Microstructures and magnetic fabrics of the Ngaoundéré granite pluton (Cameroon): Implications to the late-Pan-African evolution of Central Cameroon Shear Zone
Introduction
The break up of West Gondwana which took place during the Mesozoic led to the disruption of the Brasiliano-Pan-African belt into Africa and South America. During the last decades, several studies were conducted for a better reconstruction of the Pre-Mesozoic correlation between these two continents (De Witt et al., 2008, Van Schmus et al., 2008, Guimaraes et al., 2009, Archanjo et al., 2013, Ganade et al., 2016). There is no consensus, however, for the adjustment between the geological domains identified on both sides of the equatorial Atlantic Ocean. This is mainly due to the limited data in central Africa, one of the most poorly understood parts of West Gondwana.
Large-scale shear zones are currently considered as key features to pre-drift reconstruction (De Witt et al., 2008). Numerous shear zones are identified in the Pan-African domain of Adamawa-Yadé (Central-North Cameroon) among which the Adamawa fault, also known as the Central Cameroon Shear Zone, or CCSZ (Ngako et al., 1991), is one of the most important. This NE-SW trending major lithospheric structure extends from south Sudan to Cameroon (Dumont, 1986, Ngako et al., 1991, Ngako et al., 2003, Fig. 1) and continues into northeastern Brazil (De Witt et al., 2008, Van Schmus et al., 2008). In Cameroon, high-grade and migmatitic gneisses, and numerous granitic intrusions outcrop near the CCSZ and its branches.
From the structural point of view, due to a lack of macroscopic deformation structures recorded in the granitic intrusions that outcrop along the Central-North Cameroon shear zones, the evolution of the CCSZ is mostly known through the studies of gneissic rocks (Njonfang et al., 2006, Njonfang et al., 2008, Ganwa et al., 2011, Kankeu et al., 2012). Accordingly, the last stages of this shear zone are relatively poorly known. According to Ngako et al. (2008), the emplacement of granitic intrusions in the Pan-African domains of Cameroon was controlled by the action of the various dextral wrench faults. It is therefore inferred that such intrusions behave as “invisible” markers of the action of strike-slip faulting during the Pan-African/Braziliano orogenesis. To reveal these markers and constrain the relationship between the emplacement of granite and the regional tectonics, several studies (Archanjo et al., 2008, Archanjo et al., 2013, Majumder and Mamtani, 2009, Dawaï et al., 2013, Adissin Glodji et al., 2014, Olivier et al., 2015 amongst others) demonstrate the usefulness of combining microstructural analyses and magnetic fabrics. In this study we adopt a similar approach. The Ngaoundéré pluton being apparently isotropic and spatially closely associated with the CCSZ, the structural relationships of this pluton with the shear zones may help to understand its role during the late Pan-African orogeny, hence to contribute to a better reconstruction of West Gondwana puzzle.
Section snippets
Adamawa-Yadé domain
The Central African Fold Belt (CAFB) is a major feature that resulted from the collision between the Congo craton, the West-African craton (Toteu et al., 2004) and the Saharan metacraton (Ngako et al., 2008, Liégeois et al., 2013). In Cameroon, this belt is commonly subdivided into three main domains (Toteu et al., 2004, Ngako et al., 2008, Van Schmus et al., 2008): (i) the South Cameroon domain lies south of the Sanaga fault; (ii) the Adamawa-Yadé domain, also known as Central Cameroon domain,
Geology and petrography
The Ngaoundéré pluton is a part of the Adamawa-Yadé batholiths, located near the Central Cameroon shear zones system. It occurs in and around the city of Ngaoundéré. According to the geological map (1/500,000) of Lasserre (1961), the Ngaoundéré pluton is intrusive in syn-tectonic granitoids, i.e. mesoscopically deformed. The extensive Cenozoic basaltic cover and soil make the contact between the pluton and the host rocks hardly observable. Therefore, the limits of the Ngaoundéré pluton reported
Microstructures
Microstructural study is an essential aspect preceding the analysis of AMS fabric. For this purpose, oriented thin-sections obtained from 19 sampling stations (13 samples for the Bt-granite and 6 samples for the Hbl-Bt-monzogranite) were realized.
In the Bt-granite, two types of microstructures were distinguished: (1) magmatic and (2) with incipient solid-state deformation at high temperature. (1) The magmatic microstructures are more abundant, characterized by mineral phases showing imbricate
Sampling and measurement
In the present study, 101 samples have been collected in 46 different sites in the Ngaoundéré pluton (Fig. 3b), ∼0.2 km to ∼1.2 km apart from each other. Two to four cores were extracted from each site with a portable drilling machine and non-magnetic drills and were oriented in the field using a magnetic compass. Each core was then cut into 2.2 × 2.5 cm cylindrical specimens, using a non-magnetic dual-blade saw, giving a total of 205 oriented specimens used for magnetic fabric measurements.
Significance of microstructures
Based on the isotropic appearance and geochemical data, the Ngaoundéré pluton was considered as a post-tectonic intrusion (Tchameni et al., 2006). The present microstructural study strongly suggests that the pluton has undergone a continuous deformation from the magmatic state to the solid state at high temperature during magma emplacement. The magmatic origin of the magnetite grains, as proved by their (sub)euhedral shapes, implies that the main carriers of the ASM signal have crystallized
Conclusion
This study demonstrates the importance of magnetic fabric and microstructural analyses for decrypting the internal structures of plutons devoid of macroscopic markers of deformation. This new investigation applied to the Ngaoundéré pluton indicates that this calc-alkaline and high-K pluton is not post-tectonic but rather late-tectonic. Its emplacement was influenced by the dextral transpressive tectonics related to the NE-to ENE directed Central Cameroon Shear Zone which remained active until
Acknowledgments
This publication was made possible thanks to the BEST scholarship provided by the IRD-DPF to the first author. Comments and suggestions by Philippe Olivier and anonymous reviewers, and editorial handling by Damien Delvaux are gratefully acknowledged.
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2020, Journal of African Earth SciencesCitation Excerpt :A comprehensive study of petrogenesis, fabric pattern and emplacement mechanisms of post-orogenic granite plutons as well as their thermal aureoles in the context of the regional tectonometamorphic evolution allows an overall interpretation of the geodynamic scenario and magmatism at the final stages of orogenic processes (e.g. White and Chappell, 1983; Pearce et al., 1984; Whalen et al., 1987; Barbarin, 1999; Asrat et al., 2003). In the East African Orogen (EAO) individual stages of geodynamic evolution and the associated syn- to post-tectonic magmatic activity are among the issues of ongoing discussion (e.g. Tadesse et al., 1997; Alemu, 1998; Asrat et al., 2003; Jacobs and Thomas, 2004; Kusky et al., 2003; Hargrove et al., 2006; El-Bialy and Streck, 2009; Eyal et al., 2010; De Wall et al., 2011; Farahat and Azer, 2011; Greiling et al., 2014; Khalil et al., 2015; Azer et al., 2016; Dawaï et al., 2017). The EAO was developed as a Neoproterozoic accretion-type orogen (ca. 850 to 540 Ma) involving the assemblage of oceanic arc/back-arc basin closures, continental fragments and suture zones (for a general review see Fritz et al., 2013).
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2018, Journal of Structural GeologyCitation Excerpt :According to the present authors, in the western Cameroon domain, the opposite shear sense could be linked to multi–stage mylonitization compatible with successive shearing events. Dawaï et al. (2017), from a synthesis of previous research works (e.g. Ngako et al., 1991; Njonfang et al., 2008; Njanko et al., 2006; Ganwa et al., 2011; Kankeu et al., 2012) carried out on some granitoids along the CCSZ, noted that (i) the foliations also have NE- to ENE-strikes and moderate to steep dips and (ii) the lineations also have subhorizontal NE- to ENE-trends, almost parallel to the fold axes. In the present study, the magmatic and magnetic fabrics in the pluton strikes NNE-SSW with mostly moderate-to steep-dips (Fig. 10) and the magnetic lineation is subhorizontal with NNE trends almost parallel to the fold axes and stretching lineation.