Microstructures and magnetic fabrics of the Ngaoundéré granite pluton (Cameroon): Implications to the late-Pan-African evolution of Central Cameroon Shear Zone

https://doi.org/10.1016/j.jafrearsci.2017.02.027Get rights and content

Highlights

  • Microstructures and magnetic fabrics of the Pan-African Ngaoundéré pluton in Central-North Cameroon are investigated.

  • Microstructures reveal a continuum of deformation from the magmatic state to the high temperature solid state.

  • Magnetic fabrics are mostly consistent with structures of the country rocks.

  • Our results suggest that the emplacement of Ngaoundéré pluton was controlled by dextral transpressive tectonic of the CCSZ.

Abstract

The Ngaoundéré granite pluton, in Central-North Cameroon, located near the Central Cameroon Shear zone (CCSZ), and previously studied for its petrography and geochemistry, is characterized by the absence of macroscopic markers of deformation. In this study, we report microstructures and magnetic fabrics (AMS) of this pluton and discuss the relationship with the Pan-African evolution of the CCSZ. The pluton consists of a porphyritic Hbl-Bt-monzogranite at its rim and a porphyritic biotite-granite at its core, a petrographic distribution denoting a normal zoning pattern, i.e. more silicic toward the centre. As expected, magnetic susceptibilities values also exhibit a zoning pattern in agreement with petrographic zonation. Thermomagnetic data indicate that this pluton is dominantly ferromagnetic in behaviour. As indicated by its microstructures, the pluton has suffered a continuum of deformation from the magmatic state to the high temperature solid-state during magma crystallization and solidification. The magnetic foliations dominantly strike NE-SW and dip moderately to steeply and the lineations mostly plunge shallowly to the NE or SW, roughly parallel to NE-to ENE-trending Central Cameroun Shear Zone (CCSZ). The foliation poles define a girdle pattern with a zone axis (52°/11°) rather close to the best line of the lineations (44°/21°). These fabrics correlate with the structures of the country rocks ascribed by several workers to a regional transpression. Toward the margins of the pluton, particularly the northern one, the lineations tend to rotate from NE to N in azimuth. This change is interpreted as due to strain partitioning, simple shearing with NE-SW extension being relayed by compression toward the northern pluton border. This new magnetic fabric study suggests that the Ngaoundéré pluton (poorly dated at c. 575 Ma) was emplaced during the late stages of the CCSZ dextral transpressive movement. It also provides some more constraints on the correlation between the CCSZ system and the shear zone system of NE-Brazil.

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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