Magma source and evolution of late neoproterozoic granitoids in the Gabal El-Urf area, eastern desert, Egypt: geochemical and Sr-Nd isotopic constraints

Granitoids in the Gabal El-Urf area in Eastern Egypt consist of a monzogran ite pluton, belonging to the Younger Granite province, emplaced in granodio ritic rocks. Whole rock Rb-Sr dating indicate ages of 650 +/- 95 Ma and 600 +/- 11 Ma for the granodiorites and monzogranites, respectively. The grano diorites (65-70% SiO2) are calc-alkaline and metaluminous with low Rb/Sr, T h and Nb contents, moderate enrichment in the LILE (K2O, Rb, and Ba) and di splay most of the chemical and field characteristics of syn-to late-tectoni c I-type granitoids described elsewhere in the Arabian-Nubian Shield. The m onzogranites (72-77 % SiO2) are metaluminous to mildly peraluminous, highly fractionated and depleted in Al2O(3), MgO, CaO, TiO2, Sr and Ba with corre sponding enrichment in Rb, Nb, Zr, and Y. They can be correlated with the u ndeformed post-orogenic granites in the Arabian-Nubian Shield that chemical ly resemble A-type granites emplaced in extensional settings. The mineralog ical and chemical variations within the granodiorites and monzogranites are consistent with their evolution by fractional crystallization. The granodi orites have a low initial Sr-87/Sr-86 ratio (0.7024) and high epsilon(Nd) v alues (+ 6.9 - + 7.3) and are significantly different from those (initial 8 7Sr/86Sr ratio = 0.7029, epsilon(Nd) values = + 5.2 - + 5.8) of the monzogr anites. These data suggest a predominant mantle derivation for both granite types and demonstrate that they originated from different source materials . The granodiorite melt was most probably generated through vapour-saturated partial melting of an early Neoproterozoic depleted mafic lower-crust reser voir due to crustal thickening associated with orogenic compression and/or are magma underplating. The mineralogical and geochemical data of the A-typ e monzogranites are consistent with their derivation as a residual granitic liquid from a LILE-enriched mafic magma through crystal-liquid fractionati on of plagioclase, amphibole, Fe-Ti oxides and apatite. The parental mafic magma was originated in the upper mantle due to crustal thinning associated with extension in the late stage of the Neoproterozoic crustal evolution o f northeastern Egypt.