EVIDENCE FOR PLATE TECTONICS AND EVOLUTION OF THE MIDDLE SEGMENT OF THE YARLUNG ZANGBO SUTURE ZONE TIBET

Ophiolites, melanges and blueschist as rock records of plate tectonics have been found and recognized in the middle segment of the Yarlung Zangbo suture zone in southern Tibet. 1. Ophioltes extend in a nearly E-W directinn along the Yarlung Zangbo River valley. They occur as a series of imbricated thrusting slices which consist of various units of rocks and were clearly dismembered by strong thrusting. However, in the Ringbun-Xigaze-Ngamring area, a complete and continuous sequence of the original ophiolite section can be observed. In ascending order（from south to north） the sequence is as follows: 1. ultramafic rocks; 2. cumulate; 3. sheeted sill or dike swarms, 4. mafic volcanic lavas; 5. abyssal marine seposits. 2. Melanges in the middle segment of the suture zone can be divided into two types: serpentinite melanges and argillo-arenaceous melanges, which occur parallelly to the ophiolite zone. The argillo-arenaceous melanges are the main part in the tectonic melange zone. Tectonic melanges show stronger and stronger deformation and mixing from south to north. 3. A high-pressure and low-temperature metamorphic belt, mainly consisting of glaucophane schist, stilpnomelane schist. glaucophane-stilpnomelane schist, lawsonite-bearing metagabbro and chloritoid schist, occurs to the sotth of the ophiolite complex, and can be subdivided into two metamorphic zones: blueschist zone and greenschist zone. The high-pressure f acies series consists of the glaucophane schist facies and greenschist facies. According to the above-mentioned evidence, a two-stage model of evolution of the middle segment of the Yarlung Zangbo suture zone has been proposed in the paper. The first stage witnessed the subduction of the oceanic crust of Tethys northwards beneath the Gangdise continental plate during 90-50 Ma（from the Late Cretaceous to Eocene）, and the second stage was a continent-to-continent collision period of the Indian and Gangdise plates in 50-20 Ma（from the Late Eocene to Miocene）. A series of typical geological features were formed in the two stages.