Geology of the Democratic Republic of the Congo

In Kasai, the metamorphic basement rocks of the Angolan Shield are bounded by a fault at 4 degrees south latitude, with underlying poorly exposed migmatite and gneiss and overlying rocks from the past 541 million years of the Phanerozoic. The oldest rocks in the DRC are the 3.4 billion year old Upper Luanyi granite gneiss, which contains pegmatites, metamorphosed to amphibolite grade metamorphic facies. Faults are believed to separate these rocks from the Kanda Kanda gneisses, which contain lenses of pink alaskite gneiss, probably formed near granulite grade metamorphism and originating from charnockite, gabbro and norite rocks in the Kasai-Lomami group. Geologists estimate that the Kanda Kanda gneisses are formed between 3.4 and 2.82 billion years ago.

The Kasai-Lomami group has two rock units. One is a mafic unit with gabbro, norite, amphibolite and anorthosite. Taken together, they are the remains of magma intrusions metamorphosed to granulite grade. The second unit contains dark gneiss and aluminium-rich granulite, which may have originated from sediments, along with metadolerite dikes. The metamorphism and deformation that affected both units likely happened 2.8 billion years ago.[1]

The West Congolian mobile belt became a foreland basin during the Pan-African orogeny and forms the granitoid Chaillu Massif spanning into the Republic of Congo through Gabon. The massif has a north-south foliation and two generations of granitoids. Gray granodiorite shifts to quartz, diorite, biotite and amphibolite-rich types, cut by veins of pink, potassic migmatite. The Chaillu Massif granitoids are 2.7 billion years old and engulfed pre-existing schist and greenstone.[2]

In northeastern DRC, Archean gneisses and granite-greenstones terranes are common. The Bomu Gneissic Complex and the West Nile Gneissic Complex basement gneiss rocks are 3.5 billion year old, while the more scattered Ganguan Greenstone Belt in the west and the Kibalian Greenstone Belt in the east formed between 3.2 and 2.6 billion years ago. The Bomu Gneissic Complex outcrops near the Bomu River and the Uele River at the border with the Central African Republic. Out of four named types of gneiss, the Bomu gneiss, rich in amphibolite and pyroxene is the largest group in a synform structure in northern DRC. The different gneisses probably began to form 3.5 billion years ago from oceanic source material and were later intruded by tonalite around 3.41 billion years ago.

Granulite rocks in the same region as the West Nile Gneissic Complex pre-date the Watian orogeny 2.9 billion years ago and contain charnockitic dolerite dikes, with isoclinal folding of the rock. After the Watian orogeny, volcanic and sedimentary rocks in what is now northeast DRC metamorphosed into the Western Gray Gneissic Group with biotite, hornblende and microlite. The subsequent Auran orogeny 2.68 billion years ago gave these rocks a northeast plunging fold.

The Ganguan Greenstone Belt is 3.2 billion years old and contains sericite quartzites, quartz phyllite, talc schist poor in quartz and chlorite schist. The Kibalian Greenstone Belt appears at surface level to be separate belts, divided up by the Upper Congo Granitoid Massif, but geologists believe it is actually a continuous greenstone belt with mafic and intermediate volcanic rocks in the west and banded iron formations in the east. Granitoid rocks are the most common in northeastern DRC, probably formed from monzonite granites and tonalities.[3]

The Lufilian Arc, now located between central Zambia and DRC's Shaba Province formed in an extension zone in the Neoproterozoic, during the Damara-Lufilian-Zambezi orogeny. The Lufilian Arc separates the Congo Craton from the Kalahari Craton and the Bangweulu Craton and overprints alignments in the rock from the Kibaran orogeny and the Irumide orogeny 1.3 to 1.1 billion years ago. Beginning 875 million years ago, up to 10 kilometers of sediment deposited in rift basins and sag basins in Katanga. In Katanga, the bottom of the sedimentary sequence is breccia, overlain by carbonaceous shales, iron formations and the remains of pyroclastic flows in the Mwashya Group. The overlying Kundelungu Supergroup starts off with glacial sedimentary rocks from the Neoproterozoic Snowball Earth event followed by a cap of carbonates.

The sediments in Katanga metamorphosed to greenschist and amphibolite grade during the Pan-African orogeny. Away from the mobile belts, the Congo Craton has large sedimentary rock formations leftover from the molasses areas associated with the different orogenies deep in the past, which include stromatolite and acritarch fossils. Some examples include the dolomitic shales, conglomerates, quartzites, siltstones and shales of the Mbuyi Mayi Supergroup in the southeast or the quartzites and carbonate assemblages of the Lindian Supergroup in the north, formed from stromatolites. [3]

Downwarping of the crust of the Congo Craton in the Mesozoic left the basin to fill with river and lake sediments in the Late Jurassic and Early Cretaceous, forming the Lualaba Series and Kamina Series. The Kwanga Series preserves fossilized freshwater fish and dates to the Late Cretaceous, along with alluvial diamonds at its base, suggesting kimberlite pipe eruptions at the time.

After a long period of relative tectonic quiet, volcanism started in the geologically recent past of the late Cenozoic in the Western Rift of the East African Rift System, at the same time that eruptions began in the Kenyan Rift. The oldest basalt at the base of the Virunga Massif, east of the current rift is 14 million years old. Alkaline basalts erupted from 13 million to nine million years ago. In the late Pliocene, the fracture pattern changed to northeast-southwest, building up a chain of volcanic mountains formed from potassic, undersaturated lava. [4]

The Wembo-Nyama ring structure in Omeonga is a possible impact crater in DRC, the World Impact Crater list by Anna Mikheeva lists it as "potential".[5]