OPEN-SYSTEM MAGMA CHAMBER PROCESS IN THE FREETOWN COMPLEX OF SIERRA-LEONE - EVIDENCE FROM ZONE-3

The similar to 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 mag-ma chamber is thought to have occurred at 2000 m, corresponding to the level of a major influx.