Search

A less prominent N-S oriented continental-scale strike-slip shear zone occurs within the Babouri-Figuil Magmatic Complex (BFMC). Studies have shown that such shallow to middle crustal levels shear zones often display long-lasting deformation, physical expression of strain localization, and mineralization. However, until dates, deformation evolution and structural mineralization of this ~ 8 km long shear zone in Babouri-Figuil remain enigmatic, which prompted its investigation. A combined structural study supported by U-Pb zircon dating on the sheared granitoids is reported. Biotite granite sampled near Sorawel village yielded zircon U-Pb concordia age of 606 ± 6 Ma, whereas hornblende-biotite granite sampled at the western corner of the Ribao massif yielded zircon U-Pb concordia age of 601 ± 2 Ma. These granites were emplaced in a transpressive tectonic regime shortly after a regional thickening event < 620 Ma. The studied microstructures demonstrate a typical transition from protomylonite to extremely deformed mylonite marked with decreased mineral sizes. On thin sections, two main zones are identified: "zone A" (low-strain zone) and "zone B" (high-strain zone). Core-mantle K-feldspar porphyroclasts within the mylonite show dislocation creep deformation, whereas recrystallized to neocrystallized aggregates indicate plastic flow by viscous grain boundary sliding. Shear sense indicators revealed both early dextral and late sinistral shearing. In all, the results revealed that the Guider-Sorawel shear zone (GSSZ) experienced ductile deformation probably under medium to high temperature conditions. The double shearing is possible during the switch of the regional maximum principal stress from N-S (~ 585 Ma) to NW-SE (< 580 Ma). The obtained dates are syn-collisional and are consistent with D2 deformation phase in other parts of Cameroon. Also, the dates marked the final tectonic college between the north Cameroon domain and the southwest Chad domain, which probably might have induced the SZ. The construction of these adjacent domains probably resulted to a sort of increased temperature and strain localization that led to the initiation of a rheologically weaken mechanical zone via which the magma generated flows to near surface crustal level. [ABSTRACT FROM AUTHOR]

The Babouri–Figuil Magmatic Complex (BFMC, adjacent to the central-south Chad domain) is located on the southern flank of the Saharan Metacraton. It is amongst the least geologically studied regions in Cameroon; only a few key research papers on geochronology and isotopes have been completed. The emplacement of granitic bodies is poorly constrained between the early and late–Neoproterozoic with inherited zircons of older Paleoproterozoic–Neoproterozoic ages. In this study, undeformed biotite granite massifs (Lombel and Badesi) were studied, and their geochronologic, geochemical and mineral chemical data, including Nd–Sr–Hf isotopes, are presented. The massifs are enriched in pink K-feldspar (orthoclase and microcline; 60–65 wt%) and plagioclase (8–10 wt%) anastomosing to intergranular quartz (18–20 wt%) and biotite (3–5 wt%) crystals. The Lombel massif yielded a Concordia age of 566 ± 2.5 Ma (MSWD = 1.6, zircon U–Pb), slightly older than the Concordia age of 556 ± 2.7 Ma (MSWD = 2.1, zircon U–Pb) obtained for the Badesi massif. These are interpreted as emplacement ages, which post–date arc magmatism by ∼ 45 Ma. The geochemistry of the massifs showed sub- to mid–alkaline, high–K alkali–calcic, and weakly peraluminous compositional characteristics. The calculated biotite temperatures ranged from 613 to 647 °C, pressures from 100 to 300 MPa, and H 2 O from 3.0 to 3.5 wt%. Whole–rock composition geothermobarometry revealed low pressure of 100–500 MPa, depths of 3.5–18 km and low oxygen fugacity (ΔQFM = −0.6), indicating reducing crystallization conditions. The estimated radiogenic isotopic values are similar for both massifs, falling in the range from initial ε Hf (t) = +2.6 to +8.4 and initial ε Nd (t) = +1.3 to +2.1, with a corresponding average initial Sr of 0.70212–0.70458. These radiogenic isotope values suggest a mantle–derived origin for this granite or the melting of a juvenile mafic crustal protolith, as confirmed by the two–stage model age obtained (TDM 2 = ∼ 1.2 Ga). Given that biotite is interstitial in alkali feldspar and plagioclase in both massifs, this suggests that biotite chemistry records conditions of magma evolution. Overall, these results confirm that the studied massifs are post–collisional products emplaced at shallow crustal levels under evolved magmatic conditions. They are coeval with a few post–magmatic intrusions in the central–south Chad and emplaced after the Sao–Francisco Congo Craton collided with the Saharan Metacraton. The ages obtained enabled a possible correlation with several Neoproterozoic post–collisional granites in Cameroon, south-central Chad, Central African Republic and Borborema Province of Brazil. The ages also revealed that continental assembly coupled with regional late–magmatism occurred during the Proterozoic. • Post-collisional A-type granite intrudes the Babouri-Figuil Complex. • They are emplaced between 566 ± 2.5 Ma and 555 ± 2.7 Ma, after orogeny. • They are peraluminous, alkali-calcic and sub-to mid-alkaline granites. • All authors contributed in this research. [ABSTRACT FROM AUTHOR]

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 SiO 2 (> 69.85 wt%), Fe-number [FeOt/(FeOt+MgO)] (ferroan), TiO 2 /MgO, and low CaO, MgO, and P 2 O 5 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 SiO 2 is consistent with fractionation of K-feldspar, plagioclase, apatite, amphibole, biotite, Fe Ti oxides, and MnO-rich phases under 882 to 967 °C, < 5 kb, and low ƒO 2 (∆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. • The Babouri-Figuil granitoids show two magmatic episodes of ~700 Ma and ~ 600 Ma. • The ages indicate that they belong to pre-, syn - to late-collisional granites. • Nd-Sr-Hf data suggest mantle derived with various degree of crustal contamination. [ABSTRACT FROM AUTHOR]