Early Triassic Volcanic Rocks in the Western Yarlung Zangbo Suture Zone: Implications for the Opening of the Neo-Tethys Ocean

In this study,zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone(YZSZ) in southern Xizang(Tibet) yielded an age of 247 ± 3 Ma.According to whole rock geochemical and Sr-NdPb isotopic data,the Early Triassic samples could be divided into two groups:Group 1 with P-MORB affinity,showing initial87Sr/86Sr ratios of 0.70253-0.70602,εNd(t) values of 4.2-5.3,(206Pb/204Pb)tratios of 16.353-18.222,(207Pb/204Pb)tratios of 15.454-15.564,and(208Pb/204Pb)tratios of 35.665-38.136;Group 2 with OIB affinity,showing initial87Sr/86Sr ratios of 0.70249-0.70513,εNd(t) values of 4.4-4.9,(206Pb/204Pb)tratios of 17.140-18.328,(207Pb/204Pb)tratios of 15.491-15.575,and(208Pb/204Pb)tratios of 36.051-38.247.Group 2 rocks formed by partial melting of the mantle source enriched by a former plume,and assimilated continental crustal material during melt ascension.The formation of Group 1 rocks corresponds to the mixing of OIB melts,with the same components as Group 2 and N-MORBs.The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental-oceanic transition zone during the initial opening of the Neo-Tethys in southern Xizang(Tibet).