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With 2 figures ZusammenfassungDie Gesteine des /igyptischen Kristallins sind in drei Hauptgruppen unterteilt: Meatiq Group (/ilteste), Abu Ziran Group und Hammamat Group (jiingste); die letzten zwei Gruppen geh6ren dem Pan-Afrikanischen Zyklus an. Die Meatiq-Gruppe stellt einen alten Sokkel dar, der in Gneisdomen auftritt. Die Abu-Ziran-Gruppe umfaBt die Geosynklinal-Assoziation, die aus einer unteren ophiolitischen Einheit besteht, iiberlagert von Sedimenten, volkanoklastischen Sedimenten und lokalen intermedi/iren Vulkaniten mit ausgepr/igten Inselbogen-Eigenschaften. Die Hammamat-Gruppe besteht aus Molassetyp klastischen Sedimenten und zeitlich assoziierte Dokhan Volcanics andesitischer his rhyolitischer Zusammensetzung. Die syn-bis sp~it-tektonische Kalkalkaligranite sind die plutonischen Aequivalente der Dokhan Volcanics.Das untersuchte Gebiet liegt im Vorland-Falten-und Decken-Gtirtel eines Kontinentalrand-Orogens. Ophiolite, insbesondere die Serpentinite, treten entlang dem Ausbii3 der [)berschiebungsflfiche zwischen dem Meatiq-Sockelstockwerk und der Abu-Ziran-Decke auf.Schwellen in Form zweier Geantiklinalen waren Zentren aktiver kalkalkaliner magmatischer T/itigkeit, begleitet zumindest yon Goldvererzungen. AbstractThe Egyptian basement rocks are gathered into three major rock groups, viz. Meatiq Group (oldest), Abu Ziran Group and Hammamat Group (youngest); the last two groups belong to the Pan-African orogenic cycle. The Meatiq Group is an old crystalline basement cropping out in gneiss domes. The Abu Ziran Group comprises the geosynclinal association which is formed of a lower ophiolite unit overlain by metasediments, volcanoclastics and locally intermediate volcanics having clear island arc characters. The Hammamat Group comprises molasse-type clastics, and penecontemporaneous Dokhan Volcanics of andesite to rhyolite composition; syn to late-tectonic calc-alkaline granites are the plutonic equivalents of the Dokhan Volcanics.The studied area lies within the foreland fold and thrust belt of a continental margin orogen. Ophiolites, particularly serpentinites, crop out along the trace of the sole thrust between the Meatiq infrastructure and the imbricated Abu Ziran nappe. Swells, developed along two geanticlines, were centers of marked calc-alkaline magmatic activity associated, at least, with gold mineralization. R~sum~Les roches du socle ~gyptien se r~partissent en trois groupes majeurs: Meatiq Group (le plus ancien); Abu Ziran Group et Hammamat Group (le plus r6cent); les deux derniers groupes appartiennent au cycle orog6nique Pan-Africain.Le Meatiq Group est un socle cristallin ancien affleurant en d6mes gneissiques. Le Abu Ziran Group comprend une association g6osynclinale form6e d'une unit6 ophiolitique *) Authors' addresses: S. EL-GABY, O. EL-NADY and A. KHUDEIR, Department of Geology, Assuit University, Assuit, Egypt.Geologische Rundschau 73, 3, 1019-1036, Stuttgart 1984 1019 Des d6mes, d6velopp6s le long de deux g6anticlinaux, ont 6t6 les centres d'une forte activit6 magmatique calco-alcaline ...

Meatiq and Hafafit core complexes are large swells in the Eastern Desert of Egypt, comprising two major tectono-stratigraphic units or tiers. The lower (infrastructure) unit is composed of variably cataclased gneissose granites and high-grade gneisses and schists. It is structurally overlain by Pan-African ophiolitic mélange nappes (the higher unit). The two units are separated by a low-angle sole thrust, along which mylonites are developed. Major and trace element data indicate formation of the gneissose granites in both volcanic arc and within-plate settings. Nevertheless, all analyzed gneissose granites and other infrastructural rocks, exhibit low initial ratios (Sr i ) (<0.7027), positive e Nd (t) (+4.9 to +10.3) and Neoproterozoic Nd model age (T DM ) (592-831 Ma for the gneissose granite samples). Although these values are compatible with other parts of the Arabian-Nubian Shield considered to be juvenile, the e Nd (t) values and several incompatible element ratios of the gneissose granites are too low to be derived from a mantle source without contribution from an older continental crust. Our geological, Sr-Nd isotopic and chemical data combined with the published zircon ages indicate the existence of a pre-Neoproterozoic continent in the Eastern Desert that started to break up at ca 800 Ma. Rifting and subsequent events caused the formation of oceanic crust and emplacement within-plate alkali basalts in the hinterland domains of the old continent. The emplacement of basaltic magma might have triggered melting of lower crust in the old continent and resulted in emplacement of the within-plate granite masses between 700 Ma and 626 Ma. The granite masses and other rocks in the old continent have been subjected to deformation during the over-thrusting of Pan-African nappes, probably because of the oblique convergence between East and West Gondwanaland. Rb-Sr isotopes of the gneissose granites in both Meatiq and Hafafit core complexes defines an isochron age of 619 Ϯ 25 Ma with Sr i of 0.7009 Ϯ 0.0017 and mean squares of weighted deviates = 2.0. We interpret this age as the date of thrusting of the Pan-African nappes in the Eastern Desert. Continued oblique convergence between East and West Gondwanaland could have resulted in the formation northwest-southeast-trending Meatiq and Hafafit anticlinoriums.

Electron microprobe analyses of chromite ores from Baramiya, Seifein, Siwigat, Ashayer, Um Salatit and Ras Shait, Egypt, reveal two compositional groups. The unaltered chromites from Baramiya and Seifein have high Al and low Cr contents [Cr/(Cr + AI) = 0.56] whereas the remainder have low Al and high Cr contents [Cr/(Cr + A1) = 0.73]. Such bimodality characterises Alpine-type peridotites. The second group probably crystallised at higher T and lower P than the first group and has a composition characteristic of a type III alpine-type peridotite, i.e. of arc or possibly ocean plateau origin but not of mid-ocean ridge origin. The crystallisation setting of the first group is less certain but not inconsistent with the same environment as the second group so that considering the likely higher pressure of crystallisation of the first group overall favours plutonic crystallisation in the roots of an arc for both groups.Rim, patchy and fracture alteration of the chromite occurred possibly partly of late magmatic (deuteric) origin but mainly connected with fluid movements, serpentinisation and tectonism. The final composition of the resultant ferritchromite is variable and depends largely on the original chromite composition; the composition of the ferritchromite developed in highly cataclased chromites deviates markedly from that of the original chromite presumably due to unmixing and migration.