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NW Sudan)
0731-72-17/83 $3.00 + 00 .0 Pergamon Press Ltd.
Journal of African Earth Sciences. Vol. I. No . 3/-1. pp. 275 to 283. 1983 Pr inted in Great Britain
Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf (SW Egypt/NW Sudan) H. SCHANDELMEIER Sonderforschungsbereich 69, Arid Areas, Technische Universitat Berlin, Ackerstrafle 71-76,0·\000 Berlin 65, Federal Republic of Germany
A.
RICHTER
Institut fiir Angewandte Geologie, Freie Universitat Berlin, Malteser Stralle 74·100,0·1000 Berlin 46, Federal Republic of Germany
G.
FRANZ
Institut fiir Angewandte Geophysik, Petrologie und Lagerstattenkunde, Technische Universitat Berlin, Straflc dcs 17. Juni 135,0·1000 Berlin 12, Federal Republic of Germany
(Received 13 October 1983) Abstract-The Uweinat basement uplift system extends from Gebel Uweinat in the west to Bir Safsaf in the cast underlying an area of roughly 40,000 km~ in which crystalline rocks arc exposed. The distribution of the basement rocks was mapped by means of field traverses combined with an interpretation of Landsat satellite images. General foliation trend is NE-SW in the Uweinat area changing to NNE-SSW in norIltern direction and to WNW-ESE in the Gebel Kamil area. In the eastern part (Bir Safsaf) it is NW-SE. Metamorphic rocks are migmatites and granitic to granulitic gneisses with subordinate amounts of amphibolite and marble . Low grade metamorphic rocks were found only in the southern area (NW Sudan). Plutonic rocks include different types of granitoids and dykes. volcanic rocks are alkali olivine basalt, phonolite, trachytes, and rhyolites . All age data for the area are compiled and together with three newly obtained ages (673 ± 65 rn.y, .rnigmatitcs, 431 ± 33, granite, 216 ± 5, rhyolite) it can be shown that magmatic activity occurred in several periods during the Phanerozoic. Some of the magmatic rocks a rc concentrated along fracture zones.
INTRODUCTION
Description ofareas investigated and working methods
IN SW EGYPT and in a minor part ofNW Sudan, crystalline rocks are exposed in the vicinity of Gebel Uweinat and in isolated outcrops west of Lake Nasser. In Egypt and Sudan, the outcrops of the Uweinat region extend 180 krn to the north-east, where they are covered by the continental strata of the Gilf Kebir formation. They occur for 175 km eastwards to Gebel Kamil and 125 km south-eastwards into Sudan (Fig. 1). A 60 km x 40 km wide outcrop of mainly granites and migmatie gneisses (Bir Safsaf complex) lies at 23°N, 29°30'E. Immediately to the north is a smaller complex (15 x 5 krn) of volcanic and subvolcanic rocks, the Nusab el Balgum (Fig. 4). Further to the north and east there are small isolated outcrops of granites, migmatites and other metamorphic rocks, extending eastwards towards Lake Nasser (23°N, 31 0 E ) . None of these areas had previously been described in detail; brief information on the 'basement outcrops' is given on the preliminary geological interpretation map of Klitzsch and List (1978) and the geological map of EI Ramly (1972). We visited selected parts of this vast region in 1980, 1981 and 1982 and the results which apply only to the investigated areas were extrapolated with the help of satellite images towards unknown areas, but these interpretations may be altered when better information becomes available. AES 1 : ) /4-C
The purpose of this paper is to record the available data about lithologies, metamorphism , radiometric ages and structures of the basement of SW Egypt between Gebel Uweinat and Bir Safsaf.
Geological frame The "East Sahara Craton" (Kroner 1979), which is largely covered by Phanerozoic sedimentary strata, extends from the Pharusian and Dahomeyan orogenic zone in the west (Fig. 2, Nos. 3 and 4) to the orogenic Hijaz Magmatic Arc (Fig. 2, No.1) and the northern continuation of the tectono-thermal Mozambique Belt (Fig . 2, No.2) in the east. The eastern and southern boundaries of the craton are still ill-defined, Collision of the West Africa Craton with the East Sahara Craton has resulted in the closing of the Pharusian ocean or graben (Bertrand et al. 1978). In the cast, new crust was accreted onto the East Sahara Craton according to Gass (1977) by continuous westward subduction of oceanic crust during the Pan-African from about 1100 to 500 Ma, but recent results (Ries 1983, Kroner et al. 1983) indicate that the Pan-African evolution in NE Africa and Arabia started around 900 Ma and not earlier. The rigid cratonic block, sandwiched between these two Pan-African mobile zones (Fig. 2) has been affected by strong intraplate tectonics (Kroner 1979) and the successive formation of large-scale fracture zones has been accompanied by intrusions of granite plutons and alkaline ring complexes. As a result of this Pan-African tectogenesis, the older crustal rocks were structurally
275
H. SCHANDELMEIER, A. RICHTER and G. FRANZ
276
DISTRIBUTION AND
OF
METAMORPHIC
MAGMATI C ROCKS OF
SW-EGYPT AND NW-SUDAN
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Fig. 1. Distribution of magmatic and metamorphic rocks of SW Egypt and NW Sudan: 1, Gebel Kamil area; 2, Peneplain Plateau; 3, Uweinat area; 4, NW Sudan area; 5, Sir Safsaf area
and metamorphically overprinted; locally partial melting of crustal material with resetting of isotopic systems (Vachette 1974, Kroner 1979) has occurred. Finally, post-tectonic granites have been intruded into the whole area between Hoggar and Red Sea. In the Phanerozoic, large scale crustal warping resulted in the formation of extensive intracratonic basins which were continuously modified by structural events throughout the whole period. The investigated area which we describe with the term'Uweinat Uplift' is situated at the eastern border of the East Sahara Craton (Fig. 2) and may therefore give some important information on the geological development of a key area near the craton-mobile zone boundary. Because most parts of the East Sahara Craton are buried beneath thick continental and partly marine Palaeozoic, Mesozoic and post-Mesozoic strata (Klitzsch et al, 1979), a satisfactory examination of the basement of the East Sahara Craton is impeded.
1 Hljaz Magmatic Arc
2
Mozambique Belt
3 Pharuslan 40ahomeyan
REGIONAL DISTRIBUTION AND PETROGRAPHY OF BASEMENT OUTCROPS
Basement outcrops in SW Egypt and adjacent Sudan arc exposed across an area of about 40,000 km 2 in five regions. The easternmost outcrops of the Uweinat inlier
5 Mauri tanldes & Rokelides
~
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~ ~ Tectono-thermal
F::::j Thermal
Fig. 2. Areas of Northern Africa and Arabia which were affected by Pan-African tectogenesis (modified after Kroner 1979, Martin and Porada 1977a).
Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf
277
20 km
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GEBEL KAMIL AREA
migmatites
Fig. 3. Geological sketch map of the Gebel Kamil area.
are in the Gebel Kamil area (Fig. 1, No.1) a rough morphology of scarps, plains and hillocks up to 200 m above terrain level. North-east of Gebel Uweinat extends the Peneplain (Fig. 1, No.2), an elevated region, topped by a flat plain, cut by frequent wadis. Immediately east of Gebel Uweinat (nON, 25°E; Fig. 1, No.3) is an area of predominantly low ground with isolated inselbergs. The area in NW Sudan (Fig. 1, No. 4) is mainly made up of linear or arcuate ranges which rise 100 m above terrain level. Four hundred and fifty km east of Gebel Uweinat the Bir Safsaf outcrops (Fig. 1, No .5) form a rather flat plain which grades northwards to an undulating boulder plain. Earlier petrographic and radiometric studies on the basement and the younger magmatic rocks were almost exclusively made in Libya (Fig. 1, west of 25°E) by Menchikoff (1927), Marholz (1968), Hunting Geology and Geophysics Ltd (1974), Klerk x and Rundle (1976) and Klerkx and Deutsch (1977). The adjacent basement in Egypt and Sudan is virtually undescribed, except by Sandford (1935).
Gebel Kamil area Metamorphic rocks. Gneissose granoblastites and granulitic gneisses occupy the western part of the Gebel Kamil area (Fig. 3) where mega folds built up several ranges parallel to the fold axes. These gneisses are mineralogically and texturally very similar to those of the Uweinat area described by Klerkx (1980) (see Gebel
Uweinat area). Granoblastic and flasertextures prevail in these rocks and consequently mineral assemblages are not representative of the regional hypersthene zone, only one sample showing a diagnostic (Winkler 1978) metamorphic assemblage with orthopyroxene. Minerals are isometric plagioclase (albite to andesine) platy quartz, K-feldspar occurring as perthite and rarely as microcline. Mafic components are clinopyroxene, orthopyroxene, hornblende, actinolite, biotite, chlorite and epidote. Pyroxenes are mostly altered to talc and hornblende to actinolite. The modal composition of the rocks is tonalitic. Anatectic rocks are exposed at the center and eastern part of the Gebel Kamil area. They can be classified as gneisses interpreted as paleosome, and true migmatites which exhibit intercalations of melanosome and leucosome with small-scale folding. The paleosomatic gneisses of granodioritic composition show gneissose as well as stromatic textures. Their fabric is granoblastic with interlobate grain boundaries. They contain sericitised plagioclase, cataclastic quartz, perthitic feldspar, biotite often altered to chlorite and sphene, white mica and epidote. Migmatites occur in all stages of migmatization; blastic growth of feldspar, schlieren and stromatic metatexites with distinctive differentiation of melanosome and leucosome, and finally as nebulitic diatexites. At one location, in the central part of the Gebel Kamil area, a sillimanite-garnet-cordierite gneiss was found. This dark brown rock shows a well-foliated flaser texture; besides the idioblastic sil-
278
H. SCHANDELMEIER, A. RICHTER and G. FRANZ
limanite, garnet and cordierite occur as well as xenoblastic quartz, K-feldspar, tabular plagioclase, biotite, spinel, white micas and various accessories. Marbles and calcsilicate rocks are also present, the former throughout the area. In most places they are in stratiform contact with the gneisses and show alternate thin greenish-brown and thick white layers. Where they are intercalated in anatectic rocks, the layers are often perpendicular to the foliation trend of the surrounding rocks and probably represent a pre-anatectic structure. Main minerals of the white layers are calcite and dolomite with minor actinolite, diopside, pargasite, olivine, spinel, members of the humite group, and phlogopite. The dark zones are enriched in almost completely serpentinized olivine, diopside, actinolite and iron oxides. Dark greenish-grey diopside-scapolite and titaniumaugite-hornblende rocks are associated with the marbles and the sillimanite-garnet-cordierite gneiss. Their main minerals are An-rich plagioclase, pyroxene which includes calcite and epidote. Minor minerals are chlorite, sericite, green spinel, actinolitic hornblende, sphene, apatite and ore. Plutonic rocks. Intrusive rocks of different, but generally granitoid composition were encountered 10 krn south of Gebel Kamil where a boulder plain is made up of coarse-grained, red granites similar to the red granites of the northern Bir Safsaf complex . These granites are very rich in perthitic feldspar, Ab-rich plagioclase and quartz. Mafic minerals are mainly biotite and small amounts of hornblende. This red granite also occurs in the south-western part of the Gebel Kamil area (Fig. 3). In the central-western part of the area a large granitic body, lacking any foliation, was found. In contrast to the red granites the rock is porphyritic and shows slight flow textures. It contains equal proportions of quartz, perth itic feldspar and zoned plagioclase. Mafic minerals are green biotite, hornblende and pyroxene. Apatite, zircon, sphene and ore are accessories. Granodiorites are not common; they occur as smaller bodies which show marginal foliation. Mineralogically they are nearly identical to the previously described plutonic rocks. Volcanic rocks and dykes . Volcanic rocks and dykes are widespread but are concentrated in the southern Gebel Kamil area and at the southern end of the Peneplain Plateau (Fig. 1). Most common are phonolites, olivine basalts and alkali trachytes which form plugs and cones up to 200 m high. Where dissected by erosion, the remnants occur as ring structures. The alkaline rocks contain abundant sanidine and/or alkali feldspar and pyroxene is present as aegirine. The phonolites contain considerable amounts of euhedral nepheline. Their texture is finely crystalline, linear fluidal; some interstitial glass may be present. Frequently these rocks contain numerous xenoliths of basement rocks. The olivine basalts are generally fine-grained with variable sized phenocrysts of olivine, which is also present in the groundmass. Fine-grained prismatic plagioclase is abundant and two generations of titan-augite are present. Isometric opaque ore is a minor constituent.
The texture of the rocks is ophitic intergranular or intersertal. Rhyolite flows cover a considerable area of the southern Gebel Kamil complex. Granophyric quartz-feldspar inter-growths are embedded as xenoliths in a devitrified matrix. Some micas, calcite and opaque ore are further constituents. Cryptocrystalline haematite causes red colouration of the rocks. At some localities rhyolitic volcanic breccia could be found. Fragments of aphanitic devitrified glass arc welded by veins of fine-grained polygonal quartz. The dykes can be subdivided into: (i) acidic composition: spherulitic to porphyritic felsites with a devitrified glass matrix and linear flow texture and porphyritic granophyrs in which K-feldspar, plagioclase and quartz are embedded in a micrographic matrix. (ii) intermediate and basic composition: porphyritic microgranodiorites, microtonalites and microdiorites, with abundant plagioclase, minor quartz and little or no K-feldspar. Biotite is present in some rocks as saghenite. In addition chlorite, amphibole, pyroxene, epidote and white mica are found. Peneplain area
Gneisses similar to the paleosomatic gneisses of the Gebel Kamil complex occupy most parts of the Peneplain (Fig. 2, No.2). 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. However. for the most part the rocks are well layered over extensive areas in which there is no fluid mixing at all. The intrusive granitoid rocks are of the same type as the reddish, K-feldspar granites of the Gebel Kamil area. Gebel Uweinat area
In the eastern and south-eastern vicinity of the Gebel Uweinat ring complex (Fig. 1) the basement is made up of gneissose granoblastites and granulitic gneisses which probably extend as far as the western part of the Gebel 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, Uweinat (22°01' N/25°09' E), around the northern slopes of Gebel Uweinat, and at the western and south-western base of this mountain. Texturally and mineralogically the anatectic rocks arc comparable to similar anatectic rocks in the Gebel Kamil area, but correlation is not yet supported by radiometric data (Table 1). A detailed description of the Gebel Uweinat ring complex was given by Klerkx and Rundle (1976).
Outline of the geology of magmatic and metamorphic units between Gebel Uwcinat and Bir Safsaf
279
Table I. Some relevant radiometric age dat a from the Uweinat-Bir Safs af upl ift Location
Karkur Murr
Rock type
Charnockitic Gneiss and retrogressive metamorphism WadiWahesh Anatectic gr anites Uwein at/Libya and tonalities Gebel Kamil M igmatite area/Egypt GebelKamil Porph. Granite mea/ Eg ypt Gebel Babein Microgranite Liby a Nusabel Balgum Alkali Subvolcanics Egypt Uwcin at/Libya Basalt Uwcinat/Libya Alkali Granite Gebel Arehenu Neph. Syenite Libya Uwcinat/Libya Phonolite UweinatlLibya Dolerite Uwcinat/Libya Syenite Uweinat/Libya Syenite Uwc inat/Libya Pegmatite UweinatlLibya Granite
A nalyzed mineral Whole rock
Age(Ma)
Method Rb/Sr
Uweinat/Libya
29~to2919
References Cahen ers/. (1983)'
2656± 1.;\2
Whol e rock
Rb/Sr
178.t± 126 isochron
Cahenelal. (1983)'
Whole rock
Rb/Sr
673 ± 56 regression line
Thi s paper
Whole rock
Rb/Sr
.;\31 ± 33 isochron
This paper
Wh olcrock
KlAr
.;\SI ± 12
Whole rock
Rb/Sr
216 ± 5 regression line
Hunting G eol ogyand Geophysics Ltd. (197,;\) This paper
Whole rock KFsp/Bio/Aegirine KFsp/KFsplAegirine
KlAr K/Ar KlAr
235±5 .;\5 ± I; .;\2± I; .;\2± I 32 ± I ; 36 ± I; .;\8± I
Klerkx and Rundle (1976) Klerkx and Rundle (1976) Klerkx and Rundle (1976)
Whole rock Whole rock Amphibole Amphibole Feldspar Feldspar
KlAr KlAr KlAr K/Ar Rb/Sr Rb/Sr
.;\6± I 32± I .;\1 ±2 .;\2±2 .;\5±.;\
Klerkx and Rundle (1976) Klerkx and Rundle (1976) Marholz(1 96S) Marholz (19 68) Marholz(196S) Marholz (196 8)
-t5±2
, Re calculated from Klerkx and Deutsch (1977).
NW Sudan area Metamorphic rocks. 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. 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. Ea stwards they grade into metasiltstones which are exposed in a flatly eroded plain of several square kilometres. 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 haernatitc , 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. They can be traced for 20 km. Magmatic rocks. Two large ring intrusions, deeply eroded , are emplaced in the center of the area, and everywhere smaller stocks of red , aplitic rocks could be found. These rocks are generally granitic in composition. The only exception is Gebel Kissu (210 3 4 ' N I 25°09'E) which is made of coarse-grained alkali syenites becoming finer grained towards the centre. Bir Safsafand Nusab el Balgum area Metamorphic rocks. The southern half of the Bir Safsaf outcrop area consists of red and subordinate grey coloured, medium- to coarse-grained gneisses and migmatites (Fig. 4). In parts these gnei sses are so massive, lacking any foliation, that their textural appearance is very similar to that of granite. Typical migmatic textures, especially nebulitic schlieren , could be observed but are not widely distributed. Pegmatitic veins and aplitic dyke s are abundant. . The composition of the rocks is mainly granitic; they probably formed under anatectic conditions and were subsequently overprinted by low grade metamorphic conditions, as documented by the new formation of muscovite and amphibole. Whether this happened during the cooling stage of the high grade metamorphism or represents two independent events, needs more detailed investigation. Plutonic rocks. The northern half of the Bir Safsaf complex consists of magmatic rocks of mainly granitic composition (Fig. 4), displaying complicated intrusive relationships. Locally small tonalitic and granodioritic stocks can be distinguished. In some areas at least four different intrusive stages are present. Hornblendebearing gneisses form xenoliths within grey, massive
280
H . SCHANDELMEIER, A . RICHTER and G. FRANZ
Nusab el Balgum
----
.--
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q
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5 ubvolcan ics Granitoids
~ Migmatites,Gneisses . - - Dykes __
Faults
Sir Salsal
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20 km
2g0 30E
Fig. .t. Geological sketch map of the Bir Safsaf area.
granites and these were subsequently intruded by pink, coarse-grained, often porphyritic granite. Aplitic and pegmatoid veins penetrate irregularly the previously mentioned rocks. The area was finally invaded by abundant acidic, intermediate and basic dykes (see next section). The grey type of 'granite' is granodioritic to tonalitic in composition.
Volcanic rocks and dykes. Volcanic rocks around Bir Safsaf were found mainly at Nusab el Balgum, which consists exlusively of volcanic porphyritic to subvolcanic rocks, which are exposed in a distinctive elongated shape extending from NNE to SSW (Fig. 4). The central part (north of the main peak) is made up of coarsegrained, often inhomogeneous rocks of alkali granitic composition (quartz, alkali feldspar, plagioclase, aegirine and/or alkali amphibole). These rocks are closely associated with grey rhyolites which have quartz and alkali feldspar as phenocrysts , some alkali amphibole and a ground mass with micrographic texture. Similar light-reddish and dark-reddish porphyries occur in the northern and southern part where they build the higher elevation of the area. Nusab el Balgum and the sediments in the immediate vicinity, as well as the granitic and metamorphic units to
the south (Bir Safsaf) have been cut by numerous dykes. No statistically significant trend in their direction was observed but D-30oNNE and 9D-140oESE striking dykes were frequently observed. The dykes are classified on mineralogical composition into four groups: (i) rhyolitic composition; these dykes are texturally and' mineralogically similar to the porphyritic rocks described above, indicating that at least those which are ncar to Nusab el Balgum may be genetically related . (ii) trachytic-phonolitic composition; they are rare (two of 22 sampled dykes) and were found in Bir Safsaf only. They show fluidal textures and are comparable to trachytic and phonolitic rocks found in the Gebel Kamil area. (iii) basaltic composition; these dykes are frequent in the Bir Safsaf area, occurring as parallel dyke swarms. (iv) basaltic composition with numerous xenoliths; these dykes occur in the central part of Bir .Safsaf showing often diffuse contacts to the country rocks.
Isolated all/crops oral/lid Bir Safsaf. The area of Abu Bayan el Wastani, 225 km north-west of Bir Safsaf (Fig. 1), the area east and south-east of Bir Safsaf were also investigated to obtain information about the isolated outcrops (Fig. 1). In addition, information on the base-
Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf ment has come from a drill hole 300 m in depth, approximately 40 km SSW of Bir Tarfawi (sample made available by courtesy of General Petroleum Company of Egypt). Comparable lithologies with similar petrographic features as in the Bir Safsaf outcrops were found (except for the volcanic and subvolcanic rocks which are typical for Nusab el Balgum only), i.e. reddish granites (sample from drill hole), grandiorites to tonalites, high grade metamorphic, massive gneisses to migmatites, and subordinately dykes of rhyolitic and basaltic composition.
281
(NE-SWfNW-SE and N-S/E-W) which probably existed since the Precambrian but which were continuously rejuvenated during several periods in the Phanerozoic. The fracture pattern is very similar to that in the Hoggar described by Bertrand and Caby (1978) although the faults are considerably smaller, and this supports the theory (Kroner 1979) that the East Sahara Craton has reacted as a single rigid block under compressive stress from the east and west and from the northeast. Area east of Gebel Kamil
STRUCTURES The eastern Uweinat basement inlier
The general foliation trend in the area is NE-SW and can be observed south of Gebel Kissu, in the western and southern parts of the Gebel Kamil outcrop area, as well as in the vicinity of Uweinat in Egypt and Sudan. This most striking structural element of the area can easily be traced on satellite images as long, resistant ridges parallel to the foliation of the gneissic rocks. Dips of the gneissic layers are mainly to NW and only subordinate to SE. Plunge of the fold axes are mostly to SW but rarely also to NE. Fold types are rhombic, tight and isoclinal and only in the western part of the Gebel Kamil area do overturned folds with axial planes dipping to NW occur. Within the central part of the Gebel Kamil area (gneisses, marbles, amphibolites) the foliation trend is WNW-ESE. Similar foliation trends and fold axes with E-W trend do occur in the migmatites south of Gebel Kamil, though they scatter more than in the previously mentioned rocks. In the NW Sudan basement area the general NE-SW trend is locally disturbed where low grade metamorphic schists are updomed or displaced by ring-like intrusive complexes. The foliation trends are slightly different in the Peneplain Plateau. The foliation of the gneisses changes from NE-SW in the southern part to NNE-SSW and finally to N-S in the northern part of the Plateau. Dips are moderate to NW and W. Distinct from the main foliation trend, we could observe locally a relict foliation which strikes at small angles to the main trend, but more detailed structural investigations will have to be carried out to reconstruct a probable older tectonic episode. Generally the major tectonic trend is consistent with the foliation of the basement gneisses south-west of Uweinat in Libya. Klerkx (1980) described NE-SW trending rocks with relics of older N-S striking isoclinal recumbent folds. The fault and joint pattern, clearly traceable on satellite images, indicates compressive stress which has resulted in shear zones that occur commonly as vertical, orthogonal fault breccia. Dyke swarms are frequent, especially in the southern Peneplain area, where they cut each other. As well as the dykes, the occurrence of younger volcanic sills and plugs is clearly bound to fracture systems. There are two major fracture systems
East of Gebel Kamil the basement is covered by a wide serir plain and no exposures for a distance of 260 km between the Kamil area and the Bir Safsaf area were found. However, unpublished gravity maps from the General Petroleum Company of Egypt and airborne magnetic maps from Conoco Cairo show undoubtedly that the Uweinat uplift extends further to the east. The sedimentary cover between the above mentioned outcrops is probably not thicker than 500 m. Structural trends in the Bir Safsaf region are not very distinctive. Within the limited outcrops of granitic gneisses, the foliation strikes mainly NW-SE and NE dips of 15-30° are common. Where the gneisses grade into migmatites, the direction of migmatic layers and schlieren varies and no statistically significant trend is obvious. A very striking feature on satellite images is the parallel E-W striking fault system that extends roughly from Baris in the north to Bir Nakhlai in the south (Fig. 1). These megafaults which reach 50 krn in length, are mostly normal faults with vertical displacements from 10 to 50 m (Abdallah et al. 1979). These authors believe that the faults are tectonically reactivated structural elements of the underlying basement rocks which were uplifted in the Upper Cretaceous. The Nusab el Balgum volcanics seem to be emplaced along such a fault system and since they are of Lower Triassic age there is strong indication that these fracture systems were already reactivated in Late Palaeozoic and Early Mesozoic times.
GEOCHRONOLOGICAL DATA There are only a few reliable radiometric data from the Uweinat area which are summarized in Table 1. In addition to the previously known ages, we obtained three new Rb/Sr whole-rock ages from the Gebel Kamil and Nusab el Balgum areas, respectively. They are described here briefly as analytical details and a fuller discussion will be published separately. Twenty krn south of Gebel Kamil eight samples from a fairly inhomogeneous migmatite were taken for Rb/Sr whole-rock age determination. The regression line yielded an age of 2588 ± 26 Ma and an initial Sr 87/Sr86 ratio of 0.6955 ± 0.0002 (MSWD = 2678). The huge scatter of the points about the line could suggest a very
282
H . SCHANDELl\IEIER, A. RICHTER and G. FRANZ
disturbed older (probably Archean) basement. Five out of the 8 points are well aligned along a line and the recalculated regression line yielded an age of 673 ± 56 Ma with an initial Sr87/S~6 ratio of 0.7050 ± 0.0004 (MSWD = 5.3, enhanced errors). The line is not far from qualifying as an isochron and this could suggest a Late Proterozoic resetting within parts of these rocks. From a very homogeneous porphyritic granite of the central Gebel Kamil area which is in sharp contact to the surrounding gneisses and migmatites, four samples define a perfect isochron (MSWD = 0.3) yielding an Upper Ordovician/Lower Silurian age of 431 ± 33 Ma and an intercept ofO. 7097 ± 0.0007. This result indicates a crustal origin of this rock. Five samples from a subvolcanic rhyolitic rock of the Nusab el Balgum define a regression line yielding an age of 216 ± 5 Ma with an intercept of 0.7054 ± 0.0025 (MSWD = 4.1, enhanced errors).
A PROPOSED SCHEME FOR THE DEVELOPMENT OF THE UWEINAT UPLIFT Based on presently available geological, petrological and radiometric data, the following provisional geological history for the Uweinat Uplift is proposed. The authors are aware, that modifications may be necessary when more information about this still poorly known area becomes available. The oldest recognizable event in the Uweinat area is the Early Proterozoic (-2700 Ma) metamorphism to granulite facies of Archean rocks, which were deposited on a now unrecognizable floor (Klerkx and Deutsch 1977). A Middle Proterozoic metamorphic event is documented by the formation of migmatites (-1800 Ma) through anatexis within these basement gneisses. The migmatization was accompanied by the formation of anatectic granites and tonalites (Klerkx and Deutsch 1977). In the Late Proterozoic (Pan-African) the area between Hoggar and Red Sea was under compressive and tensional stress (Kroner 1979); strong intraplate block faulting was accompanied by the formation of locally occurring migmatites and voluminous granites by reworking of older rocks from the lower crust. The Late Proterozoic development in the Uweinat area is still uncertain. The migmatites of the Gebel Kamil area are possibly reworked rocks of Pre-Pan-African age during the Pan-African, as indicated by the geochronological results . Conclusive results will have to be postponed until more radiometric data are available. The stratigraphic position of the low grade metamorphic rocks of the NW Sudan outcrop area is very uncertain. It is possible that these rocks, originally deposited into shallow water basins, are infolded remnants of a higher crustal level into basement gneisses. Uplift and erosion resulted in the formation of a Late Proterozoic land surface and Palaeozoic sediments were deposited in the depressions at least from the Ordovician onwards (Uweinat area). The intrusion of Ordovician
anorogenic granitic complexes (Gebel Babein; Gebel Kamil area; Table 1) falls in a time span where, after a 'quasistatic' interval (Cambrian-Lower Ordovician) , a rapid polar wandering of Gondwanaland can be observed (Briden et al. 1973). Palaeozoic sedimentation continued in the Uweinat area and most likely east of Uweinat until the Permo-Triassic. Reactivation of older fault structures combined with uplift in the Gebel Kamil and Bir Safsaf areas removed the sedimentary cover and the eroded material was transported mainly northwards into the present-day Dakhla basin (Fig. 1). Basaltic dykes (Uweinat area, 235 Ma; Table 1) and rhyolitic subvolcanic rocks (Nusab el Balgum, 216 Ma; Table I) were intruded along fracture zones in the Upper Permian and the Lower Triassic, respectively. Following subsequently, the structure of the Dakhla basin permitted continuous continental accumulation of Nubian type sediments, beginning in the Late Jurassic/ Lower Cretaceous, whose basal parts are probably reworked Palaeozoic strata (Fay and Hermann-Degen in prep.). Intercalated into the continental Nubian arc several marine horizons and the whole sequence is overlain by Lower MaestrichtianlTertiary marine sediments (Klitzsch et al. 1979). From the Upper Cretaceous until the Quaternary intensive reactivation of older zones of crustal weakness gave way to a variety of intrusive and extrusive rocks like alkali granites, syenites, basalts, trachytes and phonolites. The peak of these magmatic activities occurred around 45 Ma (Table 1) in the Eocene. The present landform of the area is mainly a product of erosive and accumulative activities of predominantly N-S blowing winds and episodic rainfalls.
CONCLUSIONS Structural and petrographic investigations on metamorphic rocks from west of Gebel Kamil area have shown that these rocks might be correlated with the Archean/Early Proterozoic granulitic gneisses from the Uweinat area previously described by Klerkx (1980). The migmatites of the Gebel Kamil area are petrographically very similar to the Middle Proterozoic migmatites of Uweinat. Rb/Sr results from these rocks indicate that they are probably of Pre-Pan-African age but contrary to the migmatites of Uweinat, the Gebel Kamil migmatites show some Pan-African resetting effects. Lithologi cally and petrographically the migmatites of Bir Safsaf are also similar to previously mentioned migmatites but their age is still unknown. More radiometric dating will have to be carried out in order to establish the location of the eastern margin of the Archean-Early Proterozoic craton which must lie east of Gebel Uweinat. Acknoll'ledgements-This work was supported by the DFG (SFB 69. A IID~). We thank our colleagues for their help during the field work and GPC (Cairo) for their help with the organization. Critical reading of an earlier draft of the manuscript by G. Mortcani and an unknown reviewer is gratefully acknowledged .
Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf
REFERENCES Abdallah, A. M., Issawi, B. and El-Hinnawi, M. 1979. Structural Geology of Abu Bayan-Bulaq Stretch, Western Desert, Egypt. Chem. Erde 38, 2-l5-25-l. Bertrand, J. M. L. and Caby, R. 1978. Geodynamic evolution of the Pan-African orogenic belt: a new interpretation of the Hoggar shield (Algerien Sahara). Geol. Rdsch. 67,337-338. Bertrand, J. M. L., Caby, R., Ducrot , J., Lancclot, J., MoussinePouch kine, A. and Saadallah, A. 1978. The late Pan-African ensialic linear fold belt of eastern Hoggar (Algeria): Geology, structural development, U/Pb geochronology, tectonic implications for the Hoggar shield. Precambrian Res. 7,349-376. Briden, J. C., Morris, W. A. and Piper, J. D. A. 1973. Regional and global implications. In: Paleomagnetic studies in the British Caledonides. Geophys. J. R. astr. Soc. 34, 107-13-l. Cahen, L., Snelling, N. J., Delhal, J. and Vail, J. R. with the collaboration of Bonhornrnc, M, and Lcdent, D. 1983. The Geochronology and Evolution ofAfrica. Clarendon Press, Oxford. EI Ramly, M. F. 1972. A new geological map for the Basement rocks in the Eastern and South-Western desert of Egypt. Anllls geol. Sun'. Egypt 2, 1-18. Fay, M. and Hormann-Degen, W. (in prep.) Maestrichtian Sand deposits of SW Egypt and a model for the basin development of the Nubian group. Gass. I. G. 1977. The evolution of the Pan-African crystalline basement in NE Africa and Arabia. J. geol. Soc. Land. 134,129-138. Hunting Geology and Geophysics, Ltd. 1974. Geology ofthe Jabal AI Uwaynat area, Libyan Arab Republic. Unpub\. Rcp., Industrial Research Centre, Department of Geological Research and Mining, Tripoli (Libya). Klerkx, J. 1980. Age and metamorphic evolution of the Basement complex around Jabal al Awaynat. In: The Geology ofLibya (edited by Salem, M. J. and Busrewil, M. T.). Vol. 3, 901-906. Klcrkx, J. and Deutsch, S. 1977. Resultats preliminaries obtenus par la methode Rb/Sr sur l'age des formations precambriennes de la region d'Uwcinat (Libye). Rapp. annu. Mus. roy. Afr. centr., Tervuren (Be/g.) Dep. Geol. Min. 1976, 83-9-l. Klcrkx, J. and Rundle, C. C. 1976. Preliminary K/Ar results on
283
different igneous rock formations from the Jebel Uweinat region (S.E. Libya). Rapp. annu. Mus. roy. Afr. centr., Tervuren (Be/g.) Dep, cest. Min. 1975,105-111. Klitzsch, E. and List, F. (Eds) 1978. Geological interpretation map of SW-Egypt, 1: 500 000, Preliminary edition, Technische Fachhochschule Berlin, Berlin. Klitzsch, E., Harms, J., Lejal-Nicol, A. and List, F. 1979. Major subdivisions and depositional environments of Nubian strata, SW Egypt. Bull. Am. Ass. Petrol. Geol. 63 (6), 967-97-l. Kroner, A. 1979a. Pan African mobile belts as evidence for the transitional tectonic regime from intra-plate orogeny to plate margin orogeny. In: Evolution and Mineralization of the Arabian-Nubian Shield (edited by Al-Shanty, A. M. S.). Pergamon Press, Oxford, 21-37. Kroner, A. 1979b. Pan African plate tectonics and its repercussion on the crust of northeast Africa. Geol. Rdsch. 68 (2), 565-583. Kroner, A., Stern, R. J. and Fleck, R. J. 1983. Chronology and mechanism of Pan-African crustal accretion in north-east Africa and Arabia. Abstracts, 12th Coli. Afr. Geology, April 1983, Brussels, 55. Marholz, W. W. 1968. Geological exploration of the Kufra region, April-May, 1965. Bull. Geol. Surv, Libya 8,1-76. Martin, H. and Porada, H. 1977. The intracratonic branch of the Damara orogen in South West Africa.-l. Discussion of geodynamic models. Precambrian Res. 5,311-338. Menchikoff, N. 1927. Etude petrographique des roches cristallines et volcaniques de la region d'Oucnat. Bull. Soc. geol. Fr., 4 Ser. 27, 337-354. Ries, A. C. 1983. Geochronology and tectonic evolution of the Late Proterozoic ofNE Africa. Abstracts, 12th ColI. Afr. Geology, April 1983, Brussels, p. 84. Sandford, K. S. 1935. Geological observations on the north-west frontiers of the Anglo-Egyptian Sudan and the adjoining part of the southern Libyan desert. Qt. JI geol. Soc. Lond. 91,323-381. Vachette, M. 1974. Repartition des ages de biotites au strontium en Afrique. C.». Znd Reun. ann. Sci. Terre, Pont-a-Mousson Nancy, 378. Winkler, H. G. F. 1978. Petrogenesiso] Metamorphic Rocks. Springer, New York.
Journal of African Earth Sciences. Vol. I. No . 3/-1. pp. 275 to 283. 1983 Pr inted in Great Britain
Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf (SW Egypt/NW Sudan) H. SCHANDELMEIER Sonderforschungsbereich 69, Arid Areas, Technische Universitat Berlin, Ackerstrafle 71-76,0·\000 Berlin 65, Federal Republic of Germany
A.
RICHTER
Institut fiir Angewandte Geologie, Freie Universitat Berlin, Malteser Stralle 74·100,0·1000 Berlin 46, Federal Republic of Germany
G.
FRANZ
Institut fiir Angewandte Geophysik, Petrologie und Lagerstattenkunde, Technische Universitat Berlin, Straflc dcs 17. Juni 135,0·1000 Berlin 12, Federal Republic of Germany
(Received 13 October 1983) Abstract-The Uweinat basement uplift system extends from Gebel Uweinat in the west to Bir Safsaf in the cast underlying an area of roughly 40,000 km~ in which crystalline rocks arc exposed. The distribution of the basement rocks was mapped by means of field traverses combined with an interpretation of Landsat satellite images. General foliation trend is NE-SW in the Uweinat area changing to NNE-SSW in norIltern direction and to WNW-ESE in the Gebel Kamil area. In the eastern part (Bir Safsaf) it is NW-SE. Metamorphic rocks are migmatites and granitic to granulitic gneisses with subordinate amounts of amphibolite and marble . Low grade metamorphic rocks were found only in the southern area (NW Sudan). Plutonic rocks include different types of granitoids and dykes. volcanic rocks are alkali olivine basalt, phonolite, trachytes, and rhyolites . All age data for the area are compiled and together with three newly obtained ages (673 ± 65 rn.y, .rnigmatitcs, 431 ± 33, granite, 216 ± 5, rhyolite) it can be shown that magmatic activity occurred in several periods during the Phanerozoic. Some of the magmatic rocks a rc concentrated along fracture zones.
INTRODUCTION
Description ofareas investigated and working methods
IN SW EGYPT and in a minor part ofNW Sudan, crystalline rocks are exposed in the vicinity of Gebel Uweinat and in isolated outcrops west of Lake Nasser. In Egypt and Sudan, the outcrops of the Uweinat region extend 180 krn to the north-east, where they are covered by the continental strata of the Gilf Kebir formation. They occur for 175 km eastwards to Gebel Kamil and 125 km south-eastwards into Sudan (Fig. 1). A 60 km x 40 km wide outcrop of mainly granites and migmatie gneisses (Bir Safsaf complex) lies at 23°N, 29°30'E. Immediately to the north is a smaller complex (15 x 5 krn) of volcanic and subvolcanic rocks, the Nusab el Balgum (Fig. 4). Further to the north and east there are small isolated outcrops of granites, migmatites and other metamorphic rocks, extending eastwards towards Lake Nasser (23°N, 31 0 E ) . None of these areas had previously been described in detail; brief information on the 'basement outcrops' is given on the preliminary geological interpretation map of Klitzsch and List (1978) and the geological map of EI Ramly (1972). We visited selected parts of this vast region in 1980, 1981 and 1982 and the results which apply only to the investigated areas were extrapolated with the help of satellite images towards unknown areas, but these interpretations may be altered when better information becomes available. AES 1 : ) /4-C
The purpose of this paper is to record the available data about lithologies, metamorphism , radiometric ages and structures of the basement of SW Egypt between Gebel Uweinat and Bir Safsaf.
Geological frame The "East Sahara Craton" (Kroner 1979), which is largely covered by Phanerozoic sedimentary strata, extends from the Pharusian and Dahomeyan orogenic zone in the west (Fig. 2, Nos. 3 and 4) to the orogenic Hijaz Magmatic Arc (Fig. 2, No.1) and the northern continuation of the tectono-thermal Mozambique Belt (Fig . 2, No.2) in the east. The eastern and southern boundaries of the craton are still ill-defined, Collision of the West Africa Craton with the East Sahara Craton has resulted in the closing of the Pharusian ocean or graben (Bertrand et al. 1978). In the cast, new crust was accreted onto the East Sahara Craton according to Gass (1977) by continuous westward subduction of oceanic crust during the Pan-African from about 1100 to 500 Ma, but recent results (Ries 1983, Kroner et al. 1983) indicate that the Pan-African evolution in NE Africa and Arabia started around 900 Ma and not earlier. The rigid cratonic block, sandwiched between these two Pan-African mobile zones (Fig. 2) has been affected by strong intraplate tectonics (Kroner 1979) and the successive formation of large-scale fracture zones has been accompanied by intrusions of granite plutons and alkaline ring complexes. As a result of this Pan-African tectogenesis, the older crustal rocks were structurally
275
H. SCHANDELMEIER, A. RICHTER and G. FRANZ
276
DISTRIBUTION AND
OF
METAMORPHIC
MAGMATI C ROCKS OF
SW-EGYPT AND NW-SUDAN
...
...
==-_==-.-c= 100 km
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.,.
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.-
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Fig. 1. Distribution of magmatic and metamorphic rocks of SW Egypt and NW Sudan: 1, Gebel Kamil area; 2, Peneplain Plateau; 3, Uweinat area; 4, NW Sudan area; 5, Sir Safsaf area
and metamorphically overprinted; locally partial melting of crustal material with resetting of isotopic systems (Vachette 1974, Kroner 1979) has occurred. Finally, post-tectonic granites have been intruded into the whole area between Hoggar and Red Sea. In the Phanerozoic, large scale crustal warping resulted in the formation of extensive intracratonic basins which were continuously modified by structural events throughout the whole period. The investigated area which we describe with the term'Uweinat Uplift' is situated at the eastern border of the East Sahara Craton (Fig. 2) and may therefore give some important information on the geological development of a key area near the craton-mobile zone boundary. Because most parts of the East Sahara Craton are buried beneath thick continental and partly marine Palaeozoic, Mesozoic and post-Mesozoic strata (Klitzsch et al, 1979), a satisfactory examination of the basement of the East Sahara Craton is impeded.
1 Hljaz Magmatic Arc
2
Mozambique Belt
3 Pharuslan 40ahomeyan
REGIONAL DISTRIBUTION AND PETROGRAPHY OF BASEMENT OUTCROPS
Basement outcrops in SW Egypt and adjacent Sudan arc exposed across an area of about 40,000 km 2 in five regions. The easternmost outcrops of the Uweinat inlier
5 Mauri tanldes & Rokelides
~
Orogenic
~ ~ Tectono-thermal
F::::j Thermal
Fig. 2. Areas of Northern Africa and Arabia which were affected by Pan-African tectogenesis (modified after Kroner 1979, Martin and Porada 1977a).
Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf
277
20 km
1..>;s<1
o
Quaternary
1'.-11
Volcanic
1:::::::::1
Nub ian sed I men t s
o
Granitoid rocks
~
Precambrian gneisses and
rocks
GEBEL KAMIL AREA
migmatites
Fig. 3. Geological sketch map of the Gebel Kamil area.
are in the Gebel Kamil area (Fig. 1, No.1) a rough morphology of scarps, plains and hillocks up to 200 m above terrain level. North-east of Gebel Uweinat extends the Peneplain (Fig. 1, No.2), an elevated region, topped by a flat plain, cut by frequent wadis. Immediately east of Gebel Uweinat (nON, 25°E; Fig. 1, No.3) is an area of predominantly low ground with isolated inselbergs. The area in NW Sudan (Fig. 1, No. 4) is mainly made up of linear or arcuate ranges which rise 100 m above terrain level. Four hundred and fifty km east of Gebel Uweinat the Bir Safsaf outcrops (Fig. 1, No .5) form a rather flat plain which grades northwards to an undulating boulder plain. Earlier petrographic and radiometric studies on the basement and the younger magmatic rocks were almost exclusively made in Libya (Fig. 1, west of 25°E) by Menchikoff (1927), Marholz (1968), Hunting Geology and Geophysics Ltd (1974), Klerk x and Rundle (1976) and Klerkx and Deutsch (1977). The adjacent basement in Egypt and Sudan is virtually undescribed, except by Sandford (1935).
Gebel Kamil area Metamorphic rocks. Gneissose granoblastites and granulitic gneisses occupy the western part of the Gebel Kamil area (Fig. 3) where mega folds built up several ranges parallel to the fold axes. These gneisses are mineralogically and texturally very similar to those of the Uweinat area described by Klerkx (1980) (see Gebel
Uweinat area). Granoblastic and flasertextures prevail in these rocks and consequently mineral assemblages are not representative of the regional hypersthene zone, only one sample showing a diagnostic (Winkler 1978) metamorphic assemblage with orthopyroxene. Minerals are isometric plagioclase (albite to andesine) platy quartz, K-feldspar occurring as perthite and rarely as microcline. Mafic components are clinopyroxene, orthopyroxene, hornblende, actinolite, biotite, chlorite and epidote. Pyroxenes are mostly altered to talc and hornblende to actinolite. The modal composition of the rocks is tonalitic. Anatectic rocks are exposed at the center and eastern part of the Gebel Kamil area. They can be classified as gneisses interpreted as paleosome, and true migmatites which exhibit intercalations of melanosome and leucosome with small-scale folding. The paleosomatic gneisses of granodioritic composition show gneissose as well as stromatic textures. Their fabric is granoblastic with interlobate grain boundaries. They contain sericitised plagioclase, cataclastic quartz, perthitic feldspar, biotite often altered to chlorite and sphene, white mica and epidote. Migmatites occur in all stages of migmatization; blastic growth of feldspar, schlieren and stromatic metatexites with distinctive differentiation of melanosome and leucosome, and finally as nebulitic diatexites. At one location, in the central part of the Gebel Kamil area, a sillimanite-garnet-cordierite gneiss was found. This dark brown rock shows a well-foliated flaser texture; besides the idioblastic sil-
278
H. SCHANDELMEIER, A. RICHTER and G. FRANZ
limanite, garnet and cordierite occur as well as xenoblastic quartz, K-feldspar, tabular plagioclase, biotite, spinel, white micas and various accessories. Marbles and calcsilicate rocks are also present, the former throughout the area. In most places they are in stratiform contact with the gneisses and show alternate thin greenish-brown and thick white layers. Where they are intercalated in anatectic rocks, the layers are often perpendicular to the foliation trend of the surrounding rocks and probably represent a pre-anatectic structure. Main minerals of the white layers are calcite and dolomite with minor actinolite, diopside, pargasite, olivine, spinel, members of the humite group, and phlogopite. The dark zones are enriched in almost completely serpentinized olivine, diopside, actinolite and iron oxides. Dark greenish-grey diopside-scapolite and titaniumaugite-hornblende rocks are associated with the marbles and the sillimanite-garnet-cordierite gneiss. Their main minerals are An-rich plagioclase, pyroxene which includes calcite and epidote. Minor minerals are chlorite, sericite, green spinel, actinolitic hornblende, sphene, apatite and ore. Plutonic rocks. Intrusive rocks of different, but generally granitoid composition were encountered 10 krn south of Gebel Kamil where a boulder plain is made up of coarse-grained, red granites similar to the red granites of the northern Bir Safsaf complex . These granites are very rich in perthitic feldspar, Ab-rich plagioclase and quartz. Mafic minerals are mainly biotite and small amounts of hornblende. This red granite also occurs in the south-western part of the Gebel Kamil area (Fig. 3). In the central-western part of the area a large granitic body, lacking any foliation, was found. In contrast to the red granites the rock is porphyritic and shows slight flow textures. It contains equal proportions of quartz, perth itic feldspar and zoned plagioclase. Mafic minerals are green biotite, hornblende and pyroxene. Apatite, zircon, sphene and ore are accessories. Granodiorites are not common; they occur as smaller bodies which show marginal foliation. Mineralogically they are nearly identical to the previously described plutonic rocks. Volcanic rocks and dykes . Volcanic rocks and dykes are widespread but are concentrated in the southern Gebel Kamil area and at the southern end of the Peneplain Plateau (Fig. 1). Most common are phonolites, olivine basalts and alkali trachytes which form plugs and cones up to 200 m high. Where dissected by erosion, the remnants occur as ring structures. The alkaline rocks contain abundant sanidine and/or alkali feldspar and pyroxene is present as aegirine. The phonolites contain considerable amounts of euhedral nepheline. Their texture is finely crystalline, linear fluidal; some interstitial glass may be present. Frequently these rocks contain numerous xenoliths of basement rocks. The olivine basalts are generally fine-grained with variable sized phenocrysts of olivine, which is also present in the groundmass. Fine-grained prismatic plagioclase is abundant and two generations of titan-augite are present. Isometric opaque ore is a minor constituent.
The texture of the rocks is ophitic intergranular or intersertal. Rhyolite flows cover a considerable area of the southern Gebel Kamil complex. Granophyric quartz-feldspar inter-growths are embedded as xenoliths in a devitrified matrix. Some micas, calcite and opaque ore are further constituents. Cryptocrystalline haematite causes red colouration of the rocks. At some localities rhyolitic volcanic breccia could be found. Fragments of aphanitic devitrified glass arc welded by veins of fine-grained polygonal quartz. The dykes can be subdivided into: (i) acidic composition: spherulitic to porphyritic felsites with a devitrified glass matrix and linear flow texture and porphyritic granophyrs in which K-feldspar, plagioclase and quartz are embedded in a micrographic matrix. (ii) intermediate and basic composition: porphyritic microgranodiorites, microtonalites and microdiorites, with abundant plagioclase, minor quartz and little or no K-feldspar. Biotite is present in some rocks as saghenite. In addition chlorite, amphibole, pyroxene, epidote and white mica are found. Peneplain area
Gneisses similar to the paleosomatic gneisses of the Gebel Kamil complex occupy most parts of the Peneplain (Fig. 2, No.2). 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. However. for the most part the rocks are well layered over extensive areas in which there is no fluid mixing at all. The intrusive granitoid rocks are of the same type as the reddish, K-feldspar granites of the Gebel Kamil area. Gebel Uweinat area
In the eastern and south-eastern vicinity of the Gebel Uweinat ring complex (Fig. 1) the basement is made up of gneissose granoblastites and granulitic gneisses which probably extend as far as the western part of the Gebel 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, Uweinat (22°01' N/25°09' E), around the northern slopes of Gebel Uweinat, and at the western and south-western base of this mountain. Texturally and mineralogically the anatectic rocks arc comparable to similar anatectic rocks in the Gebel Kamil area, but correlation is not yet supported by radiometric data (Table 1). A detailed description of the Gebel Uweinat ring complex was given by Klerkx and Rundle (1976).
Outline of the geology of magmatic and metamorphic units between Gebel Uwcinat and Bir Safsaf
279
Table I. Some relevant radiometric age dat a from the Uweinat-Bir Safs af upl ift Location
Karkur Murr
Rock type
Charnockitic Gneiss and retrogressive metamorphism WadiWahesh Anatectic gr anites Uwein at/Libya and tonalities Gebel Kamil M igmatite area/Egypt GebelKamil Porph. Granite mea/ Eg ypt Gebel Babein Microgranite Liby a Nusabel Balgum Alkali Subvolcanics Egypt Uwcin at/Libya Basalt Uwcinat/Libya Alkali Granite Gebel Arehenu Neph. Syenite Libya Uwcinat/Libya Phonolite UweinatlLibya Dolerite Uwcinat/Libya Syenite Uweinat/Libya Syenite Uwc inat/Libya Pegmatite UweinatlLibya Granite
A nalyzed mineral Whole rock
Age(Ma)
Method Rb/Sr
Uweinat/Libya
29~to2919
References Cahen ers/. (1983)'
2656± 1.;\2
Whol e rock
Rb/Sr
178.t± 126 isochron
Cahenelal. (1983)'
Whole rock
Rb/Sr
673 ± 56 regression line
Thi s paper
Whole rock
Rb/Sr
.;\31 ± 33 isochron
This paper
Wh olcrock
KlAr
.;\SI ± 12
Whole rock
Rb/Sr
216 ± 5 regression line
Hunting G eol ogyand Geophysics Ltd. (197,;\) This paper
Whole rock KFsp/Bio/Aegirine KFsp/KFsplAegirine
KlAr K/Ar KlAr
235±5 .;\5 ± I; .;\2± I; .;\2± I 32 ± I ; 36 ± I; .;\8± I
Klerkx and Rundle (1976) Klerkx and Rundle (1976) Klerkx and Rundle (1976)
Whole rock Whole rock Amphibole Amphibole Feldspar Feldspar
KlAr KlAr KlAr K/Ar Rb/Sr Rb/Sr
.;\6± I 32± I .;\1 ±2 .;\2±2 .;\5±.;\
Klerkx and Rundle (1976) Klerkx and Rundle (1976) Marholz(1 96S) Marholz (19 68) Marholz(196S) Marholz (196 8)
-t5±2
, Re calculated from Klerkx and Deutsch (1977).
NW Sudan area Metamorphic rocks. 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. 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. Ea stwards they grade into metasiltstones which are exposed in a flatly eroded plain of several square kilometres. 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 haernatitc , 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. They can be traced for 20 km. Magmatic rocks. Two large ring intrusions, deeply eroded , are emplaced in the center of the area, and everywhere smaller stocks of red , aplitic rocks could be found. These rocks are generally granitic in composition. The only exception is Gebel Kissu (210 3 4 ' N I 25°09'E) which is made of coarse-grained alkali syenites becoming finer grained towards the centre. Bir Safsafand Nusab el Balgum area Metamorphic rocks. The southern half of the Bir Safsaf outcrop area consists of red and subordinate grey coloured, medium- to coarse-grained gneisses and migmatites (Fig. 4). In parts these gnei sses are so massive, lacking any foliation, that their textural appearance is very similar to that of granite. Typical migmatic textures, especially nebulitic schlieren , could be observed but are not widely distributed. Pegmatitic veins and aplitic dyke s are abundant. . The composition of the rocks is mainly granitic; they probably formed under anatectic conditions and were subsequently overprinted by low grade metamorphic conditions, as documented by the new formation of muscovite and amphibole. Whether this happened during the cooling stage of the high grade metamorphism or represents two independent events, needs more detailed investigation. Plutonic rocks. The northern half of the Bir Safsaf complex consists of magmatic rocks of mainly granitic composition (Fig. 4), displaying complicated intrusive relationships. Locally small tonalitic and granodioritic stocks can be distinguished. In some areas at least four different intrusive stages are present. Hornblendebearing gneisses form xenoliths within grey, massive
280
H . SCHANDELMEIER, A . RICHTER and G. FRANZ
Nusab el Balgum
----
.--
t~~
q
D
5 ubvolcan ics Granitoids
~ Migmatites,Gneisses . - - Dykes __
Faults
Sir Salsal
'===~-'"""'==~-~'
20 km
2g0 30E
Fig. .t. Geological sketch map of the Bir Safsaf area.
granites and these were subsequently intruded by pink, coarse-grained, often porphyritic granite. Aplitic and pegmatoid veins penetrate irregularly the previously mentioned rocks. The area was finally invaded by abundant acidic, intermediate and basic dykes (see next section). The grey type of 'granite' is granodioritic to tonalitic in composition.
Volcanic rocks and dykes. Volcanic rocks around Bir Safsaf were found mainly at Nusab el Balgum, which consists exlusively of volcanic porphyritic to subvolcanic rocks, which are exposed in a distinctive elongated shape extending from NNE to SSW (Fig. 4). The central part (north of the main peak) is made up of coarsegrained, often inhomogeneous rocks of alkali granitic composition (quartz, alkali feldspar, plagioclase, aegirine and/or alkali amphibole). These rocks are closely associated with grey rhyolites which have quartz and alkali feldspar as phenocrysts , some alkali amphibole and a ground mass with micrographic texture. Similar light-reddish and dark-reddish porphyries occur in the northern and southern part where they build the higher elevation of the area. Nusab el Balgum and the sediments in the immediate vicinity, as well as the granitic and metamorphic units to
the south (Bir Safsaf) have been cut by numerous dykes. No statistically significant trend in their direction was observed but D-30oNNE and 9D-140oESE striking dykes were frequently observed. The dykes are classified on mineralogical composition into four groups: (i) rhyolitic composition; these dykes are texturally and' mineralogically similar to the porphyritic rocks described above, indicating that at least those which are ncar to Nusab el Balgum may be genetically related . (ii) trachytic-phonolitic composition; they are rare (two of 22 sampled dykes) and were found in Bir Safsaf only. They show fluidal textures and are comparable to trachytic and phonolitic rocks found in the Gebel Kamil area. (iii) basaltic composition; these dykes are frequent in the Bir Safsaf area, occurring as parallel dyke swarms. (iv) basaltic composition with numerous xenoliths; these dykes occur in the central part of Bir .Safsaf showing often diffuse contacts to the country rocks.
Isolated all/crops oral/lid Bir Safsaf. The area of Abu Bayan el Wastani, 225 km north-west of Bir Safsaf (Fig. 1), the area east and south-east of Bir Safsaf were also investigated to obtain information about the isolated outcrops (Fig. 1). In addition, information on the base-
Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf ment has come from a drill hole 300 m in depth, approximately 40 km SSW of Bir Tarfawi (sample made available by courtesy of General Petroleum Company of Egypt). Comparable lithologies with similar petrographic features as in the Bir Safsaf outcrops were found (except for the volcanic and subvolcanic rocks which are typical for Nusab el Balgum only), i.e. reddish granites (sample from drill hole), grandiorites to tonalites, high grade metamorphic, massive gneisses to migmatites, and subordinately dykes of rhyolitic and basaltic composition.
281
(NE-SWfNW-SE and N-S/E-W) which probably existed since the Precambrian but which were continuously rejuvenated during several periods in the Phanerozoic. The fracture pattern is very similar to that in the Hoggar described by Bertrand and Caby (1978) although the faults are considerably smaller, and this supports the theory (Kroner 1979) that the East Sahara Craton has reacted as a single rigid block under compressive stress from the east and west and from the northeast. Area east of Gebel Kamil
STRUCTURES The eastern Uweinat basement inlier
The general foliation trend in the area is NE-SW and can be observed south of Gebel Kissu, in the western and southern parts of the Gebel Kamil outcrop area, as well as in the vicinity of Uweinat in Egypt and Sudan. This most striking structural element of the area can easily be traced on satellite images as long, resistant ridges parallel to the foliation of the gneissic rocks. Dips of the gneissic layers are mainly to NW and only subordinate to SE. Plunge of the fold axes are mostly to SW but rarely also to NE. Fold types are rhombic, tight and isoclinal and only in the western part of the Gebel Kamil area do overturned folds with axial planes dipping to NW occur. Within the central part of the Gebel Kamil area (gneisses, marbles, amphibolites) the foliation trend is WNW-ESE. Similar foliation trends and fold axes with E-W trend do occur in the migmatites south of Gebel Kamil, though they scatter more than in the previously mentioned rocks. In the NW Sudan basement area the general NE-SW trend is locally disturbed where low grade metamorphic schists are updomed or displaced by ring-like intrusive complexes. The foliation trends are slightly different in the Peneplain Plateau. The foliation of the gneisses changes from NE-SW in the southern part to NNE-SSW and finally to N-S in the northern part of the Plateau. Dips are moderate to NW and W. Distinct from the main foliation trend, we could observe locally a relict foliation which strikes at small angles to the main trend, but more detailed structural investigations will have to be carried out to reconstruct a probable older tectonic episode. Generally the major tectonic trend is consistent with the foliation of the basement gneisses south-west of Uweinat in Libya. Klerkx (1980) described NE-SW trending rocks with relics of older N-S striking isoclinal recumbent folds. The fault and joint pattern, clearly traceable on satellite images, indicates compressive stress which has resulted in shear zones that occur commonly as vertical, orthogonal fault breccia. Dyke swarms are frequent, especially in the southern Peneplain area, where they cut each other. As well as the dykes, the occurrence of younger volcanic sills and plugs is clearly bound to fracture systems. There are two major fracture systems
East of Gebel Kamil the basement is covered by a wide serir plain and no exposures for a distance of 260 km between the Kamil area and the Bir Safsaf area were found. However, unpublished gravity maps from the General Petroleum Company of Egypt and airborne magnetic maps from Conoco Cairo show undoubtedly that the Uweinat uplift extends further to the east. The sedimentary cover between the above mentioned outcrops is probably not thicker than 500 m. Structural trends in the Bir Safsaf region are not very distinctive. Within the limited outcrops of granitic gneisses, the foliation strikes mainly NW-SE and NE dips of 15-30° are common. Where the gneisses grade into migmatites, the direction of migmatic layers and schlieren varies and no statistically significant trend is obvious. A very striking feature on satellite images is the parallel E-W striking fault system that extends roughly from Baris in the north to Bir Nakhlai in the south (Fig. 1). These megafaults which reach 50 krn in length, are mostly normal faults with vertical displacements from 10 to 50 m (Abdallah et al. 1979). These authors believe that the faults are tectonically reactivated structural elements of the underlying basement rocks which were uplifted in the Upper Cretaceous. The Nusab el Balgum volcanics seem to be emplaced along such a fault system and since they are of Lower Triassic age there is strong indication that these fracture systems were already reactivated in Late Palaeozoic and Early Mesozoic times.
GEOCHRONOLOGICAL DATA There are only a few reliable radiometric data from the Uweinat area which are summarized in Table 1. In addition to the previously known ages, we obtained three new Rb/Sr whole-rock ages from the Gebel Kamil and Nusab el Balgum areas, respectively. They are described here briefly as analytical details and a fuller discussion will be published separately. Twenty krn south of Gebel Kamil eight samples from a fairly inhomogeneous migmatite were taken for Rb/Sr whole-rock age determination. The regression line yielded an age of 2588 ± 26 Ma and an initial Sr 87/Sr86 ratio of 0.6955 ± 0.0002 (MSWD = 2678). The huge scatter of the points about the line could suggest a very
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H . SCHANDELl\IEIER, A. RICHTER and G. FRANZ
disturbed older (probably Archean) basement. Five out of the 8 points are well aligned along a line and the recalculated regression line yielded an age of 673 ± 56 Ma with an initial Sr87/S~6 ratio of 0.7050 ± 0.0004 (MSWD = 5.3, enhanced errors). The line is not far from qualifying as an isochron and this could suggest a Late Proterozoic resetting within parts of these rocks. From a very homogeneous porphyritic granite of the central Gebel Kamil area which is in sharp contact to the surrounding gneisses and migmatites, four samples define a perfect isochron (MSWD = 0.3) yielding an Upper Ordovician/Lower Silurian age of 431 ± 33 Ma and an intercept ofO. 7097 ± 0.0007. This result indicates a crustal origin of this rock. Five samples from a subvolcanic rhyolitic rock of the Nusab el Balgum define a regression line yielding an age of 216 ± 5 Ma with an intercept of 0.7054 ± 0.0025 (MSWD = 4.1, enhanced errors).
A PROPOSED SCHEME FOR THE DEVELOPMENT OF THE UWEINAT UPLIFT Based on presently available geological, petrological and radiometric data, the following provisional geological history for the Uweinat Uplift is proposed. The authors are aware, that modifications may be necessary when more information about this still poorly known area becomes available. The oldest recognizable event in the Uweinat area is the Early Proterozoic (-2700 Ma) metamorphism to granulite facies of Archean rocks, which were deposited on a now unrecognizable floor (Klerkx and Deutsch 1977). A Middle Proterozoic metamorphic event is documented by the formation of migmatites (-1800 Ma) through anatexis within these basement gneisses. The migmatization was accompanied by the formation of anatectic granites and tonalites (Klerkx and Deutsch 1977). In the Late Proterozoic (Pan-African) the area between Hoggar and Red Sea was under compressive and tensional stress (Kroner 1979); strong intraplate block faulting was accompanied by the formation of locally occurring migmatites and voluminous granites by reworking of older rocks from the lower crust. The Late Proterozoic development in the Uweinat area is still uncertain. The migmatites of the Gebel Kamil area are possibly reworked rocks of Pre-Pan-African age during the Pan-African, as indicated by the geochronological results . Conclusive results will have to be postponed until more radiometric data are available. The stratigraphic position of the low grade metamorphic rocks of the NW Sudan outcrop area is very uncertain. It is possible that these rocks, originally deposited into shallow water basins, are infolded remnants of a higher crustal level into basement gneisses. Uplift and erosion resulted in the formation of a Late Proterozoic land surface and Palaeozoic sediments were deposited in the depressions at least from the Ordovician onwards (Uweinat area). The intrusion of Ordovician
anorogenic granitic complexes (Gebel Babein; Gebel Kamil area; Table 1) falls in a time span where, after a 'quasistatic' interval (Cambrian-Lower Ordovician) , a rapid polar wandering of Gondwanaland can be observed (Briden et al. 1973). Palaeozoic sedimentation continued in the Uweinat area and most likely east of Uweinat until the Permo-Triassic. Reactivation of older fault structures combined with uplift in the Gebel Kamil and Bir Safsaf areas removed the sedimentary cover and the eroded material was transported mainly northwards into the present-day Dakhla basin (Fig. 1). Basaltic dykes (Uweinat area, 235 Ma; Table 1) and rhyolitic subvolcanic rocks (Nusab el Balgum, 216 Ma; Table I) were intruded along fracture zones in the Upper Permian and the Lower Triassic, respectively. Following subsequently, the structure of the Dakhla basin permitted continuous continental accumulation of Nubian type sediments, beginning in the Late Jurassic/ Lower Cretaceous, whose basal parts are probably reworked Palaeozoic strata (Fay and Hermann-Degen in prep.). Intercalated into the continental Nubian arc several marine horizons and the whole sequence is overlain by Lower MaestrichtianlTertiary marine sediments (Klitzsch et al. 1979). From the Upper Cretaceous until the Quaternary intensive reactivation of older zones of crustal weakness gave way to a variety of intrusive and extrusive rocks like alkali granites, syenites, basalts, trachytes and phonolites. The peak of these magmatic activities occurred around 45 Ma (Table 1) in the Eocene. The present landform of the area is mainly a product of erosive and accumulative activities of predominantly N-S blowing winds and episodic rainfalls.
CONCLUSIONS Structural and petrographic investigations on metamorphic rocks from west of Gebel Kamil area have shown that these rocks might be correlated with the Archean/Early Proterozoic granulitic gneisses from the Uweinat area previously described by Klerkx (1980). The migmatites of the Gebel Kamil area are petrographically very similar to the Middle Proterozoic migmatites of Uweinat. Rb/Sr results from these rocks indicate that they are probably of Pre-Pan-African age but contrary to the migmatites of Uweinat, the Gebel Kamil migmatites show some Pan-African resetting effects. Lithologi cally and petrographically the migmatites of Bir Safsaf are also similar to previously mentioned migmatites but their age is still unknown. More radiometric dating will have to be carried out in order to establish the location of the eastern margin of the Archean-Early Proterozoic craton which must lie east of Gebel Uweinat. Acknoll'ledgements-This work was supported by the DFG (SFB 69. A IID~). We thank our colleagues for their help during the field work and GPC (Cairo) for their help with the organization. Critical reading of an earlier draft of the manuscript by G. Mortcani and an unknown reviewer is gratefully acknowledged .
Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf
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