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Enclaves and their bearing on the origin of the Cornubian batholith, southwest England

Published online by Cambridge University Press:  05 July 2018

James A. Stimac ,
Alan H. Clark ,
Yanshao Chen  and
Sammy Garcia
James A. Stimac
Affiliation:
Earth and Environmental Sciences, MS-D462, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Alan H. Clark
Affiliation:
Department of Geological Sciences, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Yanshao Chen
Affiliation:
Department of Geological Sciences, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Sammy Garcia
Affiliation:
INC-15, MS-G776, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
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Abstract

Enclaves of diverse origin are present in minor amounts in the coarse-grained biotite granites of the Cornubian batholith, southwest England. The most common enclave type is layered, rich in biotite, cordierite and aluminosilicates, and has textures and compositions that reveal variable degrees of melt extraction from metasedimentary source rocks. Rare sillimanite-bearing enclaves represent residual material, either from the region of magma generation or its ascent path, but most such enclaves were probably derived from the contact aureole closer to the present level of exposure. These non-igneous enclaves (NIE) and their disaggregation products are present in all major plutons, comprising from < 2 to 5 vol.% of the granites. Enclaves of igneous origin are also present in all major plutons except Carnmenellis, generally comprising < 1 vol.% of the granites. The most common type is intermediate in composition, with microgranular texture, and mineral compositions and textures consistent with an origin by magma mixing. Large crystals of K-feldspar, plagioclase and quartz, common in these microgranular enclaves (ME) but absent in NIE, represent phenocrysts derived from the silicic end-member during magma mixing events rather than products of metasomatism as suggested previously. Although the composition of the mafic end-member (basaltic or lamprophyric) involved in the mixing process is poorly constrained, the presence of ME in the granites, and the preponderance of mantle-derived mafic rocks in the coeval Exeter Volcanics, indicate that mafic magma injection into the crust was a factor in the generation of the batholith. Advection of sub-crustal heat provides an explanation for large-volume crustal melting in regions of relatively thin crust such as southwest England.

Keywords

enclavesmagma mixingS-type graniteCornubian batholith
Type
Geochemistry
Information
Mineralogical Magazine , Volume 59 , Issue 395 , June 1995 , pp. 273 - 296
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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Footnotes

*

Present address: Dept. of Physics, University of Toronto, 60 St. George St., Toronto, Ontario, Canada M5S 1A7

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