Constraints of Mantle and Crustal Sources Interaction during Orogenesis of Pre-and Post-collision Granitoids from the Northern Arabian-Nubian Shield:A Case Study from Wadi El-Akhder Granitoids,Southern Sinai,Egypt

The Egyptian older and younger granitic rocks emplaced during pre-and post-collision stages of Neoproterozoic Pan-African orogeny,respectively,are widely distributed in the southern Sinai Peninsula,constituting 70% of the basement outcrops.The Wadi El-Akhder,southwestern Sinai,is a mountainous terrain exposing two granitoid suites,namely the Wadi El-Akhder Older Granites(AOG)and the Homra Younger Granites(HYG).The AOG(granodiorites with subordinate tonalite compositions)have geochemical characteristics of medium-K calc-alkaline,metaluminous to mildly peraluminous granitoids formed in an island-arc environment,which are conformable with well-known Egyptian older granitoids rocks,whereas the HYG display calc-alkaline to slightly alkaline nature,peraluminous syeno-,monzogranites and alkali feldspar granites matching well those of the Egyptian younger granites.With respect to the AOG granitoids,the HYG granites contain lower Al_(2)O_(3),FeO^(*),MgO,MnO,CaO,TiO_(2),Sr,Ba,and V,but higher Na_(2)O,K_(2)O,Nb,Zr,Th,and Rb.The AOG are generally characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values(e.g.,negative Nb and Ta anomalies).The geochemical features of the AOG follow assimilation-fractional crystallization(AFC)trends indicative of extensive crustal contamination of magma derived from a mantle source.The chemical characteristics of the AOG are remarkably similar to those of subduction-related granitoids from the ArabianNubian Shield(ANS).The compositional variations from monzogranites through syenogranites to alkali feldspar granite within HYG could not be explained by fractional crystallization solely.Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments,which is similar to the most of younger granitic suites in the ANS.

Origin and Petrogenetic Implications of Spessartine Garnet in Highly-Fractionated Granite from the Central Eastern Desert of Egypt

A highly-fractionated garnet-bearing muscovite granite represents the marginal granitic facies of the Abu-Diab multiphase pluton in the Central Eastern Desert of Egypt. New electron microprobe analyses(EMPA) and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) data from garnets are reported, in order to constrain their origin and genesis. Garnet in the Abu-Diab host granite is euhedral to subhedral, generally homogeneous and, in rare cases, it shows weak zonation. The garnet contains appreciable amounts of MnO and FeO, with lesser amounts of MgO and CaO, yielding an end-member formula of Sps61–72Alm25–35Prp1–4Adr0–1. Moreover, it is depleted in large ion lithophile elements(LILE) with lower values of Ba, Nb and Sr relative to the primitive mantle. Additionally, it contains high concentrations of HREE and Y and their REE pattern shows strong negative Eu anomalies. The garnet was crystallized under relatively low temperature(646°C–591°C) and pressure(< 3 kbar) conditions. The textural and chemical features indicate that the garnet is magmatic in origin and is chemically similar to that from highly-fractionated A-type granite. It was probably formed at the expense of biotite in a highly-evolved MnO-rich magma and/or by hydroxyl complexing of Mn during the ascending fluid phases.