Two‐stage growth of porphyroblastic biotite and garnet in the Barrovian metapelites of the Imjingang belt, central Korea

Abstract

AbstractPorphyroblastic biotite and garnet in the Barrovian metapelites of the Imjingang belt, Korea, were investigated to unravel the sequence and mechanism of mineral growth. Poikiloblastic biotite contains straight inclusion trails (Si) discontinuous to the major foliation, and develops clear zones at the grain margin. These microstructures suggest an initial growth of biotite between two contractional deformations (Dn−1 and Dn) followed by an overgrowth during Dn. Although garnet poikiloblasts contain variable Si patterns, their major growth is likely to have occurred during Dn on the basis of compositional relationships among variable garnet types. Early poikiloblasts of both minerals were formed by chemical replacement of the matrix that consisted mainly of chlorite, muscovite and quartz. Subsequent growth of biotite was governed by a crack‐filling mechanism, and was accompanied by the production of extensional cracks inside or around biotite, providing fluid pathways. The overgrowth of garnet was favoured at the biotite–garnet interface, and the consequence was a partial replacement of inclusion‐poor garnet after biotite subsequent to Dn. In addition, clear zones and pressure shadows as well as the matrix around biotite porphyroblasts were replaced by garnet, suggesting an inheritance of various pre‐existing microstructures in the Si pattern of garnet. Further attention is thus required for any attempt to delineate the microstructural interaction between deformation and metamorphism, particularly in a sample containing early‐grown porphyroblasts. Microstructural evidence for the two‐stage growth of biotite and garnet is present up to the kyanite zone, indicating that this growth mechanism is prevalent during progressive metamorphism of Barrovian metapelites.