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Rubidium-strontium age studies and geochemistry of acid veins in the Freetown complex, Sierra Leone

Published online by Cambridge University Press:  05 July 2018

R. D. Beckinsale ,
J. F. W. Bowles ,
R. J. Pankhurst  and
M. K. Wells
R. D. Beckinsale
Affiliation:
Institute of Geological Sciences, Gray's Inn Road, London WIX 8NG
J. F. W. Bowles
Affiliation:
Institute of Geological Sciences, Gray's Inn Road, London WIX 8NG
R. J. Pankhurst
Affiliation:
British Antarctic Survey
M. K. Wells
Affiliation:
University College, Gower Street, London WC1E 6BT
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Summary

The stratigraphical limits on the age of the Freetown intrusion, Sierra Leone, are very wide, yet the intrusion has not previously been accurately dated by isotopic methods, despite a number of attempts. Rubidium-strontium dating of acid veins contemporaneous with the early stages of the prolonged cooling history of the intrusion provides an age of 193 ± 3 Ma. The veins consist of quartz and orthoclase with relict minerals, principally plagioclase, from thehost gabbro. Electron-microprobe analysis of the altered minerals of the veins, and the petrography of the vein and adjacent host gabbro clearly demonstrate that the veins were formed from a granitic fraction, differentiated in situ from the surrounding solid gabbro with the assistance of a hydrous fluid phase within the incipient vein. This assertion is supported by the identical, low value of the initial 87Sr/86Sr ratio (0·70389) obtained from both the acid and basic rocks, and the technique described here may be useful in dating other, similar, intrusions.

Type
Research Article
Information
Mineralogical Magazine , Volume 41 , Issue 320 , December 1977 , pp. 501 - 511
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1977

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