Early Mesozoic metamorphism and tectonic significance of the eastern segment of the Lhasa terrane, south Tibet

Abstract

The metamorphic belt in the Basongco area, the eastern segment of Lhasa terrane, south Tibet, occurs as the tectonic blocks in Paleozoic sedimentary rocks. The Basongco metamorphic rocks are mainly composed of paragneiss and schist, with minor marble and orthogneiss, and considered previously to be the Precambrian basement of the Lhasa terrane. This study shows that the Basongco metamorphic belt experienced medium-pressure amphibolite-facies metamorphism under the conditions of T=640–705°C and P=6.0–8.0kbar. The inherited detrital zircon of the metasedimentary rocks yielded widely variable 206Pb/238U ages ranging from 3105Ma to 500Ma, with two main age populations at 1150Ma and 580Ma. The magmatic cores of zircons from the orthogneiss constrain the protolith age as ca. 203Ma. The metamorphic zircons from all rocks yielded the consistent metamorphic ages of 192–204Ma. The magmatic cores of zircons in the orthogneiss yielded old Hf model ages (TDM2=1.5–2.1Ga). The magmatic zircons from the mylonitized granite yielded a crystallization age of ca. 198Ma. These results indicate that the high-grade metamorphic rocks from the Basongco area were formed at early Jurassic and associated with coeval magmatism derived from the thickening crust. The Basongco metamorphic belt, together with the western and coeval Sumdo and Nyainqentanglha metamorphic belts, formed a 400-km-long tectonic unit, indicating that the central segment of the Lhasa terrane experienced the late Paleozoic to early Mesozoic collisional orogeny.