Original paper

Petrological and geochemical constraints on the tectonomagmatic evolution of the late Neoproterozoic granitoid suites in the Gattar area, North Eastern Desert, Egypt

El-Sayed, M.M.; Shalaby, M.H.; Hassanen, M.A.

Abstract

Late Neoproterozoic granitoid rocks from the Gattar area, north Eastern Desert of Egypt, belong to three distinctive magmatic suites, namely: (1) calc-alkaline older granitoids, (2) mildly alkaline subsolvus younger granitoids, and (3) alkaline hypersolvus younger granitoids. These suites are genetically unrelated. The older granitoids consist of three main varieties: quartz-monzodiorite, quartz-monzonite and granodiorite. They show enrichment of LIL elements, high LREE/HREE ratio, with little or no Eu anomalies, and HFS elements are generally low, and further, exhibit metaluminous (ASI < 1) affinity with i-type features. the parental magma for this suite is most probably of mantle derivation. the most mafic quartz monzodiorite represents 25 % batch melting of garnet lherzolite lower crustal source followed by fractional crystallization producing quartz-monzonite and granodiorite emplaced in a volcanic arc environment. The Gattar younger granitoids have metaluminous to slightly peraluminous and alkaline affinities. They can be classified as mildly alkaline subsolvus and alkaline hypersolvus granites. The younger granitoids cannot be related to the older granitoids by simple fractionation from one parental magma. Hypersolvus granites have higher contents of Rb, Y, and Nb and lower contents of TiO2, Sr, Ba and REE and have more pronounced negative Eu anomalies than the subsolvus granites. Subsolvus granites have post-orogenic (A2-subtype) features, while hypersolvus granites have anorogenic (A1-subtype) characters. Mildly alkaline subsolvus granites have been generated by fractional crystallization of mainly K-feldspar, plagioclase, biotite, hornblende and accessory phases such as allanite, titanite and zircon from a mafic magma in order to generate their chemical variations. The alkaline hypersolvus granites, on the other hand, could be generated by about 10% batch melting of amphibolite in the lower crust. The presence of leached biotite-hematite-fluoride clots in the hypersolvus granites suggests that fluid fractionation and fluorine complexing have played some role during the evolution of the hypersolvus granites.

Keywords

gattaregyptgranitoidsgeochemistrypartial meltingfractional crystallization