Petrology and geochemistry of UHP-metamorphosed ultramafic–mafic rocks from the main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH), China: Fluid/melt-rock interaction: Mafic–ultramafic complex from CCSD-MH

Abstract

Mafic–ultramafic rocks recovered between 600 and 690 m in the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) Project in the Sulu terrane consist of garnet peridotite, garnet pyroxenite, garnet clinopyroxenite and eclogite. All of the rocks have undergone ultrahigh pressure (UHP) metamorphism with peak conditions of at least ∼750–850 °C/3.5–4.0 (GPa). The protoliths were mainly lherzolite, websterite, minor clinopyroxenite and harzburgite, with major and trace element compositions characteristic of crustal layered intrusions. The low Mg# of the whole rocks and mafic minerals, layered textures, and adjacent lithology pairs of garnet peridotite - garnet pyroxenite and garnet pyroxenite–eclogite pair probably reflect fractional crystallization of the protolith magma. Whole-rock MgO and FeO abundances decrease and Al 2O 3, CaO and SiO 2 increase toward the mafic endmembers. Extensive metasomatism and metamorphism influenced REE and trace element patterns. At least three episodes of fluid/melt metasomatism are recognized: a partial melting event that depleted K and Rb during the UHP metamorphism; a retrograde metasomatism that occurred during exhumation, leading to the formation of serpentine + phlogopite veins; and a late subgreenschistfacies hydrothermal event that formed carbonates and calc–silicate minerals. The protoliths of the garnet-bearing mafic–ultramafic rocks of the CCSD-MH are interpreted as crustal cumulates, which were subducted during collision between the Yangtze and Sino-Korean Cratons in the Triassic. During subduction, the original ultramafic rocks of the CCSD-MH were transformed to garnet peridotites and pyroxenites.