Records of Indosinian orogenesis in Lhasa terrane, Tibet

Abstract

Based on the deformation characteristics of the ductile shear zones in Sumdo (松多) Group, the quartz fabric by EBSD (electron backscatter diffraction), the data of muscovite 40 Ar- 39 Ar geochronology (220-230 Ma) from ductile shear zones and the zircon SHRIMP U-Pb chronology (190 Ma) of granites in Sumdo region, Lhasa (拉萨) terrane is thought to have experienced an important Indosinian orogenic event at 220-230 Ma, which caused the closure of the paleo-Tethys Ocean along the tectonic zone of eclogite and the collision between northern part and southern part of the Lhasa terrane. The zircon SHRIMP U-Pb chronology of 190 Ma for biotite adamellite, with the distributing charac- teristics of the granite massif intruding in Sumdo Group, indicates that the biotite adamellite should be the late orogenic or post-orogenic granite resulting from the Indosinian orogenesis. The discovery of Indosinian orogenic belt in Lhasa terrane expansed the southern boundary of Indosinian orogenic belt in Qinghai (青海)-Tibet plateau to Lhasa terrane from Qiangtang (羌塘) terrane, which changed the understanding about the distribution of Indosinian orogenic belt in Qinghai-Tibet plateau and extended the "T" type Indosinian orogenic belt in China. The study is very important for the formation and dis- tribution of paleo-Tethys Ocean in Tibet. The ancient terrane framework and evolution of Qinghai-Tibet plateau need further research.