The Early Paleozoic Subashi ophiolite in the West Kunlun Orogenic Belt (northwestern Tibetan Plateau): Implication for the tectonic evolution of the Proto-Tethys

Abstract

The West Kunlun Orogenic Belt (W-KOB) in the northwestern Tibetan Plateau retains critical information about the tectonic evolution of the Proto- and Paleo-Tethys during the Early Paleozoic to Mesozoic. Although a trench-arc-back arc basin model was used to address the tectonic evolution of Proto-Tethys, records about the evolution of this ocean basin remain ambiguous, particularly no studies about ages and tectonic affinities of ophiolites in the W-KOB are available. In this paper, we present new results on whole-rock elemental and Sr-Nd isotopic composition of basalts, and U-Pb zircon ages of gabbro, diabase and basalt from the Subashi ophiolite. The basalts are characterized by low REE concentrations, depleted LREEs in the chondrite normalized REE patterns, and unfractionated HFSEs in the primitive mantle normalized trace element spider diagrams, resembling normal mid-ocean ridge basalts. They have constant 87Sr/86Sr(i) values of 0.70472–0.70590 and positive εNd values of +7.5 to +7.9. Accordingly, it can be concluded that the Subashi ophiolite was derived from a depleted spinel lherzolite mantle source in a mid-ocean ridge setting. Zircon grains separated from basalt, diabase and gabbro have 206Pb/238U ages of 455 ± 4 Ma, 451 ± 6 Ma, and 446 ± 6 Ma, respectively, which are taken to be the crystallization ages of these rocks. Together with previously published data, our new data propose that the Mazha-Kangxiwa-Subashi ophiolitic mélange zone marks the existence of the Proto-Tethys ocean basin during the Early Paleozoic.