Paragenetic Relationships among Staurolite and Aluminosilicate Polymorphs in the Barrovian Metapelites of the Imjingang Belt, Central Korea

Mineral parageneses, inclusion relationships and microstructures in the Barrovian metapelites of the Imjingang belt, Korea, were investigated to delineate the exhumation process of a crustal section thickened during the collisional orogeny. In particular, the timing of metamorphism was constrained by the in-situ SHRIMP dating of monazite grains in kyanite-zone schists. In a staurolite-zone metapelite, andalusite occurs as large poikiloblast containing biotite, garnet, staurolite and rare kyanite as inclusions, suggesting the low- P overprint after the Barrovian-type metamorphism. In the kyanite-zone schists, two types of inclusion assemblages are distinct: (1) high-P, staurolite-free assemblage of biotite and garnet within kyanite porphyroblasts; and (2) medium-P one of biotite, kyanite and garnet within staurolite. Kyanite is also present in andalusite-rich quartz veins of the kyanite zone. These veins are deformed or boudinaged on the outcrop scale. The coexistence of two aluminosilicate polymorphs is in contrast with the lack of sillimanite. Thus, the cooling path of the metapelites is characterized by a near-isothermal P-T path passing through the kyanite-andalusite boundary. The U-Th-Pb isotopic ages of monazite dated from three kyanite- zone schists define two distinct age groups. The 206Pb/238U ages of monazite were dated at 255 ± 5 Ma and 239 ± 4 Ma, respectively, and the 208Pb/232Th ages at 252 ± 3 Ma and 238 ± 5 Ma. All the above features in conjunction with available P-T-ometric and thermochronometric data suggest that the high-P peak metamorphism (~11 kbar and 670 °C) occurring at ~252 Ma was overprinted by the medium-P one (~6.5 kbar and 630 °C) at ~238 Ma. The subsequent cooling to 500 °C occurred at ~230-225 Ma. The latest Permian age of monazite is coeval with that of peak metamorphism, dated from the overgrowth rim of zircon in a paragneiss of the Imjingang belt (253 ± 2 Ma), but the heat and fluid sources for the Middle Triassic overprint are poorly constrained. The absence of paragonite and the low celadonite component of muscovite (less than ~3.1 Si / 11 O atoms) argue against the supply of fluid via the breakdown of hydrous minerals. On the other hand, Triassic igneous activities in the vicinity of the Imjingang belt, recently dated at 237-232 Ma, might have provided heat and fluid necessary for the growth of staurolite and monazite at mid- crustal depths. In summary, our result suggests that the Imjingang belt has experienced a near-isothermal decompression from ~11 to 6.5 kbar at ~252-238 Ma, and the formation of staurolite porphyroblasts enclosing biotite, kyanite and garnet is possibly attributed to the thermal flux associated with Middle Triassic magmatic activity.