Geochemistry and petrogenesis of the late proterozoic kid volcanics: evidence relevant to arc-intra-arc rifting volcanism in Southern Sinai, Egypt

Abstract

The Kid volcanosedimentary succession in Southern Sinai is one of several Late Proterozoic volcanic sequences in the Arabo-Nubian Shield. It comprises varieties ranging from basalt to dacite/rhyolite and equivalent pyroclastics constituting a bimodal sequence with dominated subaerial to shallow marine depositional nature. Based on field, petrographical and chemical evidence, the Kid volcanic suite is stratigraphically divisible into three volcanic sequences; Lower, Middle and Upper volcanic sequences. Consideration of major elements' chemistry, rhyolite-dominated bimodal volcanism, high Zr/Y ratio and the marked enrichment in potassium and HFS elements emphasis that the rocks of the lower and middle volcanic sequences are of high-K. Calcalkaline characters developed in continental margin are setting. The stratigraphically higher rocks of the upper volcanic sequence display LIL and HFS elements enrichment typical of intraplate continental alkali basalts. The bimodal nature of the volcanic suite with abundant felsic pyroclastic materials, the presence of intraplate alkali basalts and alkali diabasic dykes suggest that rifting occurred within the continental margin arc. Modelling of the basic rocks using Ba/Y and Zr/Nb ratios in a variable-depth melting models in the stability fields of both garnet and spinel indicate that the high-K calcalkine basalts (LVS) are derived through 7 %–11 % partial melting from spinel Iherzolite source at shallow depth followed by limited fractional crystallization of olivine and clinopyroxene. The alkali basalts and trachybasalts (UVS) are the products of smaller (5 %–10 % and 7 %–15 %) degrees of melting of a garnet-Iherzolite source at relatively greater depth and later modified by varying degrees of magma replenishment and mixing analogue to the open system fractionism.