Petrogenesis Of Triassic Granitoids And Rhyolites In The East Kunlun Orogenic Belt And Their Tectonic Implications

The East Kunlun Orogenic Belt(EKOB),an important part of the Greater Tibetan Plateau,is an ideal region for understanding the tectonic evolution of the Anyemaqen Ocean.Here,we present zircon U-Pb ages,bulk-rock major and trace element analyses and Sr-Nd-Hf isotope compositions on representative samples of the syn-collisional Dulan batholith at the eastern end of the EKOB.The zircon U-Pb age data indicate that the bulk of the Dulan batholith was emplaced at 240-235 Ma.The granitoids have high-to medium-K and metaluminous characteristics.They are enriched in large ion lithophile elements(LILEs)and light rare earth elements(LREEs)and depleted in some high field strength elements(HFSEs,e.g.,Nb and Ta),while having a flat heavy REE(HREEs)pattern.The mafic magmatic enclaves(MMEs)share the same age,mineralogy and indistinguishable Sr-Nd-Hf isotopes with their granitoid hosts except for the higher HREE abundances.We show that the MMEs represent cumulate formed at earlier stages of the same magmatic system.The trace element data(e.g.,Nb/Th,Ta/U)and inherited mantle isotopic characteristics of the Dulan batholith are also consistent with an origin via partial melting of the last fragments of underthrusting ocean crust.Simple mass balance calculations using the Sr-Nd-Hf isotopic data show that ~ 85% Paleo-Tethys MORB and ~15% mature crustal material(the Proterozoic gneiss of the study area)contribute to the source of the granitoids.The Dulan batholith shows compositional similarities to the bulk continental curst with inherited mantle isotopic signatures.The syn-collisional felsic magmatism must have contributed to the net continental crust growth in the EKOB.We infer that the Kunlun and Qinling orogens may actually be one single orogen offset later by the Wenquan fault system.Based on the research on the typical peralkaline rhyolites from the Glass House Mountains in Australia,we collected Triassic volcanic rocks from the EKOB.The rhyolites that exposed near Xiangride area have spectacular columnar joints.The zircon U-Pb dating result shows that these rhyolites are formed in the Late-Triassic(~ 209Ma).According to the high Nb and Ta content and very low Ba,Sr,Eu,P and Ti content features,we infer that the petrogenesis of the rhyolites are similar to the typical peralkaline rhyolites which are originated from mantle-drived basaltic magma.The widespread felsic volcanic rocks which belong to the Elashan formatin in the EKOB erupted during Late-Triassic(232Ma-225 Ma).Compared with the near-peralkaline rhyolites,we argue that these volcanic rocks are originated from the lower crust under the post-collision extentional tecnonic setting.