الفهرس 
GEOLOGICAL SETTINGOnly 14 pages are availabe for public view
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Abstract
The present work deals with the geology of the Pan-African basement rocks northwest of Gabal .El
Urf in the north Eastern Desert of Egypt. The .
study area covers about 850 knl starting from the area west of Gabal El
Urf to the Basement-Nubia Sandstone contact and is delineated by
latitudes 27° 48’ 51” & 28° 05’ 00” N and longitudes 32° 35’ 00” &
32° 54’ 28” E.
Detailed field studies revealed that this area is covered by metavolcanic rocks, which are next to
the granitoids in areal extent. Arc granitoids, Dokhan volcanics, molasse-type Hammamat
sediments, a small fresh gabbro stock cover a relatively limited part of the map. Finally, the
area is dominated by a variety of post- to late- tectonic granilnids (younger granites).
The different rock units encountered are grouped into three petrotectonic assemblages. Starting
with the oldest, these are:- a) the calc alkaline assemblage, b) the molasse-type Hammamat
sediments and c) the late intrusives.
The calc-alkaline assemblage includes the metavolcanic rocks, the arc granitoids and the Dokhan
volcanics.
The metavolcanic rocks form a sort of continuous series of slightly metamorphosed volcanic rocks
intruded by the different granitoids. They include submarine lava flows essentially of intermediate
composition (met-andesite- meta-dacite) and a minor basic variety (metabasalt- meta basaltic
andesite), together with their pyroclastic equivalents which prevail along the northern border
of the main belt. The low grade regional
,.. metamorpheism of the greenschist facies to which these metavolcanic
· rocks were subjected did not oblitrate their origional volcanic forms.
Geochemically, they are low-K, calc-alkaline in character with the exception of the metabasalts
and metabasaltic andesites which exhibit both calc-alkaline and, tholeiitic affinities and
represent the first stage of island-arc volcanism. The systematic variation in the chemistry of the
metavolcanic rocks may indicate that they were differentiated from a basaltic parent magma.
The arc granitoids are represented by a quartz diorite - tonalite association. Petrochemical
studies showed that these granitoids fall in the field of volcanic arc magma, they are
calc-alkaline in nature, metaluminous emplaced at compresional environment. They are similar in
there characters to the G1 group of the Egyptian granitoids which are subduction related.
The Dokhan volcanics in the study area pertain to the more felsic varieties of this rock unit. In
the main exposure, they form an alternation of rhyolite and dacite lava flow and tuffs with
intervening quartz porphyry and bands of feldspathic arenite. This indicates that eruption of the
Dokhan volcanics was active while the depositon of the Hammarnat sediments was still in progress.
These volcanics plot in the upper boundary of the medium K-field of the type locality, they are
calc alkaline developed in the late stage of an island-arc development.
The molasse type Hammamat sediments pertain to the Igla Formation, · they are closely
associated with the Dokhan volcanics and form. a repeatedly alternating succession
of bedded clast- supported conglomerates, sandstones and siltstones of red and
purple colours. The detrital modes of the fine conglomerates and sandstones plot on the QFL
diagram in the undissected arc field and the transitional arc fidd respectively.
Within such active magmatic arcs, sediment sources :are mainly in the volcanics capping
igneous belts and granitic plutons of the arc roots.
Geochemically, these sediments are immature, their materials :are derived from two contrasting
provenances, namely; uplifted .older ensimatic oceanic island arcs and younger continental island
arc (Dokhan volcanics).
The small gabbro intrusions is transitional in composition from tholeiitic to calc-alkaline,
evolved in a within plate geotectonic environment.
Plutonism in the study area was terminated by the emplacement of three phases of the younger
granites; the oldest is a granodiorite followed by huge biotite and biotite hornblende plutons and
finally a leucocra.tic phase of monzogranite. On the QAP modal classification diagram, these
granites fall in the fields of medium K- granodiorite and high K- calc alkaline monzonite.
Geochemically, these granites are alkaline to calc-alkaline, essentially peraluminous, formed also
in an environment of compression at a relatively shallow depth. They are comparable in there
characters to the G2 group of the Egyptian granites produced by crustal anatexis as a result of the
thickening of the crust at a late phase of orogeny.
Dyke swarms in the Eastern Desert is highest in the grantoids which are more susceptible to
fracturing and dyke intrusion relative to the mafic rocks. This is the case in the study area where
a relatively large number of dykes varying in composition, traverse the granitoids particularly the
monzogranites.
. .
The geological history of the area started with the development of an
island-arc system formed of a variety of basic to intermediate metavolcanic rocks. This was
accompanied by a compressional deformation event which resulted in tight folds trending essentially
NW in the metavolcanics. Low grade metamorphism was synchronous with this event.
Arc granitoids were emplaced followed by the eruption of the Dokhan volcanics in the late stage of
the island arc. Harnmamat molasse debris started to accumulate in an intermontane small basin while
volcanicty was still active, and hence the temporal association of the lower horizons of these
sediments with the Dokhan volcanics.
The intrusion of the small gabbroic stock, the younger granites and the series of dykes, terminate
the igneous activity in the region.
In a later stage probably during the Phanerozoic, the different rock units were dissected by a
system of normal faults following two main trends; a NE or ENE normal to the Red Sea which is
younger than the NW - SE trend.
The pink younger granite in the northern part of the study area and environs is now quarried as an
ornamental stone from a number of sites.