A MIDDLE TRIASSIC RIFTED CONTINENTAL MARGIN SEQUENCE WITHIN THE WESTERN YARLUNG ZANGBO SUTURE ZONE, TIBET: RECORD OF DIACHRONOUS OPENING OF NEOTETHYS

We have investigated an “ophiolitic mélange” unit, which occurs within the western segment of the Yarlung Zangbo Suture zone (YZSZ) in the Tibetan–Himalayan orogenic belt, along two N-S geological profiles near the 82°E Longitude. YZSZ in this region is subdivided into two subparallel belts of late Jurassic–Cretaceous ophiolites and mélange complexes that are separated by a ribbon continent of East Gondwana origin (Zhongba microcontinent, ZMC). The mélange complex in the Northern belt is juxtaposed against the Gangdese magmatic arc along dextral oblique faults in the north and tectonically overlies ZMC to the south. The ZMC comprises a metamorphosed and deformed early Paleozoic granitoid basement, overlain by recrystallized limestone, quartzite and phyllite units of a passive margin sequence. Upper Paleozoic lithological units examined within the mélange along both profiles consist mainly of neritic limestone, siliceous limestone and mudstone that are stratigraphically intercalated with massive lava flows and basaltic tuffaceous breccias. Geochemically, these volcanic rocks represent within plate alkaline basalts (WPAB), characteristic of rifted continental margins. In-situ U-Pb zircon dating of the basaltic tuffaceous braccias has yielded crystallization ages of 245.1±2.5 Ma. We interpret this volcanic–sedimentary sequence as a rifted northern margin of the ZMC within the proto– Neotethyan realm in the early Mesozoic. Compared to the similar records of the eastern and central segments of the YZSZ, this rifting event in the west appears to have occurred ~30 m.y. later, suggesting diachronous rift-drift tectonics along the E–W-oriented Neotethys.