JOURNAL OFGEOPHYSICAL RESEARCH, VOL.108,NO. B9,2423, doi:10.1029/2002JB001876, 2003 Magmatic history of the northeastern Tibetan Plateau George E. Gehrels DepartmentofGeosciences,UniversityofArizona,Tucson,Arizona,USA An Yin DepartmentofEarthandSpaceSciences,UniversityofCalifornia,LosAngeles,California,USA Xiao-Feng Wang ChineseAcademyofGeologicalSciences,InstituteofGeomechanics,Beijing,China Received13March2002;revised4January2003;accepted16January2003;published12September2003. [1] ThenortheasternmarginoftheTibetanPlateauisunderlainbytheQaidamandQilian terranes, which consist primarily of mid-Proterozoic through lower Paleozoic oceanic and arc-type assemblages that have been accreted to the southern margin of the Tarim/Sino- Korean craton. Most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the Tarim/Sino- Korean craton during early Paleozoic time. The main components are interpreted to have formedeitherasanarchipelagoofvolcanicarcsandbackarcbasins,orasabroadexpanse of accretionary complexes. Ourgeochronologic data support a model, suggested by Sobel and Arnaud [1999], in which the Qaidam and Qilian terranes are separated from the Tarim/Sino-Korean craton by a mid-Paleozoic suture that closed along a southwest dipping subduction zone. The basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of (cid:1)920– 930 Ma granitoids. Early Paleozoic arc-type magmatism occurred between (cid:1)480 and (cid:1)425 Ma, apparently sweeping southwestward across much of the Qilian and Qaidam terranes. Accretion-related magmatism along the inboard margin of the Qilian terrane occurred between (cid:1)423 Ma and (cid:1)406 Ma. Following Silurian-Devonian accretion, the region has experienced late Paleozoic and Mesozoic uplift and erosion and has been severely overprinted by Tertiary thrusting, uplift, and strike-slip motion along the Altyn Tagh fault. Correlation of geologic features and magmatic histories between the Altun Shan and the Nan Shan suggests that the eastern Altyn Tagh fault has a total left-lateral offsetof(cid:1)375km. INDEXTERMS:8105Tectonophysics:Continentalmarginsandsedimentarybasins (1212); 8110 Tectonophysics:Continentaltectonics—general (0905);8125Tectonophysics:Evolution of theEarth;8157Tectonophysics:Platemotions—past (3040);KEYWORDS:tectonics,China,Tibet,Paleozoic magmatism Citation: Gehrels,G.E.,A.Yin,andX.-F.Wang,MagmatichistoryofthenortheasternTibetanPlateau,J.Geophys.Res.,108(B9), 2423,doi:10.1029/2002JB001876, 2003. 1. Introduction Harrison,2000],whichareexposedintheAltunShan,Nan Shan, and Qilian Shan (Figure 1). These mountainous [2] The northeastern portion of the Tibetan Plateau con- regions are underlain largely by accreted rocks of mid- tains a variety of crustal fragments that have been accreted Proterozoic to mid-Paleozoic age, Devonian through Creta- to the southern margin of the Tarim/Sino-Korean craton ceousmarineandcontinentalplatformalstrata,andTertiary (Figure 1). The cratonal regions northeast of the plateau synorogenicdeposits.Figure2showsthedistributionofthe consist largely of Precambrian crystalline basement, over- main assemblages in the area and provides a brief descrip- lain locally by Proterozoic, Paleozoic, and Mesozoic plat- tion of their lithotectonic constituents. formal strata [Liu, 1988; Yin and Harrison, 2000]. These [3] An additional element in the region is the Qaidam rocks generally occur at elevations of <2 km with low basin, which is a low-relief basinal terrane within the topographic relief and little sign of active deformation northeasternTibetanPlateau(Figure1).Manyworkershave (Figure1).Tothesouthandwestofthecratonsisacollage suggested that the Qaidam terrane is underlain at least in of arc-type, ocean floor, and shallow marine assemblages, part by Precambrian continental basement [Li et al., 1978; referred to as the Qilian and Qaidam terranes [Yin and Yin and Nie, 1996], although Sengo¨r and Natal’in [1996] suggested that it is underlain by Paleozoic arc and accre- Copyright2003bytheAmericanGeophysicalUnion. 0148-0227/03/2002JB001876$09.00 tionarycomplexesandHsu¨ etal.[1995]concludedthatitis ETG 5 - 1 ETG 5- 2 GEHRELS ETAL.:TIBETANMAGMATISM Figure 1. Sketch maps showing (a) location of the Altun Shan, Nan Shan, and Qilian Shan in the northeastern portion of the Tibetan Plateau and (b) location of the study area with respect to the main crustal fragments and sutures in the area. Black dashed box shows the area of Figure 2. Digital topography in Figure 1a is from the GLOBE project of the National Geophysical Data Center (http:// www.ngdc.noaa.gov/seg/topo/globe.shtml).TerranesandsuturesinFigure1barefromYinandHarrison [2000].IYS,Indus-Yalusuture;BNS,Bangong-Nujiangsuture;JS,Jinshasuture;KS,Kudisuture;SQS, South Qilian suture; NQS, North Qilian suture. underlain by Paleozoic oceanic crust that formed in a back the older accreted terranes [Liu, 1988], and by previous arc setting. Resolving between these three possibilities is thermochronologicstudies.SobelandArnaud[1999],Sobel one of the main objectives of the present study. etal.[2001],andDelville etal.[2001]usedthermochrono- [4] According to most previous syntheses, arc-type and logic data from the Altun Shan and nearby ranges to show oceanicrocksinthenortheasternpartoftheTibetanPlateau that a regional metamorphic event occurred in the region formed within an early Paleozoic convergent margin that between (cid:1)435 Ma and (cid:1)380 Ma. This metamorphism is was built along or outboard of the southern and southwest- interpreted by these workers to record accretion of the arc- ern (using present coordinates) margin of the Tarim/Sino- type terranes to the Tarim/Sino-Korean craton. A third Koreancraton[Lietal.,1978;Hsu¨ etal.,1995;YinandNie, objective of the present study is to determine the history 1996; Xia et al., 1996; Sengo¨r and Natali’in, 1996; Sobel ofmagmatismduringandimmediatelypriortothisSilurian- andArnaud,1999;Xuetal.,2000;Yangetal.,2000,2001; Devonian accretionary event. Yin and Harrison, 2000]. These syntheses support three [7] Followingmid-Paleozoicaccretion,rocksinthestudy fundamentally different models for the main processes by area have experienced several additional phases of defor- which the arc-type and oceanic terranes were created and mation, metamorphism, and uplift. Sobel et al. [2001] and eventually accreted. Delville et al. [2001] present thermochronologic data that [5] Hsu¨etal.[1995]proposedthattheTarim/Sino-Korean record cooling events during late Paleozoic through Late cratonwasrimmedtothesouthbyanarchipelagoofoceanic Jurassic time, and the entire region has clearly experienced arcsandbackarcbasins.Thissubductionsystemfacedsouth profound deformation and uplift during Tertiary India-Asia (dippednorth)overall,butcollapseofsmalleroceanicbasins collision. In the study area, Tertiary deformation is clearly withinthearchipelagooccurredalongsubductionzonesthat evidenced by the abundant thrust faults within the Altun may have faced south [Yin and Harrison, 2000; Xu et al., Shan,NanShan,andQilianShan,andbystrike-slipmotion 2000;Yangetal.,2001],bothsouthandnorth[Lietal.,1978; on the Altyn Tagh and related faults (Figure 2). These XiongandConey,1985],ornorth[Xiaetal.,1996].Asecond Tertiary tectonic features have been described in consider- model, proposed bySengo¨randNatal’in [1996],holdsthat abledetailbyPeltzerandTapponnier[1988],Tapponnieret early Paleozoic magmatism migrated progressively south- al. [1990], Meyer et al. [1996], Me´tivier et al. [1998], and ward from the Tarim/Sino-Korean margin, with magmatic Tapponnier et al. [2001]. A final objective of the present arcsbeingbuiltonslightlyolderaccretionarycomplexes.A studyistousethemagmaticandgeologicrecordsnorthand thirdmodel,presentedbySobelandArnaud[1999],suggests south of the Altyn Tagh fault to help constrain the amount that a major mid-Paleozoic suture separates the oceanic of offset on the eastern segment of the fault. terranes from the Tarim/Sino-Korean craton, and that this sutureclosedalonganorthfacing(southdipping)subduction 2. Present Study system.Testingthesethreemodelsisasecondmajorobjec- tiveofthepresentstudy. [8] This study uses U-Pb geochronology to address [6] The timing of accretion of arc systems to the Tarim/ questions concerning the history of subduction- and accre- Sino-Korean craton is relatively well established by the tion-related magmatism in the northeast Tibetan Plateau. occurrence of Devonian strata that unconformably overlie Twenty-three samples have been collected and analyzed. GEHRELS ETAL.:TIBETANMAGMATISM ETG 5 - 3 Figure2. Mapofthemaintectonicassemblagesandfirst-orderstructuresinthenortheasternpartofthe TibetanPlateau(adaptedfromLiu[1988]).GeographicnamesintextandonmaparefromU.S.National Imagery and Mapping Agency [1989]. Terrane names and divisions are adapted from Yin and Harrison [2000]. The two Early to Middle Ordovician age samples labeled ‘‘X’’ are from E. Cowgill (written communication, 2002). The results of our analyses are discussed in the following 3. Results section, with analytical information presented on Pb/U 3.1. Archean-Early Proterozoic Crystalline Basement concordiadiagrams,describedinAppendixA,andreported in auxiliary material Table A1.1 [9] The crystalline basement of the Tarim/Sino-Korean craton is exposed in several isolated areas along the north- eastern margin of the Tibetan Plateau [Liu, 1988]. Where 1Supporting tables are available at ftp://agu.org/apend/jb/2002 seen during this study, the basement consists of three main JB001876. components: layered diorite-quartz diorite gneiss, massive ETG 5- 4 GEHRELS ETAL.:TIBETANMAGMATISM disturbance.Thedifferenceinbehaviormayresultfromthe higher uranium concentration in the younger zircons from sample2. 3.1.3. Sample 3 [12] A second sample of quartz dioritic orthogneiss was collected from a small body exposed near the southern margin of Tarim basement in the northeastern Altun Shan (Figure2).AsshownonFigure4,thegrainsyieldarangeof highly discordant ages that do not lie along a simple discordia. The minimum age of the sample is constrained by the 207Pb*/206Pb* age of the oldest grain, which is (cid:1)2642 Ma (Table A1). A possible explanation of the complex discordance pattern relies on U-Pb data from a sample of crosscutting granite (sample 4). As described below, this sample yields a well-defined discordia array withinterceptsat1877±10Maand375±42Ma,whichare interpreted as the ages of crystallization and Pb loss (respectively). These data indicate that the gneiss must be older than (cid:1)1877 Ma, and must also have experienced the same isotopic disturbance at (cid:1)375 Ma. One possible history, shown on Figure 4, is that the zircons in sample 3 Figure 3. Pb/U concordia diagram of samples 1 and 2. crystallized originally at (cid:1)2720 Ma (the upper intercept of Note that uncertainties in this and following concordia thetwoleast-discordantgrains),experiencedPbloss(and/or diagrams are at the 95% confidence level. Data reduction zircon growth) at (cid:1)1877 Ma, and were then isotopically and plotting are from programs of Ludwig [1991a, 1991b]. disturbed at (cid:1)375 Ma. The 2926 ± 10 Ma age for sample 1 is interpreted mainly 3.1.4. Sample 4 from the 207Pb*/206Pb* age of a concordant grain (analysis [13] This sample was collected from a 3-m-wide dike of 6inTableA1),whereasthe1728±25Maageforsample2 nonfoliated pinkish leucogranite that intrudes across the is determined from the upper intercept of a discordia foliation and layering of the Archean gneiss described through all analyses. above (sample 3). Eleven single grains were analyzed, of which six were strongly abraded prior to analysis. The pinkishgranitetoquartzalkalisyenite,andswarmsofmafic analyses define a well-constrained discordia with intercepts dikes.ThiscomplexisshownonthemapsofLiu[1988]as of 1877 ± 10 Ma and 375 ± 42 Ma (Figure 3). The upper Archean/Proterozoic in age, and ages of (cid:1)2462 Ma (U-Pb interceptisinterpretedastheageofcrystallization,whereas onzircon)and2787±151Maand2792±208Ma(Sm-Nd) arereportedbyWangetal.[1993]andCheandSun[1996]. Our samples have been collected from three areas, as described below and shown on Figure 2. 3.1.1. Sample 1 [10] This sample was collected from a layered diorite- quartz diorite gneiss exposed in a basement uplift within the southern Tarim basin (Figure 2). Six large euhedral crystalswereanalyzed,fourofwhichwereabradedpriorto analysis. One of the grains (analysis 6) yields a concordant age of (cid:1)2926 Ma, which we interpret as the crystallization age, whereas all of the others are moderately discordant (Figure3).Thediscordiahasalowerinterceptof(cid:1)1701Ma, which presumably records Pb loss during emplacement of graniticdikesthatintrudethegneiss(seesample2discussion below). 3.1.2. Sample 2 [11] Intrusive into the gneiss described above is a small granite body, which was collected as sample 2. Nine individualgrainswereanalyzed,ofwhichfourwereabraded. The analyses are all highly discordant, but define a well- constrained discordia with intercepts of 1728 ± 25 Ma and 305 ± 31 Ma. The upper intercept is interpreted as the crystallization age, and the lower intercept is interpreted astheapproximateageofPbloss.Alikelyinterpretationis Figure4. Pb/Uconcordiadiagramofsamples3and4.See that Pb loss in the zircons from sample 1 occurred during text for discussion of the possible age for sample 3. The emplacement of this younger granite, but that zircons 1877 ± 10 Ma age for sample 4 is determined from the in the gneiss were not affected by the (cid:1)305 Ma isotopic upper intercept of a regression through all of the analyses. GEHRELS ETAL.:TIBETANMAGMATISM ETG 5 - 5 Figure5. Pb/U concordiadiagram ofsample5.The1870 ± 7 Ma age for this sample is interpreted from the upper intercept of a discordia through all analyses. the lower intercept apparently records the age of Pb loss and/or new zircon growth. 3.1.5. Sample 5 [14] Sample5wascollectedfromacoarse-grainedquartz alkali syenite located along the southernmost margin of the Tarim craton in the northeastern Altun Shan (Figure 2). Nine abraded single grains were analyzed (Figure 5). A discordia through these analyses yields an interpreted crys- tallization age of 1870 ± 7 Ma and a Pb loss age of 819 ± 611 Ma. 3.2. Mid-Proterozoic Plutons 3.2.1. Sample 6 [15] A large body of porphyritic granite that intrudes marble, quartzite, and metapelite of the Xorkol sequence was sampled along the southern margin of the northeastern AltunShan(Figure2).Eightsinglegrainswereanalyzed,of which three were abraded prior to analysis. Five of the grains yield concordant analyses of 922 ± 6 Ma, and two additional analyses define a discordia with an upper inter- cept (interpreted as the average age of inherited compo- nents) of 1884 ± 185 Ma (Figure 6a). An additional grain yields much younger ages, presumably due to Pb loss. 3.2.2. Sample 7 [16] A small granitic pluton that intrudes marble, quartz- ite, and pelitic schist of the Central Qilian sequence was sampled for U-Pb analysis. Nine single grains were ana- lyzed, all of which were strongly abraded prior to analysis. The analyses range from concordant to moderately discor- dant, and define a discordia with intercepts of 922 ± 5 Ma and (cid:2)7 ± 33 Ma (Figure 6b). 3.2.3. Sample 8 [17] This sample comes from a heterogeneous intrusive Figure 6. Pb/Uconcordiadiagramofsamples(a)6,(b)7, bodylocatedwestofthetownofDaQaidaminthewestern and(c)8.The922±6Maageforsample6isinterpretedfrom Nan Shan (Figure 2). This body is important because it theclusteroffive concordant analyses, whereas the agesof clearlyintrudesintotheeclogite-bearingultramaficrocksin 922±5Maforsample7and928±10Maforsample8are the region [Yang et al., 2001] (Appendix A). Ten abraded basedonregressionsthroughthediscordantanalyses. ETG 5- 6 GEHRELS ETAL.:TIBETANMAGMATISM Figure 7. Pb/U concordia diagram of samples (a) 9, (b) 10, (c) 11, and (d) 12. The indicated ages for samples 9, 10, and 12 are based on the clusters of concordant ages, whereas the 482 ± 11 Ma age for sample 11 is based on the upper intercept of a regression through the discordant analyses. single grains were analyzed, all of which are discordant tals were analyzed, all of which were abraded prior to (Figure 6c). The upper intercept of 928 ± 10 Ma is analysis. The grains are all concordant at 480 ± 6 Ma interpreted as the crystallization age, whereas the lower (Figure 7d). interceptof291±49MaisinterpretedastheageofPbloss. 3.3.5. Sample 13 [22] A sample has also been analyzed from a small body 3.3. Early to Middle Ordovician Plutons of reddish granite that crops out near the village of 3.3.1. Sample 9 Lapeiquan in the northeastern Altun Shan (Figure 2). The [18] Intrusive into volcanic and sedimentary rocks of the body intrudes volcanic and sedimentary rocks of the Dang He Nan Shan volcanic terrane is a nonfoliated quartz Lapeiquan volcanic terrane. One multigrain fraction and diorite pluton (Figure 2). We collected a sample of this eleven single grains were analyzed, with abrasion of four pluton from the northwesternmost Nan Shan. Two multi- grain fractions and four single grains were analyzed. As shown on Figure 7a, all of the analyses are essentially concordantandyieldanageof482±5Ma.Thereisnosign of inheritance or Pb loss. 3.3.2. Sample 10 [19] This sample was collected from a coarse-grained granite located in the Yema Shan metamorphic belt, which is in the northern Nan Shan (Figure 2). Seven individual crystalswereanalyzed,allofwhichareconcordantat472± 6 Ma (Figure 7b). 3.3.3. Sample 11 [20] Anonfoliatedgranitebodythatintrudesvolcanicand sedimentary rocks of the Lapeiquan volcanic terrane was sampled in the northeastern Altun Shan (Figure 2). Seven multigrain fractions were analyzed, of which one is appar- ently concordant and the rest are moderately discordant. A regression throughtheanalysesyieldsanupperinterceptof 482±11Maandalowerinterceptof62±180Ma(Figure7c). These are interpreted, respectively, as the ages of crystal- lizationandPbloss. 3.3.4. Sample 12 [21] This sample is from a large body of nonfoliated, Figure 8. Pb/U concordia diagram of sample 13. This medium- to coarse-grained granite that intrudes blueschist- sampleyieldsaverypoorlyconstrainedageof479±64Ma faciesmetasedimentaryandmaficmetavolcanicrocksofthe based on the upper intercept of a regression through the YemaShanmetamorphicbelt.Sevenindividualzirconcrys- highly discordant analyses. GEHRELS ETAL.:TIBETANMAGMATISM ETG 5 - 7 Figure 9. Pb/U concordia diagram of samples (a) 14, (b) 15, and (c) 16. The indicated ages are determined from the clusters of concordant analyses. larger grains (Table A1). All of the analyses are highly belongtotheNorthQilianmelangecomplex.Sixindividual discordant(Figure8).Theupperinterceptof479±64Mais crystals were analyzed, all of which are apparently concor- interpreted as the approxim ate age of crystallization, dant at 424 ± 4 Ma (Figure 9c). There is no sign of whereas the lower intercept of 75 ± 65 Ma is interpreted inheritance in the zircons analyzed. as the approximate age of Pb loss (Figure 8). 3.5. Silurian-EarlyDevonianPlutonsWithInheritance 3.4. Late Ordovician-Silurian Plutons 3.5.1. Sample 17 With No Inheritance [26] Thissamplewascollectedfromanonfoliatedgranite 3.4.1. Sample 14 body that occurs along the northern margin of the Lapei- [23] This sample was collected from a large granite body quan volcanic terrane in the Altun Shan (Figure 2). The near the town of Da Qaidam in the western Nan Shan plutonintrudesmetavolcanicandmetasedimentaryrocksof (Figure2).Fourindividualzirconcrystalswereanalyzed,all probable arc affinity, and is structurally juxtaposed against of which overlap at an age of 442 ± 4 Ma (Figure 9a). The Precambrian basement of the Tarim craton. Four single shift of two analyses to the left of concordia (Figure 9a) grains (cid:1)80 m in length were analyzed, as well as three presumably results from low Pb signal intensities (low U multigrain fractions consisting of three grains (cid:1)60 m in concentration) and high common Pb concentrations. length (Table A1). Two single grains and two multigrain 3.4.2. Sample 15 fractions yield concordant analyses, with an age of 413 ± [24] Intrusive into the Qaidam metamorphic belt is a 5Ma,andadditionalanalysesrevealthepresenceofinherited smallbodyoffoliatedquartzdiorite(Figure2).Wesampled componentswithanaverageageof(cid:1)2.1Ga(Figure10). this body from the same locality (within (cid:1)100 m) that a 3.5.2. Sample 18 K-Ar age of 1120 Ma is shown on the geologic map of the [27] A large body of granite was sampled in the north- area [Regional Geological Survey Team of Qinghai Bureau easternmost Qilian Shan (Figure 2). The pluton intrudes of Mines and Geology (RGST-QBMG), 1986]. Itwashoped Paleozoic metasedimentary rocks of the North Qilian com- that oursample would help constrain the ageof Precambri- plex. Five multigrain fractions and five single grains were an(?) basement beneath the Qaidam basin. Five zircon analyzed, of which five are concordant and fiveare moder- crystals were analyzed, all of which are concordant at 425 atelytohighlydiscordant(Figure11).Theconcordantgrains ±3Ma(Figure9b).Thedifferencebetweenthisageandthe yieldaninterpretedcrystallizationageof406±12Ma,and Precambrian K-Ar age is most likely due to analytical the discordant grains contain inherited components of mid- problems with the older data (e.g., excess Ar). Proterozoic and Late Archean age. 3.4.3. Sample 16 3.5.3. Sample 19 [25] This sample was collected from a large granite body [28] This sample comes from a large granite body in the inthenorthernmostNanShan,northoftheAltynTaghfault northeastern Qilian Shan (Figure 2). The body is shown on (east of the town of Subei) (Figure 2). The granite intrudes the map of Liu [1988] as intruding sedimentary rocks of metasedimentary and metavolcanic rocks that probably Paleozoicage,whichweinterpretaspartoftheNorthQilian ETG 5- 8 GEHRELS ETAL.:TIBETANMAGMATISM Figure 10. Pb/U concordia diagram of sample 17. The 413±5Maageforthissampleisdeterminedfromthefour Figure 12. Pb/U concordia diagram of sample 19. The concordantanalyses.Theupperinterceptof2110±381Ma 422 ± 6 Ma age for this sample comes from the cluster of isbasedonaregressionthroughthediscordantanalysesand five concordant analyses. A regression line through the the cluster of concordant analyses. oldest grain and the cluster of concordant grains yields an upper intercept of 2446 ± 200 Ma. complex. Nine single grains were analyzed, of which five are apparently concordant at 422 ± 6 Ma (the interpreted discordant(Figure13).Thelowerinterceptof423±6Mais crystallizationage)andfourarediscordant(Figure12).The interpreted as the age of crystallization, whereas the upper discordance pattern suggests that both Pb loss and inheri- intercept of 1845 ± 86 is interpreted as the average age of tance are present. inheritedcomponents. 3.5.4. Sample 20 [29] A small granite body was sampled in the northern- 3.6. Late Paleozoic Plutons most Qilian Shan (Figure 2). The rock intrudes lower [30] Samples were collected from plutons along the PaleozoicvolcanicandsedimentaryrocksoftheNorthQilian northern margin of the Qaidam basin in an effort to complex. Six multigrain fractions were analyzed, two of determine the ages of crystalline basement that extends which are nearly concordant and four of which are highly Figure 13. Pb/U concordia diagram of sample 20. The Figure 11. Pb/U concordia diagram of sample 18. The 423 ± 6 Ma age for this sample is determined from the 406±12Maageforthissampleisderivedfromthecluster lower intercept of a regression through the discordant of five concordant analyses. Inherited components have analyses. The upper intercept age of 1845 ± 86 Ma is apparent ages of mid-Proterozoic and Late Archean. interpreted as the average age of inherited components. GEHRELS ETAL.:TIBETANMAGMATISM ETG 5 - 9 Figure 14. Pb/U concordia diagram of samples (a) 21, (b) 22, and (c) 23. The ages for these three samples come from the clusters of concordant analyses. beneath the Qaidam basin. Two of the bodies sampled had dioritic gneisses that yield Late Archean (2.7–2.9 Ga) previouslyyieldedPrecambrianK-Arages,whichprovided crystallization ages. The U-Pb systematics of zircons in support for the interpretation that Qaidam is floored by these rocks were disturbed during or slightly prior to Precambrian basement. emplacement of widespread (cid:1)1.7–1.9 Ga granitic bodies. 3.6.1. Sample 21 Our data record a second phase of isotopic disturbance [31] This sample was collected from a medium-grained during mid-Paleozoic time (lower discordia intercepts of leucogranite exposed along the north central margin of the (cid:1)305and(cid:1)375Maforsamples2and4).Thisdisturbance Qaidam basin (Figure 2). The pluton intrudes various early apparently involved low-temperature Pb loss, perhaps due Paleozoic(?)dioriteandquartzdioritebodiesthatareexposed to hydrothermal activity, as an 40Ar/39Ar (biotite) age from alongthenorthernmarginoftheQaidammetamorphicbelt. basement rocks near sample 4 [Sobel and Arnaud, 1999] Five abraded single grains were analyzed, and all are con- indicatesthattemperaturesintheserockshavenotsurpassed cordantat270±4Ma(Figure14). (cid:1)300(cid:2)C since Early Proterozoic time. 3.6.2. Sample 22 [35] Interestingly, there is no sign of early Paleozoic [32] A second sample was collected from an unnamed magmatism in the basement complexes that we have stud- range along the northern margin of the Qaidam basin ied.Plutonsofthisagewouldbeexpectedtooccurinthese (Figure 2). Much of this range is underlain by a large and areas according to common models that portray northeast homogeneous body of nonfoliated quartz diorite. Our sam- dipping subduction beneath a southwest facing (in present ple was collected from the same locality as a K-Ar age of coordinates) magmatic arc developing along the margin of 1474 Ma [RGST-QBMG, 1986]. Five single zircon grains the Tarim/Sino-Korean craton during Ordovician-Silurian were analyzed, all of which are concordant at 272 ± 8 Ma time. The lack of evidence for this magmatic arc raises the (Figure 14). The relatively low precision of these analyses possibility that subduction occurred mainly along a north- results from the low U concentration and high common Pb east facing (southwest dipping) convergent margin con- content of the zircon grains. structed along the leading edge of the Qilian and Qaidam 3.6.3. Sample 23 terranes.Thepresenceofthesearc-relatedassemblagesnow [33] Athirdsamplefromthisplutonicsuitewascollected resting structurally on Tarim/Sino-Korean basement is pre- from the same general region as sample 22 (Figure 2). The sumably a result of mid-Paleozoic obduction followed by bodysampledisaquartzdioritelayerwithinagranodiorite- Tertiary thrusting. quartz diorite-diorite intrusive complex. A K-Ar age of 4.2. Precambrian(?) Basement of Qaidam 794Mawasreportedfromthesamelocality[RGST-QBMG, 1986].Fivesinglezircongrainswereanalyzed,allofwhich [36] Previous workers have suggested that the Qaidam basin is underlain by Precambrian continental basement [Li are concordant at 270 ± 4 Ma (Figure 14). et al., 1978; Yin and Nie, 1996], Paleozoic convergent margin assemblages [Sengo¨r and Natal’in, 1996], or 4. Implications oceanic crust formed in a Paleozoic back arc basin [Hsu¨ 4.1. Precambrian Rocks of the Tarim/Sino-Korean et al., 1995]. The interpretation that Precambrian basement Craton ispresentisbasedinparton(cid:1)794,(cid:1)1120,and(cid:1)1474Ma [34] Our geochronologic analyses reveal a regionally K-Aragesfromgranitoidsalongthenorthernmarginofthe consistent history of igneous activity for the southern Qaidam basin [RGST-QBMG, 1986]. Given that our Tarim/Sino-Koreancraton.Oldestarelayeredquartzdioritic- samples from these same bodies yield ages of (cid:1)425 Ma ETG 5- 10 GEHRELS ETAL.:TIBETANMAGMATISM and (cid:1)270 Ma, we have not been able to document the existence of Precambrian basement beneath northern Qai- dam. Our ages, combined with the widespread occurrence of arc-type volcanic assemblages in the country rocks to these plutons [Liu, 1988], instead support the interpretation that Qaidam is underlain by latest Proterozoic(?)–early Paleozoic convergent margin assemblages [Sengo¨r and Natal’in, 1996]. 4.3. Mid-Proterozoic Magmatic Activity [37] The recognition of (cid:1)920–930 Ma igneous rocks in the study area has important implications for the tectonic evolution of this part of the Tibetan Plateau. Intrusion of a mid-Proterozoic granitoid into the ultramafic rocks near Da Qaidam (sample 8) is particularly important, as these Figure 15. Plot of apparent Pb/U ages of plutons along a ultramaficrockshavebeeninterpretedbyYangetal.[2001] northeast-southwestsectionacrosstheNanShanandQilian torecordamajorearlyPaleozoiccollisionaleventbetweenthe Shan (A-A’ on Figure 2). Sample numbers are shown for Qaidam and Qilian terranes. Our data indicate instead that each age, with two additional unlabeled samples from high-pressure metamorphism, and hence any collisional E. Cowgill (written communication, 2002). Samples along events along this boundary, occurred during or prior to theAltynTaghfaultandintheAltunShanarenotincluded mid-Proterozoic time. The early Paleozoic U-Pb and Ar-Ar becauseofuncertaintiesintheirprojectedpositionalongthe ages reported by Yang et al. [2001] from these rocks lineofsection.Thesouthwestward youngingfrom(cid:1)480to may record emplacement of Ordovician plutons that are (cid:1)440 Ma is interpreted as a record of migration of arc widespread in the region (e.g., sample 14), rather than the magmatism, whereas the northeastward jump between timingofhigh-pressuremetamorphism. (cid:1)440 and (cid:1)423 Ma may record the onset of accretion- [38] The occurrence of coeval (cid:1)920–930 Ma granitoids related crustal melting. Note that uncertainties in age and intrusiveintoshallowmarinestrataoftheQilian terrane,as position are generally smaller than the symbols. well as into ultramafic rocks of the Qaidam terrane, indi- catesthatmuchoftheregionmaybeunderlainbythesame mid-Proterozoic and perhaps older basement. On the basis of the lithologic descriptions of Liu [1988] and our own scale of Tucker and McKerrow [1995]), and E. Cowgill mapping, a reasonable scenario is that this basement con- (writtencommunication,2002)hasdeterminedPb/Uagesof sists of oceanic assemblages (e.g., slices of oceanic crust, (cid:1)459and(cid:1)461Maontwoadditionalbodiesinthecentral (cid:1)920 Ma magmatic arcs(?), thick piles of turbidites, and Nan Shan (two samples labeled ‘‘X’’ on Figure 2). These sequences of shallow marine strata) that were amalgamated bodies yield zircons that show little evidence of incorpora- into a coherent crustal fragment by mid-Proterozoic time. tion of older crustal material. This crustal fragment apparently did not form in proximity to the Tarim/Sino-Korean craton because (1) there is no [41] The younger plutons are more felsic in composition (generallygranitesandleucogranites)andyieldzirconsthat evidence of (cid:1)920–930 Ma magmatism in the cratonal commonly contain significant inherited components. Inher- areas, (2) the shallow marine sedimentary sequences and itance ages suggest incorporation of zircons primarily of metaturbidites in the Qilian and Qaidam terranes yield Late Archean and Early Proterozoic age (Figures 10–13), detritalzirconagesthatarequiteunliketheagesofigneous which is consistent with derivation from the nearby Tarim/ rocks in the Tarim/Sino-Korean craton [Gehrels et al., Sino-Korean cratonal crust. These bodies yield ages of 2003],and(3)itisclearfromgeologic,thermochronologic, (cid:1)424 Ma to (cid:1)406 Ma (mid-Silurian to Early Devonian). andpetrologicevidencethattheQilianandQaidamterranes areseparatedfromtheTarim/Sino-Koreancratonbyamajor [42] AplotofPb/Uagesalonganortheast-southwestline ofsectionacrosstheQilianShanandNanShan(Figure15) mid-Paleozoic suture [Liu, 1988; Sobel and Arnaud, 1999; emphasizes the spatial difference between the older and Yang et al., 2001]. youngersuitesofplutons,andalsosuggeststhatmagmatism migrated southwestward between (cid:1)480 Ma and (cid:1)440 Ma. 4.4. Early Paleozoic Magmatic Activity Thesamesouthwardyoungingtrendisalsoapparentinages [39] Early Paleozoic plutons in the study area are ofplutonsintheAltunShanandalongtheAltynTaghfault, emplaced into the mid-Proterozoic and older(?) basement but these ages are not shown on Figure 15 because of described above, and into lower Paleozoic marine strata, uncertainties in their projected paleoposition along the line arc-type volcanic rocks, and accretionary complexes. The of section. plutons can be divided into an older suite of dioritic to [43] ThepatternsofearlyPaleozoicplutonismintheregion, granitic bodies that occur throughout the study area, and a combinedwiththeconstraintsderivedfromgeochronologic younger suite of granitic to leucogranitic bodies that occur studiesofPrecambrianrocksintheregion,asoutlinedabove, only along the northeastern margin of the Qilian Shan and yield the preferred model shown in Figure 16. Following Altun Shan (Figure 2). SobelandArnaud[1999],wesuggestthatOrdovician-Early [40] The older plutons range from (cid:1)482 to (cid:1)425 Ma Silurian magmatism in the region occurred in a northeast (Early Ordovician to mid-Silurian according to the time- facing magmatic arc that formed in response to southwest