CHIME dating and age mapping of monazite in granulites and paragneisses from the Hwacheon area, Korea: implications for correlations with Chinese cratons

摘要

The basement of the Hwacheon area in the northern part of the Gyeonggi massif in the Korean Peninsula comprises granulite and gneiss complexes that are separated by an extensive mylonite zone. Granulites distant from the mylonite zone show little evidence of a retrograde overprint beyond minor growth of biotite along garnet grain boundaries. Monazite grains enclosed in garnet show a core to rim decrease in Y, and those in the matrix show a peripheral enrichment of Y on low-Y interiors. Similarly, garnet grains decrease in Y from core to rim, with peripheral Yenrichment, which can be attributed to early to peak garnet growth and post-growth modification, respectively. The three chemical domains representing stages of monazite growth show no detectable age difference, together yielding a CHIME age of 1868 ± 24 Ma. Granulites near the mylonite zone show growth of cordierite and biotite aggregates around garnet grains, and yield Permo-Triassic monazite domains on and within predominantly Paleoproterozoic monazite grains. Mylonitized granulites have a metamorphic overprint characterized by the growth of the sillimanite and K-feldsper assemblage at ca. 241 Ma, subsequent to ductile shearing. Pelitic gneisses in the gneiss complex contain zircon grains with Neoproterozoic detrital cores and monazite grains with unequivocal metamorphic ages of ca. 245 Ma. Geological, petrological and chronological evidence suggests differing evolutional histories between two crustal domains prior to amalgamation. The widespread occurrence of Permo-Triassic paragneisses and mylonites in the Gyeonggi massif can be explained by detachment of the metamorphosed upper crust of the South China block, which was thrust over the Paleoproterozoic granulite complex at ca. 241 Ma during the Sulu orogeny. Permo-Triassic metamorphic overprints on the latter are attributed to contact metamorphism from the hot allochthon.