A middle Triassic seamount within the western Yarlung Zangbo suture zone, Tibet: The earliest seafloor spreading record of Neotethys to the North of East Gondwana

Abstract

Initial opening time of Neotethyan ocean in the eastern Gondwana realm is still controversial. Ocean plate stratigraphy (OPS) forming in or near middle ocean ridges is of significance to trace the early stage of seafloor spreading. An over 10-km-long and ~ 5-km-wide Gongzhu seamount OPS is well preserved in the western Yarlung Zangbo suture zone (Southern Tibet), and consists mainly of neritic limestone, siliceous limestone, mudstone, mud-riched basaltic tuff, globular basalt and basaltic breccia interbedded with basaltic lava flows, and basaltic tuffaceous breccia. Geochemical and isotopic compositions of basaltic tuffceous breccias and basaltic lavas exhibit OIB-type affinities, basaltic lavas display slightly high (87Sr/86Sr)t ratios of 0.70430–0.70523, low (143Nd/144Nd)t ratios of 0.51244–0.51247 and εNd(t) values of +2.21 to +2.84, initial ratios of 206Pb/204Pb = 17.571–18.176, 207Pb/204Pb = 15.612–15.652, and 208Pb/204Pb = 37.290–38.706. These features are similar to those of alkaline basalts from the Vening-Meinesz Seamount Province in the northeast Indian ocean. The Gongzhu basaltic rocks are geochemically comparable with the mixing products of melts derived from the Pacific/Atlantic MORB and a metasomatized subcontinental lithosphere mantle. In-situ U--Pb zircon dating of two basaltic tuffaceous braccias has yielded mean ages of 245.1 ± 2.5 Ma and 241.9 ± 4.1 Ma. We propose that 272–290 Ma Panjal tholeiitic lavas exposed in the Kashmir region of NW India in the Himalayan orogenic belt developed during rifting of the Gondwana continental margin, whereas the slightly younger, 242–245 Ma Gongzhu seamount OPS formed during the initial seafloor spreading stage of the opening of the Neotethyan ocean.