Petrogenesis And Tectonic Implications Of The Early Cretaceous Granitoids In Xiongmei Area,Tibet

The northward subduction of the Bangonghu-Nujiang Tethys Ocean has been widely accepted,supported by a large amount of geological evidence.However,the southward subduction of the Bangonghu-Nujiang Tethys Ocean is still debated.During the subduction process of the oceanic plate,a arc magma-forearc basin-accumulation wedge-trench system will be formed.Therefore,the arc-trench system is the basic method for identifying oceanic slab subduction which occurred in the genlogic history.This paper investigates the petrogenesis of the Early granitioids by zircon U-Pb geochronology,geochemistry and Sr-Nd-Pb-Hf isotope study of a set of peraluminous granites in the Xiongmei-Bange area of the North Lhasa block,and further provides new evidence for the existence of southward subduction of the Bangonghu-Nujiang Tethys Ocean.There are five Early Cretaceous granitoids in the study area.They are mainly monzonitic granites and granodiorites,and are alkaline to calcium alkaline series and metaluminousto peraluminous.Chondrite-normalized REE patterns show enriched light rare earth elements,but depleted heavy rare earth,with slight to obvious Eu negative anomaly.The spider diagram of the trace element display enrichment of large ion lithophile elements,but depletion of high field strength elements.The ages of Shenzhong rock mass,Resuo rock mass,Tangjiangqiongguo rock mass,Bange rock mass and Bamucuo rock mass are 113.3±1.4 Ma,116.2±2.1 Ma,110.1±1.1Ma,112.2±2.8 Ma,116.7±1.2,Ma,respectively.Five rock masses are coeval.The values of(⁸⁷Sr/⁸⁶Sr)_i are from 0.7077 to 0.7148 with an average value of0.7110;the values of?_Nd?t?are from-11.2 to-5.9 with an average value of-8.6,the?_Hf?t?values are from-9.7 to 3.8 with an average of-2.1.The values of ²⁰⁸Pb/²⁰⁴Pb Pb are from 38.90 to 40.11 with an average value of 39.33;The values of ²⁰⁷Pb/²⁰⁴Pb are from 15.66 to 15.77 with an average value of 15.72;The values of ²⁰⁶Pb/²⁰⁴Pb are from 18.69 to 19.91 with an average value of 19.01.Sr-Nd-Hf-Pb isotopic characteristics of the granites in the study area demonstrate that the peraluminous rocks are mostly possibly soruced from the oceanic sediments from the accretionary weged and the metaluminous rocks are probably form the lower crust of the Northern Lhasa Terrene with involvement of a certain amount of oceanic sediments.The identification of the accretionary wedge is the most direct evidence for the southward subdution of the Bangonghu-Nujiang Tethys ocean.Based on provious works,we conclude that Bangonghu-nujiang ocean southward sudcted during middle to late Jurassic;the that Bangonghu-nujiang ocean closed approximately at 140 to 130 Ma;at approximately 130 Ma the subdcted Bangonghu-nujiang oceanic slab began to roll back with small amount of magmtism;until to approximately 115 Ma,the subdcted Bangonghu-nujiang oceanic slab broke off companied with a mass of magmatisms.