Geochemistry And Geochronology Of The Granitic Rocks In Gongzhucuo,Western Tibet: Constraints On The Geodynamic Setting

In the western Yarlung zangbo suture,there are widespread outcrops of Zhongba terrane between southern and northern tectonic mélange.In recent years,Zhongba terrane has become a research topic,and its genesis and evolution are controversial.Study of Zhongba terrane have profound significance.The Gongzhucuo granitoid pluton,located in the Zhongba terrane,southern tectonic mélange,consists mainly of two-mica granite.The two-mica granites of the northern granitoid pluton,are composed by quartz(about 40 percent),plagioclase(about 35 percent),muscovite(about 10 percent),biotite(about 7 percent),K-feldspar(about 5 percent),and accessory minerals such as zircon,garnet,sphene ect(about 3 percent).The samples of western Gongzhucuo granitoid pluton have almost the same petrological characteristics as the northern two-mica granite.Zircon U-Pb ages of 439-470 Ma for the samples from the northern two-mica granite indicate that this plunton was emplaced during the Middle-Late Ordovician.The two-mica granites of the northern Gongzhucuo granitoid pluton are high-K cale-alkaline and peraluminous,with K2O/Na2 O > 1 and A/CNK=1.47~1.82.These rocks are enriched in large-ion lithophile elements(such as Rb,K ect),light rare-earth elements [(La/Yb)N=4.79~19.37],and depleted in high field-strength elements(such as Nb,Ta,Zr,Ti ect).They show similar chondrite normalized REE patterns,with slight negative Eu anomalies(δ Eu=0.72~0.84).Such geochemical signatures indicate that the Gongzhucuo granitoid pluton are S-type granite in character and were derived mainly from anatexis of ancient crustal materials(such as greywacke).Preliminary mantle-normalized trace element patterns spider diagram,R1-R2 diagram,Rb-(Y+Nb)and Nb-Y diagram indicate the Gongzhucuo granitoid pluton is formed in sys-collision setting.The Gongzhucuo granitoids are comparable to the coeval Early Palaeozoic volcanic rocks reported in the blocks of the southern Tibetan Plateau.It may suggeste that the Gongzhucuo granitoids are derived from that the collision between the Zhongba terrane and the north margin of the eastern Gondwana after the proto-Tethyan Ocean crust subducted.