Original paper

Petrogenesis and age dating of continental Mesozoic basalts, Um Bogma area, Sinai, Egypt

Abou El Maaty, Mostafa A.; Ali-Bik, Mohamed W.; Sadek, Mohamed F.


Numerous small Mesozoic basaltic outcrops in the form of plugs, sills, dykes and lava flows intrude and overlie the Carboniferous sandstone (CSS) in the Um Bogma area, west central Sinai. They represent sporadic small-scale fissure-fed eruptions along deep detachment faults over a period of about 65 Ma, encompassing two main volcanic phases (243-233 Ma and 199-178 Ma). The petrogenesis and the results obtained from K-Ar dating of these outcrops at Wadi Mokattab (WM) and Wadi Baba (WB) are discussed. The radiometric data records a new volcanic event in Sinai at 199 ± 3 Ma and substantiates the age of a previously known basaltic phase at about 182 Ma. Geochemically, these volcanic rocks pertain to the low P2O5-TiO2 (LPT) transitional basalts of mainly tholeitic affinity. They display a limited compositional range with the absence of more felsic varieties, suggesting equilibrium crystallization sequences rather than fractional crystallization as the dominant fractionation mechanism. Simulation of equilibrium crystallization processes and evolution trajectories of these basalts have been modeled. Phase equilibrium calculations demonstrate an earlier olivine over-saturation stage followed by a sub-cotectic (olivine + plagioclase) interval in the least evolved varieties of WM basalts. In the WB basalts, plagioclase is the first crystallized phase, followed by a cotectic (plagioclase + olivine) stage.


petrogenesisk-ar datingmesozoicbasaltum bogma