Origin And Petrogenesis Of Mantle-derived Magmatism From Oceanic Subduction-related And Collisional Porphyry Deposits In Gangdese Belt

Mantle-derived magmatism triggerd by lithosphere breakoff or the tearing or thinning,play an important role on formation of mineralization.Both oceanic subduction-related and collisional porphyry deposits can be found within Gangdese belt.Gangdese belt,thus can be regarded as a ‘natural laboratory' of study for deep process between two type of porphyry deposit.However,previous studies mainly focused on geochronological and tectonic setting of felsic magmatic rocks within deposits,ignored the importance of mafic magmatism and related deep process beneath the Gangdese belt.This study selected Jurassic hornblende gabbro from oceanic subduction-related deposits and Miocene lamprophyres from collisional porphyry deposits to reveal their origin and petrogenesis,lithospheric architecture and associations between subduction-related or collisional porphyry depositsIn post-collision environment,widespread Miocene lamprophyres and ultrapotassic rocks and their entrained xenoliths provide information on the composition,structure,and thermal state of the sub-continental lithospheric mantle in southern Tibet.The ultrapotassic rocks along the Lhasa block delineate two distinct lithospheric domains with different histories of depletion and enrichment.The eastern ultrapotassic rocks reveal a depleted,young,and fertile lithospheric mantle.The western ultrapotassic rocks and their peridotite xenoliths reflect a refractory harzburgitic mantle refertilized by ancient metasomatism.These data integrated with seismic tomography suggest that upwelling asthenosphere was diverted away from the deep continental root beneath the western Lhasa block,but rose to shallower depths beneath a thinner lithosphere in the eastern part.Heating of the lithospheric mantle by the rising asthenosphere ultimately generated the ultrapotassic rocks with regionally distinct geochemical signatures reflecting the different nature of the lithospheric mantle.In oceanic subduction environment,we report Xietongmen diorites,formed at 185 Ma.Their whole-rock major-and trace-element compositions,Sr-Nd isotopes,in situ zircon Hf-and plagioclase Sr-isotope compositions,and major-and trace-element compositions of amphiboles to constrain the magmatic emplacement and evolution related to the porphyry Cu-Au mineralization in the Xietongmen region.These isotopic results are similar to coeval mantle-derived hornblende gabbros and the Sangri volcanic rocks in the Gangdese belt.Amphibole clots in the Xietongmen diorites have higher MgO and Ni contents,and REE patterns different from those of magmatic and idiomorphic amphiboles in the host diorites,suggesting that the clots are xenocrysts.All the petrological and geochemical evidence suggests that the Xietongmen mineralized diorites were derived from a Jurassic hydrous mantle wedge induced by slab dehydration.These magmas contain higher contents of water and other volatiles and have higher oxidation states.These conditions favored the transfer of metals(such as Cu,Au)to the upper crust,and then formed the Jurassic porphyry Cu-Au deposits.We recently identified hornblende gabbro in the Dongga area,southern Lhasa sub-block.This hornblende gabbro is dominated by hornblende and plagioclase,dated at Early Jurassic(ca.180–190 Ma).It has depleted isotopic signatures.Our results integrated with published data suggest a model of Gangdese juvenile crustal growth by a subduction-related water-enriched mantle wedge.The hydrous partial melting of the lithosphere mantle was triggered by the dehydration of a Neo-Tethyan oceanic slab.This mafic magmatism emplaced in the middle-lower crust of intraoceanic arcs or active continental margins,leading to Jurassic juvenile crustal growth in southern Tibet.