Spatial distribution, P-T-t paths, and tectonic significance of high-pressure mafic granulites from the Daqingshan-Wulashan Complex in the Khondalite Belt, North China Craton

High-pressure (HP) mafic granulites in the Daqingshan-Wulashan Complex from the central segment of the Khondalite Belt of the North China Craton occur as irregular lenses or deformed dike swarms within dioritic-granitic gneisses and meta-sedimentary rocks. The HP maflc granulites contain three distinct mineral assemblages: a peak HP granulite-facies assemblage (Mj) that consists of garnet + clinopyroxene + plagioclase + amphibole +/- quartz +/- Fe-Ti oxides; a post-peak decompression assemblage (M (2)) that is characterized by symplectites of orthopyroxene + clinopyroxene + plagioclase +/- amphibole +/- Fe-Ti oxides; a late cooling assemblage (M-3) represented by symplectites of amphibole + plagioclase + Fe-Ti oxides. Pseudosection modeling using THERMOCALC in the NCFMASHTO system and conventional thermobarometers constrain the P-T conditions of the M-1, M-2, and M(3)assemblages to P = 1.25-1.40 GPa and T = 760-815 degrees C, P = 0.72-0.80 GPa and T = 825-860 degrees C, and P = 0.69-0.81 GPa and T = 645-730 degrees C, respectively. In-situ U-Pb dating of zircons show that the protolith ages of the HP mafic granulites are 2100-2000 Ma and the timing of peak HP granulite-facies metamorphism is 1960-1940 Ma. The medium-to low-pressure amphibolite-to granulite-facies retrogression occurred mainly from 1920 to 1820 Ma. Petrographic investigations, mineral compositions, metamorphic reaction histories, P-T calculations, and geochronological data define a clockwise P-T-t path involving near-isothermal decompression and near-isobaric cooling, which suggests that the Daqingshan-Wulashan Complex underwent initial crustal thickening from 1960 to 1940 Ma, followed by relatively rapid exhumation, cooling, and retrogression from 1920 to 1820 Ma. This P-T-t path was likely the result of the Paleoproterozoic continental-continental (or arc) collision during the amalgamation of the North China Craton.