Congo Craton

The Congo craton includes a generally poorly known and underexplored region of Archean rocks that are exposed around the Congo basin in central Africa. The craton extends from the Kasai region of the Democratic Republic of the Congo into Sudan, Angola, Zambia, Gabon, and Cameroon, and is known for containing many greenstone belts, albeit deeply weathered under thick profiles of laterite soil. Most parts of the Congo craton are known from separate regions that outcrop around the Congo basin, and these areas are generally known as cratons or blocks, even though they are likely continuous at depth.

The Kasai (and northeast Angolan) block is exposed over an area 270 miles (450 km) across by 210 miles (350 km) north-south in Kasai, Democratic Republic of the Congo, Lunda, and Angola. The area is overlain by a thick Phanerozoic cover, so most of the Archean rocks are restricted to river valleys. The Archean rocks are divided into three main divisions, including the circa 3.4 billion-year-old Luanyi tonalitic-granodioritic gneiss; two belts of younger, strongly metamorphosed (granulite facies) rocks; and a still younger granitoid and migmatite complex known as the Dibaya Complex. The age of the granulite facies events are constrained to be between 2.77 and 2.84 billion years, with a lower pressure and temperature (retrogressive) metamorphic event at 2.68 billion years ago. The late-stage Dibaya Complex includes calc-alkaline granites and gneisses that are strongly deformed and mylonitized locally, with deformation and migmatization at 2.68 billion years ago. These different assemblages are cut by the unde-formed circa 2.59 Ga Malafundi granites.

The Gabon-Chaillu block consists of two roughly elliptical areas each a couple of hundred miles (several hundred km) across, in Cameroon,

Congo Tectonics
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Map of the Zimbabwe craton showing the distribution of the old gneissic Tokwe terrane, the northern and southern magmatic belts, and shelf-type associations. Numbers correspond to individual greenstone belts: 1 Mount Darwin, 2 Chipuriro, 3 Harare, 4 Chegutu, 5 Midlands, 6 Gweru-Mvuma, 7 Shurugwe, 8 Bubi, 9 Bulawayo, 10 Filabusi, 11 Gwanda, 12 Antelope, 13 Lower Gwanda, 14 Tati, 15 Vumba, 16 Mweza, 17 Buhwa, 18 Belingwe, 19 Masvingo, 20 Mutare. Inset is a tectonic cross section showing evolution of the Zimbabwe craton, with the northern magmatic belt evolving as an Andean-style arc above a major subduction zone from 2.7 to 2.6 billion years ago, while the southern magmatic belts represent where a small ocean basin closed in the same interval.

Equatorial Guinea, Gabon, and Congo. Rocks in this block include an assemblage of 2.8-3.2 billion-year-old charnockite, migmatite, gneiss, greenstone belts, and late-stage 2.7 billion-year-old granitoid plutons. Greenstone belts in the block include complexly folded pillow basalts, rhyolites, quartzite, and banded iron formation, in structural contact with granitoid gneiss and granodiorite. High-grade meta-morphism occurred at 2.9 Ga, corresponding to other high-grade metamorphic events known across central Africa. Much of the northern part of the block was reworked by strong northeast-trending folds, faults, and regional metamorphism at 500 million years ago, in a part of the Central African belt, related to the late Proterozoic-early Paleozoic amalgamation of the supercontinent of Gondwana.

The Kibalian block covers an area about 500 miles (800 km) long by 300 miles (500 km) wide in northern Zaire and the southern Central African Republic, to Lake Mobutu in Uganda. The Kibale and Uele Rivers flow across the block, providing good exposures of the basement rocks. The block is bordered in the north by folded Late Protero-zoic rocks, by the West Nile gneiss Complex on the west, and on the south by strata of the Congo basin. The Kibalian block contains a granite-greenstone assemblage, with granitoids falling into three groups. An older tonalite-trondhjemite-granodio-rite group (TTG) has an age of 2.8 billion years, and younger granites have been dated to be 2.5 billion years old.

The greenstone and schist belts form structurally complex assemblages of mafic schists, intruded by 2.9 billion-year-old tonalites. Of the 11 major greenstone belts, most have a similar rock assemblage, including the lower Kibalian sequence, consisting of mafic to intermediate volcanic rocks and banded iron formation, overlain by the upper Kibalian sequence, consisting of andesite, quartzite, and banded iron formation. At Mambasa the lower Kibalian sequence is thought to overlie unconform-ably the 3.35 Ga Ituri metasedimentary-rich basement gneiss, and is in turn intruded by the 2.5 Ga old Mambasa granite.

A 600 x 300-mile (1,000 x 500-km) area on the central plateau of Tanzania and east of Lake Victoria is known as the Tanzania block. The southeastern part of the block near Dodoma contains mainly granitoids and migmatitic gneisses, with remnants of schist or greenstone belts. These schist belts contain assemblages of quartzite, banded iron formation, schists that locally bear corundum, amphibolite, and mafic and ultramafic gneiss. The granitoid gneisses are about 2.6 billion years old, and the craton is intruded by circa 1.8 Ga late-stage granites. Kimber-lite pipes locally bring up fragments of older, circa 3.1 Ga gneiss, the oldest rocks recognized in the Tanzania block. Schist belts in the central plateau region of Tanzania are very similar to the schist belts of southeast uganda, and this forms the basis of correlating the Tanzania and Kibalian blocks as part of the larger Congo craton.

WEST AFRICAN CRATON

Archean rocks form large sections of the basement rocks between the Gulf of Guinea and the Atlas Mountains. These rocks form parts of the Man and Reguibat shields of the West African craton, and parts of the Tuareg shield.

The Man shield forms the southern part of the West African craton along the Gulf of Guinea and includes the Archean Liberian (Kenema-Man) domain in the west and the Eburnean (Baoule-Mossi) domain in the east. The Liberian domain occupies most of sierra Leone, Liberia, Guinea, Ivory Coast, and part of Guinea Bissau. Rocks in this domain include many structurally complex greenstone remnants consisting of ultramafic rocks, mafic volcanic rocks and banded iron formations, granitoid gneiss, and granites, forming a classical granite-greenstone terrain. some of the gneisses have been dated to be 3.2-3.0 billion years old, whereas the greenstones are intruded by granites that are 2.7 billion years old. Greenstone belts in the west are generally metamorphosed to amphibolite facies, comparatively large, being up to 80 miles (130 km) long, and up to 4 miles (6.5 km) in structural thickness. Greenstone belts in the southeast are smaller, being up to 25 miles (40 km) long, thinner, and show a wide range of metamorphic grades from greenschist to granulite facies. Greenstone belts in the east have more quartzite and pelite than those in the west, and are structurally discordant with basement gneiss and granitoids. structures across the Man shield generally strike northerly and formed in a strong tectonic event at 2.75 billion years ago, known as the Liberian orogeny, that is superimposed on structures from an older event, known as the Leo-nean orogeny.

The westernmost part of the Man shield consists of three narrow belts of Proterozoic-Paleozoic tectonic activity, known as the Rokelides. Conglomerates, sandstones, arkose, and volcanic rocks of the Rokell River Group unconformably overlie the older Kenema basement and are increasingly deformed and metamorphosed to the west. The western margin of the Rokell River Group is marked by large thrusts where recumbently folded klippen of intensely deformed and metamorphosed sedimentary and volcanic rocks of the late Archean-Proterozoic Marampa Group were thrust to the east over the Man shield. Farther west, a 180-mile (300-km) long belt of granulite facies Archean metasedimentary rocks of the Kasila Group represent the core of a deeply eroded orogen. The eastern boundary of the Kasila Group is a 3-mile (5-km) wide mylonite belt, interpreted as an Archean suture that formed when the West African craton collided with the Guiana shield of south America.

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