Petrography and geochemistry of the Ngaoundéré Pan-African granitoids in Central North Cameroon: Implications for their sources and geological setting.
Résumé
The Nagoundéré Pan-African granitoids in Central North Cameroon belong to a regional-scale massif, which is referred to as the Adamawa-Yade batholith. The granites were emplaced into a ca. 2.1 Ga remobilised basement composed of metasedimentary and meta-igneous rocks that later underwent medium- to high-grade Pan-African metamorphism. The granitoids comprise three groups: the hornblende–biotite granitoids (HBGs), the biotite ± muscovite granitoids (BMGs), and the biotite granitoids (BGs). New Th–U–Pb monazite data on the BMGs and BGs confirm their late Neoproterozoic emplacement age (ca. 615 ± 27 Ma for the BMGs and ca. 575 Ma for the BGs) during the time interval of the regional tectono-metamorphic event in North Cameroon. The BMGs also show the presence of ca. 926 Ma inheritances, suggesting an early Neoproterozoic component in their protolith. The HBGs are characterized by high Ba–Sr, and low K2O/Na2O ratios. They show fairly fractionated REE patterns (LaN/YbN 6–22) with no Eu anomalies. The BMGs are characterized by higher K2O/Na2O and Rb/Sr ratios. They are more REE-fractionated (LaN/YbN = 17–168) with strong negative Eu anomalies (Eu/Euasterisk operator = 0.2–0.5). The BGs are characterized by high SiO2 with K2O/Na2O > 1. They show moderated fractionated REE patterns (LaN/YbN = 11–37) with strong Eu negative anomalies (Eu/Euasterisk operator = 0.2–0.8) and flat HREE features (GdN/YbN = 1.5–2.2). In Primitive Mantle-normalized multi-element diagrams, the patterns of all rocks show enrichment in LILE relative to HFSE and display negative Nb–Ta and Ti anomalies. All the granitoids belong to high-K calc-alkaline suites and have an I-type signature. Major and trace element data of the HBGs are consistent with differentiation of a mafic magma from an enriched subcontinental lithospheric mantle, with possible crustal assimilation. In contrast, the high Th content, the LREE-enrichment, and the presence of inherited monazite suggest that the BGs and BMGs were derived from melting of the middle continental crust. Structural and petrochemical data indicate that these granitoids were emplaced in both syn- to post-collision tectonic settings.
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