The tertiary igneous geology of Strathaird, Skye

Abstract

This study is concerned with the Tertiary igneous rocks found within an area some twenty square miles in extent on the south-east side of the Skye central intrusion complex. The Tertiary rocks include basic lavas, a small intrusion of granophyre, the margin of the Cuillin Gabbro, and numerous minor intrusions. Extrusive rocks occupy an area of some seven square miles, and have an aggregate thickness of about 1,000 feet. The greater part of the sequence consists of alkaline olivine basalts, but there is also one horizon of trachybasalt and thin group of picritic flows. Pyroclastic rocks are rare, being limited to one small agglomerate vent and an isolated lens of tuff. It has been found possible to divide the sequence into several mappable units based on groups of flows possessing common petrographical features. The laves which lie within half a mile of the Cuillin Gabbro have been metamorphosed by that intrusion, while an outer zone of lower temperature alteration includes the whole of the strathaird lava outcrop. Within the thermal aureole a narrow zone of pyroxene hornfels along the contact with the Gabbro is followed outwards by a broader zone charaterized by the presence of an actinolitic amphibole. Within this pyroxene hornfels zone the lavas have been thoroughly recrystalized to high temperature mineral assemblages, but within the amphibolezone equilibrium was only locally attained, and many of the lavas still posses much of their original texture and mineralogy. The transition from unmetamorphed lava to pyroxene hornfels was not accomplished by independently under its own particular pressure-temperature conditions. On the basis of both chemical and petrographical evidence it is considered probable that a small amount of silica was added to at least some of the lavas during metamorphism. The internal structure and composition of the Cuillin Gabbro do not come within the provenence of this thesis, but some attention has been given to the relationships between the Gabbro and its country rocks, especially in the neighbourhood of camasunary. In this latter area there is an extensive development of basic minor instructions pre-dating the Gabbo, and metamorphosed in a similar manner to the nearby lavas. Most of these small intrusions are emplaced into Torridonian sandstones, which locally extend up to several hundred feet from the contact, the Torridonian shows clear evidence of having been partially melted by the high temperatures induced by the intrusion of gabbro magma, and many of the early intrusions are veined and broken into fragments by the mobilized material. The vein material is chemically more basic in composition than the sediments from which it was derived, partly due to reaction with the basic minor intrusions. Parallel to the contact of the Cuillin Gabbro, in Strathaird, there is a series of irregularly shaped outer-ops of granphyre which are thought to be parts of a single intrusion exposed in the vicinity of its roof. The granophyre is fine-grained, is unusual in containing paramorphs after tridymite, and differs chemically from other Skye granitic rocks. Through there is sometimes a well-defined, chilled margin, many of the contacts against sedimentary rocks are complex and highly xenolithin. The granophyre post-dates the Cuillin Gabbro, but is earlier than the large granitic intrusions which lie to the north and north-east of the areas. One of the most impressive features of the local geology is the great profusion of north-west dykes, all but a few of which are basic in composition. Most of the dykes are later date than the Cuillin Gabbro, but their distribution shows that they are closely related to that centre. Other minor intrusions include a few acid and basic skills which are, in general, of earlier date then the dyke swarm, while basic cone-sheets occur in the vicinity of the Cuillin Gabbro and are later than most of the north-west dykes. In Strathaird, erosion to a relatively deep level allows the structural relationships between lavas, intrusions, and pre-Tertiary sediments to be more easily examined than elsewhere in Syke. Most of the sediments are of Jurassic age, and these rest uncon-formably on Torridonian sandstones. There were several phases of warping in Mesozic times, but the main period of deformation occurred after the outpouring of the lavas and gave rise to large arcuate folds. It is believed that these folds are one result of the stresses set up by the intrusion of the Cuillin Gabbro.