Metasomatism of garnet peridotite from Jiangzhuang, southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental lithosphere

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Abstract

The Jiangzhuang ultrahigh-pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse-grained garnet porphyroblasts and a fine-grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti-clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large-ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti-clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite-free peridotites, the phlogopite-bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite-bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite-free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite-bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile-bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.

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