The Pan-African fayalite quartz-monzonite from north-central basement of Nigeria

Enigmatic fayalite and orthopyroxene-bearing quartz-monzonite, locally named bauchite, is identified at the lowest exposed structural level of the Pan-African basement in Nigeria. This very iron-rich rock challenges the typical Bowen's reaction series, which suggests that olivine and quartz should not coexist. Nigeria's Precambrian basement consists of a series of metamorphosed magmatic and sedimentary rocks including schists, quartzites, amphibolites, and calc-silicates, transitioning into a granitoid-gneiss complex, designated as the Bauchi complex, in the north-central region. The lowest structural level of this complex consists, from bottom to top, in bauchite, hornblende-biotite granite and biotite granite, which is in contact with granulite facies migmatites. Earlier studies attributed bauchite formation to the impregnation of granites by iron-rich fluids and argued that the coexistence of ortho- and clino-pyroxenes with fayalite and quartz suggests deep-crustal magmatic emplacement (≈30 km depth).

Our field investigations indicate that bauchite and surrounding granite, crosscuts the regional scale NW-SE trending foliation of the host migmatites, which is consistent with an intrusive plutonic body. The preferred orientation of feldspar phenocrysts in bauchite but also in granites, delineates a shallow-dipping magmatic foliation and a regional-scale domal structure. Bauchite, exposed in the core of the dome, has a granular texture, with microcline and albite phenocrysts in a matrix of fayalite, ortho- and clino- pyroxenes, hornblende, biotite, and quartz. The accessory minerals present are zircon, apatite, magnetite, ilmenite, and titanite. At the lowest structural level, green bauchite dominated by fayalite and pyroxenes grades in brown bauchite characterized by a larger amount of hornblende and biotite. Textural analysis indicates a magmatic layering delineated by the alternation of fayalite-pyroxenes and microcline-albite layers. Interstitial quartz shows no signs of intracrystalline deformation, consistent with late crystallization from a melt. Hornblende shows lobate contacts with feldspars and typically forms a corona around fayalite and pyroxenes. Biotite is present as euhedral crystals in contact with hornblende. Microcline is typically bordered by myrmekite. These textures point to a reaction between fayalite-pyroxenes and microcline-albite layers leading to the crystallization of hornblende, biotite and quartz. Bauchite samples have an average SiO₂ content of 65%, a high FeO/MgO ratio (14-17), and low Mg/(Fe+Mg) ratios (0.09-0.12). Their average K/(Na+K) is 0.49, with K₂O exceeding 4%, making them highly potassic. The SiO₂ content negatively correlates with most major oxides except K₂O and Na₂O, which show positive correlation. Trace elements data show high concentrations of Rb, Ba, K, and Zr, along with negative anomalies in Nb, Sr, P, Ti, Gd, Lu, and Y but positive anomalies in Zr, pointing to a deep-seated, alkaline magma. These features are consistent with an origin of bauchite resulting from interaction between an exotic iron-rich mantle derived alkaline magma and a felsic hydrous crustal one.