Age, geochemistry and tectonic setting of Buqingshan ophiolites, North Qinghai-Tibet Plateau, China

Abstract

The Buqingshan ophiolite complex is a sector of the A'nyemaqen ophiolite belt in the East Kunlun southern marginal suture zone in the northern part of the Qinghai-Tibet Plateau. The WNW-trending ophiolite complex consists of metaperidotite, gabbro, diabase, pillow basalt, massive basalt and pelagic sedimentary rocks including radiolarian chert. We have determined that Early Paleozoic and Early Carboniferous–Early Permian ophiolites are present in this ophiolite complex that were previously thought of Permian–Middle Triassic age. A zircon U–Pb age of 467.2±0.9 Ma for gabbro, and a Rb–Sr isochron age of 481±130 Ma for diabase and gabbro have been obtained from the Early Paleozoic ophiolite slice, and Middle–Late Ordovician acritarchs have been found in the mélange matrix that envelopes the sliver. We interpret the first two ages to represent the formation age of the ophiolite. Granodiorite–tonalite plutons with a zircon U–Pb age of 402±24 Ma intrude the slice. Early Carboniferous–Early Permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice. Geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle. Most mafic rocks in the Early Paleozoic ophiolite are of N-MORB type with small amount of T-MORB, and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination. The basalts in the Early Carboniferous–Early Permian ophiolite have similar characteristics to the mafic rocks and also show DUPAL anomaly and a mixed source of DMM and EM II. The original formation environments of the two-stage ophiolites are both mid-ocean ridges. These lines of evidence suggest that a mature Early Paleozoic Kunlun–Qilian–Qinling ocean basin and a mature Paleotethyan ocean basin once existed in the study area.