The petrology and structure of the tertiary volcanic rocks of west-central Skye, N. W. Scotland

Abstract

An area of approximately 125 square kilometres of Lower Tertiary volcanic and associated intrusive igneous rocks in the Minginish district of west-central Skye was investigated. Detailed mapping of the lava field revealed that it was possible to divide the volcanic sequence into a series of units or groups; each characterised by associations of lava types and separated from one another by locally developed pyroclastic and sedimentary rocks including fluvial conglomerates. The nature, provenance and regional implications of the presence of various clasts within the conglomerates is discussed. Penecontemporaneous, subaerially weathered flow tops and fossil lateritic soils were also recognised and investigated. An overall volcanic stratigraphy is established and structural aspects are considered. Whole-rock and primary mineral chemical data indicate that the majority of lavas belong to mildly alkaline to transitional alkaline (basalt-hawaiite-mugearite-benmoreite- trachyte) suites but that rare tholeiites and true transitional basalts are also present. Comparisons with other Hebridean suites points to important regional variations. The wide spectrum and locally almost random distribution of lava types encountered in these suites and the presence of tholeiites late in the Skye suite implies a rather complex petrogenesis. This is reasoned to have involved various criteria, the most important of which were probably differing degrees of progressive partial and batch melting of a pre dominantly garnet Iherzolite upper mantle, varying ascent rates, polybaric crystal fractionation, re-equilibration of high-pressure phase to relatively low-pressure ones in an intricate series of upper crustal magma reservoirs, crustal contamination and possibly also batch mixing. The structure and petrology of the minor intrusions and immediately adjacent Plutonic rocks is also considered and an overall igneous sequence postulated. The volcanic sequence was variously affected by zeolite facies metamorphism resulting in the wide-spread systematic deposition of zeolite and associated mineral assemblages in vesicles and veins; a large-scale hydrothermal circulatory system having been established in the area subsequent to burial. Superimposed upon this is a thermal-metasomatic, contact aureole developed around the Cuillin Hills Intrusive Complex. In this aureole, a narrow, irregular zone of high- grade basaltic hornfels is succeeded outwards by zones characterised by the appearance of distinctive mineral assemblages at the expense of primary minerals and in the amygdales. The metamorphism is not considered to have been one of progressive change; each zone therefore developing independently. Consideration of the geochemical data for the meta-volcanics indicates that some degree of mobilisation of certain elements normally considered relatively unaffected under such conditions, characterises the aureole.