Geochemistry of magmatic rocks and time constraints on deformational phases and shear zone slip in the Méiganga area, central Cameroon

摘要

The Meiganga area is situated in the Adamawa–Yade domain (AYD) of the Pan-African fold belt in Cameroon. The AYD is characterized by abundant plutonic rocks that intruded Palaeoproterozoic gneisses. It is cut by the transcurrent Central Cameroonian Shear Zone (CCSZ). The studied area is made up of metadiorite (MD), pyroxene-bearing granite (PGr) and biotite-muscovite granite (BMGr), hosted in a metasedimentary sequence of amphibole-biotite gneisses. The granitoids are metaluminous to slightly peraluminous, and mainly of I-type (ASI ≤ 1.1), representing a high-K calc-alkaline to shoshonitic suite. They were derived from crustal protoliths (BMGr), from rocks of mantle origin (MD, PGr), and/or from the remelting of metabasalt or metatonalite (MD, PGr). Four successive deformational phases (D1, D2, D3, and D4) are present in the Méiganga area. The S1 foliation is formed by tectonic transposition of the S0 primitive surface represented by contacts between the gneiss and intercalated amphibolites.The synmigmatitic D2 deformational phase is characterized by S2 banded schistosity, S2/C2 sinistral shear planes, and F2 folds with axes parallel to a L2 stretching lineation.Imprints of the D2 and subsequent deformational phases are similar in the metadiorite and host rocks, implying syn-D2 emplacement and crystallization of the metadiorite; therefore 614–619 Ma ~(207)Pb/~(206)Pb zircon evaporation ages obtained for this rock type date the syn-D2 magmagenesis. Similarly, the D3 phase of the PGr is 601 ± 1 Ma, dated by the ~(207)Pb/~(206)Pb evaporation method. D4 is a late-stage brittle deformational phase. Sinistral movement of the CCSZ is associated with D2, whereas its latest activity, characterized by dextral slip, cannot be older than emplacement of the 558 ± 2 Ma BMGr (~(207)Pb/~(206)Pb zircon evaporation age).