Late Triassic back-arc spreading and initial opening of the Neo-Tethyan Ocean in the northern margin of Gondwana: Evidences from Late Triassic BABB-type basalts in the Tethyan Himalaya, Southern Tibet

Abstract

The tectonic evolution of the Neo-Tethyan oceanic plate between the Indian Plate and the Lhasa Terrane is a key issue for the evolution of Gondwana and formation of the Tibetan Plateau. In this paper we report on the geochemistry, SrNd isotope compositions, and zircon UPb dating of Late Triassic basalts from the Nieru Formation (T3n) in the Kampa region, southern Tibet. The basalts have relatively low contents of MgO, TiO2, and total alkalis (K2O + Na2O), and they have affinities to tholeiitic basalt. They exhibit weakly fractionated rare earth element (REE) patterns with slight depletions in LREEs and slightly negative Eu anomalies on a chondrite-normalized diagram. On a primitive-mantle-normalized spider diagram, they are characterized by slight enrichments in large ion lithophile elements (LILEs) and relatively flat patterns of high field strength elements (HFSEs), except for depletions in Ba, Nb, Ta, and Ti. Their initial 87Sr/86Sr ratios vary from 0.718133 to 0.738977 and εNd(t) values are relatively depleted (4.09–5.22), similar to mid-ocean ridge basalts in the Yarlung Zangbo ophiolite. We propose the T3n basalts were derived from a shallow and depleted mantle source by relatively high-degree partial melting in the spinel stability field. They underwent slight crustal contamination as well as the fractional crystallization of clinopyroxene. The T3n basalts are similar to typical back-arc basin basalts (BABB) such as the Okinawa BABB. This indicates an extensional back-arc-basin setting along the northern margin of Gondwana during the Late Triassic. The initial opening of the Neo-Tethyan Ocean was related to the southwards subduction of the Paleo-Tethyan oceanic plate and back-arc-basin spreading during the Late Triassic. Magmatic activity along the passive continental margin records key information on continental break-up and incipient ocean development.