Geochronology and Geochemistry of the Subduction-related Rocks with High Sr/Y Ratios in the Zedong Area: Implications for the Magmatism in Southern Lhasa Terrane during Late Cretaceous

The southern Lhasa Terrane is famous for its huge magmatic belt which records the magmatism during Mesozoic and Cenozoic. Although the Mesozoic continental-margin setting in the southern Lhasa Terrane has been identified,details of this tectonic setting and the evolution history during the Late Cretaceous remain unclear. To further constrain these issues,we present zircon LA-ICP-MS U-Pb,Hf isotopic and geochemical data of the Gongbari dacites(of the Sangri Group) which intruded by Paleocene granodiorites from the eastern part of the southern Lhasa Terrane,Tibet. New age data indicate that the dacites were generated at ~95.4 Ma,which suggests the Sangri Group volcanism may last to Late Cretaceous. The Gongbari dacites are characterized by high Sr(428–758 ppm) contents,low concentration of heavy rare earth elements and Y(e.g. Yb=0.78–1.14 ppm; Y=8.85–11.4 ppm) with high Sr/Y(41.91–67.59) and La/Yb(22.64–30.64) ratios,similar to those of adakite. The rocks are calc-alkaline,metaluminous,enriched in LILEs,depleted in HFSEs,and have positive εHf(t) values(+7.7 to +11.6). The Gongbari dacites were probably produced by partial melting of young and hot subducted Neo-Tethyan oceanic crust under amphibolite to garnet amphibolite-facies conditions. Though the Gangdese Mountains may have formed before Indo-Asian collision,the southern margin of Lhasa Terrane might not go through obviously crustal thickening during the northward subduction of NeoTethyan oceanic lithosphere.