Mafic-ultramafic rocks in the Buqingshan Complex of the East Kunlun Orogen, northern Tibetan Plateau: remnants of the Paleo-Tethys Ocean

The Buqingshan Complex, the eastern part of the Muztagh–Buqingshan–Anemaqen ophiolitic mélange zone (MBAM), contains a series of oceanic rocks that represent the remnants of the Paleo-Tethys Ocean. Field investigations reveal that the mafic-ultramafic rocks consist mainly of serpentinite, diabase, massive basalt, and pillow basalt. The serpentinites show similar geochemical characteristics to abyssal peridotites, which is probably the residual part of the depleted upper mantle after 17–20% melt extraction. The mafic rocks have geochemical affinities similar to both normal mid-ocean ridge basalt (N-MORB) and oceanic island basalt (OIB). The rocks with N-MORB features (diabases, massive basalts, and pillow basalts) are associated with serpentinites and cherts, characterizing the sequence of MOR-type ophiolites. The OIB-type pillow basalts are characterized by enrichment of Nb, Ti, and LREEs. They are associated with massive bioclastic limestones, limestone breccias, basalt breccias, and matrix-supported conglomerates, resembling a typical seamount sequence of bottom pillow lavas–cap carbonate–slope facies deposits. Zircon U–Pb dating of the diabases yields a Late Carboniferous age at 303 ± 3 Ma. The reported biostratigraphic age data from the overlying limestone suggest the formation age of the basalt basement to be the Early Permian. Linking all the geological, geochemical, and geochronological data together, these pieces of evidence suggest that the Late Palaeozoic N-MORB- and OIB-type rock assemblages in the MBAM are most likely to be fragments of mid-ocean ridge oceanic lithosphere and seamount, respectively. Such new observations also represent the two main stages of the evolution of the Paleo-Tethys Ocean: the spreading of the Carboniferous mid-ocean ridge and the Early Permian plume magmatism.