Polyphase garnets from western Ireland: two-phase intergrowths in the grossular-almandine series
Yardley, Bruce W.D.; Condliffe, Eric; Lloyd, Geoff E.; Harris, David H.M.
European Journal of Mineralogy Volume 8 Number 2 (1996), p. 383 - 392
published: Apr 26, 1996
manuscript accepted: Nov 9, 1995
manuscript received: Nov 9, 1995
ArtNo. ESP147050802007, Price: 29.00 €
Abstract Low-grade metasediments of the Clew Bay Complex on south Achill Island, Co. Mayo, Ireland, include sparse garnet-bearing schists in which individual garnet grains are polyphase. The central kernel is pyrope-rich (up to 40mol.% pyrope), and has a corroded outline. Overgrowing this is a zone of spessartine-almandine, which grades out into grossular-almandine, comprising the bulk of each grain. The kernel of each garnet is homogeneous, but kernels of adjacent grains show appreciable differences in composition, for which reason they are interpreted as detrital relicts. Back-scattered electron imaging reveals that the grossular-almandine dominated portion is composed of two distinct phases, intimately intergrown on a scale of a few micrometers. One phase has grossular 22mol.% grossular, and the compositional difference between immediately adjacent material is typically around 4mol.%. 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 slightly lower grades than is normal, because detrital garnet kernels, which are in crystallographic continuity with the rest of the grain, acted as nuclei. Four possible interpretations of the origin of these textures are considered, but none is entirely satisfactory. They are: a) equilibrium growth below a hypothetical solvus, b) rapid, non-equilibrium growth, c) unmixing of a metastable precursor, d) replacement of pre-existing heterogeneity in the rock matrix. Irrespective of origin, the presence of subtle heterogeneities in the garnet emphasises the need for circumspection in the use of low-grade garnets for geobarometry