Petrology and geochemistry of the Yoro-Yangben Pan-African granitoid intrusion in the archaean Adamawa-Yade crust (Sw-Bafia, Cameroon)

摘要

South of Bafia (Cameroon), the Pan-African Yoro-Yangben massif is emplaced in the Adamawa-Yade block, an archaean-paleoprotrozoic micro-continent that detached from the Congo craton during the early Neoproterozoic. This emplacement within a micro-continent raises the problematic of its (i) origin either by the melting of a subducted slab and the mantle wedge, or by the melting of the base of a thick crust or both, and (ii) bearing on the evolution of the Pan-African orogeny in Cameroon. Granodiorites, quartz monzonites, quartz diorites and adamellites occur as massive bodies crosscut by dykes of gabbros and granites. The texture varies progressively from granular bands at the core of the massif to foliated texture towards the border. Mineralogical composition is constant in all the rock types (Am + Bt + PI + Kfs + Qtz + Op + Sph + Zrn + Ap), except the adamellites which are amphibole-free. Chemically, these potassic rocks are either magnesian and metaluminous, or ferroan and peraluminous. Quartz diorites, granodiorites and granites are High-K calc-alkaline whereas quartz monzonites, adamellites and gabbros are shoshonitic. Most incompatible elements (e.g. Nb, Zr) show an erratic variation with increasing silica contents. Granites rare earth elements (REE) patterns are highly fractionated with a positive Eu anomaly (Eu/Eu-star = 2.50-2.04) compared to less fractionated REE patterns of adamellites, quartz monzonites, and granodiorites. The negative anomalies in U, Nb, Ta and Ti shown by gabbros, quartz diorites, quartz monzonites and granodiorites spider diagrams as Th, K, Pb, Zr and Hf positive anomalies for the adamellites and granites patterns are consistent with magmas derived from a subduction setting. The low Sr-87/Sr-86((600)) initial ratio (0.697-0.706) and epsilon Nd-i values (-0.82706-1.03707) of the rocks plot in the mantle array. However, a wide variation of the Sr-87/Sr-86 ratio values with constant epsilon Nd-i suggests an enrichment in radiogenic Sr-87 likely from an old crust displaying high Rb/Sr ratio. The variations of the La/Yb, Y/Nb and Zr/Nb ratios preclude the fractional crystallization process. Moreover, these characteristics, combined with the erratic (Zr, Nb) suggest particular magmatic sources and complex fusion processes to the Yoro-Yangben heterogeneous granitoids. The lower range of Ba/Nb values (130-135) and the occurrence of shoshonitic magmas likely point to the melting of an oceanic slab; while the higher range (225-230) indicates a blend of melts from oceanic slab and the associated sediments. Granite compositions are similar to melting liquids of metagreywackes. La/Sm and Sm/Yb ratios in gabbros, quartz monzonites and quartz diorite indicate an origin by a batch melting of phlogopite-amphibole-gamet lherzolite, whereas granodiorites and adamellites parent magmas may have originated from a batch melting of a metasomatized spinel lherzolite. Whole rock Sm-Nd and Rb-Sr results yield a unique age of 600 Ma, assigned to the collision time between the West-African and the Congo cratons. Thus, the Yoro-Yangben intrusion is part of the numerous Pan-African granitoids in Cameroon, South of Chad and East of Nigeria.