Geochronological and geochemical constraints on

TECTONICS, VOL. 22, NO. 4, 1037, doi:10.1029/2002TC001466, 2003

Geochronological and geochemical constraints on Mesozoic suturing in east central Tibet Franc¸oise Roger,1,2 Nicolas Arnaud,3,2 Stuart Gilder,4 Paul Tapponnier,1 Marc Jolivet,5 Maurice Brunel,5 Jacques Malavieille,5 Zhiqin Xu,6 and Jingsui Yang6 Received 21 October 2002; revised 17 March 2003; accepted 17 April 2003; published 15 August 2003.

[1] This paper reports isotopic, major and minor element geochemistry of igneous and metamorphic rocks from the Kokoxili and Yushu regions of central and eastern Tibet. The first region lies along the Kunlun suture, which separates the Bayan Har-Songpan Ganze (Songpan) terrane from the Tarim and Qaidam blocks. Two Kokoxili granitoids yield U-Pb zircon dates of 217 ± 10 and 207 ± 3 Ma (Late Triassic), which represent the time of emplacement, and Rb-Sr isochron dates of 195 ± 3 and 190 ± 3 Ma (Early Jurassic), which are interpreted as cooling ages. The geochemical signatures of these granitoids suggest that they are related to subduction continuing into the Late Triassic. In the Yushu area, three samples help constrain the age of the Jinsha suture, which separates the Songpan terranes from the Qiangtang blocks. A leucocratic granite and an orthogneiss in the suture zone yield U-Pb zircon dates of 206 ± 7 and 204 ± 1 Ma, respectively, and a paragneiss south of it, a U-Pb monazite date of 244 ± 4 Ma. The existence of coeval magmatism in both the Jinsha and Kunlun sutures suggests that the two subduction zones were simultaneously active. Combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions, we propose that the Kunlun and Qinling block boundaries, which were distinct in the Permian, subsequently formed a continuous, Late Triassic, northward subducting plate margin. Our data suggest that the Jinsha suture correlates with the Benzilan and Nan-Uttaradit sutures, which together belong to a major INDEX TERMS: 8102 Late Triassic subduction zone. Tectonophysics: Continental contractional orogenic belts; 1035 Geochemistry: Geochronology; 1040 Geochemistry: Isotopic composition/chemistry; 8110 Tectonophysics: Continental 1 Laboratoire de Ge´ochronologie et Tectonique, UMR 7578, Univ. Paris 7-IPG Paris, Paris, France. 2 Also at Laboratoire Dynamique de la Lithosphe`re, UMR 5573, Universite´ Montpellier II, Montpellier, France. 3 Laboratoire ‘‘Magmas et Volcans,’’ UMR 6524, Universite´ Blaise Pascal, Clermont Ferrand, France. 4 Laboratoire de Pale´omagne´tisme et Ge´odynamique, UMR 7577, Universite´ Paris 7-IPG Paris, Paris, France. 5 Laboratoire Dynamique de la Lithosphe`re, UMR 5573, Universite´ Montpellier II, Montpellier, France. 6 Ministry of Lands and Resources, Beijing, China.

Copyright 2003 by the American Geophysical Union. 0278-7407/03/2002TC001466$12.00

tectonics—general (0905); 8157 Tectonophysics: Plate motions— past (3040); KEYWORDS: geochronology, geochemistry, granitoid, Jinsha suture, Kunlun suture, Late Triassic subduction zone. Citation: Roger, F., N. Arnaud, S. Gilder, P. Tapponnier, M. Jolivet, M. Brunel, J. Malavieille, Z. Xu, and J. Yang, Geochronological and geochemical constraints on Mesozoic suturing in east central Tibet, Tectonics, 22(4), 1037, doi:10.1029/ 2002TC001466, 2003.

1. Introduction [2] The great elevation of the Tibetan Plateau results mostly from the Tertiary India-Asia collision, but its geology reveals a mosaic of blocks welded during Paleozoic and Mesozoic accretion [Sengo¨r, 1984]. Figure 1 shows the simplified tectonic boundaries between major Asian tectonic blocks as generally accepted by most workers [Sengo¨r, 1984; Matte et al., 1996; Leloup et al., 1995]. North and south China, Tarim, India and Kontum constitute the most ancient cratons, with suites of tectonostratigraphic terranes lying in their midst. Though the boundaries and plate tectonic histories of some of these terranes are still uncertain, there is little doubt that greater Asia grew through Phanerozoic welding of such blocks and terranes south of the Siberian craton. [3] The Kunlun Mountain Range forms the topographic and tectonic boundary that separates the Tibetan terranes from the Qaidam and Tarim blocks to the north. It contains the traces of at least two suturing events [Harris et al., 1988a, 1988b; Matte et al., 1996; Yang et al., 1996]. South of the Kunlun Range lies a geologically complex region referred to as the Bayan Har terrane (in the west) and the SongpanGanze terrane (in the east). It is likely that they form a single terrane as their Mesozoic stratigraphies are alike and no known suture separates them. We refer to both as the Songpan terrane. Farther south the Jinsha suture separates Songpan from the Qiangtang block (Figure 1). We present here new geological, geochemical and geochronological data from igneous and metamorphic rocks collected along the boundaries of the Kunlun-Songpan and Songpan-Qiangtang blocks. We compare these data with other extant evidence and use them to discuss and improve the picture of the EarlyMiddle Mesozoic paleogeographic evolution of Tibet.

2. Geological Framework 2.1. Songpan Terrane, Kunlun Mountains, and Geology of Jingyu-Kokoxili Region [4] The Songpan terrane is chiefly composed of a uniform sequence of dark slates with brownish sandstone beds

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Figure 1. Simplified tectonic map of central Asia. Permo-Triassic ophiolites in Kunlun Mountains and along Jinsha Suture are: 1- Dagiubu, 2- Heichigou, 3- Buqinshan, 4- Xiadawu, 5- Majixueshan, 6- Anyemaqen.

that form a 2000 km long belt. Some interbedded lavas and local intrusions of granitoids are also found [Pearce and Mei, 1988]. This Triassic flysch sequence is dated mostly with fossils in olistoliths south of Golmud [Yin et al., 1988]. The facies indicate progressively southward deepening. [5] The northern limit of the terrane lies along the Kunlun range, which follows the active sinistral Kunlun fault [Tapponnier and Molnar, 1977; Kidd and Molnar, 1988;

Van der Woerd, 1998; Van der Woerd et al., 2000]. The geology of the range varies along strike. Paleozoic and Mesozoic sedimentary rocks in the range are intruded by magmatic rocks, the latter recording tectonothermal events with ages ranging from 500 Ma to 90 Ma [Academia Sinica, 1976; Geological Publishing House, 1991]. Yang et al. [1996] identified one Early Carboniferous ophiolite belt along the northern margin of the western Kunlun (70–

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Figure 2a. Simplified geological map of Kokoxili area [after Jolivet et al., 2003].

80
E), north of the fault, and two ophiolite belts, along and south of the fault, in the eastern Kunlun: one (Early Carboniferous) in the middle of the range and another (Early Permian to Middle Triassic), 1200 km long south, of it. Most known Permo-Triassic ophiolites (Heichigou, Buqinshan, Xiadawu, Majixueshan and Anyemaqin (Maxin) are found in the eastern Kunlun. Another one (Daguba), lies much farther west. Several other maficultramafic bodies are reported south of the latter, north of Ulugh Muztagh [Geological Publishing House, 1991; Burchfiel et al., 1989; Yang et al., 1996] (Figure 1). In between the eastern ophiolite group and Daguba, in the remote area of Kokoxili, we collected two granitoids to better define the age of subduction and subsequent welding. [6] The Jingyu lake basin lies between two active branches of the Kunlun fault (Figure 2a). Tertiary red

conglomerates and sandstones within the basin are overthrust by Permian limestones in the north and by Songpan Triassic slates and conglomerates in the south. Permian and Triassic rocks east of the lake are intruded by the 6004 mhigh Wei Xue Shan plutonic massif. The plutons consist mostly of granites with abundant microgranular dioritic enclaves, cut by a few tourmaline-rich leucogranitic dikes. We collected an undeformed coarse-grained granite (AT118) composed of quartz, orthoclase, plagioclase (oligoclase) and biotite with mm- to cm-sized feldspar phenocrysts. Accessory minerals include zircon, titanite, apatite and opaques. We also collected an undeformed granodiorite (AT113) in the NW corner of the Jingyu basin. This granodioritic intrusive is surrounded by a hornfels-facies contact metamorphic aureole, and contains dioritic and microdioritic enclaves. Its mineralogical composition includes quartz,

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orthoclase, zoned plagioclase (andesine) and biotite; accessory minerals are zircon, apatite and opaques. 2.2. Qiangtang Block, Jinsha Suture and Geology of Yushu Area [7] South of the Songpan terrane lies the Qiangtang block (Figure 1), whose Late Carboniferous glacio-marine sequences and paleo- fauna and flora attest to a Gondwanian origin [e.g., Chang and Cheng, 1973; Chang et al., 1986; Metcalfe, 1988]. Overlying these cold water deposits in the west is a thick sequence of Late Permian to Jurassic limestones and shales that are locally interbedded with lava flows [Matte et al., 1996]. These units are unconformably overlain by Cretaceous-Paleocene red conglomerates and sandstones, which are in turn unconformably covered by Eocene rhyolites [Norin, 1946; Roger et al., 2000a]. West of the Lhasa-Golmud road blueschist-facies metamorphic rocks within Qiangtang indicate complex Early Mesozoic deformation. Such high pressure, low temperature metamorphic rocks have been successively interpreted to represent pre-Devonian basement [Cheng and Xu, 1986], a Triassic suture zone [Li et al., 1995], a collapsed Early Permian-Late Triassic extensional basin [Deng et al., 1996] and part of a Late Triassic-Early Jurassic core complex [Kapp et al., 2000]. Using Ar-Ar, Li et al. [1995] and Kapp et al. [2000] dated different blueschist samples at 223 ± 4, 222.8 ± 0.4 and 205 ± 0.3 Ma on amphibole and white mica. [8] East of the Lhasa-Golmud road, the limit between the Songpan and Qiangtang terranes is marked by the Jinsha suture, a linear ophiolitic/melange zone with peridotites, radiolarites, gabbro and basalts, interlayered with volcaniclastic sediment and intruded by granitoids. Ophiolite outcrops near the Lhasa-Golmud Geotraverse are rare, and lie NW and ESE of Erdaogou [Pearce and Deng, 1988] (Figure 1). SE of Yushu, on the other hand, the SE-trending, 40 km wide, Jinsha ophiolite belt becomes continuous and is traceable over a length of 700 km [Geological Publishing House, 1991]. It appears to continue into Yunnan as the Benzilan suture [Leloup et al., 1995]. East of Yushu, we collected three samples that help constrain the age of the Jinsha-Benzilan suture. South of the strongly folded Songpan Triassic flysch, the suture is marked here by gabbros and steeply dipping pillow lavas and radiolarites interbedded with thin quartzite and white marble horizons (Figure 2b). Also present are serpentinized, antigorite-bearing peridotites. [9] Amphibole-rich leucocratic plutons intrude these rocks, from which we extracted sample QGS34 15 km east of Yushu near the Yangtze river. This slightly deformed leucocratic granite intrusion contains quartz, plagioclase, microcline, biotite, green hornblende, muscovite and epidote, with allanite and zircon as accessory minerals. The southern edge of the suture belt is strongly sheared by the Xianshui He strike slip fault. This fault has probably had significant motion in the Tertiary, locally complicating the juxtaposition of typical subduction-related rocks and ultramafic associations marking the suture. Slices of flazer gabbros, orthogneisses and schists are juxtaposed in the fault zone. The orthogneiss sample collected 100 km ESE of Yushu (QGS38) (Figure 2b) contains quartz, plagioclase,

green hornblende, biotite and minor microcline and epidote (zoisite). Zircon is the main accessory phase. South of Yushu, Triassic andesites and agglomerates lie between the southern and northern branches of the Xianshui He Fault. Yet farther south, within the Qiangtang block, PermoTriassic limestones overlie the basalts. This sedimentary sequence is unconformable on greenschists and crystalline basement that includes foliated paragneisses, intruded by mildly deformed granites. QGS37 was taken from a foliated basement paragneiss. It contains quartz, chloritized biotite, muscovite, cordierite, sericitised plagioclase and some K-feldspar. Zircon and monazite are the main accessory minerals. The occurrence of cordierite implies that peak metamorphic T
C and P were greater than 500
C and less than 5 kbar, respectively [e.g., Winkler, 1979]. [10] A synthetic cross section between the Kunlun range and the Jinsha suture is shown in Figure 3. The cross section broadly runs N-S (Figure 1) and shows the major units and sutures. Note that because the Jingyu area shown on Figure 2a is small compared to the Jinsha suture zone, the former setting corresponds roughly to the northern part of the cross section.

3. Geochronology 3.1. Results [11] Mineral separates were obtained by standard rock crushing, density and magnetic separation techniques with final selection by hand-picking under binocular lenses. The U-Pb and Rb-Sr analytical methods are those outlined in detail by Roger et al. [2000b]. 3.1.1. Kunlun Range 3.1.1.1. U-Pb Analyses [12] The zircons selected for U-Pb analyses were euhedral, unbroken, inclusion- and crack-free grains, as transparent as possible. For AT113 north of Jingyu lake, six zircon fractions were analyzed, with weights ranging from 0.029 to 0.112 mg (Table 1). All the fractions plot near concordia, with 206Pb/238U (R8) and 207Pb/235U (R5) ages ranging between 200 and 208 Ma (Table 1 and Figure 4a). These data define a regression age of 207 ± 3 Ma (MSWD = 5.4). Two fractions (n
1 and n
2) lie on concordia, with R8 ages of 206 ± 1 Ma and 205 ± 1 Ma (Table 1 and Figure 4a). For the Wei Xue Shan granite (AT118), six zircon fractions, ranging in weight from 0.027 to 0.102 mg, were analyzed (Table 1). The data define a normal discordia, with an upper intercept at 217 ± 12 Ma (MSWD = 3.3) (Figure 4b). One titanite fraction (n
13), with yellowish sub-euhedral grains, yields a concordant date with R8 = 212 ± 1 Ma and R5 = 213 ± 4 Ma (Table 1 and Figure 4b). For accessory minerals with low radiogenic lead contents (apatite, epidote, titanite), the R8 apparent age is generally considered more reliable than R5 because R8 is less sensitive to common lead contamination [Mattinson, 1978]. A linear regression of the six zircon fractions and single titanite fraction yields 217 ± 10 Ma (MSWD = 3.2). 3.1.1.2. Rb-Sr Analyses [13] Table 2 lists the Rb-Sr analytical results for the Wei Xue Shan (AT118) and Jingyu (AT113) granites. For

ROGER ET AL.: CHRONOLOGY OF MESOZOIC SUTURES IN TIBET

Figure 2b. Simplified geological map of Yushu area [Geological Publishing House, 1991].

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Figure 3. Synthetic geological cross section between the Kunlun range and the Jinsha suture. 1) Gneiss rocks (basement of the Qaidam Block). 2) Permian-Triassic sedimentary rocks. 3) Triassic subductionrelated granites. 4) Ophiolites and upper Paleozoic sedimentary rocks of the Kunlun-Anyemaquen suture zone. 5) Basement of the Triassic fold belt. 6) Triassic flyschoids sequences of the Bayan Har accretionary wedge. 7) Mesozoic granites. 8) Neogene continental basins. 9) Ophiolites and sedimentary rocks of the Jinsha-Benzilan suture zone. 10) Basement of the Qiangtang Block. 11) Paleozoic granites. 12) Deformed sedimentary cover of the Qiangtang Block. 13) Tertiary granite. 14) Eocene continental deposits. AT118, whole rock, feldspar, and four different size fractions of biotite together yield a linear regression age of 195 ± 3 Ma and initial 87Sr/86Sr of 0.7069 ± 2 (±2s) (Figure 4d). For AT113, whole rock, feldspar, and five fractions of biotite yield a best-fit regression age of 190 ± 3 Ma with an initial 87Sr/86Sr of 0.70746 ± 8 (±2s) (Figure 4c). 3.1.2. Jinsha Suture 3.1.2.1. U-Pb Analyses [14] Seven zircon fractions and one single zircon, ranging in weight from 0.012 to 0.681 mg, were analyzed from the leucocratic granite closest to Yushu (QGS34) (Table 1). The data define a discordia, with a lower intercept at 206 ± 7 Ma and an upper intercept at 1176 ± 320 Ma (MSWD = 18.2, Figure 5a). The most discordant fraction (n
17) corresponds to a non-abraded single grain with an S-type morphology [Pupin, 1980], while fine zircon needles (n
14, 15 and 20) and a single fraction (n
19) of fine P and G types are subconcordant. Following Silver and Deustsch, [1961], we think that the small grains crystallized during the last magmatic stage and incorporated small amounts of inherited lead. [15] The zircons from the orthogneiss (QGS38) farther east in the suture zone have U and Pb concentrations of 688 – 506 and 23 –17.7 ppm (respectively) and probably exhibit both inheritance and post-crystallization lead loss (Table 1 and Figure 5c). Modest amounts of inherited Pb likely account for the discordance of points 23, 25 and 26. A line fit through these three points intersects concordia at 204 ± 1 Ma and 2265 ± 232 Ma (MSWD = 0.4) and thus suggest a minimum emplacement age of ca 204 ± 1 Ma. Moreover fractions 22 and 2 appear free of inherited components and align along a reference line from zero to 204 Ma, suggestive of recent lead loss superimposed on a similar emplacement age. [16] Both zircons and monazites were extracted from the paragneiss of the Qiangtang basement (QGS37). Six zircon fractions and one single zircon (n
41) were analyzed for U-Pb (Table 1). The data define a discordia, with a lower

intercept at 682 ± 25 Ma and an upper intercept at 2460 ± 74 Ma (MSWD = 110) (Figure 5e). The large scatter in the age distribution (as reflected by the large MSWD) prohibits one from fitting a single linear regression and is suggestive of multiple inheritance implying that the calculated age of 682 ± 5 Ma has no geological significance. Eight monazite fractions, ranging in weight from 0.11 to 0.60 mg, were analyzed (Table 1). Although two monazite grains (28 and 34) plot above the concordia (due to U loss or incomplete dissolution), the majority of the data define a normal discordia with an upper intercept at 244 ± 4 Ma (MSWD = 0.5) (Figure 5f ). 3.1.2.2. Rb-Sr Analyses [17] For the leucocratic granite (QGS34), the whole rock, one feldspar and four different size fractions of biotite yield a age of 196 ± 4 Ma and an initial 87Sr/86Sr ratio of 0.70837 ± 2 (±2s) (Table 2 and Figure 5b). For the orthogneiss (QGS38), the whole rock, one feldspar, and three fractions of biotite yield a best-fit regression age of 176 ± 3 Ma with an initial 87Sr/86Sr ratio of 0.70794 ± 4 (±2s) (Table 2 and Figure 5d). 3.2. Interpretation of Age Data [18] Because U/Pb closure temperatures in zircon are >800
C [Pidgeon and Aftalion, 1978; Dahl, 1997; Lee et al., 1997], the 217 ± 10 and 207 ± 3 Ma ages in Kokoxili likely represent the time of granite emplacement. Taking 325 ± 25
C as the closure temperature for the Rb-Sr system in biotite [Purdy and Ja¨ger, 1976; Harrison et al., 1978], we interpret the Rb-Sr internal isochron dates (195 ± 3 and 190 ± 3 Ma) to clock the cooling of the Jingyu and Wei Xue Shan granites. Cooling curves can be used to calculate exhumation rates using the mineral-pair method if the closure temperature of the minerals is known and if the local geothermal gradient during cooling can be estimated. This allows one to translate temperature-time histories into depth-time histories. Such apparent exhumation rates can

Zirc Zirc Zirc Zirc Zirc Zirc Zirc Zirc

Zirc Zirc Zirc Zirc Zirc

Mon Nab medium Trp yellow Mon Nab medium Trp yellow Mon Nab medium Trp yellow Mon Ab small Trp yellow Mon Ab small Trp yellow Mon Ab small Trp yellow Mon Ab medium Trp yellow Mon Ab medium Trp yellow Zirc Ab medium Trp pink lpr Zirc Ab medium Trp pink spr Zirc Ab small Trp pink ndl Zirc Ab small Trp pink spr

14 15 16 17 18 19 20 21

22 23 24 25 26

27 28 29 30 31 32 33 34 35 36 37 38

Nab medium Trp clrls Ab small Trp clrls ndl Ab medium Trp clrls lpr Ab small Trp clrls ndl Ab medium S-type Trp

Ab small Trp clrls ndl Ab small Trp clrls ndl Ab medium Trp clrls spr single Ab S-type Trp Ab medium Trp clrls lpr Ab small P-G-type spr Ab small Trp clrls spr Ab small Trp clrls lpr

Zirc Ab small Trp clrls ndl Zirc Ab medium Trp clrls ndl Zirc Ab medium milky Zirc Ab medium Trp clrls ndl Zirc Ab medium Trp clrls Zirc Ab large Trp clrls Ti Ab Xeno Trp yellow

small Trp clrls ndl small Trp clrls ndl medium Trp clrls medium Trp clrls ndl large Trp clrls ndl small Trp clrls

7 8 9 10 11 12 13

Ab Ab Ab Ab Ab Ab

Zirc Zirc Zirc Zirc Zirc Zirc

Sample and Description

1 2 3 4 5 6

Number

0.0524 0.0607 0.0380 0.0285 0.0473 0.0410 0.0508 0.0116 0.0274 0.0305 0.0075 0.0555

0.0362 0.0080 0.0756 0.0526 0.0539

0.0781 0.0742 0.0549 0.0165 0.0128 0.0403 0.0223 0.6810

0.1019 0.0920 0.1707 0.0477 0.0271 0.0725 0.3058

0.1122 0.0539 0.0673 0.0636 0.0376 0.0289

Weight, mg

7384 4522 8612 5420 5334 6123 5384 5073 492 532 442 697

642 506 558 688 594

515 492 654 736 289 163 314 511

725 1058 2249 1115 1195 1777 56

806 1144 709 920 1330 954

U

705.2 681.5 683.4 741.1 703.0 733.0 718.1 671.0 70.7 84.2 53.4 92.3

201 16.8 17.7 23.0 20.4

18.0 17.5 24.7 29.5 10.7 6.0 11.0 18.3

20.9 29.0 46.0 32.9 33.0 38.5 2.01

25.6 36.1 22.2 29.5 42.4 29.6

Pb rad. 206/204 (raw) %err

207*/235

%err

Suture: Yushu Area; QGS 38: Orthogneiss 1206 0.03086 0.6 0.2155 169 0.03260 0.5 0.2311 2959 0.03051 0.5 0.2126 1814 0.03232 0.5 0.2255 4736 0.03347 0.5 0.2494 Suture: Yushu Area; QGS 37: Paragneiss 2950 0.02773 1.0 0.1950 3232 0.04487 1.0 0.3161 3009 0.02446 1.4 0.1713 3038 0.03881 2.0 0.2739 5380 0.04054 1.7 0.2860 3260 0.03712 1.0 0.2621 2567 0.03896 1.0 0.2740 4257 0.04132 0.6 0.2915 166 0.13881 0.7 1.5669 3276 0.15008 0.4 1.9098 627 0.11849 0.5 1.1550 7073 0.12907 0.4 1.4160

Jinsha 3.2 3.2 3.0 2.6 1.5 3.2 4.2 0.1 28.8 1.2 4.1 0.6

1.1 1.1 1.5 2.2 1.7 1.8 1.1 0.7 0.8 0.4 0.6 0.5

0.8 0.6 0.5 0.6 0.5

Area; QGS 34: Leucocratic Granite 0.03275 0.7 0.2288 0.8 0.03358 0.7 0.2342 0.7 0.03585 0.7 0.2714 1.1 0.03917 0.5 0.2990 0.5 0.03355 0.8 0.2405 0.9 0.03354 0.5 0.2357 0.7 0.03333 0.6 0.2351 0.6 0.03394 0.6 0.2438 0.6

Jinsha Suture: Yushu 0.8 1254 0.3 2730 0.1 496 2.1 682 2.0 240 0.9 305 2.4 385 0.1 4184 Jinsha 0.8 5.7 0.2 0.6 0.1

Area; AT 118: Wei Xue Shan Granite 0.02991 0.5 0.2088 0.5 0.02845 0.5 0.1982 0.5 0.02104 0.8 0.1482 0.9 0.02987 0.5 0.2091 0.5 0.02702 0.6 0.1890 0.8 0.02233 0.9 0.1564 0.9 0.03337 0.5 0.2332 2.1

Area; AT 113: Jingyu Granodiorite 0.03252 0.5 0.2253 0.5 0.03238 0.5 0.2247 0.5 0.03204 0.6 0.2251 0.7 0.03260 0.7 0.2276 0.7 0.03192 0.6 0.2234 0.7 0.03162 0.5 0.2216 0.5

206*/238

Atomic Ratiosb,c (±2s)

Kokoxili 1635 1943 893 661 426 503 60

Kunlun Belt: 0.8 0.9 3.3 3.0 4.6 5.0 2.57

Kunlun Belt: Kokoxili 0.2 4483 0.7 2762 0.2 1658 2.9 648 1.1 1837 1.6 1014

Pb not rad

Concentrations, ppm

Table 1. Analytical Results of U-Pb Age Determinationsa

0.05100 0.05110 0.05079 0.05115 0.05117 0.05122 0.05101 0.05117 0.08187 0.09229 0.07070 0.07957

0.05066 0.05141 0.05054 0.05061 0.05405

0.05068 0.05058 0.05491 0.05535 0.05199 0.05098 0.05116 0.05209

0.05062 0.05051 0.05110 0.05077 0.05074 0.05081 0.05068

0.05025 0.05034 0.05095 0.05064 0.05075 0.05083

207*/206*

0.4 0.4 0.5 0.7 0.4 0.5 0.4 0.4 0.8 0.5 0.5 0.5

0.6 0.5 0.5 0.5 0.5

0.5 0.5 0.8 0.5 0.5 0.7 0.5 0.5

0.5 0.5 0.5 0.5 0.7 0.5 1.0

0.5 0.5 0.5 0.5 0.5 0.5

%err

176.3 282.9 155.8 245.6 256.2 234.9 246.4 261.0 838 901 722 783

195.9 206.8 193.8 205.0 212.2

207.8 212.9 227.1 247.7 212.7 212.7 211.3 215.1

190.0 180.9 134.2 189.7 171.7 142.3 211.6

206.3 205.4 203.3 206.8 202.6 200.7

206*/238

180.9 278.9 160.5 245.8 255.4 236.4 245.9 259.8 957 1084 780 896

198.2 211.1 195.8 206.5 226.1

209.3 213.7 243.8 265.6 218.8 214.9 214.4 221.5

192.5 183.6 140.4 192.8 175.8 147.6 212.8

206.3 205.8 206.1 208.2 204.7 203.2

207*/235

240.6 245.2 231.3 247.7 248.6 250.6 241.2 248.7 1242 1473 949 1186

225.4 259.1 219.9 223.1 373.0

226.2 221.7 408.5 426.5 285.0 239.8 248.0 289.5

223.7 218.5 245.5 230.2 229.1 232.4 226.5

206.4 210.5 238.6 224.3 229.5 232.9

207*/206*

Apparent Ages, Ma

0.94 0.95 0.94 0.95 0.97 0.95 0.93 0.86 0.64 0.92 0.60 0.98

0.67 0.57 0.96 0.84 0.94

0.82 0.98 0.65 0.50 0.89 0.50 0.92 0.70

0.86 0.91 0.93 0.82 0.53 0.97 0.60

0.98 0.88 0.80 0.81 0.97 0.82

Rho

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a Individual analyses were performed on euhedral, unbroken, crack-free from the best quality grains of the population. Abbreviations are as follows: Zirc, zircon; Mon, monazite; Ti, titanite; Ab, abraded; Nab, not abraded; xeno, xenomorph; Trp, transparent; clrls, colorless; lpr, long prism; spr, short prism; ndl, needle. Definition are as follows: small: grains 50 – 100 mm in length; medium: 100 – 150 mm in length; large: >150 mm in length. b Ratio corrected for mass discrimination (±0.1%/amu for Pb and U), spike contribution, 10 pg (zircon) or 50 pg (monazite, titanite) for Pb blank, 1 pg for U blank and initial common Pb [Stacey and Kramers, 1975]. c The asterisk or rad.: radiogenic.

0.95 0.88 0.91 1482 1033 2361 1044 814 2168 848 736 1971 0.5 0.3 0.5 0.09270 0.07368 0.15129 0.5 0.5 0.6 1.7969 1.2282 7.4591 0.5 0.5 0.5 0.14058 0.12089 0.35758 1633 971 1244 2.3 3.9 4.7 65.6 65.0 110.1 456 544 308 Zirc Nab medium S-type pink Zirc Ab small Trp pink lrp Zirc single Ab Trp pink spr 39 40 41

0.0558 0.0207 0.0103

207*/206* 207*/235 206*/238 %err 207*/235 Pb not rad Pb rad. U Sample and Description Number

Table 1. (continued)

Weight, mg

Concentrations, ppm

206/204 (raw)

206*/238

%err

Atomic Ratiosb,c (±2s)

207*/206*

%err

Apparent Ages, Ma

Rho

ROGER ET AL.: CHRONOLOGY OF MESOZOIC SUTURES IN TIBET

also incorporate fission-track dates, which are preserved at temperatures of 250 ± 50
C for zircon [Hurford, 1986]. For apatite fission-tracks, the annealing and closure temperatures are 60
C to 120
C and 110 ± 10
C, respectively [Green et al., 1989; Corrigan, 1991]. The Fission-track analyses used in Figure 6 are these obtained by Jolivet et al. [2003]. The central zircon fission-track dates of AT113 and AT118 are 96 ± 4 Ma and 161 ± 14 Ma (±2s), respectively. The central apatite fission-track dates of AT113 and AT118 are 19.3 ± 1.8 Ma and 17.3 ± 1.8 Ma respectively, and their mean track lengths are 13.56 ± 0.23 mm (AT113) and 12.45 ± 0.31 mm (AT118), which suggests that the samples underwent rapid late cooling and exhumation [Jolivet et al., 2003]. By using an assumed mean geothermal gradient of 25
C/km [Ulmishek, 1984], the calculated thermal histories for both samples are similar (Figure 6). During the Late Triassic and Early Jurassic, after intrusion and crystallization, cooling was relatively fast (15 – 30
C/Ma) implying a corresponding fairly fast exhumation rate of 0.6 to 1.2 mm/yr. This was followed by a long period (Late Jurassic to Early Cenozoic) of very slow cooling (0 – 3
C/Ma) which translates into a very slow mean denudation rate 0.1 mm/yr. This implies in turn that cooling resulted mostly from erosion during a time of tectonic quiescence. The mean cooling rate appears to accelerate again, to 6 – 8
C/Ma in the Tertiary (after 20 Ma) (denudation rate = 0.2– 0.3 mm/yr.) [Jolivet et al., 2003], probably as a result of Tertiary faulting. [19] Most the geochronological data reported for the Kunlun range comes from the Royal Academy of Sciences and Sino-French geotraverses [Harris et al., 1988b; Matte et al., 1996; Mock et al., 1999]. In the westernmost Kunlun, along the southern margin of the range, Matte et al. [1996] reported 40Ar/39Ar, Rb/Sr and U/Pb granitoid ages spanning the period between 180 Ma and 215 Ma. Granitic plutons in the Golmud area (Figure 1) have biotite Rb/Sr ages from 190 Ma to 200 Ma and 240 Ma to 260 Ma and a U/Pb age of 240 Ma [Harris et al., 1988b]. Younger granites are exposed in the south and older ones in the north. Harris et al. [1988b] interpreted that the latter (240 Ma to 260 Ma) intruded the continental margin during active subduction, and that the former (190 Ma to 200 Ma) were emplaced in a post-collisional setting. In the Golmud region, most of the evidence points to a Paleozoic age for the basement [Coward et al., 1988] but isotopic studies on the Kunlun granites suggest reworking of more ancient Mid-Proterozoic crust [Harris et al., 1988a; Arnaud et al., 2003]. [20] In the Jinsha suture, we take the U/Pb lower intercept age of 206 ± 7 Ma obtained for the leucocratic granite sample (QGS34) to be the best estimate for the time of emplacement. The inherited Pb components in this granite suggest it was derived from a Proterozoic protolith. For the mildly deformed orthogneiss (QGS38) the 204 ± 1 Ma U/Pb age likely represents the time of metamorphism. The protolith of this orthogneiss was probably a 2.3 ± 0.2 Ga continental crust. For the Qiangtang paragneiss (QGS37), some monazite grains lie on concordia whereas the zircon grains plot within a polygon (Figures 5e and 5f ) indicating

ROGER ET AL.: CHRONOLOGY OF MESOZOIC SUTURES IN TIBET

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Figure 4. (a and b) U-Pb concordia diagrams and (c and d) Rb-Sr isochron diagrams of the Wei Xue Shan granite (AT118) and Jingyu granite (AT113) from Kunlun Mountains. W.R.: whole rock; Bt: biotite; Fd: feldspar.

that they contain a complex mix of several inherited Pb components. Thus the age defined by the monazites (244 ± 4 Ma) probably best represents the time of metamorphism. The Rb/Sr dates of 196 ± 4 Ma and 176 ± 3 Ma are interpreted as cooling ages for the leucocratic granite and orthogneiss. The isotopic data from Yushu thus suggest a period of metamorphism in the Early Triassic, followed by intrusion and relatively fast cooling (15 – 50
C/Ma) during the Early Jurassic as in the Kunlun. Such fast cooling implies that plutonism occurred in the upper and cooler levels of the crust. [21] The Litang-Batang arc, east of Yushu also contains an ophiolitic and melange series of mafic and ultramafic rocks, radiolarites, flysch and calc-alkaline granitoids. Pre-

Norian (early Late Triassic) andesitic volcanism preceded the development of a carbonate platform. This sequence has long been interpreted as reflecting a subduction/collision environment of Late Triassic/Early Jurassic age [Sengo¨r, 1984; Pearce and Mei, 1988; Harris et al., 1988a, 1988b]. The age of the Qiangtang basement is poorly known. The oldest exposed rocks identified during the 1985 Tibet geotraverse are Permo-Triassic clastic sediments interbedded with coal, limestone, and basaltic and silicic volcanics [Coward et al., 1988]. Metamorphic rocks in Qiangtang however have been interpreted to represent pre-Devonian basement [Cheng and Xu, 1986]; see also Figure 2b. U-Pb ion-microprobe ages (419 to 556 Ma) in a garnet-amphibole gneiss near Gangma Co around 34
N, 84
15’E [Kapp et al.,

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Table 2. Rb-Sr Analytical Resultsa Sample

Rb, ppm

Sr, ppm

87

Rb/86Sr

87

Sr/86Sr (±2s)

Kokoxili Area—AT 118: Wei Xue Shan Granite Feldspar 198 156 3.73 0.71702 ± 2 Whole rock 139 61 4.64 0.72015 ± 2 Biotite 1 (400/300 mm) 55 5.2 31.2 0.79887 ± 1 Biotite 2 (600/400 mm) 512 4.8 337 1.61333 ± 17 Biotite 3 (800/600 mm) 534 2.7 683 2.49128 ± 6 Biotite 4 (>800 mm) 455 2.9 529 2.18389 ± 19 Kokoxili Area—AT 113: Jingyu Feldspar 82 192 Whole rock 108 96 Biotite 1 ( 800 mm) 291 2.4

Granite 1.26 3.29 49 23.6 212 181 384

0.71087 ± 1 0.71618 ± 2 0.83420 ± 3 0.77832 ± 8 1.25557 ± 6 1.20432 ± 22 1.69277 ± 16

Yushu Area—QGS 34: Leucocratic Granite Feldspar 33.9 530 0.187 Whole rock 35.4 283 0.336 Biotite 1 (400/300 mm) 440 11.9 111 Biotite 2 (600/400 mm) 400 13.5 88.8 Biotite 3 (800/600 mm) 331 50.4 19.3 Biotite 4(> 800 mm) 406 14.1 86.4

0.70880 0.70970 1.00956 0.98241 0.74993 0.95231

± ± ± ± ± ±

1 3 4 4 3 7

Yushu Area—QGS 38: Orthogneiss Feldspar 56.4 373 0.443 Whole rock 156 450 1.014 Biotite 1 (400/300 mm) 295 17.3 50.5 Biotite 2 (600/400 mm) 323 16.1 59.4 Biotite 3 (800/600 mm) 332 33.4 30.3

0.70922 0.70967 0.82895 0.85656 0.78752

± ± ± ± ±

2 2 4 8 2

a The 87Sr/86Sr is normalized to 86Sr/88Sr = 0.1194. The maximun error for 87Rb/86Sr is ±3%, and for 87Sr/86Sr, it is given in the table, on the last digit of the values measured.

2000] suggest this gneiss may belong to an overthrust sliver of the Pan-African basement of the Qiangtang block.

4. Geochemistry [22] Whole rock samples were powdered in a tungsten carbide crusher. Powders were then dissolved in LiBO3 and HNO3. Major and trace elements were determined by inductively coupled plasma emission spectrometry (ICP) at the Nancy Centre de Recherches Pe´trographiques et Ge´ochimiques. Uncertainties based on repeat analyses of international standards are at most 5% for major elements and 15% for trace elements. Nd data were obtained using techniques described by Nakamura [1974]. Rare earth elements (REE) were extracted on cation exchange resin after dissolution. Nd was purified from Ba and other REE using Teflon powder coated with HDEHP [Richard et al., 1976]. The 143Nd/144Nd ratios are expressed here in eNd notation [DePaolo and Wasserburg, 1976]. 4.1. Jingyu and Calc-Alkaline Granitoids of the Kunlun Range [23] The major element geochemistry of both AT113 and AT118 in the central Kunlun falls within the calc-alkalic range (Figure 7a and Table 3). It is similar to that of Andean

andesites [Brown, 1982] and of the Gangdise Belt granites [Harris et al., 1988a; Debon et al., 1986]. On the Rb versus (Nb + Y) plot of Figure 7b is a useful indicator of tectonic setting for granitic rocks [Pearce et al., 1984]; both samples plot well within the volcanic arc/post-collision field (Figure 7b) and have high contents of: Rb (160 and 199 ppm), Sr (157 and 128 ppm), Ba (417 and 473 ppm), Zr (196 and 204 ppm), Hf (5.2 and 5.06 ppm), Th (20.9 and 22.2 ppm) and REE (165 to 152 ppm) (Table 3). Both have similar REE patterns with slightly negative Eu anomalies (around 0.7), typical of the upper crust (Figure 8a). REE abundances are about 100 times greater than chondrite values for La and 10 times greater for Lu. All chondritenormalized REE patterns are enriched in LREE/HREE (LaN/LuN = 10.7 and 7.7), possess moderate amounts of LREE fractionation (LaN/SmN = 3.2 and 2.9), and are weakly enriched in HREE (GdN/LuN = 2.1 and 1.7) (Figure 8a and Table 3). Such patterns suggest calc-alkaline affinity and have REE profiles close to those of the Gangdise Belt granodiorites [Debon et al., 1986]. Trace element spidergrams (Figure 8b) indicate that the Wei Xue Shan and Jingyu granites are identical to active continental margin granites with comparable SiO2 contents. These granites differ notably from crustal melts derived from collisionrelated anatexis such as the High Himalayan leucogranites (Figure 8b) in that they have much higher Th and LREE contents, and are less depleted in HFS elements (HF, REE, Y). Negative Nb anomalies are often characteristic of the continental crust, implying that it was involved at some stage in the magmatic process (Figure 8b). The Th contents in both AT113 and AT118 are typical of those observed in magmatic arcs (20 ppm) [Brown et al., 1984]. Both have initial Sr ratios of 0.706 – 0.707 and their 87Rb/86Sr ratios are relatively high (3 to 5) (Table 4). The eNd(T) and Nd depleted mantle model age (TDM) were determined only for AT113, yielding 6.1 and 1.54 Ga, respectively. The eNd(T) eSr(T) values resemble those of the intermediate crust (Figure 9). [24] Calc-alkaline magmatism is often associated with active subduction. However, the geochemical distinction between mature arc-related magmatism and certain examples of post-collisional magmatism is not clear-cut [Pearce et al., 1984]. In general post-collisional magmatism is characterized by a wide range of crustal compositions and lacks basic dykes, enclaves, or discrete plutons. [Harris et al., 1990]. The fact that many mafic dykes, enclaves, etc. are found within the Kokoxili calc-alkaline plutons we studied suggests they could be related to subduction. [25] Clearly, the geochemistry, isotopic compositions and geochronology of the Kokoxili granitoids are very close to those of the Kunlun batholith, 400 km to the east [Harris et al., 1988a, 1988b] (Figures 1 and 8). Harris et al. [1988a, 1988b] separated the Kunlun granitoids into two geochemically distinct populations based on Eu anomalies and Ce/Y. Both groups lie in the calc-alkaline and calcic realm and have trace element compositions interpreted to reflect a volcanic arc or post-collisional setting. The granitoids appear to have assimilated significant crustal sources with a limited mantle component. These geochemical constraints led Harris et al. [1988a] to conclude that the Kunlun

ROGER ET AL.: CHRONOLOGY OF MESOZOIC SUTURES IN TIBET

Figure 5. (a and b) U-Pb concordia diagrams and Rb-Sr isochron diagrams for leucocratic granite QGS34, (c and d) orthogneiss, and (e and f ) paragneiss QGS37 from Yushu area. W.R.: whole rock; Bt: biotite; Fd: feldspar.

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Figure 6. Cooling histories of Kunlun granites from multisystem geochronology (white boxes: AT113; gray boxes: AT118). Fission track data from Jolivet et al. [2003]. intrusions were derived from anatexis of a garnet-bearing source at intermediate crustal depths above a subduction zone. This interpretation would also account best with the geochemistry, ages and cooling of the Kokoxili granites. 4.2. Granitoids of the Yushu Area and Jinsha Suture [26] The granite and orthogneiss from the Jinsha suture near Yushu (QGS34 and QGS38) have calcic affinities (Figure 7a) and plot in the volcanic arc/post collisional field of Figure 7b, both of which are indicative of island arc magmas with little or no upper crustal contamination [Harris et al., 1988a]. The Yushu rocks have low Rb/Zr versus SiO2, which indicates either a source with low crustal input or a crustal source of intermediate composition such as a tonalite or amphibolite. Low 87Rb/86Sr and initial 87 Sr/86Sr of 0.707 – 0.708 are typical of intermediate rocks, whereas eNd(T) of 7.4 to 8.4 indicate more important crustal contamination (Table 4 and Figure 9). The REE patterns of the Yushu granites are slightly different from the Kunlun granitoids (Table 3 and Figure 8c). The former have no Eu anomalies and are less enriched in HREE. QGS38 has an REE pattern like that of the average Archean lower crust [Weaver and Tarney, 1980] (Figure 8c) which accords well with its upper intercept date of 2.3 ± 0.2 Ga. For the leucocratic granite QGS34, a TDM age of 1.1 Ga lies within uncertainty of the upper intercept of 1.2 ± 0.3 Ga (Table 4). These dates are similar to those of the Songpan basement, 829 ± 9 Ma (U/Pb zircon) and TDM around 1.0 Ga [Roger and Calassou, 1997], as well as to the TDM ages obtained for Eocene granites in South Yushu and near the Tanggula pass [Roger et al., 2000a]. [27] The REE pattern of the paragneiss (QGS37) south of the Xianshui He fault in Qiangtang is also similar to that

of the average upper crust [Weaver and Tarney, 1980] (Figure 8c). The eNd(T) and TDM age are 12 and 1.95 Ga. The eNd(T) and eSr(T) indicate a relatively ancient source with a great amount of crustal involvement (Table 4 and Figure 9). The TDM age of 2 Ga is of the same magnitude as the U-Pb upper intercept of 2460 ± 74 Ma.

5. Discussion [28] We now discuss the paleogeodynamic implications of the geochemical and geochronological data presented for the Late Permian-Jurassic time window. Although limited, the new data we present helps improve the Mesozoic paleogeographic picture of Tibet. In particular, it is clear that the segments of the north Tibetan sutures studied here correlate well with others to the east and west. Below we examine such correlations in the light of tectonic reconstructions. 5.1. Subduction Zones on the North and South Sides of the Songpan Terrane Wedge: 260– 210 Ma [29] The geochronologic data confirm that both the Kunlun and Jinsha sutures were simultaneously active subduction zones from 250 to 200 Ma [Harris et al., 1988a; this study]. West of Kokoxili, the Kunlun subduction zone continued north of Ulugh Mustagh, where ophiolitic remnants and metamorphic Triassic flysch have been observed [Burchfiel et al., 1989; Molnar et al., 1987]. Farther west before being offset by the Altyn Tagh fault, it stretched along the Karakax valley where granites with ages between 190 and 211 Ma are found [Matte et al., 1996]. To the east, it continued along Xidatan [Harris et al., 1988a, 1988b] and south of Anyemaqin [Yang et al., 1996]. Thus, in the Late Triassic, a continuous subduction zone extended 2500 km

ROGER ET AL.: CHRONOLOGY OF MESOZOIC SUTURES IN TIBET

Figure 7. (a) Log (CaO/(Na2O+K2O)) versus SiO2 for samples from Kunlun (AT113 and AT118, closed circles) and Yushu (QGS34 and QGS38, closed diamonds) compared with granites from the Gangdise Belt (open circles, data from Debon et al. [1986] and Harris et al. [1988a]) and Kunlun Range (open triangles, data from Harris et al. [1988a]). Dashed lines indicate a calc-alkali field boundaries. Solid lines define field of Andean andesites [Brown, 1982]. (b) Rbversus(Nb+Y) plot for samples from Kunlun (AT113 and AT118, closed circles) and Yushu (QGS34 and QGS38, closed diamond). Field boundaries from Pearce et al. [1984].

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Table 3. Chemical Compositionsa AT 113 SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2 O TiO2 P2O5 LOI Total, % La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Total (La/Lu)N (La/Sm)N (Gd/Lu)N (Eu/Eu*)N Rb Sr Ba Zr Hf Nb Ta Y Th U

AT 118

QGS34

QGS38

QGS37

67.24 15.06 4.14 0.05 1.19 3.22 2.97 4.21 0.53 0.15 1.00 99.86

ICP, % 68.18 74.51 14.83 13.65 3.66 2.53 0.04 0.03 1.06 1.44 2.43 3.24 2.96 3.9 5.02 1.1 0.50 0.26 0.14 0.13 1.05 0.62 99.87 100.41

67.7 14.95 4.27 0.09 1.47 4.17 3.22 1.91 0.39 0.16 1.59 99.92

69.76 13.1 5.59 0.08 2.22 1.31 2.44 2.69 0.7 0.21 1.83 99.99

33.6 70 8.12 29.9 6.25 1.04 5.2 0.724 4.35 0.827 2.08 0.327 2.43 0.325 165.173 10.66 3.19 2.11 0.71 160 157 417 196 5.2 9.43 2.26 23.3 20.9 1.54

ICP-MS, ppm, REE 29.4 73.33 63 129.8 7.51 11.92 29.1 35.69 5.93 3.8 0.902 1.07 4.94 2.24 0.72 0.247 4.28 1.427 0.864 0.239 2.44 0.625 0.419 0.085 2.72 0.661 0.391 0.119 152.616 261.253 7.75 63.55 2.94 11.46 1.67 2.48 0.69 0.73 199 40.7 128 284 473 430 204 202 5.06 4.79 9.56 5.57 2.85 2.1 26.7 6.05 22.2 16.44 3.36 0.79

28.1 48.49 4.86 14.85 2.49 0.75 2.026 0.307 2.053 0.416 1.085 0.194 1.386 0.207 107.214 14 6.7 1.29 0.81 75.99 231 556 109 2.74 5.28 1.3 12.7 12.25 0.89

36.3 71.63 8.16 31.01 6.06 1.27 5.98 0.8 5.3 1.09 2.82 0.47 2.92 0.45 173.81 8.32 3.56 1.75 0.82 127.3 123 466 231 5.87 9.87 2.35 30.3 15.77 2.87

a

Abbreviation LIO is loss on ignition.

along the southern margin of the eastern Kunlun (Figure 1). The relative positions of the granites, ultramafics and flyschs unambiguously indicate north-dipping subduction [Geological Publishing House, 1991; Sengo¨r, 1984; Matte et al., 1996]. [30] A distinct subduction zone existed along the northern margin of the Qiangtang continental block, south of the Songpan terrane [Sengo¨r, 1984]. The age of the QGS 34 granite near Yushu (206 ± 7 Ma) suggests that this zone was still active in the Late Triassic. Southeastwards, this subduction zone continued along the Jinsha River valley, to Benzilan, north of the Red-River Ailao Shan fault zone [Geological Publishing House, 1991; Leloup et al., 1995]. In that area, subduction dipped west, in the present geographic coordinates [Leloup et al., 1995]. West of Yushu, this subduction zone apparently extended across the Fenghuo Shan, where isolated ophiolitic remnants exist [Geological Publishing

House, 1991; Pearce and Deng, 1988]. Recent mapping and dating of blueschists (205– 223 Ma) in central Qiangtang [Kapp et al., 2000] extend the trace of the Jinsha subduction zone farther west, and suggest that it dipped under the Qiangtang block. 5.2. Correlation of Kunlun and Jinsha Sutures with Other Indosinian Sutures [31] Other regions of Asia were sites of subduction and collision in the Triassic, at the time of the so-called Indosinian Orogeny. One well-documented Indosinian suture is that along the Qinling-Dabie Shan, limit between the north and south China blocks. Estimates of the north-south China (NCB-SCB) collision range from the middle Paleozoic based on structural studies and 40Ar/39Ar metamorphic ages (314 – 348 Ma) for the Qinling belt [Mattauer et al., 1985; Faure et al., 1999; Lin et al., 2000] to the Early to Middle Mesozoic based on paleomagnetism [e.g., McElhinny et al., 1981]. The Paleozoic age was supported by the presence of a dismembered ophiolite belt, synkinematic granitoids, and Devonian molasse [Mattauer et al., 1985]. Petrochemical and geochronological study of granulites in the Tongbai area in the east Qinling suggest a phase of crustal shortening and metamorphism in the Early Silurian to earliest Devonian [Kro¨ner et al., 1993, Xue et al., 1996; Zhai et al., 1998]. Laveine et al. [1992] presented paleofloral evidence indicating communication between both blocks as early as the Late Carboniferous whereas other paleoclimatic data support an Early Mesozoic consolidation [Nie, 1991]. Sengo¨r [1984] and Hsu¨ et al. [1987] argued that collision was Middle to Late Triassic based on preliminary radiometric dating of ophiolites and eclogites in the suture and because marine sedimentation continued well into the Triassic. While the Qinling orogen clearly contains evidence for important deformation in the Paleozoic, the final deformation event resulted from the collision of the north and south China blocks in the interval between the Early Permain and the Middle Jurassic [McElhinny et al., 1981; Lin et al., 1985; Zhao and Coe, 1987, 1989; Lin and Fuller, 1990; Enkin et al., 1992; Yang et al., 1992; Gilder et al., 1993, 1999]. Paleogeographic reconstructions based on paleomagnetic data show that the collision occurred diachronously from east to west with major rotation occurring near the Triassic-Jurassic boundary [Zhao and Coe, 1987, 1989; Enkin et al., 1992; Yang et al., 1992; Gilder et al., 1993, 1999]. The existence of 1) a regional angular unconformity between the Middle and Late Jurassic, 2) concordance of Late Jurassic poles and discordance of Middle Jurassic poles from the north and south China blocks, and 3) a cusp in the Middle to Late Jurassic part of the north China block apparent polar wander path suggests that the collision ended near the Middle to Late Jurassic boundary [Gilder and Courtillot, 1997]. Moreover, geochronological studies of ultra-high-pressure (UHP) metamorphic rocks in the Qinling orogen indicate that collision was active in the Late Triassic [Ames et al., 1993, 1996; Eide et al., 1994; Rowley et al., 1997; Hacker et al., 1998]. In conclusion, the abundant multidisciplinary data have now converged showing that the north-south China block

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Figure 8. (a and c) Chondrite normalized REE plot. Gray region delineates range from Quxu intrusion (Gangdise Belt) (data from Debon et al. [1986]) and stippled region is Kunlun batholith [Harris et al., 1988a]. (b and d) Spidergrams for both samples normalized by Ocean Ridge Granite [Pearce et al., 1984]. Gray field = volcanic arc/post-collision granites, stippled field = High Himalaya leucogranites. collision began at least by the Late Permian with the most active period in the Late Triassic to Early Jurassic. Older deformation episodes are present in the suture [Zhai et al., 1998; Meng and Zhang, 1999, 2000] and are probably related to terrane/active margin tectonics active prior to the NCB-SCB collision [Hsu¨ et al., 1987]. Such ages, and the fact that the Kunlun-Anyemaqin suture continues along

strike into the Qinling-Dabie suture imply there existence of a single, continuous subduction zone along the two belts between the Late Permian and the Early Jurassic. [32] Unraveling the timing and geometry of the JinshaBenzilan suture is hindered by younger tectonic deformation due to the India-Asia collision. Tertiary left-lateral movement of 700 km along the Ailao Shan-Red River fault has

Table 4. Sm-Nd and Rb-Sr Isotopic Data of Whole Rock Sample

T, Ma

AT113 AT118 QGS34 QGS38 QGS37

207 217 206 204 244

147

Sm/144Nd 0.1296 / 0.0660 0.1039 0.1211

143

Nd/144Nd

0.512219 ± 10 / 0.512013 ± 9 0.512117 ± 20 0.511886 ± 7

eNd(0)

eNd(T)

7.8 / 11.8 9.8 14.3

6.1 / 8.4 7.4 12.0

TDM,

Rb/86Sr

87

3.290 4.640 0.336 1.014 3.715

0.71618 0.72015 0.70970 0.70966 0.74336

87 Ga

1.54 / 1.09 1.32 1.95

Sr/86Sr

Sr/86Sr

87 (0)

± ± ± ± ±

2 2 3 2 2

(T)

0.70650 0.70583 0.70872 0.70672 0.73047

eSr

(T)

29.6 20.2 61.1 32.8 370

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Figure 9. The eSr(T)-eNd(T) plot for samples from Kunlun and Yushu (this study) compared with Lhasa Terrane (gray field) and Kunlun Terrane near Golmud (data from Harris et al. [1988a]). UC, IC, M indicate upper crust, intermediate crust and upper mantle at 240 Ma, crustal extraction at 1100 Ma. Dashed lines show mixing curves between average upper crust and depleted mantle and intermediate crust and depleted mantle. Crustal parameters from Weaver and Tarney [1980].

displaced geologic features that were once continuous across the fault [Tapponnier et al., 1986, 1990; Yang and Besse, 1993; Gilder et al., 1996a; Leloup et al., 1995, 2001]. Among them are the several suture zones that cross Indochina and peninsular Malaysia. The most prominent zones are the Song Ma and Song Da in Vietnam and the Nan-Uttaradit zone in Thailand, [Sengo¨r, 1984, 1987; Hutchison, 1989a]. Suturing along the Song Ma zone may have occurred as early as the Devonian-Early Carboniferous [Deprat, 1914; Helmcke, 1985; Sengo¨r et al., 1988; Hutchison, 1989b; Metcalfe, 1995]. This hypothesis is supported by the occurrence of YunnanolepiformAntiarch (placoderm fish) on either side, suggesting that Indochina was land-linked to south China in the Mid-Devonian [Janvier et al., 1994; Thanh et al., 1996]. Laveine et al. [1994] provided paleobotanical evidence for an Early Carboniferous Indochina-south China link, compatible with the time proposed earlier by Hutchison [1989b] and Mouret [1994]. [33] Geochronological data, however, suggests that the Song Ma suture was reactivated during the Indosinian orogeny [Lepvrier et al., 1997; Roger et al., 2000b]. The Nan-Uttaradit suture zone is usually interpreted to extend south into the Bentong-Raub zone in Malaysia [Hutchison, 1983; Metcalfe, 2000] and north into the Luang PrabangDien Bien Phu or Changning-Menglian suture in Laos and western Yunnan [Huang, 1984; Sengo¨r and Hsu¨, 1984, Barr and Macdonald, 1987; Wu et al., 1995] (Figure 1). The collision may have occurred in the Late Paleozoic [Helmcke, 1985] but a Middle Triassic age is generally favored [Sengo¨r and Hsu¨, 1984] with a final stage near the Triassic-Jurassic boundary [Hahn, 1985]. In northern Thailand, the Lampang-Jinghong-Lincang volcanic belt

(Figure 1) formed during early Middle Triassic convergence as constrained by a U-Pb-zircon date of 240 ± 1 Ma from a rhyolite [Barr et al., 2000]. Granites of the Main Range Batholith in Thailand associated with the Nan-Uttaradit suture are dated at 230 ± 9 Ma, 207 ± 14 Ma, and 205 ± 4 Ma supporting a Late Triassic to Early Jurassic collision [Liew and Page, 1985; Darbyshire, 1988; Dunning et al., 1995] and consistent with a west-dipping subduction zone beneath the eastern margin of the Shan-Thai terrane [Leloup et al., 1995; Singharajwarapan and Berry, 2000]. [34] Thus, of the two main sutures that cut SE Asia, one is of probable Late Devonian to Early Carboniferous age with reactivation in the Early Mesozoic, and the other formed between the Late Permian and the Early-Middle Mesozoic. The Jinsha-Benzilan suture extended along the Nan-Uttaradit suture, between the Indochina and Shan-Thai blocks prior to being offset 700 km by motion on the Red River Ailao Shan fault [Briais et al., 1993; Leloup et al., 1995]. 5.3. Paleogeographic Implications [35] In order to better understand the space-time relationship of the numerous Asian sutures/subduction zones, we made paleomagnetic-based reconstructions for the Late Permian (260 Ma) and the Early Jurassic (186 Ma) (Figure 10). The reconstructions include 700 km of Cenozoic left-lateral strike slip faulting on the Red River fault zone and assume that Kazakhstan moved rigidly with Siberia. For the Late Permian we use the Siberian reference pole of Van Der Voo [1993], those by Enkin et al. [1992] for the north and south China blocks (NCB and SCB), and that by Huang et al. [1992] for Qiangtang. The Late Paleozoic

ROGER ET AL.: CHRONOLOGY OF MESOZOIC SUTURES IN TIBET

Figure 10. Paleogeographic reconstructions of major Asian blocks in (a) Late Permian and (b and c) Early Jurassic. The difference between the models in Figures 10b and 10c is the choice in poles for the Eurasian plate and the way we have reconstructed the southern margin of Siberian. Figure 10b uses the Eurasian reference pole from Besse and Courtillot [1991] with the southern margin of Siberian resembling the reconstruction of Enkin et al. [1992]. Figure 10c uses preliminary Early to Middle Jurassic reference pole from Tarim [Gilder et al., 1996a, 1996b] fixed to Eurasia, with our preferred reconstruction of the southern margin of Siberia (as in Figure 10a). Abbreviations for plates (using present-day boundaries) are Eur, Europe; Sib, Siberia; Kaz, Kazakhstan; Indo, Indochina; IND, India; Lh, Lhasa; Mon, Mongolia; NCB, north China Block; SCB, south China block; Ta, Tarim; Qa, Qaidam; QI, Qiangtang, ST, Shan Thai; SG, Songpan Ganze; Ir, Iran and Jun, Junggar.

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ROGER ET AL.: CHRONOLOGY OF MESOZOIC SUTURES IN TIBET

reconstruction is similar to those published by Zhao et al. [1990] and Huang et al. [1992] which show that the NCB, SCB, Qiangtang and Indochina blocks were in relatively close proximity at equatorial latitudes. It differs with the reconstruction of Enkin et al. [1992] in that they position Qiangtang farther to the north, situate NCB, Mongolia, etc. closer to Siberia-Kazakhstan, and link them with a land bridge. Figure 10a shows no such bridge although communication between them may have been possible via island arcs, etc. The Siberian continent lies some 30
north, clearly separated from the equatorial assemblage, whereas Zhao et al. [1990] placed them much closer together. Notable differences with previous reconstructions are that Indochina is separated from the Qiantang by a subduction zone and that subduction also occurs along the southern margin of the NCB. This way, the Nan-Uttaradit and the Jinsha-Benzilan subduction zones form the same plate boundary. [36] The two Early Jurassic reconstructions presented make use of the NCB and SCB poles reported by Gilder and Courtillot [1997], and that from Lin and Watts [1988] for Qiangtang. The difference between the two is the choice of pole for the Siberian plate. Figure 10b uses the Siberian pole from Besse and Courtillot [1991] and resembles the reconstruction of Enkin et al. [1992] except for the position of Qiangtang, and the emplacement of the subduction zones, in light of more recent paleomagnetic data from Qiangtang, and of our new ages and structural data concerning the sutures. Using the Besse and Courtillot [1991] pole, Siberia lies farther from the Chinese landmasses in the Early Jurassic than it did in the Late Permian. Reconstruction 10b was contested by Nie et al. [1994], because there is no evidence for a 3000 km-long northtrending strike slip fault within across the dominant E-W structure fabric east and north of the Tarim. Furthermore, the validity of using the synthetic Eurasian poles for Siberia has been questioned because the data constraining it come either from westernmost Europe or from poles transferred from other continents. Cogne´ et al. [1999] recently suggested that the European poles are not valid for Asia due to non-rigid behavior of the Eurasian plate, even during the Cenozoic. Because this vast plate spans over 90
in longitude, and because the Early Jurassic poles come solely from the western part of the plate, a slight relative rotation between western Europe and Asia has a large impact on the paleolatitude of Siberia. For these reasons, we base an alternative reconstruction (10c) on a preliminary Early to Middle Jurassic pole from Tarim [Gilder et al., 1996b].

Because only minor amounts of relative motion (300 km) [e.g., Avouac and Tapponnier, 1993] occurred between Tarim and Siberia since the Jurassic, this pole probably serves as a more realistic reference for Siberia despite its relatively large uncertainty (14
). Using this pole places Siberia at more temperate latitudes and brings the southern margin of the NCB into coincidence with Kunlun, resulting in a 3000 km-long, continuous Kunlun-Qinling suture zone. Figure 10c differs from the reconstruction of Metcalfe [1988, 1995], which does not include the 700 km Tertiary offset along the Red River fault and infers suturing along the Jinsha to have been complete before the Late Triassic. At 186 Ma we consider that Qiangtang and south China were in close proximity, consistent with paleomagnetic data and Mesozoic sedimentary facies [Yin et al., 1988]. According to reconstruction 10c, the Qiangtang-south China block rotated about 1
/m.y. with respect to NCB from 240 Ma to 160 Ma [Zhao and Coe, 1987, 1989; Gilder and Courtillot, 1997]. Assuming the eastern end of the Qiangtang block was 3000 km from the Euler pole implies it would move at