Carvalho et al 2008 (3)

Journal of African Earth Soences. Vol 31, No 2. pp. 383-402, 2000 o 2000 Elsev~er Science Ltd

pll:so899-5382(00)00095-6 All rights reserved Pruned I” Great Brmm

0899.5362100 $- see front matter

Geochronological review of the Precambrian in western Angola: links with Brazil

H. DE CARVALHO’,*, C. TASSINAR12, P.H. ALVES’,

F. GUIMARWES3 and M.C. SIM8ES” ‘Centro de Geologia, lnstituto de Investigacso Cientffica Tropical, Alameda Afonso Henriques 41-4O D, 1000-I 23 Lisboa, Portugal

*Centro de Pesquisas Geocronolbgicas, lnstituto de Geociencias, Universidade de Slo Paulo, CP 11348-CEP 05422-970, Sao Paula, Brazil

3Servico Geologico de Angola, Caixa Postal 1260, Luanda, Angola 41nstituto Geologic0 e Mineiro, Rua da Amieira, 4465-021 S6o Mamede de Infesta, Portugal

ABSTRACT-In this study the main features of the Precambrian of western Angola are presented as a way of outlining the position of the Congo Craton and coastal Pan-African fold belts. These were separated from the Sio Francisco Craton and Atlantic Brasiliano fold belts in Brazil by the opening of the South Atlantic. The main events and cycles that affected the Precambrian lithostratigraphical units in western Angola, can be correlated with similar events in the northern part of the Congo Craton and in northwestern Namibia. Using these correlation and geochronological studies in the Luanda-Malanje region, two tectono-metamorphic events (the Lunda-Cuango-Malanje and the Malanje-Andulo events) are defined and several Precambrian lithostratigraphical units are reinterpreted. The margin of the Congo Craton in northwestern Angola (Luanda-Malanje region) is located between the Eburnian porphyroblastic gneisses and the Pan-African migmatites. The southern margin of this craton alternates between the Angolan and Brazilian coasts; then, after a brief incursion into Namibia, this limit runs northeast through southeast Angola until it meets the Lufilian Arc. @ 2000 Elsevier Science Limited. All rights reserved.

RESUME-Dans cette etude, les principaux aspects du Precambrien de I’Ouest de I’Angola sont presentes, avec le but de souligner la position du craton du Congo et des ceintures c&i&es Panafricaines separees, par I’ouverture de I’Atlantique Sud, du craton de Sao Francisco et des ceintures atlantiques du Brasiliano au Bresil. On presente la correlation parmi les principaux Bvenements et cycles qui ont marque les unites lithostratigraphiques Precambriennes de I’Ouest de I’Angola et les Bvenements similaires dans le nord du craton du Congo et dans le Nord-Ouest de la Namibie. Utilisant ces correlations et les etudes geochronologiques faits dans la region de Luanda-Malanje, on peut definir deux dvenements tectono-metamorphiques fles Bvenements de Lunda-Cuango-Malanje et de Malanje-Andulo) et re- interpreter plusieurs unites lithostratigraphiques Precambriennes. La bordure du craton du Congo au nord-ouest de I’Angola (region de Luanda-Malanje) est sit&e entre les gneisses porphyroblastiques de I’Eburneen et les migmatites Panafricains. La bordure sud de ce craton dessine une courbe rentrante soit en Angola, soit au Bresil; apres une breve incursion en Namibie, cette limite se prolonge vers le nord-est a travers le Sud-Est de I’Angola jusqu’a I’Arque Lufilien. o 2000 Elsevier Science Limited. All rights reserved.

(Received 29/l 1196: revrsed version received 21/3/00: accepted 22/3/00)

* Corresponding author [email protected]

Journal of African Earth Sciences 383

H. CARVALHO et al.

INTRODUCTION Andulo event’ (ca 2680-2593 Ma: see later). They

The events and cycles, which affected the Precam- car-respond, in the Democratic Republic of Congo

brian lithostratigraphical units of western Angola, (DRC) respectively, to the Musefu charnockitisation

are redefined in this paper. Tectonogenetic cycles episode (ca 2820 Ma: see Discussion; Cahen et a/.,

of Eburnian (2200-I 800 Ma), Kibaran (I 400-I 000 1984) and to the Moyo episode aged 2680 + 5 Ma Ma) and Pan-African (900-500 Ma) ages characterise (U-Pb concordant age: Cahen et a/., 1984) to the Precambrian evolution of Angola (Carvalho, 2593 f 92 Ma (Rb-Sr whole rock isochron, R, = 0.702, 1984; Carvalho and Alves, 1993). The Palaaoprotero- MSWD = 0.85: Delhal et al., 1975). zoic (Eburnian) event was the most important, affec- The revised Precambrian stratigraphy of western ting most rocks of the country, while the Mesopro- Angola and the definition of the margins of the Sao terozoic (Kibaran) and Neoproterozoic (Pan-African) Francisco Craton in Brazil (Trompette, 1994) have events were only locally significant. helped to establish the limit of the Congo Craton in

The two earliest major tectono-metamorphic events Angola and its connection with Brazil. The limit that are named here as the ‘Lunda-Cuango-Malanje event’ is proposed here is based on a different interpretation (ca 2900 Ma: see Discussion) and the ‘Malanje- from those considered until now.

I 12” I’4 * 16”

I 18’ 20’ 22’ 24’

I DEMOCRATIC REPUBLIC OF CONGO

5’

htAC6TA t c’

“’ OMPUPA CAHAM

NAMIBIA

CUANGO 9”

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Figure 1. Simplified geographical map of Angola.

384 Journal of African Earth Sciences


GENERAL GEOLOGY OF THE CONGO CRATON IN WESTERN ANGOLA

The region discussed in this paper extends eastwards from the Atlantic Ocean to 18’E, being limited to the north by the DRC border ( -6OS) and to the south by the Namibian border (- 17’3O’S), covering approximately 700,000 km2 (Fig. I).

Three Precambrian geological units were at first considered in Angola (Mouta, 1954). Later, Carvalho (1982) defined more than 40 lithostratigraphical units, some of them dated by the Rb-Sr method. Later still, Carvalho (1984) established in more detail the Precambrian stratigraphy in Angola.

In this paper, a new stratigraphical interpretation of the Precambrian in western Angola is discussed as a basis for outlining the margin of the Congo Craton. Geochronological data recently obtained in the Luanda- Malanje region (northwest Angola), and also presented here, contribute to this new interpretation. The lithostratigraphical units of Angola were, in general, dated by the Rb-Sr method; most units extend into other countries (DRC and Namibia) where they were also dated using Rb-Sr (whole rock) and U-Pb (zircons).

In general, the Rb-Sr whole rock isochrons define the ages of magmatic or metamorphic events which caused isotopic homogenisation of Sr on a whole rock scale. The Rb-Sr age indicates the time when the rocks cooled through the Rb-Sr critical tempera- ture, generally related to the last rock-forming metamorphic episode. In several cases, the Rb-Sr whole rock isochron ages are somewhat younger than the upper intercept age in a U-Pb Concordia diagram. This difference is due to the fact that the U-Pb system in zircons normally has a higher closing tempera- ture than the Rb-Sr whole rock system. In rocks which have a very slow cooling history, like some plutonic or high-grade metamorphic rocks, this difference could be significant.

In this paper it is assumed that the Rb-Sr whole rock isochron age is the age of the major rock-forming episodes that acted in western Angola. However, when the MSWD is very high, the values obtained must be considered as minimum ages. All Rb-Sr ages mentioned in the text have been standardised using a decay constant of Rb = 1.42 x 10-l’ a-‘.

PRECAMBRIAN STRATIGRAPHY OF WESTERN ANGOLA

The Central and Cuango Shields (Archaean) The Central Shield (Figs 2 and 3) occupies the region between 9O and 17OS. Its northern zone is essentially made up of rocks from the Granite, Gneiss and Migmatite Complex, intruding the Gabbro, Norite and Charnockite Complex (Table I) and locally well-

preserved in some kilometre-sized occurrences (Car- valho, 1982, 1984). Between the Cuanza River and 11 OS, basic and charnockitic rocks occur in greater abundance than granitoid and gneissic rocks, though these become predominant again in the southern zone of this shield (Andulo region; Figs 1 and 3). In this region, Y. Vialette and H. Carvalho (unpubl. data) have determined a Rb-Sr value of 2520 f 36 Ma for the granitoids and porphyroblastic gneisses (six point whole rock isochron, R, = 0.7006, MSWD = 14.8: Carvalho, 1984). These granitoids, gneisses and migmatites are, according to J. Delhal (in Carvalho, 19841, lithologically and petrologically similar to those of the ‘Dibaya Granite and Migmatite Complex’ of the DRC. On the other hand, recent isotopic studies have been made in the northwestern zone of the Central Shield and will be referred to later.

The Cuango Shield outcrops in just two places (Figs 2 and 3). The northern occurrence consists exclusively of rocks from the Granite, Gneiss and Migmatite Complex and has been correlated with the ‘Dibaya Granite and Migmatite Complex’ by Delhal and Fieremans (I 964). The Granite, Gneiss and Migmatite Complex was dated in the DRC by Delhal et a/. (19751, and, after being recalculated by Cahen et al. (I 9841, the dates are 2680& 5 Ma (U-Pb concordant age for migmatites) and 2593 + 92 Ma (Rb-Sr whole rock isochron from nine granitoid samples, R, = 0.702, MSWD = 0.85). These rocks extend on to the Luanda-Malanje region under the Pan-African and post-Pan-African cover (Figs 2 and 3; Carvalho, 1982). In the Cuango River, around the town of Cuango (Figs 1 and 21, another occurrence is essentially made up of rocks from the Gabbro, Norite and Charnockite Complex. An age of 2822 + 66 Ma (1 1 point whole rock Rb-Sr isochron, RI = 0.7008, MSWD=0.17) was given by J. Torquato (in Delhal et al., 19761 to the charnockitisation event in the Cuango Shield, while the latter authors attri- buted similar ages to the same event in Kasai, DRC (2772 + 28 Ma, Rb-Sr isochron, Ri =0.7005, MSWD = 2.48; 2889 Ma, U-Pb concordant age on zircon). Based on these results, Cahen et al. (I 984) considered 2820 Ma as a good approximate age for this charnockitisation event that extends to Cameroon, Gabon, the DRC and Angola. In southern Cameroon, Toteu et a/. (1994) document three distinct rock- forming or high-grade metamorphic events: - 2900 Ma, -2050 Ma and -600 Ma (U-Pb analyses of zircons and Sm-Nd whole rock analyses).

Pre-Eburnian granitisation and migmatisation lithostratigraphical units This section includes the Schist, Quartzite and Amphi- bolite Complex with marble layers, the Jamba Group,


H. CARVALHO et al.

: : 0 6 . . .

.

ttgure Z. Lieotectonic sketch map of Angola. 1: Phanerozotc cover. Pan-African: 2: northwestern metasedimentary fold belt; 3: Luanda Fold Belt; 4: lona Fold Belt; 5: Calueque metasedimentary fold belt; 6: Alto Zambeze metasedimentary fold belt. Kibaran: 7: Lubango Zone; 8: Alto Zambeze metasedimentary foldbelt; 9: northwestern zone; 10: coastalpolyorogenic belt; 7 1: Alto Zambeze polyorogenic belt; 12. central Eburnian zone; 13: Lubango Eburnian andpre-Eburntan (7) zone; 14: Lunda metasedtmentary Eburnian zone,’ 15: northwestern metasedimentary Eburntan zone; 16: Cassinga Zone. Neoarchzan: 17: Central Shield; 18: Cuango Shield; 19: Lunda Shield.

the Chivanda-Negola-Utende-Cela Supergroup, the quartz-feldspar porphyries and assocrated rocks and

The Schist, Quartzite and Amphibolite Complex with

the Gabbro-Anorthosite Complex of southwest Angola. marble layers is described in coastal polyoro-genie

Located within an area between 1 O”-1 73 and 11 O- granrtoids, gneisses and sedimentary sequen-ces (see

18OE (Fig. 2), all but the first occur in the core of the later). The Jamba Group (Korpershoek, 19841, which

Eburnian granitoids, sometimes as kilometre-sized occurs between 15’S and 16OS and nearly 16OE,

relics unaffected by granrtisation and migmatisation. and the Chivanda Group of the Chivanda-Negola-Utende- Cela Supergroup are made up of epimetamorphic,

386 Journal of African Earth Soences


12' I 1

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50 I DEMOCRATIC REPUBLIC OF CONGO

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11”

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r--- ------- i 13

IH I 16

'7 r- NAMIBIA 12" 14" 16"

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18' 20" 0 I 24"

Figure 3. Geological sketch map of western Angola. 1: Phanerozoic cover; 2: Pan-African rocks; 3: Kibaran rocks; 4: Kunene Basic Complex; 5: anorogenic and Late Eburnian granitoids and othogneisses of the Namib event; 6: Eburnian granitoid rocks; 7: Eburnian metasedimentary rocks; 8: undifferentiatedrocks affected by Eburnian granitisation andmigmatisation; 9: Gabbro-Anorthosite Complex of southwest Angola; 10: Schist, Quartzite andAmphibolite Complex with marble layers; 11: polyorogenic granitoids, gneisses and migmatites; 12: undifferentiated Granite, Gneiss and Migmatite Complex and Gabbro, Norite and Charnockite Complex; 13: Cassinga gneiss, migmatite andgranitotd rocks.

greenstone belt-type, volcano-sedimentary rocks (Table I).

According to Bassot et al. (I 9811, in the outcrop south of Cassinga (Figs 1 and 31, the Jamba Group trends north-south and shows epizonal metamorphism; this group lies discordantly over the Cassinga gneiss, migmatite and granitoid rocks, which show meso- and catazonal metamorphism and trend northeast-southwest. Both the Cassinga gneiss, migmatite and granitoid

rocks and the Jamba Group have not been isotopically characterised yet. However, due to lithological simi- larities, the correlation between the Jamba Group (Table 1) and the Rio das Velhas Supergroup in Brazil was suggested by Carvalho and Alves (1993). Felsic volcanic rocks from this supergroup have yielded U-Pb (zircon) concordant ages of 2776 + 6 Ma, and Pb-Pb isochron minimum ages of 2883 +6 and 2912 Ma (Machado etal., 1992a, 1992b, in Teixeira etal., 1996).


Table 1. Main Precambrian lithostratigraphical units of western Angola

Lithostratrgraphrcal unrt Lrthology Age (Ma)

PanAfrIcan Damaran basic rocks gabbro, serpentrnrte, dolente 509+26 (Rb-Sr 5 w.r.1 , R,=0.722,

MSWD = 0.60: Torquato. 1974)

Damaran Granrtord granrte, granodronte, syentte 540+115 (Rb-Sr 6 w.r I., R,=O 713,

MSWD = 1 .O. Torquato, 1974)

Noqur-type Grancte alkaline and hyperalkakne granrtes ca 730 (U-Pb on zrrcons and Rb-Sr) (Cahen et a/ , 1976)

West Congo Gabbro-Nonte gabbro, nonte, dolente Damara Supergroup gnerss, amphrbolrte. marble, dolomrte, ca 1100 (U-Pb on zrrcons)(Hugh and Schalk,

quartzrte, sandstone, conglomerate 1973, rn Krbner, 1982) to ca 509 (Rb-Sr w.r.r.,

R,=O 722, MSWD=0.60: Torquato, 1974)

West Congo Supergroup shale, quartzrte, Irmestone, srltstone, ca 1000 to ca 600

Kibaran

chert, mrxtrte, arkose, mafrc lava, ICahen et al ., 1976) stromatolmc limestone

Luanda Fold Belt metasedrments, mrgmatrtes, granrtords ca 684 (Rb-Sr w.r.1.. R,=0.7085. MSWD=0.76: thus paper)

northwest granrtord rocks alkalrne and talc-alkalrne granite, 1027 f 56 (U-Pb on zrrcons) orthognerss (Cahen et al , 1978)

nortrte and dolente nonte, dolente, basalt (~111s and dykes) 11 19+44 (Rb-Sr 5 w.r.r., R,=0.707. MSWD = 0.391: Carvalho et al., 1987)

southwest subophmc subophmc gabbro, dolente (~111s and ca 1200 (K-Ar) (Carvalho et al., 1987)

gabbro and dolerrte dykes) southwest red granrtes and granite, granodronte, syenrte, 1400-1300 (Rb-Sr w r I., R,=O 705-O 706,

assocrated rocks granrte-porphyry MSWD=0.33, 0.79, 1.50: Carvalho etal.. 1987 Mayumbran rhyolrte, andesrte, quartzrte, serrcmc z 1027 f 56 IU-Pb on zircons)

shale, phyllrte (Cahen ef al., 19781

Leba-Tchamalrndr Formatron dolomrtrc lrmestone wrth stromatolttes 1411 +24-l 119+44 (Fib-Sr 7 and 5 w.r.r., Chela Group conglomerate, quartzrte. sandstone, R,= 0.705 and 0 707. MSWD =0.32 and 0.39,

srltstone, volcanrc and volcano- sedrmentary rocks

respectrvely, Carvalho et al , 1987)

Cahama-Ot]rn]au Formatton conglomerate, sandstone, shale, quartzrte

Kunene Basic Complex whrte anorthosrte, troctolrte

ca 1700 lfreld evrdence: this paper) to ca 1302 (Rb-Sr w.r.i., R,=0.707. MSWD=1.50.

Carvalho ef al., 1979) ca 1500 IU-Pb. F.J Burger, rn Allsopp, 1975)

to 1302*20 (Rb-Sr 6 w r I., R,=0.707, MSWD = 1.50: Carvalho et al , 1987)

Anorogenrc Caraculo-Chrprndo granrtords medrum- to coarse-grarned porphyrrc 1686+69 (Rb-Sr w.r.1.. R,=0.703,

and/or Late

Eburnran

Brbala and central-west

regron granrtords

Leucocratrc granite

granrtord MSWD =0 41 Torquato and Carvalho, 1992) to porphyrrc granrtord 1596?86 (Rb-Sr 4 w.r.1.. R,=O 713,

MSWD=0.20: Torquato and Olrvetra, 1977)

fine- to medium-grarned equrgranular 1763*21 (Rb-Sr 8 w.r.1.. R,=O 713,

Eburnian

(Macota-type) granite Ruacana Orthognerss augen gnerss

northwest syenrte syenite and granite Qurbala-type Granite porphyroblastrc granrte and

MSWD =0 77: Carvalho et al , 19791 ca 1800 (U-Pb on rrrcons) (Tegtemeyer and Kroner, 1985)

ca 1960 IU-Pb on zrrcons)(Cahen et al., 1984)

1847+62 (Rb-Sr 5 w.r.r., R,=O 795, granodronte MSWD=0.60. Carvalho et al.. 1979)

Regional Granrte equrgranular brotrte granrte, 2191 +60 (Rb-Sr 4 w.r.1.. R,=O 7037, granodrorrte, tonalrte MSWD=0.75: Torquato et al.. 19791

Eburnran Gnerss and gnerss, mrgmatite, granrte, ca 2200 (this paper) Mrgmatrte granodrorrte Caraculo-Brbala meta- conglomerate, quartzlte, volcanrc >1686?69 (Rb-Sr w r I, R,=0.703, sedrmentary occurrences rocks (~111s and dvkes) MSWD =0.413: Torquato and Carvalho, 1992) Tchrpa-lona Formatron quartzrte, arkose, conglomerate, correlated wrth the Khoabendus Formatron

chert, marble (rare), gramtold, (Namrbra). ca 1800 (SACS, 1980) porphyry, rhyolrte, acid tuff

Bale Group and conglomerate, srltstone, arkose, 2162+99 (Rb-Sr 5 w.r.1, R,=0.7137, central-west occurrences sandstone, greywacke, shale, MSWD=0.701) to 1915*58 (Rb-Sr 5 w.r I.,

white quartzite R, = 0.7300, MSWD = 0.086)

(Torquato and Olrverra, 1977) Lulumba and Uonde Groups quartzrte, arkose, conglomerate,

shale, srltstone, talc-sandstone, mafrc and felsrc volcanrcs,

ca 2100 to ca 1960 (Cahen et a/ , 1979)

w.r.1.: whole rock rsochron.

carbonaceous shale, silty shale

Table 1. continued

Lrthostratigraphical umt Lrthology Age (Ma)

Units with Gabbro-Anorthostte aabbro, norite, anorthosrte, troctolrte, ca 2100 IK-Ar) - poorly Complex of southwest dunrte, hornblendite, pyroxenite, (F. Srlva, in Torquato et a/., 1979)

defined Angola subophitrc gabbro, olrvrne dolente

ages but quartz-feldspar porphyries granite and granodrorne porphyries, 22lSiSO (Rb-Sr 4 w.r.i., R,=0.709,

pre-Eburnian and associated volcanrc rhyolrte, dacite. andesite. rgnrmbrrte, MSWD=O.ll: Carvalho et al., 1979)

granitrsatron rocks felsrte

and Chrvanda-Negola-Utende- conglomerate, quartzite, schist, pillow > 2219 f SO (field evidence)

migmatisatron Cela Supergroup lava, tuff, vrtrophyre, itabrrite (Carvalho, 1984; this paper)

Schist, Quartzrte and marble, chert, schist, quart&e, >ca 1826 (Rb-Sr 8 w.r.i., R,=0.713,

Amphibolrte Complex with phyllite, para- and ortho-amphlbolite MSWD = 0.77: Carvalho et al ., 1979)

marble layers (tholeiitic basalt, komatstic peridotite) > 2219 f SO (Carvalho and Alves. 1990) Jamba Group schrst, greywacke, pillow lava, chert, correlated with the Rio das Velhas Supergroup

jasprlrte, itabrrrte, dacite. tuff, (Brazrl) > 2192 (Terxeira. 1996) quartzrte, pelite

Malanje- Andulo event Migmatite Complex

Lunda- Gabbro, Norite and

Cassrnga gneiss, mrgmatite gneiss, migmatite, quartzite. correlated wrth the Kanda-Kanda Gnerss (DRC) and granitoid rocks granitolds 3100-2800 (Delhal, 1991) northwest relrcs of old metaquartzite, schrst, metalrmestone, correlated with the pre-Zadrnean and Zadrnean metasediments talc-silicate rocks, rtabirite, para- (DRC) 22100 (Delhal and Ledent, 19741, and

amphtbolrte, gneiss, drorite, with volcano-sedrmentary sequences in Bahra granodionte (Brazrl) Archasan or Palaeoproterozorc

(Trompette, 1994)

Granrte. Gnerss and granitord, granite-tonalrte mrgmatitic ca 2600 fthrs paper) gneiss, migmatrte

gabbro, nonte, anorthosrte, gneiss, 2822+66 (Rb-Sr 11 w.r.i., R,=0.7008,

Cuango- Charnockrte Complex amphibolitised basic rocks, MSWD=0.17: Delhal etal.. 1976) Malanje event amphrbolites, enderbites,

charnockrtes, rare granulates

w.r.i.: whole rock isochron.

Bassot et al. (1981) determined an age of 1884 + 39 Ma (Rb-Sr whole rock, R, = 0.706, MSWD = 1 .O, recalculated) for migmatites and granitoids of the Cassinga gneiss, migmatite and granitoid rocks, and reported it as due to an Eburnian rehomogenisation affecting the analysed rocks.

The tonalites and granodiorite gneiss of the Kanda- Kanda region (DRC, near the northeast border of Angola), the age of which, based on Sm-Nd results, is between 2800 and 3100 Ma (Delhal, 19911, may correspond to the Cassinga gneiss, migmatite and granitoid rocks for the following reasons:

ij Carvalho (I 982) presented a structural sketch of Angola where the Precambrian rocks from the Alto Zambeze region (I 1, Fig. 2) have similar trends to those of the Cassinga Zone (16, Fig. 21, i.e. the foliation strikes west-northwest-east-southeast, which is not concordant with the Eburnian granitoids that surround them (trending north-northwest- south-southeast), or with the Cassinga gneiss, migmatite and granitoid rocks (see the Central and Cuango Shields), which show different directions

northeast-southwest. In Fig. 2 it seems that the rocks of the Cassinga Zone (16, Fig. 2) extend to the east- north-east through the Alto Zambeze region (11, Fig. 2) and further into the DRC at west Shaba. This concept is also referred to by Delhal (I 991). Delhal and Liegeois (1987) carried out Rb-Sr isotope studies in the Alto Zambeze region of Angola and obtained diverse results for the gneisses, inferior to those obtained in the same rocks from west Shaba. They thought that this was due to an unequal and substantial loss of radiogenic Sr, related to an event, at ca 2000 Ma, that was more intense in the Alto Zambeze region than in west Shaba.

- mainly north-south, north-northeast-south- southwest and north-northwest-south-southeast.

It is suggested here that this event was related to the Eburnian cycle, strongly represented in central Angola (12, Fig. 2) and extending southwards to the Cassinga Zone. This would mean that the loss of radiogenic Sr in the Cassinga Zone would be much more intense than in the Alto Zambeze region, and that the Cassinga gneiss, migmatite and granitoid rocks may correspond to the tonalite and granodiorite gneiss of the Kanda-Kanda region in the DRC, the age of which is between 2800 and 3100 Ma (see earlier).

iii Carvalho (1984) reports that the lithostrati- The Chivanda Group lies unconformably on rocks graphical units of the eastern border of Angola of the Jamba Group (Table I) (Basset et al., 1981; (entering from the DRC) extend into Angola in some- Korpershoek, 1984) and also occurs in the core of what parallel stripes, trending approximately the central Eburnian granitoid rocks (Figs 2 and 3).


H. CARVALHO et al.

The Chivanda-Negola-Utende-Cela Supergroup is older than the quartz-feldspar porphyries and asso-ciated volcanic rocks.

Quartz-feldspar porphyries and associated volcanic rocks The quartz-feldspar porphyries and associated volcanic rocks (Table I), intruded by the Eburnian granites, have also been strongly affected by the Eburnian granitisation and migmatisation processes. Isotopic studies ascribed to these rocks a 2219 + 90 Ma age (4 point whole rock Rb-Sr isochron, R, = 0.709, MSWD = 0.11: Carvalho et al., 1979). In the opinion of Catvalho and Alves (1993), this unit may be related to the Gabbro-Anorthosite Complex of southwest Angola as a felsic stage within the main basic magmatic emplacement.

The three liihostratigraphical units outlined above belong to undifferentiated rocks affected by Eburnian granitisation and migmatisation considered in Fig. 3.

Gabbro-Anorthosite Complex of southwest Angola The Gabbro-Anorthosite Complex of southwest Angola (Table 1; Carvalho and Alves, 1990) is intruded by Eburnian granitoid rocks in several places. The K-Ar ages obtained by F. Silva (in Torquato et a/., 1979) on plagioclases in the northern part of this basic complex are: 1964 f 61 Ma in one gabbro (K = 0.295, Arrad= 4.06, Aratm= 12.59); and 2102+51 Ma (K= 0.121, Arra,=1.952, Arat,,,= 8.361) and 2157 +43 Ma (K =O. 121, Arrad = 1.952, Arat,,, = 8.361) in two anorthosites. For the authors, the two last results may correspond to the age of the anorthosites.

In the Cahama region (Fig. I), the Gabbro-Anortho- site Complex is intruded by granites that Torquato and Salgueiro (1977) correlated with the leucogran- ites from the Macota region (see later). During this study, several outcrops in the Macota-lona-Tchipa- Cahama region (Fig. 1) were examined and were found to be lithologically similar to the Macota leuco- granites, whose age is ca 1700 Ma (see later), and they are also intrusive in the Gabbro-Anorthosite Complex (Carvalho and Alves, 1990).

Bearing in mind the relationships mentioned earlier, it is considered here that the basrc rocks occurring to the north of the Serpa Pinto Line (slightly north of 17”s; as defined by Simpson, 1970) have preceded Eburnian granitisation and migmatisation. It is suggested, therefore, that they should be designated the Gabbro-Anotthosite Complex of southwest Angola.

Coastal polyorogenic granitoids, gneisses, migmatites and metasedimentary sequences These rocks constitute the coastal polyorogenic belt (Fig. 2) and occur more or less continuously along the


Angolan coast (Fig. 3). Exceptions are the Luanda- Malanje region, where they are interrupted by rocks affected by the Lunda-Cuango-Malanje and Malanje- Andulo events, and the region to the east of Namibe and Tombua (Fig. I), where the Schist, Quartzite and Amphibolite Complex with marble layers occurs (see earlier).

The radiometric age of the gneisses and migmatites (ca 2 150 Ma: U-Pb concordant on zircons and Rb-Sr whole rock isochrons, R, = 0.704, MSWD = 1.8: Delhal and Ledent, 1976) was determined from rocks of the Boma and M’Pozo-Tombagadio regions in the DRC, near the Angolan border. Gneisses and migmatites in the Luanda-Malanje region have yielded Eburnian (ca 1790 Ma) and Pan-African ka 684 Ma) ages (see later).

Several outcrops of Eburnian, Kibaran and Pan- African age are intrusive into the coastal polyorogenic granitoids, gneisses and migmatites in northwestern Angola, between the border with the DRC and 9’s. In the same region, the northwest relics of old metasediments (Table 1) occur in the core of the coastal polyorogenic rocks. The age of these metasediments has not yet been determined in Angola, but they may correspond to the pre-Zadinean and Zadinean (>2100 Ma: Delhal and Ledent, 1976) metasedimentary rocks In the DRC referred to by Cahen et a/. (1979). The northwest relics of old metasediments may correspond to the volcano-sedimentary sequences in Bahia (Brazil), which are poorer in ultramafic facies and for which there still is some dispute concerning their Archaean versus Pala.?opro- terozoic age (Trompette, 1994).

For the Vista Alegre granitoids (granites and granodiorites), Cahen et al. (1979) ascribed an age of ca 1900 Ma (see later).

To the south of the Luanda-Malanje region, the granitoid occurrences become progressively more frequent than the gneiss-migmatite rocks. In the Benguela region (Fig. I), Y. Vialette (mph/. data) determined for the granitoids, gneisses and migmatites a value of 2230 f 160 Ma (6 point whole rock Rb-Sr isochron, R, =0.705, MSWD = 13.8). The considerable errors obtained may be due to the fact that these rocks are not cogenetic. Mendes (1968) obtained in biotites, from the granitoids that occur nearly 10“55’S, a value of ca 500 Ma (model ages with R, = 0.7 12). For a granitoid gnerss located nearly 12 ‘20’S, the same author obtained values of ca 1500 Ma for the whole rock and ca 500 Ma for the muscovite of the same rock (model ages with R,=0.712).

Between 14’30’s and the Namibian border, the gneiss-migmatite rocks of the coastal polyorogenic belt become less prominent and give way to granitoids


of granodioritic, dioritic, tonalitic and charnockitic compositions. Nevertheless, those few migmatite occurrences, like that found in the Macota area, clearly represent the effects of two tectono- metamorphic events: the last event has been estimated as‘ being 1826 + 48 Ma old (6 point whole rock Rb-Sr isochron, R, =0.704, MSWD = 3.8: Car- valho et a/., 19791, while a pre-Eburnian age has been suggested for the older migmatisation event (Carvalho and Alves, 1990). At least two metasedimentary sequences have been found in this region: a discordant conglomerate-quartzite sequence and a volcano-sedimentary sequence, both of Eburnian age. They lie discordantly over the Schist, Quartzite and Amphibolite Complex with marble layers and are older than the Caraculo Granite. This granite was isotopically studied by Torquato and Carvalho (I 992) and yielded an age of 1686 + 69 Ma (Rb-Sr whole rock isochron, R, =0.703, MSWD =0.413).

The Schist, Quartzite and Amphibolite Complex with marble layers (Table 1; Carvalho, 1984; Carvalho and Alves, 1993) only occurs in southern Angola (see earlier). From field observations (Carvalho and SimBes, 19721, the marbles occur interbedded with schists and quartzites and are intruded by basic and ultrabasic rocks (related to the Gabbro-Anorthosite Complex of southwest Angola), which were affected by the Eburnian migmatisation in the Macota area. The migmatites yield a ca 1826 k 48 Ma age (Car- valho et a/., 1979) (see earlier). It is possible that the marbles might be younger than the Schist, Quar- tzite and Amphibolite Complex, given the fact that intrusions of quartz-feldspar porphyries and associated volcanic rocks (2219 + 90 Ma: Carvalho et a/., 1979) were only found in ortho-amphibolites from that complex (Carvalho and Simdes, 1972).

In southwesternmost Angola, Torquato (1974) referred to a thermotectonic event for the first time (the Namib event) with an age of ca 1700 Ma determined from several granitoid rocks. Later, Carvalho (I 984) reported that this event extends northwards, from the border with Namibia to at least 10°S. Finally, Carvalho and Alves (I 993) showed that the event extends to the northwestern region of Namibia to at least the ‘Franzfontein Granitoid Suite’ occurrences; these authors suggest that the Namib event could include Late Eburnian and/or anorogenic granitoids, whose ages are bracketed between 1650 and 1800 Ma.

As mentioned earlier, the Caraculo Granite yielded an age of 1686 f 69 Ma (Torquato and Carvalho, 1992). For the granite that occurs between Huambo and Cassinga (Fig. I) and is intrusive into two Ebur- nian units (namely the Chipindo volcano-sedimentary rocks and the Regional granites), an age of 1596 +

86 Ma has been determined (4 point whole rock Rb- Sr isochron, R, = 0.713, MSWD =0.20: Torquato and Oliveira, 1977). Further to the south of Angola and extending beyond the Namibian border (Fig. 3), the Ruacana Augen Gneiss may represent a mag-matic event of ca 1800 Ma (concordant U-Pb on zircons: Tegtemeyer and Kroner, 1985). A gneiss from northwest Namibia has yielded concordant U-Pb (zircon) ages of 1470-I 408 Ma (Burger, unpubl. data, in Allsopp, 1975). The ‘Franzfontein Granitoid Suite’ of Namibia (Geological map of Southwest Africa/ Namibia, 1980) has yielded a U-Pb (zircon) concordant age of 1662 + 30 Ma (Burger et a/., 1976).

Considering all the data presented here suggests that the Namib event may have occurred between ca 1800 and ca 1500 Ma, affecting a 50-I 00 km wide strip of Angola-Namibia, between 13OS and 21 OS, nearing the ‘Franzfontein Granitoid Suite’. It is also suggested that the rocks with ages in the 1800- 1700 Ma range may be related to the last stages of the Eburnran cycle, while those yielding ages from 1700 to 1500 Ma may correspond to an anorogenic event. The granitoids yielding ages from 1500 to 1400 Ma may be related to the magmatic emplacement of the Kunene Basic Complex.

Eburnian metasedimentary sequences and granitoid rocks In northwestern Angola, the Eburnian metasedimentary rocks occur in a 40-60 km wide north-northwest- south-southeast strip about 100 km away from the coast. These sedimentary rocks comprise the Lu- lumba and Uonde Groups (Table 11, which are bracketed between ca 2150 Ma (age of the gneiss and migmatites in the DRC, where the Lulumba Group lies discordantly: see earlier) and ca 1960 Ma (Cahen et a/., 1979). In fact, in the Luanda-Malanje region, the Vista Alegre granitoids are frequently intrusive into the rocks of the Eburnian Lulumba Group (Scher- merhorn, 1976) and the porphyroblastic gneisses (Fig. 4). The age of emplacement for the Vista Alegre granitoids is ca 1960 Ma (5 point whole rock Rb-Sr isochron, R,=0.704, MSWD=2.7: Cahen et al., 1979). U-Pb concordant zircon ages are between 1967 Ma for zero intercept and 2047 Ma for a 356 intercept (Cahen et al., 1979).

Eburnian metasedimentary rocks are also found as isolated outcrops in the core of the Eburnian central zone (Fig. 2). According to Carvalho (19841, they constitute the central-west occurrences (Table 1) in central-western Angola, and also the Bale Group (Table I) described by Korpershoek (I 984). The Bale Group includes Au mineralisation and occurs between 13OS and 16OS, framed by the Kunene and Cubango Rivers (Figs I,3 and 4). In the Chipindo region

Journal of African Earth Sciences 39 1

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(Fig. I), Rb-Sr ages determined by Torquato and Oliveira (1977) on the metasediments of the Bale Group were 2162 + 99 Ma (5 tuffite-shale specimens, R,=0.7137,MSWD=0.701)and1915~ 58Ma(5 black-schist specimens, R, = 0.7300, MSWD = 0.086). The granite intrusive into these metasediments yielded a Rb-Sr age of 1596 + 86 Ma (4 point whole rock isochron, R, =0.713, MSWD= 0.20). According to Torquato and Oliveira (op. cit.), the age of the Bale Group would therefore be bracketed between ca 2 100 and ca 1596 Ma.

The Eburnian metasedimentary rocks of the Cas- singa Zone extend southwestwards to the northwest of the Calueque region, and on to Namibia, to the south of lona (Figs 1 and 3). South of 16OS, these rocks constitute the Tchipa-lona Formation (named by Carvalho and Alves, 1993; Table I). These authors have suggested a correlation between the Tchipa-lona Formation with the Khoabendus Group (ca 1800 Ma, concordant U-Pb on zircons age: SACS, 1980).

Rb-Sr values obtained for the migmatites from the Eburnian central zone (Figs 2 and 3) were as follows: 2206 + 197 Ma in the Quipungo region (Rb- Sr whole rock isochron, R, =0.7073, MSWD = 13.9: Torquato et al., 1979); and 1914 f 28 Ma in the southeast and near Serra da Ganda (Rb-Sr, 7 point whole rock isochron, R, =0.7045, MSWD = 3.20: Carvalho et al., 1979; Fig. 1). The considerable error in the first determination and the high MSWD values indicate that the analysed samples are not isotopically homogeneous. According to Torquato et a/. (1979), the Eburnian gneisses and migmatites progressively grade into the Regional granites, or are intruded by them. This suggests an age of ca 2200 Ma for those gneisses and migmatites, as it is close to that determined for the granitisation/migmatisation processes in the Boma and M’Pozo-Tomba- gadio regions, DRC, near the Angolan border (ca 2150 Ma, Rb-Sr whole rock isochron, R, =0.704, MSWD= 1.8; concordant U-Pb on zircons and sphenes: Delhal and Ledent, 1976).

In the Quipungo region, the Regional Granite (Table 1) has yielded a Rb-Sr age of 2191 f 60 Ma (4 point whole rock isochron, R, = 0.7037, MSWD = 0.75: Torquato et a/., 19791, while in the Jamba region its Rb-Sr age is 1853?74 Ma (6 point whole rock isochron, R, =0.7076, MSWD = 0.72: Carvalho et al., 1979), and in the Cela-Cariango region it yielded a Rb-Sr age of 2236+48 (6 point whole rock isochron, R,=0.7036, MSWD = 1.61: Silva and Kawa- shita, 1978).

The Quibala granites (Table I) have yielded Rb-Sr ages (Carvalho et al., 1979) of 2243 ?49 Ma (4 point whole rock isochron, R,=0.7008, MSWD= 2.46) in Serra da Ganda and 1847 + 62 Ma (5 point

whole rock isochron, R, = 0.7095, MSWD = 0.60) in the Chicala region.

Kibaran metasedimentary sequences, granitoids and basic rocks: the Kunene Basic Complex On the margins of the Kunene River, the rocks of the Kunene Basic Complex (Table I) post-date the Eburnian granitisation/migmatisation processes. In northern Namibia, gneisses apparently intruded by the Kunene Basic Complex have yielded ages of 1470-I 408 Ma (concordant U-Pb on zircons: Bur- ger, unpubl. data, in Allsopp, unpubl. data, 1975). According to the latter author, these data suggested a maximum age of ca 1500 Ma for the Kunene Basic Complex, which until then had been considered to be of pre-Eburnian age, together with the Gabbro- Anorthosite Complex of southwest Angola.

The red granites and associated rocks (1400-I 300 Ma, see later), intrusive into the Gabbro-Anorthosite Complex of southwest Angola, may be related to a felsic stage associated with the magmatic processes responsible for the Kunene Basic Complex. The age of the Kunene Basic Complex is therefore Kibaran, bracketed between 1500 Ma and 1400-I 300 Ma. Here it is suggested that only basic rocks of this age, on either side of the Kunene River, should be considered as belonging to the Kunene Basic Complex.

The red granites and associated rocks (Table 1) occupy a considerable area in southwest Angola and northwest Namibia (represented as ‘syenite and foiaite’ on the Geological map of Southwest Africa/ Namibia, 1980). The lithotypes associated with the red granites are red granite porphyries and syenites (Carvalho et al., 1987). Rb-Sr ages based on whole rock isochrons for the red granites from the Ompupa region and from the south of Otjinjau (Carvalho et al., 1987) are 1407 + 26 Ma (R, = 0.706, MSWD = 0.79) and 1411 + 24 Ma (R, =0.705, MSWD =0.32), respectively. Rb-Sr determinations on red granite porphyries from north of Chitado have yielded an age of 1302 f 20 Ma (R, = 0.707, MSWD = 1.50). The Matala red granite porphyries were isotopically studied by Bassot et a/. (19811, and the Rb-Sr ratios recalculated to yield an age of 1338 f 1 12 Ma (R,=0.716, MSWD=0.6).

The Rb-Sr ages determined up to now for the red granite porphyries of southwest Angola seem to be less reliable than those obtained for the red granites. Field evidence shows that the red granite porphyries occur near the contact with older rocks and that the transition to the red granites is gradual (Carvalho and Alves, 1993). The red granite porphyries may represent different crystallisation stages in the contact zone between the intrusive magma and the wall rocks, which might explain why these red granite


H. CARVALHO et al.

porphyries are neither petrologically nor isotopically homogeneous.

The Cahama-Otjinjau Formation (Table 1) lies discordantly over the Cahama leucogranite (see earlier) and is intruded by the red granites, which grade into red and dark granite porphyries in the contact zone, north of Chitado. Considering the field evidence, the Cahama-Otjinjau Formation is older than the Chela Group; in fact the Cahama-Otjinjau Forma- tion is intruded by the Kibaran red granites and red granite porphyries (see earlier), which are also found as boulders within the Chela Group basal conglomerate (Carvalho et al., 1987).

Sills and dykes of subophitic gabbros and dolerites occurring south of 12’S are ca 1200 Ma old (K-Ar ages: Silva et a/., 1973). These rocks intrude into the red granites near the Ompupa region (Fig. I), the age of which is 1407 f 26 Ma (Carvalho et a/., 1987; see earlier), and are, in their turn, intruded by the ca 1100 Ma old norites and dolerites.

Kibaran metasedimentary rocks occur mainly in southwest Angola. Three major occurrences deserve special reference, all located between 12”s and the border with Namibia, to the west of 17OE: they constitute the Chela Group, which is made up of sedimentary and volcano-sedimentary rocks (Car- valho, 1984; Carvalho and Alves, 1993). The age of the Chela Group (Table 1) has been bracketed between ca 1400 Ma and ca 1100 Ma, as it contains clasts of the ca 1400 Ma old red granites yet is also intruded by sills and dykes of the norites and dolerites unit, which has yielded a Rb-Sr age of 1119 f. 44 Ma (5 point whole rock isochron, R,=0.7078, MSWD=0.39: Carvalho eta!., 1987).

The Leba-Tchamalindi Formation conformably over- lies the two occurrences of the Chela Group (Fig. 1) and is essentially made up of dolomitic limestones (Table 1).

Kibaran granitoid rocks are also abundant in the north, from 7O3O’S to the border with the DRC. The Lufico orthogneisses can be followed from northwest Angola to the DRC, where their age has been given as 1027 + 56 Ma (concordant U-Pb on zircons: Cahen et a/., 1978). These granitoids are intrusive into metasediments that are an extension, in Angola, of the Mayumbien unit of the DRC (Figs 2 and 3). Towards the south, neither the granitoids not the metasediments extend near 7OS (Figs 2 and 3), perhaps due to the eastward displacement (Fig. 8) between two major fault systems in the Luanda-Malanje region (see Discussion).

Pan-African rocks Lying unconformably on the Lulumba and Uonde Eburnian metasediments, and on the granitoids and


gneisses of northwest Angola, occur Pan-African metasediments that constitute the West Congo Supergroup (Figs 3 and 4, Table 1), comprising the Terreiro, Alto Chiloango, Xisto-Calcario and Xisto- Gresoso Groups (Stanton et al., 1963). The age of the West Congo Supergroup lies between ca 1027 Ma and ca 564 Ma. The basal Sansicua Group of the West Congo Supergroup lies, in the DRC, over the Mativa Granite. Four zircon fractions of this granite and two of the Kianga Microgranite define a concordant U-Pb age of 1027 Ma (Cahen et al., 1978, in Cahen et al., 1984). On the other hand, Cahen et al. (1976) indicate an age between 750 and 564 Ma for the later stages of metamorphism of the West Congo Orogeny.

Several outcrops of granitoid rocks intrude the coastal polyorogenic granitoids, gneisses and migmatites, between 9’30’s and the border with the DRC. The alkaline and/or peralkaline granite occurrence at Noqui outcrops on both sides of the border between northwest Angola and the DRC. Cahen et al. (1976) ascribed an age of ca 752 Ma (concordant U-Pb on zircons and Rb-Sr whole rock isochrons) for the origin of this granite. This granite is also intrusive into the West Congo Supergroup.

Near the coast in southwesternmost Angola, the Pan-African metasedimentary rocks make up the Damara Supergroup (Table I), which is younger than ca 1010 Ma and older than ca 509 Ma. In fact, the maximum age of the Nosib Group (base of the Damara Supergroup) is given by the age of under- lying older units. The pre-Nosib acid volcanics yielded a Pb-Pb minimum zircon age of 1010 t 7 Ma (Hugo and Schallk, 1973, in Kroner, 1982). On the other hand, a ring granite intrusion in the southwest of Angola, yielding an age of 509 + 15 Ma (6 point Rb-Sr whole rock isochron, R, = 0.722, MSWD = 0.60: Torquato, 1974), metamorphosed the Damaran metasediments.

Besides being responsible for the intrusion of basic and granitoid rocks, the Pan-African cycle also caused intense granitisation and migmatisation in the southwest of Angola. Rb-Sr results obtained by Torquato (1974) ascribed an age of 509 f 26 Ma (5 point whole rock isochron, R, =0.72, MSWD = 0.60) to the basic intrusions, while conferring an age of 540 + 15 Ma (6 point whole rock isochron, R, =0.713, MSWD = 1 .O) to the granitisation and migmatisation processes.

Recent geochronological studies in the Luanda- Malanje region This region includes the northwestern part of the Central Shield and the northern part of the poly- erogenic granitoids, gneisses and migmatites, as well

87 Sr/ 86%

0.796

0.772

629 CD-79

/ 116 MS -74


Table 2. Rb-Sr analytical data for 14 whole rock samples from the Luanda-Malanje region

Sample Rock Rb Sr 87Rb/86Sr 87Sr/86Sr Lab. kwn) bpm) No.

118-MS-74 granitoid 159 154 3.021 + 0.085 0.81274 + 0.00007 11682 132-MS-74 granitoid 42 337 0.358 &O.OlO 0.71666 +0.00007 11683 629-CD-79 granitoid” 134 121 3.235 f 0.009 0.82625 f 0.00013 11684 1018-MF-80 granitoid 31 283 0.317 zk 0.009 0.7 1860 f 0.00006 12152 553-MF-80 granitoid* 137 300 1.328 + 0.037 0.75483 kO.00007 12157 1080-CD-79 porphyroblastic gneiss 98 385 0.741 * 0.021 0.72493 * 0.00007 11622 1086-CD-79 porphyroblastic gneiss 190 448 1.232 kO.035 0.73705 +0.00011 11687 1088-CD-79 porphyroblastic gneiss 151 183 2.402kO.068 0.76550~0.00010 12158 1089-CD-79 porphyroblastic gneiss 92 430 0.62OkO.018 0.72128~0.00006 12155 1573-MF-81 porphyroblastic gneiss 40 703 0.166 + 0.005 0.70972 * 0.00007 11681 1725-81 porphyroblastic gneiss 88 441 0.578 + 0.016 0.72086 f 0.00009 12156 760-81 migmatite 138 352 1.134kO.032 0.71950~0.00006 11620 790-81 migmatite 130 202 1.869 f 0.053 0.71717+0.00007 11621 687-MF-81 migmatite 201 49 12.009 kO.336 0.82477 +0.00021 12154

*: with some slightly orientated minerals.

as the supracrustal Eburnian and Pan-African rocks of northwest Angola (Fig. 4). Besides its importance for revealing the oldest rocks in Angola, the Luanda- Malanje region is important for defining the boun- daries of the Congo Craton and, consequently, its connection with the Sgo Francisco Craton. The rock samples used in these studies are from the Geo- logical Survey of Angola.

I I / R, = 0.7053 +, 0.000 4 The results obtained, although consistent with the

ages of the same formations in the DRC, do not have a high quality probably because the samples analysed, in spite of belonging to the same lithostratigraphical unit, are sometimes taken up to 125 km apart. I 0.00 0.70 1.40 2.10 2.80 3.50

The isotopic ratios for the five granitoid samples (118 to 553, Table 21, collected in the Zenza do Itombe-N’Dalatando-Lucala area (Fig. 41, though thrice estimated, are somewhat dispersed in the Rb- Sr diagram and only a regression line has been obtained with a value of 2560 + 50 Ma (R =0.7053 + 0.0004, MSWD = 16.0, Fig. 5). The high MSWD value may be related to the fact that some of the samples were collected far from each other, as mentioned earlier. On the other hand, these granitoid rocks had previously witnessed a long crustal history and lack isotopic homogeneity (see Discussion). The value obtained must be considered to be a minimum age.

Figure 5. Whole rock Rb-Sr isochron diagram for granitoids from the Zenza do ltombe-N’Dalatando-Lucala area.

The Rb-Sr value determined in the Zenza do Itombe- N’Dalatando-Lucala area (2560 f 50 Ma, Fig. 4) and Andulo region (2520& 36 Ma: 6 point Rb-Sr whole rock isochron, R, =0.7006, MSWD = 14.8: Carvalho, 1984; see earlier) are similar to that obtained by Delhal et al. (1975) for the ‘Dibaya granites’ (2593 +

92 Ma: R,=0.702, MSWD =0.85: Delhal et al., 1975). These granites and ‘migmatites’ extend from Kasai (DRC) to the Cuango River, and Malanje and Andulo regions, as the Angolan extension of the ‘Dibaya Granite and Migmatite Complex’. The age of this complex is between 2593 and 2680 Ma (see earlier) and is related to the Moyo event in the DRC (ca 2680 Ma: see Introduction). In this paper it is proposed that the same event in Angola be designated the ‘Malanje-Andulo event’.

Six samples of porphyroblastic gneisses (I 080 to 1725, Table 2) were collected in the ucua-Quilombo dos Dembos area, corresponding to the Eburnian domain extending from the town of ucua to 20 km south of Quilombo dos Dembos and near to the


H. CARVALHO et al.

0.770

t 87 sr/ =Sr

T (MO) = 1790 + 32

R, = 0.7055 t 0.0002 MSWD = 0.78

‘=Rb/=Sr

0.00 0.50 1.00 I.50 2.00 2.50

Figure 6. Whole rock Rb-Sr isochron diagram for porphyroblastic gneisses from the l&a-Quilombo dos Dembos area.

0.830 4

87 Sr/ 86 Sr 607 MF-81

0.804 --

0.778 --

=684+ 20 R, = 0.7085 -+ 00005 MSWD = 0 75

67 0.700 ’ Rb/ 86%

0.00 3.00 6.00 9.00 12.00 15.00

Figure 7. Whole rock Rb-Sr lsochron diagram for migmatltes from the Caxito-Zenza do ltombe area.

Eburnian metasedimentary rocks (Fig. 4). For the gneisses, a well-defined Rb-Sr whole rock isochron was obtained yielding an age of 1790+32 Ma (R, =0.7055 +- 0.0002, MSWD = 0.78, Fig. 6). This age is inferior to the age determined for the granitisation and migmatisatron near the Angola- DRC border, in the Boma and M’Pozo-Tombagadio regions (ca 2150 Ma: Delhal and Ledent, 1975; see earlier). This fact is perhaps due to the loss of radiogenic Sr during the Pan-African Orogeny (see Discussion).

The only three migmatite samples (687 to 760, Table 2) from the Caxito-Zenza do ltombe area, near the Meso-Cenozoic sedimentary rocks (Figs 4 and 8), defined a Rb-Sr isochron yielding an age of 684+20 Ma (R,=0.7085 +-0.0005, MSWD=0.76, Fig. 7). These migmatites occur in the Luanda Pan- African Fold Belt (Fig. 2). Despite the fact that the samples were collected 60 km from each other, they all belong to the same lrthostratigraphical unit

and represent the first evidence of the Pan-African/ Brasiliano tectono-metamorphic episode found in the basement of northwest Angola (see Discussion). The analysed migmatites are sometimes of calc- silicate mature and bear such minerals as garnet, cordierite and andalusite, due to the metamorphic contact between Pan-African granitoids and metasedimentary rocks. According to Trompette (1994), the Aracuai-West Congo Fold Belt consists of highly metamorphosed metasediments and Pan-African granitoids and migmatites. The migmatites of the Luanda Pan-African Fold Belt are lithologically similar to those that occur on the Aracuai Brasiliano Fold Belt in Brazil. The age determined in Angola, despite the fact that the three analysed rocks occur 60 km apart, seems correct and is similar to those determined in Lower Zaire, DRC (730-600 Ma: Cahen et a/., 1976), and southern Cameroon (ca 600 Ma: Toteu et al., 1994).

In the Central Shield (Fig. 2), no isotopic ages have so far been determined for the Gabbro, Norite and Charnockite Complex. Nevertheless, in the Zenza do Itombe-N’Dalatando-Lucala area, as well as in the Andulo region, field evidence indicates that these rocks are older than the intrusive Granite, Gneiss and Migmatite Complex, i.e. they would be older that ca 2600 Ma (see Discussion).

DISCUSSION

The western and southern margins of the Congo Craton in Angola will be defined using the isotopic data so far available for the Precambrian lithostratigraphical units of western Angola mentioned earlier. Where radiometric dates are not available, clear field relationships have been used to infer the age ranges of tectonic/metamorphic/magmatlc events.

The oldest rocks found in Angola so far belong to the Gabbro, Norite and Charnockite Complex. For these rocks an age of ca 2822 Ma was determined (in the Cuango Shield, Fig. 2), similar to those obtained by Delhal and Ledent (1975) and Laserre and Soba (1976) in southern Cameroon and northern Gabon, respectively. Cahen et al. (I 984) regressed the data determined by Delhal and Ledent (1975) and Laserre and Soba (1976) and obtained a com- posite age of 2900 + 44 Ma (R, = 0.7011, MSWD = 1.05), “which is considered to be an excellent approximation to the age of chamockitization”. How- ever, Toteu et a/. (I 994), working in neighbouring Cameroon, concluded “that theplutonism andmeta- morphism probably occurred very close together at -2900 Ma IlJ-Pb, Sm-Nd)“. These authors also document two other events occurring at ca 2050 Ma and ca 600 Ma.



In Gabon, Bassot et al. (1987) have determined the ages of several metamorphic events: the first at ca 3150 Ma; the second related with granitisation events occurring between 2850 and 2800 Ma; and the third associated with the emplacement of ultrabasic and talc-alkaline rocks, ca 2780 Ma and 2750- 2700 Ma ago, respectively.

Up to 1986, only one stage of metamorphism (named the Musefu event at ca 2820 Ma) had been recognised in the DRC and the Cuango region, An- gola (Delhal et al., 1976). This event was related to the formation of the Gabbro, Norite and Charnockite Complex and affects similar rocks in the Lunda area (Delhal, 1973). It also extends into the Cuango (Delhal et al., 1976) and Malanje-Dondo regions (Delhal and Fieremans, 1964), and then southwards (Carvalho, 1982, 1984) at least into the Andulo region (Figs 1, 2 and 3). It is suggested here that this event in Angola be named the ‘Lunda-Cuango- Malanje event’. Pursuing their research on the Gabbro, Norite and Charnockite Complex, Delhal et a/. (1986) note that Sm-Nd model Ct,,) give dates for the acid granulites from the Kasai Complex that agree with previous Rb-Sr and U-Pb ages of - 2.8 Ga for the early and main stage of granulite-facies metamorphism. These authors also state that “a Sm-Nd study (whole rock and internal isochrons, and to, ages) of the associated basic granulites (meta-gabbro-norite, . . .I demonstrates that the second stage of granulite metamorphism, corresponding to regional deformation of all the rocks, is near/y coeval with the emplacement of the basic rocks at -2.4 Ga”.

No age determinations have been made so far on the Gabbro, Norite and Charnockite Complex in the Luanda-Malanje and Andulo regions, but field evidence shows that this complex is older than the rocks of the Granite, Gneiss and Migmatite Complex in these two areas. In fact, in Angola the latter complex contains xenoliths from several facies belonging to the Gabbro, Norite and Charnockite Com- plex. The Granite, Gneiss and Migmatite Complex in the Luanda-Malanje and Andulo regions represents an extension of the ‘Dibaya Granite and Migmatite Complex’ of Kasai (ca 2.7-2.6 Ga: Delhal, 1991). Therefore, the Gabbro, Norite and Charnockite Com- plex in northwest Angola is older than the second stage of granulite-facies metamorphism (2.4 Ga: Delhal et a/., 1986).

Taking into account the above considerations, if 2.4 Ga is the age for the emplacement of the Kasai gabbro-norites, then correlation of these basic rocks in the Luanda-Malanje and Andulo regions to the second stage would be impossible, because field evidence (see earlier) proves that they are older than the ‘Dibaya Granite and Migmatite Complex’ (ca

2.7-2.6 Ga). On the other hand, according to Toteu et al. (I 9941, not only the charnockitisation event - as it was considered up to 1986 by Delhal and Ledent (1975) and Cahen et al. (1984) - but also the plutonism and metamorphism occurred very close together at - 2.9 Ga (see earlier). Then the Gabbro, Norite and Charnockite Complex in northwestern Angola (Luanda-Malanje and Andulo regions) may be related to the older, rock-forming or high-grade metamorphic event in southern Cameroon at - 2.9 Ga old (Toteu et al., 1994). Nonetheless, in this paper it is suggested that this age, as well as those determined for the charnockitisation and the main stage of granulite-facies metamorphism (ca 2820 Ma: Cahen et al., 1984; -2.8 Ga: Delhal et a/., 19861, correspond to the same event.

Granites and migmatites of the ‘Dibaya Granite and Migmatite Complex’ of Kasai plotted on the same whole rock isochron have yielded an age of 2648 + 22 Ma (R, =0.7013, MSWD = 1.94: Delhal et a/., 1975). According to the same authors, these granites (and also tonalite and quartz diorites in northeast Lunda: Delhal, 1973) could be slightly younger than the so-called ‘migmatites’ (granitic to tonalitic migmatitic gneisses). Some representative granitic to tonalitic migmatitic gneisses plot significantly above the whole rock Rb-Sr 2593 + 92 Ma isochron (Ri = 0.702, MSWD = 0.85) defined for the granites. On the other hand, sphenes, zircons and apatites from some granites and some ‘migmatites’ have yielded a mean concordant U-Pb age of 2680 Ma. In the opinion of Delhal et a/. (19751, “the migmatites were formed from materials which had a long crustalhistory before 2680 Ma, which were not entirely homogenisedat that date”. This fact, together with the different ages obtained for the granites and ‘migmatites’ of the ‘Dibaya Granite and Migmatite Complex’ (Delhal et a/., 1975), might explain the high MSWD values for the Luanda-Malanje and Andulo granitoids. These values, but mostly the field evidence, confirm the extension of the ‘Dibaya Granite and Migmatite Complex’ of the DRC up to northwest Angola (Luanda-Malanje and Andulo regions).

The Malanje-Andulo event corresponds, in our opinion, not only to the Moyo episode responsible for the ‘Dibaya Granite and Migmatite Complex’ in the DRC (Delhal et a/., 19751, but also to the last stage of the (talc-alkaline) granitisation in Gabon, referred to by Bassot et al. (I 987).

The effects of the Eburnian cycle, which followed the Malanje-Andulo event, were felt all over Angola and were responsible for its geological framework, es- pecially through significant rock-forming events, intense metamorphism and radiogenic Sr losses in older rocks. This cycle is also a very important stratigraphical

Journal of African Earth Sciences 39 7

H. CARVALHO et al.

Meso-Cenozoic

0 Polyorogenic granrtoids. gneisses and mrgmatltes

Lx] Fold trends tn the Pan-African and Eburnlan metasedimentary rocks

\T Fault

z. -- Inferred fault

N Displacement sense of the block between faults

Figure 8. Tectonic sketch of northwesr Angola (modifiedafter Stanton et al., 19631.

reference, since Eburnian granitisation and migmatisation are subsequent to the major litho-stratigraphical units of central-western Angola, for which few isotopic ages are available as yet.

Three particular units affected by the Eburnian granitisation and migmatisation processes deserve special reference, given their economic significance:

$ the Jamba Group: a greenstone belt with banded iron formations;

iii the Bale Group: with Au mineralisation; and iiil the Gabbro-Anorthosite Complex of southwest

Angola: for its ornamental stones, and Ti- and Ni- bearing rocks.

The K-Ar plagioclase values obtained from anorthosites from the northern area of the main outcrop indicate an age of ca 2000 Ma for the basic complex in that area (see earlier). The designation ‘Gabbro-Anorthosite Complex of southwest Angola’ should be exclusively for those northern and central parts of the complex. In the southern part, the rocks have yielded Kibaran ages, ranging from ca 1500 to ca 1300 Ma, and are included in the Kunene Basic Complex as stated earlier.

To the north of the Luanda-Malanje region (Figs 1 and 41, the polyorogenic granitoids, gneisses and migmatites were strongly affected by Eburnian tectonics and metamorphism, giving rise to granites with microcline porphyroblasts (Korpershoek, 1964) and migmatites similar to those of the Boma and M’Pozo-Tombagadio region in the DRC. In this zone, the isotopic age proposed for granitisation and migmatisation was ca 2 150 Ma (Delhal and Ledent, 1976). On the geological map (Carvalho, 19821, these ca 2150 Ma old rocks occurred in Angola only in the area between the border with the DRC and - 8OS. The present Rb-Sr results on porphyroblastic gneisses of &ua-Quilombo dos Dembos (ca 1800 Ma), in the Luanda-Malanje region, show that the Eburnian granitisation and migmatisation event extended southwards, nearly to the Lombige River (Fig. 4). However, this Eburnian age is significantly lower than the values previously obtained for the granitisation and migmatisation processes in the Boma and M’Pozo-Tombagadio regions (ca 2 150 Ma: Delhal and Ledent, 1976; see earlier) and for the high-



\ S - Salvador -.,- __,-A-*

Lx, L - Luanda ,/-

4MlUlA ; Na - Namibe

\ .L 1 SP - SZro Paul0

200’ RJ - Rio de Janeiro

Figure 9. Diagram showing the connections between the Congo Craton and Pan-African belts of Angola, and the southern margin of the SrTo Francisco Craton and coastal Brasiliano belts of Brazil.

grade metamorphic event in southern Cameroon (ca 2050 Ma: Toteu et al., 1994). The lower values obtained in northwest Angola are possibly related to important radiogenic Sr loss during the Pan-African Orogeny (see earlier). In fact, the Vista Alegre granitoids (ca 1960 Ma: Cahen et al., 19791, are intrusive into the Ijcua-Quilombo dos Dembos porphyroblastic gneisses, which will therefore be older than ca 1960 Ma. The Vista Alegre granitoids seem to be an extension, to the north of 9OS, of the two main Eburnian granitoid occurrences in the central region, namely the Quibala-type Granite and the Regional Granite.

The Kibaran and Pan-African cycles have only a local effect, being best represented in the southwest and northwest of Angola, respectively. Some units formed during these cycles help delineate some segments of the Congo Craton. For example, the

Kibaran Chela Group outcrops in the border area between Angola and Namibia (Carvalho and Alves, 19931, and establishes the southern limit of the Congo Craton.

In the Luanda-Malanje region, the most evident structural directions vary as follows (Fig. 8). To the north of the Bembe Fault System, most structures strike north-south or north-northwest-south-southeast: to the south of this fault system, they change abruptly to northwest-southeast, which can be well- recognised in the Eburnian and Pan-African metasedimentary rocks. Structures then follow north-northwest-south-southeast or north-south trends up to near the N’Dalatando Fault System: a clear northeast-southwest deflection is recognised in the fold trends in the metasedimentary rocks near that fault system. This structural setting, together with the Pan-


H. CARVALHO et al.

African ages given for the migmatites, suggest that the area located between these two major fault systems in the Luanda-Malanje region has experi- enced an eastward displacement (Fig. 8). On the basis of this evidence, it is suggested here that the Luanda Pan-African Fold Belt and the Aracuai Fold Belt in Brazil are one and the same (see earlier). In fact, according to Trompette (1994) the Pan-African Aracuai-West Congo Fold Belt is cut in half by the opening of the South Atlantic Ocean, and the metamorphism and lithology are similar in Angola and Brazil.

LIMIT OF THE CONGO CRATON IN ANGOLA AND LINKS WITH BRAZIL

The margins of the ‘%o Francisco Craton, surrounded by the Brasiliano fold belts in Brazil, and their connection with the Congo Craton and the Pan-African fold belts in Africa, have been recently studied by Trompette (1994). He is also the author of the simplified geological map of the Congo Craton, which was fundamental to the work of several authors. This simplified map shows the basic geological setting of the Congo Craton, surrounded mainly by Pan- African fold belts and including rocks of essentially ArchEan and Eburnian ages. Carvalho and Tassinari (1992) made the southern connection between the Pan-African/Brasiliano fold belts that flank the Congo Craton in Angola, and the Atlantic margin of the SBo Francisco Craton in Brazil. This paper suggests an interpretation concerning the connection between Angola and Brazii. Isotopic studies in the Luanda-Malanje region, together with this study’s reinterpretation of Precambrian lithostratigraphical units from western Angola, make it possible to outline the western margin of the Congo Craton in Angola.

The southwestern margin of the Pan-African lona Fold Belt (Fig. 2) passes northwards, through the Cabo Frio region (northeast of Rio de Janeiro, Fig. 9), eastwards of the coastal Brasiliano Fold Belt. There, a zone of Eburnian granitoid rocks was identi-fied by isotopic studies yielding Trans-amazonian (Eburnian) ages (1981 f 18 Ma, concordant U-Pb on zircons; 1799 +- 22 Ma, Rb-Sr whole rock isochron, R, = 0.706, MSWD=0.57: Zimbres et a/., 1990), and also presenting structural trends striking north-west- southeast, not northeast-southwest (charac-teristic of the Brasiliano Fold Belt).

Continuing northwards, under the Phanerozoic (Meso-Cenozoic) cover, this margin comes close to the Central Shield in the Luanda-Malanje region. North of the Central Shield, the margin of the Congo Craton passes between the Pan-African migmatites in the

400 Journal of African Earth Scrences

Caxito-Zena do ltombe area, and the Eburnian porphyroblastic gneisses in the Ijcua-Quilombo dos Dembos area (Fig. 4), and it possibly extends northwards under the Phanerozoic (Meso-Cenozoic) rocks (Figs 3 and 8). This suggests that the connection in Angola with the SBo Francisco Craton would be located under the Phanerozoic (Meso-Cenozoic) cover, near the border with the DRC (Figs 5, 8 and 9).

In the southernmost part of Angola, the limit of the Congo Craton IS located between the lona Fold Belt (Pan-African) and older rocks. This limit then crosses into Namibia, returning to Angola as the Calueque Pan-African metasedlmentary belt (Fig. 2). From there It continues under the Kalahari Super-group and Phanerozoic cover (Carvalho, 1984) to the Alto Zambeze region, between the Pan-African and Kibaran Fold Belts (Fig. 2), and on to the Lufilian Arc.

ACKNOWLEDGEMENTS

This work was developed as part of the ‘Angola-Brazil Geological Correlation Project’. The Geological Survey of Angola, the Geochronological Research Center of the Sgo Paulo University (Brazil) and the Geological Center of IICT (Tropical Sciences Research Institute, Portugal) have co-operated in this project.

The authors acknowledge the support of the lnstituto da Cooperacso Portuguesa (Portugal). The authors also thank the technicrans of the Geochrono- logical Research Center (University of Sio Paulo, Brazil) for the analytical work. The original draft of this manuscript benefited from suggestions by A. Kr6ner, which are greatly appreciated. G.F.W. Menge contributed with some personal communications to this paper, for which we thank him. We are also grateful to our colleague Isabel R. Costa for the helpful discussion and a critical re-writing of the English manuscript. The authors would like to thank G.J.H. Oliver for the suggestions and constructive reviews that greatly improved this work. Editorial handling - G. J. H. Oliver

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