الفهرس 
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Abstract
The present thesis deals with the geological, structural,
petrographical, geochemical and radiometrical studies of the basement rocks of Urn Safi area which
is located in the Central Eastern Desert, Egypt between Long. 34° 03’ and 34° 14’ E, Lat. 25° 15’
and 25° 24’ 20” N.
The basement rock units in the study area can be chronologically arranged beginning with the oldest
as follows:- allochthonous ophiolitic assemblage (dismembered ophiolitic rocks and melange), arc
assemblage [metavolcanics (meta-andesites), volcaniclastic and volcano-sedimentary association,
older granitoids, volcanic rocks (rhyolite and pyroclastics)), molasse sediments, younger gabbros
(noritic troctolite, normal gabbro and hornblende gabbro), younger granites (biotite granite,
perthite leucogranite and muscovite leucogranite), post granite dykes and veins and trachyte
plugs.
The granitoid rocks of Urn Safi area have been subdivided into three categories, (based on field
relationships, petrography and geochemical characteristics) termed magmatic cycles emplaced during
successive tectonic events. Granites of the first magmatic cycle (diorite, quartz diorite, tonalite
and gneissic granodiorite) have calc-alkaline, metaluminous, 1-type character and emplaced during
pre-plate collision to post collision uplift regime under a high water vapour pressured (5-10 k
bar) and high temperature (800-840 °C). Biotite granite and perthite leucogranite represent the
second magmatic cycle, they characterized by calc-alkaline, metaluminous magma and emplaced during
late-orogenic regime. Granites of the third magmatic cycle are mainly peraluminous muscovite
leucogranite in composition, crosscutting all other rock types in the region and were emplaced
during syn-collision regime. The latter two magmatic cycles were emplaced at moderate water-vapour
pressure (2-5 k-bar) and temperature (760-800 °C).
The fresh rhyolite rocks of the studied area are originated by a combination of fractional
crystallization of lithospheric source and crustal contamination, whereas the magma was intruded
at active continental margin environment.
The structural evolution of Urn Safi area passed through three main phases of deformation,
accompanying regional metamorphism. (I) The first phase of deformation (DI) was associated with the
first folding phase (F1), contemporaneous with thrusting and coeval with low-grade regional
metamorphism of green schist to amphibolite facies. (2) The second deformation phase (D2) is
widespread and manifested by abundant tight overturned and isoclinal folds, plunging SE at moderate
angles. This folding can be inferred from stereoplot for S1
surfaces most
probably related to shortening oriented NE-SW. (3) The third phase of deformation (D3) is less
intensity than the second phase (D ). It is proved
2
by open symmetrical and asymmetrical fold (F3) around axial plane
strikes NE-SW and dips about 70°-80° to SE. The deformation during this
phase appears less intense and its main effect is mainly expressed in the partial distribution of
hinges of meso-folds along NE-SW trending axial plane. The D3 deformation was followed by a
structural episode of faulting in several trends of strike-slip faults.
The average eU- and eTh-contents increase from older granitoids (3 ppm eU & 7 ppm eTh) through
biotite granites (6 ppm eU & 14 ppm eTh), perthite leucogranites (7 ppm eU & 15 ppm eTh),
muscovite leucogranites (9 ppm eU & 17 ppm eTh) to rhyolite (26 ppm eU & 50 ppm eTh). Four
different types of radioactive anomalies are identified; two within rhyolite (tunnel and trenches),
one in greisen (greisenized microgranite crosscut rhyolite rocks) and the last one along the
muscovite
leucogranite shear zone.
The high radioactive intensity of granitoid and rhyolite rocks in the studied area are
discussed under three concepts namely, the leaching, the syngenetic and the epigenetic
concepts. The source of uranium in shear zone (average of eU= 288 ppm) may come
by leaching of surrounding rocks (muscovite leucogranites). These coincide with
presence of secondary uranium minerals (beta-uranophane and zippeite). The enrichment of
radioactive minerals in rhyolite (tunnel and trenches) is due to syngenetic concept. These
coincide with the presence of radioactive columbite, yttrialite, betafite, thorite and
uranothorite. In the study area the epigenetic concept explained by greisenization effect due
to a process of hydrothermal alteration by action of water vapour containing fluorine
of microgranite apophyses in rhyolite.