Geochemistry and Petrogenesis of the Neoproterozoic Granitoids in the Central Eastern Desert, Egypt

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Abstract Neoproterozoic rocks in the Umm Gheig Province (UGP), central Eastern Desert, comprise five plutonic rock suites, i.e. a gabbro-diorite suite (GDS), an older granitoid suite (OGS), an I-type younger granitoid suite (I-YGS), an A-type younger granitoid suite (A-YGS) and a garnet-bearing granitoid suite (GBGS). The GDS exhibits a tholeiitic affinity generated in a volcanic arc, subduction-related environment. The unfractionated REEpattern of the gabbro {(La/Lu) N =0.6}differs significantly from the fractionated pattern of the diorite {(La/Lu) N =5.07}. They are, however, characterized by flat HREE{(Gd/Lu) N =1.24 and 1.45, respectively}and the absence of a Eu anomaly (Eu/Eu*=1.03 for both). The rocks of the GDSdo not represent a primary magma, and the gabbro could be generated by high degree melting of a garnet-free source followed by crystallization of clinopyroxene and olivine. The dioritic melt could be produced by melting of gabbroic rocks. The granitoids can be divided into four general suites emplaced late- (OGS, I-YGS and GBGS) and post-tectonically (A-YGS), each of which shows some variations in their bulk chemical compositions. All the granitoid suites display ferriferous and metaluminous affinities and belong to the non-alkaline series:OGS and I-YGS represent calc-alkaline magmatism while A-YGSand GBGS encompasses highly fractionated calc-alkaline magmatism. There is substantial uniformity in REEpatterns between the OGS, I-YGS and A-YGS, with no real change in light to heavy REEratios. Chondrite-normalized REEpatterns of the GBGSis characterized by strong enrichment in HREE and pronounced negative Eu anomalies (Eu/Eu*=0.095) compared to the other granitoid suites. The geochemistry and petrological characteristics of the granitoid suites suggest close genetic relationships and it is suggested that they were formed from a single parent magma, over a relatively short period of time. Major and trace element variations in the different granitoid suites indicate that they can be modelled by fractional crystallization of plagioclase, K-feldspar, biotite and hornblende. Variation in REEpatterns are governed by fractionation of accessory phases such as apatite, monazite and zircon which occur as inclusions in phenocryst phases.