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STRUCTURAL EVOLUTION IN THE PALAEOPROTEROZOIC BASEMENT (BANDED IRON FORMATION AND RELATED ROCKS) OF SW EGYPT

MOSTAFA, MOSTAFA FAHMY

German Title: STRUKTURENTWICKLUNG IM PALAEOPROTEROZOIKUM GRUNDGEBIERGE IM BEREICH DES BIF, SW ÄGYPTEN

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Abstract

Structural evolution in the Palaeoproterozoic Banded Iron Formation of SW Egypt The exposed basement of the Western Desert of Egypt is part of the pre-Pan-African East Sahara Craton. The Gabel Uweinat-Bir Safsaf Aswan Uplift is situated at the eastern fringe of this craton, and its high-grade metamorphic and granitoid rock associations are markedly distinct from the metavolcanic-metasedimentary-ophiolitic sequences of the Eastern Desert of Egypt (Nubian Shield). Crystalline basement rocks cover an area of some 40,000 km2 in SW Egypt. The fieldwork and mapping of the basement rocks in the present area were carried out during the winters 1998, 1999 and at last in 2001 for altogether about 10 months of fieldwork. A detailed field geologic-structural map to the scale of 1:60,000 and two cross sections, perpendicular to the regional structural trend for the whole area, are prepared using vertical aerial photographs and Landsat images. Detailed petrographic studies of the different rock units were carried out to determine their compositional character and the effect of deformation on each rock unit. Some 125 rock specimens representing the different rock units were collected and more than 130 thin sections were examined petrographically using the polarizing microscope. Microstructure studies and (analyses) attempted to determine the structural evolution and possible transport direction from about 40 oriented thin sections parallel and perpendicular to the foliation. Further structural data, from the anisotropy of the magnetic susceptibility (AMS) was determined from 31 oriented samples, which were cored and cut (200 cylinders). The magnetic susceptibility measurements were analyzed with the Kappa bridge. The BIF is exposed as an upper unit, while anatexite forms a middle one, underlain by ultramafic-mafic units. The BIF shows large quartz crystals with wavy extinction and undeformed recrystallized quartz. The Anatexite sequences were affected by granulite facies metamorphism, followed by a retrograde metamorphism. The petrographic study shows some orthopyroxene crystals altered to hornblende, biotite and chlorite. The structural analysis of the area indicated that it was subjected to major tectonic deformation including folding, overthrusting, shearing and faulting. The area is built up of thrust slices of the BIF and Anatexite, which extend for more than 25 km in N15°E direction, dipping to the west with angles between 30° to 70°. The thickness of slices horses is up to 4 km. The thrust sequence in this area has about 20 km width. Thrust Faults have been derived from overturned folds, the shear surface replacing one of the limbs of the fold. The structure is dominated by a series of nearly parallel, minor thrust faults, or high angle reverse faults, which dissect the rock into slices, sheets, plates, blocks or wedges that are approximately equidistant and have the same displacement and are all steeply inclined to the WNW (285°). Restoration of this thrust sequence shows that the anatexite forms a basement to the overlying BIF. Both metamorphic banding in the anatexite and the layering in the BIF restore to a horizontal primary orientation. The Deformational Evolution Three major deformation phases can be discerned in the study area (D1, D2 and D3), the three phases D1, D2 and D3 affected only the basement rocks and are followed by Phanerozoic brittle deformation affecting both the basement and the overlying sedimentary rocks in the investigated area. The D1 structures indicate a crustal thickening followed by crustal thinning and developed the melano- and leucosome in the Anatexite sequences. A foliation (S1) observed in the thin section of the BIF as simple shear mylonitization zones. Generally in (D1) the Anatexite bands are parallel with the mylonitic shear zones in the BIF. The D2 structures dominate in the BIF as a micro and macro folding (F2) associated with low angle thrust or shear zones (S2). In the Anatexite and the Ultramafic-mafic sequences minor folds with the same characters as in the BIF are observed. Generally in (D2) the BIF and Anatexite bands had E-W strike and dipped shallowly to the south (S2), and contain minor folds (F2) with E-W axial surfaces. The D1 and D2 deformations are both of them overprinted by the D3 structures, which refolded the F2 at a perpendicular direction of the fold axes by F3. The D3 affected refolding and thrusting at the BIF, Anatexite sequences and Ultramfic-mafic bands as F3 and S3. The F3 has NNE trend and plunge with an angle of 15°. The foliation S3, is parallel to the fold axial surfaces and thrust surfaces. The dip angle ranges between 30° and 70° to the WWN (285°).

Translation of abstract (German)

Strukturenentwicklung im Bereich der Banded Iron Formation in Süd-West Ägypten, Uweinat Gebiet. Zusammenfassung In der Region des Gabal Uweinat ist das älteste in NE-Afrika bekannte Grundgebirge aufgeschlossen. Sm/Nd Alter zeigen eine Krustendifferentation zwischen 3200 und 3000 Ma. Im Rahmen der Arbeit werden hierin die tektonische Entwicklung und vor allem die regionalen Strukturen, Mikrostrukturen sowie die Mineralisationen untersucht. Eine geologische Kartierung wurde unter Zuhilfenahme von Luftbildern im Maßstab 1:60.000 und Satellitenbildern in Maßstab 1:100.000 durchgeführt. Die wichtigsten geologischen Einheiten in Arbeitsgebiet die Banded Iron Formation (BIF), die „Anatexite Sequences“ und die Ultramafiachen Gesteine, sind welche überwiegend Gneis-Reiche Abfolge beinhalten. Geländebeobachtungen zeigen, dass die BIF NNE-SSW streichen und um 65° nach Westen einfallen. Der Mächtigkeit der BIF schwankt zwischen 10 und 300 Meter. Sie erstreckt sich über mehr als 20 Kilometer im Streichen. Die BIF besteht aus eisenreichen Bändern mit Magnetit und Hämatit, welche mit quarzreichen Bändern wechsellagern. Die Anatexite Sequences ist meist in den Tälern als kleinere morphologische Erhebungen aufgeschlossen, die aus hellen (Leucosome) und dunklen Bändern (Melanosome)bestehen. Die Bänderung im BIF und die Bänderung im Anatexite ist subparallel. Ultramafiachen Gesteine wurden besonders im südlichen Teil des Gebietes als kleine aufschlösse zwischen den oberen Teil des BIF und untern Teil des anatexite aufgenommen. Neben der früheren Gesteine stehen in manchen Teilen des Gebietes jüngere Intrusiv- und Extrusivegesteine an. Die Deformationenphasen im Arbeitsgebiet sind sehr komplex, und zeigen verschiedene Arten von Falten und Störungen. Drei Deformationsphasen sind nachgewiesen. Die verschiedenen Deformationsphasen prägen den BIF, Anatexite Sequences und Ultramafiachen Gesteine mit fast den gleichen Deformationen.

Item Type: Dissertation
Supervisor: GREILING, REINHARD
Date of thesis defense: 26 June 2003
Date Deposited: 07 Jul 2003 10:53
Date: 2003
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institut für Geowissenschaften
Subjects: 550 Earth sciences
Controlled Keywords: Banded Iron Formation, Anatexite, Uweinat SW Egypt
Uncontrolled Keywords: Steep folds , east vergent folds , thrust , thrust slices , retrograde metamorphism
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