The iron-rich alkaline Bauchi Complex (North-Central basement of Nigeria): an olivine-clinopyroxene-quartz monzonite emplaced into the partially molten root at the junction of the Pan-AfricanTrans-Saharan and Central African Orogenic Belts

The Pan-African Fe-rich alkaline Bauchi Complex (Central Basement of Nigeria) is located at the junction of the N-S Trans-Saharan Belt (TBS) and the E-W Central African Orogenic Belt (CAOB). It is made of a peculiar metaluminous olivine-clinopyroxene-quartz monzonite-monzogabbro suite hosted by migmatitic paragneiss. Previous models for the Bauchi Complex have debated whether it corresponds to a mantle or crustal-derived magma modified by metasomatism. Our integrated field, airborne geophysical (radiometric and magnetic), and geochemical data point to a more complex hybridization process. The diffuse contact between the migmatitic paragneiss and the biotite heterogeneous granite, grading into monzogranites reflects a progressive metamorphic gradient. The NW-SE trending moderately dipping syn-migmatitic foliation of migmatitic paragneiss is transposed into the dominantly NE-SW trending shallow dipping magmatic foliation of the Bauchi Complex indicating a syntectonic intrusion of the Bauchi Complex in partially molten rocks. The anatectic origin of the heterogeneous granites is further supported by their S-type, peraluminous (A/CNK > 1), magnesian, calc-alkaline signature. In contrast the monzonite-monzogabbro of the Bauchi Complex display a metaluminous (A/CNK < 1), intermediate-to-mafic, transalkaline signature characteristic of I- or A-type magma derived from partial melting of either a mafic lower-crust or metasomatized, enriched lithospheric mantle (SCLM) under reducing conditions. The higher potassium content of the monzonite-monzogabbro, ranging from 3.18 to 4.6 wt%, compared to the one of the host migmatite ranging from 2.13 to 2.33 wt%, precludes an origin from fractional crystallization of a magma derived from the paragneiss. The interaction between the deep-seated and the locally derived crustal anatectic melts, marked by transitional monzogranites with intermediate signatures facilitates mixing and hybridization. This results in the observed spatial polarity from the host migmatitic paragneiss to the monzonite-monzogabbro of the Bauchi Complex. These data suggest that the monzonite-monzogabbro of the Bauchi Complex intruded the partially molten root of the Pan-African orogenic belt during the transition between tectonic accretion of the N-S trending Trans-Saharan Belt and activation of NE-SW trending shear zones parallel to the northern margin of the Congo craton. • Bauchi Complex lies at the junction of the Trans-Saharan and Central African belts. • Integrated geophysical and geochemical data reveal a complex hybridization process. • I- or A-type metaluminous suite marks a transition from S-type crustal anatexis. • Potassium enrichment precludes origin from simple fractionation of host paragneiss. • Dextral shearing parallels the Central African Shear Zone.