Geochemistry and Petrogenesis of the Late Jurassic-Early Cretaceous Mansouri Ring Complex, Southeastern Desert, Egypt

Abstract

The Mansouri Ring Complex is an eccentric mass (9 × 6 km) located in the extreme southwestern part of the Eastern Desert of Egypt. The complex exhibits a poorly defined ring structure and a limited variety of rock types: trachytes, alkali feldspar syenites and rhyolitic dykes. The rocks are hypersolvus, porphyritic and less commonly show trachytic textures. The complex has a high fracture density consisting of two dominant fault systems trending west-northwest and east-west. The rocks are silica-oversaturated; trachytes and syenites are mildly metaluminous to peralkaline, while rhyolites are mainly peraluminous due to the relative loss of a peralkaline fluid phase. The rocks are enriched in high field strength elements (Nb, Zr, Y) reflecting a within-plate tectonic setting. The curvilinear trends defined by some major and trace element variations, together with the decrease in the K/Rb ratio and concomitant increase in Rb/Ba and Rb/Sr ratios, indicate the important role of feldspar fractionation during the evolution of the Mansouri Complex. In some Mansouri rocks, radioactive anomalies are mainly associated with shear zones aligned along the east-west direction and characterised by abnormal accumulations of U- and Th-bearing accessory minerals like monazite, allanite, apatite and titanite. The Mansouri Ring has carbonatite rocks of calcitic, calcitic-dolomitic and dolomitic varieties; the latter is widely abundant. Apatite, monazite, pyrochlore, sphalerite, pyrrohotite and magnetite are the common accessory minerals associated with the Mansouri carbonatites. The U and Th contents of the carbonatites range from 10–25 ppm and 40–250 ppm, respectively. The U and Th contents of the alkali feldspar syenites range from 1–9 ppm and 3–10 ppm, respectively. The low Y/Nb ratios (< 0.6), together with the relatively low initial 87Sr/ 86Sr ratio (0.705) of the Mansouri rocks, substantiates the dominant role of mantle-derived magma in their genesis.