A unique Paleoproterozoic HP-UHT metamorphic event recorded by the Bengbu mafic granulites in the southwestern Jiao-Liao-Ji Belt, North China Craton

摘要

The Wuhe Complex in the Bengbu area of the Jiao-Liao-Ji Belt, southeast North China Craton, contains garnet-bearing mafic granulites that have undergone high-pressure (HP) and ultrahigh-temperature (UHT) metamorphism. These granulites also experienced partial melting and occur as lenses within marbles. Petrographic observations and quantitative phase equilibria modeling reveal clockwise P-T paths, involving an inferred HP stage followed by decompressional, medium-pressure, granulite-facies metamorphism and subsequent cooling. The HP assemblage of garnet + clinopyroxene + plagioclase + K-feldspar +/- amphibole +/- quartz +/- rutile indicates P-T conditions of 840-980 degrees C and 12-17 kbar. This was followed by post-peak, near-isothermal decompression with the development of orthopyroxene + clinopyroxene + plagioclase + K-feldspar + garnet + amphibole + ilmenite at 850-960 degrees C and 7-10 kbar, resulting in the development of orthopyroxene rims on resorbed garnet. Pyroxene and ternary feldspar thermometry yielded high temperatures of similar to 1150 degrees C and 1055-1087 degrees C at 10 kbar, respectively, which constrain the minimum crystallization temperatures of the igneous protoliths. The host and lamellae of the pyroxene and ternary feldspar are relict magmatic minerals/textures that survived metamorphism due to the silica-undersaturated bulk-rock conditions. Zr-in-rutile thermometry yielded temperatures of similar to 935 degrees C and 800 degrees C, with the former being consistent with the predicted peak metamorphic temperatures. Small amounts of melts (up to 5%) were generated during decompression of the Bengbu mafic granulites. The generated partial melts were mainly (quartz) monzonite at 900-920 degrees C, and the silica contents of the melts were controlled by the quartz stability field in P-T pseudosections. The partial melts were enriched in Na and strongly depleted in Fe-Mg at the peak pressure of similar to 14 kbar and 920 degrees C, and later evolved to Fe-Mg-rich and high-K compositions during decompression. The melt compositions in the studied rocks are similar when the pressures reached similar to 9 kbar. The modal proportion of amphibole increased as the melt H2O content decreased at lower pressures, indicating that the limited H2O remaining in the host rocks was consumed to produce amphibole. U-Pb geochronology of zircon containing inclusions of clinopyroxene, plagioclase, and apatite constrains the timing of metamorphism to 1930-1840 Ma, as is the case for HP granulites from Shandong, Liaoning, and southern Jilin in the central and northeastern Jiao-Liao-Ji Belt. The Wuhe HP-UHT mafic granulites were ultimately sourced from upwelling asthenosphere-derived magma at similar to 2.1 Ga, which intruded and crystallized at shallower depths. The igneous protoliths were then buried to middle-lower crustal levels and experienced HP-UHT granulite-facies metamorphism and partial melting at 1.95-1.90 Ga related to continental subduction and overthickening. The HP-UHT mafic granulites were rapidly exhumed at similar to 1.85 Ga and generated small volumes of (quartz) monzonite during decompression. The newly discovered Paleoproterozoic HP-UHT mafic granulites associated with partial melting suggest that the continent materials were deeply subducted to the lower crustal levels and that additional heating was not involved.