Petrogenesis and tectonic setting of A-type granites in the Babouri-Figuil Magmatic Complex (North Cameroon): Constraints from whole rock geochemistry, zircon U[sbnd]Pb geochronology and Sr-Nd-Hf isotopes

Abstract

The tectonics of the Central Africa Orogenic Belt (CAOB, in Cameroon, Chad, and the Central Africa Republic) is poorly constrained. For this reason, a study was carried out at the Cameroon-Chad border from biotite granite and hornblende-biotite massifs that intruded a gneiss-amphibolites basement of the NW Cameroon domain. Together, we report data on the whole-rock major and trace element compositions, mineral chemistry, and zircon U/Pb ages, supported by Nd-Sr-Hf isotopes. Both rocks are calc-alkaline, except for one sample with a Rittmann Serial Index (σ) > 3.5 (alkaline), high-K, and marginally peraluminous. The biotite granite is alkalic-calcic, whereas hornblende-biotite granite is alkalic. These rocks are characterized by high contents in SiO2 (> 69.85 wt%), Fe-number [FeOt/(FeOt+MgO)] (ferroan), TiO2/MgO, and low CaO, MgO, and P2O5 contents. Their petrological and geochemical features, such as early formed sodic and iron-rich amphibole, late crystallized biotite, and high Ga/Al ratios, are consistent with A-type granite formed from F and Cl enriched melts. Biotites compositions are close to those of annite, and amphiboles to ferro-amphibole, ferro-edenite, and ferro-gedrite types. The correlation for some major elements against SiO2 is consistent with fractionation of K-feldspar, plagioclase, apatite, amphibole, biotite, FeTi oxides, and MnO-rich phases under 882 to 967 °C, < 5 kb, and low ƒO2 (∆QFM = −0.55) at shallow crustal levels between <1 and 7 Km. Zircon U/Pb geochronology indicates two magmatic episodes for the biotite granite (RBO-27 and SMO-8) 602 ± 1.6 Ma, 602 ± 1.8 Ma, and (DBE-9) 707 ± 2.5, whereas hornblende-biotite granite (HRI-23) yielded 604 ± 1.0 Ma, respectively. The younger group yielded moderately to notably depleted initial Hf and Nd values [εHf(t) = − 3.4 to +1.8, av. = − 1.80; εNd(t) = − 2.46 to - 0.95, at 1.34 to 1.51 Ga], and low to moderate calculated initial Sr values (87Sr/86Sr(i) = 0.66781–0.70287). The older A-type granite yielded highly depleted initial Hf values (εHf(t) = + 7.74 to +9.30). The strong positive Pb anomaly for these granites indicates involvement of either crustal component or melt from the subducted slab. Given that the obtained initial Hf and Nd values are enriched relative to the depleted mantle (εHf(t) ~ +14; εNd(t) ~ +6.5), it therefore, suggest that these granites could be derived from low-degree partial melting of a juvenile crust extracted from the mantle.