Late Cretaceous Magmatism And Its Geological Significance In Shenma Area,Southern Tibet

The Late Cretaceous extensive magmatic“flare-up” event occurred in southern Lhasa Terrane has been widely regarded as a record of the northward subduction of the Neo-Tethan Ocean.In this paper,systematic petrology,LA-ICP-MS zircon U-Pb geochronology,Hf isotope,whole rock and mineral geochemistry of the Shenma pluton from the southern Lhasa terrane have been carried out,based on the existing research data,the characteristics of magma source and genetic information are analyzed in order to provide more evidence for the deep dynamic mechanism of magmatism in the late Cretaceous in the southern Lhasa terrane.The samples obtained in this paper are mainly diorite,dark diorite dyke and granite vein.The diorites are mainly composed of plagioclase,potash feldspar,biotite,hornblende and quartz.The dark diorite dykes are mainly composed of plagioclase,orthoclase,biotite,hornblende and quartz.The granites are mainly composed of orthoclase,quartz,plagioclase,hornblende and biotite.LA-ICP-MS zircon U-Pb dating results show that the diorite age is88.0±1.0 Ma,the dark diorite dike age is 79.1±0.7 Ma,and the granite vein age is 66.6±4.2 Ma.Diorite belongs to high potassium calcium alkaline series,high aluminum(Al₂O₃=15.15%?17.10%),high Mg^#(44.37?47.85),weak Eu negative anomaly,with positive?Hf(t)(12.2?14.2),the biotite shows magnesium rich characteristic,the MgO content is 10.43%?11.50%,belongs to magnesium biotite,hornblende is rich in magnesium,calcium,iron,potassium and sodium;The dark diorite veins are medium potassium calc-alkaline series,high aluminum(Al₂O₃=15.38%?17.36%),high Mg^#(40.51?45.38),high La/Yb,Sr/Y ratio,low Y content and HREE content,which are consistent with the geochemical characteristics of Edak rock,the content of biotite Fe O is 19.96%?20.31%,which belongs to iron biotite;Granite veins belong to high potassium calc-alkaline series,with relatively low Mg^#(27.71?35.21),with Eu weak negative anomalies.The samples are enriched in Rb,Ba,Th,U,K and other large ion lithophile elements,and depleted of Nb,Ta,Ti and other high field strength elements.Isotopic analysis indicates that the late Cretaceous magmatic rocks in Shenma area have the characteristics of depleted mantle source.The dioritic rocks are regarded as the products of partial melting of mantle wedge materials,and their formation is controlled by the northward subduction of the Neo-Tethys oceanic ridge.Granite veins(66Ma)in the later period were in the arc environment associated with normal northward subduction of the New Tethys Ocean.