Ultramafic and mafic protoclastic intrusions in the Dahomeyan gneiss of Ghana

Abstract

SummaryCharacteristics of mineralogy and texture, together with systematic changes in the enstatite content of orthopyroxene and in the anorthite content of plagioclase, indicate that intrusions of pyroxenite, norite and gabbro, and diorite and leucocratic diorite in the Dahomeyan gneiss of Ghana are genetically related, and that they belong to a subalkaline comagmatic series derived from a parental mafic magma. Partial recrystallization (deuteric) of the igneous rocks during and after intrusion and protoclasis produced minerals characteristic of temperatures higher than those that existed during the medium-grade regional metamorphism of the Dahomeyan gneiss, thus indicating that emplacement of the igneous rocks occurred late during, or perhaps after, the metamorphism. Contact metamorphism appears to have resulted in higher than normal anorthite contents of the plagioclase in the surrounding Dahomeyan gneiss.