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Geochemistry of Crustally Derived Leucocratic Igneous Rocks from The JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. BI3, PAGES 21,483-21,502, DECEMBER 10, 1990 Geochemistryof CrustallyDerived Leucocratic Igneous Rocks From the Ulugh MuztaghArea, NorthemTibet andTheir Implications for the Formation of the Tibetan Plateau L. W. MCKENNA 1 Departmentof Earth, Atmosphericand Planetary Science,Massachusetts Institute of Technology,Cambridge J. D. WALKER Department of Geology, Universityof Kansas,Lawrence Igneous rocks collectedfrom the Ulugh Muztagh, 200 km south of the northem rim of the Tibetan Plateau (36ø28'N, 87ø29'E), form intrusive and extrusivebodies whose magmas were producedby partial melting of upper-crustal,primarily pelitic, source rocks. Evidence for source composition includeshigh initial 87Sr/86Sr ratios (-0.711 to 0.713), 206pb/204pb ratios of 18.72,207pb/204pb of 15.63and 208pb/204pb of 38.73. The degree of meltingin thesource region was increased by significant heating via in situ decay of radioactive nuclides; a reasonable estimate for the heat productionrate in thesource is 3.9x 10'6 W/m3. Thecrystallization ages and cooling ages [Burchfiel et al., 1989] of the earliest intrusive rocks within the suite suggestcrustal thickening began in the northernTibetan Plateaubefore 10.5 Ma, with maximum averageunroofing rates in this part of the Tibetan Plateaufor the period between 10.5 and 4 Ma at approximately< 2 mm/yr. The Ulugh Muztagh flows are at the northernedge of a widely distributedfield of Plio-pliestocenevolcanic rocks in the north-central Tibetan Plateau. The crustally derived rocks described here are an end- member componentof a wide mixing zone of hybrid magmas; the other end-memberforms mantle- derived, potassicbasanites and tephrites exposedin the central section of the Plio-Pleistocenefield. The compositional trends in these belts strike east-west, at high angle to the N30E strike of the Plateau itself. Considerationof the chemical data and publishedgeophysical data argue that the sub- Plateau mantle is mechanically detachedfrom the overlying continental lithosphere, and that in this section of the plateau the thermal structureof the asthenosphereis not responsiblefor the formation or maintenanceof the plateau's topography. INTRODUCFION allow inferencesto be madeon boththe compositionof the Tibetan crust and the character of Miocene to Pliocene The collision(between 40 and 50 Ma) and subsequent magmatism.Aspects of the structuralgeology in the Ulugh convergence of the Indian subcontinent and the Eurasian Muztagharea are discussedby Burchfielet al. [1989]. continentcreated both the HimalayanRange and the Tibetan The Tibetan Plateau is typically divided into three Plateau[Molnar, 1988]. Althoughrecent years have seen an structuralblocks; from north to south these are the Kunlun, enormousincrease in our knowledgeof the geologyof the Qiangtangand Lhasablocks (Figure 1; Changet al. [1986]). greater Himalayan orogen, little is still known of the The JinshaSuture, dated as end-Triassic[Harris et al., 1988], geologyof the Tibetan Plateauand its surroundingareas. is the surfacetrace of the north-dipping[Harris et al., 1988] Our limitedknowledge is derivedprimarily from teleseismic or south-dipping [Pearce and Mei, 1988] subductionzone data, a smallnumber of seismiclines, and remotesensing which separatedthe Kunlun and Qiangtangterrains prior to studiesof the area[Molnar, 1988] and, with theexception of collision. As describedby Burchfielet al. [1989], Ulugh the Royal Society-AcademicaSinica Geotraverse results, few Muztaghlies astridea seriesof ophioliticfragments which directedfield observationsare available. The samplesof may mark the western extension of the Jinsha Suture into igneousrocks discussedin this report were collectedfrom this area. The mean age of crustalmaterial in the Kunlun the Ulugh Muztagh region of the north-central Tibetan blocksome 500 km eastof UlughMuztagh, along the Royal Plateau(Figure 1) and providean opportunityto constrain SocietyGeotraverse route, is constrainedby the isotopic both the thermal structureof the Tibetan crust and mantle, composition and ages of syn-collision to post-collision and the rates of crustal unroofing within the northern granitiods exposed in the Kunlun Mountains to be mid- Tibetan Plateau. In addition,analysis of thesesamples Proterozoic[Harris et al., 1988]. Neodymiummodel ages of sedimentaryrocks collectedalong the Geotraverseroute also ! Nowat the Department of Geology, University of Kansas, give mid-Proterozoicages. Accordingto the geologicalmap Lawrence of the Tibetan Plateau [Ministry of Geology and Natural Resources, 1980], country rocks of the Kunlun terrain Copyright1990 by the AmericanGeophysical Union includeCarboniferous to Permianrocks juxtaposed with units Paper 90JB01427 of Triassicand Cretaceousage. Southof the suture,upper 0148-0227/90/90JB-01427 $05.00 Triassic [Burchfiel et al., 1989] to lower to middle 21,483 21,484 MCKENNA AND WALKER: GEOC'ttEMISTRYOF LECOUCRATICIGNEOUS ROCKS 50ø/• T USSR T T T T T T 500krn o ß ß ß 30ø• • ß ß . ß ß .... CHINA 2oo INDIA 75øE -L ' "• 85o • J. •5 ø 95 ø 105 ø Fig. 1. Locationmap and regionalgeologic setting of Ulugh Muztagh ("Great Snowy Mountain"), which is situated 200 km south of the northern edge of the Tibetan Plateau, central Asia. Areas of Cenozoic magmatismare shaded,basins are stippled,strikeslip faults are shown with half arrows indicating relative movements,thrust faults are shownwith barbs on upper plate. Dashedlines with open barb are suturezones; from north to south they are the Jinsha,Banggong and Indus-ZangpoSutures, respectively. International boundariesare shownby thin lines, degreesNorth latitude and East longitudeare also shown. Cretaceoussediments form the majorityof the pre-Cenozoicleucitites, phonolites and pyroxeneandesites; these samples rocks. The apparentdiscrepancy of the age of the suture are discussed in more detail below (see Regional (end Triassic)and that of rocks which it truncates(middle Relationships). Additional major element determinationsof Cretaceous)is probablydue to the poorlyknown ages of the a subset of these samples, along with new trace element sedimentaryunits in this largelyinaccessible region. data, were includedby Pearce and Mei [1988], who discussed The broadstructure of the TibetanPlateau as illustratedby the major and trace element chemistryof volcanicrocks geophysicaldata was recentlyreviewed by Molnar [1988]. encounteredalong the 1985 Royal Society-AcademiaSinica From his review of seismic data, Molnar concluded that the GeotraverseRoute, approximately500 km east of Ulugh depth to the Moho in the Ulugh Muztagh region of the Muztagh. north-centralTibetan Plateau(36ø28'N, 87ø29'E)is some65 Country rocks in the study area proper consist of + 5 kin. While sucha crustalthickness would generally be metamorphosed,openly folded, Triassicsedimentary rocks, consideredabnormally high, it appearsto be 5 kin, and intrudedby granitoidrocks of Mioceneage [Burchfielet al., perhaps10 km, thinnerthan the crustof surroundingareas 1989]. Theseintrusives were thoughtby theseworkers to withinthe plateau. Molnar [1988] alsoconcluded that upper be responsiblefor the andalusite-grade,regional-scale asthenospherictemperatures for this area are higher than contact metamorphismof the country rocks in the area. surroundingareas, and cautiouslysuggests temperatures at Mineralseparates of theseintrusive rocks give 40Ar/39Ar the crust-mantleboundary may be as high as 1300 K. The coolingages of 10.5 to 8.4 Ma ([Burchfielet al., 1989], see neotectonicsof this area of the plateauare dominatedby of this paper for a summaryof the geochronologicdata). north-directed thrusting of the Kunlun Shan and Tibetan The intrusive rocks are overlain, above a local Plateauover the Tarim Basinat a rate estimatedby Molnar et unconformity,by boulderconglomerate overlain in turnby a al. [1987] at 6 + 4 mm/yr. sequenceof now dissectedextrusive rocks, principally ash The earliest(and, until recently,the only) reportsof the flow tuffs and flows. These flows are dated at 4.0 + 0.1 Ma geology of the area are those of Littledale [1886], (40Ar/39Ar,biotite fusion and K-feldspar, [Burchfiel et al., Backstrom and Johanssen[1907] and Norin [1946]. 19891). Somewhatmore recently, Deng [1978] reportedthe Field relationsallow divisionof the magmaticrocks into petrologyand major elementchemistry of a numberof threegroups: (1) intrusivesamples BKSP, UBTG, 2MGR, samples of Plio-Pleistocene volcanic rocks from a transect and QTD which intrudethe metamorphosedbasement; (2) southof UlughMuztagh. These rocks included ultra potassic samples (defined below as the PotassiumPoor samples) MCKENNAAND W,M.,KER: GEOCItEMISTRY OF LECOUCRATIC IGNEOUS ROCKS 21,485 which crop out as small plugs (UM10, KSPO) and dikes in Appendix 1. The extrusive samples, with the exception (QTL) that intrude the Triassic sandstoneseast of the Ulugh of UMQP, all contain similar phenocryst assemblages of Muztagh; and (3) the capping Ulugh Muztagh extrusive quartz+sanidine+plagioclase(An20_40) +cordierite+biotite+ series(MV1B, UM1B, MV2, UMVU, UM3V, UMQP). tourmaline (rare) in microlitic to glassy groundmasses.The rocks occur as ash flow tuffs (UMVU, UM3V, MV2, and UMQP) and rhyolitic flows (UM1B, MV1B). Similar modal abundances for five of the samples suggests that Samples were taken as 1 to 3 kg blocks from outcropsor, syneruptionalcrystal sorting was not significant in the ash in the caseof samples UM10, BKSP, and UBTG, in moraine flow tuffs, except for UMQP which has no modal plagioclase and stream depositsbelow outcrops,using
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