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The Petrology and Geochemistry of the St. David's Granophyre and the Cwm Bach Rhyolite, Pembrokeshire, Dyfed

Published online by Cambridge University Press:  05 July 2018

T. W. Bloxam  and
M. H. J. Dirk
T. W. Bloxam
Affiliation:
Department of Geology, University College, Swansea
M. H. J. Dirk
Affiliation:
Department of Geology, University College, Swansea
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Abstract

A petrological and geochemical study has been made of the late Precambrian St. David's granophyre (SDG) and the Cwm Bach rhyolites (CBR). The SDG is trondhjemitic, peraluminous, and chemically zoned with lower Na/K ratios in its interior portions. Although the SDG contains only minor amounts of K, the Rb content is relatively high and ratios of immobile versus mobile elements indicate that K has been removed. The CBR are flow-banded and autobrecciated subaerial rhyolite flows which are more K-rich and less altered than the SDG, lying close to the ternary minimum composition of the system Q-Or-Ab-(H2O). K mobility is considered to result from the reaction K-feldspar→ K-mica which released excess K ions, imparting peraluminous compositions to the rocks. On the basis of this reaction the original K content of the rocks has been estimated, the values for which are more compatible with immobile element abundances and with normal igneous trends. Similar abundances of the relatively immobile elements Ti, Zr, Sr, Y and Nb, together with K-corrected K/Rb ratios, suggest that the SDG and CBR may be comagmatic volcanic-arc-collision-type granite and rhyolite.

Keywords

St. David'sgranophyreCwm BachrhyolitePembrokeshireDyfedWales
Type
Petrology and Geochemistry
Information
Mineralogical Magazine , Volume 52 , Issue 368 , December 1988 , pp. 563 - 575
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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References

Allen, P.M.e. al. (1985) British Geol. Surv. Mineral Reconnaissance Report No. 78.Google Scholar
Ambrust, G.A. and Gannicott, D.A. (1980) Econ. Geol. 75, 466-77.CrossRefGoogle Scholar
Beswick, A.E. and Soucie, G. (1978) Precambrian Res. 6, 235-48.CrossRefGoogle Scholar
Bloxam, T.W. (1981) J. Geol. 89, 754-64.CrossRefGoogle Scholar
Brown, G.C., Cassidy, J., Tindle, A.G., and Hughes, D.J. (1979) J. Geol. Soc. London, 136, 745-53.CrossRefGoogle Scholar
Coleman, R.B. and Donato, M.M. (1979) In Trondhjemites, dacites and related rocks (Barker, F., ed.), Amsterdam, Elsevier Press.Google Scholar
Coleman, R.B. and Donato, M.M. and Peterman, Z.E. (1975) J. Geophys. Res., 80, 109-91.8.Google Scholar
Davies, J.F. (1980) Econ. Geol. 75, 760-70.CrossRefGoogle Scholar
Grant, R.W.E., and Whitehead, R.E.S. (1979) Can. J. Earth Sei. 16, 305-11.Google Scholar
Donnelly, T.W. (1966) Mem. Geol. Soc. Am. 98, 85-176.Google Scholar
George, T.N. and Pringle, J. (1970) British Regional Geology, South Wales (HMSO).Google Scholar
Green, J.F.N. (1908) Q. J. Geol. Soc. London, 64, 363-83.CrossRefGoogle Scholar
Heier, K.S. and Adams, J.A.S. (1964) Phys. Chem. Earth, 5, 253-381. (Pergamon Press).CrossRefGoogle Scholar
Hicks, H. (1878) Q. J. Geol. Soc. London, 34, 147-69.CrossRefGoogle Scholar
Hicks, H. (1886) Ibid. 42, 351-6.Google Scholar
Jakes, P. and White, A.J.R. (1972) Geol. Soc. Am. Bull., 83, 29-40.CrossRefGoogle Scholar
James, R. and Hamilton, D.L. (1969) Contrib. Mineral. Petrol. 21, 111-41.CrossRefGoogle Scholar
Jones, O.T. (1938) Q. J. Geol. Soc. London, 94, 60-110.Google Scholar
O'Connor, J.T. (1965) U.S. Geol. Surv. Prof. Paper 525B, 79-84.Google Scholar
Olade, M.A. and Fletcher, W.K. (1975) Econ. Geol. 70, 152-1.CrossRefGoogle Scholar
Olade, M.A. and Fletcher, W.K. (1976) Ibid. 71, 733-48.Google Scholar
Patchett, P.J. and Jocelyn, J. (1979) J. GeoL Soc. London, 136, 13-19.CrossRefGoogle Scholar
Pearce, J.A., Harris, N.B.W., and Tindle, A.G. (1984) J. Petrol 25, 956-83.CrossRefGoogle Scholar
Peterson, M.D. (1983) J. Geochem. Explor. 19, 615-17.CrossRefGoogle Scholar
Pitcher, W.S. and Berger, A.R. (1972) The geology of Donegal (Wiley, New York and London).Google Scholar
Rast, N. and Crimes, T.P. (1969) Tectonophys 7, 277-307.CrossRefGoogle Scholar
Rast, N. and Crimes, T.P. O'Brien, B.H., and Wardle, R.J. (1976) Ibid. 30, 315-38.Google Scholar
Schleicher, H. and Lippolt, H.J. (1981) Contrib. Mineral. Petrol. 78, 220-4.CrossRefGoogle Scholar
Shaw, D.M. (1968) Geochim. Cosmochim. Acta, 32, 593–602.CrossRefGoogle Scholar
Streckeisen, A.L. (1973) Geotimes, 18, 263-0.Google Scholar
Thorpe, R.S. (1972) Bull. Geol. Soc. Am., 83, 366-38.CrossRefGoogle Scholar
Beckinsale, R.D., Patchett, P.J., Piper, J.D.A., Davies, G.R., and Evans, J.A. (1984) J. Geol. Soc. London, 141, 521-36.CrossRefGoogle Scholar
Tuttle, O.F. and Bowen, N.L. (1958) Geol. Soc. Am. Mem. 74.Google Scholar
Vance, J.A. (1961) Bull. Geol. Soc. Am. 72, 172-38.CrossRefGoogle Scholar
Winchester, J.A. and Floyd, P.A. (1977) Chem. GeoL 20, 325-43.CrossRefGoogle Scholar
Wright, A.E. (1969) In North Atlantic Geology and Continental Drift (Kay, M. ed.) Am. Assoc. Petrol. Geol. Mem. 12, 931-09.Google Scholar
Yoder, H.S. and Eugster, H.P. (1955) Geochim. Cosmochim. Acta, 8, 225-80.CrossRefGoogle Scholar