El Gharbawy R.I1*. and El Maadawy W.M.2
(1 Faculty of Science, Ain Shams Univeristy, Cairo, Egypt; 2 Nuclear Materials Authority, P.O. Box 030, Maadi, Egypt )
Abstract There are two main granitic rocks cropping out in the study area: 1) the syn-orogenic granites are moderately weathered, jointed, exfoliated and characterized by low relief. These rocks are subdivided into tonalite and granodiorite. They are essentially composed of plagioclase, quartz, biotite, hornblende and potash feldspar; and 2) the post-orogenic granites, characterized by high relief terrain and represented by monzogranite, syenogranite and alkali granite. The monzogranites suffered hydrothermal alteration in particular along joints, faults, shear zones and fractures, which recorded the highest values of radioactivity, reflecting the role of post-magmatic alteration processes in the enhancement of radioactivity. The hydrothermal alteration (desilicification and hematitization) resulted in the formation of mineralized (altered) granites.The altered granites are enriched in TiO2, Al2O3, FeOT, MnO, MgO, Na2O, Rb, Sr, Y, Zr, Zn, Ga and Co and depleted in SiO2, CaO, P2O5, Nb, Pb, Cu, Ni and Cr relative to the fresh monzogranite. The investigated granites contain basic xenoliths as well as pockets of pegmatites. Perthites, quartz, plagioclase and sometimes biotite, represent the essential constituents. Some accessory minerals like zircon are metamicted reflecting their radiogenic nature. The alkali granites are characterized by the presence of aegirine, rebeckite and arfvedsonite. Both syn- and post-orogenic granites show some variations in their bulk chemical compositions. The older granitoids are metaluminous and exhibit characteristics of I-type granites and possess an arc tectonic environment. On the other hand, the younger granites are peraluminous and exhibit the characteristics of post-collisional granites.It is interpreted that radioactivity of the studied rocks is mainly controlled by both magmatic and post-magmatic activities. Frequently, the post-orogenic granites host zoned and unzoned pegmatite pockets. Some of these pockets anomalously attain high radioactivity. The syenogranites and the pegmatites are characterized by high contents of SiO2 and K2O and low CaO and MgO. They have transitional characters from highly fractionated calc-alkaline to alkaline. The alkali granites related to A2-subtype of A-type granites. The post-orogenic granites were originated from magma of dominant crustal source materials and related to post-collisional setting under extensional environment.
Key words synorogenic; postorogenic; granite; hydrothermal; pegmatite
* Corresponding author, E-mail: elgharbawy19@hotmail.com
CHINESE JOURNAL OF GEOCHEMISTRY Vol. 31, No. 3, 2012, page 242-259
© Science Press and Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2012
