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The Lundy granite: a geochemical and petrogenetic comparison with Hercynian and Tertiary granites

Published online by Cambridge University Press:  05 July 2018

Maurice Stone
Maurice Stone*
Affiliation:
Earth Resources Centre, University of Exeter, North Park Road, Exeter EX4 4QE
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Abstract

New chemical data show that the two main granite types (G1 and G2) cannot be discriminated, but that microgranite sheets/dykes (G3) are significantly different and more evolved, largely as a result of biotite, accessory mineral, and plagioclase fractionation. The Lundy granite is similar to other Tertiary granites of Scotland and Ireland, in age, setting, possible high-temperature mineralogy, relationship to basic magmatism, and REE patterns. These features and a highly evolved chemistry suggest derivation from an unexposed more ‘primitive’ granite that, in turn, had a basaltic parentage. However, similarities with the nearby S-type Hercynian granites, such as high aluminium saturation index (and normative corundum), high trace alkali, Nb, and F contents, low Zr, and high initial Sr ratio suggest a significant crustal component. The problem is resolved by proposing either mixing of silicic magma derived by strong fractionation of basaltic magma with anatectic magma from a pelitic/semi-pelitic crustal source, or fractionation of basaltic magma heavily contaminated by assimilated crustal material. Both origins would yield the high REE contents and fiat REE patterns of a ‘primitive’ granite magma. Fractionation, perhaps of hornblende initially, and later, of biotite and accessory minerals together with feldspars, would produce the small volume of highly fractionated Lundy granite.

Keywords

Lundy granitegeochemistrypetrogenesisCornubian graniteTertiary granite
Type
Petrology and Experimental Studies
Information
Mineralogical Magazine , Volume 54 , Issue 376 , September 1990 , pp. 431 - 446
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1990

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