How was the Triassic Songpan-Ganzi basin filled? A provenance study



[1] The Triassic Songpan-Ganzi complex comprises >200,000 km2 of 5–15 km thick turbiditic sediments. Although surrounded by several magmatic and orogenic belts, the Triassic high- and ultrahigh-pressure Qinling-Tongbai-Hong'an-Dabie (QTHD) orogen, located several hundred kilometers to the east, was proposed as its major source. Middle to Late Triassic samples from the northern and southern Songpan-Ganzi complex, studied using detrital white mica 40Ar/39Ar ages, Si-in-white mica content, and detrital zircon U/Pb ages, suggest that the northern Songpan-Ganzi deposystem obtained detritus from the north: the north China block, east Kunlun, northern Qaidam, Qilian, and western Qinling; the southern Songpan-Ganzi deposystem was supplied from the northeasterly located Paleozoic QTHD area throughout the Ladinian and received detritus from the Triassic Hong'an-Dabie orogen during the Carnian, indicative of exhumation of the orogen at that time. The QTHD orogen fed the Norian samples in the southeastern southern Songpan-Ganzi deposystem, signifying long drainage channels along the western margin of the south China block. An additional supply from the Emeishan magmatic province and/or the Yidun arc is suggested by the paucity of white mica in the southern Songpan-Ganzi deposystem. Mica ages of Rhaetian sediments from the northwestern Sichuan basin best correlate with those of the Triassic QTHD orogen. Our Si-in-white mica data demonstrate that the high- and ultrahigh-pressure rocks of the Hong'an–Dabie Shan were not exposed in the Middle to Late Triassic.

1. Introduction

[2] The Songpan-Ganzi complex (SGC) contains deformed and locally metamorphosed Middle to Late Triassic (∼230–206 Ma) turbidites at the eastern Tibetan Plateau (Figure 1) [e.g., Huang and Chen, 1987; Harrowfield and Wilson, 2005; Huang et al., 2003]. Although the SGC represents a robust record of Middle to Late Triassic sedimentation, widespread low-grade metamorphism and alteration overprints and obscures the provenance signature recorded by sandstone framework grain composition and has impeded efforts to determine the origin of its detritus.

Figure 1.

Topographic map (National Geophysical Data Center) showing the orogens and cratons of central and eastern China. Dashed lines highlight sutures between major tectonic blocks, solid lines are major active faults. The white shaded area indicates the Songpan-Ganzi complex. Box indicates study area.

[3] In this study, we combine the provenance information provided by single-grain laser fusion 40Ar/39Ar ages and Si content of detrital white micas with that of detrital U/Pb zircon ages of samples collected at the same locality. White mica and zircon differ in their chemical and physical properties, and the presence of each is a function of the effects of physical and chemical weathering processes (e.g., climate, source terrain, composition, transport distance and duration), geologic reworking processes (e.g., metamorphic overprint), and cooling of source areas. Zircon is common in many source rocks and resistant to weathering and abrasion, thus the zircon content in sediments is high relative to other mineral phases that are more susceptible to weathering during transport and short-term storage [e.g., Hubert, 1962; Morton and Hallsworth, 1999]. White mica is also a useful provenance indicator, because it generally withstands one cycle of weathering, erosion, transport, and deposition, and is datable by the 40Ar/39Ar method. Ar/Ar ages of white mica date cooling through temperatures of ∼350°C [McDougall and Harrison, 1998] and thus can provide evidence for metamorphic source rocks that did not experience a phase of zircon growth. In addition, the Si-in-white mica content can distinguish high- and ultrahigh-pressure metamorphic phengite from Barrowian metamorphic or igneous muscovite [e.g., Massonne and Szpurka, 1997; Grimmer et al., 2003]. We use these two mineral phases to study the provenance of the SGC that is surrounded by a complex assembly of Precambrian blocks and Precambrian to Triassic magmatic and metamorphic belts (Figure 2).

Figure 2.

(a) Geological map highlighting possible source areas surrounding the Songpan-Ganzi complex modified from Yin and Harrison [2000], Liu et al. [2006], Hacker et al. [2004] and Ratschbacher et al. [2003, 2006]. (b) Different tectonic units of the Qinling-Tongbai-Hong'an-Dabie orogen after Hacker et al. [2004] and Ratschbacher et al. [2003, 2006].

[4] The SGC is ideal for a provenance study because the basin received relatively uniform, fine-grained sand primarily by turbidity currents, with little evidence for debris flows or other sediment gravity flow deposits [Weislogel et al., 2007]. Thus the mineral content of the SGC is not biased by significant grain size variation or hydraulic segregation of the original sediment load. Additionally, the SGC siliciclastic deposits are composed of marine deposits that combine sediment from the entire drainage network, including trunk river system and tributary drainages, and thus are able to provide integrated information about the source region [e.g., Bernet et al., 2004].

2. Geological Setting

[5] Covering a triangular area >200,000 km2 and with thicknesses of 5–10 km and up to 15 km [Gansu Bureau of Geology, 1972; Lai et al., 1992; Zhou and Graham, 1996; Chang, 2000; Geological Brigade of Northwest Sichuan, 2003], the volume of SGC detrital material has been estimated at ∼2.0 × 106 km3 [Nie et al., 1994]. In the eastern SGC (Figure 1), where this study was carried out, Middle Triassic strata are composed of thin-bedded calciclastic turbidite, pebble to cobble limestone conglomerate, and limestone olistostromal deposits up to ∼3 km thick. Conformably to disconformably overlying these deposits are ∼10 km of Upper Triassic turbiditic sandstone and slate [Chinese Geological Society, 2004].

[6] The SGC has been interpreted as a remnant ocean basin between the colliding south and north China blocks [Zhou and Graham, 1996], as a Permian-Triassic rift basin [Chang, 2000], and as a back-arc basin [Şengor et al., 1988]. The Late Permian Emeishan magmatic flood province, located within the western south China block, has been attributed to a mantle plume that was suggested to be responsible for the breakup of the south China block and formation of the Songpan-Ganzi ocean basin; Late Permian basaltic rocks present in the SGC basin and the Yidun area (Figures 1 and 2) may be distal members of the Emeishan continental flood basalts [Song et al., 2004]. SGC basin inversion and closure occurred during the Late Triassic–Early Jurassic, creating the Songpan-Ganzi fold-thrust belt; it resulted from subduction along the Jinsha-Benzilan suture in the south(west) and the Kunlun-Anyemagen-Qinling suture in the north, causing development of folds and thrusts that displaced strata of the SGC onto the margin of the south China block (Figure 2) [e.g., Li and Zeng, 1995; Bruguier et al., 1997; Roger et al., 2004; Harrowfield and Wilson, 2005]. The SGC was intruded by Late Triassic–Early Cretaceous granitoids during and after this fold-and-thrust episode; the granitoids are interpreted to be partial melts generated by crustal thickening that occurred during the shortening event [e.g., Bureau of Geology and Mineral Resources Sichuan Province (BGMRS), 1991; Roger et al., 2004; Harrowfield and Wilson, 2005]. Metamorphic grade varies across the SGC; in general, metamorphic overprint is stronger along the margins where mudstones were metamorphosed to phyllite [Chang, 2000; Huang et al., 2003].

[7] The SGC is situated at the junction of the north China block (composed of the west, east, and trans-north China blocks), the Qaidam block, the south China block (including the Yangtze and the Cathaysia blocks), and the Qiangtang block (Figures 1 and 2); it is bounded to the north by the Early Paleozoic to Early Mesozoic east Kunlun magmatic and metamorphic belt [e.g., Arnaud et al., 2003; Liu et al., 2005], to the northeast by the Paleozoic to Early Mesozoic QTHD orogen [e.g., Mattauer et al., 1985; Ratschbacher et al., 2003; Hacker et al., 2004], to the northwest by the Paleozoic north Qaidam and north and south Qilian metamorphic belts [e.g., Gehrels et al., 2003; Liu et al., 2003; Song et al., 2005], to the southeast by the latest Triassic Longmen Shan fold-thrust belt [e.g., Burchfiel et al., 1995], and to the southwest by the Yidun arc and Qiangtang metamorphic belt [e.g., Watson et al., 1987; Şengor et al., 1988; Kapp et al., 2003; Reid et al., 2005]. As no remnants of feeder systems to this immense basin are preserved, all of these areas represent potential sources of sediment for the SGC turbidites.

[8] Interpretations of the origin and the tectonic significance of the sedimentary rocks of the SGC were mainly based on sediment composition studies. Proposed source areas include: (1) the north China block, based on sandstone framework-grain modal data [Zhou and Graham, 1996] and U/Pb detrital zircon ages [Bruguier et al., 1997; Weislogel et al., 2006]; (2) the south China block, based on U/Pb detrital zircon ages [Bruguier et al., 1997; Weislogel et al., 2006]; (3) the Late Ordovician–Early Jurassic QTHD orogen, based on sandstone framework-grain modal data [Zhou and Graham, 1996] and U/Pb detrital zircon ages [Weislogel et al., 2006]. The latter orogen has been divided into the north China block Proterozoic-Silurian passive margin (Kuanping unit), an Early Paleozoic intraoceanic arc (Erlangping unit), the Qinling microcontinent of south China block affinity including Paleozoic flysch (Liuling, Nanwan, and Foziling units), a Devonian Andean-type arc superimposed onto these units, and a remnant Devonian-Permian accretionary wedge complex (Huwan mélange) that was strongly overprinted in the Triassic, and the Triassic Wudang-Hong'an-Dabie high- (HP) and ultrahigh-pressure (UHP) metamorphic south China block (Figure 2b) [Ratschbacher et al., 2003, 2006]; (4) magmatic arcs that bound the SGC to the southwest (Early Triassic–Jurassic Yidun arc) and northwest (composite Early Paleozoic and Late Paleozoic–Triassic east Kunlun belt), based on sandstone and mudstone geochemical data [Gu, 1994]; and (5) the Triassic Hong'an-Dabie HP and UHP metamorphic complex, based on the comparable volumes of SGC turbidite deposits and rocks eroded to expose the Dabie HP-UHP rocks [Yin and Nie, 1993, 1996; Nie et al., 1994; Zhang et al., 2002], and the synchronicity of the SGC deposits and initial exhumation of the Hong'an-Dabie rocks [e.g., Ayers et al., 2002; Hacker et al., 1998, 2000; Li et al., 2000].

[9] Recently, Weislogel et al. [2006] used detrital U/Pb zircon ages combined with paleocurrent data to discriminate distinct depocenters within the SGC. Throughout the Middle and Late Triassic a northern deposystem provided detrital zircon ages consistent with QTHD orogen and north China block sources. A southern deposystem exhibited changing detrital zircon age populations over time. Early deposits contain zircons with ages consistent with sources in the QTHD orogen; later the detrital zircon ages changed to indicate a combination of QTHD orogen and north China and south China block sources. So far, the detrital zircon record cannot explicitly link the SGC to the exposure of HP and UHP rocks of Hong'an-Dabie [Bruguier et al., 1997; Weislogel et al., 2006].

3. Analytical Techniques

[10] At nine localities across the SGC, we collected fine-grained sandstones for white mica 40Ar/39Ar geochronology; at four of these localities, a companion sample was collected for detrital zircon U/Pb analysis. Our samples represent each of the recently established Middle to Upper Triassic stratigraphic formations of siliciclastic turbidites of the northern and southern SGC (Table 1 and Figure 3) [BGMRS, 1991; Chang, 2000] and also include one Upper Triassic (Rhaetian) fluvial sandstone from the northwestern margin of the Sichuan basin (Table 1 and Figure 3). The limited white mica content of the sandstones impeded direct analysis of thick sections. Therefore >400 μm mica grains were separated from 2 to 3 kg samples using standard techniques, handpicked, and ultrasonically cleaned. The white mica fraction was randomly split into two aliquots, one used for age determination and another used to establish the chemical composition. The first aliquot was packed in pure Cu foil for neutron irradiation. The second aliquot was embedded in epoxy and polished for electron microprobe analysis. Zircon analysis samples were separated for the detrital fraction following the procedures of DeGraaff-Surpless et al. [2002].

Figure 3.

Simplified geological map of the study area locating samples in the Songpan-Ganzi complex and the Sichuan basin. Geology modified after Chinese provincial maps [BGMRS, 1991; Bureau of Geology and Mineral Resources of Shaanxi Province, 1991]. Single-grain 40Ar/39Ar detrital white mica ages, Si-in-white mica content, and detrital zircon U/Pb ages are shown as probability plots with respect to their stratigraphic position. Mean age or age range of major age populations are also shown. Paleocurrent data are from Weislogel et al. [2006]; n, number of measurements; m, mean; std, standard deviation.

Table 1. Sample Location and Stratigraphic Formation
SampleRock FormationaLongitudeLatitude
  • a

    T2, Middle Triassic; T3, Upper Triassic.

E13T3 Xinduqiao30°36.394101°07.003
E16 (UTYa3)T3 Yajiang30°06.078101°00.998
E22 (02UTZ4)T3 Zhuai31°51.956100°46.429
E26T3 Zhuai31°51.335102°18.150
E28T3 Zagunao32°47.131103°36.422
E30 (02MTZ1)T2 Zagashan32°47.863103°37.472
E38T3 Xujiahe32°25.357105°38.871
E40 (02UTZu1)T3 Zhouni34°28.394104°05.040
E46T3 Kache34°28.505102°28.401

3.1. The 40Ar/39Ar Single-Grain Dating

[11] We determined 234 single-grain white mica ages using the laser fusion technique at the 40Ar/39Ar geochronology laboratory at the Geological Survey of Norway. The analytical protocol follows Eide et al. [2002, 2005]. In short, the sample packages were irradiated at the McMaster Nuclear Reactor facility, Hamilton, Canada. We used Taylor Creek Rhyolite sanidine with an age of 27.9 Ma [Duffield and Dalrymple, 1990] as neutron fluence monitors. Uncertainties in J values are 0.1–1% and are incorporated as 1% error in J in the age calculations. Single mica grains were fused under ultrahigh vacuum conditions using a 20 W CO2 laser (Merchanteck Dual-Wave Laser Ablation station) operated at infrared wavelengths using power conditions of 16 Hz and 4–10% power (typically yielding 0.2–0.8 W at fusion). Data reduction included corrections for interfering isotopes, mass discrimination, error in blanks and decay of 37Ar.

3.2. Si-in-White Mica Microprobe Analysis

[12] Aliquots of the same white mica grain size fraction used for dating were analyzed with the JEOL JXA-8900R electron microprobe at Freiberg. Analytical details are given by Grimmer et al. [2003]. Grains with the least apparent alteration were probed from grain mounts. In general two spots, placed between core and rim, were measured to check reproducibility and compositional zoning. The celadonitic substitution (Al[VI] + Al[IV] → (Mg,Fe)[VI] + Si[IV]) of muscovite plays an important role during prograde HP-UHP metamorphism [e.g., Massonne and Szpurka, 1997]. Phengite is the intermediate member (0.2 < × < 0.8) of the muscovite-celadonite solid solution series, thus the Si content in white mica is used to identify phengite grains.

3.3. U-Pb Single-Zircon SHRIMP Dating

[13] Zircon age determinations were performed at Stanford University using the Sensitive High Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG) operated at the specifications outlined by DeGraaff-Surpless et al. [2002]; ages were calibrated with reference to a standard zircon sample R33, a quartz diorite of the Braintree complex, Vermont [Black et al., 2004]. Individual zircon grains were chosen for analysis at random in order to prevent bias in the sampled population. Cathodoluminescence imaging of the samples was performed so that internal heterogeneities and compositional domains present within the zircons could be observed and coherent zircon domains could be targeted for analysis. All ages were corrected for common Pb using 206Pb/204Pb. In general, depending on U concentration and common Pb contamination, ages younger than 1000–800 Ma are based on common Pb-corrected 206Pb/238U ratios, whereas ages older than 1000–800 Ma are based on common Pb-corrected 207Pb/206Pb ratios. Discordance either due to inheritance, Pb loss, multiple phases of grain growth, or both was encountered in ∼20% of the grains analyzed. Zircon age results described in the text were vetted to exclude analyses with >20% discordance or >10% reverse discordance. Results from one locality (UTYa3/E16) were presented by Weislogel et al. [2006]; the other three samples are new data presented here for the first time.

4. Results

4.1. The 40Ar/39Ar White Mica Geochronology

[14] White mica 40Ar/39Ar results are listed in Table 2 and shown as relative probability versus age plots [Ludwig, 2003] in Figure 3. There are four major age populations: the youngest population (220–205 Ma) constitutes 13–25% of all dated grains in the late Carnian and Norian samples (E13, E16, E26) of the southern SGC. The second age population (270–230 Ma) occurs in most southern SGC samples (E13, E16, E22, E26, E30; 6–39%) and in those of the northwestern Sichuan basin (E38, 42%). The third age population (370–320 Ma) occurs in all SGC samples (E13, E16, E22, E26, E28, E30, E40, E46, 3–48%). The fourth major age population (430–380 Ma) constitutes 52–65% of all dated grains in the Ladinian through Carnian samples of the southern SGC (E22, E28, E30), 28% in the northwestern Sichuan basin (E38), 42–70% in the Norian of the northern SGC, but only 4–13% in the Norian of the southern SGC samples (E13, E16). Three auxiliary age populations occur: a tight peak at 300 Ma (E13, E16, E40), ages of 730–720 Ma (E30 and E46), and 650–500 Ma ages (E13, E28, E40).

Table 2. Single-Grain Laser Fusion 40Ar/39Ar Results of Detrital White Mica
Sample40Ar/39Ar38Ar/39Ar37Ar/39Ar36Ar/39Ar39Ar, 10−12 molPercent 40Ar*40Ar*/39ArKAge, Ma±1σ
J = 0.006062, Weight 5 mg
J = 0.006116, Weight 5 mg
J = 0.006324, Weight 5 mg
J = 0.006398, Weight 5 mg
J = 0.006376, Weight 5 mg
J = 0.006369, Weight 5 mg
J = 0.006282, Weight 5 mg
J = 0.006253, Weight 5 mg
J = 0.00634, Weight 5 mg

4.2. Si-in-White Mica Microprobe Analysis

[15] The microprobe analyses of white mica from seven samples of the SGC and the Sichuan basin are shown in Figure 3 and are listed in the auxiliary material. In general, the white micas exhibit low Si content (low: <3.3 atoms pfu; intermediate: 3.3–3.5 atoms pfu; high: >3.5 atoms pfu) with mean values of 3.02 to 3.09 atoms pfu. Phengites are rare: sample E22 shows small peaks at intermediate to high Si content (3.35–3.53 atom pfu); E46 and E28 show a small peak at high Si content (3.55 atoms pfu).

4.3. U/Pb Detrital Zircon Geochronology

[16] Zircon age populations fall into four groups (Figure 3 and Table 3). The youngest major age population is at 460–420 Ma and forms a major component (13–34%) of three of the four samples (E30/02MTZ1, E22/02UTZ4, E40/02UTZu1). The second age population covers 920–650 Ma and is a major component (23–37%) of all samples. A third population is at 1.1–0.9 Ga; it includes 6–16% of all dated grains in samples E22/02UTZ4, E16/02UTY3, and E40/02UTZu1 and is the least abundant of the four major populations. The oldest population is present in E30/02MTZ1, E22/02UTZ4, and E16/02UTY3 and contains 2.0–1.8 Ga ages. Other zircon ages are represented in some of the samples. There is a population of 295–250 Ma that constitutes 13% of E22/02UTZ4 and 6% of E30/02MTZ1. Sample E16/02UTY3 contains four zircons at 260–220 Ma. Archean and Paleoproterozoic grains were found in every sample, however, only two samples, E22/02UTZ4 and E40/02UTZu1, have populations of Archean grains that make up >5% of all dated grains. Sample E22/02UTZ4 has ages between 2.7 and 2.5 Ga that make up 8% of the dated grain, whereas sample E40/02UTZu1 has a slightly older population ranging from 2.8 to 2.6 Ga that constitutes 11% of the distribution. The Th/U ratios indicate that the bulk of zircons are derived from igneous source rocks, with only ∼20 zircons exhibiting Th/U < 1.

Table 3. U-Pb SHRIMP Geochronologic Analysesa
Spot NameIsotopic RatiosApparent Ages, Ma
U, ppm204Pb/206PbU/ Th207Pbb/ 235U± %206Pbb/ 238U± %Error Corrected206Pbb/ 207Pbb± %206Pb/ 238U± 1σ206Pbb/ 207Pbb± 1σPercent DiscordanceBest Age± 1σ
  • a

    Italicized data were omitted because of large measurement error or large discordance. Discordance is not reported for ages ≥1.0 Ga. Read −2.7E-05 as −2.7 × 10−5.

  • b

    Ages corrected for common Pb. Errors on SHRIMP-determined ages are 1σ.

Yajiang Formation (UTYa3), Middle-Late Norian, N = 63
02Y3-32638.3E-051.61.1630.1320.70.07278315   78315
02Y3-41023.8E-041.41.0660.1420.40.06583417   83417
02Y3-51352.9E-041.20.5350.0820.40.05547310   47310
02Y3-67571.5E-042.10.3030.0420.60.0532625   2625
02Y3-7535.9E-051.11.1550.1230.50.07475619   75619
02Y3-854−1.1E-041.21.3540.1420.60.07384320   84320
02Y3-91961.3E-051.11.1230.1220.70.07275015   75015
02Y3-101501.5E-052.21.3230.1520.70.07287717   87717
02Y3-114397.4E-051.91.0930.1220.70.07273814   73814
02Y3-1313091.6E-032.30.2460.0320.30.0562194   2194
02Y3-14805−1.5E-052.60.5320.0720.80.0614338   4338
02Y3-18881.4E-031.20.22280.0430.10.04272527   2497
Y3-28321.5E-041.11.0820.1210.40.0617384   7384
Y3-1011971.3E-056.81.5310.1500.50.0719163   9163
Y3-171383.6E-0544.70.8330.1010.40.0635847   5847
Y3-21791.0E-320.41.0930.1220.50.06374411   74411
Y3-223453.4E-041.70.2470.0410.20.0572313   2313
Y3-25201.3E-032.40.71290.1140.10.052969822   69822
Y3-273401.0E-320.80.5220.0710.40.0624164   4164
Y3-286214.2E-0519.90.5420.0710.40.0624393   4393
Y3-311562.6E-041.31.0130.1210.40.0637459   7459
Y3-321572.6E-041.00.9930.1210.40.0637408   7408
Y3-363208.8E-053.90.4730.0610.30.0633874   3874
Y3-402791.3E-041.30.9230.1110.40.0626527   6527
Zhuai Formation (02UTZ4), Middle-Late Carnian, N = 57
Z4-93941.0E-321.30.2930.0410.30.053253229359 2532
Z4-147834.6E-041.30.2750.0410.20.055259245120 2592
02Z4-1312965.6E-051.40.2930.0420.70.052264520244 2645
Z4-2844002.8E-042.20.3120.0400.20.052272126442 2721
Z4-52761.0E-322.80.3430.0510.40.053292334868 2923
02Z4-69671.0E-043.70.3230.0520.70.052294518442 2945
Z4-427342.5E-040.70.3820.0500.20.052325133040 3251
02Z4-71881.1E-031.30.30120.0520.20.04123327−241291 3327
02Z4-8B1534.4E-052.00.5250.0720.40.064411951597 4119
Z4-81266.5E-041.60.4950.0710.30.0544266276103 4266
Z4-133217.5E-043.50.4290.0710.10.0494304−78230 4304
Z4-228371.0E-320.80.5210.0710.40.051431239328 4312
Z4-124393.4E-041.50.4840.0710.20.054431322198 4313
Z4-104361.5E-042.10.5220.0710.40.052435336043 4353
Z4-3620871.0E-3215.10.5410.0700.40.061440143617 4401
02Z4-85661.9E-042.90.5330.0720.70.052455832850 4558
Z4-243682.5E-042.40.5930.0810.20.053494435872 4944
Z4-30690−3.6E-061.70.9210.1110.40.061660367423 6603
Z4-375594.3E-042.10.9930.1110.20.063693471758 6934
Z4-235082.7E-043.31.0520.1110.30.072697482639 6974
02Z4-128081.1E-040.41.0420.1220.80.0617171373429 71713
02Z4-17704−1.1E-053.21.1720.1320.90.0717651485524 76514
Z4-212381.4E-041.81.1130.1310.30.063782667854 7826
Z4-114596.0E-054.81.1710.1310.40.071785479926 7854
Z4-3211582.7E-053.41.1610.1300.40.061789376219 7893
Z4-344626.1E-051.11.1810.1310.50.071793678725 7936
02Z4-101112.3E-051.21.3730.1420.70.0728701788550 87017
02Z4-11125.9E-041.51.2250.1520.40.0648931859694 89318
Zagashan Formation (02MTZ1), Middle-Late Ladinian, N = 61
02MTZ1-17951.7E-045.00.1930.0310.30.053180219271 1792
02MTZ1-26108−6.9E-041.70.33170.0420.10.0562546586354 2525
02MTZ1-555911.0E-322.10.3030.0410.30.053267228063 2672
02MTZ1-24401.0E-322.10.3590.0530.30.0582879384190 2869
02MTZ1-174396.2E-054.90.4930.0710.40.052417431255 4184
02MTZ1-352821.9E-043.90.4940.0710.30.053430426480 4335
02MTZ1-383786.7E-050.90.5530.0710.40.062444442953 4444
02MTZ1-392839.2E-052.80.5530.0710.30.062449442865 4495
02MTZ1-32192−2.7E-041.30.6060.0710.20.0634536608130 4515
02MTZ1-182691.0E-321.20.5430.0710.40.053454533660 4565
02MTZ1-592571.0E-320.50.7720.0910.40.062573661548 5726
02MTZ1-2374.4E-041.51.0180.1230.30.07572419649174 72619
02MTZ1-91111.0E-321.81.0930.1220.50.0737311180461 72911
02MTZ1-232979.1E-051.31.0820.1210.40.062745673039 7457
02MTZ1-19963.5E-041.31.0980.1220.20.07475412717163 75512
02MTZ1-12491.0E-321.01.1950.1320.50.0747751685785 77217
02MTZ1-16122−6.6E-051.71.1930.1310.40.0737741184863 77211
02MTZ1-522061.0E-321.41.1720.1310.50.072786878942 7868
02MTZ1-581441.0E-321.51.1630.1310.50.062786976850 78710
02MTZ1-445132.7E-052.11.1920.1310.50.071792681028 7926
02MTZ1-572725.0E-053.31.1920.1310.40.072793780938 7927
02MTZ1-155871.6E-041.11.2020.1310.30.071797580339 7975
02MTZ1-613121.0E-322.11.2320.1310.50.072812681932 8127
02MTZ1-56126−5.5E-050.71.2330.1310.40.0738161080958 81611
02MTZ1-30841.6E-041.91.1840.1420.40.0738181270379 82113
02MTZ1-291421.0E-321.01.2730.1410.50.0728331082350 83310
02MTZ1-25961.0E-321.51.3130.1410.50.0738421286257 84112
02MTZ1-601161.0E-321.01.2830.1410.50.0728551179351 85711
02MTZ1-222779.0E-053.91.3820.1510.40.072888786640 8898
Zhouni Formation (02UTZu1), Middle-Late Norian, N = 50
02ZU1-3123−1.0E-041.30.5840.0720.50.064427966376 4279
02ZU1-104831.9E-0511.81.4010.1500.40.071877491920 8774
02ZU1-134202.4E-041.21.1220.1300.20.062774373640 7743
02ZU1-152864.8E-050.61.1120.1210.40.072743479534 7434
ZU1-22643.7E-041.50.5140.0710.30.054443427488 4434
ZU1-31016.0E-042.00.52110.0720.20.05114417343247 4417
ZU1-44243.9E-051.40.6420.0810.40.062499453142 4994
ZU1-64536.1E-054.30.6520.0810.40.062500454242 5004
ZU1-74009.7E-051.50.5220.0710.40.052433440550 4334
ZU1-91851.0E-322.20.6030.0710.40.063459555562 4595
ZU1-11945.5E-041.30.4850.0720.30.055452792124 4527
ZU1-123965.7E-051.60.5220.0710.30.052439336050 4393
ZU1-142807.3E-051.50.5330.0710.40.063431444163 4314
ZU1-15392.2E-041.31.3360.1520.40.06590219757108 90219
ZU1-162271.0E-321.40.5630.0710.40.063452543758 4525
ZU1-18364−1.3E-052.81.5110.1610.50.071938692125 9386
ZU1-195721.0E-322.00.5720.0710.50.062444453038 4444
ZU1-232814.3E-052.914.2210.5310.80.19127422027809 96534
ZU1-243465.2E-053.81.5620.1610.40.072951796534 9517
ZU1-25260−3.5E-041.80.5950.0710.20.0654435623110 4435
ZU1-273921.0E-321.11.0920.1210.50.061751574530 7515
ZU1-281691.0E-321.81.5730.1610.40.0729451099346 99346
ZU1-29951.0E-321.11.1030.1220.50.0737311182364 73111
ZU1-315331.0E-321.10.5320.0710.40.062430344039 4303
ZU1-331801.0E-320.90.5330.0710.40.063433542269 4335
ZU1-351273.5E-040.90.4580.0720.30.0584088168177 4088
ZU1-363696.7E-051.20.5130.0610.40.063400453157 4004

5. Discussion

[17] Figure 3 summarizes our new U/Pb zircon and 40Ar/39Ar white mica ages, and the Si-in-white mica data. Probability density plots of each sample are shown with respect to stratigraphic age, geologic formation name, and geographic location. Because of analytical limitations and the rare white mica content in the samples, the number of analyzed grains per sample is less than the value (∼100 [Vermeesch, 2004]) necessary to guarantee that no fraction of the population comprising more than 5% of the total is missed at the 95% confidence level. However, this study aims to identify major source areas that fed the SGC; we base our discussion on linking major age populations of our samples with those of possible source areas and do not discuss single-grain ages. We accept that minor source areas might have contributed to the sedimentation but cannot be identified in this study.

[18] The data are interpreted in the context of the paleocurrent data of Weislogel et al. [2006] shown in Figure 3 and published radiometric ages of potential source areas (Figures 2 and 4). Seven orogenic belts or segments of them and two crustal blocks were discriminated (Tongbai-Hong'an-Dabie, Qinling, east Kunlun, northern Qaidam, Qilian, Qiangtang, Yidun, north China, south China; Figures 2 and 4 and reference list in Text S1 in the auxiliary material). The literature compilation reflects the fact that bedrock thermochronology studies are heterogeneously distributed; e.g., 272 ages were published from the Hong'an-Dabie section of the QTHD orogen but only 22 ages from the Qiangtang metamorphic belt. This bias in the quality of the definition of age populations should be kept in mind when interpreting provenance.

Figure 4.

Compilation of published ages of crustal blocks and Paleozoic-Triassic orogenic belts surrounding the Songpan-Ganzi complex outlining potential source areas. (a) Qiangtang block and Yidun arc; (b) Qaidam block and north Qaidam metamorphic belt; (c) Qinling section of the Qinling-Tongbai-Hong'an-Dabie orogen; (d) east Kunlun magmatic and metamorphic belt; (e) Qilian Shan orogen; (f) Tongbai-Hong'an-Dabie section of the Qinling-Tongbai-Hong'an-Dabie orogen; (g) south China block; (h) north China block. A complete reference list for the used literature is given in Text S1 in the auxiliary material.

Figure 4.


5.1. Southern Songpan-Ganzi Complex

[19] The Ladinian sample (E30/02MTZ1) is dominantly (70% of all dated grains) composed of Silurian–Early Devonian (430–385 Ma) white mica ages that are consistent with ages from the compound Paleozoic orogenic belt that lines the southern margin of the north China block (northern Qinling-Tongbai-Hong'an, north Qilian, and east Kunlun, Figures 4c–4f). It also contains a population (23%) of Late Permian–Early Triassic mica ages (265–240 Ma), corresponding to ages in the Huwan mélange and the Qinling microcontinent in Hong'an [Xu et al., 2000; Ye et al., 1993; Niu et al., 1994] and the Triassic HP-UHP belt of the QTHD (Figure 4f) [Webb et al., 1999; Hacker et al., 2000]; it also corresponds to granitic gneisses from east Kunlun (Figure 4d) [Liu et al., 2005]. Zircon from this sample yielded primarily (37%) 880–720 Ma ages; such zircon ages occur frequently in the south China block (Figure 4g) and the metamorphosed south China crust found in the QTHD orogen [e.g., Hacker et al., 2000; Chen et al., 2003]. They also occur in the Qinling microcontinent (Figures 4c and 4f) [Ratschbacher et al., 2006]. Another significant zircon age component of ∼1.9 Ga (14%) is consistent with a derivation from the trans-north China block. The population of ∼440 Ma zircon could be derived either from the Qilian Shan (Figure 4e) [e.g., Gehrels et al., 2003; Song et al., 2005, 2006], east Kunlun (Figure 4d) [Chen et al., 2002; Cowgill et al., 2003], or the Qinling microcontinent (Figures 4c and 4f) [Reischmann et al., 1990; Kröner et al., 1993; Lerch et al., 1995].

[20] The early Carnian sample (E28) is located adjacent to and up section from E30 in the northeastern part of the southern SGC (Figure 3). White mica ages are almost exclusively (92%) Silurian-Carboniferous (420–325 Ma), corresponding to the Paleozoic orogenic belts along the southern margin of the north China block (Figures 2 and 4b–4f). No zircon age data from the lower Carnian SGC strata were acquired at this location; however, zircon ages from the Ladinian through Carnian rocks were located farther west (Figure 3) exhibit major populations of Permian (280–260 Ma) and Paleozoic (500–300 Ma) ages but lack a significant population of Precambrian zircons [Weislogel et al., 2006].

[21] The sediments represented by the Ladinian and early Carnian samples of the southern SGC were most likely fed from the south China block fragments that were accreted to the north China block during the Paleozoic in the QTHD orogen; i.e., the Qinling microcontinent, the Early Ordovician intraoceanic arc (Erlanping unit), the superimposed Devonian arc, the Devonian-Permian fore arc (Liuling, Nanwan, and Foziling units), and the Huwan mélange (Figure 2, 4c, and 4f).

[22] The two late Carnian samples (E22/02UTZ4, E26) are located in the central part of the southern SGC (Figure 2). Similar to sample E28, 94% of the detrital mica ages of E22 are Silurian-Carboniferous (420–325 Ma), corresponding to the Paleozoic belts along the southern margin of the north China block (Figures 4b–4f). Only two micas yielded Late Permian–Early Triassic (250–242 Ma) ages. Paleocurrent data from this area indicate a supply from the northeast [Weislogel et al., 2006]; the intermediate to high Si-in-white mica values of E22 correspond to Paleozoic phengites reported from Hong'an (Figure 4f) [Xu et al., 2000] or north Qilian (Figure 4e) [Song et al. [2006] (see below). Zircon ages from this sample location are primarily 900–660 Ma (23%) and 495–325 Ma (23%); these ages correspond to crystalline basement ages of the south China block and its reworked counterparts in the QTHD orogen [e.g., Hacker et al., 2000, 2004; Chen et al., 2003] and to north Qinling, east Kunlun, Qaidam, and Qilian [e.g., Lerch et al., 1995; Xue et al., 1996; Gehrels et al., 2003] (Figures 4b–4f). Another significant component (15%) at 2.0–1.8 Ga stems from the north China block. The population of 295–250 Ma zircon could be derived either from east Kunlun (Figure 4d) [Cowgill et al., 2003], Hong'an-Dabie (Figure 4f) [e.g., Hacker et al., 1998; Ayers et al., 2002], or north Qaidam (Figure 4b) [Gehrels et al., 2003]. The Archean–Early Proterozoic (3.2–2.4 Ga, 10%) zircon ages correspond to ages of the north China block (Figure 4h). The broad peak of Paleozoic mica ages is in contrast to the tighter and ∼30 Ma older peak of zircon ages; this suggests slow cooling within the Paleozoic orogen. Zircon data from the time-equivalent Zhuai Formation farther to the north (UTZh7 of Weislogel et al. [2006]; Figure 3) show a broad age range between 440 and 250 Ma, corresponding to the Paleozoic orogenic belts (east Kunlun, north Qinling, Qilian, and Qaidam) located to the north of this sample and the QTHD belt (Figures 4b–4f); this sample, however, does not contain Precambrian zircons.

[23] In sample E26, 34% of mica ages are younger than its stratigraphic age. The sample location is close to a granite body and was probably thermally effected. Thin sections of a sample derived from the same outcrop shows grow of new white micas, which supports a thermal overprint. We therefore do not interpret the ages of E26 geologically.

[24] We suggest that the late Carnian samples mainly share their source with the Ladinian to early Carnian samples, i.e., the Paleozoic QTHD belt. The likely greater supply of Triassic detritus in the late Carnian than in the Ladinian–early Carnian samples suggests an eastward and southward shift of the drainage area to the Triassic metamorphic belt of the QTHD orogen; rapid exhumation in Hong'an-Dabie probably caused a more restricted drainage area [e.g., Amidon et al., 2005].

[25] The Middle (E16) and late Norian (E13) samples are located in the southeastern part of the southern SGC (Figure 3). The white micas comprise a broadly distributed Triassic age population (250–207 Ma; ∼50%), corresponding to ages in the Triassic HP-UHP metamorphic Hong'an–Dabie Shan (Figure 4f) [e.g., Webb et al., 1999; Hacker et al., 2000] and the Triassic basement domes of the south China block [Ratschbacher et al., 2003]. The Triassic white mica ages also correspond to ages form the west Yidun metamorphic and magmatic belt [Reid et al., 2005] and the Qiangtang belt [Kapp et al., 2000, 2003; Li et al., 2006] located to its west (Figure 2, 3, and 4a). However, the east Yidun volcanic arc, active in the Middle and Late Triassic, likely prohibited sediment supply from areas to its west and probably did not contain white mica-bearing rocks (Figure 3). Samples E16 and E13 also contain Paleozoic white mica age populations at 370–330 Ma (48%) and 390–370 Ma (20%), respectively. These ages correspond to ages in the Qinling (Figure 4c) [Mattauer et al., 1985] and in the east Kunlun belt (Figure 4d) [Liu et al., 2005]. Zircons from locality E16 (UTYa3 of Weislogel et al. [2006]) primarily contain Neoproterozoic (∼750 Ma) and Paleoproterozoic (∼1.9 Ga) age populations. The Neoproterozoic population is evidently derived from the rift-related rocks of this age that are common in the south China block (Figure 4g); Neoproterozoic ages occur in the Qinling microcontinent but mostly in the Triassic belt of the QTHD orogen. They also occur in the basement domes of the Longmen Shan that is closest to our sample locality (Kangding area of Figure 3 [Liu et al., 2003]). Paleoproterozoic ages were reported from the south China block basement but such ages are typical for the trans-north China block [e.g., Guo et al., 2005; Kröner et al., 2006], and for volcanic rocks along the southern margin of the north China block (Figure 4h) [Zhao et al., 2004]. The Permo-Triassic zircon age population (260–219 Ma; 6%) tightly overlaps the mica age population (250–207 Ma) and thus suggests a supply from a rapidly exhuming orogen. The overall meager mica content in these samples indicates an additional source that lacks white mica, or destruction of mica due to extreme environments. Such sources could have been the Permo-Triassic Emeishan magmatic province that is located to the southeast, at the western margin of the south China block (Figure 2) and yielded whole rock, biotite, hornblende, and plagioclase 40Ar/39Ar ages of 256–246 Ma [Boven et al., 2002; Lo et al., 2002] and the east Yidun arc that provided 240–216 Ma zircon ages [Reid et al., 2005; Liu et al., 2006]. Paleocurrent data from the localities of samples E16 and E13 suggest a source area in the southeast [Weislogel et al., 2006] (Figure 3). However, these southeastern directions in the southernmost SGC likely need to be rotated counterclockwise by ≤50° in the light of GPS and paleomagnetic data that indicate clockwise rotation around the eastern Himalayan syntaxis in this area [e.g., Shen et al., 2005; Sato et al., 1999; Otofuji et al., 1990]. Taking this into account, a source from the northeast, i.e., the QTHD belt, is likely.

[26] The zircon ages indicate derivation from combined south China and north China block sources, i.e., from the entire QTHD belt (Figures 4c and 4f). Locality E16/UTYa3 records a significant increase in Triassic mica ages and the first occurrence of Late Triassic zircons [Weislogel et al., 2006]. The overlap of Late Permian–Late Triassic mica and zircon ages indicates that the southern SGC deposystem was most likely fed by the rapidly exhuming Triassic QTHD orogen. A supply from this orogen implies a long and narrow drainage system following the strike of the orogen but also significant southward channeling of the sediments along the western margin of the south China block (Figure 5). We suggest that the triple-junction configuration, constituted by the east trending Kunlun-Anyemagen-Qinling suture/orogen and the south-southwest trending margin of the south China block played a significant role for channeling the sediments into the southeastern part of the southern SGC [e.g., Wang et al., 2001].

Figure 5.

Summary of suggested source areas for the southern Songpan-Ganzi complex in the (top) Ladinian–early Carnian, (middle) late Carnian, and (bottom) the northern and southern Songpan-Garzi complex in the Norian and the northeastern Sichuan basin in the Rhaetian.

5.2. Northern Songpan-Ganzi Complex

[27] In contrast to the Norian samples of the southern SGC, no Permo-Triassic white mica ages occur in the two Norian samples (E40/02UTZu1 and E46) of the northern SGC (Figure 3). Most ages are Silurian-Devonian (435–380 Ma; 40–70%), corresponding to the Paleozoic belts along the southern margin of the north China block (Figures 2 and 4h), the east Kunlun [e.g., Gehrels et al., 2003], Qilian [Song et al., 2006], and Qinling-Tongbai-Hong'an (Figures 4c–4f) [e.g., Xu et al., 2000; Ratschbacher et al., 2003]. Additionally, both samples yielded ages of 370–320 Ma (13–25%). Such Late Devonian–Carboniferous mica ages have rarely been reported; two ages from the Kuangping and Qinling units of northern Qinling (Figure 4c) [Mattauer et al., 1985; Zhang et al., 1991], one age from south Qilian (Figure 4e) [Liu et al., 2003], and one from east Kunlun (Figure 4d) [Liu et al., 2005] are known.

[28] Similar to the mica age distribution, the E40/02UTZu1 zircon age distribution also lacks Permo-Triassic ages but consists of 34% Ordovician-Silurian zircons (peak at 440 ± 10 Ma), corresponding to the HP-UHP metamorphic belts and the granitoids in northern Qaidam (Figure 4b) [e.g., Gehrels et al., 2003; Song et al., 2005, 2006] and northern Qilian (Figure 4e) [e.g., Cowgill et al., 2003; Song et al., 2004]; only one zircon age from this range is known from the east Kunlun [Chen et al., 2002], a few from the Huwan mélange [Jian et al., 2001; Gao et al., 2002], and the UHP unit in the Dabie Shan (Figure 4f) [Rowley et al., 1997; Ma et al., 2005]. The remaining populations are composed in about equal proportions of Archean (2.8–2.6 Ga; 11%), Mesoproterozoic (1.0–0.9 Ga; 16%), and Neoproterozoic grains (775–730 Ma; 11%). The Archean zircon grains are most likely derived from the north China block [e.g., Kröner et al., 2005; Gao et al., 2006]. The Mesoproterozoic and Neoproterozoic ages correspond to crystalline blocks that are involved in the Paleozoic orogenic belts of Qilian [Mao et al., 2000; Tseng et al., 2006], Qaidam [Gehrels et al., 2003], and Qinling [Chen et al., 1992; Ratschbacher et al., 2003] (Figures 4e, 4b, and 4c, respectively).

[29] The major zircon age population is ∼30 Ma older than the major mica age population, indicating that the source area is composed of rocks that cooled slowly during the Silurian-Devonian. This is consistent with derivation from north Qaidam and Qilian rocks produced by the amalgamation of the Qilian and Qaidam microcontinents with the north China block. Derivation from the Qinling rocks is less likely due to the paucity of Late Ordovician–Silurian zircon ages in the QTHD belt. The ∼80 Ma gap between the depositional age of samples E46 and E40/02UTZu1 and the youngest mica age population (370–320 Ma) indicates that the source rocks of these micas had been at shallow crustal depths for a significant time prior to exhumation and erosion.

[30] Zircons from four Late Triassic (Ladinian through Norian) samples of the northern SGC, located west (UTKa1, UTZh1, UTZho2) and east (UTNa2) of locality E40/02UTZu1 (Figure 3) show significant Late Permian–Early Triassic (280–245 Ma) age populations [Weislogel et al., 2006]. The occurrence of detrital zircon U/Pb ages that are younger than the detrital white mica 40Ar/39Ar ages can be explained either by a volcanic/magmatic source that did not contain white mica, recycling of a sedimentary source, or weathering during erosion and transport that destroyed mica in the sediment load. The Late Permian–Early Triassic east Kunlun arc and the Early Triassic granitoids of west Qinling are suggested to be the major source for the northern SGC. The lack of those zircon ages in sample E40/02UTZu1 is probably due to its location between the granitoids of west Qinling and east Kunlun (see Figure 3), or due to changes in the source area. We conclude that the northern SGC was fed by the Paleozoic orogenic belt along the southern margin of the north China block (east Kunlun, north Qaidam and Qilian, northern units of Qinling) and by the Late Permian–Early Triassic Kunlun arc (Figure 5). This implies that major exhumation and erosion in the Late Triassic was not only taking place along the QTHD belt due to the collision of the north China and south China blocks but occurred also north of the SGC, along the Paleozoic belts west of the QTHD belt; the latter might reflect northward subduction beneath an increasingly mature wedge-arc complex north of the Kunlun-Anyemagen-Qinling suture.

5.3. Sichuan Basin

[31] We analyzed mica grains from one Rhaetian (Late Triassic) sandstone sample (E38) located in the northwestern Sichuan basin. The white mica data show two major age populations: (1) Late Permian–Late Triassic (260–220 Ma, 59%) corresponding to ages from the Triassic HP-UHP belt of the QTHD orogen (Figures 4c and 4f), the southern margin of the Qinling microcontinent that was affected by a Late Permian–Triassic thermal event (Liuling and Nanwan units, Huwan mélange [e.g., Hacker et al., 2000; Ratschbacher et al., 2006]), and the east Kunlun (Figure 4d) [Liu et al., 2005]. (2) Silurian–Middle Devonian (440–380 Ma; 35%) corresponding to the Erlangping, Qinling, and Liuling units, and the Huwan mélange of the QTHD belt [Xu et al., 2000; Zhang et al., 1991; Ratschbacher et al., 2003] and the east Kunlun arc [Chen et al., 2002; Wang et al., 2003]. An east Kunlun source is improbable due to the sample location in the Sichuan basin that was likely separated from sources to the (north)west by an ocean basin as well as by the evolving Longmen Shan thrust belt in the Late Triassic (Figures 2 and 3).

[32] We suggest that the northern Rhaetian Sichuan basin was sourced by the QTHD orogen (Figure 5). The low Si contents in the micas suggest that the HP-UHP rocks of the Triassic Hong'an-Dabie orogen were not exposed at the surface in the Late Triassic (see below).

5.4. Comparison of Zircon and Mica Grain Age Data

[33] The combination of detrital zircon and white mica age data allows general statements regarding the contribution of first-cycle and recycled sediment sources to the SGC deposits. In addition, the Si-in-white mica content allows discrimination of HP-UHP phengitic white mica from low-pressure metamorphic or igneous muscovite [e.g., Massonne and Szpurka, 1997; Grimmer et al., 2003].

[34] Mica ages are predominantly Paleozoic, whereas zircon ages are primarily Precambrian; Precambrian ages are only sparsely represented in the mica age data. This suggests that Precambrian micas vanished during recycling of sedimentary rocks derived from the original Precambrian sources or were reset by later metamorphism. Where zircon ages exhibit a cluster at ∼445–430 Ma, mica ages are younger by approximately 30 Ma. Assuming the same source rocks for the micas and zircons, this age difference is interpreted to be indicative of slow cooling in the source region, likely along the Paleozoic orogenic belts that formed by accretion of microcontinents (Qinling, Qaidam, Qilian) against the southern margin of the north China block [Wang et al., 2005; Ratschbacher et al., 2003]. The Late Paleozoic–Triassic zircon and mica age clusters are less separated; this suggests more rapid exhumation in the hinterland, consistent with geochronometric data from the Triassic HP-UHP metamorphic belt of the QTHD orogen [Li et al., 2000; Hacker et al., 2000, 2004].

5.5. Does the SGC Contain HP-UHP Rocks From the Hong'an-Dabie Shan?

[35] Zircon U/Pb ages of crustal rocks of the Hong'an–Dabie Shan record protracted UHP metamorphic growth at 240–225 Ma [e.g., Hacker et al., 2006]. Together with Ar/Ar phengite cooling ages at ∼235–225 Ma, they suggest rapid exhumation from mantle to lower crustal depths; exhumation through the crust at ∼225–195 Ma was less rapid [e.g., Eide et al., 1994; Hacker et al., 2000; Ratschbacher et al., 2006]. Studies published so far indicate that the UHP rocks reached the surface not earlier than Jurassic [Grimmer et al., 2003; Wang et al., 2003; Wan et al., 2005; Li et al., 2005].

[36] Samples of our study show in general low Si-in-white mica content and thus exclude supply from a major HP-UHP source area; this conforms to a post-Triassic surface exposure of the Hong'an–Dabie's UHP rocks. Southern SGC sample E22 (upper Carnian) yielded a small population of intermediate to high Si content micas and mica age populations of 325–425 Ma and 240–250 Ma. Ar/Ar ages of phengite and muscovite of the UHP Hong'an–Dabie belt are younger (240–195 Ma, Figure 4f [e.g., Eide et al., 1994; Hacker et al., 2000; Ratschbacher et al., 2006]) and thus can most probably be excluded as a source. Similarly, a source from the westerly located central Qiangtang metamorphic belt, with phengite ages of ∼220 Ma [Kapp et al., 2003], can be excluded. Paleozoic phengite cooling ages were reported from eclogites in the Huwan mélange in the QTHD orogen (350–430 Ma, Figures 2 and 4f [Xu et al., 2000]) and from the north Qilian belt (400 Ma, Figure 4e [Song et al., 2006]). As the central southern SGC was mainly sourced from the northeasterly QTHD belt, we suggest that the intermediate to high Si content micas in the southern SGC were derived from the Huwan mélange. Furthermore, our results indicate that detritus of the Hong'an–Dabie HP–UHP rocks was neither deposited in the Middle and Late Triassic SGC basin nor in the uppermost Triassic strata of the Sichuan basin. That conforms to recent provenance studies, suggesting that the HP–UHP detritus arrived during the Jurassic in the foreland basins to the south and east [Grimmer et al., 2003] and to the north of the Dabie Shan (Hefei basin [e.g., Li et al., 2005]). These provenance studies imply that the vast amounts of Triassic sediments deposited in the SGC basin were derived from the Paleozoic QTHD orogen and the orogenic belts farther west (Qilian, Qaidam, east Kunlun). During the Late Triassic and with progressing collision of the south China and the north China blocks, the southern part of the QTHD and the roof of the HP–UHP rocks, representing south China block, were increasingly included into erosional denudation. The inference gained from the available provenance studies (little erosional denudation of the Hong'an–Dabie Shan in the Triassic but a substantial one in the Jurassic) support structural and geochronologic models that attribute Triassic exhumation of the HP–UHP rocks of the Hong'an–Dabie Shan to tectonic rather erosional denudation [e.g., Hacker et al., 2004; Ratschbacher et al., 2006].

6. Conclusion

[37] The combined interpretation of detrital zircon U/Pb and white mica 40Ar/39Ar ages allows the differentiation between cratonal blocks and orogenic belts that surround the SGC as potential source areas. Figure 5 illustrates our model of source area evolution for particular SGC deposystems during the Middle to Late Triassic. The northern SGC was a distinct deposystem, fed mainly from the north Qaidam and Qilian metamorphic belts, the east Kunlun arc, the western Qinling, and the north China block located in the north. There is no evidence that the Norian samples of the northern SGC received detritus from the Hong'an–Dabie section of the Triassic QTHD orogen. The Late Permian–Early Triassic Kunlun arc and the Early Triassic granitoids of west Qinling are major source areas, as indicated by a significant detrital zircon population with these ages but the lack of coeval white micas.

[38] The Ladinian through Carnian samples of the southern SGC indicate a supply from the Paleozoic belt of the QTHD orogen and a southward and eastward shift of the drainage area into the Triassic metamorphic belt of Qinling–Tongbai-Hong'an-Dabie; the north and south China blocks also contributed. Samples located farther to the west in the southern SGC indicate the same supply from the QTHD belt but a mix with detritus derived from the east Kunlun arc to the north. Norian samples from the southern part of the southern SGC indicate a strong input from the entire QTHD belt and the south China block, indicating long and narrow drainage channels along the northern and western margin of the south China block. The paucity of white mica in these sediments support an additional supply from the Emeishan magmatic province and/or the Yidun arc that probably did not yield white mica. The white mica 40Ar/39Ar ages of the latest Triassic (Rheatian) sample from the northwestern margin of the Sichuan basin are most consistent with the mica ages reported from the QTHD orogen. However, the Si content of the white micas gives no evidence that the Triassic HP-UHP rocks of the Hong'an–Dabie Shan were already exposed at the surface in the Late Triassic; this conforms to recent provenance studies, suggesting that the HP-UHP detritus arrived in the foreland basins to the south, east, and north of the Dabie Shan in the Jurassic. The inference that erosional denudation of the Triassic Hong'an-Dabie orogen became pronounced only in the Jurassic also supports structural and geochronologic models that attribute Triassic exhumation of the HP-UHP rocks of the Hong'an–Dabie Shan to tectonic rather erosional denudation.

[39] The samples of this study are located closer to the eastern margin of the SGC than most of the samples of the detrital zircon study of Weislogel et al. [2006]. They indicate a more significant component of south China block derived material in the eastern region of the SGC than compared with its interior regions; this is particularly so for the southern SGC. This is consistent with the results from samples located along the eastern margin of the SGC analyzed by Bruguier et al. [1997], who interpreted the detrital zircon grains to be derived from the south China block. Recent investigations of the provenance of the SGC detritus [Gu, 1994; Zhou and Graham, 1996; Bruguier et al., 1997; Weislogel et al., 2006], including this study, demonstrate the variability of provenance across the complex and support the interpretation of the SGC as an amalgamation of multiple deposystems.


[40] Associate Editor Eric Kirby and two anonymous reviewers provided many pertinent comments. DFG grants Ra 442/19 and 25 supported E.E. and funded the 40Ar/39Ar and Si-in-white mica work. Detrital zircon geochronology was supported by the NSF grant EAR-0408752, the Stanford-China Geosciences Industrial Affiliates Program, Stanford University McGee Fund, and Geological Society of America Student Research Grant 7306-02. We thank Brad Ito, Bettina Weigand, and Frank Mazdab of the Stanford University Micro-Analytical Center for assistance. Guidance in the field by Hengshu Yang of the northwest Sichuan Geological Bureau is greatly appreciated.