Nseka-Mbemba, P.,  Glorie, S.,  Archibald, D.,  De Grave, J.,  Baudet, D.,  Delvaux, D.,  Fernandez-Alonso, M.,  Kongota-Isasi, E.,  Love-Pay, A. & Tack, L. 2018. ‘The Mao gneiss dome (Bas-Congo region, DR Congo): missing link between the geologic setting of the Boma and Matadi areas in the West Congo Belt’. CAG 27 Portugal, Aveiro abstracts: 5.

Article in a scientific Journal / Article in a Journal

The Pan African West Congo Belt (WCB) is part of the Araçuaí-West Congo Orogen (AWCO) formed during Gondwana amalgamation (550 Ma). The AWCO is underlain by 2.1 Ga Eburnean-aged gneissic basement. The WCB extends from SW Gabon to NW Angola along the Atlantic Ocean. The NNW-SSE striking segments of the fold-and-thrust belt show eastward decreasing deformation and regional metamorphism. The westernmost part of the Bas-Congo region of DR Congo reveals the deepest crustal segment of the belt (amphibolite facies) (Tack et al., this meeting). In the Boma area (westernmost Bas-Congo region), 930-870 Ma bimodal rocks of the Shinkakasa Plutonic Complex (SPC) displaying magma mixing features intrude 2069 Ma orthogneissic basement (Pedrosa-Soares et al., this meeting). In the Matadi area, fifty km to the East, metaquartzites of the Mesoproterozoic Nzadi Group (Baudet et al., this meeting) overlie gneissic basement and are intruded by the 1.0 Ga Noqui peralkaline granite (De Grave et al., this meeting). In between, the Mao area (20 x 20 km) is poorly known because of difficult access. The preliminary geologic sketch map is based on remote sensing of the 1970-ies supported by little field data. It shows complex gneiss dome morphology, locally surrounded by metaquartzites and intruded by a circular alkaline granite (7 km diameter). New U-Pb single zircon dating by LA-ICPMS has been performed respectively on the three rock types. The 2036 ± 11 Ma augengneiss of the dome is in line with available ages for the Eburnean-aged basement of the WCB. The 1008 ± 10 Ma and 990 ± 10 Ma ages for the alkaline granite are in line with the 1.0 Ga Noqui peralkaline granite (Matadi area). The maximum deposition age of the Matadi metaquartzites is 1126 ± 29 Ma and the provenance analysis histogram is in line with those of the metaquartzites of the Matadi type area. In addition, field mapping in progress reveals bimodal intrusive rocks reminiscent of the Tonian SPC within the gneiss dome. The Mao area thus shows a geologic setting combining features of both the Boma and Matadi areas. Unlike the Matadi area and because of the deeper level of crustal exposure, the Mao granite intrudes the Mao basement dome, which itself is surrounded by the overlying Matadi metaquartzites. Similarly to the Boma area, the Mao dome is intruded by Tonian intrusive rocks of the SPC-type. In the WCB the Mao dome is not an isolated feature. A similar gneiss dome morphology extends to the North in DRC (Luki-Temvo dome) and probably even in southern Congo-Brazza (Mfoubou area). It abounds in NW Angola (Noqui-Tomboco geologic sheet map). In the easternmost part of the Araçuai Belt of Brasil basement domes have not been recognized. The geodynamic origin of the 2.1 Ga gneissic domes surrounded by Late Mesoproterozoic metaquartzites and intruded by 1.0 Ga (per)alkaline granites is debatable. It may be explained in terms of tangential extensional detachment of the Eburnean-aged basement of the AWCO followed by updoming as a result of emplacement of (per)alkaline granites (Theunissen et al., this meeting).