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Compositions of trioctahedral micas in the Cornubian batholith

Published online by Cambridge University Press:  05 July 2018

M. Stone ,
C. S. Exley  and
M. C. George
M. Stone
Affiliation:
Department of Geology, The University, Exeter EX4 4QE
C. S. Exley
Affiliation:
Department of Geology, The University, Keele, Staffs ST5 5BG
M. C. George
Affiliation:
Department of Geology, The University, Exeter EX4 4QE
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Abstract

New major and trace element chemical analyses of trioctahedral micas of the Cornubian batholith emphasize their enrichment in a ‘trace-alkali element’ association (Li, Rb, Cs, F, Nb, Mn) and their deficiency in a ‘femic element’ association (Zr, Ce, Th as well as Mg and Ti) compared with micas from many other granite suites, although there are similarities with some Hercynian granites and with rare metal pegmatites. The new data demonstrate a continuous series from siderophyllite through zinnwaldite to lepidolite, principally as a result of Li-R2+ substitution as indicated by Foster (1960), although a more complete replacement is (Li, Al) = (Fe2+, Fe3+, Ti). It is suggested that the ranges of these micas are based upon the Li content in the unit cell formula and the ratio of Li to R2+, in effect, a compromise between the ranges proposed by Foster (1960) and Rieder (1970). Microprobe analyses lack Li2O, but can be plotted on FeO-SiO2 and FeO-Al2O3-SiO2 diagrams (wt. or atom %) in order to locate compositions within the trioctahedral LiFe micas and to distinguish between lepidolite and muscovite.

An examination of 55 new mica analyses shows that hornfels biotites are richer in Mg and that the Cornubian Type A granite (as classifed by Exley and Stone, 1982) micas are richer in Ti and Fe compared with those of Type B granites. Micas from microgranite dykes appear to be poorer in femic elements and richer in trace alkalis and F compared with their Type B granite hosts, consistent with their differentiation from the latter. Mica chemistry is consistent with the magmatic evolution of the A-B-microgranite sequence in the biotite granites, the transformation of Types B to D upon emplacement of E, the derivation of Types E from B by extreme differentiation or metasomatic transformation and mobilization, and the in situ differentiation of Types G from E.

Keywords

micasCornubian batholithgraniteCornwall
Type
Petrology
Information
Mineralogical Magazine , Volume 52 , Issue 365 , April 1988 , pp. 175 - 192
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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