DATING POLYMETAMORPHISM IN A GRANULITE-AMPHIBOLITE: EARLY PROTEROZOIC AND PERMO-TRIASSIC CHEMICAL AGES OF MONAZITE

New chemical ages of monazites have documented a Permo-Triassic age for the timing of exhumation and retrogression of the Hwacheon Granulite Complex (HGC), near Demilitarized Zone, South Korea. Our previous studies have documented the Paleo-proterozoic peak metamorphism of the granulite facies as well as the presence of diverse Protetozoic-Archean protoliths: Sm-Nd garnet age of 1779°¾190 Ma from mafic granulites, SHRIMP-II U-Pb zircon overgrowth age of 1872 +11/-9 Ma, and diachronous core ages of ca. 2300, 2450, 2650 and 2900 Ma. However, the timing of amphibolite facies metamorphic overprinting has not been known, although this retrograde event is distinct in both mineral assemblages and reaction textures. To attack this problems, we have determined U-Th-Pb chemical ages of zoned monazite, using electron microprobe and back scattered electron image (BSEI). Zoning of monazite, easily seen in the BSEI, is strongly dependent on Th contents varying from 5 to 17 wt%. Although zoning patterns are irregular, Th-poor domains are prevalent, and are partly surrounded by Th-rich rims.

Our preliminary result on chemical ages of monazite grains from both granulites and amphibolite facies gneisses defines two distinct age groups: early Proterozoic and Permo-Triassic. The older chemical age of ~1860-1766 is consistent with the SHRIMP-II U-Th-Pb monazite age. The younger age of ~220 Ma appears to be prominent in high-strain zones surrounding the HGC, rather than the amphibolite facies metamorphism. However, both structural and temporal relationships responsible for the exhumation of the HGC in the host Gyeonggi massif are compatible with those reported in the western Imjingang belt, which crosses most of the Korean Peninsula. It is thus likely that the HGC represents the reworked crystalline basement exhumed during the Permo-Triassic collisional orogeny. This thermo-tectonic event is coeval with the continental collision between the Sino-Korea and Yangtze cratons at ca. 210-240 Ma. Hence, both the Imjingang belt and the Gyeonggi massif corresponding to thin- and thick-skinned orogens, respectively, are probably the eastward continuation of the Chinese continental collision belt in the Korean Peninsula.