Mineralogy and Bulk‐Rock Geochemistry of Mafic Rocks From Bocaranga, Adamawa‐Yadé Domain of Central African Republic: Evidence of Paleao‐Oceanic Crust on the Northern Edge of the Congo Craton

ABSTRACT The Bocaranga area belongs to the northern part of the Adamawa‐Yadé Domain (AYD) in the Central African Republic (CAR) where the occurrence and petrogenesis of mafic rocks still remain a subject of debate. New field observations, petrographic, mineralogical, and geochemical data of mafic rocks from Bocaranga show palaeo‐oceanic crust character within the Adamawa‐Yadé domain on the northern margin of the Congo Craton. These mafic rocks consist of amphibolites (biotite‐amphibolites, garnet‐amphibolites, epidote‐amphibolites and actinolite‐amphibolites) and amphibole‐pyroxenites associated with gneisses of trondhjemite, tonalite, and granodiorite compositions (TTG). These mafic rocks display basalts, dioritic basalts, and diorites geochemical character, sub‐alkaline to tholeiitic N‐MORB and E‐MORB affinities, with Th (0.03–1.60 ppm) and U (0.01–0.49 ppm) contents very close to the average values for oceanic crust (Th = 0.40 ppm, U = 0.12 ppm). Amphibolites derived from 5% to 15% degrees of partial melting of a mantle source are enriched in LREE relative to HREE ([La/Sm] N = 1.53–2.04; [Gd/Yb] N = 0.98–1.54; [La/Yb] N = 1.60–3.80), show slight Nb, Zr, Hf, and Ti negative anomalies, as well as low ratios of (Dy/Yb) N (1.1–1.3) < 1.6 and of Nb/Y (0.1–0.2) which are compatible with the contribution of subduction components. These data are different from those from the Ngaye amphibolites (Cameroon) and those in the Adamawa‐Yadé domain, which are almost similar to those of the Banda tholeiites in south‐eastern CAR on the Congo Craton edges, in terms of petrography and geochemistry. Amphibole‐pyroxenites show a relatively low REE fractionation ([La/Sm] N = 0.53; [Gd/Yb] N = 1.04; [La/Yb] N = 1.0), derived from a relatively high degree of partial melting (35%) which is compatible with their low HFSE levels and low ratios in (La/Yb) N = 0.58. Similar results are described in neighbouring regions (Sudan, Chad and Cameroon) for the Bayuda Desert amphibolites, the South Ouaddaï amphibolites, the Ngaye amphibolites, Banda, and Mbomou amphibolites (Congo Craton). The petrographic and mineralogical data indicate that mafic rock from Bocaranga underwent prograde‐(peak?)‐retrograde metamorphism whose prograde‐peak phase was attained in the upper amphibolites – to low granulite facies, coeval to the continent–continent collision between the Saharan Metacraton and the Congo Craton. The retrograde phase followed peak metamorphism in amphibolite facies for P – T conditions of 1.4 – 5.2 kbar and 742°C – 876°C related to decompression coeval to the post – collisional history of Western Gondwana.