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Abstract
The present thesis deals with the geology, petrography, geochemistry and
radioactivity of Wadi El-Morakh basement rocks which are located south Taba, along
the western coast of the Gulf of Aqaba between 2920 and 2930 N, and longitudes
3441 and 3454 E. The studied area covers about 215 Km2.
Detailed field work revealed that the investigated area is built up of the
following Precambrian metamorphic and igneous rocks (from old to young): 1)
Metasediments (schists); 2) Metadiorites; 3) Paragneisses and Migmatites; 4) Foliated
Granitoids; 5) Younger granites (phase II); 6) Volcanic rocks; 7) Younger granites
(phase III); 8) Pegmatites as well as post-granitic dykes. The studied schists are
derived, by metamorphism, from greywackes and pelites or pelitic greywackes that
were originally derived from felsic and intermediate igneous provenance and
deposited in a tectonic setting related to an active continental margin regime. The
studied metadiorite is characterized by a subalkaline tholeiitic affinity and, based on
Meschede (1986) discrimination diagrams, is mainly developed in “volcanic arc”
tectonic environment.
In the study area, the gneisses comprise mainly paragneisses and migmatites,
however, it is difficult to separate them into two separate mapple units. Two varieties
of paragneisses were recorded (biotite-gneiss and staurolite–garnet–biotite gneiss).
The sedimentary origin for these gneisses is indicated by the presence of aluminumrich
mineral assemblage such as garnet and staurolite of the pelitic rocks (e.g. Wadi
Tweiba paragneiss), the common occurrence of schist xenoliths within gneisses, the
high percent of quartz and biotite and the presence of sedimentary structures. The
migmatites are well recognized and the main structural types are: banded (stromatic),
ptygmatic, ophthalmitic (augen), fleck, schlieren, and folded.
Metamorphic studies revealed that metamorphism in the studied area belongs
to three phases: two progressive and one retrogressive, grading from the
greenschist facies to the amphibolite facies. Phase I includes all the metamorphic
processes that transferred the sedimentary pile of the pre-migmatite parent rock into
the schist. Phase II includes the migmatization of the schist into migmatite. Phase III
includes all retrograde metamorphism that occurred in the area.
The foliated granitoids constitute the largest rock unit in the study area. and
continue southward beyond its limits. They have fine-streaks of quartz and
plagioclase concordant with the foliation giving them in some places a banded
appearance. The lithologic composition of the foliated granitoids varies from
granodioritic to quartz-dioritic. They are peraluminous in nature and display trace
element chemistry characteristic to volcanic arc and syn-collision tectonic settings.
The younger granites (phase II) are mainly biotite granites while the (phase III)
younger granites are mainly biotite-muscovite granites. These younger granites are
peraluminous in composition, fractionated from calc-alkaline magma generated in a
tectonic setting related to volcanic arc and syn-collision settings i.e. compressional
settings.
The volcanic rocks comprise an association of intercalated basic and acidic
volcanics. These volcanic rocks show a transitional mildly alkaline to calc-alkaline
nature and were erupted in a tectonic setting similar to recent volcanic arc lava and
away from the field of within plate lava.
The pegmatites strike generally NE-SW, NW-S E and rarely N-S. Pre-, Syn-,
and Post-tectonic pegmatites were recorded.
Structurally, these rocks were subjected to various tectonic cycles since their
formation. Detailed structural analyses of small scale and macrostructural elements in
the area, indicate three possible tectonic phases, D1, D2 , D3 trending NW-SE, SE- N
W and NE-SW respectively.
Concerning radioactive elements, all the rocks cropping out in the studied
area have no economic potentialities. The uranium content ranges from 0.8 to 4.7
ppm and thorium ranges from 2 to 10.3 ppm. Their concentrations and ratios indicate
magmatic origin and also point to post-magmatic processes that affected the uranium
distribution.