الفهرس 
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Abstract
Wadi Abu Hamr area is occupied by igneous and metamorphic rocks
belonging to late Precambrian units. It is located between latitudes 26°34’, and 26°36’N, and longitude 33° 31’ and 33° 46’E, near the northern boundary of the Central Eastern Desert domain. It is characterized by high rugged topography includes different mountain as G.Um Tagher, G.Abu Furad, G, Wasif, G.Wairah and and G.Muhammed Rabah. The present work is devoted to study the geology, structure, petrography, geochemistry of the arc rocks as well as the mineralogical characteristics of the exposed rocks in the study area. The exposed rock types in the mapped area can be classified and arranged from oldest to youngest as: ultramafics (serpentinite and talc carbonate) and metagabbros, metavolcanic, represented by metabasalts and metabasaltic-andesites, meta-andesites and metadacites, older granites represented by quartz diorites and tonalites, Dokhan volcanics represented by basaltic-andesites, andesites dacites and rhyolite, Hammamat sediments are represented by conglomerate, sandstone, siltstone, younger gabbro, younger granites represented by monzogranites and finally dikes.
The ultramafic rocks crop out in three parts in the studied area, the first one presents as an elongated mass, extending NNE-SW on the northwestern corner of the map, and the other ones crop out as isolated two small masses in the Central Western part of the mapped area, cut by Wadi Abu Hamr in contact with metagabbros, metavolcanics and quartz diorite-tonalites, they form hillocks of moderate relief, these masses are composed of serpentinized peridotites. The serpentinized peridotites are olive-green to black on fresh surfaces. In some places, they are cavernous, buff-colored talc-carbonate rocks. These rocks include few lenses of fresh, coarse-grained pyroxenites and have small chromite segregations. These rocks are extensively sheared, and foliated. Their contact with the overlying metagabbros to the east is heterolithic, or a nonconformable surface and intruded by the granitoid rocks in the west side of the mapped area.Petrographically, The serpentinite rocks are massive, fine-grained and greenish black to dark green. They are composed of antigorite which is the most common minerals with variable amounts of chrysotile and lizardite. chromite is the most abundant accessory mineral, beside, magnetite and hematite. The talc-carbonates rocks are fine to medium-grained and reddish brown to yellowish buff in color. Carbonate minerals are represented by magnesite and dolomite which form irregular grains. Talc appears as minute flakes or radiating bunches, with a circular bladed habit. Antigorite is subordinate and forms fine flakes. Fine dusty grains of magnesite and hematite are sporadically dispersed throughout the rock.
The metagabbros are present in the western part of the studied area, cut by, Wadi El Pula, Wadi Abu Hamr and at the end of Wadi Safaga, contacted with serpentine. They are also present as small outcrops in the eastern part of the mapped area and traversed by W. Rabah. They are characterized by moderate topography and occur as massive and compact rocks of medium- to coarse-grained sizes, in grey to dark-grey color. These rocks are commonly fractured and highly altered, especially along their contacts and fault planes. They are intruded by quartz diorite-tonalities, especially from the north with granitic offshoots and apophasis. Petrographically, these rocks are mainly equigranoblastic in texture, with subordinate ophitic and poikiloblastic varieties. They are formed mainly of altered plagioclases, amphiboles and very little relics of pyroxene, together with subordinate to accessory chlorite, epidote, carbonate, apatite and opaques. The mineral assemblage indicates a metamorphism in the green schist-lower amphibolites facies similar to that of the ophitic metagabbros of the Eastern Desert of Egypt.(Takla et al., 1981 and 1990).
The exposed metavolcanics cover the vast area in the central part of the mapped area, and cut by Wadi Safaga and Wadi Abu Furad as well as the mouth of Wadis Al Bulah and Abu Furad. These metavolcanics are intruded by quartz diorite-tonalites with a sharp contact and a fault contact with Hammamat sediments. These rocks are highly weathered and jointed. They are of low to moderate topography, fine grain sizes and greenish black to dark black in color. Occasionally, they are highly sheared, fractured, and altered, particularly along the fault planes, where brecciation, mylonitization and kaolinitization are well developed.
At the eastern part, due east of W. Rabah, the metavolcnics are intruded by younger gabbros and monzogranite and intersected with dikes and offshoots of them.
Petrographically, the matavolcanic rocks in the studied area are distinguished to metabasalts and metabasaltic andesites, meta-andesites and metadacites. The metabasalts and metabasaltic andesites rocks are greyish green to dark green with brownish green weathered surface. These rocks are characterized by porphyroblastic and amygdaloidal textures. They are essentially composed of plagioclases, hornblende and pyroxene phenocrysts embedded in a fine-grained groundmass. The groundmass consists mainly of small plagioclases crystals, epidote, zoisite, saussurite, tremolite-actinolite and chlorite as well as iron oxides. Opaques are the dominant accessory minerals while chlorite, epidote and saussurite occur as secondary minerals.
Meta-andesites are greyish green and display brownish green weathered surface. They are commonly porphyblastic and essentially composed of plagioclase and hornblende phenocrysts embedded in a fine-grained groundmass. The groundmass is composed of the same mineral constituents, with little amounts of interstitial quartz and clusters of epidote as well as iron oxides.
The metadacites are generally porphyroblastic and made up mainly of quartz, plagioclases and biotite. The phenocrysts consist mainly of large crystals of quartz and plagioclase. Chlorite and sericite are present as secondary minerals. They are represented by very fine grains forming the groundmass of the rock. The iron oxides are accessory minerals.
The quartz diorite-tonalites crop at the western part of the studied area and cut Wadi Abu Hamr, and appear in the western part of Wadi El Pula and intrud the metagabbros and metavolcanics. They have a sharp nonconformally contact with Hammamat sediments. Quartz diorite-tonalite rocks are characterized by highly weathered, foliated, jointed, and fractured. They also have metavolcanic xenolithes. These rocks are intruded by numerous acidic dikes.
The quartz-diorites rocks are massive, medium to coarse-grained and grey in color. Microscopically, they are hollocrystalline with hypidiomorphic to panidiomorphic granular textures. These rocks are essentially composed of plagioclases, hornblende, quartz and potash feldspars with minute crystals of biotite. Epidote and opaque minerals as well as zircon, titanite and apatite are accessories.
Tonalites are mainly composed of plagioclases, quartz, hornblende, biotite and potash feldspars, while sphene and apatite are accessories. These rocks show gneissic structures.
The Dokhan volcanics constitute a definite and well-marked association corresponding to the well-known basaltandesite and rhyolite association. They represent an important stratigraphic unit in the basement complex of Egypt, relatively older than the Hammamat group.
The Dokhan volcanics represent a bimodal suite composed of basic eruption followed by felsic eruption with their pyroclastics. They are followed unconformably by Hammamat sediments. The acidic varieties of these rocks generally have pink brownish-red and red colors. The basic types are grey greenish in colors. The Dokhan volcanic rocks form moderate relief ridges. They show banding generally striking E-W with a gentle dip either to the 300-400, The Dokhan volcanics are generally massive, and sometimes foliated, especially the pyroclastic and tuff members. Open folds were recognized in the tuffaceous rocks. The upper and thinner sequence is intercalated with the Hammamat sediments in a fault bounded basin.
In the studied area, the Dokhan volcanics have a porphyritic nature and form a thick sequence of volcanic flows and minor pyroclastics that are predominantly basaltic and rhyolitic in composition. They are cropping out at the northeastern side and extending further east beyond the eastern limit.
The Basaltic andesite rocks have a distinctive porphyritic texture defined by abundant phenocrysts of plagioclases together with amphibole and augite, set in a fine-grained groundmass of the same minerals. The accessory minerals include opaques and apatite. Secondary minerals include saussrite, epidote and chlorite. While the andesite rocks are the dominant variety of the Dokhan volcanics in the studied area. They display colors ranging from dark grey to dark green. These lavas typically display glomero-porphyritic texture and flow structure defined by the parallel alignment of the tabular plagioclase phenocrysts. The Basaltic andesite rocks are composed of plagioclase and hornblende phenocrysts embedded in fine-grained groundmass.
The acidic variety of these rocks are represented by dacites and rhyiolites. The Porphyritic dacites are composed mainly of plagioclases, quartz, biotite and potash feldspars, as well as a few hornblende phenocrysts, embedded in a fine grained groundmass; which is formed of small laths of plagioclases, quartz, biotite, epidote and chlorite. Apatite, zircon, and sphene are common accessories, while this rock type is sometimes fractured; the fractures are filled with secondary muscovite and quartz. While the rhyolites are porphyritic, leucocratic, and composed essentially of K-feldspar, quartz with minor amount of plagioclases, and some biotite phenocrysts embedded in a fine-grained groundmass of quartz and K- feldspar. Spherulites are common and are composed of radiating and fibrous aggregates of alkali feldspar and quartz intergrowths. The main accessories are apatite, sphene and zircon, whereas the secondary minerals are sericite, chlorite and kaolinite.
The Hammamat sediments are well exposed to the east of the central part of the mapped area, cut by Wadi Safaga. They occur as an elongate strip, covering an area of about 10km2 (2km wide and 5 km long), and trending in N-S to NNE-SSW direction. The sediments exhibit a relatively moderately topography. The weathering products of the Hammamat sediments are blocks and spheroidal masses. The sequence of Hammamat sediments in the studied area represented from bottom to top as conglomerates, sandstones, green siltstones.
The conglomerate represents 5% of the Hammamat sediments, and is exposed at the contact with the monzogranite on the eastern and western shoulders of Wadi Safaga. It represents the flanks of a major asymmetric syncline. These conglomerates vary in color from grey to reddish. They are polymictic: included clasts are mostly volcanic, and rarely granitic in the Wadi Safaga outcrop. The clasts are angular to subrounded, illsorted, and hard and compacted, in a fine-grained siliceous matrix. Normal graded bedding is recorded, particularly at the eastern and western sides. Petrographically, this conglomerate is indurated pebble and cobble. The finer pebble conglomerate is hard, and reddish-grey in color. and consists mainly of pebbles of 1 cm to 3 cm in diameter, embedded in a reddish, hematitic or clayey matrix, in which chlorite is a major constituent. This clastic rock is mainly composed of round to subround quartz and plagioclase crystal fragments, and purplish porphyritic andesites and dacites porphyries lithic clasts.
The sandstones represent 5% of the Hammamat sediments and are present in the eastern, and western shoulders parts of Wadi Safaga. These beds are generally thin to thick, ranging from 10cm to 100cm in thickness; but in few outcrops the thickness of sandstones reach 15m. The sandstones appear to be restricted to medium to coarse, moderately to poorly sorted sand, and frequently exhibit graded bedding. The sandstones vary from grey to green in color. Petrographically, sandstones are composed of angular to subangular crystals and lithic fragments embedded in a matrix of sericite, chlorite, epidote and sphene. The clasts exhibit undulose extinction and other deformational features. Plagioclase fragments are abundant and display sasuritization , kinking, and bent twinning; equally abundant quartz exhibits wavey extinction and incipient recrystallization. cloudy orthoclase and bent chlorite flakes are accessories.
The siltstones represent approximately 90% of the Hammamat outcrops. The siltstones are restricted as thin beds in the eastern part of the main outcrop. The siltstones constitute the thickest Hammamat outcrops; the beds are generally very thin to very thick ranging from 3cm to 2m, and exceptionally may reach up to 70m. They are well bedded and show color variation from green to purple, due to presence of chlorite and hematite, respectively, while the siltstones are typically of greyish-green to dark green color. Numerous pyrite crystal cubes are recorded in some green siltstones.
These rocks are hard, and reddish-purple or green in color. Few of these rocks are invaded by quartz and calcite veins. Microscopically, the siltstones consist of minute, angular to subangular, and poorly sorted grains of quartz and minor feldspar. The matrix is usually rich in iron oxides, chlorite, and sericite.
The younger gabbros are located at the mouth of Wadi Rabah and located to the east of Wadi Safaga. These rocks are characterized by a low to moderate topography and intruded by the monzogranites from the north. Also, the younger gabbros intrude the quartz diorite-tonalites and metavlocanics from the northwest, west, south and east respectively. Show bouldery appearance and spheroidal blocks and traversed by pegmatite veins and dike swarms. They are fine to medium grained in size and dark grey to light grey in color. These rocks are hypidiomorphic, with dark greyish green color (meso- gabbro). They are composed of plagioclases, hornblende, opaques and relics of pyroxene. Ophitic and subophitic textures are the predominant features.
The monzogranites are characterized by higher topography than the surrounding country rocks such as the quartz diorite-tonalites, the metavolcanics and the younger gabbros. The rocks are located in the mouth of Wadi Rabah in the north and the end of Wadi Waeara in the south. These rocks are of reddish pink to pink color, medium- to coarse-grained and characterized by vertical jointing and bouldery weathering with monumental shapes as well as cavernous weathering and exfoliations. They intrude all the previously described rocks and send several offshoots into them. Xenoliths of the older rocks exhibiting different shapes (irregular, oval or elongated) and sizes (from 5 to 20 cm in diameter) and of different degrees of assimilation and digestion are present. The monzogranite is a medium to coarse grained, greyish pink in color, and show hypidiomorphic equigranular with myrmekitic texture. Microscopically, they are composed essentially from quartz, microcline perthite, microcline, plagioclase and biotite, whereas sphene and allanite are found as accessory minerals.
The recorded structures in the studied area are classified as bedding geometry, bedding internal structures and gravitational deformation.
Bedding is the most important feature, being a primary planar structure, inherited in a certain rock from the time of its formation. In the study area, bedding thickness exceeds one cm in some outcrops of siltstones, so can be defined as thick-bedded. On the basis of field observation, bedding is a more appropriate description than foliation. The strike of the beds varies from ENE-WSW and dip amounts are subhorizontal to subvertical.
The graded bedding in the study area is shown in sandstone outcrops. It is regularly well sorted, from coarse at the base to fine at the top. This indicates that the beds are right-way-up.
The cross-bedding which is recorded in the study area, is of ”tabular type” according to Mckee and Weir (1953, p382). It is bounded by ordinary horizontal subparallel bedding planes or strata with no cross- stratification.
Ripple marks are excellently exposed in the green siltstones, in the central part of Wadi Safaga. The ripples are asymmetric and strike in the mein direction of E-W; they have a gentle slope of 40°N and a steep slope of 50°S. The ripple marks indicate a normal position of the bedding, and the current direction in induced to be from south to north.
Gravitational deformation is concerned only with deformation which took place, while the sediments were still in the environment of deposition, thus excluding tectonic and other later deformation. The resulting gravitational slide or slump produces folds and faults in the affected materials.
Folds are delineated in a wide distribution over the study area. The folds are recognized in both the major and mesoscopic scales.
Major folds are exposed in parallel, plunging major synclines. The axial traces are oriented in the E-W direction, and plunge 15 towards the E-W, while the limbs dip by 40 to70 toward the north direction. The axial trace extends for about 3km. The core of this fold is occupied by the Hammamat sediments. The folds are isoclinal, open, asymmetrical and of different wave lengths. The exposures of these major folds are well observed in Wadi Safaga.
The metavolcanics and Hammamat sediments show numerous fold exposures of different styles. The folds range in size from few centimeters up to few meters. The recorded fold styles are open concentric, closed, and tight. The folds are generally asymmetric, and the axial planes show different inclinations. The hinges are generally rounded.
Two types of foliations are principally recorded in the study area. The Slaty Cleavage is a penetrative foliation occurring in the fine-grained rocks, primarily the siltstones of the Hammamat sediments, and some tuffs affiliated to the metavolcanics, and the second one is crenulation cleavage which are produced by microfolding (crenulation) of “preexisting, continuous cleavage” (Davis and Reynolds, 1996, p. 431). This foliation is penetrative and only recorded in the fine-grained tuffaceous metavolcanics, slates, phyllites and schists. It is “marked by wider cleavage domains” (opt. cit.), on the limbs of the microfolds.
Penetrative planar fabric elements, such as foliations and bedding are usually combined with penetrative linear fabric elements. These linear structures are considered as strain markers defining the axes of folding in different phases (Hatcher, 1990). In the study area, the following lineation varieties are recognized: 1-Mineral lineations parallel to fold axes, 2- Pencil-like lineations and 3- Boudinage.
Faults are essential structural features in the study area. These faults have been well studied in the present work, with emphasis on fault slip. Slickensides and grooves which are commonly associated with brittle faulting. These kinematic indicators indicate the direction and sense of maximum resolved shear stress in that plane (Carey and Brunier, 1974). Slickensides are often composed of fibrous crystals of chlorite, epidote, calcite, and quartz that stretch from one side of the fault plane to the other. Strike-slip faults are the most effective type and are encountered in all the rock units with heavy concentration. These faults are represented in both orders of sinistral and dextral senses of movement. Fewer normal faults are also recorded.
Geochemically, the metagabbros were originated from calc-alkaline magma and developed in the arc basalt field environment. The geochemical classification of the studied Dokhan volcanic rocks are classified to basic and felsic rocks. The basic to intermediate varieties are composed from basaltic, to basaltic andesites, and andesite magma, while the felsic Dokhan volcanics are composed dacite, and rhyolite magma. The chemical nature and behavior of the magma during its evolution for the study volcanic rocks clears that the Dokhan volcanics were originated from calc-alkaline magma. The studied Dokhan volcanic rocks also clear that they cover a wide range of A/CNK, reflecting the metaluminous to slightly peraluminous character of the basic rock sample and peraluminous character for the felsic rocks. The relatively high K2O (3.01–1.14wt %), Ba (913–495ppm) and Sr (1088–570ppm) contents and the high ratios of Zr/Y (6.35–14.5) of the Dokhan volcanic rocks are all likewise typical of calc-alkaline magmas formed in continental arcs. The Dokhan volcanics erupted in an arc environment with thicker (continental) crust. Also, the Dokhan volcanics evolved mainly through fractional crystallization of the petrographically observed phenocryst assemblage, which are plagioclase + augite + hornblende + magnetite in the intermediate Dokhan volcanic rocks and plagioclase + K-feldspar + biotite + apatite + magnetite in the acidic varieties.
The mineral constituents in the studied area are iron oxides minerals (magnetite, hematite, ilmenite, and chromite, garnet, sulfide minerals, (pyrite, brass, and chalcopyrite), titaum minerals (rutile), titanite (sphene), zircon, phosphate minerals (monazite, apatite, allanite) and green silicates (biotite-phlogobite, epidote and actinolite).