Archean Basement outcrops of the East Sahara Ghost Craton (ESGC)
 

in the northern part of Uweinat-Howar Uplift (Egypt/Libya/Sudan country corner)

Norbert Brügge, Germany
Dipl.-Geol.

Upload:
01.07.2017

 

A. East Sahara Ghost Craton (Saharan Metacraton)
 

A "Saharan Metacraton" is suspected in central North Africa in a 500,000 square kilometer area. This craton, also referred to as the "East Sahara Ghost Craton", is largely mysterious because it is covered by very thick younger sediment deposits.
This craton, consisting of a Archean–Paleoproterozoic crust and probably divided in several blocks, is bounded by the West African Craton in the west and Congo Craton in the south with its peripheral orogenetic belt zones. In the east, the striking orogetic rifting zone of the "Nubian Arabian Shield" limits the ESGC. The orogenetic zone was shaped in the Neoproterozoic time, and contains remains of cratonic and oceanic components accompanied by magmatic intrusions. In connection with this, large parts of the adjoining "East Sahara Ghost Craton" could be changed by tectonic and metamorphic processes during the Neoproterozoic (Pan-African orogeny).


Unknown are central parts of the "East Sahara Ghost Craton", which could be still present in the pristine state without changes due to the orogenetic influence of the Neoproterozoic
.
But, according Abdelsalam et al. (2011) there are S-wave velocity anomalies in the 175–250 km depth of the mantle that are much lower than those typical of other cratons. The anomalous upper mantle structure of the "East Sahara Ghost Craton" might be due to partial loss of its cratonic root. Possible causes of such modification include mantle delamination or convective removal of the cratonic root during the Neoproterozoic due to collision-related deformation. Partial loss of the cratonic root resulted in regional destabilization, most notably in the form of emplacement of high-K calc-alkaline granitoids.

Therefore, unique outcrops in the Western Desert are of particular interest
. These are located in the northern section of the so-called Howar-Uweinat Uplift, in which the cratonic basement was raised tectonically up to terrain level and is called "Uweinat-Kamil Inlier". The most outcrops of this cratonic crust are located in Jebel Kamil and Jebel Uweinat.

Of interest are also outcrops of metasediments within the cratonic crust in the SW of Jebel Kissu and at Jabal Arkenu ("Infra-Cambrian"), as they are also known at the base in the Kufra basin.

 

 

B. Uweinat - Howar Uplift

 

The Uweinat-Howar Uplift between Kufra basin in the west and Dakhla basin in the east is a tectonically elevated block. Along main faults (possibly staggered), the basement has been lifted up to 2000 m compared to the Basement of the basins, and lie near surface level now (about +500m SL). The cause of the uplifting is unknown, but a continuously active plume in the earth's mantle is assumed by the author due to the long-lasting processes of uplifting (Ordovizium-Carboniferous) and magmatism.
The overlying Palaeozoic sediments were uplifted along with the basement. They are still in remains (plateaus) present, despite continuous erosion.

         

 

C. Uweinat-Kamil Inlier
(Jebel Uweinat, Jebel Kamil, Jebel Nazar, Jabal Arkenu)

Karmakar & Schenk (2015):  "The Uweinat-Kamil basement complex, in the central part of the East Sahara Ghost Craton (ESGC) in NE Africa, is an unique inlier-outcrop in the craton that contains rocks with Archean formation ages, and is hence a key to understanding the ancient crustal evolution of the otherwise enigmatic and poorly known ESGC. The craton is thought to have been decratonized during the Neoproterozoic after thickening as a result of Pan-African collisional events along its margins.

Textural and compositional relationships preserved in the metapelitic granulites from the Uweinat-Kamil inlier suggest a two-stage metamorphic evolution of the rocks:
Stage I saw the growth of sapphirine + quartz + garnet 1 at ~10 kbar and ~1050°C, from an initial assemblage containing kyanite, sillimanite ± biotite, which are now preserved only as inclusions in porphyroblastic garnet 1. This stage was followed by near-isobaric cooling stabilizing the assemblage garnet 1 + sillimanite 1 ± orthopyroxene 1 ± sapphirine coexisting with melt.
Stage II saw the breakdown of this assemblage forming a variety of symplectite assemblages (orthopyroxene 2 + cordierite or orthopyroxene 2 + cordierite + sapphirine ± sillimanite 2 or cordierite + spinel) through near-isothermal decompression from ~9 kbar to ~6 kbar at ultrahigh temperatures of 900–1000°C. This was followed by near-isobaric cooling of the rocks to temperatures of 700°C at ~5.5–6 kbar, as evidenced by the growth of garnet 2 and the formation of late-stage biotite, owing to back-reaction of melt with residual garnet 1 and symplectite minerals.
The second stage of the evolution is also observed in the associated metabasic granulites. Complete to partial replacement of garnet 1 porphyroblasts and clinopyroxene by orthopyroxene + plagioclase + hornblende ± spinel symplectites represents a stage of near-isothermal decompression, whereas the growth of garnet 2 around the symplectitic minerals represents a stage of isobaric cooling. Texturally controlled in situ Th–U–total Pb monazite dating of the metapelitic granulites reveals the polymetamorphic nature of the rocks.
Stage I occurred at ~2600 Ma, as indicated by monazite inclusions within garnet 1 porphyroblasts (coexisting with sapphirine + quartz), and represents a previously unknown Neoarchean ultrahigh-temperature metamorphism.
Stage II occurred 700 Ma later at ~1900 Ma, as indicated by monazite grains in the symplectites and matrix, and represents a previously uncharacterized Paleoproterozoic ultrahigh-temperature isothermal decompression event.
No evidence of any metamorphism during the Neoproterozoic has been found. In this context, it is possible that the proposed decratonization occurred during a Paleoproterozoic decompression event instead of in the Neoproterozoic. The Paleoproterozoic evolution of the Uweinat–Kamil inlier is very similar to that described from other Paleoproterozoic orogenies across the world."

Previous works
The earliest scientific geologic report on area, that already mentioned the leptitic character of the basement along the slopes of the Jebel Uweinat massif is that by Menchikoff (1927). His early study included major-element analyses of 12 metamorphic and magmatic rocks. Mahrholz (1968) published some radiometric age data from samples collected in the Uweinat region. More information was supplied by Hunting Geology & Geophysics Ltd. (1974) who conducted exploration work on behalf of the Libyan Government. Klerkx and co-workers carried out fieldwork on the basement southeast of Jebel Uweinat during the mid and late seventies. Klerkx & Deutsch (1977), Klerkx (1980), Klerkx & Rundle (1976) improved the knowledge on this remote area considerably, providing fundamental information on its crustal composition.
Based on petrography and Rb/Sr whole rock age data, Klerkx (1980) recognized two major basement series: a) The granulitic Karkur Murr series at the eastern and southeastern slopes of Jebel Uweinat. b) The migmatic Ain Dua series, which crops out along the northern, western and southern margin of the massif.
In addition to geochronologic work on basement rocks, Klerkx & Rundle (1976) dated a number of magmatic rocks by the K/Ar method. Schandelmeier et al. (1983) and Schandelmeier & Darbyshire (1984) investigated the eastern extension of the area.

Finally are distinguished two basement formations by their lithofacies and metamorphic history:

1) the high grade granulitic Granoblastite Formation (Karkur Murr Series) as lower unit, overlain by
2) the clearly remobilized Anatexite Formation (Ain Dua Series):

 

 Isotopic age determination for the basement complex of the Uweinat-Kamil inlier
 

Location

Rock type

Method

Age (Ma)

Reference

Jebel Kamil

Anorthositic gneiss

U/Pb

2629 - 2063

Sultan et al. 1994

Anorthositic gneiss

U/Pb

2141 - 1922

Sultan et al. 1994

Jebel Uweinat (Wadi Wahesh)

Mylonite

RB/Sr, biotite regression line

2637 (+/-393)

Cahen et al., Klerkx & Deutsch 1977

Jebel Uweinat

Granulitic gneiss

RB/Sr, regression line

2556 (+/-142)

Cahen et al., Klerkx & Deutsch 1977

Granulitic gneiss

RB/Sr, model ages

2919-2904

Cahen et al., Klerkx & Deutsch 1977

Granulitic gneiss

Sm/Nd model ages

3200 - 3000

Harris et al. 1984

Jebel Uweinat (South of)

Migmatite biotite gneiss

RB/Sr, isochron

1784 (+/-126)

Cahen et al., Klerkx & Deutsch 1977

Granodiorite gneiss

K/Ar biotite

1878 (+/-64)

Hunting Geology & Geophysics Ltd. 1974

 

1. Jebel Kamil (loc 1)
 

Further detailed investigations of the Basement in a limited area in the Jebel Kamil were made by Mostafa F. M. Elkady within the scope of a dissertation and published in 2003. Elkady has been in the Jebel Kamil for a period of 10 months for fieldwork, and he made detailed petrographic studies of different rock units to determine their compositional character and the effect of deformation on each rock unit:

"The major rock assemblages of the Jebel Uweinat-Jebel Kamil basement inlier were subdivided by Klerkx (1980) and Richter (1986) on the basis of their rock types and metamorphic grade into three units namely, the Granoblastite Formation, the Anatexite Formation and the Metasedimentary Formation. The Granoblastite Formation (Karkur Murr series) consists mainly of a group of pyroxene ganulites containing charnockitic, noritic, and diopsidic gneisses and metaquartzites. The Anatexite Formation (Ain Dua series) is dominated by migmatites interpreted as anatectic granulites and contains abundant supracrustal intercalations. The Granoblastite Formation (Karkur Murr series) underwent granulite facies metamorphism, high pressure-high temperature, while low pressure-high temperature for the Anatexite Formation (Ain Dua series).
The intrusive rocks are among the most prominent features of the Jebel Uweinat region. They intruded most of the basement in the study area. Based on the general geologic setting, microscopic appearance and bulk composition, at least three principal suites were recognized by Richter (1986), these are:

1- Grey-green, calc-alkaline granitoids
2- Red, alkaline granites (sensu strictu)
3- Porphyritic, calc-alkaline granitoids (Paleozoic intrusions)

The major style of the folding of the basement is tight to tight-isoclinal folds. NNE to NE fold axis trends dominate, but locally they change to E-W. Axial surfaces are frequently overturned to the east. The deformation of the area occurred in the Early to Middle-Proterozoic, accompanied with and succeeded by a regional anatectic event (Klerkx 1980, Cahen et al 1984).

The structural analysis of the area indicated that it was subjected to major tectonic deformation including folding, overthrusting, shearing and faulting. The tectonic evolution can be concluded as follows:
1- The formation of continental crust occurred of the study area 3200 to 3000 Ma ago (Harris et al. 1984).
2- Deposition of Banded Iron Formation in continental marginal basins. The time of this deposition may be between 2900 and 1800 Ma in accordance with the world-wide deposition of the BIF (Windley 1979).
3- In the time-span from 1974 Ma to 1800 Ma crustal thickening followed by crustal thinning (D1), led to the pro-grade and subsequent retrograde metamorphism in this period. In this period developed the anatexite bands in the deeper units and the above-mentioned shear zones in the BIF.
4- Somewhat later, further crustal shortening (D2) occurred in north-south direction by regional folds with E-W fold axes, associated with pure shear. In this period the area was still subjected to retrograde metamorphism.
5-Continued crustal shortening due to forces from west-east direction (D3) caused folding with NNE-SSW fold axes plunging to the north or south directions with shallow angles. Continued folding processes caused a series of reverse faults. This group of faults extends in the NNE-SSW direction with dip angles between 25° and 65° to the NW. These types of faults are prevailing in the area.
6- During the Pan-African times a series of granitic intrusions intruded into the Anatexite Sequences and the BIF.

Rocks
1. Ultramafic-Mafic and Calc-Silicate Rocks
The ultramafic-mafic and calc-silicate rocks (UM) are exposed as spots or bands beneath the Anatexite Sequence in some localities of the area, especially in the southern part. These rocks represent less than 5% of the rock units here. The rocks are grey to green, fine to medium-grained. In the southern part of the area some bands trend NNW-SSE with 45°dip angle to the SW. These bands consist mainly of forsterite and/or spinel marble, minor serpentinite and high deformed, fine-grained gabbro-norite. To the southwest of the present area, the exposures of the UM are represented by bands and spots of highly deformed gabbroic rocks, serpentinite and talc-carbonate, grading upward, and alternating with the melansome in the Anatexite Sequence.
These rocks show rusty brown, brownish green and greyish weathered surfaces with brownish, white veinlets giving a characteristic mesh structure. In places, these rocks contain black nodules of forsterite or spinel minerals. The contact between the UM and the underlying BIF is tectonic. The tectonic contact is a thrust, parallel with the main thrusts elsewhere in the investigated area.
Generally these rocks consist essentially of interlayered wherlite (olivine + clinopyroxene) and clinopyroxenite and gabbro. Olivine compositions are quite forsteritic. The rocks are variably transformed into foliated metamorphic equivalents (talc-tremolite serpentinite, talc-serpentine tremotitite, hornblendite, amphibolite) characterized by pervasive metamorphic recrystallization under amphibolite facies metamorphism. The presence of olivine and spinel in these ultramafic-mafic rocks indicate that their parent rock was formed at a relatively high temperature and high pressure, and originated in the upper mantle.


2. Anatexite Sequence
In the investigated area the Anatexite Sequence is usually found as isolated exposures, consisting of leucosome and melanosome bands. The leucosome bands with felsic components are buff grey to white, medium to coarse grained, and composed mainly of quartz and alkali-feldspar. The melanosome bands of mafic components are greyish green to dark green, fine to medium-grained, and are composed mainly of amphibole and plagioclase. Both bands are arranged in layers or schlieren and as patches.
A unique feature of the Anatexite Sequences is the replacement of a primary prograde mineralogy by later retrograde mineral assemblages, or even more than one.
The Anatexite Sequence covers about 30% of the outcrop surface in the investigated area. The banding trends NNE-SSW and dips about 65° toward NW. The thickness of the leucosome and melanosome bands varies between a few centimeters to a few meters. Generally the leucosome bands show medium to coarse-grained or pegmatitic textures, while the melanosome exhibits finer-grained texture.
The contact with the underlying ultramafic-mafic rocks is mostly a secondary, transitional contact with an increase in the mafic minerals toward the gabbroic, serpentinite and related rocks. The direct contact between the Anatexite Sequence and the overlying BIF is very rarely observed. Tectonic contact between the Anatexite Sequence and the lower part of the BIF indicate detachment thrust fault, due to the early south-north maximum stress. The Anatexite Sequences is overlain by metapelitic bands alternating with iron-rich and silica-rich bands. This alternation contains a high amount of garnet and subordinate orthopyroxene minerals (enstatite and hypersthene), and calcite.

The characteristic feature of the Anatexite Sequence is the predominance of migmatic gneiss, i.e. diatexites, metatexites and metablastites. Whereas the highly mobilized, medium-grained diatexites are of a homogeneous to nebulitic texture, rarely showing foliation, metatexites and metablastites are well-foliated and a two-phase nature - melanosome and leucosome - becomes obvious, arranged in the form of layers or schlieren and patches.
These rocks exhibit seriate granoblastic or porphyroblastic and in places equigranular textures, and are composed mainly of intermediate plagioclase, orthopyroxene, hornblende, biotite, and chlorite with subordinate amounts of quartz in the melanosome, and alkali-feldspar, quartz, orthopyroxene, biotite, and chlorite in the leucosome. In general, plagioclase is idioblastic and strongly saussuritisized. Later albite or albite-rich plagioclase is unaltered. Antiperthite was occasionally formed.
Alkali-feldspar is variably shaped and appears as microcline, perthite or mesoperthitic-chessboard-albite. The perthites tend to form megablasts. Two types of quartz dominate in the leucosome, the older one is deformed and shows large grains with wavy extinction, while the later phase one is fine grained and recrystallized.
Quartz occurs as anhedral grains, which are stretched and display ribbon structure and predominantly is rich in inclusions. Brownish-green biotite is the dominant mafic component and shows replacement by intergrown, subparallel chlorite. The chlorite, sphene and/or epidote are dominant in most samples. A similar alteration may be seen in the subordinate green hornblende. Orthopyroxene is represented by hypersthene which occurs as skeletal-shaped anhedral grains, partially altered to talc and chlorite. Cordierite, if present, occurs as anhedral grains or irregular porphyroblastic grains with numerous inclusions of quartz.


3. The Banded Iron Formation (BIF)
The BIF represents about 30 % of the rock units cropping out in the investigated area. The BIF is built up of accumulated clastics of psammitic and subordinate pelitic character. The BIF bands are well-foliated and generally fine-grained. The lower part of the BIF exhibits a frequently cataclastic metamorphism, but some parts still show weak graded bedding. The BIF succession consists of variegated bands with colours of yellow, red, brown, black and grey. The iron-rich bands are composed of black magnetite and dark greyish brown and red hematite alternating with quartz bands, whereas the silica-rich bands consist mainly of quartz or chert and jaspilite.
Generally, the chert bands dominate in the upper parts of the BIF while the quartz bands (metamorphosed chert bands) increase toward the base. Chert and jaspilite bands occupy the top of the formation. The chert contains more hematite and shows remnants of thin layering in contrast to the quartz rich bands in the middle and lower part of the BIF succession. On the basis of their structural position the BIF beds seem to be younging toward west. The iron oxides and magnetic minerals in the BIF bands are distributed in different proportions.
The BIF differs from top to base as follows:

3.1 Meta-Cherts
The meta-chert is hard, extremely dense or fine-grained crystalline rocks and shows alternations of microcrystalline quartz and relics of primary deposition. The meta-chert is composed mainly of quartz, hematite and subordinate amounts of opal and chalcedony. The crystalline silica bands are a few millimeters in thickness, while the dense silica bands exhibit thicknesses ranging between a few centimeters and more than one metre. These bands show colour variations from the top to the base depending on the iron oxide content, which increases from top to base. Consequently colours range from light yellow at the top to yellow, red or brown at the base. In the field, the chert is exposed as high ridges composed of well banded chert or large chert fragments with more than 20 centimeters of diameter. Highly sheared and brecciated bands occur mostly adjacent to shear zones are parallel with the banding and major thrusts and show minor cleavage parallel to the major thrust surfaces. The thickness of the chert bands ranges between 5 and 10 meters, extends for a few km in NNE-SSW direction. In some places of the study area the chert bands alternate with jaspilite bands. The quartz grains are sometimes stained with reddish brown colour as an indicator for hematite minerals, giving rise to jasper, a variety of chert associated with iron oxide ores and containing iron-oxide impurities that give it various colours (here red or yellow to brown). The chert bands show well developed bands in some exposures or highly brecciated bands in others. The fragments have diameters from a few millimeters to more than 70 centimeters. These fragments are welded with hematite and iron oxides. This breccia represents a fault breccia. The chert bands have mostly low magnetic susceptibility in comparison with the other BIF bands.

3.2. Well-Banded Iron-Silica Bands
The well banded iron-silica bands show a gradational boundary with the underlying fuchsite bearing quartz bands over a few meters contact. In the lower part of these bands the silica-rich bands dominate as a few meters thick, while the iron rich bands are a few centimeter thick. In the upper part, however the thickness of the iron-rich bands increases to few meters of magnetite-hematite rich bands, while the silica rich bands are only a few centimeter thick.
Lower part of the iron-silica-rich bands often contains highly altered mylonite zones due to an earlier shear zone. The upper parts of these bands alternate by with overlying chert bands. The magnetite-hematite rich bands are interlayered with brown to yellow chert bands. The thickness of the chert bands ranges between a few centimeters, at the border with the magnetite-hematite rich bands, to more than five meters toward the top of the BIF and exhibit a red to yellow colour due to the increase in iron oxides.
All of BIF bands strike NNE parallel to the regional strike direction and have dips ranging between 10° and 70° to the WNW. Most of the BIF bands are separated by shear zones and mylonite bands, developed during the south-north maximum stress.

Well-banded iron-silica bands often underlie the chert-jaspilite bands and are composed primarily of magnetite-hematite-rich bands alternating with quartz-rich bands. The magnetite-hematite-rich bands show alternation with quartz bands. The thickness of quartz bands increases to the east and the thickness of the iron rich bands decreases in that direction- This alternation of magnetite-hematite-quartz bands is more than 50 meters thick. It is parallel to the overlying chert bands, and the trend is 10°/70°.
The magnetite-hematite bands constitute hard rocks of very fine-grained iron-rich greyish black bands. The mineral assemblage is mainly deformed quartz bands alternating with iron-rich bands mainly composed of hematite, goethite and magnetite. The magnetite is clearly observed to be the original iron mineral together with hematite. Hematite, in general, is the predominant mineral. Goethite occurs as secondary mineral after magnetite and hematite. Graphite occurs in most of the BIF samples.
The graphite occurs as spots of varying sizes, aligned in the bands and might have had a biogenic source. In places, it is found as cavity filling. At the surface this appears as weathered cavities with high concentration of graphite. These bands have a very high magnetic susceptibility due to the high amount of magnetite, while in the quartz rich bands the magnetic susceptibility is intermediate.


3.3. Fuchsite-Bearing Quartz Bands
Fuchsite bearing quartz bands have a thickness of up to 100 meters. They alternate with deformed quartz bands. The fuchsite bearing quartz bands are a light green and consist mainly of quartz, fuchsite and iron oxide. The deformed quartz bands are a white to grey and consist mainly of quartz with subordinate amounts of iron oxides. Generally, the bands exhibit penetrative foliation parallel to the primary banding. The thicknesses of the alternation between the fuchsite bearing quartz and the deformed quartz bands range between 3 and 1 meters respectively. The quartz bands are overlain by well banded iron-silica bands.
Fuchsite-bearing quartz bands often follow in the succession, overlying the iron-silica rich bands, but are exposed mostly in the extreme eastern parts of the study area.
Generally, these bands have the same trend as the BIF bands and occupy large bands of up to 500 meters thick. They are white, grey, greenish grey or green fine to medium-grained in primary bands, few centimeters thick. The fuchsite-bearing quartz bands consist mainly of quartz, chlorite, fuchsite, altered feldspar minerals, clay minerals, and minor iron oxide. In these bands a penetrative foliation is parallel with the primary banding, and having the same trend as the BIF bands, mostly with 15° N and dips 60° to the west.

3.4. Metapelitic Volcanosedimentary Bands Alternating with Iron-Silica Bands
Metapelitic volcanosedimentary bands represent the base of the BIF sequence. They alternate with iron-silica bands (BIF), and vary in thickness between 1 and 5 meters. They are fine to medium-grained, and white, brown and grey. These bands have the same trend as the BIF bands (10°/60° west). Lithic tuff bands, brownish grey to white and consist of very fine quartz and altered feldspar dominate these bands. They are 3 meters thick in average and alternate with quartz bands with thicknesses of up to 1 metre, BIF bands up to 0.5 metre and sometimes thin lamina of meta-chert with a few centimeters thick.
The iron-silica rich bands differ in composition with the higher part of the BIF sequence. They contain, sometimes, subordinate amounts of garnet, ortho-pyroxene, amphiboles, highly altered feldspars, chlorite, and relatively much magnetite. Graphite is rarely observed in these bands.
Within the BIF occasional mylonitic bands between 1 and 3 meters in thickness can be traced. These bands show reddish white colour or greyish red colour. Normally, they are fine- to very fine-grained, consisting mainly of crushed (mylonitized) quartz with subordinate amounts of magnetite, hematite and secondary goethite bands. The mylonitic bands are parallel to the BIF bands as well as to the foliation, and represent shear zones within the BIF. They show quartz porphyroclasts surrounded generally by foliated fine-grained groundmass of quartz-magnetite rich bands, which show earlier folding, formed during earlier deformations.
The bands in metapelite-volcanosedimentary rocks are generally parallel with the main thrusts and the fold axial surfaces in the area. Often, zones of fault breccia cut through the BIF bands. Some zones are welded by secondary goethite. These fault breccias generally prevail at E-W trending wrench faults, and can be considered as polyphase faults".
 

 

Ultramafic and calc-silicate rocks

 UM: Calcite-dolomite

Anatexite rocks

   (red) granite ?

BIF: Quartz Bands

BIF: Metapelite Bands

 BIF: with graphite

BIF: Iron-Silica Bands

BIF: Meta-Cherts

 BIF: Quartz Bands ?

 Boundary

 

2. Jebel Uweinat (loc 2a, 2b, 2c, 2d)
"In the eastern and south-eastern vicinity of the Jebel Uweinat ring complex (Tertiary syenite) the basement is made up of gneissose granoblastites and granulitic gneisses which probably extend as far as the western part of the Jebel Kamil outcrop area. They were described in detail by Klerkx (1980). Their microfabric is characterized by granoblastic to flaser textures where quartz is always present in elongated to platy grains or in ribbon-like aggregates subparallel to the foliation. The gneisses contain orthopyroxene, garnet and biotite as common constituents and variable amounts of plagioclase and K-feldspar. Anatectic rocks are exposed in the Karkur Talh (22°01' N/25°09' E), around the northern slopes of Jebel Uweinat, and at the western and south-western base of this mountain. Texturally and mineralogically the anatectic rocks are comparable to similar anatectic rocks in the Jebel Kamil area.
Paragneisses are exposed around the southernmost part of the NW Sudan basement outcrop area. They form a system of large scale folds trending NE-SW. The rock suite embraces quartz-mica schists, biotite-hornblende-garnet gneisses and hornblende gneisses. They show gneissose to flaser textures, some with small-scale folding due to a second cleavage.
The schists contain abundant quartz but no feldspar; white mica, well aligned along cleavage plains, is present and in one sample kyanite and some tourmaline were observed. The gneisses contain microcline, perthite and smaller amounts of plagioclase and quartz. Biotite, hornblende and garnet arc the mafic minerals."

No further scientific investigations have been carried out since 1980s. Consequently, only a few photographic evidence are to be found of Basement rocks. There are only photos of participants of trekking tours (including the author).

Anatexite: Mouth of Karkur Talh

Granulite gneiss ?: Western branch of Karkur Talh

 

Granulite gneiss ?: Slop of southeastern Uweinat

Anatexite?: Mouth of W. Wahesh

Migmatite ?: SE Uweinat (Wadi Waddan site)

 

 

 

         

 BIF: Uplifted inside the Uweinat granite-dome

 

Granulite gneiss ?: Ain Zueia, Ain Dua Gneiss or granite ?  BIF: loc unknown

4. Gilf Kebir (loc 3a, 3b)
In the southern foreland of the Gilf Kebir plateau, the author has documented two previously unknown locations with outcrops of BIF (Iron-Silica-Bands).

BIF: Southern foreland of Gilf Kebir (near Aqaba passage)

BIF: Southern foreland of Gilf Kebir ("Eight Bells")

5. Jebel Nazar (loc 4)
Further outcrops of Basement occur in the southern Jebel Nazar, in an area with granitoide intrusions. "The Basement is represented by metabasites, gneisses and BIF-sediments. Gneisses similar to the paleosomatic gneisses of the Jebel Kamil complex occupy most parts of the peneplain. Structural evidence gives the impression that at least some of these gneisses have passed melting conditions. Besides these in-situ migmatites, injection migmatites with raft textures have been found at the eastern foreland of the peneplain. Intrusive granitoid rocks are of the same type as the reddish, K-feldspar granites in the Jebel Kamil area."

 

BIF: Meta-Cherts ?

Granitoids (unclassified)

6. Jabal Arkenu (loc 5)
"Undifferentiated rocks of the craton occupy the central and eastern part of the Jabal Arkenu area and include amphibolite, migmatite (anatexite), quartzite, quartzofeldspathic gneisses, biotite gneisses, diorite greisses, granitic gneisses, quartz-magnetite (BIF) and porphyroblastic granitoids. These rocks are highly metamorphosed and have been subjected to faulting and folding."

"To the north-east of the Jabal Arkenu ring complex there are obviously further outcrops of Basement ("Marble"). Photomicrographs of a metabasite collected NE of Jabal Arkenu illustrate a paragenesis of epidote and sericite. In addition, chlorite occurs as fracture fills. Paragenesis of green amphibole (possibly actinolite), turbid plagioclase megacrysts and alkali feldspar. Note that opaque minerals are also important; accounting for up to 5% of the surface are of this slide. Composite crystal comprising an orthopyroxene core and a reaction rim of green amphibole (possibly actinolite), with evidence for additional overgrowth of epidote."

 

Amphibolite

NE of Jabal Arkenu: "Marble"

Thin section of a Metabasite

Quartz-Magnetite (BIF)

 

D. Intracratonic Metasediments (of not sure explained age)

1. Area eastern of Jebel Kissu
"The probably youngest and clearly bedded Metasedimentary Formation is located in the Northern Sudan, eastern of Jebel Kissu. Richter (1986) noted an "itabiritic sequence" of iron quartzite that crops out for several kilometers in the area. It consists mainly of quartz bands alternating with bands of hematite and goethite with relics of titanomagnetite. Bands of marble and amphibolites are present and represent a minor proportion of the Metasedimentary Formation. The Metasedimentary Formation is a low to medium grade supracrustal sequences. The tectonic environment of these rocks is interpreted as an intracontinental rift (Richter 1986).
The central and eastern part of this area is occupied by rocks of lower metamorphic grade. At the central part fine-grained phyllites with the typical mica glance on the cleavage planes occur. Eastwards they grade into metasiltstones which are exposed in a flatly eroded plain of several square kilometers. Their colour changes from white to yellow, violet and grey; small scale sedimentary layering is preserved in these rocks. Quartz is the major constituent and minor minerals are sericite, chlorite and biotite. Homogeneously distributed, submicroscopic haematite , limonite and bituminous substances cause the variable colours.
In the southern part of the area quartz-mylonites and quartzites are common. The stratiform beds have developed in some layers a fine-grained, granoblastic texture with flattened components. Quartz shows undulatory extinction and cataclasis as well. In other layers detrital quartz and minor feldspar and sheet silicates are embedded in a cryptocrystalline matrix of quartz and ore. Small scale alternation of ore and quartz layers can be seen; the latter is frequently jaspilitic, thus it seems warrantable to refer to these rocks as itabirites."


2. "Infra-Cambrian" metasediments (Jabal Arkenu) (loc 6)
"A sedimentary succession of presumed Infracambrian age crops out along the eastern side of Jabal Arkenu igneous ring complex. These partly deformed rocks of the so-called "Arkenu Formation" are of low metamorphic grade. The formation comprises slightly recrystallized, interbedded sandstones, rare conglomerates and mudrocks. Sandstones are buff yellow, fine grained and medium-bedded, often structureless and stacked in uninterrupted units. Within these sandstones, sedimentary structures include both tabular cross-beds and possible ripple cross-lamination. The mudrocks form thick accumulations, and comprise maroon, lilac and dark grey silty shale. Subtle changes in grain size between beds include variations in silt content, reflected in the weathering pattern of the mudrocks where silty laminae stand proud of more clay-rich horizons. A single conglomerate horizon was observed on the eastern side of the exposure. The grain size of this conglomerate is variable with both pebbly and granular clasts set within a sandstone matrix. Pebbles are well rounded with a maroon coloration. In thin section, the pebbles are shown to be metamorphosed quartz arenitic sandstones, comprising quartz grains with undulose extinction and sutured contacts. By comparison, the sandy matrix of the conglomerate shows some later quartz overgrowth, but sutured contacts are few."
 

 

"Infra-Cambrian": Metasediments

Ferruginous conglomerate

  Conglomerate

Banded Marble

 

E. Some important sources

The Pre-Cambrian in North Africa
H.M.E. Schürmann --
E.J. Brill, Leiden, 1974, ISBN 90 04 03694 6

Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf (SW Egypt/NW Sudan)
H. Schandelmeier, A. Richter & G. Franz -- Journal of African Earth Sciences 1: 275-283, 1983 (PDF)

The Saharan Metacraton
M.G. Abdelsalam et al. --
Journal of African Earth Sciences 34(3):119-136, 2002 (PDF)

Structural Evolution in the Palaeoproterozoic Basement (Banded Iron Formation and related Rocks) of SW Egypt
M.F. Mostafa Elkady -- Dissertation, University Heidelberg, 2003
http://archiv.ub.uni-heidelberg.de/volltextserver/3573/1/Structural_Evolution_of_SW_Egypt.pdf

Petrographische Kartierung von granulitfaziellen Gestein im Jebel Uweinat Basement, SW Ägypten
Katharina Wulff -- Diplom-Arbeit,  Universität Kiel, 2003 (abstract)

Field-based investigations of an "Infracambrian" clastic succession in SE Libya and its bearing on the evolution of the Al Kufrah Basin
D. Le Heron et al. -- Geological Society London Special Publications 326(1): 193-210, 2009 (PDF)

Upper mantle structure of the Saharan Metacraton
M. G. Abdelsalam et al. -- Journal of African Earth Sciences 60: 328–336, 2011 (PDF)

Neoarchean UHT Metamorphism and Paleoproterozoic UHT Reworking at Uweinat in the East Sahara Ghost Craton, SW Egypt:
Evidence from Petrology and Texturally Controlled in situ Monazite Dating
Shreya Karmakar  & Volker Schenk
  -- J. Petrology 56 (9): 1703-1742, 2015 (PDF)