Petrogenesis Of The Gabbro-granite Composite Plutons In The Eastern Segment Of The Gangdese Belt And Its Tectonic Implications

Magmatic arc developed on the convergent plate margins recorded the information of crust-mantle interaction and provides insights into regional magmatism and tectonics.The Gangdese magmatic arc has now become an ideal object in tracing the Neo-Tethys subduction and subsequently India-Eurasia collision.In this paper,we report petrography,geochronology,lithogeochemistry,Sr-Nd-Hf isotopic data,and mineral chemistry for the Siborongqu composite pluton and Zhengga composite pluton,in the eastern segment of the Gangdese belt,aiming to constrain the petrogenesis and to discuss its tectonic implications.The investigation results indicate that the Siborongqu composite pluton is composed of Early Jurassic gabbro-granite complex(188?185 Ma)and Eocene granitoids(?47 Ma).Mafic microgranular enclaves(MMEs)with various plastic shape are generally dispersed in both stages of granitoids.The Early Jurassic complex consists predominantly of hornblende gabbros and tonalities.They can geochemically be grouped into a set of calc-alkaline magmatic rocks with arc signatures,and show depleted Sr-Nd-Hf isotopic compositions.The Eocene granitoids constitute dominantly of monzogranites and granodiorites.They are more alkaline and show somewhat enriched Sr-Nd-Hf isotopic compositions with respect to the Early Jurassic tonalities,and can be grouped into calc-alkaline or high-K calc-alkaline I-type granites.Zhengga composite pluton is mainly composed of Late Cretaceous hornblende gabbros gabbro-granite complex(?91 Ma),and Late Cretaceous quartz diorites(?85Ma)and Paleocene biotite granites-garnet-bearing monzogranites-garnet-bearing two-mica monzogranites(64?61 Ma).Late Cretaceous hornblende gabbros can geochemically be grouped into calc-alkaline magmatic rocks with arc signatures,and show depleted Sr-Nd-Hf isotopic compositions.While the quartz diorites show an obvious adakitic affinities.Paleocene biotite granites-garnet-bearing monzogranitesgarnet-bearing two-mica monzogranites can be grouped into calc-alkaline I-type granites with arc signatures and show depleted Sr-Nd-Hf isotopic compositions.It is worth noting that some of the Paleocene granitoids show tetrad REE patterns.Combined with our data and previous studies,we concluded that the Early Jurassic gabbroic rocks were generated by partial melting of the overlying depleted mantle wedge which was metasomatized by aqueous fluids released from the subducted slab.The coexisting Early Jurassic tonalites were produced by mixing of felsic magmas derived from partial melting of juvenile lower crust and mafic magmas related to the gabbroic rocks.The petrogenesis of Early Jurassic complex is controlled by the northward subduction of the Neo-Tethyan oceanic slab.The Late Cretaceous gabbroic rocks were generated by partial melting of the overlying depleted mantle wedge which was metasomatized by aqueous fluids released from the subducted slab,as well as the Early Jurassic gabbroic rocks.The Late Cretaceous adakitic quartz diorites were derived from partial melting of subducted slab.The quartz diorites found in the region are consistent with the mid-ocean ridge subduction model,given that the rocks indicate high temperatures and require mantle contributions that could be attributed to mantle up-welling through a slab window.As for the Paleocene granitoids,which were derived from partial melting of juvenile lower crust.The trace element data constrain the parental magmas to having undergone extensive magmatic differentiation.Early Paleocene slab roll-back would have significantly enhanced the upwelling of asthenosphere and supplied a long-lived heat source for magmatic differentiation in southern Lhasa.The Eocene granitoids were derived from partial melting of juvenile lower crust triggered by underplating of a contemporaneous mafic magma,and with somewhat involvement of enriched components from the subducted Indian continental crust.From the perspective of tectonic evolution,the Eocene granitoids correspond to the upwelling of asthenosphere after break-off of the Neo-Tethyan oceanic slab.