Petrogenesis and tectonic implications of Triassic mafic complexes with MORB/OIB affinities from the western Garze-Litang ophiolitic melange, central Tibetan Plateau

摘要

Although numerous Paleo-Tethyan ophiolites with mid-oceanic ridge basalts (MORE) and/or oceanic-island basalt (OIB) affinities have been reported in the central Tibetan Plateau (CTP), the origin and tectonic nature of these ophiolites are not well understood. The petrogenesis, mantle sources and geodynamic setting of the mafic rocks from these ophiolites are unclear, which is the main reason for this uncertainty. In this paper, we present new geochronological, mineralogical and Sr-Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garze-Litang suture zone (GLS), a typical Paleo-Tethyan suture crossing the CTP. Zircon LA-ICP-MS U-Pb ages of 234 +/- 3 Ma and 236 +/- 2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes, respectively. The basalts and gabbros of the Chayong complex exhibit enriched MORB (E-MORB) compositional affinities except for a weak depletion of Nb, Ta and Ti relative to the primitive mantle, whereas the basalts and gabbros of the Xiewu complex display distinct E-MORB and OIB affinities. The geochemical features suggest a probable fractionation of olivine clinopyroxene plagioclase as well as insignificant crustal contamination. The geochemical and Sr-Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB-type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithospheric mantle metasomatized by OIB-like components. The ratios of Zn/Fe-t, La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet + minor spinelbearing peridotite or spinel +/- minor garnet-bearing peridotite. We propose that back-arc basin spreading associated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times, and the decompression melting of the depleted MORE-type asthenosphere mantle and partial melting of sub-continental lithosphere were metasomatized by plume-related melts, such as OIB s, which led to the generation of the Chayong and Xiewu mafic melts. (C) 2016 Elsevier B.V. All rights reserved.