Pan-African post-orogenic gabbro cumulates from Sinai massif, “Egypt”: geochemistry and mineral chemistry

Abstract

In the Pan-African event “Younger Gabbro” cumulates are considered as intrusive postorogenic rocks that correspond to the late phase of calc-alkaline magmatic activities during the Cordilleran stage (655-570 Ma). The present work deals with the Rod Dashab “Younger Gabbro” to study mineral chemistry, genetic aspects and its emplacement in the continental crust. The observed crystallization order of cumulus phases is plagioclase-olivine-clinopyroxene-hornblende — and/or tschermakitic hornblende; the intercumulus phases are clinopyroxene-hornblende-magnetite-sulphides. Zoning is well developed in cumulus amphiboles. Zoned crystals have hornblende and/or tschermakitic hornblende cores rich in Fe/Mg, Al and Ti and actinolite and/or actinolitic hornblende rims which are depleted in these elements and enriched in Mg. This zonation expresses a systematic increase of oxygen fugacity during magma fractionation towards the more siliceous residue. Major and trace element abundances in the studied gabbro cumulates indicate differentiation from a subalkaline basaltic melt of continental type. During the fractionation process the magma has been developed from low to relatively high oxygen fugacity conditions (i.e., olivene-pyroxene gabbros, hornblende-pyroxene gabbros and hornblende gabbros respectively). The investigated gabbro cumulates are rock-types with high and low contents in the incompatible trace elements which generally may reflect a variably veined upper mantle source for the parent magma of “Younger Gabbro”.