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Geodynamic significance of post-Variscan intrusive and extrusive potassic magmatism in SW England

Published online by Cambridge University Press:  03 November 2011

P. T. Leat ,
R. N. Thompson ,
M. A. Morrison ,
G. L. Hendry  and
S. C. Trayhorn
P. T. Leat
Affiliation:
Department of Geology, Imperial College of Science and Technology, Prince Consort Road, London SW7 2BP, England.
R. N. Thompson
Affiliation:
Department of Geology, Imperial College of Science and Technology, Prince Consort Road, London SW7 2BP, England.
M. A. Morrison
Affiliation:
Department of Geological Sciences, University of Birmingham, Birmingham B15 2TT, England.
G. L. Hendry
Affiliation:
Department of Geological Sciences, University of Birmingham, Birmingham B15 2TT, England.
S. C. Trayhorn
Affiliation:
Department of Geology, Imperial College of Science and Technology, Prince Consort Road, London SW7 2BP, England.
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Abstract

Post-Variscan magmatism in SW England involved the synchronous emplacement of basaltic and potassic lavas, minette dykes and the Cornubian granite batholith at c. 290 Ma. The basaltic and potassic rocks have high contents of Ni and Cr, which suggest that both are not excessively fractionated. The basaltic lavas are moderately enriched in LREE and LIL elements relative to HREE, whereas the chemically-varied potassic lavas are more strongly enriched in LREE and LIL elements, with notable depletions in Nb, Ta and Ti relative to LREE. These features are consistent with the view that these rocks are subduction-related. Possibly the potassic rocks were derived from an ultimate source in lithosphere subducted or downthrust during the Variscan orogeny. The source of the basaltic rocks was probably in the asthenosphere. The minette dykes are chemically similar to the potassic lavas, suggesting that they are genetically related. Most dykes occur in a zone up to 25 km wide around the margin of the granite batholith, in a “shadow-zone” relationship. The granite batholith (c. 48,000 km3) is moderately enriched in Th and HFS elements, but is strongly enriched in Rb. Rb-Th relationships indicate an origin for the granite by fractionation from potassic magma in addition to melting of crust.

Keywords

assimilationbasaltCornubian batholithgraniteminettepetrogenesisrhyoliteRubidiumshoshoniteS-type graniteThorium
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 77 , Issue 4 , 1987 , pp. 349 - 360
Copyright
Copyright © Royal Society of Edinburgh 1987

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