XVI.—Metamorphism of Tertiary Lavas in Strathaird, Skye

Abstract

SynopsisIn Strathaird, Tertiary lavas occupy an area of about 7 square miles and are up to 1000 ft. thick. Most of the sequence consists of alkaline olivine basalts but there is also a flow of trachybasalt and a thin group of picritic lavas. Pyroclastic rocks are rare.The area lies along the south-eastern margin of the Cuillin gabbro and thermal metamorphism is evident to a varying degree over the whole of the lava outcrop. In the immediate vicinity of the gabbro the lavas have been recrystallized to pyroxene-bearing hornfelses, and these rocks are followed outwards by a broader zone characterized by the presence of actinolite, chlorite, albite and epidote. In the outermost of the three zones, which extends for at least 3 miles from the gabbro, thermal effects are limited to the local growth of chlorite, albite and epidote. When compared with Turner's (1958) scheme of metamorphic facies the inner zone is equivalent to the pyroxene hornfels and sanidinite facies whereas the two outer zones form subdivisions of the albite-epidote hornfels facies. The hornblende hornfels facies appears to be only poorly represented by a few hornfelses found on the outer boundary of the inner zone.The metamorphism was not progressive since the mineral assemblages of each zone developed independently from unmodified lavas. There is evidence that a small amount of silica was added to some of the inner zone lavas during metamorphism.Eight new chemical analyses of metamorphosed basalts are presented, together with one new analysis of a metamorphosed bole.