POLYPHASE GARNETS FROM WESTERN IRELAND - 2-PHASE INTERGROWTHS IN THE GROSSULAR-ALMANDINE SERIES

Low-grade metasediments of the Clew Bay Complex on south Achill Island , Co. Mayo, Ireland, include sparse garnet-bearing schists in which in dividual garnet grains are polyphase. The central kernel is pyrope-ric h (up to 40 mol.% pyrope), and has a corroded outline. Overgrowing thi s is a zone of spessartine-almandine, which grades out into grossular- almandine, comprising the bulk of each grain. The kernel of each garne t is homogeneous, but kernels of adjacent grains show appreciable diff erences in composition, for which reason they are interpreted as detri tal relicts. Back-scattered electron imaging reveals that the grossula r-almandine dominated portion is composed of two distinct phases, inti mately intergrown on a scale of a few micrometers. One phase has gross ular < 21 mol.%, the other has > 22 mol.% grossular, and the compositi onal difference between immediately adjacent material is typically aro und 4 mol.%. Two types of intergrowth occur: a laminar pattern that is apparently crystallographically controlled,and a more irregular flame texture. The textures may have originated during garnet growth at sli ghtly lower grades than is normal, because detrital garnet kernels, wh ich are in crystallographic continuity With the rest of the grain, act ed as nuclei. Four possible interpretations of the origin of these tex tures are considered, but none is entirely satisfactory. They are: a) equilibrium growth below a hypothetical solvus, b) rapid, non-equilibr ium growth, c) unmixing of a metastable precursor, d) replacement of p re-existing heterogeneity in the rock matrix. Irrespective of origin, the presence of subtle heterogeneities in the garnet emphasises the ne ed for circumspection in the use of low-grade garnets for geobarometry .