Geology of the loch Ailsh complex, Assynt

Abstract

The Loch Ailsh intrusion, Assynt, was interpreted by Phemister (1926) as a stratified laccolite, the basal ultrabasic, intermediate, and upper leucocratic portions being differentiated at depth and intruded separately. The whole complex rested on a thrust plane. The leuco-syenites are here divided into three intrusions, SI, S2 and S3. S3 is latest and roofs S1-2. Their contacts are sharp or gradational, when feldspars of S1-2 type are enclosed as xenocrysts in S3. Basic alkaline rocks form a discontinuous zone at the top of S2; calc-silicate rocks occur at the same horizon. Similar basic types grade into limestone xenoliths in the north of the intrusion. A magnetometer survey showed anomalies corresponding to the outcrop of the ultrabasic rocks allowing their continuation to be traced. The magnetic properties of these rocks require that the induced component of the magnetization be used in interpretation of the anomalies. They are caused by a steeply inclined sheet. Further ultrabasic masses were proved by excavation. Brief petrographic description of the rook types is made with modal data. Cell dimensions of pyroxenes from metamorphosed limestones and basic and ultrabasic types are identical. The mafic aggregates in the basic rocks represent the remains of material of metamorphic origin. The alkali feldspars are described in terms of their appearance in thin section, optic axial angle, bulk composition and X-ray powder and single crystal properties. They are low albite-microcline and low albite-orthoclase perthites in the compositional range Or(_27)-Or(_41). Only S1 and S2 have mono-clinic material in the potassium phase. There may be a systematic distribution of microcline obliquities with locality. Little modification of feldspars was found at contacts between syenites and xenocrysts preserve higher temperature features than host material. Minimum crystallization temperatures are suggested. The variation depended on concentration of volatiles during cooling. A mechanism is suggested for the metastable preservation of orthoclase. The ultrabasic rocks are a skarn between limestone and syenite. The basic alkaline rocks are hybrids of feldspathic magma contaminated by metamorphic material, and represent the remains of a roof to S2, disrupted by S3, The mass does not have a major thrust at its contacts and is a stock like body with stoping its mode of emplacement.