5.2.1. Adakite and Nonadakite Characteristics in Negros Arc Silicic Lavas
 Several high silica (>56% SiO2) lavas from Cuernos and Ampiro-Malindang possess some of the geochemical features of rocks termed adakites, which are explained as direct melts of subducted oceanic crust [Defant and Drummond, 1990, 1993]. These Cuernos and Ampiro-Malindang lavas have high Sr concentrations (500–680 ppm), low Y and Yb contents (11–18 ppm and 0.6–1.7 ppm, respectively), and moderate Sr/Y ratios (27–52) (Table 1; Figure 9) Sajona et al. [1993, 1994, 2000a, 2000b]. However, they lack other chemical features critical for adakites. Negros Arc silicic lavas have low La/Yb ratios (< 20), high Ba/La and Pb/Ce ratios, and relatively flat to slightly concave HREE patterns (Figures 5 and 6), unlike typical adakites. Cuernos silicic lavas, alleged to have adakite-like characteristics, have low MgO (average ∼2.5%), Ni (∼7 ppm), and Cr (∼17 ppm) similar to nonadakitic lavas of Northern Negros. Only those from Ampiro-Malindang have MgO ∼3.5%, Ni ∼41 ppm, and Cr ∼100 ppm similar to other proposed adakites [e.g., Martin, 1999]. We emphasize, as discussed previously, that this enrichment can be explained by incorporation of ultramafic basement by silicic differentiates from more mafic magmas. It is important to note that Cuernos and Ampiro-Malindang silicic lavas with adakite features have Sr, Nd, and Pb isotopic ratios that fall within or close to the range of values in Southeast Sulu Sea Basin basalts (Figure 7); this would be consistent with an origin by partial melting of the subducted crust. However, their isotopic compositions can also be explained by derivation from basaltic magmas generated from the mantle wedge having isotopic characteristics similar to the Southeast Sulu Sea basalts. This interpretation is supported by their isotopic similarity to associated mafic lavas (Table 2), which are not adakitic.
Figure 9. Plots of (a) La/Yb versus Yb and (b) Sr/Y versus Y for silicic (SiO2 > 56%) Negros Arc lavas. Also shown are batch partial melting curves for an average Southeast Sulu Sea crust with residues of varying amphibole:garnet:clinopyroxene proportions: A (75:7:18); B (55:15:30); C (15:45:40); D (0:15:85); E (0:20:80); F (0:50:50). Numbers adjacent to tick marks in top plot (Figure 9a) are melting percentages. Typical adakite field in lower plot (Figure 9b) is from Defant et al. [1991b].
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5.2.2. Melting Model Calculations
 To test the proposed origin for some of the silicic lavas in Cuernos and Ampiro-Malindang by melting of basaltic crust, we compare concentrations and ratios of several key trace elements in Negros Arc lavas with those calculated for partial melts of the subducted Southeast Sulu Sea Basin crust using the batch equilibrium melting equation of Shaw . Source compositions used in modeling are average concentrations of the basin's basaltic lavas reported by Spadea et al. ; mineral-melt partition coefficients used are in Table 3.
Table 3. Source Basaltic Crust Compositions and Mineral-Melt Partition Coefficients Utilized in Partial Melting Models
|Average Composition||Sr||Y||La||Yb||Ba||Pb||Ce||Nb|| || || |
|SE Sulu Sea Basalta||132||20.4||3.28||2.07||25||0.92||8.84||2.5|| || || |
|Rutile|| || || || || || || ||26.5||44|| || |
 Figures 9a and 9b show variations in Negros Arc silicic lavas of La/Yb versus Yb and Sr/Y versus Y; these variations are criteria commonly used to distinguish melts of subducted crust from other arc lavas [Defant and Drummond, 1990; Martin, 1999]. Superimposed on these plots are calculated partial melting curves for the Southeast Sulu Sea basaltic crust at the appropriate pressure (depth) conditions beneath the arc volcanoes. Low Yb values and La/Yb ratios in most silicic lavas from Cuernos and Ampiro-Malindang can be reproduced by partial melting of the crust with residues containing variable proportions of hornblende (amphibole), garnet, and clinopyroxene (Curves A, B, and C; Figure 9a), similar to models proposed by other studies in this area and elsewhere [e.g., Sajona et al., 2000a, 2000b; Defant and Drummond, 1990, 1993]. However, calculated melting curves for the majority Negros Arc silicic lavas require significant amounts (>55%) of residual amphibole in the subducted crust beneath the Negros Arc volcanoes. Experimental data indicate that amphibole stability in subducted basaltic crust is unlikely to exceed depths of ∼80 to 85 km (pressure of ∼25 to 26 kbar [Rushmer, 1991; Tatsumi and Eggins, 1995; Drummond et al., 1996]). Cuernos, which is closer to the trench than Ampiro-Malindang, is already ∼100 km above the slab according to Bautista .
 Model calculation results for eclogite residues composed essentially of garnet and clinopyroxene (Curves D, E, and F) are also problematic because these are inconsistent with experimental results of dehydration melting of basaltic rocks at high pressures. Rapp et al. [1991, 1999; Rapp and Watson, 1995] and Sen and Dunn  have produced adakitic magmas by 10–40% partial melting of basaltic compositions at pressures ≥20 kbar leaving an eclogitic residue of 40–60% garnet, 60–40% clinopyroxene and accessory rutile. However, Yb and La/Yb values in some of Cuernos and Ampiro-Malindang silicic lavas within the 10–40% experimental melting range have only ≤ 20% residual garnet (Curves D and E). A few silicic lavas from Ampiro-Malindang that can be matched with a reasonable residual eclogite mineralogy (e.g., 50% garnet, Curve F) require very high degrees (≥50%) of partial melting, which should generate mafic melts (i.e., SiO2 < 56% [Rapp and Watson, 1995]).
 In general agreement, the Sr/Y and Y of silicic lavas in Negros Arc likewise cannot be reproduced by model partial melts of average Southeast Sulu Sea crust (Figure 9b). Only Curve D plots closest to the Ampiro-Malindang silicic lavas, but it has only 15% residual garnet. The other combinations of garnet amphibolite to eclogite residues (Curves A, B, C, E, F) cannot duplicate the Sr/Y and Y of Negros Arc silicic lavas. The low Y values of the model melts are due to the fact that their Y contents are significantly lower than the natural data at similar Sr/Y ratios, even though a low Y partition coefficient for garnet in silicic partial melts (∼4.7 [Jenner et al., 1993]) was used.
 Another useful geochemical tracer of melts of subducted crust is Nb/Ta ratio because it is not changed significantly from source value during moderate to high degrees of melting and subsequent low pressure crystallization of typical arc minerals [Stolz et al., 1996; Eggins et al., 1997; Munker, 1998]. Aqueous fluids equilibrated with residual rutile do not contain appreciable amounts of Nb and Ta and their Nb/Ta ratio are slightly decreased due to more effective retainment of Nb in rutile [Brenan et al., 1994]. In contrast, silicic melts in equilibrium with residual rutile carry small amounts of Nb and Ta and their Nb/Ta ratio are significantly increased due to higher compatibility of Ta in rutile [Green and Pearson, 1987; Jenner et al., 1993]. Thus as proposed by Stolz et al. , partial melts of the oceanic crust with rutile in the residue would exhibit Nb/Ta ratios significantly greater than subducted crust and chondritic primitive mantle values (17.7 for N-MORB crust; 17.5 for primitive mantle [Sun and McDonough, 1989]). Although Prouteau et al.  contend that Nb/Ta ratios cannot be an indicator for slab melting in subduction zones because of uncertainty in rutile-melt partitioning coefficients, several studies of arc lavas suggest otherwise. Metasomatism of the mantle wedge by high Nb/Ta oceanic crust melts best explain high Nb/Ta ratios in Sunda Arc basalts (up to 33 [Stolz et al., 1996]) and Cambrian arc lavas from New Zealand (up to 25 [Munker, 1998]). A similar mechanism of mantle metasomatism, involving partial melts of sediments equilibrated with rutile, has been proposed to explain the higher range of Nb/Ta values in Mariana arc lavas [Elliot et al., 1997; Peate and Pearce, 1998].
 The Nb/Ta ratios of adakite-like and nonadakite silicic and mafic lavas in Negros Arc are slightly less than the chondritic primitive mantle ratio of 17.5 (silicic lavas: 13.5–16; mafic lavas: 13.7–16.4; Table 4). These are lower than the 29–23 range of Nb/Ta values for 10–40% model partial melts of the Southeast Sulu Sea crust assuming an average Nb of 2.5, an N-MORB Nb/Ta of 17.7, and a rutile-bearing eclogite residue (garnet:clinopyroxene:rutile = 59.5:39.5:1, with rutile weight% following Stolz et al.  and Rapp et al.  and overall phase proportions similar to Rapp et al. [1991, 1999]). It is reasonable to assume the presence of rutile in the slab as this is a common accessory mineral in the residues of high pressure dehydration and partial melting experiments of basaltic rocks [e.g., Kogiso et al., 1997; Rapp et al., 1991, 1999; Sen and Dunn, 1994] and in exhumed subducted slabs [e.g., Sorensen and Grossman, 1989, 1993]. Moreover, the presence of Nb depletion relative to other incompatible trace elements in all of the analyzed Negros lavas (Figures 5 and 6) strongly suggest residual rutile in the subducted slab [Brenan et al., 1994; Stalder et al., 1998]. Reducing the proportion of rutile to about 0.25% would only lower values of model melts at 40% melting to about 21. As mentioned earlier, significant residual amphibole is unlikely at ∼100 km slab depth beneath the Negros Arc, but even assuming presence of ∼75% amphibole, which slightly prefers Nb to Ta [Klein et al., 1997], together with small amount of rutile (0.25%) would still lead to a high Nb/Ta ratio of ∼ 19 at 40% melting. Finally, incorporation of bulk sediment with a low Nb/Ta ratio (∼12.7) in partial melts of the crust to force their Nb/Ta to lower values is inconsistent with higher Nb values of both the model melts and sediment than the Negros Arc silicic lavas. Thus Nb/Ta data do not support an oceanic crust melt origin for the silicic lavas in Negros Arc. On the contrary, the similarity in Nb/Ta ratios of silicic lavas with associated mafic lavas strongly suggests a cogenetic relationship.
Table 4. Nb, Ta, and Nb/Ta Ratios for Negros Arc Lavas and Southeast Sulu Sea Sediments Based on ICPMS Analysis
|Southeast Sulu Sea|
|768B-38X-1, 34-40|| ||Claystone||8.44||0.66||12.7|
|768C-33R-2, 22-28|| ||Claystone||6.69||0.54||12.5|
|768C-72R-1, 82-88|| ||Brown claystone||3.76||0.26||14.6|