GEOLOGY, GEOCHEMISTRY AND RADIOACTIVITY OF DYKE SWARMS AT WADI EL AKHDAR AREA, SOUTHCENTRAL SINAI, EGYPT

Wadi El Akhdar area is dissected by two main dyke trends cutting mainly the older granites. The
oldest dykes trend NE-SW comprising swarms of nearly parallel basic dykes (basalt). The intermediate
dykes trending NW-SE (basaltic-andesite and trachy-andesite) cut the former dyke trend. The major dykes
trending NE-SW are acidic (rhyolite and granophyre) representing the terminate emplacement of the studied
dykes. The rocks of the dyke swarms were derived from high K-calc-alkaline magma and subjected to
fractional crystallization. They are of hyperaluminous in nature and were erupted in within-plate tectonic
and volcanic arc granitoid environments. In the studied dykes, Rb/Sr ratio for the basic and intermediate
members are far <0.7 indicating direct derivation from the upper mantle; while acidic ones with Rb/Sr ratio
up to 10.7 values suggest the presence of felsic materials in their source and were derived from sialic crust
or affected by crustal contamination.
The field radiometric survey and laboratory radiometric measurements indicate that the rhyolite and
granophyre dykes are the most important from the radioactive point of view. They are characterized by
the presence of uranium and thorium minerals, together with some rare metal minerals. The radioactive
minerals are lithologically controlled due to post magmatic processes which cause addition of uranium.
Uranophane, uranothorite, columbite, zircon and monazite are the main radioactive minerals in the
examined acidic dykes.