Remnants of a Late Triassic ocean island in the Gufeng area, northern Tibet: Implications for the opening and early evolution of the Bangong–Nujiang Tethyan Ocean

Abstract

In this paper we present new major and trace element compositions of basaltic rocks in the Gufeng ocean island (GFOI) area in the western segment of the Bangong–Nujiang Suture Zone, northern Tibet. Our aim was to assess the genesis of these rocks and discuss the implications of this new dataset for the evolution of the Bangong–Nujiang Tethyan Ocean. An ocean-island-type double-layer structure comprising a basaltic basement and an oceanic sedimentary cover sequence found within the GFOI provides direct evidence for the interpretation that the assemblage is a typical ocean island. The basalts in the GFOI can be divided into three types (named G1, G2 and G3 basalts), and these basalts range in composition from MORB to OIB types, which is typical of ocean islands. The G1 basalts have MORB-type affinities, possibly indicating the existence of MORB oceanic crust under the GFOI. The G2 basalts represent the early stage of formation of the GFOI, and are produced by the interaction of rising OIB-type basaltic magma and the existing MORB oceanic crust. The G3 basalts are typical OIB basalts and they are the products of the direct eruption of OIB-type basaltic magmas. The G3 basalts have high (La/Yb)N (12.3–14.4), (Ce/Yb)N (10.8–11.8), (La/Sm)N (2.39–2.76), and (Sm/Yb)N (4.89–5.23) ratios, indicating the presence of oceanic lithosphere below the GFOI with a thickness of 50–60km. Geochemical analyses of the GFOI cherts show that they contain terrigenous material, indicating the GFOI formed close to a continental margin. Norian conodont fossils within the GFOI limestones indicate the GFOI formed during the Late Triassic. These data, combined with geological evidence and a half-space model of lithosphere cooling, where the thickness of the oceanic lithosphere is determined from the age of the lithosphere, indicate that the western segment of the Bangong–Nujiang Tethyan Ocean opened initially in the late Permian, expanded rapidly during the Early–Middle Triassic, and was a mature ocean during the Late Triassic.