22 research outputs found
Syn-kinematic ferroan high-K I-type granites from Dschang in southwestern Cameroon: U–Pb age, geochemistry and implications for crustal growth in the late Pan-African orogeny
No full textGermany Academic Exchange Organisatio
Nature, origin and significance of the Fomopéa Pan-African high-K calc-alkaline plutonic complex in the Central African fold belt (Cameroon)
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Petrology and Sr–Nd Isotopes of High–Grade Amphibolites From Bankim Area (Central Cameroon Shear Zone), Cameroon: Constraints on Petrogenesis and Geodynamic Implications
No full textABSTRACT 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
Petrography and geochemistry of amphibolites from the Fomopéa Pluton (West Cameroon): Origin and geodynamic setting
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Comment on “The Adamawa–Yade, a piece of Archaean crust in the Neoproterozoic Central African orogenic belt (Bafia area, Cameroon)“, by Jacqueline Tchakounté et al. [Precambrian Research 299 (2017) 210–229]
No full textThe mineralogy and geochemistry of a subvolcanic alkaline complex from the Cameroon Line: The Nda Ali massif, southwest Cameroon
No full textThe Pan-African Kekem gabbro-norite (West-Cameroon), U-Pb zircon age, geochemistry and Sr-Nd isotopes: Geodynamical implication for the evolution of the Central African fold belt
No full textInternational audienceThe Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U-Pb zircon analyses provide an age of 576 ± 4 Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ). The REE patterns display enriched LREE (LaN/YbN = 14.2-23.5) while HREE present a nearly flat profile (DyN/YbN = 1.3-1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb-Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6-10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068-0.7082), low εNd (−5 to −9) and old Nd TDM model ages (1.6-1.8 Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust. The ca. 576 Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle
40Ar–39Ar ages from the Sabongari and Nana igneous complexes within the central part of the Cameroon Line (Central Africa)
No full textLes orthogneiss calco-alcalins de Foumban–Bankim : témoins d'une zone interne de marge active panafricaine en cisaillement
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