Mesozoic Volcanic And Sedimentary Rocks On The Southern Margin Of Lhasa Terrane,southern Tibet: Geochronology,geochemistry And Tectonic Implications

The Lhasa Terrane,which serves as the southernmost tectonic unit of the Asian continent,is one of the most important constituent of the Tibetan Plateau.Understanding geological history of the Lhasa Terrane prior to India-Asia collision is essential for improved models of the continent-continent collision,and can provide background information for the mineralization in the Tibetan Plateau.However,there are still ongoing issues that concern the Mesozoic tectonic background of the Lhasa Terrane,and how the juvenile crust was accreted.For this reason,we carried out detailed geochronology and geochemistry studies on the volcanics embedded in the Early Jurassic Yeba Formation and Sangri Group,and detrital sedimentary rocks deposited in the Early Cretaceous Chumulong Formation and Late Cretaceous Shexing Formation,southern Tibet,aiming to shed light on these issues.The volcanic rocks in the Sangri Group mainly comprise basalt,basaltic andesite and andesite,whereas those in the Yeba Formation are dominated by basalt,basaltic andesite,dacite and rhyolite,forming a bimodal suite with eruption age of 183-174 Ma.All the basaltic rocks(including basalts and basaltic andesite from both the Yeba Formation and Sangri Group)have an arc-like fingerprint with enriched LILEs,LREEs and depleted HFSEs.They also have depleted whole-rock Sr-Nd and zircon Hf isotopic compositions.All these results indicate that the basaltic rocks were derived from the metasomatic mantle wedge with enrichment of subduction components.The silicic volcanic rocks(including dacite and rhyolite from the Yeba Formation)did not result from fractional crystallization from the coeval basaltic magmas,but more consistent with the ones being derived from a mixed source of meta-basalts and metagreywackes through anatexis.The intermediate volcanics(andesite from the Sangri Group)have characters of Mg-andesite,which should result from the interaction between slab melts and mantle wedge peridotite.All the results indicate that the volcanic rocks formed in the back-arc region resulted from northward subduction of the Neo-Tethyan seafloor.The Cretaceous sedimentary rocks on the southern Lhasa Terrane deposited in continental arc settings.The Chumulong Formation samples yielded a depositional interval of 121-112 Ma with main detrital provenance from central and northern parts of the Lhasa Terrane.The Shexing Formation samples deposited between 88 Ma and 70 Ma with dominant detrital provenance from magmatic arcs of the southern Lhasa Terrane.The detrital sedimentary rocks illustrate a Cretaceous tectonic scenario of the southern margin of the Lhasa Terrane.In the Early Cretaceous,the Lhasa terrane was lower in the south and higher in the north with relatively low relief in topography with tropical weather.When going into the Late Cretaceous,the southern margin of the Lhasa Terrane underwent significant crustal uplift and destruction processes.Combined with existing data,we first suggest that the Lhasa Terrane had undergone multiple stages of “arc – back-arc basin system” from Triassic to Paleogene,and propose an “back-arc crustal accretion model” for the Lhasa terrane.In the mid-late Triassic,an immature arc triggered by northward subduction of the Neo-Tethyan ocean was developed in the southern margin of the Lhasa Terrane with scarce silicic rocks being generated.The Jurassic arc – back-arc basin display more lithological maturity,which more silicic rocks were generated.The Cretacesou arc – back-arc basin recorded a magmatic flare-up event,in which magmatic rocks with variable composition were formed.In our “back-arc crustal accretion model”,mantle-derived magma underplated into the back-arc region and triggered anatexis of lower crust under high temperature and low pressure conditions in the crustal extensional episode,forming silicic magmas.Subsequent crustal compression buried these juvenile materials deep in the ground to prevent from subduction erosion.As the slab roll-back went on,a new episode of crustal extension was developed in the terrane.Juvenile materials were accreted onto the southern margin of the Lhasa terrane under cycles of tectonic swiches from extension to compression,and were preserved to this day after ultimate closure of the Neo-Tethyan Ocean basin and India-Asian collision.