A Petrological Study of Mafic Hypabyssal and Ultramafic Plutonic Rocks of the Cuillin Igneous Complex, Isle of Skye, Scotland

Van Claydon, Russell (1990) A Petrological Study of Mafic Hypabyssal and Ultramafic Plutonic Rocks of the Cuillin Igneous Complex, Isle of Skye, Scotland. PhD thesis, University of Glasgow.

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The Lower Tertiary Cuillin Central Igneous Complex of Skye was mapped by Harker between 1895 and 1900. He concluded that the ultrabasic and basic rocks were formed from repeated injections of heterogeneous magma which initially was ultrabasic and became progressively more evolved with time. It was not until the late 1950's and early 60's that the complex was again investigated, when a number of researchers proposed that many of the different rock types of the complex could be related by differentiation processes which were almost exclusively envisaged to be gravity settling of early. cumulus phases accompanying crystal-liquid fractionation. The object of the present investigation was two-fold, namely to characterize the geochemistry of some of the myriad of mafic minor intrusions which transect and are associated with the plutonic rocks of the complex, and, secondly to remap the ultrabasic rocks (mainly peridotites)which cropout in An Garbh-choire and its environs within the Southern Cuillin. In the latter, particular emphasis has been placed on lithological and structural relationships within the peridotites to ascertain the processes involved in their genesis and what inferences may be drawn in the broader context of the evolution of the complex as a whole. Most notable of the minor intrusions is a suite of basic cone-sheets which are broadly arcuate in plan and which appear to dip to a common focal point coincident with the centre of the complex, at an approximate depth of 4km. Cross cutting field relationships confirm that the cone-sheets were emplaced in a protracted, progressive event over a period of time, rather than in separate isolated events as proposed by previous workers. This is supported by major, trace and rare-earth-element geochemical evidence which fails to distinguish any significant criteria for variation of the parental magma type with time. It is shown that the parental magma to the cone-sheets is indistinguishable, albeit more evolved, from the previously identified Preshal Mhor Type (Esson et al 1975) which tholeiitic in nature. Although geochemical modelling has shown that much of the intrasuite variation displayed by the cone-sheets may be produced by redistribution of the observed phenocryst assemblage of principally plagioclase, with minor amounts of olivine, the well constrained trends of the suite to more evolved representatives requires the involvement of a phase whose Ca/AI ratio exceeds unity. It is proposed, on the basis of major, trace and rare earth element data, that the occult phase is a (monoclinic) calcium-rich pyroxene. Modelling of crystal-liquid fractionation utilising these three phases produces predicted magma compositions which are in close agreement to those actually observed. Therefore, it is concluded that crystal-liquid fractionation, involving plagioclase, pyroxene and minor olivine, in the calculated proportions 55:35:10 was the dominant process involved in the evolution of the suite. Some samples show plagioclase phenocrysts present in quantities greater than is called for by the 3-phase cotectic signature displayed by the normative geochemistry of the suite. Mechanisms for this selective fractionation are discussed and it is tentatively suggested that gravity settling of the dense pyroxene and concomitant flotation of the less dense feldspar occurred. As this mechanism can be shown not to have occurred in-situ a storage-processing reservoir must have existed at a relatively shallow but greater depth. A comparative geochemical study of certain so-called Intrusive Tholeiite Sheets (Hutchison 1964, 1966) was also undertaken. Data reported here indicate certain geochemical similarities between the Intrusive Tholeiite Sheets and the Cone-sheets. However, no simple evolutionary model could be constructed which explains the geochemical evolution of these sheets. This, in part may be due to the advanced state of secondary alteration within most of the material analysed. In contrast to the liquid compositions generally preserved within the cone-sheets, the peridotites of An Garbh-choire are considered to represent the solid residuum of extreme crystal-liquid fractionation. They do, however, preserve evidence for both-syn and postcumulus processes. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Geology, Petrology
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78142
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:39
Last Modified: 30 Jan 2020 15:39
URI: https://theses.gla.ac.uk/id/eprint/78142

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