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The Significance of Almandine Garnets in the Lundy and Dartmoor Granites

Published online by Cambridge University Press:  05 July 2018

Maurice Stone
Maurice Stone*
Affiliation:
Department of Geology, University of Exeter, North Park Road, Exeter EX4 4QE
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Abstract

Almandine garnets in the cordierite-bearing granite of Sweltor Quarry, Dartmoor, contain < 10 mol. % of the spessartine end-member, whilst those in the Lundy granite have c. 10 mol. % spessartine. Experimental work indicates that such compositions can grow in equilibrium with siliceous melts at depths of 18–25 km. This evidence, reaction rims, lack of marked zoning and comparison with garnets in other siliceous calc-alkaline siliceous rocks point to a genesis involving partial melting of the ‘local’ lower crust. A restite origin rather than direct crystallization from magma is favoured but the evidence is equivocal. The Dartmoor granite (Hercynian) is a typical peraluminous late- to post-tectonic S-type granite. The S-type character of the Lundy (Tertiary) granite is revealed by the occurrence of garnet and topaz together with biotite enriched in Rb, Cs and F, despite its close association with Tertiary basic magmatism in an anorogenic setting.

Keywords

almandinegarnetgraniteDartmoorLundy
Type
Mineralogy
Information
Mineralogical Magazine , Volume 52 , Issue 368 , December 1988 , pp. 651 - 658
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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References

Allan, B.D. and Clarke, D.B. (1981) Occurrence and origin of garnets in the South Mountain batholith, Nova Scotia, Canada. Can. Mineral. 19, 19-24.Google Scholar
Birch, W.D. and Gleadow, A.J.W. (1974) The genesis of garnet and cordierite in acid volcanic rocks: evidence from the Cerberean cauldron, central Victoria, Australia. Contrib. Mineral. Petrol. 45, 1-13.Google Scholar
Bott, M.H.P., Day, A.A., and Masson-Smith, D. (1958) The geological interpretation of gravity and magnetic surveys in Devon and Cornwall. Phil. Trans. R. Soc. London, 251 A, 161-91.Google Scholar
Brammall, A. and Bracewell, S. (1936) Variability of garnets in granites. Mineral. Mag. 24, 254-6.Google Scholar
Brammall, A. and Bracewell, S. and Rama Ran, B. (1936) The variable composition of cordierite in the Dartmoor granites. Ibid. 24, 257- 9.Google Scholar
Brammall, A. and Bracewell, S. and Harwood, H.F. (1923) The Dartmoor granite: its mineralogy, structure and petrology. Ibid. 20, 39- 53.Google Scholar
Brammall, A. and Bracewell, S. and Harwood, H.F. (1932) The Dartmoor granites: their genetic relationships. Q.J. Geol. Soc. London, 88, 171-237.Google Scholar
Chappell, B.W. and White, A.J.R. (1974) Two contrasting granite types. Pacific Geol. 8, 173-4.Google Scholar
Charoy, B. (1986) The genesis of the Cornubian batholith (South West England): the example of the Carnmenellis pluton. J. Petrol. 27, 571-604.Google Scholar
Chatterjee, M. (1929) The accessory mineral assemblage of the Bodmin Moor granite (Cornwall). Proc. Geol. Assoc. 40, 147-52.Google Scholar
Darbyshire, D.P.F. and Shepherd, T.J. (1985) Chronology of granite magmatism and associated mineralization, S.W. England. J. Geol. Soc. London, 142, 115-9. 77.Google Scholar
Dollar, A.T.J. (1941) The Lundy Complex: its petrology and tectonics. Q.J. Geol. Soc. London, 97, 397-7.Google Scholar
Edmonds, E.A., Wright, J.E., Beer, K.E., Hawkes, J.R., Williams, M., Freshney, E.C., and Fenning, P.J. (1968) Geology of the Country around Okehampton. Mem. Geol. Surv. Gt Brit.Google Scholar
Edmonds, E.A., Wright, J.E., Beer, K.E., Hawkes, J.R., Williams, M., Freshney, E.C., and Fenning, P.J. Williams, B.J. and Taylor, R.T. (1979) Geology of Bideford and Lundy Island. Mem. Geol. Surv. Gt Brit.Google Scholar
Exley, C.S. and Stone, M. (1982) Hercynian intrusive rocks. In Igneous Rocks of the British Isles (Sutherland, D. S., ed.), Wiley, London, pp. 287–320.Google Scholar
Exley, C.S., Stone, M., and Floyd, P.A. (1983) Composition and petrogenesis of the Cornubian granite batholith and post-orogenic volcanic rocks in Southwest England. In The Variscan Fold Belt in the British Isles (Hancock, P. L., ed.), Adam Hilger Ltd., Bristol, pp. 153-77.Google Scholar
Ferry, J.M. and Spear, F.S. (1978) Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contrib. Mineral. Petrol. 66, 113— 17.CrossRefGoogle Scholar
Fitton, J.G. (1972) The genetic significance of almandinepyrope phenocrysts in the calcalkaline Borrowdale volcanic group, Northern England. Ibid. 36,231-48.Google Scholar
Flood, R.H. and Shaw, S.E. (1975) A cordierite-bearing granite suite from the New England batholith, N.S.W., Australia. Ibid. 52, 157-64.Google Scholar
Green, T.H. (1976) Experimental generation of cordierite- or garnet-bearing granitic liquids from a pelitic composition. Geology, 4, 85-8.Google Scholar
Green, T.H. (1977) Garnet in silicic liquids and its possible use as a PT indicator. Contrib. Mineral. Petrol. 65, 59- 67.Google Scholar
Green, T.H. and Ringwood, A.E. (1968a) Origin of garnet phenocrysts in calc-alkaline rocks. Ibid. 18, 163-74.Google Scholar
Green, T.H. (1968b) Genesis of the calcalkaline igneous rock suite. Ibid. 18, 105-62.Google Scholar
Hall, A. (1965) The origin of accessory garnet in the Donegal granite. Mineral. Mag. 35, 628-33.Google Scholar
Hampton, C.M. and Taylor, P.M. (1983) The age and nature of the basement of southern Britain: evidence from Sr and Pb isotopes in granites. J. Geol. Soc. London, 140, 499-50..Google Scholar
Indares, A. and Matignole, J. (1985) Biotite-garnet geothermometry in the granulite facies: the influence of Ti and A1 in biotite. Am. Mineral. 70, 272-8.Google Scholar
Kistler, R.W., Ghent, E.D., and O'Neil, J.R. (1981) Petrogenesis of garnet two-mica granites in the Ruby Mountains, Nevada. J. Geophys. Res. 86, 105-91.06.Google Scholar
Leake, B.E. (1968) Zoned garnets from the Galway granite and its aplites. Earth Planet. Sci. Lett. 3, 311-16.CrossRefGoogle Scholar
Lyons, J.B. and Morse, S.A. (1970) Mg/Fe partitioning in garnet and biotite from some granitic, pelitic, and calcic rocks. Am. Mineral. 55, 231-45.Google Scholar
Manning, D.A.C. (1983) Chemical variation in garnets from aplites and pegmatites, peninsular Thailand. Mineral. Mag. 47, 353-8.Google Scholar
Miller, C.F. and Stoddard, E.F. (1981) The role of manganese in the paragenesis of magmatic garnet: an example from the Old WomanPuite Range, California. J. Geol. 89, 233-46.Google Scholar
Miyashiro, A. (1955) Pyralspite garnets in volcanic rocks. J. Geol. Soc. Japan, 61, 463-70.CrossRefGoogle Scholar
Rickwood, P.C. (1968) On recasting analyses of garnet into end-member molecules. Contrib. Mineral. Petrol. 18, 175-98.CrossRefGoogle Scholar
Schneider, G. (1975) Experimental replacement of garnet by biotite. Neues Jahrb. Mineral., Mh. 110.Google Scholar
Stone, M. and Exley, C.S. (1986) High heat production granites of Southwest England and their associated mineralization: a review. Trans. Inst. Min. Metall. 95, B25-36.Google Scholar
Stone, M. and Exley, C.S. and George, M.C. (1988) Composition of trioctahedral micas in the Cornubian batholith. Mineral. Mag. 52, 175-92.Google Scholar
Thompson, A.B. (1976) Mineral reactions in some pelitic rocks: II. Calculation of some P-TX (Fe-Mg) phase relations. Am. J. Sci. 276, 425-54.Google Scholar
Vennum, W.R. and Meyer, C.E. (1979) Plutonic garnets from the Werner batholith, Lassiter Coast, Antarctic Peninsula. Am. Mineral. 64, 268-73.Google Scholar
Winkler, H.G.F. (1976) Petrogenesis of Metamorphic Rocks. 4th ed. Springer, Heidelberg.CrossRefGoogle Scholar
Wood, C.P. (1974) Petrogenesis of garnet-bearing rhyolites from Canterbury, New Zealand. New Zealand J. Geol. Geophys. 17, 759-87.CrossRefGoogle Scholar
Zeck, H.P. (1970) An erupted migmatite from Cerro del Hoyazo, SE Spain. Contrib. Mineral. Petrol. 26, 225- 46.Google Scholar