Petrology and Sr–Nd Isotopes of High–Grade Amphibolites From Bankim Area (Central Cameroon Shear Zone), Cameroon: Constraints on Petrogenesis and Geodynamic Implications

Abstract

ABSTRACT The Bankim area is located in the Tikar Plain, on the N50E branch of the Central Cameroon Shear Zone (CCSZ) where amphibolites occur, but their relationship with the general Pan–African metamorphism history is poorly known. The petrography, mineral chemistry, geochemistry and Sr–Nd isotope systematics of amphibolites from the Bankim area are presented in this paper in order to constrain their petrogenesis and geodynamic setting during the Pan–African orogeny. They are garnet amphibolites (GA), pyroxene amphibolites (PA) and biotite amphibolites (BA) occurring as metric flagstones showing relic of S 1 foliation, lense–like, or as sheared sinistral and dextral and/or boudinaged enclaves. In GA, garnet occurs as lobate porphyroblasts (Alm 54‐58 Gr 25‐27 Pyr 10‐12 ∙ Sp 3‐5 ). It is usually surrounded by a corona made up of hornblende, plagioclase [andesite (Ab 67‐59 An 32‐46 Or 0.5 ) to oligoclase (Ab 72 An 27 Or 0.6 )], quartz and ilmenite; the whole defining a kelyphitic microstructure (characterised by garnet + amphibole + plagioclase + opaques mineral (ilmenite) parageneses). In PA, clinopyroxene (diopside; Ca 49‐50 Mg 29‐30 Fe 19‐22 ) is mostly relic–like and displays hornblende or plagioclase (andesine (Ab 65‐67 An 32‐34 Or 0.6‐0.8 ) to oligoclase (Ab 69‐70 An 29‐30 Or 1‐1.2 )) corona showing hornblende + clinopyroxene + plagioclase + opaque minerals paragenesis. Field and microstructural data indicate that these rocks recorded a polyphase prograde–peak followed by retrograde metamorphism. The metamorphic prograde phase and peak P–T conditions exceeding 5.2 kbar/789°C occurring during the D 1 deformation phase (613–600 Ma) correspond to the collisional stage of the Pan–African orogeny within the granulite facies. Retrograde metamorphism occurred from the granulite facies to the amphibolite facies under P–T conditions of 1.4–5.2 kbar/789°C–513°C. These changes were synchronous with the early sinistral syn–D 2 (590–580 Ma) and the late dextral syn–D 3 (580–545 Ma) ductile shear deformation phases. These deformation phases, which occurred as a result of exhumation during the crustal re–equilibration related to relaxation and erosion during the post‐collisional evolution of Western Gondwana. The geochemical data indicate that the above amphibolites derived from the metamorphism of mafic protolith which represents an ancient Proterozoic crust with model ages of T DM = 1.9–1.6 Ga and initial 87 Sr/ 86 Sr 600 ratios of 0.70626–0.70894. At the end of the prograde and retrograde metamorphism of this ancient crust, it was stretched, sheared, dismembered and scattered along the Pan–African mobile zone during the CCSZ reactivations.