Petrology, Geochemistry And Petrogenesis Of The Granitoid In Southern Yanhu Of Gangdese, Tibet

Gangdese Belt in Tibet has long been perceived as the frontier of the Eurasian Continent before the northward drift-collision of Indian Continent in Cretaceous. However, its lithospheric structure, impact on previous subduction, the growing histories and the contact with Northern Qiangtang Terrane have resulted in confusion and dispute. The rock samples from the granitoid pluton and its association collected from Southern Yanhu, Northern Gangdese in Tibet are firstly investigated in this study, in which the subjects of U-Pb isotope chronology, petrology, elemental geochemistry and Sr-Nd-Hf isotopic geochemistry are involved. Correlated with the discovered U-Pb dating, geochemical characters of the Hf isotope, the purpose of this study endeavors to bring up the geological constraints for the structural activities and magmatic evolution of the Northern Gangdese and Bangong–Nujiang Tethyan Ocean.The 15 representative samples, in terms of lithology, are divided into 9 biotite monzogranites, 3 basalts and 2 rhyolites. The method of Zircon U-Pb dating reports the age of biotite monzogranites 107.2±2.6 Ma, basalt 116.9±2.8 Ma, as consistent as other sources of data. The biotite monzogranite samples from the Southern Yanhu Pluton compositionally characterize itself by high content of SiO2 (73.08%~78.38%), high K2O (4.3%~4.56%) and low P2O5 (0.02%~0.11%), whereas the contiguous basalt bears the chemical compositions of SiO2 45.9%~51.15%, K2O 0.31%~1.19%, MgO 6.24%~11.25%,Mg# value 58~65. The rhyolite has apparently high values of SiO2 component (75.54%~75.56%), whose Al saturation index (A/CNK) of felsic sample is in the range of 1.01~1.07. With regard to their trace element features, the biotite monzogranite samples are detected to be enriched in Rb, Th, U and Pb, but severely depleted of Ba, Nd, Ta, Sr, P, Ti and Eu, which is categorized into I-type granite due to the fact that it is slightly peraluminous and has been highly fractionated. Comparing with the mature continental crust-oriented materials in Gangdese emphasized by the heavily peraluminous granite from Ningzhong in Early Jurassic, the Southern Yanhu Pluton displays theεNd(t) value close to zero (-0.4~+0.6) and much lower (87Sr/86Sr)t value (0.70282~0.70624). But the measurement of zircon Hf isotope in the granite exhibits wide range ofεHf(t) values (+5.5~+9.7), and the relatively young crustal model ages (547~827Ma). The above zircon Hf isotope values and the bulk-rock of zircon saturated temperature (732~795℃) demonstrate the mantle-derived materials most likely involved in the petrogenesis of the Southern Yanhu Pluton. The southward subduction of Bangong–Nujiang Tethyan oceanic lithosphere is set as the initial geodynamic movement for the formation of the Southern Yanhu Pluton. The slabs of the oceanic crust were subjected to breaking off during the subduction, which caused the materials from the asthenosphere to uplift. The huge heat generated during the process facilities the lithospheric mantle undergo anatexis, during which basaltic magmas formed. The formation of basalts was naturally produced by the extrusion of the magmas. Meanwhile, the basaltic magmas from mantel mixed with its upper felsic magmas which were supplied by the melted crust through anatexis. One part of the hybrid melts erupted onto the surface and crystallized into the rhyolite nearby, and the rest of it contributed to the formation of Southern Yanhu Pluton through fractional crystallization.