THE NEOPROTEROZOIC DUBR INTRUSIVES, CENTRAL EASTERN DESERT, EGYPT: PETROLOGICAL AND GEOCHEMICAL CONSTRAINTS ON THE EVOLUTION OF A MAFIC-FELSIC SUITE

The Neoproterozoic magmatism in the Dubr area took place in two suites namely: (1) a mafic suite of a gabbro-diorite complex and (2) a felsic suite of a granodiorite and monzogranite composition. Pressures estimated for the gabbro and diorite rocks show that they have originated over a wide range of pressures (1.5-6.5 kbar). The gabbro and diorite rocks predict crystallization temperatures ranging from 820 to 940oC and 660 to 710oC, respectively. The rare-earth element abundances of the Fe-tholeiite gabbro-diorite complex display fractionated {(La/Lu)n=1.53-2.96} and LREE-enriched {(La/Sm)n=1.12-2.62} patterns. The REE patterns of the gabbro-diorite complex suggest that it can be generated from a spinel-bearing mantle source. Transition metal contents of the gabbro-diorite complex are variable and low (Ni, 9-133 ppm; Cr, 20-325 ppm; Sc, 17.7-34 ppm), indicating that none of the samples are primitive. The gabbro-diorite complex may have formed via relatively high degree partial melting (25-30%) of an enriched mantle source followed by fractional crystallization of olivine, Cr-spinel with minor clinopyroxene and plagioclase. Chemical evidences imply that crustal contamination is likely to be a minor factor in trace element variation of the gabbro-diorite complex. The granodiorite and monzogranite are calc-alkaline showing metaluminous affinity for the granodiorite and a slightly peraluminous nature for the monzogranite. They display trace element and REE characteristics of I-type granites formed in a subduction-related arc environment. The hornblende of the granodiorite and monzogranite yielded pressures of an average 3 kbar and 2.5 kbar, respectively. Amphibole-plagioclase geothermometer yielded crystallization temperatures ranging from 680 to 700oC and 650 to 675oC for the granodiorite and monzogranite, respectively. The co-linear trends on variation diagrams for the granodiorite and monzogranite indicate a genetic link between them through fractional crystallization. It appears likely that the restite-unmixing model is responsible for the petrological and chemical features of the granodiorite. The most probable source for the granodiorite is a dioritic rock. The monzogranite was derived as a result of crystallization of plagioclase, biotite, hornblende, quartz, ilmenite and magnetite phases from a granodioritic melt.