Geochemical and petrological evidence of calc-alkaline and A-type magmatism in the Homrit Waggat and El-Yatima areas of eastern Egypt

The Neoproterozoic plutonic complex in the Homrit Waggat and El-Yatima area s, central Eastern Desert of Egypt, comprises a deformed calc-alkaline I-ty pe metagabbro-diorite complex and tonalite-granodiorite suite invaded by fe lsic high level intrusions of A-type characteristics. The metagabbro-diorit e complex exhibits petrological and geochemical characteristics of mantle-d erived island-are basalts, and its magma was derived possibly from partial melting of a mantle wedge above an early Pan-Af rican subduction zone. The roc ks of the tonalite-granodiorite suite have a wide range of SiO2 (62-71% ), and K, Rb and Ba enrichment relative to Nb and Y. Their chemical variati ons suggest that they are not related to the gabbro-diorite complex, but mo st probably derived by partial melting of the amphibolitic lower crust in a subduction zone. The A-type granites are mainly syeno- and alkali-feldspar granites characterised by sub- and hyper-solvus textures, late magmatic in terstitial biotite and interstitial or vein-fluorite. They are geochemicall y evolved (SiO2 = 74-78%), metaluminous to mildly peraluminous, enriched in Fe, Y, Nb, Rb, Zr and F, and depleted in CaO, MgO, Ba and Sr. Although the y can be classified as A-type and within-plate granites, the least differen tiated samples have F, Nb, Y and Rb contents similar to those in the surrou nding I-type tonalite-granodiorite suite. This similarity suggests that hig h concentrations of these elements in the A-type granite are mostly related to unusual fractionation processes rather than to source rock (A-type sour ce). This simply indicates that these granites are I-type and their classif ication as A-type reflects the process of evolution. A petrogenetic model o f dehydration partial melting of an early Pan-African lower crust along maj or shear zones in a postcollisional environment to produce granodioritic me lt seems likely. Fractional crystallisation of this granodioritic melt gave silicic granites, during which a F-rich fluid phase was evolved. Late magm atic F-rich fluid-rock interaction and F complexing played an important rol e in the evolution and chemical characterisation of the A-type granites. (C ) 1999 Elsevier Science Limited. All rights reserved.