HERCYNITE–BEARING PELITIC GNEISS AND ASSOCIATED CHARNOCKITE FROM THE SOUTHWESTERN YEONGNAM MASSIF, KOREA: IMPLICATIONS FOR PALEOPROTEROZOIC HT (-UHT) METAMORPHISM

The Yeongnam massif located in the southeastern part of the Korean Peninsula largely consists of Paleoproterozoic metamorphic rocks and various types of granitoids including charnockite. In the present study, we report the occurrence of hercynite-bearing HT (-UHT) pelitic gneiss and associated charnockite from an outcrop in the south central Yeongnam massif. Hercynite–bearing pelitic gneiss occurs as enclaves or intercalations within the charnockite. The hercynite–bearing pelitic gneiss consists of retrogressed and recrystallized porphyroblastic garnet grains, symplectic intergrowth of fine-grained hercynite and felsic minerals (plagioclase and quartz?) including opaques and felsic layers which are developed between retrogressed garnet grains and symplectites and consist of plagioclase, quartz, biotite, orthopyroxene and opaques. The charnockite is composed of microperthitic K-feldspar, plagioclase, quartz, biotite, orthopyroxene and subordinate opaque minerals and zircon. SHRIMP zircon U–Pb age determination carried out for the charnockite reveals an emplacement age of 1879±20 Ma. The conventional biotite-garnet geothermometer yields a maximum temperature of around 1,000℃ and the Ti-in-Biotite thermometer yields a temperature of about 850 °C. Although it is problematic to consider the estimated temperatures as peak metamorphic conditions because of highly retrogressed effect, the hercynite-bearing assemblage and its association with charnockite indicate that the peak metamorphic temperature was fairly high. We suggest that the HT (-UHT) metamorphic event was associated with emplacement of the charnockite around 1.9 Ga when widespread magmatic activity in the south central Yeongnam massif raised the geothermal gradient. The high-grade metamorphism may correlate with the assembly of the supercontinent Columbia.