Discussion

Abstract

Dr J. T arney said that while he accepted that the simplest interpretation of the geophysical data to the north of the Ben Stack line suggested a model in which the Laxfordian gneisses were a supracrustal series underlain by a pyroxene-granulite basement at a depth of about 3 km, he wondered whether a model in which there was a gradational increase in density with depth would be equally compatible with the data. Would this not allow for a larger amount of suitable material to be present at depth from which to derive the granitic intrusions in the vicinity of the Ben Stack Line ? Is it not possible that granulite facies metamorphism of such material at a depth of, say, 6 km could yield granitic fluids plus a higher density granulite facies residuum ? Nevertheless he agreed with the general point that higher density granulite facies gneisses must underlie the Laxfordian grey gneisses and that the latter were not simply ‘made-over’ Scourian. Work which Mr. S. J. Moorhouse has been doing on the Lewisian inliers in the Moines, near Tongue, immediately to the east of the area, has shown that many of the Lewisian slices there correspond geochemically with the Laxfordian grey gneisses of the Rhiconich area. However in the Borgie inlier, some of the Lewisian granulites have the high K/Rb and Ba/Rb and low Rb/Sr, K/Ba and K/Sr ratios characteristic of the Scourian assemblage. This area is structurally well to the north of the Ben Stack Line. The evidence suggestes