XVIII.—A Re-examination of Barrow's Older Granites in Glen Clova, Angus

Abstract

SynopsisBarrow describes from Glen Clova an important Older Granite intrusion, the muscovite-biotite-gneiss which he holds responsible for the formation of staurolite, kyanite and sillimanite zones of progressive regional metamorphism in the district. He states that the gneiss was filter-pressed during emplacement so that potash was strained off and concentrated in the south-eastern outcrops of the intrusion leaving its north-western portions enriched in soda. He claims that by the same process the separate intrusive areas comprising the gneiss have often been marginally enriched in potash and pegmatite.The factual basis of the filter-press hypothesis does not withstand careful re-examination. The individual intrusive areas of muscovite-biotite-gneiss bear no marginal zones of potash and pegmatite enrichment like those envisaged by Barrow. The muscovite-biotite-gneiss in its entirety comprises two distinct units, a heterogeneous group of banded quartz-oligoclase-mica-gneisses that were intensely deformed plastically during high-grade metamorphism, and a later series of non-migmatitic microcline-granite intrusions. In neither unit can any regular variation in potash/soda ratio be detected in passing directly from north-west to south-east.New data are provided concerning the distribution and mode of occurrence of Barrow's metamorphic zonal index minerals. Sillimanite, as fibrolite, often replaces mica and accompanies kyanite in specimens from many localities.