Bimodal volcanics from Isbil stratovolcano, Dhamar–Radáa volcanic field, Yemen Republic: contribution to stratigraphy, geochemistry, and eruption evolution

The present paper addresses the geochemical characteristics and stratigraphy of the bimodal volcanics, including silicic (felsic) volcanics and basaltic lava flows constituting Isbil stratovolcano. It is located in the eastern part of Dhamar–Rada volcanic field (DRVF), SE, Yemen. The study area has been subjected to tensional tectonic regime involving extensive effusive-extrusive volcanism (in major) that unconformably overlies Paleozoic and Mesozoic strata of structural elements. These volcanic centers are restricted along strike-slip faults and near to a detachment fault that were feeders for cinder cones and silicic caldera. NNW, NW, and E-W are the three major trends of faults recognized and these are related to the progressive rifting of Red Sea and Aden Gulf. Four volcanic sequences have been well identified and recognized, namely, (1) the lower mafic sequence and (2) the middle silicic sequence. This is followed by (3) upper mafic sequence and (4) upper silicic volcanics sequence crosscutting the surrounding rocks. The stratigraphic and structural characteristics of the substrata are responsible for this eruptive complex in Isbil volcano, forming a shift in depositional environment from subaqueous to subaerial regime. On the basis of the geochemical data, the present silicic lava (rhyolite and dacite) has high A/CNK ratios, Ga/Al ratios, and concentrations of high field strength elements (HFSEs) such as Zr and Nb, typical of peraluminous domain giving rise to within plate magmatism. The mineralogical and chemical characteristics of the silicic rocks are best interpreted as hybrid products of crystal fractionation, coupled with crustal assimilation and/or magma mixing derived from heterogeneous source. On the other hand, the basaltic lava flows exhibit alkaline nature, characterized by enrichment in LILE, LREE, and HFS elements and depletion in HREE. The very low Sm/Nd and Rb/Sr ratios may point to lithospheric mantle origin. The thermal and mass exchange between mantle-derived basaltic magma and crustal material above a mantle plume may be controlled the compositional evolution of the Isbil volcano.