Petrological and geochronological evidence for Paleoproterozoic granulite-facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton

To better understand the formation of high-temperature metamorphic rocks, we present a detailed petrological and geochronological study of recently discovered cordierite-bearing granulites of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton. Petrographic observations indicate that four distinct mineral assemblages are present in the granulites: pre-peak amphibolite facies (M-1), peak granulite facies (M-2), post-peak decompression (M-3), and late cooling retrogression (M-4). M1 is preserved as fine-grained inclusions in the cores of garnet grains, represented by quartz + plagioclase + biotite + ilmenite. M-2 is interpreted to have comprised garnet + sillimanite + plagioclase + quartz + biotite + ilmenite + melt. M-3 was characterized by the formation of cordierite + sillimanite symplectites and cordierite + quartz coronas replacing garnet. M-4 is indicated by the formation of staurolite, accompanied by the crystallization of melt. A combination of multi-equilibria geo-thermobarometers and pseudosection modeling constrains the P-T conditions of the M-1, M-2, M-3, and M-4 stages to P = 0.66-0.71 GPa and T = 620-650 degrees C, P = 0.96-1.10 GPa and T = 790-840 degrees C, P = 0.62-0.65 GPa and T = 725-785 degrees C, and P = 0.43-0.55 GPa and T = 595-625 degrees C, respectively. Zircon and monazite U-Pb dating yields three distinct and meaningful ages for the granulites: (1) 2200-2100 Ma for the granulite protoliths, (2) ca. 1945 Ma for the near-peak metamorphism, and (3) 1851-1839 Ma for the post-peak and late retrogressive metamorphism. Thus, a clockwise P-T-t path is determined for the cordierite-bearing granulites of the South Liaohe Group. Petrological and geochronological evidence from regional metamorphic rocks in the Jiao-Liao-Ji Belt suggests that the belt experienced a continuous orogenesis from 1950 to 1800 Ma.