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