Hostname: page-component-cd9895bd7-mkpzs Total loading time: 0 Render date: 2024-12-28T02:48:58.835Z Has data issue: false hasContentIssue false
  • English
  • Français

Petrogenetic implications from biotite compositional variations in the Cornubian granite batholith

Published online by Cambridge University Press:  05 July 2018

M. Stone
M. Stone*
Affiliation:
Camborne School of Mines, University of Exeter, Redruth, Cornwall TR15 3SE, UK
Get access Rights & Permissions [Opens in a new window]

Abstract

Lithium oxide contents of biotites in some Cornubian granites which have been studied by electron microprobe are estimated from their SiO2 and F contents using samples formerly analysed by XRF and wet chemistry. Better formulae which include Li show that most of these biotites are lithian siderophyllites/ lithian Al-annites although some Dartmoor samples, especially those in the basic microgranite (ME) enclaves, are closer to Al-annites. Dartmoor samples are slightly more basic and poorer in Alvi than those from the Isles of Scilly and Carnmenellis granites agreeing with the broad differences in the host rocks. Biotites in volcanic clasts from the New Red Sandstone are significantly different in having much higher Si, Ti, Mg, F and ΣY and lower Alvi contents than those of the granite biotites: these are lithian Fe-Mg biotites. These clasts are believed to be the volcanic equivalents of an evolving Dartmoor magma (Awad et al., 1996). If so, their biotites reflect the composition of the granite biotites at the time of magma emplacement. They subsequently evolved as they equilibrated with the cooling granite to give the present biotite compositions of the Dartmoor granites.

Keywords

lithium oxidebiotitegranitevolcanic clast
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 4 , August 2000 , pp. 729 - 735
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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

Awad, N.T., Merefield, J.R., Scrivener, R.C. and Stone, M. (1996) Geochemistry and petrogenesis of volcanic clasts in the Permian breccias around Exeter. J. Geol. Soc., 153, 669–72.CrossRefGoogle Scholar
Chen, Y., Clark, A.H., Farrar, E., Wasteneys, H.A.H.P., Hodgson, M. J. and Bromley, A.V. (1993) Diachronous and independent histories of plutonism and mineralization in the Cornubian batholith, southwest England. J. Geol. Soc., 150, 1183–91.CrossRefGoogle Scholar
Chesley, J.T., Halliday, A.N., Snee, L.W., Mezger, K., Shepherd, T.J. and Scrivener, R.C. (1993) Thermochronology of the Cornubian batholith in southwest England: implications for pluton emplacement and protracted hydrothermal mineralization. Geochim. Cosmochim. Acta, 57, 1817–37.CrossRefGoogle Scholar
Darbyshire, D.P.F. and Shepherd, T.J. (1994) Nd and Sr isotope constraints on the origin of the Cornubian batholith, SW England. J. Geol. Soc., 151, 795–802.CrossRefGoogle Scholar
Speer, J.A. (1984) Micas in igneous rocks. Pp. 299–356 in: Micas (Bailey, S.W., editor). Reviews in Mineralogy, 13. Mineralogical Society of America, D.C.CrossRefGoogle Scholar
Stone, M. (1995) The main Dartmoor granites: petrogenesis and comparisons with the Carnmenellis and Isles of Scilly granites. Proc. Ussher Soc., 8, 379–84.Google Scholar
Stone, M. (1997) A geochemical dichotomy in the Cornubian batholith. Proc. Ussher Soc., 10, 206–10.Google Scholar
Stone, M., Exley, C.S. and George, M.C. (1988) Compositions of trioct ahedral micas in the Cornubian batholith. Mineral. Mag., 52, 175–92.CrossRefGoogle Scholar
Stone, M., Klominsky, J. and Rajpoot, G.S. (1997) Composition of trioctahedral micas in the Karlovy Vary pluton, Czech Republic and a comparison with those in the Cornubian batholith, SW England. Mineral. Mag., 61, 791–807.CrossRefGoogle Scholar
Scrivener, R.C. (1994) Presidential Address: aspects of Devon geology from Pengelly to the present day. Rept and Trans. Devonshire Assoc. for Adv. of Science, 126, 1–16.Google Scholar
Tindle, A.G. and Webb, P.C. (1990) Estimation of lithium contents in trioctahedral micas using microprobe data: application to micas from granitic rocks. Eur. J. Mineral., 2, 595–610.CrossRefGoogle Scholar
Tischendorf, G., Gottesman, B., Förster, H.-J. and Trumbull, R.B. (1997) On Li-bearing micas: estimating Li from electron microprobe analyses and an improved diagram for graphical representation. Mineral. Mag., 61, 809–34.CrossRefGoogle Scholar
Tischendorf, G., Förster, H.-J. and Gottesmann, B. (1999) The correlation between lithium and magnesium in trioctahedral micas: Improved equations for Li2O estimation from MgO data. Mineral. Mag., 63, 57–74.CrossRefGoogle Scholar
Ward, C.D., McArthur, J.M. and Walsh, J.N. (1992) Rare earth element behaviour during evolution and alteration of the Datmoor granite, SW England. J. Petrol., 33, 785–815.CrossRefGoogle Scholar