Chronology And Geochemistry Of Intrusive Rocks In The Aweng-Tso Area And Its Geological Significance

There are two NWW-SEE trending intrusive batholiths,namely,Aweng-Tso batholith and Pa'a batholith in Aweng-Tso area which locates in the central-northern Lhasa Terrane.The study of intrusive batholiths in Aweng-Tso area is still limited.Lack of detailed petrological,geochronological and geochemical study directly confine the interpretation of magmatic activity and metallogenic dynamic background.Based on geological survey,we collect the host granodiorite and dioritic enclaves of Aweng-Tso batholith and Pa'a batholith respectively,and carry out whole-rock geochemistry,zircon U-Pb geochronology and Hf isotopes analysis.The zircon U-Pb geochronology results show that the Aweng-Tso batholith emplaced at early Cretaceous(?119Ma)while the Pa'a batholith emplaced at late Jurassic(?156Ma).Accroding to the geochemical results,the host granodiorite of Aweng-Tso batholith is calc-alkaline I-type granite and its alumina saturation indices is metaluminous to peraluminous,besides,the host granite exhibits high Sr abundance and low Y abundance,showing typical adakites characteristics and its enclaves are metaluminous,gabbroic-dioritic rocks.The host granodiorite of Pa'a batholith are peraluminous,highly potassic calcalkaline ?-type granites and its dioritic enclaves are metaluminous dioritic rocks.Both the host granodiorite and dioritic enclaves of Aweng-Tso batholith and Pa'a batholith are rich in light rare earth element and large ion lithophile element,while relatively depleted in heavy rare earth element and high field strength elements,with insignificant europium anomaly.The geochemical characteristic of Aweng-Tso batholith and Pa'a batholith coincides with that of arc-related magma.The results of zircon Hf isotope of the host granodiorite and dioritic enclaves in the Aweng-Tso batholith exhibits positive episilon ?Hf(t)values,and relatively young crustal model ages(0.57?1.08Ga).We can conclude that Aweng-Tso adakitic magma showing the characteristics of new crust.While the measurement of zircon Hf isotope in the granodiorite exhibits negative episilon ?HKt)values,and relatively old crustal model ages(1.69?1.98Ga),indicating that the magma generated from the melting of ancient crust crystalline basement.Both the host granodiorite and dioritic enclaves of Aweng-Tso batholith and Pa'a batholith emplaced at the same time and share the identical geochemical characteristics.Those characteristics above and petrography study indicate that the cause of formation of enclaves in Aweng-Tso batholith and Pa'a batholith are magma-mixing.There are magmatic rocks which emplaced at late Jurrasic to early Cretaceous in the central and northern Lhasa Terrane.Together with previous study of these magmatic rocks and this paper,we can conclude that there is southward subduction of BangongNujiang oceanic crust from late Jurassic to early Cretaceous and there are more and more mantle-derived materials joining-in during the southward subduction.Combining the connection between ore deposits and its crustal nature in Lhasa terrance,we suspect that in Aweng-Tso area,the existence of new crust to the north of Shiquanhe ophiolitic melange belt indicates potential of forming copper-gold deposit,while the existence of ancient crust to the south of Shiquanhe ophiolitic mléange belt indicates potential of the forming lead-zinc-silver-iron polymetallic deposit.