Genetic mineralogy and metamorphic evolution of mafic high-pressure (HP) granulites from the Shandong Peninsula, China.

Mafic high-pressure (HP) granulites as enclaves and irregular vein-like blocks are widespread distributed in TTG gneiss in the Pingdu-Laixi-Laiyang-Yantai area of Shandong Peninsula, southeastern Sino-Korean craton. On the basis of petrography, mineral transformation, metamorphic reaction integrated with garnet, clinopyroxene and plagioclase zonation patterns, four stages of the metamorphic evolution have been identified in mineral assemblages, including early prograde metamorphic stage (M-1), peak HP granulite-facies metamorphic stage (M-2), near isothermal decompression granulite-facies retrograde stage (M-3), and late amphibolite-facies retrograde stage (M-4). The P-T conditions for each metamorphic stage were estimated using thermodynamical and experimental geothermometers and geobarometers. The core of garnet, together with mineral inclusions, is regarded as a prograde assemblage (M-1) of garnet + clinopyroxene + plagioclase +/- quartz, which formed at 740 similar to 770 degrees C and 0.9 similar to 1.0GPa. The peak HP granulite-facies metamorphism (M-2) is defined by high-Ca cores in garnet, high-Al cores in clinopyroxene and high-Na cores in plagioclase +/- quartz as matrix minerals, yielding 850 similar to 880 C and 1.45 similar to 1.65GPa. Symplectites or coronas of orthopyroxene + clinopyroxene + plagioclase + magnetite +/- amphibole around garnet porphyroblasts indicate garnet decompressional reactions (M,) occurring at P-T conditions of 780 similar to 830 degrees C and 0.65 similar to 0.85GPa. Symplectites of amphibole + plagioclase + magnetite +/- quartz surrounding garnet formed at a cooling retrograde stage (M-4) with P-T conditions of 590 similar to 650 degrees C and 0.62 similar to 0.82GPa. These data indicate that the mafic HP granulites record an integrated P-T path including prograde, near-isothermal decompression (ITD) and near-isobaric cooling (IBC) evolutional process. The clockwise ITD and IBC P-T paths of the mafic HP granulites in the Shandong Peninsula support a geodynamic model of crust thickening followed by extension in a continent-continent collisional orogen.