Structural and Magnetic Study of the Pan-African Bandja Granitic Pluton
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Manuscript Click here to download Manuscript: Manuscript 2.docx Click here to view linked References 1 Transpressional granite-emplacement model: structural and 1 2 2 magnetic study of the Pan-African Bandja granitic pluton (West- 3 3 Cameroon) 4 5 a,b, a,c d e f a 6 4 Yakeu Sandjo A. F. *, Njanko T. , Njonfang E. , Errami E. , Rochette P. , Fozing E. 7 8 5 a Laboratory of Environmental Geology, The University of Dschang, P.O. Box 67, Dschang, 9 10 6 Cameroon 11 12 b 13 7 Ministry of Water Resources and Energy, DR/C, Gas and Petroleum Products Service, P.O. 14 15 8 Box: 8020 Yaoundé, Cameroon 16 17 c 18 9 Ministry of Scientific Research and Innovation, DPSP / CCAR, P.O. Box 1457, Yaoundé, 19 20 10 Cameroon 21 22 d 23 11 Laboratory of Geology, ENS, The University of Yaoundé I, P.O. Box 47, Yaoundé, 24 25 12 Cameroon 26 27 13 e Department of Geology, Faculty of Sciences, The Chouaïb Doukkali University of El Jadida, 28 29 30 14 P.O. Box: 20, 24000 El Jadida, Morocco 31 32 15 f CEREGE UMR7330 Aix-Marseille Université CNRS, 13545 Aix-en-provence, France 33 34 35 16 Corresponding author: Yakeu Sandjo A. F. ([email protected]) 36 37 38 17 39 Abstract 40 41 18 The Pan-African NE-SW elongated Bandja granitic pluton, located at the western part of the 42 43 19 Pan-African belt in Cameroon, is a K-feldspar megacryst granite. It is emplaced in banded 44 45 46 20 gneiss and its NW border underwent mylonitization. The magmatic foliation shows NE-SW 47 48 21 and NNE-SSW strike directions with moderate to strong dip respectively in its northern and 49 50 22 central part. This mostly ferromagnetic granite displays magnetic fabrics mostly carried by 51 52 53 23 magnetite and characterized by (i) magnetic foliation with best poles at 295/34, 283/33, and 54 55 24 35/59 respectively in its northern, central and southern part and (ii) a subhorizontal magnetic 56 57 58 25 lineation with best line at 37/8, 191/9 and 267/22 respectively in the northern, central and 59 60 26 southern part. Magnetic lineation shows a “S” shape trend that allows to (1) consider the 61 62 63 64 1 65 27 complete emplacement and deformation of the pluton during the Pan-African D2 and D3 1 2 28 events which occurred in the Pan-African belt in Cameroon and (2) reorganize Pan-African 3 4 5 29 ages from Nguiessi Tchakam et al. (1997) compared with those of the other granitic plutons 6 7 30 in the belt as: 686 ± 17 Ma (Rb/Sr) for D1 age of metamorphism recorded in gneiss; and the 8 9 10 31 period between 604-557 Ma for D2-D3 emplacement and deformation age of the granitic 11 12 32 pluton in a dextral ENE-WSW shear movement. 13 14 15 33 Keywords 16 17 34 Granite; AMS; Pan-African; Bandja; Cameroon 18 19 20 21 35 1. Introduction 22 23 36 Spatial and temporal relationships between granites and regional tectonic structures suggest 24 25 37 granite emplacement during compression rather than extension in many convergent orogenic 26 27 28 38 belts (Hutton, 1997; Brown and Solar, 1998). Granite emplacement and deformation during 29 30 39 regional tectonic events is a challenging study, because granitic rocks do not always develop 31 32 33 40 mesoscopic scale deformation fabrics. However, microstructural studies of granites can help 34 35 41 to identify magmatic or solid-state deformation fabrics (Simpson, 1985; Paterson et al., 1989; 36 37 38 42 Bouchez et al., 1992). 39 40 43 Detailed lithotectonic study of the Pan-African orogeny (mainly along the N-S, N30°E and 41 42 44 N70°E shear zones) in Cameroon (Fig. 1) reveals that it is marked by the emplacement of 43 44 45 45 many plutonic massifs of granitic composition. The petrology and mesoscopic to microscopic 46 47 46 structural features of a few granites have been studied (the Ngondo plutonic complex, the 48 49 50 47 west-Tibati pluton, the two-mica granites from the Nkambé area and the Fomopéa granitic 51 52 48 pluton (Fig. 2)) respectively by Tagné-Kamga (2003), Njanko et al. (2006), Tetsopgang et al. 53 54 55 49 (2006) and Kwékam et al. (2010)). However, their tectonic evolution remains poorly 56 57 50 understood, due to the lack of detailed structural data (e.g. anisotropy of magnetic 58 59 60 51 susceptibility study on rocks). Indeed, the application of the anisotropy of magnetic 61 62 63 64 2 65 52 susceptibility (AMS) to the study of granitoids has led to a better understanding of the 1 2 53 tectonic history of plutons elsewhere and recently in Cameroon (Kankeu et al., 2009; Njanko 3 4 5 54 et al., 2010; Kankeu et al., 2012; Dawaï et al., 2013). Moreover, AMS is a widely accepted 6 7 55 technique and a number of studies have investigated the relationship between AMS fabric and 8 9 10 56 strain (Hrouda and Janak, 1976; Borradaile, 1988; Rochette et al., 1992; Tarling and Hrouda, 11 12 57 1993; Borradaile and Henry, 1997; Mamtani et al., 1999). 13 14 58 The aim of this paper is to combine field structures, microstructures and magnetic data to 15 16 17 59 reconstruct the kinematic of emplacement of the Bandja Pan-African pluton and show its role 18 19 60 in the Pan-African history of the west Cameroon. 20 21 22 23 61 2. Regional geological setting 24 25 26 62 2.1 The Pan-African belt in Cameroon 27 28 63 The Pan-African orogenic belt of Cameroon (Fig. 1) is one of the two main domains (together 29 30 64 with the Archaean Ntem Complex) proposed by previous research works for the Precambrian 31 32 33 65 basement rocks in Cameroon. It is characterized by medium- to high-grade metamorphism, 34 35 66 polycyclic deformation, possible reworking of older crust and generation of new granitic 36 37 38 67 magmas. The Pan-African orogenic belt is also known as Pan-African North Equatorial Fold 39 40 68 Belt (Nzenti et al., 1988) or Pan-African Belt of Central Africa (PBCA; Penaye et al., 1993; 41 42 69 Toteu et al., 2001). 43 44 45 70 According to Ngako and Njonfang (2011), PBCA is a three-plate collision belt involving 46 47 71 three major landmasses. The model proposed by these authors includes: the Sao-Francisco- 48 49 50 72 Congo Craton (SFCC), the Eastern Saharan Block (ESB) and the West African Craton 51 52 73 (WAC) (Fig. 1a). Collision in the north-south direction involved ESB and the northern margin 53 54 55 74 of the SFCC originally differentiated into a basin and a range province. This collision 56 57 75 generated an indent and intense deformation in the Cameroon domain (PBCA), following 58 59 76 penetration of the Saharan rigid prong into the SFCC active margin between 640 and 580 Ma 60 61 62 63 64 3 65 77 (Ngako and Njonfang, 2011). Successive tectonic events recorded in this active margin 1 2 78 involved: (i) crustal thickening, (ii) left- and right-lateral conjugate wrench movements, and 3 4 5 79 (iii) right-lateral wrench movements in the N70°E to EW direction. According to Ngako and 6 7 80 Njonfang (2011), structural and kinematic data show that, although collision between the 8 9 10 81 Trans-Sahara active margin and the WAC started at ca 630 Ma, deformation in north-western 11 12 82 Cameroon was mostly controlled by a NNE-SSW stress direction until ca 590 Ma. Further 13 14 83 deformation in this domain is recorded by a regional clockwise rotation compatible with a 15 16 17 84 NW-SE stress direction suggesting that the late tectonic event was the only one controlled by 18 19 85 the prominent active role of the WAC. 20 21 22 86 PBCA comprises three lithotectonic domains (Fig 1b): the northern Cameroon domain, the 23 24 87 central Cameroon domain and the southern Cameroon domain. 25 26 27 88 The northern Cameroon domain or Poli Group, also known as west Cameroon domain (WCD 28 29 89 (Fig. 1b); Toteu et al., 2004; Penaye et al., 2006) is limited to the south by the Buffle Noir- 30 31 90 Mayo Baléo (BNMB) shear zone. It is characterized by Meso- to Neo-proterozoic volcano- 32 33 34 91 sedimentary basins that were deformed and metamorphosed into schists and high-grade 35 36 92 gneisses (Nzenti et al., 1988). It represents an early Neoproterozoic back-arc basin formed 37 38 39 93 between 830 and 665 Ma (Toteu et al., 1990; Penaye et al., 2006; Toteu et al., 2006), that 40 41 94 includes: detrital and volcaniclastic deposits, metavolcanics (tholeitic basalts and calc-alkaline 42 43 44 95 rhyolites), and pre-, syn- to late-tectonic calk-alkaline intrusions (diorites, granodiorites and 45 46 96 granites). These intrusions, emplaced between 660 and 580 Ma (Toteu et al., 2001), form a 47 48 49 97 NNE-SSW corridor. 50 51 98 The central Cameroon domain, to which belongs the Bandja pluton, also known as Adamawa 52 53 99 domain, is located between the BNMB shear zone to the North and the Sanaga shear zone 54 55 56 100 (SSZ) to the South (Fig. 1b). It includes huge Pan-African batholiths and large scale 57 58 59 60 61 62 63 64 4 65 101 Paleoproterozoic remnants that were emplaced and metamorphosed during the Pan-African 1 2 102 tectonic evolution.