Petrogenesis of the Alkaline Basalts, West Shalatein, Egypt: Evidence from Clinopyroxene Chemistry PDF

Keywords

Egypt, West Shalatein, Basalts, Petrogenesis, Chemistry, Clinopyroxene

Abstract

The Paleogene volcanic and subvolcanic alkaline basalt rocks of the west Shalatein comprise mainly Olivine-rich and olivine dolerite and/or basalt and normal basalt. The pyroxenes of the ultrabasic and basic rocks follow the augite-diopside-hedenbergite fractionation trend which is significant for the undersaturated rock suites with low silica activity. The studied basalts are characterized by the presence of very peculiar zoned clinopyroxenes of megacrysts and groundmass grains. This pyroxene exhibit clear decrease of Si, Mg, Ca and Cr from cores to rims. The Ti and Al content increases in pyroxenes with crystallization, but the coprecipitation of other Ti- and Al-bearing phases deplete the pyroxenes of outer rims as well as the ground mass in these elements in the later stage of fractionation. The incorporation of both Ti and Al into the pyroxene lattice is indicative of nonequilibrium crystallization (e.g. sector zoning). The clinopyroxenes exhibit a trend of fractionation analogous to that of the non-orogenic, intraplate-type basalts from undersaturated alkaline suites. Cr-bearing cores of clinopyroxene reveal that the present basalt were derived from deep-seated mantle source magma under early high pressure and low fO2 conditions.