PS-99-5CPS-99-1PS-99-3PS-99-2PS-99-20BPS-99-22PS-99-9CP35CP22CP27CP29FO-99-1FO-99-3FO-99-4BCR-1bBCR-1BHVObBHVO
  • a

    Major elements in wt% were analyzed by XRF. Trace elements reported in ppm were analyzed by ICP-MS. Rock unit abbreviations are Plutons (Pl), Plateau Basalts (PB), Cinder Cones (CC), Paleozoic Basement (Paleoz).

  • b

    Reproducibility and precision are given by the average concentrations and standard deviations of multiple digestions (n = 5) of U.S. Geological Survey rock standards BCR-1 and BHVO.

UnitPlPlPlPlPlPBPBPBPBPBPBPBPBPBPBCCCCCCCCCCCCCCCCCCCCPaleozPaleozPaleozn = 5n = 5
Long96.52196.46396.48896.42496.41096.75196.69796.65896.55896.54596.77896.58296.60196.84396.77696.94096.81396.90496.51796.51996.86397.11796.90097.16797.23397.50596.81396.791    
Lat19.79219.69319.68919.60119.66319.89219.83919.77619.92919.71319.66919.88919.87119.96619.66519.62319.66119.66419.80119.79519.68619.55019.36719.35019.31719.92419.66119.670    
LocalityCandelariaLa LuzCalicheC. BernardilloC. CanteraCoyotitlánJuniqueMesa FarfánR. NuevoSan MiguelMafafasSta. LucreciaEl BejucoC. EspaldillaEl PatioEl EsquilónAgua SuelosEl ÓrganoLos AtlixcosLos AtlixcosC. AcatlánLa PresaMahuistlánJesús MaríaProvidenciaChachaltzinChachaltzinChachaltzin    
SiO250.7750.0648.2449.7164.7846.3852.3544.8945.6544.3159.2648.2047.4745.5147.1148.7248.7156.4452.9551.9454.3948.3349.3150.1649.7166.2471.4253.77    
TiO21.041.050.940.830.362.381.972.482.962.791.142.082.151.451.941.562.091.161.411.401.141.761.881.451.650.780.371.55    
Al2O317.6017.0216.5714.8616.5815.9916.8316.7015.2114.9417.8217.3717.0114.7016.0316.5316.0016.7117.2116.9316.6117.5316.1716.4116.7814.3214.9214.31    
Fe2O3*9.129.768.089.353.4111.739.5611.0512.9912.526.1911.3211.819.9710.4511.3310.657.388.889.417.8110.4711.579.1110.156.073.0913.34    
MnO0.170.160.210.160.130.160.180.180.180.160.150.160.190.160.170.160.160.120.140.150.120.150.160.150.150.150.040.19    
MgO4.144.812.9010.261.577.974.684.677.278.531.354.426.0311.117.738.286.665.454.856.416.268.417.007.396.622.390.694.01    
CaO8.209.079.589.943.3510.496.489.009.0011.353.689.359.9511.659.589.5010.297.548.828.157.548.968.439.949.773.850.355.49    
Na2O3.663.021.532.795.592.664.084.292.532.705.243.673.122.042.852.772.813.763.253.343.383.363.653.453.322.054.724.72    
K2O1.181.533.241.272.461.022.241.521.531.053.351.590.991.232.570.561.371.411.091.201.370.810.920.971.432.683.800.09    
P2O50.450.250.300.220.200.570.611.130.550.470.580.820.830.460.700.190.520.390.290.350.30   0.420.140.070.19    
Loi0.352.76 0.030.96 0.783.641.640.620.470.34 0.97   0.150.16 0.09    2.031.232.89    
Total96.6799.4991.5999.4199.3999.3599.7699.5499.5299.4499.2399.3299.5499.2599.1299.5999.27100.5099.0499.2899.0199.7799.1099.04100.00100.69100.70100.55    
Sc20.426.116.333.111.827.917.515.221.332.39.224.323.336.226.628.825.821.525.122.419.526.724.829.627.813.14.639.132.92.2930.70.9
V250259211249902671312232183215226325126724820125115517818015522821422121675203094094.923145
Cr42.328.75.5735.518.1219.7144.016.5151.4229.80.182.564.6477.7268.5315.9146.1176.683.2154.8228.3271.1208.5349.9262.179.27.41.812.54.41292.811.6
Co26.829.620.941.89.643.425.728.543.552.06.435.135.644.939.849.036.023.628.133.529.343.640.334.934.012.83.623.036.00.7744.20.5
Ni24.019.83.6220.87.591.561.610.278.8121.4 26.735.2180.9107.7145.963.051.731.188.9102.2119.1121.284.655.049.26.38.29.81.52119.03.8
Cu98.083.183.686.516.237.831.244.346.447.89.134.037.658.866.469.846.541.536.638.134.542.346.840.731.664.417.952.822.73.08148.60.7
Zn96.2116.978.673.043.496.690.2114.097.777.992.6111.878.167.967.171.781.781.670.689.372.376.1116.874.182.2122.040.098.4130.93.9899.412.9
Ga22.221.118.616.928.820.522.422.920.821.323.522.119.517.219.919.321.019.620.320.419.419.120.918.820.018.714.017.322.30.0821.70.2
Li12.247.5114.788.3216.254.1615.3810.725.365.3013.036.035.905.347.225.784.5510.765.727.328.075.577.917.147.5510.4312.2614.2512.600.524.690.41
Be1.791.571.401.233.521.483.882.271.602.085.483.992.071.403.241.211.001.751.731.681.47    2.180.840.931.780.211.090.14
B6.96.0  6.83.89.5 3.84.66.6 5.75.28.713.84.5 5.36.74.7    6.96.33.54.51.333.01.1
Rb33.3631.0998.7537.0966.1919.3038.2234.2922.4115.9098.5556.0418.2329.0981.939.8434.3832.6918.4918.4026.8014.2011.0918.0130.9097.30109.392.3647.040.249.310.11
Sr751621317485906783788103077372364078910907519873617426105806235325684664975683991601313354.333995
Y24.223.020.720.216.927.729.933.928.127.539.436.627.725.031.224.427.723.228.829.822.923.831.426.129.235.118.116.737.90.3327.40.2
Zr26.984.087.6112.680.9193.1378.2302.1268.5197.2489.9287.3192.7155.0284.8131.1245.7209.2197.2211.9193.7162.7208.7175.8222.7231.8152.849.2194.81.91176.80.9
Nb8.8321.029.666.5326.5422.3457.7854.2541.9131.1856.7829.5028.1924.6031.669.0927.2817.2010.6612.2313.0215.2614.9312.2712.2317.176.561.6013.450.1720.030.11
Mo1.481.270.211.320.151.284.111.662.391.933.300.911.790.981.470.681.701.591.031.021.190.950.921.170.970.290.544.541.510.051.080.10
Sn1.561.281.041.091.051.642.522.092.201.852.632.111.501.252.361.171.723.461.341.401.361.361.511.191.582.751.590.952.340.062.100.10
Sb0.070.040.690.050.160.030.110.050.060.040.080.040.040.030.110.020.040.050.040.030.040.030.030.030.020.340.451.080.620.000.160.01
Cs2.471.017.211.762.390.601.380.900.180.551.460.910.730.983.560.241.330.780.360.270.670.440.160.600.446.602.180.130.950.020.100.02
Ba625527600446268838051553727846611336704824801002153503525403447489250295300502584851486792.451333
La28.9624.2416.2616.5646.3126.1347.9552.1030.6928.2876.4237.7242.5434.4252.7111.3135.1228.6423.8426.1425.1817.1320.8117.5828.7837.2623.925.3525.320.1515.710.14
Ce59.9348.4235.2935.8672.9756.8596.50105.6665.9759.03129.3482.0086.2065.62109.9725.7273.0358.1750.2656.7552.1537.6546.4539.0964.2278.7644.3312.5853.200.1738.370.49
Pr7.706.004.624.639.237.4511.1712.568.177.7017.3210.5310.697.9313.883.488.996.966.607.506.444.926.165.118.489.095.271.746.840.055.380.08
Nd32.0424.3019.5719.6234.3233.0643.4350.6134.3033.4664.1544.4643.6232.1256.3615.5037.7327.8027.2731.5126.4120.9826.7121.7735.6734.2219.368.1128.640.2024.680.27
Sm6.635.134.434.445.967.238.1710.057.367.4411.799.548.746.5311.584.037.595.605.766.575.344.916.055.007.817.104.012.206.600.066.170.05
Eu1.911.541.451.271.612.342.622.962.412.373.092.812.641.963.181.402.341.641.761.961.631.631.911.622.281.471.020.751.990.022.070.03
Gd5.844.723.984.134.686.817.068.606.816.899.588.297.535.869.504.406.684.905.576.174.854.805.804.876.766.283.532.546.720.056.200.05
Tb0.830.710.620.620.581.001.051.221.010.991.361.191.060.851.260.710.950.730.850.910.720.750.930.760.990.970.540.431.070.010.950.01
Dy4.494.053.603.522.685.325.576.445.555.337.236.395.444.636.184.365.284.074.935.224.094.315.514.485.435.833.112.886.330.035.320.06
Ho0.850.800.710.700.491.001.061.171.041.011.411.231.010.901.100.891.000.791.011.030.810.831.100.911.021.200.610.651.300.011.000.01
Er2.292.221.921.901.192.552.852.962.682.523.803.192.562.332.682.442.562.122.732.802.182.243.002.452.683.351.681.943.570.022.520.03
Yb2.042.041.751.791.002.092.522.482.282.033.722.832.102.062.232.282.221.982.572.582.062.042.772.292.413.311.651.963.370.012.060.02
Lu0.300.300.260.280.150.300.380.360.330.290.560.430.310.300.330.340.330.300.380.390.310.300.420.350.360.490.270.320.510.0030.290.005
Hf1.022.292.422.922.944.427.746.565.974.7710.596.524.203.536.553.055.354.804.434.774.473.664.654.015.265.783.881.484.930.054.480.05
Ta0.531.100.560.371.211.293.412.932.561.783.231.591.381.291.770.521.510.910.580.640.700.870.840.700.721.110.600.110.810.011.230.01
W0.410.280.310.300.160.180.690.300.170.280.850.230.280.270.490.160.260.300.190.170.250.140.180.150.140.960.846.920.430.010.240.01
Tl0.160.201.220.210.310.110.180.170.060.090.590.170.070.140.630.070.110.200.120.090.200.090.060.140.250.690.900.020.320.0020.050.003
Pb11.186.456.338.7818.033.025.886.612.522.4914.435.434.845.4412.113.904.688.325.315.517.352.783.924.214.7023.156.516.6013.620.082.010.30
Th5.694.342.884.476.493.246.807.953.493.1113.884.326.306.0711.701.394.784.512.843.333.741.912.011.983.9811.838.271.236.000.021.240.02
U1.471.200.891.454.601.002.051.921.130.934.121.371.681.593.360.411.471.230.710.860.990.630.510.651.042.092.360.591.640.0110.410.011
Table 3. Sr, Nd, and Pb Isotopes of the Palma Sola Magmatic Suites and Paleozoic Basementa
 87Sr/86Sr206Pb/204Pb207Pb/204Pb208Pb/204Pb143Nd/144NdTDM,b Ga
  • a

    Reported values are not age corrected and taken as initials. No absolute ages were determined in this study. The 2σ errors for individual Sr and Nd measurements are multiplied by 106. Reproducibility and precision of Pb isotopes are given by the double-spike fractionation corrected composition of the NBS-981 standard 206Pb/204Pb = 16.9356 ± 0.0048, 207Pb/204Pb = 15.4912 ± 0.0047, 208Pb/204Pb = 36.7025 ± 0.014 (2σ, n = 13).

  • b

    Depleted mantle model ages of Paleozoic samples calculated following Goldstein et al. [1984].

Plutons
PS-99-210.7038971618.65815.58538.4020.51286616 
PS-99-30A0.7037191218.70915.60138.4650.51284214 
PS-99-310.7038051018.67315.59238.426   
PS-99-370.7040141018.73115.60438.5070.51282812 
PS-99-380.7035381018.65215.57438.3590.51290320 
 
Plateau Basalts
PS-99-140.7035208   0.51288314 
PS-99-180.7032601218.87015.60538.5450.51291214 
PS-99-190.703759818.88715.59638.6100.51290432 
PS-99-250.7030961018.96315.59238.5340.51294112 
PS-99-350.7032301018.84915.58938.4830.51294922 
PS-99-60.7039191618.80015.60138.5330.51284916 
PS-99-260.7040501018.73715.59938.4540.51283514 
PS-99-270.7035931218.80215.59538.4960.51289814 
PS-99-110.7038771018.76215.59538.4940.51282622 
PS-99-5C0.7041441018.76115.60238.5350.51285318 
 
Cinder Cones
PS-99-10.7035771018.76115.59838.4970.51289914 
PS-99-30.7035591018.80215.59338.4860.51286112 
PS-99-20.7045351818.73015.61238.5100.51270116 
PS-99-20B0.7041731018.76815.63438.5900.51271012 
PS-99-220.7043731218.74115.60938.4890.51268912 
PS-99-90.7044431018.73115.61038.4910.51270512 
CP350.7034561918.72815.59838.4270.51291210 
CP220.704194818.77015.60838.5170.5127599 
CP290.7041811718.74415.60238.4940.51281310 
 
Paleozoic Basement
FO-99-10.7147671018.80615.66538.8130.512061141.9
FO-99-30.7097811218.92615.63738.7970.512512121.1
FO-99-40.7071431018.59015.62038.3290.512734141.3

[16] Petrographic features of the Palma Sola rock suites have been described by previous workers in a number of publications [Cantagrel and Robin, 1979; Negendank et al., 1985; López-Infanzón, 1991]. Modal analyses and CIPW norms of the rocks analyzed in this study are presented in Table 1. The mineral assembly of the plutonic rocks is essentially composed by plagioclase, clinopyroxene, and a small amount of biotite; with the exception of the most evolved plutonic sample that contains hornblende and biotite without clinopyroxene (sample PS-99-38). Olivine and plagioclase are ubiquitous phenocrysts in the plateau basalts and cinder cones, and several samples also contain a small amount of Cpx ± Opx phenocrysts. The phenocryst assembly of plateau basalts and cinder cones is usually contained in a microcrystalline groundmass made of plagioclase, olivine, sparse pyroxene and opaque minerals. About half of the plateau basalts analyzed in this study are nepheline-normative. In contrast, most of the cinder cones and all of the plutons are hypersthene-normative.

[17] The geochemical differences among the sequences are clearly portrayed in their major and trace element variation diagrams (Figure 2). Plateau basalts are significantly distinct from the rest of the units and show differentiation trends compatible with the alkaline series. In contrast, plutons and cinder cones are subalkaline. The subalkaline rocks do not show iron enrichment during differentiation processes, plotting within the calc-alkaline field on an AFM diagram (not shown). Plateau basalts also have lower SiO2 and Al2O3, but higher Fe2O3, TiO2, MnO and P2O5, at a given value of MgO when compared to the plutons and cinder cones. Trace element concentrations of the plateau basalts are relatively enriched when compared to the plutons and cinder cones. Highly incompatible trace elements like Ba, Th, and Nb typically increase with decreasing MgO contents in all sequences. In contrast, relatively more compatible elements like Zr, Hf, Y and the HREE typically decrease with decreasing MgO in the plutons, but they remain somehow constant in the cinder cones.

image

Figure 2. Major (wt%) and trace element (ppm) variation diagrams of the Palma Sola magmatic suites. Alkaline and subalkaline division from Irvine and Baragar [1971].

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[18] Distinctions among the three suites are also observed in their trace element patterns (Figure 3). Plutonic rocks show prominent enrichments of LIL elements and strong Nb-Ta and Zr-Hf troughs (Figure 3a), a pattern typical of arc volcanics. In contrast, plateau basalts show more variable trace element systematics (Figure 3b): a set of lavas show Nb-Ta enrichments and very small or nonexistent Pb and Sr positive spikes, a pattern commonly observed in intraplate, oceanic-island basalts (OIB). Another set of plateau basalts have trace element signatures with high LILE/HFSE ratios and an unusual flat trend in the normalized concentrations of Rb, Ba, Th and U (Figure 3b, sample PS-5c). In the field, both sets of lavas are interstratified with no clear correlation between stratigraphic position and geochemical character. Notably, the contrasting trace element patterns of the plateau basalts often occur at the same MgO content. Rocks erupted from cinder cones also show two kinds of trace element patterns (Figure 3c): the most primitive rocks (SiO2 between 48% and 50%) generally show a very weak subduction signature, but the “subduction signal” gradually becomes stronger in more evolved rocks (SiO2 between 50% and 56%).

image

Figure 3. Trace element and REE patterns of the Palma Sola magmatic suites. Primitive mantle values from Sun and McDonough [1989]. Chondrite values from McDonough and Sun [1995] (a) plutonic rocks; (b) plateau basalts; (c) cinder cones.

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[19] Chondrite-normalized REE patterns of the different suites are shown as insets in Figure 3. All rocks show LREE enrichment over the HREE. Chondrite-normalized La/YbN ratios of most of the plutonic rocks vary from 6.2 to 9.6, although the most evolved plutonic rock (PS-99-38) is strongly depleted in HREE (La/YbN = 31.5). In contrast, plateau basalts tend to have more fractionated REE patterns than the rest of the sequences (La/YbN = 8.5–16). Cinder cones are once again less fractionated, with La/YbN ratios similar to most of the plutonic rocks (La/YbN = 3.4–10.7). Eu/Eu* anomalies of the plutonic rocks and plateau basalts vary from 1.05 to 0.88, and do not show a coherent correlation with differentiation indices. In contrast, Eu/Eu* anomalies of the cinder cones vary from 1.02 to 0.94 and correlate positively with MgO contents (not shown).

[20] Relationships among Sr-Nd-Pb isotope ratios and possible end-members are shown in Figure 4. Overall Sr-Nd isotopic compositions of the Palma Sola volcanic rocks show a tight negative correlation (Figure 4a). Plateau basalts with OIB-like trace element patterns (i.e., sample PS-99-25, see Figure 3) have lower 87Sr/86Sr and higher 143Nd/144Nd isotopic ratios than the rest of the samples, but they are more enriched than the East Pacific Rise (EPR) mid-ocean ridge basalts (MORB). Pb isotopic compositions of the plateau basalts are also much more radiogenic than EPR MORB (Figures 4b and 4c), and form a distinct correlation in the 206Pb/204Pb-207Pb/204Pb-208Pb/204Pb space that trends toward the composition of the bulk subducted sediment (A. B. LaGatta et al., Changing sediment contributions across the Mexican Volcanic Belt, manuscript in preparation, 2003) (hereinafter referred to as LaGatta et al., manuscript in preparation, 2003). The Sr, Nd and Pb isotopic variations of the plateau basalts correlate with each other and with subduction signatures (Figures 4d and 5). Plutonic rocks plot within the field of the plateau basalts in the Sr-Nd array (Figure 4a), but form a different correlation trend in their Pb isotopic compositions that extends to unradiogenic Pb isotope ratios (Figures 4b and 4c). Like most of the Pb isotopic data of the TMVB, the Pb isotopic composition of the plutonic rocks form a correlation array bracketed between the bulk subducted sediment and EPR MORB. Low silica cinder cones plot within the plateau basalts field in the Sr-Nd isotopic composition, whereas more evolved cinder cones extend to a significantly more enriched isotopic values. Cinder cones do not show a coherent trend in their Pb isotopic compositions, plotting mostly within the field of the plateau basalts. Higher silica rocks, however, tend to have higher 207Pb/204Pb ratios than the bulk sediment, and point toward the composition of the local continental crust.

image

Figure 4. Isotope variation diagrams of the Palma Sola volcanics and possible end-members. (a) Sr-Nd isotopic variation diagram; (b–c) Pb isotopes variation diagram. The yellow field represents the Pb isotopic composition of most samples within the current volcanic front of the TMVB (A. B. LaGatta and A. Gómez-Tuena, unpublished data); (d) Pb-Nd isotopic variation diagram. Also shown: data field from 5°–15° North East Pacific Rise MORB (PETDB, 2002, http://petdb.ldeo.columbia.edu/); weighted bulk sediment composition from DSDP site 487 (LaGatta et al., manuscript in preparation, 2003); late-Paleozoic rocks from the Teziutlán massif (this study); Sr-Nd data field of lower crustal xenoliths with Grenville-age Nd model ages collected in northern Mexico [Ruiz et al., 1988a, 1988b; Schaaf et al., 1994]; Pb and Nd isotopic data field of the Grenville-age Huiznopala Gneiss (Precambrian crust) [Lawlor et al., 1999]; depleted MORB-mantle (DMM), enriched mantle I (EMI), enriched mantle II (EMII) and HIMU mantle components [Zindler and Hart, 1986], and Northern Hemisphere reference line [Hart, 1984].

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image

Figure 5. Ba/Nb ratios of the plateau basalts and plutonic rocks correlate with 206Pb/204Pb ratios (and Sr-Nd isotopic enrichment). Cinder cones do not show a coherent correlation, plotting mostly within the enriched tip of the plateau basalts.

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5. Geochemical Affinities and Systematics

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[21] On the basis of experimental work and the geochemical characterization of several volcanic arcs, there is currently a wealth of information describing the roles played by the different elements and isotopic ratios in the arc environment. In this paper we generally follow the same rationale used by Class et al. [2000] and Hochstaedter et al. [2001] to differentiate the roles of the different components and processes involved in the arc magma petrogenesis: (1) The HFS elements and the HREE are generally not considered to be mobile in fluids [Brenan et al., 1995c] nor are they significantly enriched in sediments [Plank and Langmuir, 1998], and therefore can be used to some extent to infer the characteristics of the mantle sources. (2) Elements like Ba, Sr, and Pb, and to a lesser extent Rb and U, are mobilized by fluids [Brenan et al., 1995a, 1995c; Keppler, 1996; Ayers et al., 1997; Stadler et al., 1998]; thus their relative enrichments can be used as proxies of the fluid contributions. In addition, the altered oceanic crust and sediment pile sampled near the Middle America Trench off the coast of Acapulco (DSDP site 487) have different isotopic compositions [Verma, 1999a; LaGatta et al., manuscript in preparation, 2003]. Therefore the isotopic ratios of Sr and Pb can be used to differentiate between different fluid sources coming off the subducted slab. (3) The LREE and Th are commonly enriched in the oceanic sediments [Plank and Langmuir, 1998] but are not considered to be highly mobile in fluids [Brenan et al., 1995c; Johnson and Plank, 1999], and therefore sediment-melt contributions should be recognizable by the relative enrichments of these elements [Vroon et al., 1995; Class et al., 2000]. Furthermore, Nd additions from the sediments should bring unradiogenic Nd isotopic compositions into the mantle wedge. (4) Melts coming from the metamorphosed subducted slab usually show steep REE patterns due to a garnet-rich residue, positive Sr spikes, high Sr/Y ratios, low concentrations of the HREE, and isotopic compositions similar to MORB or to altered oceanic crust [Defant and Drummond, 1990]. (5) The participation of the continental crust in the petrogenesis should be recognizable when the isotopic modifications are correlated with the process of crystal fractionation [DePaolo, 1981].

6. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[22] The main goal of this section is to recognize the different components and processes that generated the geochemical variations observed in the three different magmatic suites described above and to quantify the participation of the different fluxes.

6.1. Crustal Contamination

[23] Hildreth and Moorbath [1988] argued that most of the chemical and isotopic variability of continental arc magmas develops at the crust-mantle boundary in zones of melting, assimilation, storage and homogenization (MASH), and that the processes that take place in the deep mantle are largely obscured. Magmas ascending from such zones can be later compositionally modified as they fractionate and assimilate upper crustal rocks, following Assimilation Fractional Crystallization (AFC) paths. Magmas formed in this way should be physical mixtures between mantle and lower crustal melts. Melting of the lower crust requires heat from the intruding magma, a process that induces its crystallization (i.e., differentiation), and generates a melt with higher silica than the original magma. Therefore arc signatures formed by a MASH process should be more pronounced in more evolved rocks and true basalts would rarely erupt. In contrast, magma formation by mantle metasomatism in subduction zones should initially create a basaltic melt in which the “arc signature” depends on the amount of fluxing from the slab, a process that can significantly modify the isotopic and trace element composition of the resulting melts, with only minor effects on the SiO2 and MgO contents. Therefore the chemical variations induced by subduction should be distinguishable from those associated with MASH processes or upper crustal contamination, because the physical processes of enrichment are not only different but should also bring contrasting chemical patterns, even though the participating components can have similar compositions.

[24] An accurate evaluation of magma genesis in continental arcs thus requires that the different components involved are known or can be reasonably inferred with the geologic information available. Unfortunately, the characteristics of continental basement rocks underneath the TMVB are largely unknown because most of them are covered by Mesozoic to Holocene sedimentary and volcanic sequences. Even so, different lines of evidence indicate that the volcanic arc is emplaced over large crustal masses or terranes, with contrasting ages and geologic characteristics [Sedlock et al., 1993; Ortega-Gutiérrez et al., 1994]. According to these studies, the basement rocks of eastern Mexico are largely Grenville age (∼1 Ga) gabbros, charnokites, anorthosites, and metapelites with metamorphic peaks in the granulite facies that belong to the so-called Oaxaquia microcontinent [Ortega-Gutiérrez et al., 1995]. Lower crustal xenoliths have been also recovered in several areas of northern Mexico and, although they are generally more mafic than the exposed terranes, their Nd model ages are generally consistent with the Grenville-age Oaxaquia microcontinent. Several studies have dealt with the geochemistry of the xenoliths and high-pressure terranes, and there is an extensive data base available in the literature [Ruiz et al., 1988a, 1988b; Roberts and Ruiz, 1989; Schaaf et al., 1994; Lawlor et al., 1999]. In general, Oaxaquia rocks show the typical characteristics of lower crustal materials: high Ba/Th and low Th/Nd ratios, negative Pb spikes, and strong positive Sr and Eu anomalies (Figure 6). Importantly, all lower crustal materials analyzed so far in Mexico have ‘enriched’ Sr and Nd isotopic compositions but unradiogenic Pb isotopes when compared to the Palma Sola rock suites (Figure 4). The closest outcrops of Grenville-age rocks are 220 km to the NW and 250 km to the south of Pico de Orizaba volcano, in Molango and Oaxaca, respectively, but Precambrian rocks have also been verified in exploratory wells along the northern Gulf coast. It is therefore usually assumed that the Oaxaquia microcontinent underlies most of eastern Mexico, even beyond the continental margin [Keppie and Ortega-Gutiérrez, 1995; Ortega-Gutiérrez et al., 1995].

image

Figure 6. Trace element patterns of possible crustal end-members and weighted bulk subducted sediment. Average Oaxaquia composition calculated from Lawlor et al. [1999], weighted bulk sediment data (LaGatta et al., manuscript in preparation, 2003), and sample FO-99-1 from the local Paleozoic basement is the component used in the AFC modeling.

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[25] In the Palma Sola area the oldest exposed rocks are Mesozoic to mid-Tertiary marine to continental sandstones, but their relative volume is small compared to the magmatic portion of the stratigraphic record. Exploratory wells have verified the existence of late Paleozoic plutonic and metasedimentary rocks underlying the Mesozoic sediments [Jacobo, 1985], and it has been proposed that rocks with similar age and lithologic characteristics extend to the northwest of the Cofre de Perote stratovolcano, in the so-called Teziutlan Massif [López-Infanzón and Torres-Vargas, 1984; López-Infanzón, 1991]. The structural base of the Teziutlan massif is made of a strongly tectonized metamorphic complex of mica-schists, plutons (granodiorites to granites) and metavolcanic rocks (andesites to dacites) that have been dated between 259 and 252 Ma [López-Infanzón and Torres-Vargas, 1984; López-Infanzón, 1991]. In order to better constrain the participation of the crust in the petrogenesis of the Palma Sola volcanic rocks, we analyzed three metamorphic samples from the structural base of the Teziutlan Massif (Tables 1, 2, and 3). The samples are metasedimentary mica-schists (sample FO-99-1) and slightly foliated and chloritized metavolcanics (samples FO-99-3 and FO-99-4). The trace element characteristics of the Paleozoic basement rocks show typical upper crustal systematics with high LILE/HFSE ratios and negative anomalies in Sr and Eu (Figure 6). Isotopic data for these rocks is also variable but they have high 87Sr/86Sr and 207Pb/204Pb, coupled with low 143Nd/144Nd, and similar 206Pb/204Pb when compared to the Palma Sola volcanics (Figure 4).

[26] Variations of Sr, Nd, and Pb isotopic ratios with differentiation indices are shown in Figure 7. Isotope ratios and fractionation indices of the plateau basalts are uncorrelated. Primitive rocks with OIB and arc-like trace element patterns exist with almost the same MgO content (see Figure 3), a variation that few would attribute to crustal assimilation. Field evidence also indicates that the plateau basalts were most likely fed by dyke systems, since no central volcanic structures were associated with them. Also, at least one lava flow contains abundant spinel-lherzolite mantle xenoliths. These features indicate a rapid ascent to the surface and thus participation of the continental crust in their petrogenesis appears to be negligible. We thus conclude that the compositional variations of this suite are most likely mantle derived features.

image

Figure 7. Variations of Nd, Sr, and Pb isotopes with differentiation indices. Plateau basalts do not show a coherent correlation between isotopic enrichment and indices of fractionation. Cinder cones and plutons do show correlations between these parameters but in opposite directions. Evolved cinder cones appear to be consistent with the assimilation of an enriched upper crustal component, similar to the local Paleozoic basement. In contrast, the plutons require a depleted lower crustal component or the participation of melts coming from the subducted MORB (see text).

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[27] Plutonic rocks and cinder cones do show correlations among the isotopic compositions and indices of differentiation, but they follow diverging trends (Figure 7). Sr and Nd isotopic composition of the cinder cones appear to be consistent with contamination by an isotopically enriched crustal component. The 206Pb/204Pb ratios in the cinder cones do not follow a simple correlation trend with MgO or SiO2 contents, but incipient correlations can be seen in the 207Pb/204Pb and 208Pb/204Pb ratios. The poor correlations observed in the 206Pb/204Pb ratios probably indicate the lack of a significant compositional contrast between the mantle wedge (modified by subduction or not) and the crustal contaminants.

[28] An Assimilation Fractional Crystallization (AFC) model [DePaolo, 1981] that uses the most depleted and primitive cinder cone lava flow as starting magma composition (sample CP-35), and the average composition of the Paleozoic crust as a contaminant, fails to explain the trend of enrichment in the Sr-Nd-Pb isotopic space (not shown). If an average lower crustal composition is used as a contaminant (i.e., Oaxaquia), the AFC model predicts a trend that strongly modifies the 206Pb/204Pb isotopic composition of the contaminated rocks (orange line in Figure 8a). This is because the Palma Sola cinder cones and Oaxaquia have very different Pb isotopic compositions. Instead, the 206Pb/204Pb ratios of the cinder cones remain fairly constant with differentiation, thus making the Oaxaquia lower crust an implausible contaminant. Using the most enriched Paleozoic crustal sample as a contaminant (FO-99-1), the resulting trend passes closer to most of the data in Sr-Nd-Pb isotopic space, and also can explain the variations in concentrations and ratios of most of the incompatible trace elements (Figures 8 and 10). Even though no unique crustal end-member analyzed so far can fully describe the compositional systematics of the cinder cones, and it is somehow unclear the extent of which sample FO-99-1 represents the basement lithology of the Palma Sola area, it seems nonetheless reasonable to conclude that at least the most evolved cinder cone lavas appear to be contaminated with an upper crustal component, similar in composition to the most enriched portions of the nearby Paleozoic basement.

image

Figure 8. Crustal contamination models (AFC). All models assume sample CP-35 (MgO = 8.4) as starting magma composition. Green lines consider sample FO-99-1 from the local Paleozoic basement as a contaminant. Orange lines use the average Oaxaquia crust as a contaminant. All models assume a r (Ma/Mc) value of 0.8. Bulk D values used are DSr = 0.307, DNd = 0.142, DPb = 0.128, DBa = 0.104, DLa = 0.047, compatible with crystallization of 35% Ol, 34% Cpx, 20% Opx, 10% Plag 1% Ilmenite. The enriched tip of the models represent a value of Ma/Mm = 1 (a 1:1 mixture of mantle and crustal materials) and 80% of magma remaining (F). All parameters are from DePaolo [1981]. (a) AFC models of Pb-Nd isotopic compositions. Contamination with a Precambrian lower crustal composition (Oaxaquia) strongly modifies the Pb isotopic composition of the resulting melts. (b) AFC modeling of Sr-Nd isotopic compositions; (c) 143Nd/144Nd versus Ba, (d) 143Nd/144Nd versus La.

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[29] Plutonic rocks follow an opposite correlation trend to the cinder cones, with the most evolved plutonic sample (PS-99-38) being the most isotopically depleted (Figure 7). This particular rock sample also has highly fractionated Gd/Yb ratios, low HREE concentrations and positive Sr anomalies (Figure 3a). If the geochemical characteristics of the plutons are controlled by contamination, MASH processes, or crustal melting, then the composition of the crust involved has to be isotopically depleted in Sr, Nd and Pb, and it must have retained residual garnet, otherwise the highly fractionated Gd/Yb ratios and positive Sr spikes would not be preserved. In other words, an overthickened and isotopically depleted lower crust should be invoked. Clearly, this crustal component has to be different from the local Paleozoic rocks that apparently contaminated the Quaternary cinder cones. Lower crustal xenoliths and high pressure Precambrian terranes in Mexico (i.e., Oaxaquia), although very depleted in Pb isotopes, are all significantly more enriched in their Sr and Nd isotopic composition than the plutons making them again unlikely candidates (Figures 4 and 8). On the other hand, the contemporary crustal thickness in the Palma Sola area reaches only 20 km [Molina-Garza and Urrutia-Fucugauchi, 1993], a low pressure for garnet to be stable during melting of a mafic lower crust [Rapp and Watson, 1995]. With these considerations, the geochemical characteristics of the plutons can be equally explained by: (1) assimilation and/or melting of a garnet-bearing and isotopically depleted lower continental crust that was delaminated and recycled into the mantle shortly after the plutons were emplaced or (2) melting of the subducted oceanic crust and the formation of adakite magmas [Kay, 1978; Defant and Drummond, 1990] metasomatizing the subarc mantle wedge. Given that there is little geological evidence to support the existence of a thick, garnet bearing lower crust in this area, and the delamination scenario is ad hoc with no supporting evidence, the second possibility will be further explored below.

6.2. Mantle Wedge

[30] The composition of the mantle wedge beneath the eastern TMVB can be investigated by comparing the isotopic compositions, and the concentrations and ratios of the HFSE and the HREE in volcanic rocks with known petrologic reservoirs. Plateau basalts from the Palma Sola area show significantly higher Nb concentrations and Gd/Yb ratios than the Pacific MORB, with values that overlap OIB reservoirs (Figure 9a). Nb/Ta ratios of the Palma Sola volcanic rocks (∼18 on average) also tend to be higher than the average EPR MORB (∼15.5 on average), overlapping only with the most enriched portions of the EPR (Figure 9b). Data arrays involving Pb isotopes of the plateau basalts do not point toward the MORB field, but rather they terminate (or begin) at high 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in samples with the smallest subduction signatures (Figures 4 and 5). The most primitive plutons and cinder cones mostly share these elevated values in their Pb isotopes, indicating that this mantle end-member is contributing to some extent to all rock suites. All these characteristics suggest that the source region of the eastern TMVB is more likely related to an enriched OIB-like mantle wedge that has been compositionally modified by various subduction components.

image

Figure 9. Mantle source. (a) Comparison of trace element patterns: MORB, E-MORB and OIB [Sun and McDonough, 1989], average alkaline basalt from the Mexican basin and range province (Ventura, San Luis Potosí) [Lassiter and Luhr, 2001] and sample PS-99-25. (b) Nb/Ta versus Gd/Yb ratios of the Palma Sola volcanics and EPR MORB between 5°–15° N (PETDB, 2002, http://petdb.ldeo.columbia.edu/). Plateau basalts have higher Gd/Yb ratios at similar values of Nb/Ta than the rest of the magmatic suites. This is indicative of melting of a deeper region of the mantle, where garnet is more abundant. Higher Nb/Ta and Gd/Yb ratios of the most isotopically depleted pluton (sample PS-99-38) could be an indication of melts coming from the subducted MORB, in which garnet and rutile are residual phases. (c) Gd/Yb ratios do not show a coherent correlation with subduction signatures (i.e., Ba/Nb) indicating that Gd/Yb ratios are probably not a proxy for extent of melting, but provide some insight into the relative depth from which these magmas were extracted.

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[31] Cinder cones and most of the plutons have similar Nb/Ta ratios to the plateau basalts but lower Gd/Yb ratios (Figure 9b). While elevated Nb/Ta ratios appears to be an ubiquitous mantle feature shared by all rock suites, variations toward higher Gd/Yb ratios in the plateau basalts provide some insight into the mantle melting process. Higher Gd/Yb ratios in the plateau basalts could be controlled by lower extents of melting or by a relatively larger amount of modal garnet in the source at higher pressures [Green and Ringwood, 1970]. Even though the Gd/Yb ratios alone cannot distinguish between these two process, the relative concentrations of major elements like SiO2 and Fe2O3 at constant MgO contents are sensitive to variations in pressure and water contents in the sub-arc mantle source [Gaetani and Grove, 1998]. Cinder cones and plutons have higher SiO2 and lower Fe2O3 (higher SiO2/Fe2O3 + MgO ratios) than the plateau basalts at similar MgO contents (Figure 2). Experimental studies have shown that the effect of adding water to mantle peridotite at fixed pressures is to increase the silica activity of the melt, so that water-rich mantle melts should display higher SiO2 and lower Fe2O3 contents that those formed by anhydrous melting [Gaetani and Grove, 1998]. On the other hand, these experiments showed that the same composition is obtained by decreasing pressure at constant water contents. It has been also shown that the amount of fluid fluxing into the wedge should be positively correlated with the extent of mantle melting [Stolper and Newman, 1994; Reiners et al., 2000]. Following this rationale, the geochemical features of the cinder cones and most of the plutons (with lower Gd/Yb ratios) could be consistent with H2O-rich magmas generated by larger extents of partial melting than the plateau basalts at similar pressures. Nonetheless, Gd/Yb ratios do not correlate with other indicators of fluid contributions (i.e., Ba/Nb ratios) within and among rock suites (Figure 9c). Cinder cones and plateau basalts with similar Ba/Nb ratios have different Gd/Yb ratios, while most of the plutons have higher Ba/Nb ratios at similar Gd/Yb values to the cinder cones. These systematics thus imply that the relative concentrations of SiO2 and Fe2O3, combined with Gd/Yb ratios, are not proxies for extent of melting or water contents in the source but provide an insight on the relative depth from which these magmas were extracted. We thus conclude that the geochemical characteristics of the plateau basalts are consistent with melting of a relatively deeper region in the mantle wedge where garnet was more abundant. The significant overlap in isotopic composition and Nb/Ta ratios of the most primitive cones and plutons with the plateau basalts indicates that the subarc mantle column has a similar bulk chemical composition, but that changes in mantle mineralogy at different pressures have a strong effect on the REE patterns of the resulting partial melts. We further suggest that the mantle wedge below the Palma Sola region most likely melted by a combination of processes. The existence of interstratified plateau lavas with and without subduction signatures indicates that slab fluxing into the mantle is a heterogeneous process, and that at least a small part of this wedge melted by decompression and not by fluxing.

[32] An exception to this general rule is sample PS-99-38, the isotopically most depleted pluton. This rock has the highest Gd/Yb and Nb/Ta ratios of the whole data set. The partition coefficients of Nb and Ta in mantle mineralogies are so similar that the ratios between these two elements should not be significantly affected by melting or subsequent fractional crystallization. The only known mineral phase that can selectively fractionate Ta from Nb is rutile in the presence of a silicate melt [Green, 1995]. Experimental studies have shown that rutile is not a likely residual phase in the source of basaltic melts, as TiO2 contents in basalts are too low, but rutile appears to be stable during partial melting of hydrous basalts and eclogites [Green and Pearson, 1986; Ryerson and Watson, 1987]. Therefore the highly fractionated Gd/Yb and Nb/Ta ratios of this sample are strong indications that both garnet and rutile were probably residual phases during partial melting of an eclogite or a garnet amphibolite source.

6.3. Subduction Components

[33] While most researchers agree that the source region of convergent margin volcanism is the mantle wedge, and that element fluxing from the slab is the main mechanism triggering partial melting, the specific transport process of the subduction components remains controversial. Are elements transported by hydrous fluids or by water-rich silicate melts? And if a distinction can be drawn between these two processes, which reactions are involved and under which conditions? Coupling the geological information and geochemistry of arc lavas with experimental constrains provide some insights into these questions.

[34] In this section we discuss the different contributions from the subduction environment to the Palma Sola volcanics. Although the following figures will show data from the Quaternary cinder cones, the interpretations and discussions regarding the subduction systematics are based on the petrologic characteristics of the plutonic rocks and the plateau basalts because, as mentioned before, there is strong evidence that the most enriched cinder cones are contaminated with the upper continental crust.

[35] Trace element ratios between Th, Pb, and Nd are instructive because they allow us to recognize the contributions from the different subduction components. Th and Nd are considered to be fluid immobile, whereas Pb is soluble in aqueous fluids [Brenan et al., 1995c]. Therefore the Th and Nd budgets of the magmas should be mainly controlled by their abundances in the mantle wedge plus the subducted bulk sediments or their partial melts [Plank and Langmuir, 1993; Elliott et al., 1997; Class et al., 2000]. The Pb contributions, on the other hand, can be either controlled by fluid additions and/or the additions from the subducted sediments. The melt/solid partition coefficients between Pb and Nd are very similar and thus fractionation between these two elements is negligible during mantle melting. Additionally, the use of these element ratios allows us to construct mixing plots with the isotopic ratios of Nd and Pb where the correlations are linear.

[36] In the Th/Nd and Pb/Nd correlation diagram (Figure 10a) all samples follow a positive correlation trend with plutonic rocks (and cinder cones) having a higher Pb/Nd at similar Th/Nd values than the plateau basalts. In general, plutonic rocks have significantly higher LILE/HFSE ratios than the plateau basalts. If the geochemistry of these sequences is not controlled by the continental crust, then we can safely assume that the plutonic rocks and the plateau basalts have been affected by two different subduction components. The compositional difference between these components is confirmed in a plot of Nd/Pb versus Pb isotopes (Figure 10b). While the plateau basalts form a tight negative correlation between the OIB-like mantle source and the bulk subducted sediments, the plutons clearly need an additional component with low Nd/Pb and unradiogenic Pb. And again, an AFC process with the local Paleozoic crust can explain the composition of the most enriched cinder cones in this diagram.

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Figure 10. Subduction components. (a) Plutons have higher Pd/Nd at similar values of Th/Nd than the plateau basalts, suggesting that the two magmatic suites were affected by two different subduction components. (b) In a simple mixing diagram of Nd/Pb versus 207Pb/204Pb, the plateau basalts are bracketed by the OIB-like mantle wedge and the subducted sediments. In contrast, the plutons require an additional component with unradiogenic Pb. This component is apparently consistent with melts coming from the subducted oceanic crust (see text). Geochemical data from late Miocene Baja California adakites [Aguillón-Robles et al., 2001] also show low Nd/Pb values and plot as an extension of the Palma Sola plutons. (c) Th/Nd versus 143Nd/144Nd variation for the Palma Sola volcanics. Because Th is relatively more incompatible than Nd during partial melting of the subducted sediment [Johnson and Plank, 1999], the negative correlation array formed by the plutons and most of the plateau basalts is interpreted as additions of sediment melts into the OIB-like mantle wedge. An AFC model with the same parameters as in Figure 8 and DTh = 0.034, can explain the different trend followed by the cinder cones. (d) The sediment component also has lower Ba/Nb (and Pb/Nd) than the bulk subducted sediment. This indicates that the sediment was significantly depleted in fluid mobile elements prior to melting.

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[37] Th/Nd ratios of plutonic rocks and most of the plateau basalts display a negative correlation with Nd isotopes (Figure 10c). The correlation trends to a higher Th/Nd ratio than the bulk sediment, a characteristic that has been previously documented as evidence that the sediment addition is in the form of a partial melt [Elliott et al., 1997; Class et al., 2000; Hochstaedter et al., 2001]. Indeed, the recently determined partition coefficients from Johnson and Plank [1999] indicate that, during partial melting of the sediment, Th is more incompatible than Nd. These results provide strong evidence that the plutonic rocks and plateau basalts have been affected by different proportions of a sediment partial melt. The Th/Nd ratios of the cinder cones are lower than those of the plateau basalts and plutons, and do not follow a simple correlation trend. A process of assimilation with an enriched crustal component will create a trend with lower Th/Nd and can explain reasonably well the geochemical variations of the most enriched cinder cones.

[38] Although it seems that both sequences, plateau basalts and plutonic rocks, received contributions from a sediment melt, plutonic rocks require a component with higher LILE/HFSE ratios, lower Nd/Pb ratios and unradiogenic Pb isotopes (Figure 10b). At first sight, these characteristics might indicate the participation of a second component rich in fluid mobile elements but isotopically depleted, a component usually associated with a fluid derived from the dehydration of the subducted oceanic crust [Miller et al., 1994]. In detail, however, the injection of isotopically depleted fluids alone does not explain the strong garnet signature and the high Nb/Ta ratio of the most depleted plutonic sample (PS-99-38) because fluids do not modify these parameters. In addition, the sole injection of MORB fluids into the mantle wedge would create a mixing array connecting the composition of the OIB mantle and MORB in the Pb isotopes correlation diagrams (Figure 4). Partial melting of subducted oceanic crust could explain the high Nb/Ta and Gd/Yb ratios and the depleted isotopic compositions but, as with the fluids, the injection of pure slab melts into the wedge would create a mixing array connecting MORB and the OIB mantle in Pb isotopic space. Therefore a pure MORB melt component injected into the mantle does not fully explain the geochemical data and a more complicated scenario should be envisaged.

[39] The plutons form a positive correlation in the 206Pb/204Pb-207Pb/204Pb diagram (Figure 4b), bracketed by the MORB and the sediments, indicating that the component we are looking for is probably a mixture between these two end-members. Figure 10b also shows that the isotopically depleted component required to explain the geochemistry of the plutons has a very low Nd/Pb ratio. The Nd/Pb ratio of average EPR MORB is very high (∼19), thus implying that during slab melting Pb must be preferentially partitioned into the liquid phase while Nd should behave relatively more compatible. Recently published partitioning data for garnet (DNd/DPb ≈ 2.12, on average), and clinopyroxene (DNd/DPb ≈ 24, on average) with hydrous tonalitic melts [Barth et al., 2002], combined with partition coefficients for amphibole and andesitic melts (DNd/DPb ≈ 5.16, on average) [Brenan et al., 1995b] indicate that Pb should indeed be preferentially partitioned into the liquid phase during hydrous melting of an eclogite or a garnet amphibolite source. Although very few data exist in the literature for Nd and Pb concentrations and isotopic ratios in adakites, it is notable that recently published geochemical data from late Miocene Baja California adakites [Aguillón-Robles et al., 2001] also show low Nd/Pb ratios, and plot mostly as an extension of the Palma Sola plutons (Figure 10b). The Baja adakites, believed to have formed by melting of a subducted East-Pacific Rise ridge, provide additional evidence supporting the interpretation that the Palma Sola plutons were influenced by a slab-melt subduction component. Assimilation of a small proportion of sediments into the slab derived melts could have played a role in decreasing the Nd/Pb ratio of the mixture, because sediments have very low Nd/Pb values. Since the Pb isotopic composition of the most depleted plutonic sample is more radiogenic than MORB, it seems likely that some sediments were in fact assimilated by the slab melts forming hybrid magmas. Pockets of melts coming from the sediments and oceanic crust could also interact with the mantle wedge in a complex way, making it extremely difficult to recognize the exact proportions or characteristics of the initial components. Even though such a scenario is difficult to visualize and even harder to quantify, it is probably the simplest way to explain the overall geochemical features of the Palma Sola plutons.

[40] Plateau basalts geochemistry appears to be related to a much simpler scenario in which sediment melts alone can explain the isotopic and trace element variations. Even so, simple mixing lines connecting the most depleted sample (the OIB-like component) and the bulk sediment (or its melt) does not reproduce the plateau basalt data because the Ba/Nb and Pb/Nd ratios of the sediment component are too high (Figures 10a and 10d). Considerably lower Ba/Nb and Pb/Nd ratios are necessary to reproduce the mixing trends. This would be possible if some of the Ba and Pb were extracted from the sediment prior to melting, while Th and Nd remained immobile. This depletion of the fluid mobile elements could be an indication that the sediment involved was significantly dehydrated before entering the melting regime of the Palma Sola volcanics.

6.4. Quantitative Modeling of Subduction Components

[41] The discussion thus far suggests the mantle wedge below the eastern TMVB is similar to an enriched OIB-like source, and that this source can melt at different depths generating magmas with distinct major and trace element characteristics. The geochemical signatures of the plutonic rocks also record the participation of at least two different slab components (MORB melts and sediment melts), while the geochemistry of the plateau basalts indicate the participation of a dehydrated sediment melt only. The chemical characteristics of the cinder cones, in contrast, appear to be largely influenced by assimilation of the upper continental crust.

[42] In order to quantify the different subduction fluxes we assume that our most isotopically depleted plateau basalt sample, that shows no subduction signature (see sample PS-99-25 in Figure 3b), represents a pristine mantle melt that records the composition of the mantle source prior being modified by the subduction agents. The trace element pattern and isotopic composition of sample PS-99-25 are similar to alkaline rocks sampled in the Mexican Basin and Range province, where modifications derived from the subduction environment are negligible [Luhr et al., 1989b; Pier et al., 1989]. This composition is also similar to the average OIB of Sun and McDonough [1989] (Figure 9a). These data and comparisons provide additional support to our assumption that the source region of sample PS-99-25 was not significantly modified by subduction. We then reconstructed the mantle composition by assuming that sample PS-99-25 is a mantle melt formed by 3% batch melting of a source made of 54% olivine, 19% Cpx, 24% Opx and 3% garnet (Table 4). Mantle mineralogy was determined using the BATCH program and the experimental constraints described by Longhi [2002] for low degree mantle melts. The choice of 3% melting was based on comparisons of incompatible elements concentrations with the alkaline basalts form Ventura and Chichinautzin volcanic fields, assumed to represent about 1–5% melting of a relatively enriched mantle source [Luhr et al., 1989b; Wallace and Carmichael, 1999]. The model peridotite composition is similar to those used in previous studies where enriched mantle domains are considered [Borg et al., 1997; Reiners et al., 2000]. Even though this mantle reconstruction is not strictly correct, as sample PS-99-25 has most likely undergone a small amount of fractionation (MgO = 7.3 wt%), we use this composition because it is convenient for the purpose of estimating subduction fluxes in samples that have suffered similar levels of fractionation (i.e., sample PS-99-5c with MgO = 7.8 wt%, see Figure 3b). Accounting for fractionation would yield a source that is roughly parallel in shape but slightly more depleted in trace element concentrations than the one used. For this reason we emphasize that the calculated contributions from the slab are maxima.

Table 4. End-Member Compositions and Partition Coefficients Used in Models
 Mantle WedgeaDmantle/meltbBulk SedcDeh SeddDsed/melteSed MeltfSlabgDslab/melth
  • a

    Mantle wedge composition is sample PS-99-25 inverted to 3% batch melting.

  • b

    Bulk partition coefficients (solid/melt) assuming a mantle mineralogy of 54% ol, 19% cpx, 24% opx and 3% gt. Mineral Kd's were taken from Hart and Dunn [1993], Kelemen et al. [1993], Johnson [1994], and Salters and Longhi [1999].

  • c

    Bulk Sediment composition from DSDP site 487 (LaGatta et al., manuscript in preparation, 2003).

  • d

    Bulk dehydrated sediment used in the models.

  • e

    Bulk solid/melt partition coefficients from Johnson and Plank [1999], except DNb = 20.

  • f

    Sediment melt assuming 5% batch melting.

  • g

    Slab composition is average EPR MORB between 5°–15°N from the PETDB database http://petdb.ldeo.columbia.edu) with slightly higher concentrations of fluid mobile elements, and higher 87Sr/86Sr isotopic ratio to account for alteration. Fresh average EPR MORB Rb = 3.59, Ba = 33.9, U = 0.16, Sr = 152.3, Pb = 0.8 and 87Sr/86Sr = 0.7026.

  • h

    Bulk partition coefficients (solid/melt) of andesitic-dacitic melts in equilibrium with a garnet amphibolite residuum (35% cpx, 30% gt, 34.8% amph, 0.2% rut). Mineral Kd's from Rollinson [1993, and references therein], Green [1995], van Westrenen et al. [2001], Barth et al. [2002], and the Geochemical Earth Reference Model (GERM) database http://www.earthref.org).

Rb0.680.0001594.150.00.7268.18.00.0306
Ba8.390.000162910.3500.00.75655.760.00.0243
Th0.110.000656.76.70.897.50.40.0709
U0.040.001843.22.20.932.40.50.1188
Nb1.390.003169.79.7200.36.90.2676
Ta      0.40.3676
La1.210.0102539.339.31.3429.76.50.2616
Ce2.910.0142648.848.81.3536.618.70.3946
Sr41.790.02481231.4150.00.51280.6200.00.1140
Pb0.170.0396941.420.01.2915.70.90.1263
Nd2.340.0394537.737.71.5325.115.30.5988
Sm0.670.063427.87.81.605.04.80.9447
Zr17.500.03624107.8107.8146.41.3780
Hf0.510.057322.42.44.01.1062
Eu0.270.085652.12.11.681.21.61.1170
Gd0.900.098678.38.31.655.16.51.8422
Tb0.160.127661.31.31.660.81.32.6402
Dy1.000.153967.87.81.684.77.84.0702
Y5.590.1737749.249.21.6929.744.04.6950
Ho0.220.181611.61.61.691.01.75.2434
Er0.600.197844.54.51.692.74.75.6504
Yb0.570.227944.34.31.682.64.16.7500
Lu0.090.241690.70.71.720.40.69.0050
87Sr/86Sr0.703096 0.708460.70846 0.708460.70275 
206Pb/204Pb18.963 18.68718.687 18.68718.303 
207Pb/204Pb15.592 15.60115.601 15.60115.483 
208Pb/204Pb38.534 38.42338.423 38.42337.749 
143Nd/144Nd0.512941 0.512520.51252 0.512520.51314 

[43] The composition of the sediment melt (Table 4) is calculated using the weighted bulk sediment composition from the DSDP site 487 (LaGatta et al., manuscript in preparation, 2003) and assuming 5% batch melting using the partition coefficients of Johnson and Plank [1999] (except Nb, see below). The composition of the slab melts (Table 4) were calculated using average and best fit mineral/melt partition coefficients of andesitic and dacitic liquids [Rollinson, 1993, and references therein; Green, 1995; van Westrenen et al., 2001; Barth et al., 2002; Geochemical Earth Reference Model (GERM) database, 2001, http://www.earthref.org/] and assuming that the metamorphosed subducted MORB is made of 35% Cpx, 30% Gt, 34.8% Amphibole and 0.2% Rutile. The bulk chemical composition of the subducted slab is an average East Pacific Rise MORB between 5°–15° North (Petrological Database of the Ocean Floor (PETDB), 2002, http://petdb.ldeo.columbia.edu/), with slightly higher concentrations of fluid mobile elements to account for hydrothermal alteration (Table 4).

[44] Since the isotopic and trace element compositional variations of the plateau basalts appear to follow simple mixing trends between the mantle source and a sediment melt, we concentrate on their characteristics before moving on to the more complex petrology of the plutonic rocks. In this case, the geochemical modifications imparted by the subduction environment are modeled by mixing different proportions of sediment melts to the reconstructed mantle source. In reality, however, sediment melts may react with the overlying peridotites, as such melts are most likely saturated with the SiO2 component [Kelemen, 1998; Rapp et al., 1999; Tatsumi, 2001]. Given that mass additions from the sediments to the mantle wedge are usually minor (<5%) [Kay et al., 1978; Bourdon et al., 2000; Class et al., 2000], and that the differences in SiO2 and MgO contents between slab-modified and unmodified samples to be modeled here are small (see samples PS-99-25 and PS-99-5c in Table 2), we assume that the mantle mineral paragenesis did not change significantly during melt/rock interaction, and thus the sediment contributions can be modeled as a simple mixing process.

[45] Figure 11 shows the geochemical modifications imparted to the mantle wedge by adding sediment melts in different proportions. In the Pb-Nd isotopic space (Figure 11a), if the bulk subducted sediment Pb/Nd ratio is considered (∼1.1), the resulting model predicts slightly lower 206Pb/204Pb values for a given 143Nd/144Nd ratio (orange lines). A similar phenomenon is observed in the Sr-Nd isotopic space (Figure 11b), where the use of the bulk sediment Sr/Nd ratio (∼6.1) predicts a trend with higher 87Sr/86Sr at a given 143Nd/144Nd value, with a slope that misses most of the data. These features validate the previous observation that a significant proportion of the fluid mobile elements were probably lost from the sediment prior to melting. Lower Pb/Nd (∼0.53) and Sr/Nd (∼4) ratios are required to accurately reproduce the Pb-Nd-Sr isotopic mixing relationships (pink lines in Figure 11). The data thus indicate that the sediment component needed to reproduce the Palma Sola plateau basalts has been significantly dehydrated with nearly 35% of the Sr and about 52% of the Pb lost in a fluid phase prior melting (Table 4). With these considerations, the isotopic data of the most enriched plateau basalts can be closely reproduced by adding less than 4% of a dehydrated sediment melt to the OIB-like source (Figure 11). Figure 12a shows that the trace element pattern of one of the most enriched plateau basalts (PS-99-5c, also see Figure 3b) can also be reproduced by adding ∼3% dehydrated sediment melt, and assuming about 3% batch melting of the metasomatized OIB-like mantle. The partition coefficients of Johnson and Plank [1999] for the sediment melts thus appear to be reasonable for most of the trace elements, with the exception of Nb in which a significantly higher D (∼20) must be assumed. This further confirms the idea that rutile is a residual phase during partial melting of the subducted sediment, so that Nb, Ta and TiO2 should not be significantly transferred to the wedge by the sediment melt input.

image

Figure 11. Isotopic modeling of subduction components. (a) Pb-Nd, (b) Sr-Nd, and (c) Sr-Pb isotopic variations. The yellow lines in (a) and (b) describe the isotopic modifications imparted to the mantle wedge by adding different proportions of non-dehydrated sediment melts (Pb/Nd = ∼1.1 and Sr/Nd = ∼6). These models largely fail to reproduce the data of the plateau basalts (see inset in Figure 11a). The pink lines consider a model in which the sediment has lost a significant proportion of the fluid mobile elements before melting (Pb/Nd = 0.53 and Sr/Nd = 4). This model accurately describes the isotopic variations of the plateau basalts by adding less than 4% of a dehydrated sediment melt (see inset in Figure 11a). The blue lines are simple mixing models of the altered oceanic crust (AOC) and sediments sampled in DSDP site 487 [Verma, 1999b; LaGatta et al., manuscript in preparation, 2003]. These models fail to reproduce the isotopic composition of the most depleted plutonic sample, suggesting that this AOC is probably not representative of the bulk oceanic crust being subducted (see text). The green lines are simple mixing models of an average EPR MORB, with a slightly higher Sr contents and 87Sr/86Sr ratio (∼0.70275), and the dehydrated sediments. Using these end-members, the most isotopically depleted plutonic sample can be reproduced by a 20:80 sediment:MORB mixture.

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image

Figure 12. Trace element modeling. (a) The trace element pattern of sample PS-99-5c (see also Figure 3) can be closely modeled by melting an OIB mantle source metasomatized with ∼3% dehydrated sediment-melt, and assuming ∼3% batch melting. Sediment melt partition coefficients are from Johnson and Plank [1999], except DNb = 20. Mantle/melt partition coefficients in Table 4. The unusual flat pattern in the normalized Rb-Ba-Th-U is an indication of the dehydrated sediment due to a significant Ba loss in a fluid phase prior to melting. (b) The trace element pattern of sample PS-99-38 can be reproduced by melting a 20:80 sediment:MORB mixture, using partition coefficients of andesitic to dacitic melts (Table 4). Slab melt residual mineralogy is assumed to be 0.35 Cpx, 0.3 Gt, 0.348 Amph, 0.002 Rut. High Nb/Ta = 22, La/Yb = 46 and Sr/Y = 53 ratios for this sample are all consistent with slab melting.

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[46] As mentioned before, the geochemistry of the plutonic rocks appear to be related to a complex mixture of slab and sediment melts metasomatizing an OIB mantle wedge. Thus modeling the proportions of the components involved in such a scenario is not a simple task. There are also many uncertainties regarding the composition of the altered oceanic crust that is being subducted. Verma [1999b] reported trace element data and isotopic compositions of altered oceanic crust (AOC) from the Cocos plate collected at DSDP site 487, and a larger trace element data set was obtained by LaGatta et al. (manuscript in preparation, 2003) on the same samples. Figure 11 shows that a mixing line connecting this AOC composition and the dehydrated sediments fails to reproduce the isotopic data of the most depleted plutonic samples because the Sr/Nd and Sr/Pb ratios of the AOC are lower than those required to satisfy the mixing relationships (blue lines in Figure 11). This AOC composition is highly depleted in REE when compared to NMORB and even more so against average EPR MORB. It is also notable that the 143Nd/144Nd ratio of this AOC is higher than most of the fresh EPR MORB samples (Figure 11). These features suggest that the samples used to define AOC, collected in the upper couple of meters of the oceanic crust, are probably not representative of the bulk oceanic crust being subducted. We thus consider it more reliable to use the extensive EPR MORB database available in the literature as a MORB end-member (PETDB, 2002, http://petdb.ldeo.columbia.edu/). If the average composition of EPR MORB with a slightly higher Sr content and 86Sr/87Sr ratio is considered as a slab end-member (Table 4), the mixing line passes close to the most depleted plutonic sample with a mixture made of 20% sediment and 80% MORB (green lines in Figure 11). A batch melting model of that mixture, that uses andesitic to dacitic partition coefficients (Table 4), can reproduce the general trace element pattern of sample PS-99-38, the most depleted pluton (Figure 12b). Strong garnet signatures, high Nb/Ta ratios, positive Sr anomalies (thus high Sr/Y ratios), and the most depleted isotopic composition compared to the other plutonic rocks all suggest that this particular sample could be related to a mixture between subducted MORB melts and sediment melts [Defant and Drummond, 1990]. The rest of the plutons could then be associated with mantle melts metasomatized with both sediment melts and hydrous slab-melts in various proportions.

7. A Tectonic-Petrogenetic Model

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[47] The magmatic record of the eastern TMVB portrays dramatic changes in composition and emplacement mechanisms in a very short time span (17 Ma). Because those changes should be ultimately linked to variations in the Pacific plate convergence dynamics, they provide insight into the thermal and tectonic evolution of the Mexican subduction zone. We therefore propose a three-step tectonic model to explain the overall temporal variations in composition of the magmatic rocks emplaced in the eastern TMVB (Figure 13).

image

Figure 13. Tectonic model. Location of cross section A–B in Figure 1. (a) During the middle-late Miocene, the volcanic front of the TMVB was located at about 500 km from the trench, nearly 100 km farther north from the current front (see Figure 1). Thus a low or essentially flat subduction angle is assumed for this period. Flat subduction could contribute to an anomalous melting of the subducted oceanic crust [Gutscher et al., 2000] and the overlying sediments. (b) By the late Miocene and into the early Pleistocene, an increase in subduction angle favored melting of a relatively deeper mantle source, and allowed a deeper transfer of the slab-derived chemical agents. The sediment solidus temperature could still be crossed at higher pressures, but slab melting will be more difficult to achieve. Slab roll-back could also induce mantle convection, decompression melting, and tectonic extension in the upper plate. (c) During most of the Quaternary, the subducted slab continued to roll back toward its present geometric configuration [Pardo and Suárez, 1995]. Slab dehydration is probably responsible for most of the volcanism along current volcanic front (i.e., Pico de Orizaba), while slab derived contributions are negligible in the back-arc region. Most of the Quaternary magmatism in the Palma Sola region appears to be formed by decompression melting of a lithospheric source, and later contaminated with the upper continental crust. (d) Average 207Pb/204Pb and Sr/Y ratios of the different units depict the overall changes in composition of magmatic rocks over a time span of ∼17 Ma. High Sr/Y and low 207Pb/204Pb are characteristic of the Miocene plutons affected by a slab-melt subduction component. The compositions of the latest Miocene-Pleistocene plateau basalts are mainly controlled by the sediment melt component. In contrast, the compositions of the Quaternary cinder cones are affected by upper-crustal contamination. (e) Sr/Y versus Y variation diagram of the Palma Sola magmatic suites, Cerro Grande volcano [Gómez-Tuena and Carrasco-Núñez, 2000], Apan Miocene volcanics [García-Palomo et al., 2002], and Palo Huerfano-La Joya-Zamorano volcanic complex (A. Gómez-Tuena et al., unpublished data, 2002). High Sr/Y ratios and low HREE concentrations are observed in middle-late Miocene dacites throughout the TMVB. This argues in favor of slab melting during the early evolution of the TMVB.

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[48] On the basis of isotopic age compilations of volcanic rocks, it has been suggested that the inception of the TMVB as a distinct E-W-trending magmatic arc occurred during the middle to late Miocene as a result of a counterclockwise migration of volcanism from the Oligocene, NW-SE-trending Sierra Madre Occidental (SMO) volcanic arc [Ferrari et al., 1994, 1999]. During the early evolutionary stages of the TMVB, volcanic rocks with prominent arc signatures were emplaced nearly 500 km from the current trench, about 100 km north of the present volcanic front (Figure 1). Large stratovolcanoes were built in Querétaro (Zamorano-La Joya-Palo Huerfano) and Puebla (Cerro Grande) during this period [Valdéz-Moreno et al., 1998; Gómez-Tuena and Carrasco-Núñez, 2000]. In the easternmost part of the TMVB, the Palma Sola plutons belong to this early phase of magmatism. The reasons why the magmatic activity migrated from the near-trench SMO to the TMVB are not yet fully understood, but on the basis of the reconstructions of the Pacific plate tectonics for that period [Mammerickx and Klitgord, 1982; Londsdale, 1991], the temporal variations in the convergence rate [Engebretson et al., 1985], and the relative timing of inland volcanism in southern and northern Mexico [Ferrari et al., 1999; Morán-Zenteno et al., 1999], it has been proposed that a transition to a low or essentially flat subduction angle must be the responsible for this migration [Morán-Zenteno et al., 1999; Gómez-Tuena and Carrasco-Núñez, 2000]. A shallow subduction angle was probably influenced by the removal of the Chortis Block [Morán-Zenteno et al., 1999], and by a significant increase in the convergence rate along the Middle American Trench during the middle Miocene [Gómez-Tuena and Carrasco-Núñez, 2000]. Once the TMVB was established as a distinct geologic entity during the middle and late Miocene, the volcanic front has been migrating southward toward its present location along the parallel ∼19°N, where the active stratovolcanoes are located (Figure 1).

[49] Recent thermal models of slab melting have suggested that during flat subduction the leading edge of the slab can be anomalously overheated allowing partial melting to occur at relatively shallow depths [Gutscher et al., 2000]. This has been recognized as a reasonable model to explain the occurrence of adakite magmatism in arcs where moderately old oceanic crust is being subducted. In this scenario, the subducted slab can remain at a nearly constant depth of ∼70 km for extended periods of time describing a P-T-t path that crosses over the basalt wet solidus at temperatures below 700°C. Indeed, the experimentally determined phase relationships of hydrated MORB indicate that melting can occur over this P-T interval with a garnet amphibolite residuum [Schmidt and Poli, 1998].

[50] The initial magmatic phase of the TMVB, as recorded in Palma Sola by the Miocene plutons, could be related to such a tectonic setting (Figure 13a). A scenario of low degree partial melts of the subducted sediments and the oceanic crust infiltrating and metasomatizing the mantle wedge appears to be consistent with the overall geochemical features of the Palma Sola plutons (Figure 13d). If this is true, then we could expect that other volcanic areas of the TMVB with roughly the same age should show those geochemical characteristics as well. Figure 13e shows the Sr/Y versus Y variation diagram of the Palma Sola volcanics, together with data from the late-Miocene Cerro Grande volcano [Gómez-Tuena and Carrasco-Núñez, 2000], the late-Miocene volcanics from the Apan region [García-Palomo et al., 2002], and from the late-Miocene Palo Huerfano-La Joya-Zamorano volcanic complex (A. Gómez-Tuena et al., unpublished data, 2002). These volcanoes are all located at nearly 500 km from the trench (Figure 1), and their isotopic ages are within the range of the Palma Sola plutons. The Miocene stratovolcanoes have high Sr/Y ratios and low HREE concentrations, characteristics that have been extensively documented and discussed as the best indicators of slab melting [Defant and Drummond, 1990]. Therefore the geochemistry of these Miocene volcanic complexes provide additional evidence supporting the notion that the flat-slab tectonic configuration could have led to an anomalous partial melting of the subducted oceanic crust.

[51] At the end of the Miocene, and during the Pliocene, a major change in the composition and emplacement mechanisms of volcanic rocks occurred in the Palma Sola area (Figure 13d). Widespread alkaline plateau basalts with both OIB and arc signatures were emplaced stratigraphically above the plutonic rocks. The geochemical compositions of plateau basalts with subduction signals appear to be related to the injection of dehydrated sediment melts into a deeper, OIB-like, mantle wedge. A group of samples, however, do not appear to have been significantly affected by the subduction agents and therefore a complex tectonic scenario of intermingling decompression melting and slab fluxing should have been in operation during this period. The fact that these plateau basalts were fed by dikes, and not through central volcanoes, supports the existence of an extensional tectonic regime that probably allowed rapid magma transit through the upper mantle and crust. We thus propose that a gradual increase in the angle of subduction must have occurred during this period (Figure 13b). Slab roll back could induce mantle corner flow, decompression melting, and tectonic extension in the upper plate. A steeper subduction angle could also influence partial melting of a relatively deeper mantle source, and allow a deeper transfer of the slab derived chemical agents. A gradual pressure increase also induces modifications in the mineralogy of the metamorphosed oceanic crust: the stability field of amphibole could be crossed closer to the trench, and the P-T path of the subducted slab would migrate away from the wet basalt solidus [Schmidt and Poli, 1998; Gutscher et al., 2000]. Higher temperatures are required for the subducted basalt to melt at greater depths, making this process less likely to occur. On the other hand, the sediment-melting experiments of Johnson and Plank [1999] showed that as temperature is increased and the sediment begins to melt, the fluid phase changes from a silica-rich aqueous fluid into a water-rich silicate melt. Therefore sediment dehydration should be a natural consequence of the continuous change in P-T conditions of the slab during subduction, and it seems inevitable that some fluid is lost from the sediment at shallower levels before the solidus is crossed. While the solidus temperature of the sediments largely depends on the starting composition [Nichols et al., 1994; Johnson and Plank, 1999], and the continuous fluid release should affect this temperature, the sediment solidus curve determined by Nichols et al. [1994] indicates that sediment melting is possible within a P-T range of 30–40 Kbars and 650–680°C. It should be acknowledged, however, that the sediment solidus reported by Johnson and Plank [1999] is ∼100°C higher under this pressure range, indicating that sediment melting can occur under multiple circumstances that are still difficult to constrain. Even though some uncertainties of the specific physical conditions of magma generation remain, the overall geochemical data of the Palma Sola plateau basalts suggest that, under higher pressures, the dehydrated sedimentary layer of the subducted slab can still melt, but apparently not the underlying oceanic crust.

[52] During the Quaternary, magmatism in the Palma Sola area was dominated by calc-alkaline cinder cones that appear to be related to melting of a relatively shallower, and probably lithospheric mantle wedge. Since subduction signatures of the most primitive rocks are small, and the most evolved cones appear to be significantly contaminated by the local continental crust, we conclude that slab input was minor (Figure 13d). During the same period of time, however, rocks with very strong subduction signatures were emplaced at the current volcanic front forming large stratovolcanoes (e.g., Pico de Orizaba). Therefore the current subduction geometry portrayed by Pardo and Suárez [1995] appears to be consistent with a scenario in which slab dehydration induces magmatism along the present volcanic front, while farther away, in the Palma Sola region, magmatism could be mostly related to an extensional back-arc with negligible slab input (Figure 13c).

8. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[53] The mantle wedge underneath the eastern TMVB is considerably more enriched than the depleted MORB source, and has the trace element and isotopic characteristics of an OIB-like mantle. Enriched mantle domains have been verified in different areas of the TMVB and in the Basin and Range province of Northern Mexico, suggesting that a large proportion of the mantle beneath Mexico is significantly enriched.

[54] The distinct volcanic sequences that were emplaced in the Palma Sola area show contrasting compositional variations that can be related to additions of different subduction components to the same enriched mantle wedge. Middle to late Miocene plutonic rocks appear to be related to the injection of slab and sediment melts that metasomatize a relatively shallow mantle wedge. In contrast, the compositional variations of the latest Miocene-Pleistocene plateau basalts are consistent with the introduction of a dehydrated sediment melt into a deeper, asthenospheric mantle wedge. The compositional variations of the Quaternary cinder cones were largely influenced by the assimilation of the upper continental crust, with only minor contributions from the subduction environment.

[55] The overall stratigraphic, volcanologic, and petrologic framework of the Palma Sola volcanics represents a portrait of the tectonic evolution of the Trans-Mexican Volcanic Belt. Gradual changes in the subduction regime, and especially in subduction angle, apparently allowed different materials to be transferred from the subducted slab into distinct portions of the mantle wedge. This in turn provides first-hand evidence of how the multiple metamorphic reactions of the subducted slab affect the compositions of volcanic rocks in a continental arc environment.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information

[56] This work is part of A. Gómez-Tuena's Ph.D. dissertation, financed by UNAM and Fulbright fellowships. AGT deeply thanks Alberto Saal for long hours of stimulating discussions. We sincerely thank John Longhi for his insight on mantle mineralogy and partition coefficients. Many thanks to Katie Donnelly, Alex Piotrowski, Beth Gier, Kathy Falato, Yongjun Su, Dave Walker, Connie Class, and to the entire LDEO petrology group for helping along the various aspects of this project, and for being such a friendly and exciting group of people. Invaluable help during sample preparation and analysis was gently provided by Rick Mortlock, Marty Fleisher, Jean Hanley, and Rufino Lozano. Constructive comments by William White, Yoshiyuki Tatsumi, Jim Luhr and two anonymous reviewers greatly improved the manuscript. This research was funded by a CONACyT grant (27642-T) to F. Ortega-Gutiérrez and by a NSF grant (EAR 96-14782) to C. Langmuir and S. Goldstein.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information
  • Aguillón-Robles, A., T. Calmus, M. Benoit, H. Bellon, R. Maury, J. Cotten, J. Burgois, and F. Michaud, Late Miocene adakites and Nb-enriched basalts from Vizcaino Peninsula, Mexico: Indicators of East Pacific Rise subduction below southern Baja California? Geology, 29, 531534, 2001.
  • Ayers, J., S. Dittmer, and G. Layne, Partitioning of elements between silicate melt and H2O-NaCl fluids at 1.5 and 2.0 GP pressure: Implications for mantle metasomatism, Geochim. Cosmochim. Acta, 59, 42374246, 1997.
  • Barth, M., S. Foley, and I. Horn, Partial melting in Archean subduction zones: Constraints from experimentally determined trace element partition coefficients between eclogitic minerals and tonalitic melts under upper mantle conditions, Precambrian Res., 113, 323340, 2002.
  • Besch, T., J. Negendank, and R. Emmermann, Geochemical constraints on the origin of the calc-alkaline and alkaline magmas of the eastern Trans-Mexican Volcanic Belt, Geofis. Int., 27, 641663, 1988.
  • Borg, L., M. Clynne, and T. Bullen, The variable role of slab derived fluids in the generation of a suite of primitive calc-alkaline lavas from the southernmost Cascades, California, Can. Mineral., 35, 425452, 1997.
  • Bourdon, B., G. Wörner, and A. Zindler, U-series evidence for crustal involvement and magma residence times in the petrogenesis of Parinacota volcano, Chile, Contrib. Mineral. Petrol., 139, 458469, 2000.
  • Brenan, J., H. Shaw, and F. Ryerson, Experimental evidence for the origin of lead enrichments in convergent-margin magmas, Nature, 378, 5456, 1995a.
  • Brenan, J., H. Shaw, F. Ryerson, and D. Phinney, Experimental determination of trace-element partitioning between pargasite and a synthetic hydrous andesitic melt, Earth Planet. Sci. Lett., 135, 111, 1995b.
  • Brenan, J., H. Shaw, F. Ryerson, and D. Phinney, Mineral-aqueous fluid partitioning of trace elements at 900°C and 2.0 GPa: Constrains on the trace element chemistry of mantle and deep crustal fluids, Geochim. Cosmochim. Acta, 59, 33313350, 1995c.
  • Cantagrel, J., and C. Robin, Géochimie isotopique du strontium dans quelques séries types du volcanisme de l'Est mexicain, Bull. Soc. Geol. France, 7(XX), 935939, 1978.
  • Cantagrel, J., and C. Robin, K-Ar dating on eastern Mexican volcanic rocks: Relations between the andesitic and the alkaline provinces, J. Volcanol. Geotherm. Res., 5, 99114, 1979.
  • Class, C., D. M. Miller, S. L. Goldstein, and C. H. Langmuir, Distinguishing melt and fluid subduction components in Umnak Volcanics, Aleutian Arc, Geochem. Geophys. Geosyst., 1, Paper number 1999GC000010, 2000.
  • Defant, M., and M. Drummond, Derivation of some modern arc magmas by melting of young subducted lithosphere, Nature, 347, 662665, 1990.
  • DePaolo, D., Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization, Earth Planet. Sci. Lett., 53, 189202, 1981.
  • Elliott, T., T. Plank, A. Zindler, W. White, and B. Bourdon, Element transport from slab to volcanic front at the Mariana arc, J. Geophys. Res., 102(B7), 14,99115,019, 1997.
  • Engebretson, A., A. Cox, and R. Gordon, Relative motions between oceanic and continental plates in the Pacific Basin, Spec. Pap. Geol. Soc. Am., 206, 59 pp., 1985.
  • Ferrari, L., V. Garduño, F. Innocenti, P. Manetti, G. Pasqueré, and G. Vaggeli, Volcanic evolution of central México: Oligocene to Present, Geofis. Int., 33, 91105, 1994.
  • Ferrari, L., M. Lopez-Martinez, G. Aguirre-Díaz, and G. Carrasco-Núñez, Space-time patterns of Cenozoic arc volcanism in central Mexico: From the Sierra Madre Occidental to the Mexican volcanic belt, Geology, 27, 303306, 1999.
  • Ferrari, L., C. Petrone, and L. Francalanci, Generation of oceanic-island basalt-type volcanism in the western Trans-Mexican volcanic belt by slab rollback, asthenosphere infiltration, and variable flux melting, Geology, 20(6), 507510, 2001.
  • Gaetani, G. A., and T. L. Grove, The influence of water on melting of mantle peridotite, Contrib. Mineral. Petrol., 131(4), 323346, 1998.
  • García-Palomo, A., J. Macías, G. Tolson, R. Valdez, and J. Mora-Chaparro, Volcanic stratigraphy and geological evolution of the Apan region, east-central sector of the Transmexican Volcanic Belt, Geofis. Int., 41, 133150, 2002.
  • Gill, J., Orogenic Andesites and Plate Tectonics, 358 pp., Springer-Verlag, New York, 1981.
  • Goldstein, S. L., R. O'Nions, and P. Hamilton, A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems, Earth Planet. Sci. Lett., 87, 221236, 1984.
  • Gómez-Tuena, A., and G. Carrasco-Núñez, Cerro Grande Volcano: The evolution of a Miocene stratocone in the Early Transmexican Volcanic Belt, Tectonophysics, 318, 249280, 2000.
  • Green, D., and A. Ringwood, Mineralogy of peridotitic compositions under upper mantle conditions, Phys. Earth Planet. Inter., 3, 359371, 1970.
  • Green, T. H., Significance of Nb/Ta as an indicator of geochemical processes in the crust mantle system, Chem. Geol., 120, 347359, 1995.
  • Green, T. H., and N. Pearson, Ti-rich accessory phase saturation in hydrous mafic-felsic compositions at high P, T, Chem. Geol., 54, 185201, 1986.
  • Gutscher, M., R. Maury, J. Eissen, and E. Bourdon, Can slab melting be caused by flat subduction? Geology, 28(6), 353535, 2000.
  • Hart, S., A large-scale isotope anomaly in the Southern Hemisphere mantle, Nature, 309, 753757, 1984.
  • Hart, S., and T. Dunn, Experimental cpx/melt partitioning of 24 trace elements, Contrib. Mineral. Petrol., 113, 18, 1993.
  • Hildreth, W., and S. Moorbath, Crustal contributions to arc magmatism in the Andes of central Chile, Contrib. Mineral. Petrol., 98, 455489, 1988.
  • Hochstaedter, A., J. Gill, R. Peters, P. Broughton, P. Holden, and B. Taylor, Across-arc geochemical trends in the Izu-Bonin arc: Contributions from the subducting slab, Geochem. Geophys. Geosyst., 2, Paper number 2000GC000105, 2001.
  • Irvine, T., and W. Baragar, A guide to the chemical classification of the common volcanic rocks, Can. J. Earth. Sci., 8, 523548, 1971.
  • Jacobo, A., Estudio petrogenético de las rocas del basamento del distrito Poza Rica, Instituto Mexicano del Petróleo, México, DF, 1985.
  • Johnson, K., Experimental cpx/ and garnet/melt partitioning of REE and other trace elements at high pressures: Petrogenetic implications, Mineral. Mag., 58, A, 454455, 1994.
  • Johnson, M., and T. Plank, Dehydration and melting experiments constrain the fate of subducted sediments, Geochem. Geophys. Geosyst., 1, Paper number 1999GC000014, 1999.
  • Kay, R., Aleutian magnesian andesites: Melts from subduction Pacific Oceanic crust, J. Volcanol. Geotherm. Res., 4, 117132, 1978.
  • Kay, R., S. Sun, and C. Lee-Hu, Pb and Sr isotopes in volcanic rocks from the Aleutian islands and Pribilof Islands, Alaska, Geochim. Cosmochim. Acta, 42, 263272, 1978.
  • Kelemen, P. B., Silica enrichment in the continental upper mantle via melt/rock reaction, Earth Planet. Sci. Lett., 164, 387406, 1998.
  • Kelemen, P. B., N. Shimizu, and T. Dunn, Relative depletion of niobium in some arc magmas and the continental crust: Partitioning of K, Nb, La and Ce during melt/rock interaction in the upper mantle, Earth Planet. Sci. Lett., 120, 111134, 1993.
  • Keppie, J., and F. Ortega-Gutiérrez, Provenance of Mexican Terranes: Isotopic Constrains, Int. Geol. Rev., 37, 813824, 1995.
  • Keppler, H., Constraints from partitioning experiments on the composition of subduction-zone fluids, Nature, 380, 237240, 1996.
  • Lassiter, J., and J. Luhr, Osmium abundance and isotope variations in mafic Mexican volcanic rocks: Evidence for crustal contamination and constraints on the geochemical behavior of osmium during partial melting and fractional crystallization, Geochem. Geophys. Geosyst., 2, Paper number 2000GC000116, 2001.
  • Lawlor, P., F. Ortega-Gutiérrez, K. Cameron, H. Ochoa-Carrillo, R. Lopez, and D. Sampson, U/Pb Geochronology, geochemistry and provenance of the Grenvillian Huiznopala gneiss of eastern Mexico, Precambrian Res., 94, 7399, 1999.
  • Londsdale, P., Structural patterns of the Pacific floor offshore peninsular California, in The Gulf and the Peninsular Province of the Californias, edited by J. Dauphin, and B. Simoneit, AAPG Mem., 47, 87125, 1991.
  • Longhi, J., Some phase equilibrium systematics of lherzolite melting: I, Geochem. Geophys. Geosyst., 3(3), 1020, doi:10.1029/2001GC000204, 2002.
  • López-Infanzón, M., Petrologic study of the volcanic rocks in the Chiconquiaco-Palma Sola area, central Veracruz, Mexico, M.Sc. thesis, Tulane Univ., New Orleans, La., 1991.
  • López-Infanzón, M., and R. Torres-Vargas, Estudio petrogenético de las rocas ígneas en el prospecto Misantla: Perote, Veracruz, Instituto Mexicano del Petroleo, México, DF, 1984.
  • Lozano, R., S. Verma, P. Giron, F. Velasco, D. Moran, F. Viera, and G. Chavez, Calibración preliminar de fluorescencia de rayos X para análisis cuantitativo de elementos mayores en rocas ígneas, Actas INAGEQ, 1, 203208, 1995.
  • Luhr, J., Extensional tectonics and the diverse primitive volcanic rocks in the western Mexican Volcanic Belt, Can. Mineral., 35, 473500, 1997.
  • Luhr, J., J. Allan, I. Carmichael, S. Nelson, and T. Hasenaka, Primitive calc-alkaline and alkaline rock types from the western Mexican volcanic belt, J. Geophys. Res., 94(B4), 45154530, 1989a.
  • Luhr, J., J. J. Aranda-Gómez, and J. Pier, Spinel-lherzoilite-bearing quaternary volcanic centers in San Luis Potosi, Mexico: I. Geology, mineralogy, and petrology, J. Geophys. Res., 94(B6), 79167940, 1989b.
  • Mammerickx, J., and K. Klitgord, North East Pacific Rise: Evolution from 25 m.y. B. P. to the present, J. Geophys. Res, 87, 67516759, 1982.
  • Márquez, A., R. Oyarzun, M. Doblas, and S. Verma, Alkalic (oceanic-island basalt type) and calc-alkalic volcanism in the Mexican volcanic belt: A case for plume-related magmatism and propagating rifting at an active margin? Geology, 27, 5154, 1999.
  • McDonough, W., and S. Sun, The composition of the earth, Chem. Geol., 120(3–4), 223253, 1995.
  • Miller, D. M., S. L. Goldstein, and C. Langmuir, Cerium/lead and lead isotope ratios in arc magmas and the enrichment of Pb in the continents, Nature, 368, 514520, 1994.
  • Molina-Garza, R., and J. Urrutia-Fucugauchi, Deep crustal structure of central Mexico derived from interpretation of Bouger gravity anomaly data, J. Geodyn., 15, 181201, 1993.
  • Morán-Zenteno, D., G. Tolson, R. Martines-Serrano, B. Martiny, P. Schaaf, G. Silva-Romo, C. Macias-Romo, L. Alba-Aldave, M. Hernandez-Bernal, and G. Solis-Pichardo, Tertiary arc-magmatism of the Sierra Madre del Sur, Mexico, and its transition to the volcanic activity of the Trans-Mexican Volcanic Belt, J. South Am. Earth Sci., 12, 513535, 1999.
  • Negendank, J., R. Emmermann, R. Krawczyk, F. Mooser, H. Tobschall, and D. Wehrle, Geological and geochemical investigations on the eastern Trans-Mexican Volcanic Belt, Geofis. Int., 24, 477575, 1985.
  • Nelson, S. A., E. Gonzalez-Caver, and T. K. Kyser, Constraints on the origin of alkaline and calc-alkaline magmas from the Tuxtla Volcanic Field, Veracruz, Mexico, Contrib. Mineral. Petrol., 122(1–2), 191211, 1995.
  • Nichols, G., P. Wyllie, and C. Stern, Subduction zone melting of pelagic sediments constrained by melting experiments, Nature, 31, 785788, 1994.
  • Ortega-Gutiérrez, F., R. Sedlock, and R. Speed, Phanerozoic tectonic evolution of Mexico, in Phanerozoic Evolution of North American Continent-Ocean Transitions, edited by R. Speed, pp. 265306, Geol. Soc. Am., Boulder, Colo., 1994.
  • Ortega-Gutiérrez, F., J. Ruiz, and E. Centeno-García, Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic, Geology, 23(12), 11271130, 1995.
  • Pardo, M., and G. Suárez, Shape of the subducted Rivera and Cocos plate in southern Mexico: Seismic and tectonic implications, J. Geophys. Res., 100, 12,35712,373, 1995.
  • Pier, J., F. Podosek, J. Luhr, J. Brannon, and J. Aranda-Gómez, Spinel-lherzoilite-bearing quaternary volcanic centers in San Luis Potosi, Mexico: 2. Sr and Nd isotopic systematics, J. Geophys. Res., 94(B6), 79417951, 1989.
  • Plank, T., and C. Langmuir, Tracing trace elements from sediment input to volcanic output at subduction zones, Nature, 362, 739742, 1993.
  • Plank, T., and C. Langmuir, The chemical composition of subducting sediment and its consequences for the crust and mantle, Chem. Geol., 145, 325394, 1998.
  • Rapp, R., and E. Watson, Dehydration melting of metabasalt at 8–32 kb: Implications for continental growth and crust-mantle recycling, J. Petrol., 36, 891931, 1995.
  • Rapp, R., N. Shimizu, M. Norman, and G. Applegate, Reaction between slab-derived melts and peridotite in the mantle wedge: Experimental constraints at 3. 8 GPA, Chem. Geol., 160, 256335, 1999.
  • Reiners, P., P. Hammond, J. McKenna, and R. Duncan, Young basalts of the central Washington Cascades, flux melting of the mantle, and trace element signatures of primary arc magmas, Contrib. Mineral. Petrol., 138, 249264, 2000.
  • Roberts, S., and J. Ruiz, Geochemistry of exposed granulite facies terrains and lower crustal xenoliths in Mexico, J. Geophys. Res., 94(B6), 79617974, 1989.
  • Robin, C., Relations volcanologie-magmatologie-geodynamique: Application au passage entre volcanismes alcalin et andesitique dans le sud Mexicain (Axe Trans-mexicain et Province Alcaline Orientale), Ph.D. thesis, Univ. de Clermont-Ferrand II, Clermont-Ferrand, France, 1982.
  • Robin, C., and E. Nicolas, Particularités géochimiques des suites andésitiques de la zone orientale de l'axe transmexicain, dans leur contexte tectonique, Bull. Soc. Geol. France, 7(XX), 193202, 1978.
  • Robin, C., and J. Tournon, Spatial relations of andesitic and alkaline province in Mexico and Central America, Can. J. Earth. Sci., 15, 16331641, 1978.
  • Rollinson, H., Using Geochemical Data, 352 pp., Addison-Wesley-Longman, Reading, Mass., 1993.
  • Ruiz, J., P. Patchett, and R. Arculus, Nd-Sr isotope constraints of lower crustal xenoliths: Evidence for the origin of mid-tertiary felsic volcanics in Mexico, Contrib. Mineral. Petrol., 99, 3643, 1988a.
  • Ruiz, J., P. Patchett, and F. Ortega-Gutiérrez, Proterozoic and Phanerozoic basement terranes of Mexico from Nd isotopic studies, Geol. Soc. Am. Bull., 100, 274281, 1988b.
  • Ryerson, F., and E. Watson, Rutile saturation in magmas: Implications for Ti-Nb-Ta depletion in island-arc basalts, Earth Planet. Sci. Lett., 86, 225239, 1987.
  • Salters, V., and J. Longhi, Trace element partitioning during the initial stages of melting beneath mid-ocean ridges, Earth Planet. Sci. Lett., 166, 1530, 1999.
  • Schaaf, P., W. Heinrich, and T. Besch, Composition and Sm-Nd isotopic data of the lower crust beneath San Luis Potosí, cetral Mexico: Evidence from a granulite-facies xenolith suite, Chem. Geol., 118, 6384, 1994.
  • Schmidt, M., and S. Poli, Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation, Earth Planet. Sci. Lett., 163, 361379, 1998.
  • Sedlock, R., F. Ortega-Gutiérrez, and R. Speed, Tectonostratigraphic terranes and the tectonic evolution of Mexico, Spec. Pap. Geol. Soc. Am., 278, 153 pp., 1993.
  • Sheth, H., I. Torres-Alvarado, and S. Verma, Beyond subduction and plumes: A unified tectonic-petrogenetic model for the Mexican volcanic belt, Int. Geol. Rev., 42(12), 11161132, 2000.
  • Siebert, L., and G. Carrasco-Núñez, Late-Pleistocene to precolumbian behind-the-arc mafic volcanism in the eastern Mexican Volcanic Belt: Implications for future hazards, J. Volcanol. Geotherm. Res., 115, 179205, 2002.
  • Stadler, R., S. Foley, G. Brey, and I. Horn, Mineral-aqueous fluid partitioning of trace elements at 900–1200°C and 3.0–5.7 GPa: New experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism, Geochim. Cosmochim. Acta, 62, 17811801, 1998.
  • Stolper, E., and S. Newman, The role of water in the petrogenesis of Mariana trough magmas, Earth Planet. Sci. Lett., 121, 293325, 1994.
  • Sun, S., and W. McDonough, Chemical and isotopic systematics of oceanic basalts: Implications for mantle compositions and processes, in Magmatism in the Ocean Basins, edited by A. Saunders, and M. Norry, Geol. Soc. Spec. Publ., 42, 313345, 1989.
  • Tatsumi, Y., Geochemical modeling of partial melting of subducting sediments and subsequent melt-mantle interaction: Generation of high-Mg andesites in the Setouchi volcanic belt, southwest Japan, Geology, 29(4), 323326, 2001.
  • Todt, W., R. Cliff, A. Hanser, and A. W. Hofmann, Evaluation of a 202Pb-205Pb double spike for high-precision lead isotope analysis, in Earth Processes: Reading the Isotopic Code, Geophys. Monogr. Ser., vol. 95, edited by A. Basu, and S. Hart, pp. 429437, AGU, Washington, D. C., 1996.
  • Torres, R., J. Ruiz, P. J. Patchett, and M. Grajales, Permo-Triassic continental arc in eastern Mexico: Tectonic implications for reconstructions of southern North America, in Mesozoic Sedimentary and Tectonic History of North-Central Mexico, edited by C. Bartolini, J. Wilson, and T. Lawton, Spec. Pap. Geol. Soc. Am., 340, 191196, 1999.
  • Valdéz-Moreno, G., G. Aguirre-Díaz, and M. López-Martínez, El Volcán La Joya, Edos. de Querétaro y Guanajuato: Un estratovolcán antiguo del cinturón volcánico mexicano, Rev. Mex. Cien. Geol., 15(2), 181197, 1998.
  • van Westrenen, W., J. Blundy, and B. Wood, High field strength element/rare earth fractionation during partial melting in the presence of garnet: Implications for identifications of mantle heterogeneities, Geochem. Geophys. Geosyst., 2, Paper number 2000GC000133, 2001.
  • Verma, S., Geochemistry of evolved magmas and their relationship to subduction-unrelated mafic volcanism at the volcanic front of the central Mexican Volcanic Belt, J. Volcanol. Geotherm. Res., 93, 151171, 1999a.
  • Verma, S., Geochemistry of the subducting Cocos plate and the origin of subduction-unrelated mafic volcanism at the front of the central Mexican Volcanic Belt, in Cenozoic Tectonics and Volcanism of Mexico, edited by H. Delgado-Granados, G. Aguirre-Díaz, and J. M. Stock, Spec. Pap. Geol. Soc. Am., 334, 128, 1999b.
  • Verma, S., Geochemical evidence for a lithospheric source for magmas from Los Humeros caldera, Puebla, Mexico, Chem. Geol., 164, 3560, 2000.
  • Verma, S., R. Lozano, P. Giron, and F. Velasco, Calibración preliminar de fluorescencia de rayos X para análisis cuantitativo de elementos traza en rocas ígneas, Actas INAGEQ, 2, 237242, 1996.
  • Vroon, P., M. Bergen, G. Klaver, and W. White, Strontium, neodymium and lead isotopic and trace-element signatures of the East Indonesian sediments: Provenance and implications for Banda Arc magma genesis, Geochim. Cosmochim. Acta, 59, 25732598, 1995.
  • Wallace, P., and I. Carmichael, Quaternary volcanism near the Valley of Mexico: Implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions, Contrib. Mineral. Petrol., 135, 291314, 1999.
  • Zindler, A., and S. Hart, Chemical geodynamics, Annu. Rev. Earth Planet. Sci., 14, 493571, 1986.

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Analytical Methods
  6. 4. Results
  7. 5. Geochemical Affinities and Systematics
  8. 6. Discussion
  9. 7. A Tectonic-Petrogenetic Model
  10. 8. Conclusions
  11. Acknowledgments
  12. References
  13. Supporting Information
FilenameFormatSizeDescription
ggge388-sup-0001-tab01.txtplain text document4KTab-delimited Table 1.
ggge388-sup-0002-tab02.txtplain text document11KTab-delimited Table 2.
ggge388-sup-0003-tab03.txtplain text document2KTab-delimited Table 3.
ggge388-sup-0004-tab04.txtplain text document3KTab-delimited Table 4.

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