Zoning in garnets from the mainland Lewisian

ABSTRACT. Two types of zoning are found in garnets from Lewisian rocks. One is due to retrogression of an earlier homogeneous garnet while the other esults from growth (or equilibration) during failing temperatures. Both zoning patterns result from metamorphism during or after the Dionard tectonic phase of the Laxfordian event. ZONING in garnets is a common phenomenon (Hollister, 1969; Edmunds and Atherton, 1971 and many others) and has been reported in garnets from some Lewisian rocks (Josey and Shaw, 1974; Wood, 1975). The pattern described by the latter authors is a two-stage growth model with early Mg-rich garnet cores giving way to late marginal increases in Fe and Ca. This zoning pattern differs from what has been termed 'normal ' zoning where Mn is enriched in the garnet centre relative to the rims, and with Fe and Mg showing a reverse tendency. However, variations of, and deviations from, this 'normal ' pattern are common. This is particularly true of garnets from rocks which have been subjected to polymetamorphism, or other P-T regimes, or are lithologically different from the pelitic assemblages that are usually studied. The garnetiferous rocks of the Lewisian complex are therefore significant since they differ on all points. Garnets, occurring in a variety of structural settings and environments ofmetamorphic mineral growth, were collected and analysed for zoning using a Cambridge Instrument Geoscan micro-analyser. Analytical procedure was that described by Smellie (1972). Geological setting The crystalline rocks of the Kylesku-Loch Inchard area consist of pyroxene granulites of the Kylesku group to the south (Khoury, 1968; Bowes and Hopgood, 1973) and amphibolite-facies gneiss of the Rhiconich group to the north (Dash, 1969; Chowdhary and Bowes, 1972). The former were metamorphosed in the Badcallian ( = early Scour-ian) event, about 2700 Ma (Moorbath et al., 1969), while isochrons of 2850Ma (Lyon and Bowes, 1977), obtained from the latter area, suggest that t ~ Copyright the Mineralogical Society it is of equivalent age. Common in the area are numerous NW-trending tholeiitic dykes which are considered by many workers (Sutton and Watson