Geochemistry and petrogenesis of Late Precambrian tonalite - granodiorite - syenogranite series at Umm Shaddad district (Egypt)
El-Sayed, M. M.; Hassanen, Alexandria M. A.; Obeid, M. A.
published: Jul 1, 1999
ArtNo. ESP154017501002, Price: 29.00 €
The rocks of the Umm Shaddad district include both syn- to late-orogenic and post-orogenic granitoids associated with volcanic rocks of basaltic composition. The basalt shows low-K tholeiite affinity and was derived from the upper mantle with subsequent modification by clinopyroxene and olivine fractionation. The syn- to lateorogenic granitoid rocks can be classified as tonalite and granodiorite, whereas the post-orogenic rocks are of syenogranite composition. The former exhibit geochemical features of I-type granites and possess a peraluminous calc-alkaline character, while the post-orogenic syenogranites represent highly fractionated rocks with A-type characteristics, and have higher LIL and HFS elements than the syn- to late-orogenic granitic rocks. Both the investigated basaltic and syn- to late-orogenic granitoid rock types exhibit subduction-zone geochemical signatures, and by comparison to modern tectonic regimes, would appear to have been emplaced during arc magmatism. The post-orogenic syenogranite shows characteristics of within-plate, A-type granites. The least fractionated tonalite can be modeled by about 25-35 % batch partial melting and subsequent fractionation from a mantle derived basaltic source. The granodiorite, on the other hand, can be produced from tonalite magma by about 21 % fractionation of albite, anorthite and hornblende (36 %, 21 % and 43 %, respectively). The highly fractionated syenogranite represents the late extreme crystal-melt fractionation from the granodioritic parent magma. The main fractionated phases were albite, anorthite, hornblende, sphene and zircon (50.5 %, 26 %, 21 %, 2 % and 0.5 %, respectively).