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

The Mansouri Ring Complex is an eccentric mass (9 x 6 km) located in the ex treme southwestern part of the Eastern Desert of Egypt. The complex exhibit s a poorly defined ring structure and a limited variety of rock types: trac hytes, alkali feldspar syenites and rhyolitic dykes. The rocks are hypersol vus, 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 m ainly peraluminous due to the relative loss of a peralkaline fluid phase. T he rocks are enriched in high field strength elements (Nb, Zr, Y) reflectin g a within-plate tectonic setting. The curvilinear trends defined by some m ajor and trace element variations, together with the decrease in the K/Rb r atio and concomitant increase in Rb/Ba and Rb/Sr ratios, indicate the impor tant role of feldspar fractionation during the evolution of the Mansouri Co mplex. 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 monazi te, allanite, apatite and titanite. The Mansouri Ring has carbonatite rocks of calcitic, calcitic-dolomitic and dolomitic varieties; the latter is wid ely abundant. Apatite, monazite, pyrochlore, sphalerite, pyrrohotite and ma gnetite are the common accessory minerals associated with the Mansouri carb onatites. The U and Th contents of the carbonatites range from 10-25 ppm an d 40-250 ppm, respectively. The U and Th contents of the alkali feldspar sy enites range from 1-9 ppm and 3-10 ppm, respectively. The low Y/Nb ratios ( <0.6), together with the relatively low initial Sr-87/Sr-86 ratio (0.705) o f the Mansouri rocks, substantiates the dominant role of mantle-derived mag ma in their genesis. (C) 2001 Elsevier Science Limited. All rights reserved .