British Antarctic Survey AND

• Main Authors: R. D. Beckinsale, J. F. W. Bowles, M. K. Wells
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Antarctic; Antarc*; British Antarctic Survey
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.615.9536; http://www.minersoc.org/pages/Archive-MM/Volume_41/41-320-501.pdf

The stratigraphical limits on the age of the Freetown i trusion, Sierra Leone, are very wide, yet he intrusion has not previously been accurately dated by isotopic methods, despite anumber of attempts. Rubidium-strontium dating of acid veins contemporaneous withthe early stages of the prolonged cooling history of the intrusion provides an age of I93 + 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 he 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 s 7Sr/86Sr atio (o.7o389) obtained from both the acid and basic rocks, and the technique described here may be useful in dating other, similar, intrusions. T H E Freetown Complex is a large basic intrusion on the western seaboard of Sierra Leone and has been described in detail in a memoir by Wells (i962). Only the eastern part is exposed on land, as spectacular mountains rising to about 9oo m above sea level. It consists of about 6ooo m of rhythmically layered sequences of troctolitic, gabbroic, and anorthositic rocks, which show variation in their mineral proportions, but no progressive (cryptic) variation in the composition of the minerals. In part this may be due to the major rhythmic units of layering having been