2003 Volume 98 Issue 6 Pages 235-244
Orthoferrosilite (denoted as ferrosilite hereafter) with Fe# [=Fe2+/(Fe2++Mg)] up to 0.8, occurs in a quartz monzonite from the Nkambe area, Cameroon, a part of the Pan-African Mobile Belt. The ferrosilite-bearing monzonite is composed of ferro-edenite/ferrohornblende, plagioclase, K-feldspar, biotite, quartz, ilmenite and accessory allanite, zircon and apatite as matrix phases. Hedenbergite, as well as quartz, calcic amphibole and biotite are found as inclusions in plagioclase. Ferrosilite reacted to form grunerite, ferro-actinolite and minnesotaite during subsolidus cooling. Both plagioclase and K-feldspar show typical igneous-type zoning patterns with decreasing anorthite and albite components from the core towards rim, respectively. The core of K-feldspar usually shows micro-perthitic texture. The ferrosilite is possibly one of the most Fe-rich magmatic orthopyroxenes, yet reported in the literature, formed during equilibrium crystallization and indicates a minimum pressure of emplacement and solidification of the magma under a pressure of about 0.5 GPa. Combinations of equilibrium relations among calcic amphiboles, biotite, feldspars and quartz give pressure and temperature conditions of 0.6-0.7 GPa and 720-760°C. The breakdown of ferrosilite to grunerite suggests that during cooling the ferrosilite-bearing monzonite body cooled below 560°C under pressures in excess of 0.20-0.25 GPa, implying the existence of a relatively low P/T gradient of 0.2 GPa/100°C during decompression of the host rock.