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Open-system magma chamber process in the Freetown Complex of Sierra Leone: evidence from Zone 3

Published online by Cambridge University Press:  01 May 2009

Christopher I. Chalokwu  and
Pamela J. Seney
Christopher I. Chalokwu
Affiliation:
Department of Geology, Auburn University, Auburn, Alabama 36849-5305, USA
Pamela J. Seney
Affiliation:
Department of Geology, Auburn University, Auburn, Alabama 36849-5305, USA
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Abstract

The ˜ 7 km thick Freetown layered complex of Sierra Leone consists of four zones each composed of a cyclically layered sequence of troctolite, gabbro, olivine gabbro, gabbronorite, and anorthosite. The complex is thought by previous workers to have solidified in situ from a single parental magma without stratigraphic changes in mineral compositions. Evidence for cryptic variation is presented based on electron microprobe analyses of mega-unit Zone 3. Two reversals in olivine forsterite content, plagioclase anorthite content, and 100 Mg/(Mg Fe2) in clinopyroxene match the variations of Ni in olivine and Cr in clinopyroxene. These changes are consistent with a magma chamber that was open to periodic influxes of new magmas, and the mixing of new and fractionated resident magmas. Expansion of the magma chamber is thought to have occurred at 2000 m, corresponding to the level of a major influx.

Type
Articles
Information
Geological Magazine , Volume 132 , Issue 3 , May 1995 , pp. 261 - 266
Copyright
Copyright © Cambridge University Press 1995

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