Field Occurrence and Petrographic Characteristics of Tertiary Volcanic Rocks and Associated Intrusions in and around Taiz City, Yemen

Abstract

Geologic and petrographic studies of Tertiary volcanic rocks and their intrusive bodies such as plutons, dykes and sills were carried out in and around Taiz city, Yemen with an aim to document their field occurrence and distribution as well as to study their mineralogical composition. This has an important bearing on the town planning in the city of Taiz as the existing buildings are collapsing due to the foundation problems. Studies on 110 exposures, carefully selected after field traverses revealed that the Tertiary rocks in the study region are mainly represented by typical bimodal mafic-felsic associations in the form of flows, plutons and dykes. Tertiary volcanic flows are characterized by alternating sequences of basic and felsic rocks and varicoloured volcaniclastic deposits which were all extruded and fed from fracturing and fissuring in the old rocks through which magma emerged in successive pulses, flooding the surrounding region. The basic flows (Tb1, Tb2 and Tb3) consist of jointed/massive basaltic rocks and volcaniclastic deposits while the felsic flows (Tr1 and Tr2) are comprised of jointed/massive rhyolite/dacite rocks and also varicoloured felsic volcaniclastic deposits. All these rocks show wide variations in their geological and petrographical characteristics such as colour, texture, heterogeneity, macro/microfractures, weathering/alteration, thickness, horizontal attitude, and intercalation with volcaniclastic deposits, repetition with depth in both vertical and horizontal directions. Volcaniclastic deposits are also characterized with great diversity in their types, colours, textural features, thicknesses, grain sizes, matrices, and degree of roundness of rock fragments and alternating and/or interlocking as well as intercalation laterally and vertically with basalt/rhyolite lava rocks. Volcaniclastic rocks were classified for the first time in the study area based on their particle sizes into different types. They range from strong, compact, welded rocks to weak, altered soils. The younger Sabir granitic pluton is represented by alkaline or peralkaline granites that are white to greyish white coloured massive, medium to coarse-grained, grading up to granite porphyry. Petrographic examinations of 52 thin sections representing the samples of basaltic lava flows, rhyolitic/dacite lava flows and younger intrusives under polarizing microscope have been carried to study their mineralogy. The dominant minerals in basaltic rocks are plagioclase, augite and olivine whereas rhyolitic rocks contain quartz, orthoclase and biotite. The main mineral constituents of younger granitic rocks are k-feldspar, quartz, hornblende and biotite. The mafic and felsic dykes almost resemble basalts and rhyolites/dacites respectively in their petrographical characteristics. XRD technique used for characterizing the minerals of volcanic soils revealed the presence of clay minerals namely montmorillonite and kaolinite as the major mineral phase in most of the samples whereas mica group of minerals such as muscovite, vermiculite and chlorite are present in minor amounts, in addition to talc, feldspar, calcite and halloysite.