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Plutonism in Pan-African belts and the geologic evolution of northeastern Africa
Earth and Planetary Science Letters, 39 (1978) 109-117 © Elsevier Sdentific Publishing Company, Amsterdam - Printed in The Netherlands
109
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PLUTONISM IN PAN-AFRICAN BELTS AND THE GEOLOGIC EVOLUTION OF NORTHEASTERN A F R I C A JOHN J.W. ROGERS 1, MOHAMED A. GHUMA 2, RICHARD M. NAGY a, J E F F R E Y K. GREENBERG 1 and PAUL D. F U L L A G A R 1 1 Department o f Geology, University o f North Carolina at Chapel Hill, Mitchell Hall 029A, Chapel Hill, NC 2 7514 (U.S.A.) 2 Department o f Geology, Faculty o f Science, University o f Al.Fateh, P.O. Box 656, Tripoli (Libyan Arab Socialist People's Jamah iria) 3 Department o f Geology, Rice University, Houston, TX 77001 (U.S.A.)
Received September 7, 1977 Revised version received December 6, 1977
Plutonic igneous rocks of Pan-African belts (500-600 m.y. age) in Africa can be described in terms of three types of assemblages: (1) migmatites formed largely by remobilization of pre-existing slalic crust during Pan-African time; (2) calcalkaline batholithic suites formed in association with subduction of oceanic crust; and (3) alkali-rich, posttectonic granites. Many suites cannot be placed precisely in one of these categories, either because of intermediate (gradational) characteristics or because of lack of adequate information. Distinction of rocks formed during PanAfrican time from older ones whose radiometric clocks were reset at that time is also difficult. The rock suites are very unevenly distributed geographically. Migmatites formed by ensialie crustal remobilization occur mostly in southern and central Africa. Calcalkaline suites occur in north Africa, particularly the northeast. Alkali-rich granites are virtually restricted to the northeastern portion of the continent, the one major part of Africa not occupied by an Archean shield. Evidence for subduction of oceanic crust is also restricted to northeastern Africa. The geologic history proposed for northeastern Africa is as follows: During, and immediately preceding, PanAfrican time an ocean basin of uncertain size and shape occurred between the West African and Nubian-Arabian shields. Subduction zones were active on the margins and within the basin. During, and at the end of, Pan-African time extensive development of alkali-richgranites was associated with cratonization of the basin. Paleozoie sediments were then deposited under platform conditions on the newly formed craton, which showed mild epeirogenlc activity throughout the Phanerozoic.
1. Introduction The Pan-African orogeny, or tectonic episode [ 1 5], is an igneous, metamorphic, and deformational event o f about 5 0 0 - 6 0 0 m.y. age that pervades much o f the African continent (Fig. 1). As shown in Fig. 1, the Pan-African belts in most o f Africa roughly surround older, mostly Archean, cratons. This geometric arrangement m a y be attributed to welding' o f the cratonic fragments together during Pan-African time or progressive remobilization o f an earlier, continent-wide, craton along the linear Pan-African belts. The latter interpretation places the present individual cratons simply as unaffected residuals.
Efforts to describe the Pan-African event in general terms, or to discuss it in terms o f m o d e m orogenic analogs, have met with little success. This lack o f success is probably related to the diversity o f processes that have characterized different portions o f the Pan-African orogeny, and it is the purpose o f this paper to discuss this diversity in terms o f the variety and significance o f the plutonic rocks associated with Pan-African terranes. This paper is based on several investigations b y the various writers. Ghuma has studied the field exposures in the Ben Ghnema area o f the northwestern Tibisti massif, and Ghuma and Rogers have investigated the petrology and geochemistry o f the rock suite. Rogers
110
\ I WAC
cc
KC
J,
Fig. 1. Outline map of Africa and part of the Arabian Peninsula showing features referred to in text. Cratons (largely Archean) are outlined by dashed lines: WAC = West African craton; CC = Congo craton; KC = Kalahari craton. Pan-African belts are shown by ruled pattern. Only the belts and areas referred to in the text are shown, rather than a complete map of the distribution of Pan-African age rocks. 1 = Rokelide belt; 2 = Dahomeyan belt; 3 = West Congo belt; 4 = Damaran belt; 5 = Katangan belt; 6 = Zambezian belt; 7 = Mozambique belt. Two areas of calcalkaline granites are shown by cross-hatched patterns: NOG = Nigerian Older Granites; SBG = Sudan Batholithic Granites. Precambrian areas of northeastern Africa are labeled: H = Hoggar; T = Tibisti; JO = Jebel Oweynat; N-AS = Nubian-Arabian shield. Approximate area of alkali granites (AG) in northeastern Africa is shown by dotted pattern. has visited the Damaran orogen o f South West Africa (Namibia) and has studied the uranium potential o f the intrusive rocks. Nagy and Greenberg have done field studies o f the Younger Granites of eastern Egypt, Rogers has visited the area, and all three have done work on the geochemistry and petrology of the Younger Granites. Fullagar has done Rb-Sr work on the Egyptian Younger Granites and the Ben Ghnema batholith.
2. Nature of the Pan-African Orogeny A number of broad and very significant observations have been made about the Pan-African orogeny.
Hurley [1,2] considers the Pan-African belts in Africa merely to be representatives of an even larger orogenic event (Pangeaic) that affected the entire supercontinent of Pangaea in the general age range of 650 to 250 m.y. ago. Clifford [3] regards the PanAfrican orogeny as the final major deformation that stabilized the African continent into a single craton. Kr6ner [4,5], conversely, regards the Pan-African deformation in the sense o f destruction of former cratons rather than as a constructional process. The reworking of earlier continental crust has been well documented in Pan-African belts in West Africa [6], the Hoggar [7], central Africa [8], southwestern Africa [9], and parts of eastern Africa [10,11]. This concept is based on several observations. One observation is the ability to correlate earlier Precambrian structures across the Pan-African belts (e.g. [ 12,13 ] ). Preservation o f old rocks and old ages in partially reset rocks has also been well documented [6,8]. Some paleomagnetic data also suggest a lack o f relative movement among the various cratons between Pan-African belts since before Pan-African time, thus demonstrating that these cratons cannot represent separate fragments welded together by the Pan-African orogeny [14,15]. Arguments for addition o f significant amounts o f new sialic material to the Pan-African belts during orogeny imply, but do not absolutely require, that there was relative motion among the various Archean cratons prior to Pan-African time. Thus, much of the argumentation about crustal reworking vs. crustal evolution centers around the question of the validity o f paleomagnetic and structural correlations across the belts. The concept of crustal addition is also supported by the primitive, mantle-derived nature o f the strontium isotopes in the late-stage granites that characterize many o f the belts [1,2]. Relative movement between opposite sides o f Pan-African belts can be interpreted in terms o f plate tectonic models of crustal suturing [16]. In addition to the question o f whether the sialic rocks of the Pan-African belts are " n e w " or "reworked", there is also the question o f the tectonic regime in which they were evolved. Various authors have described the Pan-African orogeny in terms of crustal extension, compressional remobilization, subduction of oceanic or continental crust, continental collision, etc. No general agreement has been reached,
Ill and it seems likely that different styles are characteristic of different areas.
3. Types of plutonic igneous rocks One method of characterizing the diversity of processes in Pan-African belts is by studying the igneous activity in them. For this purpose, plutonic igneous rocks of Pan-African belts can be roughly divided into three idealized categories: (1) migmatites; (2) calcalkaline batholithic suites; and (3) posttectonic, alkali granites. Properties and geographic distribution of these three assemblages are discussed below.
Migmatites. The example of migmatitic assemblages best known to the writers is the extensive G4 granites of the western part of the Damaran orogen [17]. Principal characteristics of these migmatites include: (1) association with high-grade (upper amphibolite facies) metamorphism; (2) restriction to local stratigraphic horizons within the metamorphic sequence that constitutes the wall rocks; (3) salic (alaskitic) character of most rocks, with few intermediate and virtually no mafic equivalents; (4) contact metamorphism of reactive rock types; and (5) cross-cutting relationships in some areas and concordance in others. The contact metamorphism and the presence of cross-cutting relationships indicate that the granites existed as melts injected into the wall rocks. The stratigraphic control, however, indicates that the melts were probably very locally derived. The materials remobilized to form these magmas are uncertain and may include pre-Damaran basement or early supracrustal sediments of the orogen [4,5,9, 17,18]. Crystallization ages of the granites are PanAfrican, and the initial 87Sr/a6Sr ratio of one granite is high (0.76) [19]. The implication of all of this information is that, in the Damaran belt, the PanAfrican orogeny generated granitic melts by remobilization of pre-existing crustal materials. The existence of magmas and the formation of new rocks during Pan-African time is important in view of the tendency of Pan-African orogenic activity merely to reset the clocks of older rocks without otherwise recognizably affecting their textures, struc-
tures, or compositions. This resetting has given PanAfrican ages to a wide variety of rock types, including migmatites and related materials that look very similar to the remobilized granites but actually formed much earlier, perhaps in the Archean [4,7,11, 20,21,22]. It is clear that newly created migmatite and reset older migmatite have not always been distinguished successfully in Pan-African belts. Remobilization of crustal materials, whether basement or supracrustal sedimentary rocks, is an important concept with respect to the possibility of crustal accretion during the Pan-African orogeny. Obviously, if much of the plutonism accompanying the orogeny resulted from mobilization of preexisting crustal material, then the concept that the orogeny is an ensialic reworking of earlier cratons is greatly strengthened. The question, therefore, is the extent to which these granites occur in Pan-African belts. Migmatitic, possibly remobilized, granites are widely reported from Pan-African belts from western, central, and southern Africa [4,5]. There have been hundreds of descriptions of local areas, and reference here will be made only to summaries and the more readily available literature. Among the more important occurrences are: the gneisses and migmatites of the Mozambique belt, particularly in the southern portion and in somewhat more scattered occurrences under sedimentary cover to the north [11,20,23] ; the Damaran belt [9,17,18 ] ; the Rokelide belt, whe re considerable Sr isotopic work has been done [22] ; the Dahomeyan belt [6,24,25] and parts of western Nigeria [26,27] ; and the Suggarian of the Hoggar [7]. In belts such as the West Congo and Katangan and parts of the Mozambique, Zambezian, and Damaran, migmatitic rocks may be presumed to be present at depth beneath extensive supracrustal cover [4,13]. It is particularly important to note that Pan-African-age migmatites are not reported extensively in the northern portion of the Eastern Desert of Egypt, although some may be present in areas generally mapped as "gneiss" in southern Egypt [28] and Sudan [29]. It would be a mistake to conclude that all of the migmatites mentioned above have resulted solely from remobilization of crustal materials. For example, there is complete gradation from migmatitic materials to massive, discordant igneous bodies in the
112 Pan-African of Nigeria [25]; some of these rocks are among those that contain low initial Sr isotope ratios, thus presumably indicating mantle derivation [1,2,24], but some have relatively high ratios [27]. At the present time, not enough geologic and isotopic information is available on individual migmatite occurrences to distinguish the relative abundances of crustal-/and mantle-derived migmatites throughout the Pan-African orogenic areas. Calcalkaline batholithic suites. One example of a calcalkaline Pan-African batholith that has been studied is the Ben Ghnema batholith of the northwestern Tibisti massif [30,31 ]. Its principal characteristics are: (1) typical calcalkaline trends on modal, normative, and chemical variation diagrams (very similar to such classic assemblages as the Sierra Nevada and Southern California batholiths of California); (2) typically complex batholith in the sense that it seems to consist of a number of smaller intrusive bodies; (3) discordant and massive; (4) dimensions of about 120 km north-south and 40 km east-west, with a lateral variation from silicic, granitic rock in the west to granodioritic and tonalitic rock in the east; (5) small bodies of gabbm distributed throughout the batholith that are of uncertain relationship to major batholithic phases; (6) a broad area (southern part of the batholith) that is highly pegmatized; and (7) a general age of crystallization of 550 m.y. by both Rb-Sr and K-Ar methods and an initial Sr isotope ratio of 0.705-0.706 [30,32]. This set of characteristics, particularly the analogy with the Sierra Nevada batholith, has led to the conclusion that the Ben Ghnema batholith was formed in association with the subduction of oceanic crust beneath the eastern margin of the West African craton during Pan-African time (Fig. 2) [30,32]. The batholithic magma was apparently derived from mantle or oceanic crustal material. The significance of this subduction zone for the general evolution of northeastern Africa is discussed in the final section of this paper. Calcalkaline plutonic rocks have been described from other Pan-African belts, although many of the identifications are based solely on petrographic features, and a number of localities are not well dated. The Batholithic Granite of northern Sudan [33] is apparently calcalkaline, but it does not show
the lateral compositional variation shown by the Ben Ghnema batholith; furthermore, the Batholithic Granite of northern Sudan may be older than PanAfrican, perhaps correlated with similar Older Granites of Egypt. Some rocks in the At Taif area of western Saudi Arabia are a calcalkaline assemblage associated with Pan-African subduction activity [34]. Calcalkaline assemblages in the southwestern Arabian shield have ages that may extend into the Pan-African, although they are generally older [35]. The Older Granites of Nigeria are of Pan-African age, and many are described as essentially calcalkaline [6,24,25,36,37]. They apparently have correlatives throughout much of the area of Pan-African rocks between the West African and Congo cratons. The rocks occur in a variety of intrusive and tectonic styles, ranging from relatively massive batholiths to migmatitic zones, and it is clear that part of the assemblage has been formed by crustal remobilization [27]. Comparatively small bodies of a generally calcalkaline petrography (and some chemistry on a few bodies) occur fairly broadly in other Pan-African belts, including the Damaran orogen [3,17], the Mozambique-Zambezi area of Malawi [38], and scattered areas in the Mozambique belt [5,10,11 ]. The calcalkaline assemblages, however, are not very abundant in any of these areas. As discussed above, the Ben Ghnema batholith is considered to have been associated with Pan-African subduction of an oceanic plate under an extended West African craton. Similar Pan-African (and older) subduction of oceanic material has been proposed under the western margin of the Arabian craton [35]. The amount of subduction activity in other PanAfrican belts is not known. Extensive subduction (plate-tectonics style of orogeny) has been proposed in Pan-African belts, particularly around the West African craton [16,39]. Some geologists, however, have argued that the Pan-African orogeny was largely ensialic and destructive of pre-existing cratons and have questioned whether subduction processes were important [4,5]. Unfortunately, not enough work has been done on most Pan-African calcalkaline assemb/ages, some of which are poorly exposed, to permit drawing precise conclusions about the location, age, and direction of ancient subduction zones.
113
Post-tectonic alkali granites. The typical example of post-tectonic alkali granites is the Younger Granites of Egypt. The compositions and ages of several of the plutons are shown in Table 1. The dates shown in Table 1, plus a few previously published ones [40, 41], indicate that these granites form a reasonably coherent group with a Pan-African age of develop. ment. Their principal characteristics are: (1) massive, discordant bodies; (2) most bodies rich in K, and all bodies rich in total alkali; (3) most differentiated varieties rich in incompatible (or LIL) elements; (4) clusters of separate plutons; (5) associated contact
TABLE 1 Compositions and isotopic data for Younger Granite plutons of Eastern Desert, Egypt (oxides in %; trace elements in ppm) Raba E1 Garrah * SiO2 TiO2 A1203 Fe203 ***
MgO CaO Na20 K20
76 0.1 12.5 1.0 0.03 0.53 4.0 4.7
Fawakhir **
72 0.4 14.0 2.5 0.58 1.82 4.4 3.8
Average of 20 plutons 75 0.1 12.7 1.0 0.06 0.55 4.1 4.5
Rb Ba Sr Y Zr Nb Th U
214 106 21 72 115 35 22 9
112 750 287 21 171 14
140 320 60 50 180 25
Initial 87Sr/86Sr Age (m.y.)
0.7020 580
0.7025 588
0.7029 f 580607 t t
* Raba El Garrah is one of the more felsic of the Younger Granite plutons. ** Fawakhir is one of the more mafic of the Younger Granite plutons. *** Fe203 is total iron calculated as Fe20 a. t Average of five bodies, including Raba El Garrah and Fawakhir. t t Range of five bodies, including Raba El Garrah and Fawakhir; ages were calculated using a decay constant of 1.39 X 10-11 yr-I for Rb.
metamorphism but generally an absence ofpegmatites (thus presumably emplaced at a shallow level); and (6) very minor associated intermediate and mafic rock types. In addition to their extensive development in the Eastern Desert of Egypt, similar alkali-rich granites have been reported widely throughout the northern, particularly the northeastern, portions of Africa. Other major outcrop areas include: the Tibisti area east of the Ben Ghnema batholith [42] ; the Hoggar, particularly the eastern portion [7,43,44] ; northern Sudan, where they are also referred to as Younger Granites [29,33] ; and some of the Pan-African plutons of the western portion of the Arabian shield [35,45]. The principal area of occurrence of the alkali granites is shown in Fig. 1. Outside of this area, comparable alkali-rich granites formed in Pan-African time are very scarce. The Pan-African Older Granites of Nigeria do not contain correlatives. The minor Horebis Granites of the Damaran orogen [ 17] are similar but quantitatively unimportant. Alkali-rich plutons that may extend to as great an age as PanAfrican have been described in the MozambiqueZambezi area of Malawi [38], but these rocks are nepheline-normative, quite unlike the Younger Granites of Egypt and their other north African correlatives. In short, the high-level, alkali granites of Pan-African age are virtually restricted to the area of northeastern Africa shown in Fig. 1. The source and origin of the Younger Granites and their correlatives is a major problem. New Sr isotopic data (Table 1) and previously published data [35,40, 41,46] indicate initial 87Sr/a6Sr ratios in the range of 0.702-0.706. These ratios presumably result from derivation of the melts from mantle, oceanic crust, or a sialic crust of low Rb]Sr ratio; if the source had been sial of high Rb/Sr ratio, it could not have been formed more than a short time before the melting that produced the Younger Granites. The possibility of mantle derivation is somewhat supported by some thorium and uranium data on one pluton (Table 1) from the Eastern Desert of Egypt; the Th/U ratio of the pluton is about 2.5, which is similar to that of mantle-derived basalts and much lower than the normal 4 - 6 characteristic of most differentiated granites. Other specific petrologic evidence for the source of the magmas is apparently lacking.
114 Geographic distribution. In summary, the plutonic
rock types o f Pan-African age described above are not uniformly distributed. Migmatites are abundant in southern, central, and western Africa but not in the northeast. Conversely, alkali-rich granites are essentiaUy restricted to northeastern Africa. Calcalkaline suites are also found primarily in the northeast. This distribution leads to the conclusion, discussed below, that northeastern Africa had a markedly different history from the rest o f the continent.
4. Pan-African evolution of northeastern Africa The concentration o f Pan-African-age alkali granites in northeastern Africa is interesting in view o f several features o f the geology o f the area: (1) Northeastern Africa is the only part o f Africa o f significant size that is not obviously occupied by an Archean shield. The nature o f the rocks older than Pan-African in the area has not been well determined. Summaries o f the Tibistis [30], Egypt [47], and Sudan [29] indicate that most of the older rocks are thick sequences of metavolcanic and metasedimentary rocks, calcalkaline plutonic assemblages, and "gneisses" o f uncertain origin. The possibility o f an Archean (or at least Early Protorozoic) craton in the area has been discussed and/or assumed in several papers (e.g. [4,5,48,49]). The only indications o f such early formation o f sialic crust, however, are dates o f two gneisses from Jebel Oweynat [49]. The dates are Rb-Sr model ages on a microcline and a whole rock, but data are not available to evaluate the dates, and further substantiation is necessary before they are accepted. Furthermore, a small area o f Archean rock may be only an isolated block rather than a portion o f a much larger craton. It is possible that an Archean shield is present at depth and simply covered b y later deposits; in fact, all three o f the major African cratons (Kalahari, Congo, and West African) do contain shallow Phanerozoic cover over their central portions. Nevertheless, it seems unlikely that if Archean rocks exist at depth, they do not crop out fairly extensively within an area o f several million square kilometers, particularly where that area has been affected by relatively recent rifting and block faulting. (2) As discussed above, only in northeastern Africa
is there significant evidence for the production o f calcalkaline batholithic suites by subduction o f oceanic lithosphere during Pan-African time. At this 750-
550
550
- 500
M.¥
M Y
~3 500
/4
M. Y. - P r e s e n t
Fig. 2. Diagrammatic illustration of proposed evolution of northeastern Africa. 750-550 m.y. Ocean basin between the West African craton (WAC) and the Nubian-Arabian shield (N-AS), with subduction on the margins. Subduction is also proposed in the center of the basin beneath island arcs, continental fragments, etc. The West African craton shown here includes the (generally Archean) craton mapped in Fig. 1 plus the Hoggar and other areas to the east that probably underwent ensialic reactivation in Pan-African time [7]. Thus, the Ben Ghnema batholith of the northwestern Tibistis could have been formed along the eastern margin of the craton [30,31]. 550-500 m.y. Development of stable (cratonic) area in northeastern Africa. Former ocean basin is filled by: 1 = margins and fragments of older cratons; 2 = deformed oceanic, arc, and continental-margin sediments; 3 = relatively young (molasse) sediments formed on the partially stabilized crust; 4 = alkali granites intruded into the former basin and the craton margins and closely associated with crustal stabilization. 500 m.y.-present. Mildly unstable cratonic area characterized by shelf sedimentation, minor block faulting, and limited (alkaline) magmatism. The major components are: 1 = sialic crust consisting of admixture of older continental fragments, oceanic and molasse sediments, and alkali granites; 2 = Phanerozoic basin sediments.
115 time, subduction apparently occurred under the eastern margin of the West African craton (in the northwestern Tibistis) and under the western margin of the Arabian craton (Fig. 2). Other subduction zones may have been present within the area delineated by these two zones (e.g., in the area of the Batholithic Granite of northern Sudan). (3) Pan-African time ended in widespread deposition of platform sediments throughout much of northeastern Africa; see summaries by Said [47], Goudarzi [50]. Nevertheless, northeastern Africa has probably been less tectonically stable than any other area of comparable size in Africa during Phanerozoic time. The area contains a region of structural "softness" in the Cretaceous [51 ], and a transcontinental suture and lineament virtually bisects the area [52]. The general concept that emerges from these observations is that cratonization of northeastern Africa occurred during Pan-African time (Fig. 2). Prior to that time, the area consisted of an ocean basin partly occupied by island arcs, continental fragments or nuclei, etc.; the size of the basin is unknown because of uncertainties in the relative movements of the West African and Arabian cratons during Pan-African time. Subduction on the margins of this basin, and possibly locally within it, probably continued until a widespread development of alkali granites occurred about 500 m.y. ago. Some of these granites were developed within the cratons themselves around the margins of the ocean basin (e.g., the Hoggar). The formation of the alkali granites either caused, or was associated with, crustal stabilization and the onset of platform sedimentation in the Early Paleozoic. The recency of cratonization (stabilization) in this part of Africa, as opposed to areas containing older shields, may be reflected in the mild, generally epeirogenic, Phanerozoic activity in northeastern Africa. The writers realize that this proposal of cratonization of a significant portion of Africa about 500 m.y. ago requires massive segregation of sialic material from the mantle at this time. Because of lack of sufficient information on the complete geologic history of the area, no specific mechanism is proposed here. Many geologists feel that segregation of sialic material from the interior of the earth could have taken place only during the early stages of earth history, possibly culminating at the end of the
Archean. The possibility of continued segregation is, thus, very controversial, and a complete discussion of the problems involved is beyond the scope of this paper. Despite these questions and difficulties, the writers consider cratonization during Late Proterozoic and Early Paleozoic time to be the best explanation for a significant part of the known geologic history of northeastern Africa.
Acknowledgements Financial support for different portions of this work was provided by: U.S. Energy Research and Development Administration (Department of Energy) contract E (05-1)-1661, administered by Bendix Field Engineering Corporation (to the University of North Carolina at Chapel Hill); the University of A1Fateh and the Government of Libya; and U.S. National Science Foundation grant OIP75-07943 (to the University of South Carolina), consisting of a Special Foreign Currency grant from the Division of International Programs and a dollar grant from the Division of Earth Sciences. The writers wish to acknowledge the assistance of a great many people in several countries, including: Dean Y. Meherric (Tripoli); Dr. M. Wanies (Tripoli); Mr. Rift (Sebha); Dr. E. E1 Shazly (Cairo); Dr. I. E1 Kassas (Cairo); Dr. R. Jacob (Grahamstown); and Mr. P. Woodhouse (Swakopmund).
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23 R.L. Johnson and J.R. Vail, The junction between the Mozambique and Zambezi orogenic belts, north-east Southern Rhodesia, Geol. Mag. 102 (1965) 489. 24 N.K. Grant, The Late Precambrian to Early Paleozoic Pan-African orogeny in Ghana, Togo, Dahomey, and Nigeria, Geol. Soc. Am. Bull. 80 (1969) 45. 25 P. McCurry and J.B. Wright, On place and time in orogenic granite plutonism, Geol. Soc. Am. Bull. 82 (1971) 1713. 26 M.O. Oyawoye, The basement complex of Nigeria, in: Conference on African Geology (Ibadan), T.F.J. Dessauvagie and A.J. Whiteman, eds. (University of Ibadan, Ibadan, 1972), 67. 27 O. van Breemen, R.T. Pidgeon and P. Bowden, Age and isotopic studies of some Pan-African granites from northcentral Nigeria, Precambrian Res. 4 (1977) 307. 28 M.F. El Ramly, Geological Map of the Basement Rocks in Eastern Egypt (Geological Survey of Egypt, Cairo, 1972). 29 A.J. Whiteman, The Geology of the Sudan Republic (Clarendon Press, Oxford, 1971) 290 pp. 30 M.A. Ghuma, The Geology and Geochemistry of the Ben Ghnema batholith, Tibisfi Massif, Southern Libya, Ph.D. Thesis, Rice University, Houston, Texas (1976) 185 pp. 31 M.A. Ghuma and J.J.W. Rogers, 500-600 m.y. subduction in the northwestern Tibisti massif, southern Libya, L.A.R., EOS, Trans. Am. Geophys. Union 57 (1976) 1019. 32 W.J. Pegram, J.K. Register, Jr., P.D. Fuliagar, M.A. Ghuma and J.J.W. Rogers, Pan-African ages from a Tibesti massif batholith, southern Libya, Earth Planet. Sci. Lett. 30 (1976) 123. 33 C.R. Neary, I.G. Gass and B.J. Cavanagh, Granitic association of northeastern Sudan, Geol. Soc. Am. Bull. 87 (1976) 1501. 34 F. MarzoukiandW.S. Fyfe, Pan-African plates: additional evidence from igneous events in Saudi Arabia, Contrib. Mineral Petrol. 60 (1977) 219. 35 W.R. Greenwood, D.G. Hadley, R.E. Anderson, R.J. Fleck and D.L. Schmidt, Late Proterozoic cratonization in southwestern Saudi Arabia, Philos. Trans. R. Soc. Lond., Ser. A, 280 (1976) 517. 36 R.R.E. Jacobson, N.J. SneUing and J.F. Truswell, Age determinations in the geology of Nigeria, with special reference to the Older and Younger Granites, Overseas Geol. Miner. Resour. (London) 9 (1963) 168. 37 P. McCurry, Geology of Degree Sheet 21, Zaria, Nigeria, Overseas Geol. Miner. Resour. (London) 45 (1973) 45 PP. 38 K. Bloomfield, Orogenic and post-orogenic plutonism in Malawi, in: African Magmatism and Tectonics, T.N. Clifford and I.G. Gass, eds. (Oliver and Boyd, Edinburgh, 1970) 119. 39 K. Burke and J.F. Dewey, An outline of Precambrian plate development, in: Implications of Continental Drift to the Earth Sciences, 2, D.H. Tarring and S.K. Runcorn, eds. (Academic Press, London, 1973) 1035. 40 A.H. Hashad, T.A. Sayyah, S.B. El Kholy and A. Youssef,
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Rb-Sr isotopic age determination of some basement Egyptian g~anites, Egypt, J. Geol. 16 (1972) 269. E.M. E1 Shazly, A.H. Hashad, T.A. Sayyah and F.A. Bassyuni, Geochronology of Abu Swayel area, South Eastern Desert, Egypt, J. Geol. 17 1973) 1. M. Vachette, Ages radiom6tdques des formations cdstallines d'Afrique equatoriale (Gabon, R6publique Centrafficaine, Tchad, Moyen Congo), Ann. Fac. Sci. Clermont-Ferrand 25, S6r. G~ol. 8, Etudes Geochronol. 1 (1964) 31 pp. G. Ferrara and M. Gravelle, Radiometric ages from western Ahaggar (Sahara) suggesting an eastern limit for the West African craton, Earth Planet. Sci. Lett. 1 (1966) 319. J. Boissonnas, Nouvelles pr6cisions sur les granites posttectoniques de la region de Tamanrasset (Sahara central, Algeria), C.R. Acad. Sci. Paris Ser. D (1969) 1909. R.J. Fleck, R.G. Coleman, H.R. Cornwall, W.R. Greenwood, D.G. Hadley, D.L. Schmidt, W.C. Prinz and J.C. Ratte, Geochronology of the Arabian shield, western
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Saudi Arabia: K/Ar results, Geol. Soc. Am. Bull. 87 (1976) 9. B.J. Cavanagh, An initial report on Rb/Sr geochronology of the Dunganab area of the Republic of Sudan, 18th Annu. Rep. Res. Inst. Afr. Geol., Leeds Univ. (1974) 10. R. Said, The Geology of Egypt (Elsevier, Amsterdam, 1962) 377 pp. LL. Mestraud, Afrique centrale, in: Tectonique de l'Afrique (UNESCO, Paris, 1971) 461. H.M.E. Sch~rmann, The Precambrian in North Africa (E.J. Brill, Leiden, 1974) 352 pp. G.H. Goudarzi, Geology and mineral resources of Libya - a reconnaissance, U.S. Geol. Surv. Prof. Paper 660 (1970) 104 pp. K. Burke and J.F. Dewey, Two plates in Africa during the Cretaceous?, Nature 249 (1974) 313. R.M. Nagy, M.A. Ghuma and J.J.M. Rogers, A crustal suture and lineament in North Africa, Tectonophysics 31 (1976) T67.
109
[6]
PLUTONISM IN PAN-AFRICAN BELTS AND THE GEOLOGIC EVOLUTION OF NORTHEASTERN A F R I C A JOHN J.W. ROGERS 1, MOHAMED A. GHUMA 2, RICHARD M. NAGY a, J E F F R E Y K. GREENBERG 1 and PAUL D. F U L L A G A R 1 1 Department o f Geology, University o f North Carolina at Chapel Hill, Mitchell Hall 029A, Chapel Hill, NC 2 7514 (U.S.A.) 2 Department o f Geology, Faculty o f Science, University o f Al.Fateh, P.O. Box 656, Tripoli (Libyan Arab Socialist People's Jamah iria) 3 Department o f Geology, Rice University, Houston, TX 77001 (U.S.A.)
Received September 7, 1977 Revised version received December 6, 1977
Plutonic igneous rocks of Pan-African belts (500-600 m.y. age) in Africa can be described in terms of three types of assemblages: (1) migmatites formed largely by remobilization of pre-existing slalic crust during Pan-African time; (2) calcalkaline batholithic suites formed in association with subduction of oceanic crust; and (3) alkali-rich, posttectonic granites. Many suites cannot be placed precisely in one of these categories, either because of intermediate (gradational) characteristics or because of lack of adequate information. Distinction of rocks formed during PanAfrican time from older ones whose radiometric clocks were reset at that time is also difficult. The rock suites are very unevenly distributed geographically. Migmatites formed by ensialie crustal remobilization occur mostly in southern and central Africa. Calcalkaline suites occur in north Africa, particularly the northeast. Alkali-rich granites are virtually restricted to the northeastern portion of the continent, the one major part of Africa not occupied by an Archean shield. Evidence for subduction of oceanic crust is also restricted to northeastern Africa. The geologic history proposed for northeastern Africa is as follows: During, and immediately preceding, PanAfrican time an ocean basin of uncertain size and shape occurred between the West African and Nubian-Arabian shields. Subduction zones were active on the margins and within the basin. During, and at the end of, Pan-African time extensive development of alkali-richgranites was associated with cratonization of the basin. Paleozoie sediments were then deposited under platform conditions on the newly formed craton, which showed mild epeirogenlc activity throughout the Phanerozoic.
1. Introduction The Pan-African orogeny, or tectonic episode [ 1 5], is an igneous, metamorphic, and deformational event o f about 5 0 0 - 6 0 0 m.y. age that pervades much o f the African continent (Fig. 1). As shown in Fig. 1, the Pan-African belts in most o f Africa roughly surround older, mostly Archean, cratons. This geometric arrangement m a y be attributed to welding' o f the cratonic fragments together during Pan-African time or progressive remobilization o f an earlier, continent-wide, craton along the linear Pan-African belts. The latter interpretation places the present individual cratons simply as unaffected residuals.
Efforts to describe the Pan-African event in general terms, or to discuss it in terms o f m o d e m orogenic analogs, have met with little success. This lack o f success is probably related to the diversity o f processes that have characterized different portions o f the Pan-African orogeny, and it is the purpose o f this paper to discuss this diversity in terms o f the variety and significance o f the plutonic rocks associated with Pan-African terranes. This paper is based on several investigations b y the various writers. Ghuma has studied the field exposures in the Ben Ghnema area o f the northwestern Tibisti massif, and Ghuma and Rogers have investigated the petrology and geochemistry o f the rock suite. Rogers
110
\ I WAC
cc
KC
J,
Fig. 1. Outline map of Africa and part of the Arabian Peninsula showing features referred to in text. Cratons (largely Archean) are outlined by dashed lines: WAC = West African craton; CC = Congo craton; KC = Kalahari craton. Pan-African belts are shown by ruled pattern. Only the belts and areas referred to in the text are shown, rather than a complete map of the distribution of Pan-African age rocks. 1 = Rokelide belt; 2 = Dahomeyan belt; 3 = West Congo belt; 4 = Damaran belt; 5 = Katangan belt; 6 = Zambezian belt; 7 = Mozambique belt. Two areas of calcalkaline granites are shown by cross-hatched patterns: NOG = Nigerian Older Granites; SBG = Sudan Batholithic Granites. Precambrian areas of northeastern Africa are labeled: H = Hoggar; T = Tibisti; JO = Jebel Oweynat; N-AS = Nubian-Arabian shield. Approximate area of alkali granites (AG) in northeastern Africa is shown by dotted pattern. has visited the Damaran orogen o f South West Africa (Namibia) and has studied the uranium potential o f the intrusive rocks. Nagy and Greenberg have done field studies o f the Younger Granites of eastern Egypt, Rogers has visited the area, and all three have done work on the geochemistry and petrology of the Younger Granites. Fullagar has done Rb-Sr work on the Egyptian Younger Granites and the Ben Ghnema batholith.
2. Nature of the Pan-African Orogeny A number of broad and very significant observations have been made about the Pan-African orogeny.
Hurley [1,2] considers the Pan-African belts in Africa merely to be representatives of an even larger orogenic event (Pangeaic) that affected the entire supercontinent of Pangaea in the general age range of 650 to 250 m.y. ago. Clifford [3] regards the PanAfrican orogeny as the final major deformation that stabilized the African continent into a single craton. Kr6ner [4,5], conversely, regards the Pan-African deformation in the sense o f destruction of former cratons rather than as a constructional process. The reworking of earlier continental crust has been well documented in Pan-African belts in West Africa [6], the Hoggar [7], central Africa [8], southwestern Africa [9], and parts of eastern Africa [10,11]. This concept is based on several observations. One observation is the ability to correlate earlier Precambrian structures across the Pan-African belts (e.g. [ 12,13 ] ). Preservation o f old rocks and old ages in partially reset rocks has also been well documented [6,8]. Some paleomagnetic data also suggest a lack o f relative movement among the various cratons between Pan-African belts since before Pan-African time, thus demonstrating that these cratons cannot represent separate fragments welded together by the Pan-African orogeny [14,15]. Arguments for addition o f significant amounts o f new sialic material to the Pan-African belts during orogeny imply, but do not absolutely require, that there was relative motion among the various Archean cratons prior to Pan-African time. Thus, much of the argumentation about crustal reworking vs. crustal evolution centers around the question of the validity o f paleomagnetic and structural correlations across the belts. The concept of crustal addition is also supported by the primitive, mantle-derived nature o f the strontium isotopes in the late-stage granites that characterize many o f the belts [1,2]. Relative movement between opposite sides o f Pan-African belts can be interpreted in terms o f plate tectonic models of crustal suturing [16]. In addition to the question o f whether the sialic rocks of the Pan-African belts are " n e w " or "reworked", there is also the question o f the tectonic regime in which they were evolved. Various authors have described the Pan-African orogeny in terms of crustal extension, compressional remobilization, subduction of oceanic or continental crust, continental collision, etc. No general agreement has been reached,
Ill and it seems likely that different styles are characteristic of different areas.
3. Types of plutonic igneous rocks One method of characterizing the diversity of processes in Pan-African belts is by studying the igneous activity in them. For this purpose, plutonic igneous rocks of Pan-African belts can be roughly divided into three idealized categories: (1) migmatites; (2) calcalkaline batholithic suites; and (3) posttectonic, alkali granites. Properties and geographic distribution of these three assemblages are discussed below.
Migmatites. The example of migmatitic assemblages best known to the writers is the extensive G4 granites of the western part of the Damaran orogen [17]. Principal characteristics of these migmatites include: (1) association with high-grade (upper amphibolite facies) metamorphism; (2) restriction to local stratigraphic horizons within the metamorphic sequence that constitutes the wall rocks; (3) salic (alaskitic) character of most rocks, with few intermediate and virtually no mafic equivalents; (4) contact metamorphism of reactive rock types; and (5) cross-cutting relationships in some areas and concordance in others. The contact metamorphism and the presence of cross-cutting relationships indicate that the granites existed as melts injected into the wall rocks. The stratigraphic control, however, indicates that the melts were probably very locally derived. The materials remobilized to form these magmas are uncertain and may include pre-Damaran basement or early supracrustal sediments of the orogen [4,5,9, 17,18]. Crystallization ages of the granites are PanAfrican, and the initial 87Sr/a6Sr ratio of one granite is high (0.76) [19]. The implication of all of this information is that, in the Damaran belt, the PanAfrican orogeny generated granitic melts by remobilization of pre-existing crustal materials. The existence of magmas and the formation of new rocks during Pan-African time is important in view of the tendency of Pan-African orogenic activity merely to reset the clocks of older rocks without otherwise recognizably affecting their textures, struc-
tures, or compositions. This resetting has given PanAfrican ages to a wide variety of rock types, including migmatites and related materials that look very similar to the remobilized granites but actually formed much earlier, perhaps in the Archean [4,7,11, 20,21,22]. It is clear that newly created migmatite and reset older migmatite have not always been distinguished successfully in Pan-African belts. Remobilization of crustal materials, whether basement or supracrustal sedimentary rocks, is an important concept with respect to the possibility of crustal accretion during the Pan-African orogeny. Obviously, if much of the plutonism accompanying the orogeny resulted from mobilization of preexisting crustal material, then the concept that the orogeny is an ensialic reworking of earlier cratons is greatly strengthened. The question, therefore, is the extent to which these granites occur in Pan-African belts. Migmatitic, possibly remobilized, granites are widely reported from Pan-African belts from western, central, and southern Africa [4,5]. There have been hundreds of descriptions of local areas, and reference here will be made only to summaries and the more readily available literature. Among the more important occurrences are: the gneisses and migmatites of the Mozambique belt, particularly in the southern portion and in somewhat more scattered occurrences under sedimentary cover to the north [11,20,23] ; the Damaran belt [9,17,18 ] ; the Rokelide belt, whe re considerable Sr isotopic work has been done [22] ; the Dahomeyan belt [6,24,25] and parts of western Nigeria [26,27] ; and the Suggarian of the Hoggar [7]. In belts such as the West Congo and Katangan and parts of the Mozambique, Zambezian, and Damaran, migmatitic rocks may be presumed to be present at depth beneath extensive supracrustal cover [4,13]. It is particularly important to note that Pan-African-age migmatites are not reported extensively in the northern portion of the Eastern Desert of Egypt, although some may be present in areas generally mapped as "gneiss" in southern Egypt [28] and Sudan [29]. It would be a mistake to conclude that all of the migmatites mentioned above have resulted solely from remobilization of crustal materials. For example, there is complete gradation from migmatitic materials to massive, discordant igneous bodies in the
112 Pan-African of Nigeria [25]; some of these rocks are among those that contain low initial Sr isotope ratios, thus presumably indicating mantle derivation [1,2,24], but some have relatively high ratios [27]. At the present time, not enough geologic and isotopic information is available on individual migmatite occurrences to distinguish the relative abundances of crustal-/and mantle-derived migmatites throughout the Pan-African orogenic areas. Calcalkaline batholithic suites. One example of a calcalkaline Pan-African batholith that has been studied is the Ben Ghnema batholith of the northwestern Tibisti massif [30,31 ]. Its principal characteristics are: (1) typical calcalkaline trends on modal, normative, and chemical variation diagrams (very similar to such classic assemblages as the Sierra Nevada and Southern California batholiths of California); (2) typically complex batholith in the sense that it seems to consist of a number of smaller intrusive bodies; (3) discordant and massive; (4) dimensions of about 120 km north-south and 40 km east-west, with a lateral variation from silicic, granitic rock in the west to granodioritic and tonalitic rock in the east; (5) small bodies of gabbm distributed throughout the batholith that are of uncertain relationship to major batholithic phases; (6) a broad area (southern part of the batholith) that is highly pegmatized; and (7) a general age of crystallization of 550 m.y. by both Rb-Sr and K-Ar methods and an initial Sr isotope ratio of 0.705-0.706 [30,32]. This set of characteristics, particularly the analogy with the Sierra Nevada batholith, has led to the conclusion that the Ben Ghnema batholith was formed in association with the subduction of oceanic crust beneath the eastern margin of the West African craton during Pan-African time (Fig. 2) [30,32]. The batholithic magma was apparently derived from mantle or oceanic crustal material. The significance of this subduction zone for the general evolution of northeastern Africa is discussed in the final section of this paper. Calcalkaline plutonic rocks have been described from other Pan-African belts, although many of the identifications are based solely on petrographic features, and a number of localities are not well dated. The Batholithic Granite of northern Sudan [33] is apparently calcalkaline, but it does not show
the lateral compositional variation shown by the Ben Ghnema batholith; furthermore, the Batholithic Granite of northern Sudan may be older than PanAfrican, perhaps correlated with similar Older Granites of Egypt. Some rocks in the At Taif area of western Saudi Arabia are a calcalkaline assemblage associated with Pan-African subduction activity [34]. Calcalkaline assemblages in the southwestern Arabian shield have ages that may extend into the Pan-African, although they are generally older [35]. The Older Granites of Nigeria are of Pan-African age, and many are described as essentially calcalkaline [6,24,25,36,37]. They apparently have correlatives throughout much of the area of Pan-African rocks between the West African and Congo cratons. The rocks occur in a variety of intrusive and tectonic styles, ranging from relatively massive batholiths to migmatitic zones, and it is clear that part of the assemblage has been formed by crustal remobilization [27]. Comparatively small bodies of a generally calcalkaline petrography (and some chemistry on a few bodies) occur fairly broadly in other Pan-African belts, including the Damaran orogen [3,17], the Mozambique-Zambezi area of Malawi [38], and scattered areas in the Mozambique belt [5,10,11 ]. The calcalkaline assemblages, however, are not very abundant in any of these areas. As discussed above, the Ben Ghnema batholith is considered to have been associated with Pan-African subduction of an oceanic plate under an extended West African craton. Similar Pan-African (and older) subduction of oceanic material has been proposed under the western margin of the Arabian craton [35]. The amount of subduction activity in other PanAfrican belts is not known. Extensive subduction (plate-tectonics style of orogeny) has been proposed in Pan-African belts, particularly around the West African craton [16,39]. Some geologists, however, have argued that the Pan-African orogeny was largely ensialic and destructive of pre-existing cratons and have questioned whether subduction processes were important [4,5]. Unfortunately, not enough work has been done on most Pan-African calcalkaline assemb/ages, some of which are poorly exposed, to permit drawing precise conclusions about the location, age, and direction of ancient subduction zones.
113
Post-tectonic alkali granites. The typical example of post-tectonic alkali granites is the Younger Granites of Egypt. The compositions and ages of several of the plutons are shown in Table 1. The dates shown in Table 1, plus a few previously published ones [40, 41], indicate that these granites form a reasonably coherent group with a Pan-African age of develop. ment. Their principal characteristics are: (1) massive, discordant bodies; (2) most bodies rich in K, and all bodies rich in total alkali; (3) most differentiated varieties rich in incompatible (or LIL) elements; (4) clusters of separate plutons; (5) associated contact
TABLE 1 Compositions and isotopic data for Younger Granite plutons of Eastern Desert, Egypt (oxides in %; trace elements in ppm) Raba E1 Garrah * SiO2 TiO2 A1203 Fe203 ***
MgO CaO Na20 K20
76 0.1 12.5 1.0 0.03 0.53 4.0 4.7
Fawakhir **
72 0.4 14.0 2.5 0.58 1.82 4.4 3.8
Average of 20 plutons 75 0.1 12.7 1.0 0.06 0.55 4.1 4.5
Rb Ba Sr Y Zr Nb Th U
214 106 21 72 115 35 22 9
112 750 287 21 171 14
140 320 60 50 180 25
Initial 87Sr/86Sr Age (m.y.)
0.7020 580
0.7025 588
0.7029 f 580607 t t
* Raba El Garrah is one of the more felsic of the Younger Granite plutons. ** Fawakhir is one of the more mafic of the Younger Granite plutons. *** Fe203 is total iron calculated as Fe20 a. t Average of five bodies, including Raba El Garrah and Fawakhir. t t Range of five bodies, including Raba El Garrah and Fawakhir; ages were calculated using a decay constant of 1.39 X 10-11 yr-I for Rb.
metamorphism but generally an absence ofpegmatites (thus presumably emplaced at a shallow level); and (6) very minor associated intermediate and mafic rock types. In addition to their extensive development in the Eastern Desert of Egypt, similar alkali-rich granites have been reported widely throughout the northern, particularly the northeastern, portions of Africa. Other major outcrop areas include: the Tibisti area east of the Ben Ghnema batholith [42] ; the Hoggar, particularly the eastern portion [7,43,44] ; northern Sudan, where they are also referred to as Younger Granites [29,33] ; and some of the Pan-African plutons of the western portion of the Arabian shield [35,45]. The principal area of occurrence of the alkali granites is shown in Fig. 1. Outside of this area, comparable alkali-rich granites formed in Pan-African time are very scarce. The Pan-African Older Granites of Nigeria do not contain correlatives. The minor Horebis Granites of the Damaran orogen [ 17] are similar but quantitatively unimportant. Alkali-rich plutons that may extend to as great an age as PanAfrican have been described in the MozambiqueZambezi area of Malawi [38], but these rocks are nepheline-normative, quite unlike the Younger Granites of Egypt and their other north African correlatives. In short, the high-level, alkali granites of Pan-African age are virtually restricted to the area of northeastern Africa shown in Fig. 1. The source and origin of the Younger Granites and their correlatives is a major problem. New Sr isotopic data (Table 1) and previously published data [35,40, 41,46] indicate initial 87Sr/a6Sr ratios in the range of 0.702-0.706. These ratios presumably result from derivation of the melts from mantle, oceanic crust, or a sialic crust of low Rb]Sr ratio; if the source had been sial of high Rb/Sr ratio, it could not have been formed more than a short time before the melting that produced the Younger Granites. The possibility of mantle derivation is somewhat supported by some thorium and uranium data on one pluton (Table 1) from the Eastern Desert of Egypt; the Th/U ratio of the pluton is about 2.5, which is similar to that of mantle-derived basalts and much lower than the normal 4 - 6 characteristic of most differentiated granites. Other specific petrologic evidence for the source of the magmas is apparently lacking.
114 Geographic distribution. In summary, the plutonic
rock types o f Pan-African age described above are not uniformly distributed. Migmatites are abundant in southern, central, and western Africa but not in the northeast. Conversely, alkali-rich granites are essentiaUy restricted to northeastern Africa. Calcalkaline suites are also found primarily in the northeast. This distribution leads to the conclusion, discussed below, that northeastern Africa had a markedly different history from the rest o f the continent.
4. Pan-African evolution of northeastern Africa The concentration o f Pan-African-age alkali granites in northeastern Africa is interesting in view o f several features o f the geology o f the area: (1) Northeastern Africa is the only part o f Africa o f significant size that is not obviously occupied by an Archean shield. The nature o f the rocks older than Pan-African in the area has not been well determined. Summaries o f the Tibistis [30], Egypt [47], and Sudan [29] indicate that most of the older rocks are thick sequences of metavolcanic and metasedimentary rocks, calcalkaline plutonic assemblages, and "gneisses" o f uncertain origin. The possibility o f an Archean (or at least Early Protorozoic) craton in the area has been discussed and/or assumed in several papers (e.g. [4,5,48,49]). The only indications o f such early formation o f sialic crust, however, are dates o f two gneisses from Jebel Oweynat [49]. The dates are Rb-Sr model ages on a microcline and a whole rock, but data are not available to evaluate the dates, and further substantiation is necessary before they are accepted. Furthermore, a small area o f Archean rock may be only an isolated block rather than a portion o f a much larger craton. It is possible that an Archean shield is present at depth and simply covered b y later deposits; in fact, all three o f the major African cratons (Kalahari, Congo, and West African) do contain shallow Phanerozoic cover over their central portions. Nevertheless, it seems unlikely that if Archean rocks exist at depth, they do not crop out fairly extensively within an area o f several million square kilometers, particularly where that area has been affected by relatively recent rifting and block faulting. (2) As discussed above, only in northeastern Africa
is there significant evidence for the production o f calcalkaline batholithic suites by subduction o f oceanic lithosphere during Pan-African time. At this 750-
550
550
- 500
M.¥
M Y
~3 500
/4
M. Y. - P r e s e n t
Fig. 2. Diagrammatic illustration of proposed evolution of northeastern Africa. 750-550 m.y. Ocean basin between the West African craton (WAC) and the Nubian-Arabian shield (N-AS), with subduction on the margins. Subduction is also proposed in the center of the basin beneath island arcs, continental fragments, etc. The West African craton shown here includes the (generally Archean) craton mapped in Fig. 1 plus the Hoggar and other areas to the east that probably underwent ensialic reactivation in Pan-African time [7]. Thus, the Ben Ghnema batholith of the northwestern Tibistis could have been formed along the eastern margin of the craton [30,31]. 550-500 m.y. Development of stable (cratonic) area in northeastern Africa. Former ocean basin is filled by: 1 = margins and fragments of older cratons; 2 = deformed oceanic, arc, and continental-margin sediments; 3 = relatively young (molasse) sediments formed on the partially stabilized crust; 4 = alkali granites intruded into the former basin and the craton margins and closely associated with crustal stabilization. 500 m.y.-present. Mildly unstable cratonic area characterized by shelf sedimentation, minor block faulting, and limited (alkaline) magmatism. The major components are: 1 = sialic crust consisting of admixture of older continental fragments, oceanic and molasse sediments, and alkali granites; 2 = Phanerozoic basin sediments.
115 time, subduction apparently occurred under the eastern margin of the West African craton (in the northwestern Tibistis) and under the western margin of the Arabian craton (Fig. 2). Other subduction zones may have been present within the area delineated by these two zones (e.g., in the area of the Batholithic Granite of northern Sudan). (3) Pan-African time ended in widespread deposition of platform sediments throughout much of northeastern Africa; see summaries by Said [47], Goudarzi [50]. Nevertheless, northeastern Africa has probably been less tectonically stable than any other area of comparable size in Africa during Phanerozoic time. The area contains a region of structural "softness" in the Cretaceous [51 ], and a transcontinental suture and lineament virtually bisects the area [52]. The general concept that emerges from these observations is that cratonization of northeastern Africa occurred during Pan-African time (Fig. 2). Prior to that time, the area consisted of an ocean basin partly occupied by island arcs, continental fragments or nuclei, etc.; the size of the basin is unknown because of uncertainties in the relative movements of the West African and Arabian cratons during Pan-African time. Subduction on the margins of this basin, and possibly locally within it, probably continued until a widespread development of alkali granites occurred about 500 m.y. ago. Some of these granites were developed within the cratons themselves around the margins of the ocean basin (e.g., the Hoggar). The formation of the alkali granites either caused, or was associated with, crustal stabilization and the onset of platform sedimentation in the Early Paleozoic. The recency of cratonization (stabilization) in this part of Africa, as opposed to areas containing older shields, may be reflected in the mild, generally epeirogenic, Phanerozoic activity in northeastern Africa. The writers realize that this proposal of cratonization of a significant portion of Africa about 500 m.y. ago requires massive segregation of sialic material from the mantle at this time. Because of lack of sufficient information on the complete geologic history of the area, no specific mechanism is proposed here. Many geologists feel that segregation of sialic material from the interior of the earth could have taken place only during the early stages of earth history, possibly culminating at the end of the
Archean. The possibility of continued segregation is, thus, very controversial, and a complete discussion of the problems involved is beyond the scope of this paper. Despite these questions and difficulties, the writers consider cratonization during Late Proterozoic and Early Paleozoic time to be the best explanation for a significant part of the known geologic history of northeastern Africa.
Acknowledgements Financial support for different portions of this work was provided by: U.S. Energy Research and Development Administration (Department of Energy) contract E (05-1)-1661, administered by Bendix Field Engineering Corporation (to the University of North Carolina at Chapel Hill); the University of A1Fateh and the Government of Libya; and U.S. National Science Foundation grant OIP75-07943 (to the University of South Carolina), consisting of a Special Foreign Currency grant from the Division of International Programs and a dollar grant from the Division of Earth Sciences. The writers wish to acknowledge the assistance of a great many people in several countries, including: Dean Y. Meherric (Tripoli); Dr. M. Wanies (Tripoli); Mr. Rift (Sebha); Dr. E. E1 Shazly (Cairo); Dr. I. E1 Kassas (Cairo); Dr. R. Jacob (Grahamstown); and Mr. P. Woodhouse (Swakopmund).
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