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The Tregonning granite: petrogenesis of Li-mica granites in the Cornubian batholith

Published online by Cambridge University Press:  05 July 2018

Maurice Stone
Maurice Stone*
Affiliation:
Geologisk Institut, Aarhus Universitet, C.F. Møllers Allé 8000 Aarhus C, Denmark
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Abstract

Li-mica (zinnwaldite and/or lepidolite)—topaz—albite granites in the Tregonning—Godolphin pluton and similar rocks in the St. Austell pluton appear to be petrogenetically unrelated to the spatially associated biotite granites. Evidence is provided by lack of development of Li-mica granites at roof zones of biotite granites and markedly different trends and composition fields in bivariate plots such as Li vs. Cs, Rb vs. Sr and Nb vs. Zr. Thus, differentiation of biotite granite magma is unlikely to have generated Li-mica granite magma, as also, on its own, is partial melting of biotite granite or biotiteabsent residual lower crust. However, partial melting of biotite-rich residual rocks involving biotite breakdown could yield a trace alkali- and F-enriched melt, although this would require marked femic mineral, K-feldspar and anorthite fractionation, and Na-enrichment. It is proposed that volatiles derwed from either a mantle source or the crust/mantle interface have aided metasomatism of either residual S-type crust that earlier provided S-type biotite granite magma, or basic (biotite-rich) granitoid, to produce a low-temperature, low-viscosity Li-mica granite melt that rose rapidly in the crust soon after the emplacement of associated biotite granites.

Keywords

lithiummicagraniteTregonningCornubian batholithzinnwalditelepidolite
Type
Petrology and Geochemistry
Information
Mineralogical Magazine , Volume 56 , Issue 383 , June 1992 , pp. 141 - 155
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1992

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Footnotes

*

Present address: Earth Resources Centre, University of Exeter, North Park Road, Exeter, Devon EX4 4QE, England.

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