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_Journal of Structural Geology 32 (2010) 306–320_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Emplacement and deformation of the Fomope? a pluton: Implication for the Pan-African history of Western Cameroon T. Njanko, A. Ne? de? lec*, M. Kwe? kam, R. Siqueira, L. Esteban, C. Geological Survey of Canada-Paci?c, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada; D. CSIRO, ARRC, Petroleum Resources, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia Article info Article history: Received 19 August 2008 Received in revised form 10 December 2009 Accepted 24 December 2009 Available online 6 January 2010 Keywords: AMS Cameroon Granite Pan-African Shear zone Abstract The Fomope? a pluton (622–613 Ma) is located in the western part of the Pan-African belt in Cameroon. It comprises three units: biotite-hornblende granitoids (BHG), biotite monzogranites (BmG) and edenite syenogranites (EsG). The BHG unit displays magnetic fabrics characterized by foliations gently dipping towards the ESE and ENE-trending lineations (D1 event). Microstructures are magmatic to submagmatic. In discrete N–S deformation bands (D2), lineations are rotated towards the North and microstructures indicate solid-state deformation at mid-to low-T conditions, with kinematic indicators pointing to a sinistral motion. The second unit, BmG, displays lineation trajectories suggesting emplacement during the D2 event. The EsG unit displays fabrics consistent with a later emplacement, with no superposed deformation. The last event (D3) corresponds to a dextral shear zone that runs along the southeastern border of the Fomope? a pluton. It was responsible for protomylonitic deformation of the granitic rocks in greenschist facies conditions, whereas the core of the shear zone registered higher temperatures and strain. This shear zone induced a rejuvenation of the Rb-Sr isotopic system in the pluton at ca 572 Ma. It belongs to the Central Cameroon shear zone system, regarded as the prolongation of the dextral Patos shear zone system in Brazil. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Numerous studies have underlined the interest of granitoids as crustal scale deformation markers (e.g., Gapais, 1989; Solar et al., 1998; Nascimento et al., 2004; Sadeghian et al., 2005; Archanjo et al., 2008). According to Hutton (1997) and Brown and Solar (1998), spatial and temporal relationships between granite and regional tectonic structures suggest granite emplacement during contraction rather than during extension in many convergent orogenic belts. Granites are almost invariably linked to orogenic environments, and the close correlation established between shear zone systems, transpressive convergent zones, and regions of melt transfer and granite emplacement is well documented (D’Lemos et al., 1992; Ingram and Hutton, 1994; Leblanc et al., 1996; Brown and Solar, 1998). Granite emplacement and deformation during regional tectonic events is a challenging study because granitic rocks do not always develop mesoscopic scale deformation fabrics. However, according to Simpson (1985), microstructural studies of granites can help identify magmatic or solid-state deformation fabrics. The Pan-African Belt of Central Africa (PBCA) is connected with the Neoproterozoic Brasiliano Fold Belt (Brito Neves et al., 2002; Cordani et al., 2003), as shown in Fig. 1a from Caby et al. (1991). However, it is less well known. In Cameroon, the Pan-African tectonic evolution is characterized by large scale shear zones such as the Adamaoua Shear Zone (ASZ) and the Tchollire? -Banyo fault (TBF), that have transposed early structures (Fig. 1b). Regarding the evolution of the Pan-African belt in central and south Cameroon, Ngako et al. (2003) proposed a model related to major events of transpression and transtension during shear movements. Granite plutons spatially associated with shear zones are large composite intrusions generally made up of more than two rock types. In the central domain of the PBCA, detailed petrology of some plutons is provided: e.g., the Ngondo plutonic complex, the West Tibati plutons, the two-mica granites from the Nkambe? area (Fig. 2b), respectively studied by Tagne? -Kamga (2003); Njanko et al. (2006) and Tetsopgang et al. (2006). Other studies are only available in unpublished theses. Moreover, in these works, structural aspects are generally missing due to the lack of appropriate methods. T. Njanko et al. Journal of Structural Geology 32 (2010) 306–320 307 Fig. 1. (a) Pre-drift reconstruction. (b) Geological map of Cameroon showing the major lithotectonic domains (after Toteu et al., 2001, and Ngako et al., 2008). ASZ: Adamaoua Shear Zone; CCSZ: Central Cameroon Shear Zone; GCSZ: Godé? -Gormaya Shear Zone; MNSZ: Mayo Nolti Shear Zone; Pa: Patos Shear Zone; Pe: Pernambuc Shear Zone; RLSZ: Rocher du Loup Shear Zone; SF: Sanaga Fault; TBF: Tchollire? -Banyo fault. 308 T. Njanko et al. Journal of Structural Geology 32 (2010) 306–320 Fig. 2. (a) Geological sketch map of central lithotectonic domain showing the previously studied plutons (stars) with available geochronological data; FFMZ: Fotouni-Fondjomekwet Mylonitic Zone. (b) Geological sketch map of Fomope? a granitic pluton showing the sampled stations and locations of dated samples (stars). The purpose of this paper is to present a case in which structures and microstructures are used to reconstruct the kinematics of emplacement of the Pan-African Fomope? a pluton and its subsolidus deformation history, using combined magnetic fabrics and microscopic observations. Up to now, in the PBCA of Cameroon, only Kankeu and Greiling (2006) carried out such a work on a small part of the Neoproterozoic basement of Eastern Cameroon. The present study focuses on Fomope? a pluton in West Cameroon, because of already available petrological and geochronological data (Kwe? kam, 2005; Kwe? kam et al., 2009), in addition to its proximity and accessibility near Dschang city. The paper starts by presenting the magnetic susceptibility with the magnetic mineralogy of the different rock types of the pluton; then, the anisotropy of magnetic susceptibility (AMS) data, providing magnetic foliations and lineations, are discussed with respect to the microstructures, i.e., magmatic or solid-state fabrics. The new structural data reveal a multi-phase deformation, enlightening the Pan-African history of West Cameroon that has remained poorly studied so far by comparison with southern and northern Cameroon. 2. Regional geological setting 2.1. The Pan-African belt in Cameroon Based on geochronological (Rb–Sr) works, Lasserre (1967a,b) proposed that the Precambrian basement rocks in Cameroon were divided into two domains: the Archaean Ntem Complex and the Pan-African orogenic belt or ‘‘mobile zone’’. The term ‘‘mobile zone’’, according to Anhaeusser et al. (1969), is applied to Precambrian metamorphic belts, characterized by medium-to high-grade metamorphism, polycyclic deformation, possible reworking of older crust and generation of new granitic magmas. The Pan-African mobile zone in Cameroon is also known as the Pan-African North Equatorial Fold Belt (Nzenti et al., 1988) or the Pan-African Belt of Central Africa (PBCA) (Penaye et al., 1993; Toteu et al., 2001), which will be preferentially used in this study. The PBCA in Cameroon (Fig. 1b) is divided into three main lithotectonic units: (a) a Paleoproterozoic basement predominantly composed of heterogeneous migmatitic gneisses; (b) Mesoto Neoproterozoic volcano-sedimentary basins that were deformed and metamorphosed into schists and high-grade gneisses, and (c) Pan-African granitoids that are often made of syntectonic granitoids of high-K calc-alkaline affinities. Actually, Pan-African granitoids were emplaced from the early stage of the deformation (orthogneisses) to the late uplift stages of the orogen (post-tectonic sub-circular massifs), cross-cutting the two former units. Several Pan-African granitoid intrusions were emplaced in spatial relation with the CCSZ (Central Cameroon Shear zone: Fig. 1a) or with the regional N30 x14E fault, recently reactivated as the Cameroon volcanic line. This is the case of the Fomope? a pluton. 2.2. Geology and geochronology of Fomope?a area On the reconnaissance geological map of Dumort (1968), the Fomope? a granitic pluton is recognized with a southeastern faulted border. The northern part of the pluton is hidden by tertiary basalts. According to Kwe? kam (2005) and Kwe? kam et al. (2009), the Fomope? a granitic pluton comprises three petrographic units: (i) biotite–hornblende granitoids (BHG), including diorites, quartz-monzodiorites, quartz-monzonites, granodiorites and monzogranites, (ii) biotite monzogranites (BmG) and (iii) edenite syenogranites (EsG). BHG are medium-to coarse-grained with biotite, hornblende, plagioclase, K-feldspar and quartz as main minerals. Common accessory phase minerals are magnetite, zircon, apatite, titanite and allanite. Sometimes, magmatic epidote surrounds an allanite core. This magmatic accessory mineral is indicative of deep emplacement or a high O2 fugacity of magma (Liou, 1973; Sial et al., 1999). BmG outcrop at the northeastern part of the Fomope? a granitic complex and elsewhere cross-cut by other units. 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