Abstract
Chemical analyses of garnet in pelitic rocks have been made by means of an electron probe microanalyzer. Special attention is drawn to give some clue to kinetics of garnet growth. Edges of an individual garnet crystal gave considerable chemical variation ranging from 0.5 to 2.0 wt% in MnO content. This is not due to garnet growth under disequilibrium condition but due to the fact that growth of garnet crystal took place at special edges and all parts of the crystal surface were not fomed at the same time. Near-equilibrium growth of garnet is revealed by the partition of Fe and Mg between biotite, cordierite and garnet.
Garnets in some thin section showed three kinds of zoning patterns: homogeneous, n ormal and reverse zoning patterns. Each garnet crystal showed different composition at both of center and edge of the crystal. Nucleii of garnet crystals did not start to grow at the same temperature, and cessation of garnet garnet growth occurred at different temperature for each garnet crystal.
Reversely zoned garnets which show increase of MnO content toward edge of the crystal are common in the investigated rocks. This type of zoning is never explained by the Mn-depletion model, and it is unlikely that all the reverse zonings were due to enrichment of Mn in pore fluid by supply of the element from unstalbe minerals. Probably, changes in temperature and pressure are one of the most important factors controlling zoning features of garnet.
