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

Beckinsale, R.D.; Bowles, J.F.W.; Pankhurst, R.J.; Wells, M.K.. 1977 Rubidium-strontium age studies and geochemistry of acid veins in the Freetown complex, Sierra Leone. Mineralogical Magazine, 41 (320). 501-511. 10.1180/minmag.1977.041.320.12

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Official URL: http://dx.doi.org/10.1180/minmag.1977.041.320.12

Abstract/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.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	10.1180/minmag.1977.041.320.12
ISSN:	0026-461X
NORA Subject Terms:	Earth Sciences
Date made live:	17 Oct 2019 08:13 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/525452

Item Type:

Publication - Article

Digital Object Identifier (DOI):

10.1180/minmag.1977.041.320.12

ISSN:

0026-461X

NORA Subject Terms:

Earth Sciences

Date made live:

17 Oct 2019 08:13 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/525452