Subduction-Type Magmatic Rocks in Post-Collision Stage:Evidence from Late Triassic Diorite-Porphyrite of Naomuhungou Area,East Kunlun Orogen

Zhang Mingdong; Ma Changqian; Wang Lianxun; Hao Fenghua; Zheng Shaojie; Zhang Lei

Abstract：The East Kunlun orogenic belt in the northeast of Qinghai-Tibet plateau contains large-scale crust-mantle mixed granitoid in the Late Permian and Late Triassic periods.It is an ideal region to investigate crust-mantle mixing and continental crust growth-recy-cling.In this paper,detailed petrography,LA-ICP-MS zircon U-Pb isotopic geochronology,mineral chemistry,rock geochemistry and Sr-Nd isotopes from the Naomuhungou diorite-porphyrites are reported.Zircon U-Pb ages indicate that the diorite-porphyrites were em-placed at ~215.4±3.6 Ma.Plagioclase phenocrysts mainly comprise andesine and a small amount of labradorite,characterized by nor-mal zoning and reverse zoning texture.Amphibole phenocrysts belong to the endenite.Naomuhungou diorite-porphyrites are character-ized by the metaluminous to peraluminous,relatively rich in magnesium(Mg#~5 1.2)and middle K-high K calc-alkaline.The rock samples display marked enrichment in LREE and LILE(e.g.Rb,Th,U and K),and depleted in some HREE and HFSE(e.g.,Nb, Ta,Ti),showing affinities similar to continental arc granites.In addition,the rocks have a homogeneous ISrvalue(0.708 6—0.708 8) and relatively lowεNd(t)((—5.3)—(—4.8)),and have a relatively older model age of TDM2(1.38—1.43 Ga).The above mineralogical and geochemical characteristics suggest that diorite-porphyrite might originate from the enriched mantle-derived magmatic underplating of the ancient basement crust and homogeneous mixing with crustal melts,with obvious MASH features.In combination with regional geology studies,it is concluded that the Naomuhungou diorite-porphyrites may have formed during the transitional period of Late A'nemaqen Paleo-Tethys ocean subduction and collision with the continent,and may represent the post-magmatic-response to the sutu-ring of the Bayan Har-Songpan-Ganzi-East Kunlun terrane.This study is also helpful for understanding the origin of the continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-col-lisional processes.