Hostname: page-component-cd9895bd7-q99xh Total loading time: 0 Render date: 2024-12-25T02:31:34.608Z Has data issue: false hasContentIssue false
  • English
  • Français

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
Get access Rights & Permissions [Opens in a new window]

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

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

AI-Saleh, S., Fuge, R. and Rea, W.J. (1977) Proc. UssherSoc. 4, 37-48.Google Scholar
Barriére, M. and Cotten, J. (1979) Contrib. Mineral. Petrol 70, 183-92.CrossRefGoogle Scholar
Charoy, B. (1986) J. Petrol 27, 571-604.CrossRefGoogle Scholar
Chaudhry, M.N. and Howie, R.A. (1973a) Mineral. Mag. 39, 289-96.CrossRefGoogle Scholar
Chaudhry, M.N. (1973b) Proc. UssherSoc., 2 480-1.Google Scholar
Cundy, E.K., Windle, W. and Warren, I.H. (1960) Clay Minerals Bull. 4, 151-6.CrossRefGoogle Scholar
Dangerfield, J., Hawkes, J.R. and Hunt, E.C. (1980) Proc. UssherSoc. 5, 76-80.Google Scholar
Deer, W.A. (1937) Mineral. Mag. 24, 495-502.Google Scholar
Dodge, F.C. W., Smith, V.C. and Mays, R.E. (1969) J. Petrol. 10, 250-7.CrossRefGoogle Scholar
Exley, C.S. (1959) Q.J. Geol. Soc. 114, 197-230.CrossRefGoogle Scholar
Exley, C.S. and Stone, M. (1964) R. GeoL Soc. Cornwall, 150th Anniv. Vol., 131-84.Google Scholar
Exley, C.S. (1982) Hercynian intrusive rocks. In Igneous rocks of the British Isles (Sutherland, D.S., ed.) Wiley, Chichester, 287-320.Google Scholar
French, W.J. and Adams, S.J. (1972) Analyst, 97, 828-31.CrossRefGoogle Scholar
Foster, M.D. (1960) U.S. Geol. Surv. Prof. Paper 354- E.Google Scholar
Hall, A (1971) Proc. Geol. Assoc. 82, 209-30.CrossRefGoogle Scholar
Harvey, P.K., Taylor, D.M., Hendry, R.D. and Bancroft, F. (1973) X-Ray Spectrometry,, 2 33-44.CrossRefGoogle Scholar
Haslam, H.W. (1968)J. Petrol 9, 84-104.CrossRefGoogle Scholar
Hawkes, J.R. and Dangerfield, J. (1978) Proc. Ussher Soc. 4, 158-71.Google Scholar
Jefferies, N.L. (1984) Ibid. 6,35-41.Google Scholar
Jefferies, N.L. (1985) Mineral. Mag. 49, 495-504.CrossRefGoogle Scholar
Joyce, A.S. (1973) Chem. Geol. 11, 271-96.CrossRefGoogle Scholar
Manning, D.A. C. (1981) Contrib. Mineral Petrol. 76, 205-15.CrossRefGoogle Scholar
Manning, D.A. C. (1982) In Metallization associated with acid magmatism (Evans, A.M., ed.) Wiley, Chichester, 191-203.Google Scholar
Manning, D.A. C. and Exley, C.S. (1984) J. Geol. Soc. 141, 581-91.CrossRefGoogle Scholar
Martin, J.S. (1983) An experimental and field study of late-stage Li-rich gfranites. Unpubl. Ph.D. Thesis, Univ. of Manchester.Google Scholar
Martin, J.S. (1984)Proc. UssherSoc. 6, 142.Google Scholar
Neiva, A.M. R. (1976) Mineral. Mag. 40, 453-66.CrossRefGoogle Scholar
Nockolds, S.R. and Mitchell, R.L. (1948) Trans. R. Soc. Edinb. 61, 533-75.CrossRefGoogle Scholar
Norrish, K. and Hutton, J.T. (1964) C.S.I.R.O. Div. Soils, Adelaide, S.A., Rept. 3/64,10pp.Google Scholar
Palchen, W. and Tisehendorf, G. (1974) In Metallization Associated with Acid Magmatism, Vol. 1 (Štemprok, M., ed.) Geol. Surv. Prague, 206-9.Google Scholar
Plimer, I.R. and Kleeman, J.D. (1986) Trans. Inst. Min. Metall. (Sect. B, Appl. Earth Sci.) 95, B1–5.Google Scholar
Rieder, M. (1970) Contrib. Mineral. Petrol. 27, 131-58.CrossRefGoogle Scholar
Smith, T.E., Miller, P.M. and Huang, C.H. (1982) In Metallization Associated with Acid Magmatism, Vol. 6 (Evans, A.M., ed.), Wiley, Chichester, 301–20.Google Scholar
Štemprok, M. and Šulcek, Z. (1969) Econ. Geol. 64, 392-404.CrossRefGoogle Scholar
Stone, M. (1975) Proc. GeoL Assoc. 86, 155-70.CrossRefGoogle Scholar
Stone, M. (1979) Proc. UssherSoc. 4, 370-9.Google Scholar
Stone, M. (1982) In Metallization Associated with Acid Magmatism, Vol. 6 (Evans, A.M., ed.), Wiley, Chichester, 339-55.Google Scholar
Stone, M. (1984) Proc. Geol. Assoc. 95, 28-41.CrossRefGoogle Scholar
Stone, M. and Exley, C.S. (1978) Proc. Ussher Soc. 4, 172–81.Google Scholar
Troll, G., Farzaneh, A. and Cammann, K. (1977) Chem. Geol. 20, 295-305.CrossRefGoogle Scholar
Wilson, G.C. and Long, J.V. P. (1983) Mineral. Mag. 47, 191-9.CrossRefGoogle Scholar