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Igneous and metamorphic processes in the Shap Granite and its aureole

By Stephen Lloyd Caunt

Abstract

The Shap Granite outcrops in eastern Cumbria, N.W.\ud England and is a post—orogenic granite intruded during the\ud Lower Devonian (ie 394 Ma) into rocks of Ordovician to\ud Siturian age. It is of adamellite composition and is notable\ud in having megacrysts of orthoclase which crystallised late\ud (relative to the matrix) by an essentially metasomatic\ud process. Late in its cooling history, hydrothermal fluids\ud resulted in alteration ano mineralisation in and about the\ud joint structures. Comparison of whole — rock element and\ud stable isotope data between the hydrothermally — altered and\ud non —altered granites shows that this late — stage process- was isochemical and only involved localised redistribution of the available elements, the hydrothermal fluid being derived from the granite itself and acting in a closed system.\ud \ud The associated suite of granitic dykes extends the\ud chemical range of Shap granitic rocks from the restricted\ud compositions of the granite itsetf (around 68%)to 62 to 77%\ud Si02. Mineralogical and chemical evidence suggests that the\ud Shap magma compositions were controlled mainly by biotite\ud and plagioclase fractionation over much of this silica\ud range.\ud \ud The granite is intruded about much of its outcrop into\ud rocks of the mid—Ordovician, calk—alkaline Borrowdatt\ud Volcanic Group. The aureate is lkm wide and 'generally\ud displays limited contact metamorphic reactions apart from in\ud the Blue Quarry, where higher sub—surface heat flow during\ud the granite's intrusion resulted in localised more extreme\ud metamorphic and some metasomatic conditions resulting in the\ud development of garnet veins. Analysis of 61 samples from the\ud aureole region show only very limited modifications to their\ud original calk—alkaline chemistry. Stable isotope and the\ud whole rock geochemistry indicates that the contact\ud metamorphism was essentialty, isochemical and that chemical\ud interaction with the granite did not occur. Aureole\ud metamorphic reactions took place at moderate temperatures\ud (250 — 400°C) apart from in a narrow (<1000 zone about the\ud granite contact where temperatures may have reached close to\ud those of the granite-solidus (around 600°C).\ud \ud Xenoliths from the 'granite Pink quarries at Shap contain the same mineralogy as their host granite, including\ud the megacryst orthoclase. The xenolith chemistry compares\ud well with that of the more basic dykes, with the xenoliths -\ud forming a coherent group over the compositional range 56 to\ud 674 Sia. Comparison with the country—rock chemistry shows\ud that the xenoliths were not externally derived but probably\ud represent quenched, more basic, comaqmatic melts related to\ud the generation of the main Shap Granite magma. Limited Sr\ud and Nd isotope data for the xenoliths show that these\ud isotopes were in equilibrium with the granite and lends\ud strong support for the cogeneric origins and chemical\ud relationships for the granites, aykes and xenoliths\ud together.\ud \ud The narrow aureole width is typical of those formed by\ud conductive ratner convective cooling with interaction with\ud the adjacent granite limited to thermal effects and not\ud involving the cycling of fluids through the granite or\ud aureole as in convective pluton cooling

Publisher: Earth Sciences (Leeds)
Year: 1986
OAI identifier: oai:etheses.whiterose.ac.uk:522

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