Hostname: page-component-78c5997874-dh8gc Total loading time: 0 Render date: 2024-11-10T09:48:43.760Z Has data issue: false hasContentIssue false
  • English
  • Français

The textures and genesis of metamorphic pyroxene in the Freetown Intrusion

Published online by Cambridge University Press:  05 July 2018

M. K. Wells  and
J. F. W. Bowles
M. K. Wells
Affiliation:
University College, Gower Street, London WC1E 6BT
J. F. W. Bowles
Affiliation:
Institute of Geological Sciences, Gray's Inn Road, London WC1X 8NG
Get access Rights & Permissions [Opens in a new window]

Abstract

The Freetown Intrusion, Sierra Leone, evolved by recurrent injection of olivine-bearing magma producing an aggregate thickness greater than 6000 m of rhythmically layered rocks. Sedimentation and constrained viscous flow of crystallizing magma concentrated olivine crystals during initial formation whilst concentration of volatiles locally lowered residual melt temperatures. This affected the crystallization of pyroxenes especially in pyroxene-troctolites where abundance of olivine caused deferred pyroxene growth. Three examples are described: (1) Flattened and elongated schlieren of augite enclosing plagioclase in ophitic intergrowths show preferred orientation cutting across primary layering defined by olivine, proving that pyroxene growth was secondary and influenced by stress directions in a relatively rigid rock. (2) In some pyroxene-troctolites, augite is segregated into coarse ophitic nodules which are widely and evenly spaced in a troctolitic host rock. Successful pyroxene nucleation and extensive diffusion caused development of domains of equilibration within a ‘cotectic’ pyroxene-plagioclase-volatile system. Crystallization within this was deferred compared with the surrounding olivine-plagioclase assemblage in which effects of granulitization are conspicuous. (3) Occasionally the pyroxenes are segregated in dispersed lenticular bodies of noritic pegmatite in troctolite. As in the previous cases, growth of pyroxene involved resorption and regrowth of olivine. This reflects the highest temperatures of crystallization in equilibrium with volatile-enriched pegmatite fluids. Temperatures decreased inwards giving a zonal structure and late growth of biotite, orthoclase, and apatite.

The formation of late crystalline pyroxenes and pegmatites thus involved extensive diffusion of volatile and other components accompanying the establishment of domains of reduced pressure in hot and slowly cooled rocks subjected to gradual deformation due to progressive subsidence of the central parts of the intrusion. The final fabric is therefore metamorphic resulting from annealing rather than crystal accumulation.

Type
Research Article
Information
Mineralogical Magazine , Volume 44 , Issue 335 , September 1981 , pp. 245 - 255
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1981

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

Baker, C. O. and Bott, M. H. P. (1961). Overseas Geol. Mineral. Res. 8, 260-78 [M.A. 16, 109].Google Scholar
Baker, C. O. and Marmo, V. (1956). Ann. Rept. Geol. Surv. Dep. Sierra Leone. Government Printing Department, Sierra Leone, 12 pp.Google Scholar
Beckinsale, R. D., Bowles, J. F. W., Pankhurst, R. J., and Wells, M. K. (1977). Mineral. Mag. 41, 501-11 [M.A. 78-24].CrossRefGoogle Scholar
Binns, R. A., Long, J. V. P., and Reed, S. J. B. (1963). Nature, London, 198, 777-8 [M.A. 17, 194].CrossRefGoogle Scholar
Bowen, N. L. and Schairer, J. F. (1935). Am. J. Sci. 29, 151-217.CrossRefGoogle Scholar
Bowles, J. F. W. (1976). Mineral. Mag. 40. 703-14 [M.A. 76-3633].CrossRefGoogle Scholar
Bowles, J. F. W. (1977). Ibid. 41, 103-9 [M.A. 77-2126].Google Scholar
Bowles, J. F. W. (1978). Ibid. 42, 306-12 [M.A. 78-1772].Google Scholar
Briden, J. C., Henthorn, D. I., and Rex, D. C. (1971). Earth planet. Sci. Lett. 12, 385-91.CrossRefGoogle Scholar
Brown, G. M. (1956). Philos. Trans. R, Soc. London Ser. B. 240, 153 [M.A. 14, 69].Google Scholar
Brown, G. M. (1968). Carnegie Inst. Wash. Yearb. 66, 347-53.Google Scholar
Brown, G. M. (1972). Geol. Soc. Am., Mere. 132, 523-34 [M.A. 74-72].Google Scholar
Culver, S. J. and Williams, H. R. (1979). J. geol. Soc. London, 136, 605-18 [M.A. 80-0813].CrossRefGoogle Scholar
Dixey, F. (1922). Q. J. geol. Soc. London, 78, 299-317 [M.A. 2, 315].CrossRefGoogle Scholar
Hatch, F. H., Wells, A. K., and Wells, M. K. (1972). Petrology of the Igneous Rocks. London (Allen & Unwin) 551 pp. [M.A. 73-3360].Google Scholar
Irvine, T. N. (1980). Am. J. Sci. Jackson Memorial Volume, 1-58.Google Scholar
Irvine, T. N. and Smith, C. H. (1967). In Ultramafic and related rocks, Wyllie, P. J. ed. New York (Wiley) 464 pp. [M.A. 19, 227].Google Scholar
Jones, E. J. W. and Mgbatogu, C. C. S. (1977). Nature, London, 267, 688-90.CrossRefGoogle Scholar
Krause, D. C. (1963). Nature, London, 200, 1280-1.CrossRefGoogle Scholar
McBirney, A. R. and Noyes, R. M. (1979). J. Petrol. 20, 487-554 [M.A. 80-2322].CrossRefGoogle Scholar
McConnell, R. B. (1974). Geol. Rundsch. 63, 389-430.CrossRefGoogle Scholar
Morel, S. W. (1979). Econ. Geol. 74, 1563-76.CrossRefGoogle Scholar
Umeji, A. C. (1975). Geol. J. 10, 107-30 [M.A. 77-3521].CrossRefGoogle Scholar
Wager, L. R., Brown, G. M., and Wadsworth, W. J. (1960). J. Petrol. 1, 73-85 [M.A. 14, 436].CrossRefGoogle Scholar
Wager, L. R. and Deer, W. A. (1939). Meddels. Gronland 105, No. 4, 352 pp. [M.A. 8, 27].Google Scholar
Wells, M. K. (1954). Q. J. geol. Soc. London 109, 367-95 [M.A. 12, 426].CrossRefGoogle Scholar
Wells, M. K. (1962). Geol. Surv. Dept. Sierra Leone, Short paper No. 9 (Bull. Overseas Geol. Min. Res., Suppl. No. 4) 115 pp. [M.A. 16, 305].Google Scholar
Wells, M. K. and Baker, C. O. (1956). Colon. Geol. Mineral. Res. 6, 137-58 [M.A. 14, 42].Google Scholar
Yoder, H. S. (1954). Carnegie Inst. Wash. Yearb. 53, 106.Google Scholar