The ~1.87 Ga granulite facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt and its tectonic implications

To further understand the metamorphic evolution in the southern margin of the Jiao-Liao-Ji Belt in the Liaodong area, a detailed petrological and geochronological study was performed on representative metapelites from the South Liaohe Group. The peak metamorphic stage (M1) of the samples is represented by the mineral assemblage of garnet-biotite-plagioclase-sillimanite-quartz-magnetite in sample DD24-1 and garnet-biotite-plagioclase-ilmenite-quartz in sample DD24–2. The post-peak metamorphic stage (M2) is indicated by the appearance of cordierite with the mineral assemblage of biotite-cordierite-garnet-plagioclase-K feldspar-sillimanite-quartz-magnetite-ilmenite in sample DD24-1. The P-T calculations using pseudosections and conventional geothermobarometry imply that the M1, M2 and final retrograde stage (M3) metamorphisms occurred at 865–890 °C and 6.9–9.2 kbar, 840–860 °C and 5.3–6.2 kbar, and 570–670 °C and 3.3–4 kbar, respectively. The zircon SHRIMP U–Pb age data show that M1 metamorphism occurred 1867 ± 11 Ma and 1866 ± 8 Ma for samples DD24-1 and DD24–2, respectively. Together with previous studies, this study confirms that a low-P/T type granulite facies regional metamorphic event with a clockwise P-T path occurred in the South Liaohe and the Ji'an groups during 1.87–1.84 Ga. Together with the simultaneous postcollisional igneous activity in the South Liaohe and Ji'an groups, the low-P/T type metamorphism indicates that postcollisional activity occurred in the South Liaohe and Ji'an groups during 1.87–1.84 Ga due to slab break off after the collision. Combined with the prevailing postcollisional activity in the Nangrim Massif on the Korean Peninsula, this study suggests that the Longgang Block in the North China Craton subducted under the Nangrim Massif on the Korean Peninsula.