Petrogenesis of Late Proterozoic granitoids in the Ras Gharib magmatic province, northern Eastern Desert, Egypt: petrological and geochemical constraints

The late Proterozoic magmatic province of the Ras Gharib district, northern Egyptian Shield, includes wide surface areas of granitoid rocks, that occu r in the form of separate plutons and large batholiths. Based on petrograph ic and geochemical characteristics, the granitoid rocks can be divided into three main suites, namely: a calc-alkaline suite of granodiorite, monzogra nite and syenogranite, a trondhjemite suite, and an alkali granite suite. T he granodiorite-monzogranite-syenogranite suite (63-76 % SiO2) is peralumin ous (A/CNK = 1-1.35) with small negative Eu anomalies (Eu/Eu* = 0.65-0.8), and shows moderate enrichment in the light rare earth elements (LREE) (La-N /Yb-N = 10-22), and the large ion lithophile elements (LILE). It displays c hemical characteristics of volcanic are granites. The trondhjemite suite is more silicic, muscovite-bearing, strongly peraluminous (A/CNK = 1.2-1.4), and has very low concentrations of REE (Sigma 14 REE = 29 ppm), Fe2O3* (0.9 6-1.3 %), MgO (0.19-0.45 %), Rb (50-65 ppm), and Zr (28-35 ppm). The alkali granite suite is depleted in Al2O3, MgO and enriched in Fe2O3*. It shows a more pronounced Eu anomaly (Eu/Eu* = 0.08) and high REE (Sigma 14 REE = 38 3 ppm) and HFSE contents and therefore chemically resembles A-type granites . Geochemical modelling suggests that the granodiorite-monzogranite-syenogran ite suite was formed by 35 % dehydration melting of amphibolitic lower crus t having REE contents of the exposed gabbro-diorite complex. The trondhjemi tic rocks represent a higher degree of partial melting of the same source b ut were generated under higher f(H2O). The alkali granites were derived by partial melting of the early granodiorite, and further affected by fluid me tasomatism during and after their emplacement. The metasomatic effect contr ibutes to enrichment in REE and HFSE as well as in rising their initial Sr- 87/Sr-86 ratio. Although the alkali granites show the characteristic featur es of A-type granites, they are not related to a true rifting event in the Nubian Shield, but most probably to a partial melting process along major s hear zones.