Proterozoic island arc volcanism and sedimentation in Sinai

Abstract

Three cycles of volcanism-sedimentation and magmatism are in major part responsible for the growth of the Proterozoic crust in the Sinai Peninsula. The events show a trend from early oceanic island arc calc-alkaline to younger, mostly subaerial alkaline to peralkaline volcanism and sedimentation during the culmination of arc convergence and cratonization of the Shield. Arc-related plutons show a similar evolutionary cycle with younger plutons being in general more felsic and alkaline, Sediments which formed in basins adjacent to the rapidly rising and eroding volcanoes show the expected wide spectrum of types and associations from deep marine pelagic (fore-arc?) sediments with volcaniclastic intercalations to coarse molasse-carbonate (back-arc?) sediments, with exhalite marking the interface region. Volcaniclastic and flysch-turbidite sediments with slide conglomerates are ubiquitous. Volcanism was more ignimbritic and sedimentation more continental with the progressive coalescence of the arc environment. Mafic and ultramafic rocks showing ophiolite affinities were emplaced into apparently continental crust comprising gneisses and migmatites of felsic to intermediate composition and the geosynclinal rocks. This ‘inter-arc wedge’ is furthermore covered by bedded mafic, cherty and pelagic sediments, chert breccias and greenstone thus suggesting oceanic layer 1. A metamorphic-structural discontinuity and a mylonite belt define a possible suture line. Orogenesis comprising convergence of the arc segments, deformation and metamorphism took place during the early events and ophiolite emplacement during final phases of polyphase deformation. Metamorphism was progressive and of the high T/low P type. This evolution of the Late Proterozoic took place roughly during the period 1000 to 600 Ma ago.