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The Significance of Almandine Garnets in the Lundy and Dartmoor Granites

Published online by Cambridge University Press:  05 July 2018

Maurice Stone
Maurice Stone*
Affiliation:
Department of Geology, University of Exeter, North Park Road, Exeter EX4 4QE
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Abstract

Almandine garnets in the cordierite-bearing granite of Sweltor Quarry, Dartmoor, contain < 10 mol. % of the spessartine end-member, whilst those in the Lundy granite have c. 10 mol. % spessartine. Experimental work indicates that such compositions can grow in equilibrium with siliceous melts at depths of 18–25 km. This evidence, reaction rims, lack of marked zoning and comparison with garnets in other siliceous calc-alkaline siliceous rocks point to a genesis involving partial melting of the ‘local’ lower crust. A restite origin rather than direct crystallization from magma is favoured but the evidence is equivocal. The Dartmoor granite (Hercynian) is a typical peraluminous late- to post-tectonic S-type granite. The S-type character of the Lundy (Tertiary) granite is revealed by the occurrence of garnet and topaz together with biotite enriched in Rb, Cs and F, despite its close association with Tertiary basic magmatism in an anorogenic setting.

Keywords

almandinegarnetgraniteDartmoorLundy
Type
Mineralogy
Information
Mineralogical Magazine , Volume 52 , Issue 368 , December 1988 , pp. 651 - 658
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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