富山県南東部和田川地域の泥質片麻岩中の逆累帯ざくろ石

Abstract

The Wada-gawa area, central part of the Hida metamorphic belt, consists of diverse gneisses of the upper amphibolite facies, which were later intruded by several small granitic bodies. Garnet porphyroblasts in pelitic gneisses are replaced by biotite along their rims and cracks, and are separated into smaller parts by biotites films. The replacement proceeds more remarkably in the neighbourhood of the small granitic bodies, suggesting that the replacement is closely related to the granitic intrusion. Each of the separated parts of the garnet shows a reverse zoning in which Fe and Mn contents increase and Mg content decreases from the core to the rim, although the core of some larger garnet is homogeneous. The compositional contour of the reverse zoning is concordant with irregular outline of the resorbed garnet. The mode of occurrence and the zoning pattern of garnet suggest that the reverse zoning of garnet was formed by volume diffusion in garnet crystal during the retrogression of garnet which was closely associated with the intrusion of small granitic bodies.