STRUCTURE, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE WESTERN YARLUNG ZANGBO OPHIOLITES IN SOUTHERN TIBET (CHINA), AND IMPLICATIONS FOR THE NEOTETHYAN TECTONICS

The ophiolites that crop out discontinuously for more than 2000 km along the Yarlung-Zangbo suture zone (YZSZ) in southern Tibet are the remnants of the Neotethyan oceanic lithosphere. The tectonic setting & structural relationships between the Daba-Xiugugabu (Southern belt) and Dajiweng - Saga (Northern belt) ophiolitic sub-belts in the western part of the YZSZ are the subject of debate. These two belts are separated by the Zhada-Zhongba terrane. The main argument is mainly focused on whether the ophiolites in these belts originated from the same ocean basin or from two different seaways within Tethys. We discuss in this talk the crustal architecture of the southern & northern sub-belts and their ophiolites based on several N-S structural profiles across them, as well as on the lithology, geochronology & geochemistry of the associated mafic dikes & upper mantle peridotites.

Mafic dikes in the Cuobuzha, Jianabeng & Baer ophiolites in the northern belt are characterized by high Si, Al and low Ti, K, P contents, representing calc-alkaline basalt series. In-situ analysis of zircon grains from these dikes has yielded crystallization ages of 125~128 Ma. Mafic dikes from the Dongbo, Purang & Xiugugabu ophiolites in the southern belt are characterized, on the other hand, by high Mg and Ti contents & low Si and K contents, belonging to low-K tholeiite series. Zircon grains from these dikes have yielded crystallization ages of 120~130 Ma. Dike intrusions in both belts show N−MORB REE patterns & negative Nb, Ta and Ti anomalies reminiscent of SSZ ophiolites. Harzburgitic host rocks of mafic dikes in both belts have geochemical compositions of forearc & abyssal peridotites. Based on these data & structural observations we interpret the Southern and Northern sub-belts as having formed at a SSZ setting in a single Neotethyan seaway. Detrital zircon geochronology and Hf isotope data from Permian clastic rocks in the Zhada-Zhongba terrane suggest that it was a part of the Qiangtang-Greater India-Tethyan Himalaya system during the Paleozoic, and that it was rifted off from it during the early Triassic. The Northern & Southern ophiolitic sub-belts are hence part of a single, S-directed nappe sheet derived from a Neotethyan seaway located north of the Zhada-Zhongba terrane. The Southern belt represents the tectonic outlier of this nappe.