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The geochemistry of mafic and ultramafic from the Archaean greenstone belts of Sierra Leone

Published online by Cambridge University Press:  05 July 2018

H. R. Rollinson
H. R. Rollinson*
Affiliation:
Department of Geography and Geology, The College of St Paul and St Mary, The Park, Cheltenham, Glos. GL50 2RH, UK
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Abstract

The Archaean (c. 2800 Ma) ultramafic rocks in eastern Sierra Leone cut basalt lavas and are mostly olivine-rich cumulates either iron-rich (Fo85–86) and derived from a basaltic or picritic parent, or more magnesian (Fo92–93) derived from an ultramafic melt with c. 18–25 wt. % MgO. In central Sierra Leone the ultramafic rocks are lavas predating tholeiitic basalts.

The basalts show a wide variation in Zr/Y, suggesting that garnet was present in the source region of some of these rocks but not others. This implies that melting took place at different depths in the mantle. The REE evidence for basaltic rocks in the upper part of the Nimini belt succession suggests that they were derived from a mantle source region which had already suffered melt extraction. Ti/Zr ratios in the basaltic rocks are also variable and individual belts define different trends on a Ti vs. Zr plot implying that the basaltic rocks evolved in geographically distinct magma chambers. It is likely that the basaltic rocks evolved from a parental liquid with Ti/Zr = 90 via shallow level crystal fractionation. The source region for these rocks therefore had a lower than chondritic Ti/Zr.

There are two possible models for the basaltic and ultramafic magmas in the Sierra Leone greenstone belts. First that the ultramafic and basaltic liquids were derived from mantle diapirs of differing size, but originating in the same region of the mantle. Ultramafic liquids were produced in small diapirs, which store large melt fractions, and basaltic liquids in larger diapirs which segregate larger melt fractions. A second model is based upon the double diffusion process suggested for magma chambers at mid-ocean ridges and involves a transient magma chamber from which basalts, derived from parental ultramafic liquids, are erupted, with ultramafic liquids rising directly to the surface when the magma chamber is frozen. The available data does not discriminate between these two models.

Type
Other
Information
Mineralogical Magazine , Volume 47 , Issue 344 , September 1983 , pp. 267 - 280
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1984

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References

Allegre, C. J., Treuil, M., Minster, J. F., Minster, B., and Albarede, F. (1977) Contrib. Mineral. Petrol. 60, 57–75.CrossRefGoogle Scholar
Andrews-Jones, D. A. (1968) Ph.D. thesis, Univ. Leeds.Google Scholar
Arndt, N. T. (1977) Contrib. Mineral. Petrol. 64, 205–21.CrossRefGoogle Scholar
Arth, J. G., and Hanson, G. N. (1975) Geochim. Cosmochim. Ada. 39, 325–62.CrossRefGoogle Scholar
Arth, J. G., Arndt, N. T., and Naldrett, A. J. (1977) Geology. 5, 590–4.2.0.CO;2>CrossRef2.0.CO;2>Google Scholar
Beckinsale, R. D., Gale, N. H., Macfarlane, A., Crow, M. J., Arthurs, J. W., and Wilkinson, A. F. (1980) Precambrian Res. 13, 63–76.CrossRefGoogle Scholar
Bender, J. F., Hodges, F. N., and Bence, A. E. (1978) Earth Planet. Sci. Lett. 41, 277–302.CrossRefGoogle Scholar
Bickle, M. J., Ford, C. E., and Nisbet, E. G. (1977) Ibid. 37, 97–106.Google Scholar
Cann, J. R. (1969) J. Petrol. 10, 1–19.CrossRefGoogle Scholar
Cann, J. R. (1970) Earth Planet. Sci. Lett. 10, 7–11.CrossRefGoogle Scholar
Condie, K. C. (1981) Archaean Greenstone Belts. Elsevier.Google Scholar
Dunham, K. C Phillips, R. Chalmers, R. A., and Jones, D. A. (1958) Bull. Overseas Geol. Mineral. Res. Suppl. 3, 44 pp.Google Scholar
Elliott, R. B. (1973) Contrib. Mineral. Petrol. 38, 71–9.CrossRefGoogle Scholar
Hart, S. R., and Davis, K. E. (1978) Earth Planet. Sci. Lett. 40, 203–19.CrossRefGoogle Scholar
Heinrich, K. F. J. (1964) In The Electron Microscope (McKinley, T. D., Heinrich, K. F. J., and Wittry, D. B., Eds.). New York: Wiley, 296–377.Google Scholar
Hooker, P. J., O'Nions, R. K., and Pankhurst, R. J. (1975) Chem. Geol. 16, 181–96.CrossRefGoogle Scholar
Humphries, S. E., and Thompson, G. (1978) Geochim. Cosmochim. Acta. 42, 127–36.CrossRefGoogle Scholar
Huppert, H. E., and Sparks, R. S. J. (1980a) Nature, 286, 46–8.CrossRefGoogle Scholar
Huppert, H. E., (1980b) Contrib. Mineral. Petrol. 75, 279–89.CrossRefGoogle Scholar
Jahn, B. M., and Sun, S. S. (1979) Phys. Chem. Earth. 11, 597–618.CrossRefGoogle Scholar
Jahn, B. M., Auvray, B., Blais, S., Capdevila, R., Cornichet, J., Vidal, F., and Hameurt, J. (1980) J. Petrol. 21, 201–44.CrossRefGoogle Scholar
Leake, B. E., Hendry, G. L., Kemp, A., Plant, A. G Harrey, P. K., Wilson, J. R., Coats, J. S., Aucott, J. W., Lunel, T., and Howarth, R. J. (1969) Chem. Geol. 5, 7–86.CrossRefGoogle Scholar
MacFarlane, A., Crow, M. J., Arthurs, J. W., and Wilkinson, A. F. (1981) Overseas Mem. Inst. Geol. Sci. 7, 103 pp.Google Scholar
Manno, V. (1962) Bull. Geol. Swv. Sierra Leone. 2, 117 pp.Google Scholar
Muecke, G. K., Pride, C, and Sarkar, P. (1979) In Origin and Distribution of the Elements, 2 (Ahrens, L. H., ed.). Pergamon, London.Google Scholar
Nakamura, N. (1974) Geochim. Cosmochim. Ada. 38, 757-75.CrossRefGoogle Scholar
Nesbitt, R. W., and Sun, S. S. (1976) Earth Planet. Sci. Lett. 31, 433-53.CrossRefGoogle Scholar
Nisbet, E. G., Bickle, N. J., and Martin, A. (1977) J. Petrol. 18, 521-66.CrossRefGoogle Scholar
Nisbet, E. G., and Chinner, G. A. (1981) Econ. Geol. 76, 1729-35.CrossRefGoogle Scholar
Pearce, J. A. (1978) Abstract in Rea, W. J. (ed.). J. Geol. Soc. Lond. 135, 591.CrossRefGoogle Scholar
Nisbet, E. G., and Cann, J. R. (1971) Earth Planet. Sci. Lett. 12, 339-49.Google Scholar
Nisbet, E. G., and Norry, M. J. (1979) Contrib. Mineral. Petrol. 69, 33-47.Google Scholar
Roedder, P. L., and Emslie, R. F. (1970) Ibid. 29, 275-82.Google Scholar
Campbell, I. H., and Jamieson, H. E. (1979) Ibid. 68, 325-34.Google Scholar
Rollinson, H. R. (1978) Nature. 272, 440-2.CrossRefGoogle Scholar
Rollinson, H. R. (1982) Earth Planet. Sci. Lett. 59, 177-91.CrossRefGoogle Scholar
Rollinson, H. R. and Cliff, R. A. (1982) Precambrian Res. 17, 63-72.CrossRefGoogle Scholar
Schreyer, W., Werding, G., and Abraham, K. (1981) J. Petrol. 22, 191-231.CrossRefGoogle Scholar
Simkin, T., and Smith, J. V. (1970) J. Geol. 78, 304-25.CrossRefGoogle Scholar
Smith, R. E., and Smith, S. E. (1976) Earth Planet. Sci. Lett. 32, 114-20.CrossRefGoogle Scholar
Stolper, E., Walker, D., Hager, B. H., and Hays, J. F. (1981) J. Geophys. Res. 86, 6261-71.CrossRefGoogle Scholar
Sun, S. S., and Nesbitt, R. W. (1977) Earth Planet. Sci. Lett. 35, 429-48.CrossRefGoogle Scholar
Sun, S. S., (1978) Contrib. Mineral. Petrol. 65, 301-25.CrossRefGoogle Scholar
Vallance, T. G. (1974) J. Petrol. 15, 79-96.CrossRefGoogle Scholar
Walker, D., Stolper, E. M., and Hays, J. P. (1980) EOS 61, 385 (abs.).CrossRefGoogle Scholar
Williams, H. R. (1978) Precambrian Res. 6, 251-68.CrossRefGoogle Scholar
Wilson, N. W., and Marmo, V. (1958) Bull. Geol. Swv. Sierra Leone. 1, 91 pp.Google Scholar
Wood, D. A., Gibson, I. L., and Thompson, R. N. (1976) Contrib. Mineral. Petrol. 55, 241-54.CrossRefGoogle Scholar