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

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 melange 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. (C) 2004 Elsevier Ltd. All rights reserved.