Geochemistry and petrogenesis of Late Precambrian tonalite-granodiorite-syenogranite series at Umm Shaddad district (Egypt)

The rocks of the Umm Shaddad district include both syn- to late-orogenic an d post-orogenic granitoids associated with volcanic rocks of basaltic compo sition. The basalt shows low-K tholeiite affinity and was derived from the upper mantle with subsequent modification by clinopyroxene and olivine frac tionation. The syn- to late-orogenic granitoid rocks can be classified as t onalite and granodiorite, whereas the post-orogenic rocks are of syenograni te 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 characterist ics, and have higher LIL and HFS elements than the syn- to late-orogenic gr anitic rocks. Both the investigated basaltic and syn- to late-orogenic granitoid rock typ es exhibit subduction-zone geochemical signatures, and by comparison to mod ern tectonic regimes, would appear to have been emplaced during are magmati sm. 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 deriv ed basaltic source. The granodiorite, on the other hand, can be produced fr om tonalite magma by about 21% fractionation of albite, anorthite and hornb lende (36 %, 21 % and 43 %, respectively). The highly fractionated syenogra nite represents the late extreme crystal-melt fractionation from the granod ioritic parent magma. The main fractionated phases were albite, anorthite, hornblende, sphene and zircon (50.5 %, 26 %, 21 %, 2 % and 0.5 %, respectiv ely).