P-T-T-D EVOLUTION OF BARROVIAN-TYPE METAPELITES IN THE IMJINGANG BELT, KOREA

The east-trending Imjingang belt, a possible eastward extension of the Dabie-Sulu UHP belt, consists primarily of the Barrovian-type metapelites ranging from garnet through staurolite to kyanite zones. In order to delineate the P-T-t-D path, we investigated microstructures, mineral parageneses and P-T conditions using both P-T pseudosection and P-T-ometry. Our result defines a clockwise P-T path characterized by (1) prograde garnet formation associated with crustal thickening and (2) staurolite growth during post-peak decompression. These porphyroblasts also preserve microstructural evidence for multiple episodes of deformation: (1) initial contractional deformation (Dn-1); (2) penetrative deformation (Dn) producing major foliation, Sn; and (3) extensional ductile shearing (Dn+1). In the garnet zone, microstructural relationship defines two distinct stages of garnet growth. First-stage garnet contains slightly-curved inclusion trails (Si) continuous with Sn, suggesting syn-Dn growth. In contrast, second-stage garnet is post-tectonic and grows at the expense of biotite. In the staurolite zone, garnet porphyroblasts containing planar Si oblique to Sn suggest inter-tectonic (Dn-1~Dn) growth, whereas staurolite overprinting the Sn is post-Dn. In the kyanite zone, garnet also appears to be inter-tectonic, but some garnet porphyroblasts with sigmoidal Si favor syn-Dn-1 growth. This interpretation is corroborated by progressive change in inclusion minerals from ilmenite to ilmenite+rutile within garnet. Pseudomorphic textures and inclusion assemblages further suggest that kyanite has formed at the expense of garnet and chlorite, and that staurolite has grown along decompressional P-T path in lieu of garnet and kyanite during Dn+1. Our results demonstrate not only crustal thickening process followed by extensional collapse in an orogenic belt, but also complex P-T-t-D evolution in Barrovian-type metamorphic terrane.