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Title: Petrogenetic studies of the appinite suite of western Scotland
Author: Hamidullah, S.
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1983
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Mineral and rock chemistry together with petrography have been used in association with previously determined field relationships to elucidate magma genesis, conditions of crystallisation and geotectonic environment of the appinite suite of western Scotland and northwestern Ireland. The major part of the atudy has been related to the type area of this suite, that of the Ballachulish district in the parish of Appin. In addition, the major cluster of appinites in the Loch Lomond district has been studied together with appinites from Colonsay and Ardara, Dongal. Comparison has been made with the scyelite-pyroxenite-appinitic type rock association of Central Sutherland and with the lamprophyres of the Ballachulish district and lavas of Old Red Sandstone age (late Silurian) in western Scotland. These comparative studies have been used to establish which products of the late Caledonian igneous activity were comagmatic and to determine whether the appinite suite of the type area developed under unique conditions or whether comparable maema genesis, emplacement and crystallisation conditions were widespread in Caledonideso The appinite suite of western Scotland and northwestern Ireland was developed from a volatile-rich basaltic magma. For the :Ballachulish cluster the magma. was of high-potassium calc-alkaline type and evolved towards a relatively continental environment. For the Ardara cluster it was transitional between calc-alkaline and tholeiitic and evolved towards an island arc environment while for the Loch Lomond cluster it was a normal calc-alkaline type evolved in environments intermediate between the other two. In the Ballachulish appinite suite, olivine appeared on the liquidus at about 70-80 km depth. Subsequently emplacement into high crustal level occurred. With magma uprise being impeded by structural traps of the folded Dalradian Supergroup, the rest of crystallisation occurred under decreasing temperature but highly variable PGas due to explosive activity in the subvolcanic pipes. Following olivine, the successive fractionation of clinopyroxene, amphibole, mica and plagioclase led to the progressive development of rocks from ultramafic to acidic compositions. In addition, accumulation of mafic phases occurred in the early st8.$'9sof fractionation, under increasing PGas which led to explosive activity that disrupted the early formed cumulates followed by their upward movement as large "blocks", crystal mushes and individual crystals with variable proportions of the residual liquid. Thus, appinites, kentallenites and even diorites represent "mixed rocks" in which particular phenocrysts are not in a matrix representing the particular liquid with which they were in equilibrium. Explosive activity associated with the emplacement of the marginal ultramafic rocks, appinites and majority of the diorites occurred at higher PGas (4-5 kb) than that associated with the emplacement of kenta1lenites (1 kb). The late leucocratic diorites and granodioritea represent crystallisation from a residual liquid which probably also contained a small component of a hybrid secondary liquid produced by the solution of silica from quartzite, which formed structural traps, at high P-T. Gas-streaming continued after igneous emplacement of various rock types. Corresponding petrogenetic processes were also responsible for the development of the Loch Lomond and Ardara clusters. However, due to the variable strengths of the structural traps, in the Loch Lomond cluster PGas reached before explosive breaching of the traps was generally not so high as at Eallachulish while in the Ardara cluster it was higher than both of the other two clusters. The lamprophyres and lavas of the Old Red Sandstone age (late Silurian) are genetically related to the appinite suite. The scyelite-pyroxenite-appinitic type rock association of Sutherland does not represent a cognate petrogenetic suite of rocks. Scyelite and Si02-poor appinitic type rocks are the products of crystallisation differentiation from a basaltic ma~ and are genetically related to the appinite suite. Pyroxenites and Si02-rich appinitic type rocks represent the hybrids of Ach'uain type. The basaltic magma from which the various appinite clusters crystallised were mantle-derived, at depths of ~o200 km and associated with a northwestward dipping subduction zone in late Silurian time, i.e. in the latter stages of the Caledonian orogeny .
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QE Geology Geology Mineralogy Sedimentology Geochemistry