CONTINENTAL MESOZOIC BASALTS OF UM BOGMA AREA, SOUTH SINAI, EGYPT: GEOCHEMICAL APPROACH

Continental Mesozoic volcanic activities of Sinai Peninsula are exclusively restricted in Um Bogma area. They are of basaltic composition and occur in the form of lava flows, dykes, sills and or plugs intruding or overlying the Carboniferous arenitic sandstone of Ataqa Formation. Accordingly, they exhibit grain-size variation from dolerite to porphyritic basalts. Petrographically, they are picritic basalts, olivine-rich basalts and olivine basalts. Olivine exhibits a conspicuous compositional zoning. Pyroxene is Ca-rich variety in the form of Fe-diopside and suggests an alkaline magmatic affinity. Whole-rock chemistry indicates transitional magmas with slight alkaline affinities. Previous K-Ar dating restricted the Mesozoic basaltic activities of the area in two phases: a) Middle Triassic (243-233 Ma) and b) Middle Jurassic (182 ± 7 Ma). New radiometric data enhanced a third phase of Lower Jurassic age (199 ± 3 Ma) indicating a steady crustal weakness of the area during Mesozoic. This process is continued –latter on- and culminated by Oligo-Miocene rift basalts, contemporaneously with Gulf of Suez rifting.