GEOLOGY, GEOCHEMISTRY AND MINERALOGY OF YOUNGER GRANITES FROM GABAL RABDI AREA, SOUTH EASTERN DESERT, EGYPT

The study area consists of metavolcanics and younger gabbros intruded by younger granites
and post granitic dykes. The younger granites are represented by monzogranites and syenogranites.
Petrographically, monzogranites are composed mainly of quartz, potash feldspars, plagioclase(An 9–18),
biotite and hornblende, whereas allanite, zircon and opaques (iron oxides and pyrite) are accessories. The
syenogranites are composed mainly of potash feldspars, quartz, plagioclase(An. 4–14), biotite and muscovite.
Zircon and opaques ( iron oxides and pyrite) are found as accessories.
Geochemically, the monzogranites are considered as I-type, originated from metaluminous calc-alkaline
magma and formed under an extensional regime. They are crystallized at temperature <800oC, with pressure
between 2 to 4 kb. The syenogranites are related to A-type, originated from peraluminous calc-alkaline
magma. They are post-orogenic, formed under an extensional regime and are crystallized at temperature
< 800 oC with pressure from 1 to 3 kb. The monzogranites and syenogranites have high contents of Zr,
Y, Rb, Ba and Nb but syenogranites are higher than monzogranites in above contents except Ba. These
younger granites are characterized by presence of radioactive mineral (uranophane) in association with
columbite, zircon, pyrite and allanite. Spectrometry, the monzogranites have an average content up to 7
ppm eU and 25 ppm eTh. The syenogranites have an average content up to 9 ppm eU and 23 ppm eTh.
The radioactive anomaly is related to shear zone in syenogranites (up to 98 ppm eU and 49 ppm eTh). The
origin of uranium in the shear zone is epigenetic and related to ascending hydrothermal solutions as well as
accumulation of uranium leaching from the surrounding rocks and re-deposited in the shear zone.