Geochronology,Geochemistry and Petrogenesis of the Granitoid Porphyries from Jiama Ore Deposit in Gangdese Belt

Meng Yuanku; Ma Shiwei; Xu Zhiqin; Chen Xijie; Ma Xuxuan

Abstract：The petrogenesis of Miocene porphyries is still under debate,involving four different genetic models,such as,partial melting of residual oceanic crust,partial melting of thickened lower crust,partial melting of sub-continental lithospheric mantle and partial melting of metasomatized mafic lower crust related to subducted fluids.In order to clarify the petrogenesis and tec-tonic setting,petrological analyses and zircon LA-ICP-MS U-Pb dating in this study were carried out.In addition,whole-rock major and trace elements were analyzed by means of XRF and ICP-MS methods.The results demonstrate that the porphyries formed at 16.7—14.4 Ma,showing adakitic geochemical features.Geochemical characteristics,trace elemental ratios and dis-crimination diagrams suggest that magma source of the granitoid porphyries was derived from partial melting of the lower ju-venile crustal material,whereas magma source of gabbro diorite porphyrite was sourced from the enriched lithospheric mantle. Together with published data,it is proposed that the Qinghai-Tibetan plateau was in the tectonic transformation from compres-sion to extension or strike-slip during 1 8—1 3 Ma in the early stage of Miocene.The ore-bearing porphyries ascended through vertical fractures or faults and mingled with mid-upper crustal material generating granite porphyry,monzonitic granite por-phyry,granodiorite porphyry and quartz diorite porphyrite.On the contrary,the coeval magma derived from the lithosphere mantle formed gabbro diorite porphyrite.Ore-bearing hydrothermal fluid was driven by tectonic stress and heat flow from mag-ma took place at the sandstone slate and hornfel of the Linbuzong Formation,and marble and limestone of the Duodigou For-mation characterized by inter-bedded tectonic fracture belts and collapse locations of folds,forming skarn-type copper poly-metallic ore deposits.