Studies On Geological Characteristics And Forming Age Of Southern Xiangjia Mountain Granitic Pluton At East Section Of East Kunlun Orogenic Belt

The granite formed in orogenic belt can record information of each process clearly, andstudies on whose source region features and forming mechanism have some vitalsignificances to solve scientific questions, such as deep dynamics mechanism and interactionbetween crust and mantle during collisional process. East Kunlun Orogenic Belt(EKOB) issituated at inner Qinghai-Tibetan plateau with gigantic space occupied by impulse-intrudedand multiple-lithology granites, especially widespread for Late Paleozoic–Mesozoic.The dissertation is ideologically guided by the theory of plate tectonics and continentaldynamics research, synthesizing the methods of field geological mapping indoor analysis torock samples, and studied field geology, petrography, geochemistry and LA-ICP-MS zirconU-Pb chronology of Southern Xiangjia Mountain Granitic Pluton at East Section of EKOB,and the key discussion is about their petrogenesis, diagenetic sources, forming setting andgeological importance.Petrography study showing that the plutons are composed of granodiorite, biotitemonzonitic granite, K-fieldspar granite and porphyritic monzonitic granite, in which there aremany mafic microgranular enclaves(MME) in the former two rocks. The rocks have highcontent in Al and Mg, and in which MgO content and MF*number of biotite are between thatof mantle-origin and crust-origin. Through amphibole whole-Al geothermometer, it isevaluated that the intruding depth of the host rocks is about15.47~18.25km, while thecrystallizing depth of MME is about18.31~20.78km.There are geochemically conspicuous differences between host rocks and MMEs ofdifferent rocks, which have similar trend in trace elements and REE distribution curves, suchas enrichment of LILE(Cs、Rb、Ba、U、K) and depleted in HFSE(Nb、P、Ti) except for extent.The host rocks are belong to meta-aluminous–poor peraluminous(A/CNK＜1.1), andwhose DI, FL and MF indicate a ascending trend like granodiorite�'biotite monzoniticgranite�'porphyritic monzonitic granite�'K-fieldspar granite.The LA-ICP-MS zircon U-Pb age of the host rock and MME are (251±1.9)Ma and(252.8±3.0)Ma respectively, indicating their forming age are the same in late Permian to earlyTriassic.Petrography, geochemistry and age data of the plutons indicate that the rocks arecharacterized by volcanic arc，and are the products of fractional crystallization after magmamingling when crust-partial-melt-origin magma ascending. The magma is resulted fromunderplate in crust of mantle-partial-melt-origin basaltic magma when fluid ascending to upper mantle wedge during crust dehydration, which is considered to have relationship withnorthward subduction of Buqingshan-A’nyemaqen ocean in late Permian–early Triassic.