Open access peer-reviewed chapter

Geodynamics of Precambrian Rocks of Southwestern Nigeria

Written By

Cyril C. Okpoli, Michael A. Oladunjoye and Emilio Herrero-Bervera

Submitted: 18 September 2021 Reviewed: 24 March 2022 Published: 29 September 2022

DOI: 10.5772/intechopen.104668

From the Edited Volume

Earth’s Crust and Its Evolution - From Pangea to the Present Continents

Edited by Mualla Cengiz and Savaş Karabulut

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Abstract

The geodynamics of the Southwestern Nigeria Precambrian Basement Rocks were studied with aim of understanding the evolution of rocks globally. Magnetic carriers of Precambrian Basement rocks samples collected from 110 locations were prepared for rock magnetism, optical microscopy and Scanning Electron Microscopy (SEM). The Natural Remanent Magnetisation (NRM) of the remagnetised and unmagnetised rocks are strong (0.3–1.7 A/m -< 0.5 A/m) showed northwesterly direction with moderate inclination and weak NRM with westerly shallow direction respectively. Primary and secondary NRMs are carried by maghemite, and the remagnetised and unmagnetised rocks revealed a higher coercivity for alternating field demagnetisation (<20 mT – < 10 mT median destructive field). Optical microscopy revealed maghemite, poor titanomagnetite, titanomaghemite lamellae >30 pm and finer maghemite/magnetite grains finer than 10 pm. X-ray Diffratometry (XRD) and SEM results implied NW remanence in the remagnetised rocks reside in the fine poor-maghemite during the alteration of hornblende to actinolite while the coarse-grained maghemite in both rocks carries the W remanence of a thermoremanent magnetisation acquired in the Pan – African times. Global cold collision geodynamics resulted in the generation of ultra-high pressure metamorphic complexes and remagnetisation and True Polar Wander drifts of the paleomagnetic pole move towards the equator.

Keywords

  • remagnetised
  • unremagnetised
  • pan-African
  • tectonometamorphism
  • NRM
  • orogenesis

1. Introduction

Geodynamics of the Precambrian is a fascinating and contentious topic that is now preventing us from better understanding how the Earth evolved over time. The dearth of raw data related to this tectonic regime is largely to blame for the current controversy and lack of consensus on Precambrian geodynamics. Geodynamics is the study of how the interior and surface of the Earth change through time. A time-depth diagram (Figure 1) that spans the whole history and interior of the Earth can be used to show this process schematically. For a systematic characterisation of geodynamic interactions, data points characterising the physical-chemical condition of the Earth at different depths 0 to 6000 km, for discrete times in geological time, are shown in Figure 1. (ranging from 0 to around 4.5 billion years ago).

Figure 1.

Data availability for restricting the geodynamic connection for the earth is depicted in a simplified time–depth diagram. (modified after [1]).

Geophysical data measurements, unfortunately for geodynamics, provide systematic coverage of the current Earth structure and the geological record recorded in rocks formed near the Earth’s surface (usually within a few tens of kilometres). As a result, Precambrian geodynamics remains a controversial topic. It’s also worth mentioning that four key Precambrian Earth evolution topics are among the top ten questions defining 21st-century Earth sciences [2]:

1st “What happened during Earth’s “dark period“ of the first 500 million years?” This period is critical for understanding planetary history, particularly how the Earth’s atmosphere and seas formed, yet scientists know little about it because few rocks from this age have been preserved.”

2. “How did life begin?” Remaining records of geological examinations of rocks and minerals could be used to identify where, when, and in what and what form life first arose.”

3. “How does the Earth’s interior function, and how does it impact the surface?”

Earth’s magnetic field was formed by the continual movement of the mantle and core.

How and when continents formed and were preserved throughout billions of years, as well as their future evolution.

In this study, we will concentrate on the last questions, which have improved dramatically over the last decade.

This progress has been fueled by an increase in the quality and quantity of geological, geochemical, petrological, and geochronological data for Precambrian rock complexes, as well as the ongoing development of analogue and numerical models for early Earth dynamics [3, 4]. Volcanism, seafloor building, and mountain formation are all aided by mantle convection, which has an impact on surface conditions.

Scientists, on the other hand, are unable to exactly characterise these motions or calculate how they differed in the past, making it impossible to comprehend the past and predict the Earth’s future surface environment.

How did the Earth’s plate tectonics and continents form?

Despite the fact that plate tectonic theory is widely accepted, scientists are still perplexed as to why Earth has plate tectonics and how closely it is tied to other planet features such as water content, continents, oceans, and life. Modelling has become increasingly important in generating new goods due to the shortage of empirical constraints (Figure 1).

Indeed, as Benn et al. [3] point out, one of the unique aspects of Precambrian geodynamics is that there is no thriving global geodynamics paradigm, and early Earth lithosphere tectonics differs from modern-day plate tectonics, which we can integrate and evaluate using our ever-growing set of observational and analytical data.

Several new major results have been obtained to address this particular challenge since Benn et al. [3]‘s wide summary of Archean geodynamics, based primarily on merging geochemical, geological, petrological, and geophysical data sets. “This very concise, up-to-date synthesis of Precambrian geodynamics was motivated by analogue and numerical model results.”

This research integrate modern paleomagnetic remanence, rock magnetism and optical microscopy and concepts (Plate tectonics and subduction, Orogeny and collision), petrology (metamorphic parageneses and relation with deformation), and geochronology. Data from regional literature including geophysical data are also abundantly used for synthesis and re-interpretation. The most important achievements include: the paleomagnetic studies and geodynamics on plate tectonics and subduction and orogeny and collision.

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2. Regional settings of Precambrian geodynamics

The southwestern Nigeria granitoids is within the basement complex domain that was reopened in the Pan- African time of the Neoproterozoic period. This province was located around East Saharan, southeast Congo craton and west of West African craton (Figure 2), and has a long stretch from Hoggar to Brazil, which ranges from 4000 km to an extensive orogen in hundreds of kilometres [7]. The Trans-Saharan fold belt runs north-southerly, and the reopening of this belt was due to East Saharan, Congo and West African cratons continental collision about 790 and 500 Ma [6, 8, 9]. Granitoids, growth of thrust-nappe, medium- to high-grade metamorphism, parallel orogen tectonics typifies this belt [10]. The Hoggar separated into Air, Eastern and Central polycyclic; but now called the Pharusian belt plus Laouni terrain Algeria (LATEA) microcontinent [11]. Aggregation of twenty-three micro terranes constitute eastern and polycyclic central Hoggar in the northern province [5], while in the southern block (Dahomeyide), we have the Aïr-Hoggar composed of various continental oblique collisions [12].

Figure 2.

Regional geological map of trans-Saharan metacraton/shield (modified after [5, 6]).

Nigerian sector evolved by profuse magmatism in late Neoproterozoic times at the culmination of prior basin made up of depleted Archaean crust [13]. The Nigerian part of the Dahomeyide was separated into the eastern (granulite facies) and western (greenschist to amphibolites facies) domains based on some petrological attributes [14]. The southwestern Precambrian granitoids consist of migmatite-gneisses, schists, granites and dykes [15]. Pan-African granitoids and unmetamorphosed dykes are assigned Neoproterozoic isochron [8, 16, 17, 18, 19, 20]. The Archean crust characterised the supercrustals, which later interpreted to be deposited in diverse proto ocean floors [12, 15].

Pan-African belt evolution was by Plate tectonism, which led to the active margin colliding with the Pharusian belt and passive continental margin of the West-African craton about 600 Ma [7, 21, 22, 23]. The existence of basic to ultra-basic rocks thought to be remnants of mantle diapers or paleo-oceanic crust is part of this fact, and they have complex ophiolitic characteristics. Geochronological studies have examined major magmatic complexes with their isochron ages varying from 557 ± 8 to 686 ± 17 Ma (Rb/Sr. whole rock); 640 ± 15 Ma (U–Pb), which were determined in these complexes. Deformation of migmatite-gneisses and post-tectonic uplift typifies the Pan-African fold belt in southwestern Nigeria; which consist of polycyclic orogen of Liberian (2700 ± 200 Ma), Eburnean (2000 ± 200 Ma), Kibaran (1100 ± 200 Ma), and Pan-African (600 ± 150 Ma) [7, 24]. For the Liberian and Eburnean, the International Geological Time Scale (2002) has followed the following ages: “Paleoarchean to Mesoproterozoic (3600 to 1600 Ma)”, “Mesoproterozoic to Neoproterozoic (1600 to 1000 Ma)”, “Neoproterozoic to Early Paleozoic (1000 to 545 Ma)”, and “Neoproterozoic to Early Palaeozoic (1000 to 545 Ma [25].

Ferre et al. [6] studied the northeastern Nigeria Pan-African continental collision, which resulted in high grade - high pressure and temperature (HP-HT) metamorphism up to granulite facies, migmatisation in supracrustal units of the same tectonism as the southeastern Nigeria domain [26, 27]. The extensive Archean crust of northern Nigeria was modified and remelted during the Pan-African tectonometamorphic episode. The Pan-African nappe system rejuvenates older polyorogenic times [15, 28].

Precambrian basement rocks into four units: Migmatite-Gneiss (migmatites, gneisses, granite-gneisses); Schist zones (schists, phyllites, pelites, quartzites, marbles, amphibolites); Pan African Granitoids (granites, charnockite, granodiorites, diorite, monzonites, gabbro) and Undeformed Acid and Basic dykes (muscovite, tourmaline, pegmatites, aplites, syenites, basaltic, dolerites and lamprophyre dykes). They occur as a small medium-grained rock with massive hills. This charnockites is made up of orthopyroxene, clinopyroxene, hornblende, plagioclase, alkali feldspar, magnetite, quartz and zircon. In some places, granite, porphyritic, augen gneiss, banded gneiss can be seen as low-lying outcrops and large hills (Figure 3).

Figure 3.

Geological map of southwestern Nigeria (modified from NGSA, 2010 [29]).

2.1 Gondwana configuration

Although different models exist for the absolute position of Gondwana [30] as well as the relative positioning of cratons can be done with small margins of error (Figure 4; [32]). The formation of Gondwana is often presented as a merger of East Gondwana (Antarctica, Australia, and India) with West Gondwana (those currently in Africa and South America). However, evidence, especially from the eastern Gondwana cratons, indicates that it was not a simple unification of two halves but rather a poly-phase amalgamation of cratons during the waning stages of the Proterozoic, as a result, that Gondwana was created [33].

Figure 4

Gondwana map with its cratonic nuclei positions (adapted after [31]). RP -Rio de la Plata craton; SF- Sao Francisco craton.

The Congo and West Africa cratons form part of West Gondwana and are connected through the Borborema Province in northern Brazil (Figure 4). This province was essentially an assemblage of several terrains and comprised reworked Mesoproterozoic- Neoproterozoic metasedimentary rocks and Archean-Palaeoproterozoic crystalline basement [34]. Reworking is the result of Neoproterozoic continent-continent collision, which caused extensive deformation, migmatisation, granitisation and intrusive plutons. Geochronological constraints for the different stages of deformation in the Borborema Province are provided by U-Pb radiometric ages of the granitoid plutons [35]. Ages for zircons from syn-tectonic I-type granitoids and zircons from migmatitic gneisses show that deformation started ca. 625 Ma and peaked at about 600 Ma [36]. Post-tectonic alkaline granitoids mark the final orogenic stage, and U-Pb zircon ages show that deformation had ceased around 570 Ma [36]. The Borborema Domain was correlated, predominantly based on Sm-Nd model ages and U-Pb zircon ages of Archaean-Palaeoproterozoic basement rocks in conjunction with Neoproterozoic structural tectonic data, with the Central African fold belt (Figure 4) and with the Nigerian Shield (Figure 4) in NW Africa [35].

The Central African fold belt demonstrate a poly-stage geodynamic evolution of nappe emplacement onto the Congo Craton northwardly [37]. Geochronological constraints reveal a history of individual orogenic stages broadly coeval with those of the Borborema Province: high-pressure metamorphism with granulite facies typified for syn-tectonic calc-alkaline and S-type granitoids and migmatisation occurred at 640–610 Ma, as well as post-collisional phase of exhumation and late-tectonic calc-alkaline to sub-alkaline granitoid emplacement was dated at 610–570 Ma [37]. The exact nature of the continental landmasses involved was still enigmatic. The belt could be entirely the consequence of the collision of the Congo Craton with the ill-defined Saharan Metacraton [38].

Neoproterozoic intrusions within the Nigerian Shield show a history very similar to that of the Borborema Province [34]. Combined structural data and U-Pb ages suggest that an early deformational phase took place at 640–620 Ma, peak metamorphism and syn-tectonic granitoids are positioned between 620 and 600 Ma and a post tectonic phase from 600 to 580 Ma (geochronological data synthesised by [34]). Geochronology of these plutons shows that the continental collision evolved diachronously between 620 and 580 Ma [5]. The Nigerian Shield and the Tuareg Shield were parted from the West African Craton by the Dahomeyide and Pharusian belts (Figure 4). Peak metamorphism in the Dahomeyides occurred approximately at 610 ± 2 Ma [39] to 603 ± 5 Ma [40] premised based on radiometric dating of U-Pb obtained from gneisses from granulite-facies peak metamorphic zones. The post-collisional exhumation was dated by 40Ar-39Ar muscovite ages of 587 ± 4.3 and 581.9 ± 2.4 Ma [39], which corresponds with rutile ages of 576 ± 2 Ma, which represent regional cooling below 400°C [40]. The collision of Island Arc with the West African Craton around 620 and 580 Ma, simultaneous with the height of the tectonic events in the Tuareg Shield to the east [5]. The Borborema domain evolved synchronous with the Central African Fold Belt, the Nigerian Shield (Dahomeyide Belt), and the Tuareg Shield (Pharusian Belt) strongly implies that this part of West Gondwana had amalgamated by 600 Ma, and all tectonic activity had ceased by 570 Ma.

Archaean to Mesoproterozoic granite-gneiss-migmatite complexes, greenstone belts and metasedimentary and metavolcanic units are caught up in the Brasilia Belt involving the Sao Francisco Craton and Magmatic Arc of Goias [41]. Observed data from the Paraguay-Araguaia belt that flanks the Goias Arc’s western side imply that the collision of Sao Francisco/Goias with Amazonia slightly post-dates the Brasilia event at ca 550 Ma [42, 43]. Biotite and muscovite ages around 530 Ma from Archaean basement gneisses may record late-orogenic cooling in the Araguaia belt (K-Ar) [43].

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3. Materials and methods

We make use of the following instrument to conduct measurements on some Precambrian basement rocks of southwestern Nigeria:

1. Alternating frequency, thermal demagnetisations and Spinner magnetometer for Paleomagnetic analysis.

Granite biotite granite gneiss, banded gneiss, Augen gneiss, porphyritic granite, syenite were selected based on their mineralogy, magnetic susceptibility, and natural remanent magnetisation (NRM). A combination of alternating field (AF) and thermal (TH) demagnetisation methods were employed. Their primary and secondary multi-remanence constituents were measured using the equipment. These techniques were used because constituent minerals obtained through different mechanisms have different coercivity spectra and blocking temperatures. The coercivities of magnetic minerals are involved in AF demagnetisation. The alternating field method entails exposing the specimen to increasing amounts of AF, with the waveform being sinusoidal and decreasing in magnitude linearly with time. It was used to extract remanence from grains whose coercivities were less than the peak demagnetising area. The alternating magnetic field is a quick treatment procedure likened to the thermal demagnetisation method. Test of the natural remanent magnetisation in determining the rock material is not a superimposition of several magnetic constituents, and this was done by isolating the components of stable magnetisation (CRM).

The specimens were heated to a temperature below and near ferromagnetic mineral Curie temperatures in steps of 30 and 50°C during step-by-step thermal (TH) demagnetisation and then cool in a zero magnetic field at room temperature. It gives magnetic grains blocking temperatures (Tb) lower than the temperature used to strip a portion of their normal remanent magnetisation. Step by step, temperature ranges were measured, and residual magnetisation and susceptibility were calculated. The basic measurement of NRM yields the remanent magnetisation recorded in rocks (declination, inclination, and total intensity). In the present study, the samples were AF demagnetized in 14 steps following a sequence 2.5, 5, 7.5 10, 12.5, 15, 17.5, 20, 25, 30, 40, 60, 80 and 100 mT respectively. The thermal demagnetisation was done on some selected samples in a sequence of 50°C, 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, 420°C, 440°C, 460°C, 500°C, 530°C, 550°C and 570°C respectively.

As soon as alternating field and thermal demagnetisation are treated, the rock specimen directions are studied to isolate magnetic constituents. In this paleomagnetic study, stereographic and orthogonal projections were adopted. Stereographic projection direction was characterised, magnetisation vector unit length tip was measured, the same sphere diameter aligned with the southern pole. They are the contact site with the equator plane sphere, usually referred to as a small open circle. The geographic directions of the north, east, south and west were defined. Magnetic declination ranges from 0° (N direction) to 360° clockwise, and from 0° at the edge of the equator plane to 90° at the midpoint. AF and thermal datasets were analysed with AGICO’s Remasoft 3.0 program [44] and Demagnetisation analysis in excel DAIE-v2015 program [45]. Fisher [46] ‘s statistics were employed to measure mean orientations.

2. Kaiser 785 nm Micro-Raman Microprobe system and Renishaw 830 nm inVia micro-Raman to determine iron oxides.

Granite; biotite granite gneiss; Augen gneiss and banded; banded gneiss; porphyritic granite; syenite, and amphibolite rock samples were hammered bits and pieces and selected with a solid permanent magnet in the laboratory because of their mineralogy and magnetic susceptibility in order to determine the magmatic effect of maghemite. Tiny, unpolished grains of different iron titanium oxides concentrations were affixed on carbon tape attached to a glass slide for Raman spectra measurements. In addition to optical images, micro-Raman spectroscopy of various excitation wavelengths was used. At the University of Hawai’i, various instruments were used to capture Raman spectra. Spectra with 785 and 830 nm - Kaiser Optical Systems’ micro-Raman system and Renishaw in Via microspectroscopy were used for the study. The system consists of a 785 and 830 nm Invictus diode laser, a Kaiser Holospec/Renishaw spectrometer, a spectral range of 150–3300 cm–1, a Leica microscope with imaging capabilities, as well as an Andor CCD camera. The laser light and Raman pulse are sent to the microscope and spectrometer via a 100-meter optical fibre. A 50 objective lens fixed on the microscope in backscattering geometry was used to focus the laser spot and observe the signal. The spectrometer and microscope are fixed through optical mirrors of different wavelengths and were operated using a PRIOR workstation (via WiRE 3.2 software). The spectra were imported into MATLAB 7.4.0 and Grams/AI v8.0 for normalising statistical analysis, background interference in each spectrum, as well as baseline diffraction patterns, i.e. correction and peak fitting using Gaussian and Lorentzian geometries. Background correction was done using sixth-order polynomials in both cases. Principal component analysis (PCA) and significant factor analysis (SFA) were employed to determine the principal components. Specimen were stored to avoid artefacts, and laser power had below 0.7 mW to prevent the destruction of the specimen; neutral density filters had a constant power of 675 μW; acquisition time was 60 s; spectrometer calibration before acquiring Raman spectra; and cyclohexane standard protocols were used [47, 48].

3. Scanning electron microscopy (SEM) (JEOLJSM-5900LV) and X-ray diffractometry (XRD) for magnetic mineralogy.

Gneiss, granite, biotite-granite-gneiss, charnockite, and granite were used to describe the ferromagnetic minerals based on their mineralogy and magnetic susceptibility. The thin polished sections were studied using SEM. Thus, SEM and XRD were employed in the Institute of SOEST-HIGP (Manoa, Hawaii, USA) to constrain the mineralogy of accessory minerals. SEM was used for imaging, qualitative analysis (equipped with an “EDS”) and quantitative analysis (when equipped with an “EDS/WDS”). SEM and EMPA were applied to characterise the specimen for Mineral identification; compositional information, microstructures/deformation and compositional evolution of minerals.

4. We modelled the tectonometamorphism episodes in the Precambrian era to picture the evolution of the Precambrian rocks of southwestern Nigeria and relate them to present-day orogenesis.

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4. Results and discussions

4.1 Paleomagnetic results

For samples of the same site, CO-23c subjected to thermal treatments have secondary remagnetisation averagely 70% at 300–500°C the remainder of the signal was washed up to 570°C (Figure 5d). Figure 5(d-i) demonstrated weak magnetic coercivity, unstable remanent directions and abrupt changes in intensity in Zijderveld curve plotting not directly to the origin due to tectono-metamorphic episodes. Up to 500°C, the rest of the samples retains >50% of the magnetisation is lost and it It was difficult to isolate the ChRMs (e.g., CO-23c, CO-018,CO-37a, CO-74 N and CO-100A in Figure 5a-e). The unblocking temperature revealed two distinct elements, one with natural polarity against N and the other towards NNW and NE. The second specimen has a low unblocking temperature and was thoroughly cleaned up to 300°C, while the northerly specimen reported magnetisation up to intermediate unblocking temperatures (580°C), which is referred to as characteristic remanent magnetisation (CRM). Thus, regardless of NRM decrease for the first 300 samples, a large percentage of the samples were treated to remove secondary remanence [49]. Compared to the ferrimagnetic one, the samples have a low paramagnetic effect, as shown by the similarities of the curve before and after slope correctionCO-23c, CO-018,CO-37a, CO-74 N and CO-100A samples were decomposed into two overlapping modules with medium destructive fields (MDF) ranging from 30 to 40 to 60–70 mT, and a third higher coercivity segment (MDF ~ 467 mT).

Figure 5.

(a-ce) alternate field and thermal demagnetization showing the Zijderveld orthogonal vector diagram of unremagnetised Precambrian rocks and (d-c) remagnetised rocks.

Determining the time of growth during folding is difficult because syntilting results observed in incremental tilt tests do not give a unique result. The formations of new minerals (maghemites) were demonstrated in most of the granitoids invoked for many syntilting CRMs. The remanence-carrying Fe oxide grains may have rotate during folding and tectonometamorphic episodes as a result of syntilting, which would alter the original magnetic direction. The rotated directions were not related to the ambient field during folding. Shear strain during flexural flow folding could cause a prefolding magnetization to be rotated into a syntilting configuration. Folds with different geometries and tilted thrust sheets all have the same magnetic characteristics and are probably caused by the same remagnetization events. The determined tilt test results however suggest that the CRM is pre-tilting in both the thrust sheets and a fold with a fault-bend fold geometry and syntilting in folds with a fault propagation fold geometry that probably experienced higher strains. A primary remanent magnetisation should theoretically require more stress than most rocks have been subjected to during deformation, be partially reversible and have the greatest effect on the low-coercivity. Developing a better understanding of remagnetization processes and use of palaeomagnetism for its studies, the preponderance of multi-domain and pseudo-Single domain magnetic phases and presence of maghemite suggest that the type 1 magnetite has been modified during the orogenesis. They are correlated to their respective bedding tilt orientation base on correlation fold test. The tectonic correction brought the site direction in its geographic coordinates; the same rotation was applied to the mean geographic coordinate to produce forward corrections of the mean. The remagnetised component of the granitoids was interpreted as partial thermoremanentmagnetisation (pTRM) overprint acquired during the tectonometamorphic episodes 600 ± 150 Ma (Pan –African orogeny) associated with tectonic accretion along southwestern Nigeria Precambrian shield. The overprints directions were likening to the reverse magnetization. The intermediate degree of unfolding at peak concentration could be due to subtle amounts of component mixing, diachronous magnetization acquired during a short time interval, syn-folding magnetisation acquisition, local structural differences within the fold.

Precambrian rocks of southwestern Nigeria with an intermediate unblocking temperature of 100—400°C witnessed perfect dispersed clustering in geographic coordinates after tilt correction, suggested remanence imprint after folding (Figure 5a-c).). The corresponding imprint on the paleomagnetic pole situated at 85.1°N, 183.0°E with α95 = 10.1° (dp = 12.7, dm = 8.0) in geographic coordinates, are very similar to the Basement system of Precambrian poles of southwestern Nigeria. The majority of the sites were unable to isolate the intermediate temperature component due to its remagnetisation ((Figure 5d). Therefore, this overprint was considered to be remagnetisation in Pan-African times. In Nigeria’s Eastern basement complex, NE Brazil, Central Cameroon, and much of the west Gondwana crystals provinces, different remagnetisation has been observed.

Available geologic model isochron ages, tectono-metamorphic history, and crustal evolution model that support accretional model for Paleoproterozoic and Neoproterozoic rocks with consistent older model isochron age in support of significant involvement of Archean felsic crust in their orogeny and suggested that southwestern Nigeria’s tectono-metamorphic history and crustal Nigerian active margins and Trans Saharan belt utilised U–Pb geochronological to infer the magmatism that occurred from 670 to 545 Ma ([26] and this study) for the overriding plate of Benino-Nigerian Shield. The age of ultra-high pressure metamorphic eclogites from the passive margin of the West African Craton subducted to mantle depths recently restricted the timing of crustal deformation to 600–150 Ma. The lower plate (West African Craton) and upper plate (Benino-Nigerian Shield) both experienced east-plunging continental subduction, which pushed the passive flanking margin to >90°, which suggested that granitoids subducted between 670 and 610 Ma. Both plates crystallised the Pharusian oceanic plate, while igneous rocks associated with arc magmatism include the hornblende-biotite granodioritic gneiss, dated at 610–694 Ma. Pan-African granites older than 610 Ma predate non-subduction-zone deformation. The Benino-Nigerian basement complex was formed by continental arc, according to a geochemical dataset and Sr-Nd isotopic kinematics application to the 670–610 Ma migmatite-gneisses.

Results exhibited preliminary records of exsolved maghemite in silicate, plagioclase and pyroxene minerals in the southwestern Nigeria Precambrian gneiss and granitoids. These iron oxides are seen in the pyroxene and plagioclase minerals showing good magnetic stability (Figure 6a). Figure 6a (i-iii) shows the site mean directions of the study area. Site mean direction for component i clustered around mean Dm = 325.6° lm = 28.4°(N = 12, α95 = 9.8, k = 10.93), which resulted to paleomagnetic pole located at 7.38°N. 5.57°E (A95 = 9.8, K = 12.9). Site mean direction for component ii Dm = 0.6° lm = −38.4° ((N = 7, α95 = 14.1, k = 15.24), which resulted to paleomagnetic pole located at 7.24°, N 5°E (A95 = 12.5, K = 13.4). Site mean direction for component iii is Dm = 225.8° lm = 26.4° (N = 4, α95 = 16, k = 17.93), which resulted to the paleomagnetic pole located at 8.17.5°N 4.21°E (A95 = 15, K = 13.9). Thus, site mean directions of the study area were accomplished as a reliable recorder of the old geomagnetic fields. Figure 6b-c were employed to demonstrate the approximate tilt estimates, when distinct geological criteria demonstrated folding and faulting cannot be correlated to the unit. They used to evaluate the pole position for the SW Nigerian shield, errors due to (1) the large elliptical confidence was defined for the pole position, (2) recognised tilt events, and (3) the streaked great circle distribution of reversed, normal and mixed Remanent magnetization (RM) directions towards the Pan African events must be examined. Because the pole was poorly constrained, the data can accommodate several possible interpretations. First, the unit may have gained an RM orientation during long-term cooling compared to steady African cooling, and magnetic acquisition in the gneiss may have taken place over long stretches of time, during which reversals in the Earth’s magnetic field direction occurred. Both polarities may have been recorded in individual sample owed to varying blocking temperatures. Perhaps, the field direction changed slightly during this period, then a deflection along a great circle towards the polarity which the earth’s field sustained for the shortest length of time, would be observed. Such a deflection, where changes in the field direction are represented by angular limits of mean directions from Van Der Voo, [50] and percentages represent the relative time the earth’s field spent in each polarity. These results in conjunction with demagnetization data suggest that the geomagnetic field was represented by both polarities (reversed dominating) during the magnetization of the southwestern Nigeria granitoids. The evident polar wandering path of Precambrian rocks in southwest Nigeria was caused by the effects of the mantle and superplumes. The acquired conjugate poles lie towards SW-NE at 304.8°E and 61.8°S directions (dp = 5.4, dm = 10.7); which was relatively at mean direction of 305.1°E and 64.5°S (dp = 2.3, dm = 4.5).

Figure 6.

(a) Site mean directions of the study area (b) spline apparent pole wandering APW of Africa pole and (c) VGP reconstruction of Africa databasefit (lat = 0.0, long = 151.60, angle = 27.50) with Africa, taking account of the opening of the South Atlantic.

The Precambrian Basement rocks of southwestern Nigeria demonstrated low coercivities and low unblocking temperatures especially in the coarser grains.. No younger geological event occurred in the study area. The mafic syenite dykes witnessed in the study area occurred during the last phase of deformation. The tectonometamorphic history is interpreted as remagnetisation due to Neoproterozoic Pan-African event which is correlated to Brasilian 650 ± 150 Ma. The remagnetisation was not directly attributed to tectonic stress but to fluid, chemical and viscous effects, The remagnetisation phenomenon is due to new mineral growth, whose chemical remanent magnetization (CRM) swamps that of magnetically softer earlier grains. The progressive demagnetization, along the great circle path from initial towards pressure-vessel field. Only the initial remanence and the demagnetization fields determine the remagnetisation paths

4.2 Tectonic implications

The area has been tectonically moved as a result of post-magnetization causing the Nigerian shield magnetic direction to be deviated from an original primary direction. At least three explanations for the observed SW Nigeria paleopole position exist, these are: present position acquired magnetic remanence; NW thrust to after RM acquisition brought the SW craton to its present position, or the southwestern Nigeria acquired its magnetization and was transported to the northeast >1000 km by a left-lateral transcurrent fault system, and was later thrust to the northwest to the present location of the body. The magnetic fabric exhibited by the SW Nigeria granitoids closely approximates the mineral fabric, suggesting that both were acquired during the four deformational (D1, D2, D3 and D4) events which have affected the body. Results indicate that the RM was acquired during magnetite recrystallization or cooling from metamorphic temperatures (~600°C) to maghemite. RM acquisition in the gneiss seems to have happened over a comparatively long period of time, with reversed, mixed and normal polarities represented in the magnetic signature of the unit.

The introduction of hydrothermal fluids occurred at a temperature well above the Pb–Pb closure, which corresponds to the age of the magnetic pole in southwestern Nigeria (~ 571 Ma). Granitoids emplaced over 700 Ma were not reliant on high-level hydrothermal emplacement in unmetamorphosed southwestern Nigerian Precambrian rocks, implying a Pan-African episode. Thus, the Pb–Pb date, which is younger than the 620 Ma U–Pb acquired from a deformed syenite, can provide the thrust a better constrain age. As a result, 620 Ma was proposed as the magmatic rock’s crystallisation period rather than the tectonic event’s age. The 571 Ma periods, on the other hand, proposed retrograde metamorphism at the nappe’s base. The metamorphism ranges from amphibolite-granulite and retrograde greenschist facies towards north to base of nappe respectively in Nigeria.

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5. Discusssion on Precambrian geodynamics in relation to paleomagnetism

5.1 Plate tectonics and subduction

It’s worth mentioning that thermomechanical numerical experiments were only recently employed to study the onset and patterns of Precambrian plate tectonics and subduction [51, 52, 53]. Van Thienen et al. [54] presented one of the most well-known attempts to employ numerical modelling to analyse global tectonic processes of the early Earth).Since of the considerably different temperature and viscosity conditions that existed beneath the early Earth’s mantle, and because current-day geodynamics cannot be easily projected back to the Earth’s early history. Van Thienen et al. [54] used computational thermochemical convection models with partial melting and a basic mechanism for melt segregation and oceanic crust formation to investigate an alternative set of dynamics that may have been active in the early Earth. They are: Small scale convection involving the lower crust and shallow upper mantle; Large-scale resurfacing processes in which the entire crust sinks into the (ultimately lower) mantle, forming a stable reservoir of incompatible elements in the deep mantle and segregating melt builds a fresh crust on the surface and excessive melting and crustal growth due to the intrusion of lower mantle diapirs into the upper mantle at a high excess temperature (about 250 K). This allows plumes in the Archean upper mantle to have substantially higher excess temperatures than previously thought possible based on theoretical considerations. Various geodynamical theories have predicted a dense enriched layer at the mantle’s base [55, 56]. Massive scale sinking of the thick basaltic/eclogitic crust induced by decompression melting of mantle peridotite, according to Van Thienen et al. [54], may have developed such a layer over a brief period early in the Earth’s mantle’s history. The large-scale crustal sinking model described by Van Thienen et al. [54] might thus be considered an alternative (or predecessor) to Albarede & Van der Hilst’s proposed subduction model (2002).Large-scale sinking appear like subduction and could be a Precambrian forerunner to modern plate tectonics.

In contrast to other studies, O’Neill et al. [57] presented an alternate explanation for crustal growth episodicity by providing paleomagnetic evidence for periods of rapid plate motions matching to observed peaks in crustal age distribution. The Nd and Sr. isotope ratios of many juvenile terrains [58] support the idea of increased plume activity associated with these overturns and provide a model for their cratonization. Superplumes and other ideas have been presented to explain this episodicity [59]. Plate-driven episodicity arises naturally in response to the early Earth’s high mantle temperature, and hence can explain quick pulses of plate motion and crustal formation without the need for mantle overturn events [57]. To assess the possibility of subduction in the hotter Precambrian Earth, Van Hunan and van den Berg [53] employed 2D thermomechanical models with a single subduction zone enforced by a weak fault (Figure 7).

Figure 7.

Modelling the evolution of an active continental margin with high-resolution numerical models for various mantle temperature differences (T) above current values (modified after [60]).

In contrast to Davies [61], instead of focusing on changes in crustal thickness spanning 10 to 22 km, this model ignores early upper mantle depletion; caused by rising mantle temperatures, that decided plate tectonics’ viability on a hotter Earth. Numerical results revealed no Ultrahigh-Pressure Metamorphisms (UHPM) or blueschists in most of the Precambrian: early slabs were too weak to provide a mechanism for UHPM and exhumation. Due to the lower viscosity and higher degree of melting, a hotter, fertile mantle would have resulted in a thicker crust and a thicker depleted harzburgite layer in the oceanic lithosphere, according to van Hunan and van den Berg [53]. A thicker lithosphere may have been a significant stumbling block to subduction, and Earth in the Precambrian may have been characterised by a different mode of downwelling [62] or “sub-lithospheric” subduction [53], though the conversion of basalt to eclogite may greatly relax this limitation [54, 63]. The natural reduced viscosity of the oceanic lithosphere on a hotter Earth would lead to increased Slab breakoff (Figure 7) and crustal detachment from the mantle lithosphere has occurred in some situations. Hence, lithospheric weakness may limit the feasibility of present plate tectonics on a hotter Earth. By merging knowledge from geochemical data and numerical models, Halla et al. [51] used inferences from van Hunan and van den Berg’s [53] numerical study to constrain early Neoarchean (2.8–2.7 Ga) plate tectonics. Sizova et al. [60] employed a two-dimensional (two-dimensional) petrological–morphological model. To investigate the dependence of tectonic-metamorphic and magmatic regimes at an active plate margin on upper-mantle temperature, crustal radiogenic heat production, and lithospheric weakening, a thermomechanical numerical model of oceanic–continental subduction (Figure 7) was used to conduct a series of high-resolution experiments. Based on their testing, the scientists observed a first-order change from a “no-subduction” tectonic regime to a “pre-subduction” tectonic regime, and then to the current mode of subduction (Figure 7). The first transition is gradual and occurs between 250 and 200°C over current upper-mantle temperatures, whereas the second transition is abrupt and occurs between 1 and 2°C above current upper-mantle temperatures. The change to the current plate tectonic regime occurred at 3.2–2.5 Ga, according to the link between geological evidence and model results. Convergence does not result in self-sustaining one-sided subduction in the “pre-subduction” tectonic regime (upper-mantle temperature 175–250°C above the surface), but rather two-sided lithospheric downwellings and shallow underthrusting of the oceanic plate beneath the continental plate (Figure 7b).

5.2 Orogeny and collision

Interpretations of geological, petrological, and geochemical data from Proterozoic and Archean orogenic belts revealed that the Precambrian had different tectonic kinds of orogeny than the present-day Earth [64, 65, 66]. Accretionary and collisional orogens are the two forms of Precambrian orogens [66, 67, 68]. When the oceanic crust is subducted along active continental margins, accretionary orogens form ([66] and references therein). Precambrian accretionary orogens make a significant contribution to continental expansion compared to Phanerozoic accretionary orogens due to their high rates of juvenile crust growth [66]. Several post-Archean accretionary orogens are terminated by continent-continent collisions during supercontinent formation [66]. The average terrain lifespan during the Archean is 70–700 million years, 50–100 million years during the pre-1 Ga Proterozoic, and 100–200 million years in later orogens [66]. When continents collide, collisional orogens emerge; they initially arose in the Proterozoic, but had little impact on continental growth [67, 68]. The appropriateness of studies of current collisional orogens to the Precambrian is yet unknown, given the impact of a warmer continental crust and a higher mantle on the geodynamic regime earlier in Earth history [69, 70]. Extreme ultrahigh-pressure (UHP) metamorphic rock complexes are generated and exhumed by Phanerozoic collisional orogenic systems, which also create clockwise metamorphic P–T routes. About a thousand high-pressure (HP)-ultrahigh-pressure (UHP) metamorphic terrains have been discovered around the world, the most of which are Phanerozoic in age. One is Neoproterozoic, while the other is Neoarchean to Paleoproterozoic in age [1, 71]. The lack of UHP metamorphic complexes in the Precambrian geological record indicates that another type of orogenesis predominated earlier in Earth’s history [64, 65]. Based on field results, Precambrian orogens differ greatly from current orogens. At high apparent geothermal gradients, Precambrian orogenesis made important contributions to crustal growth and magmatism [64, 66]. Four orogen categories was proposed by Chardon et al. [67] and Cagnard et al. [72] recently attempted to define Precambrian accretionary orogens using first-order structural and metamorphic traits, which represent the state of the continental lithosphere in these convergent settings involving enormous juvenile magmatism (Figure 8).

Figure 8.

Proposed classifications for Precambrian orogens Chardon et al. [67] (a) and Cagnard et al. [72] (b). (a) Orogen construction possibilities [67]: LM1 = stiff upper mantle lithosphere; LM2 = ductile, lower viscosity, lower lithospheric mantle; C = crust; LM = lithospheric mantle; LM1 = stiff upper mantle lithosphere; LM2 = ductile, lower viscosity, lower lithospheric mantle. (b) Schematic orogenic cross sections depicting the evolution of distinct orogenic styles over time. [72].

5.3 Micro-Raman spectroscopy

Figure 9 revealed Raman spectra were observed within the white matrix, pyroxene, opaque mineral pockets and diverse places around the mineral matrix. Maghemite Raman shift peaks are recorded at some points within the biotite granite gneiss, and thin section petrography of all the rock units in the study area shows the abundance of quartz, microcline and plagioclase as the major minerals that dominate the rock samples with other minor components such as hornblende, muscovite and the opaque minerals. Plagioclase, quartz, and microcline minerals were found to make up to 70% of the volume percentages of the rock in thin sections, with plagioclase being the most dominant mineral, followed by quartz microcline being the third most dominant mineral. The results of 830 nm Raman microspectroscopy of biotite granite gneiss grains have 398.8 cm–1 and 663 cm–1 indicating weak spectra while that of 785 nm have 714.8 cm–1, 720.4 cm–1 and 764.48 cm−1 indicating strong peak bands respectively (Figure 9) [47, 73, 74, 75].

Figure 9.

Micro-Raman spectroscopy of some Precambrian rocks.

Raman shifts are: 398.8 cm−1, 521 cm−1, and 714.8 cm−1 signals in P4332 mode, parallel swinging of the two tetragonal centres corresponds to polymorph Fe-O bond stretching, cubic P4332 structure, and suggested change in the tetragonal symmetry. This originates from Fe2−, Fe3− and O2− which are the bond stretching in the [γ- Fe2O3] cubic P4332/tetragonal P41212 having transformation phase of magnetite spinel of α- Fe2O3 and γ- Fe2O3 polymorphs [76]. Kaiser microprobe of 785 nm recorded Raman peaks at 522.67 cm−1 and 714.8 cm−1, 720.4 cm−1, 764.48 cm−1 for maghemite mineral grains observed in biotite granite gneiss.

Low-temperature shortage causes oxidation of magnetite in the magnetic moment, resulting in a reverse spinel structure with both Fe2− and Fe3− ions in tetrahedral positions (A) and octahedral (B) sites configuration were all factors that contributed to the presence of maghemite in the biotite granite gneiss [76]. The main minerals in granite are quartz, microcline and plagioclase, while minor minerals include hornblende, muscovite, and opaque minerals. Under plane-polarised light, the quartz mineral in the rock samples was colourless, and it appears as subhedral prismatic crystals. Microcline is typified by polysynthetic twinning in two directions (cross-hatched), one according to albite law, and the other according to pericline law (monoclinic orthoclase/sanidine transformed to microcline), whereas its polysynthetic twinning distinguished plagioclase according to albite law. Biotite is brown, yellowish-brown and reddish-brown in the thin section. It is pleochroic, occurring as plates and laths and showed elongation along the foliation plane.

Several granite grains with variable colours were subjected to two excitation wavelengths of Raman microprobe, disregarding configuration and cause of the 521 cm–1 peak. The Raman mode at 519.1 cm−1 and 521 cm−1 corresponds to polymorph Fe-O bond stretching, cubic P4332 structure, which described the tetragonal distortion symmetry. The Raman shifts of 519 cm–1, 522.67 cm–1, 663 cm–1 and 714.8 cm−1 are laser excitation wavelengths and not fluorescence induced. These bands appeared only for dark or opaque granite crystals when excited with Raman microprobe (785 and 830 nm) laser, even if the clear samples are less intense. Since the points are similar to those determined for maghemite lattice modes, and these spectra correspond to translational lattice modes in maghemite geometry [73].

Scanning electron microprobe mineralogical compositions of granitoids from the basement complex of Southwestern Nigeria were studied using scanning electron microscopy (SEM). Gneiss, granite, biotite granite gneiss, banded gneiss and charnockite predominantly recorded maghemite/magnetite, ilmenite, pyrite and poor (titano)magnetites, with differences in titanium (Ti), grain sizes content and configuration, respectively (Figure 9). Examinations of polished sections of samples from the southwestern Nigerian granitoids (Figure 10A-F) revealed: grains of maghemite and magnetite (light grey); titanomagnetite (grey) magnetite (light grey); magnetite and titanomagnetite (striations of light grey); maghemite and titanomagnetite (grey); ilmenite and pyrite observed between (white and grey) respectively.

Figure 10.

SEM results of magnetic minerals (JEOLJSM-5900LV). (a) Altered magnetite (maghemite) of gneiss (scale:10 pm), (b) titano-magnetite of granite (scale:10 pm). (c) Titano-magnetite and magnetite from dehydration of biotite in biotite granite gneiss (d) titanomaghemite and maghemite of banded gneiss (e) ilmenite of charnockite (f) pyrite of Charnockite.

Scanning electron microscope serves as a proxy in determining the diverse magnetic phases in iron titanium oxides present in the selected rock samples. Studies showed larger altered (titano)magnetite grains in the gneiss, titanomagnetite in granite, phases of titanomagnetite and magnetite in biotite granite gneiss with evidence of dehydration, maghemite and titanomaghemite in banded gneiss, ilmenite in charnockite and pyrite as seen in granite gneiss (Figure 10 a-d). The Fe-Ti-O grains indicated transformations of spinel rods, low-temperature oxidation reaction, precipitation of crystalline rock phases (exsolution) and dehydration due to tectonic-metamorphic episodes observed in studied rock samples. The abundance of precursor magnetite was susceptible to transformation than the smaller magnetite grains resulting in the pronounced formation of maghemite and titanomaghemite in the rock samples. This result correlates with the arguments of Carporzen et al. [77] that suggested tectono-metamorphism-related temperatures in the rock assemblage and heating of magnetite.

5.4 X-ray diffractometry

The magnetic minerals in the selected rock samples were investigated further using the XRD data (Figure 11 a-c) to unravel the mineralogical phases found in the granite, gneiss and charnockite. It demonstrates mainly the silicate and pseudomorphs of magnetite phases. Figure 11 a-c showed pronounced spectra of silicate phase while the smaller spectra are pseudomorphs of magnetite (magnetite and maghemite). These results are consistent with that of the Raman spectroscopy, SEM and temperature dependence.

Figure 11.

XRD of some Precambrian basement rocks the study area (a) granite (b) gneiss (c) Charnockite.

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6. Discussion of results

The Precambrian rocks witnessed remagnetisation due to four phases of tectonometamorphic episodes. Rocks like diorite, biotite granite gneiss and syenite were not remagnetised and recorded normal and reversed polarities. The documentation of preliminary paleomagnetic, geochronological and microstructural datasets for Precambrian granitoids from the Southwestern Nigeria basement complex, located on the Pan African nappe system covered the Southwestern area of Nigeria, promoted extensive greenschist hydrothermal metamorphism in the underlying cratonic basement, according to research conducted on the northern edge of the Congo craton. The tectonometamorphism has resulted in widespread remagnetisation of the granitoids in southwestern Nigeria. On amphibole grains from Precambrian rocks, 206Pb/207Pb techniques were used to date both metamorphism and magnetic resets at 571.6 Ma. The normal and reverse polarities found in late Neoproterozoic granitoids are coeval with the paleopole at 304.80E and 61.80S (DP = 5.4, dm = 10.7) meet the fifth criteria of Van der vool. These pole and specific primary poles of the Congo craton propose an elbow-shaped apparent polar wandering path ranging from 593 to 547 Ma at the Pan African tectonic metamorphism [23, 26, 42]. Raman spectroscopy revealed the presence of maghemite iron oxide minerals in most of the rocks. SEM results showed maghemite, magnetite, titanomagnetite, ilmenite and pyrite, while XRD recorded pseudomorphs of magnetite. Two-sided lithospheric downwellings and shallow underthrusting weakened the plates. When the upper mantle temperature rises over 250°C, a “no-subduction” zone emerges, in which small deformable plate pieces move horizontally. The degree of lithospheric weakening caused by the intrusion of sub-lithospheric melts into the lithosphere controls the tectonic regime. At upper-mantle temperatures of 175–160°C, a reduced melt flow leads in less melt-related weakening and more strong plates, stabilising the present subduction type even at high mantle temperatures.

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7. Conclusions

Remagnetization was prevalent in the Precambrian era. Reactivation of the Pan-African tectonics on the migmatite gneiss protolith (Eburnean granitic pluton) was not affected in numerous sites. The rocks demonstrated primary and secondary remagnetisation (normal, reversed and mixed polarities) and stability established by representative rocks of biotite granite gneiss, granite gneiss and syenite. Geochemical and isotopes parameters have revealed that the Paleoproterozoic/Eburnean orthogneiss and the granite plutons represent the same lithospheric source Paleoproterozoic/Eburnean source became molten all through the Pan-African event. The paleomagnetic pole positions of some Precambrian rocks in southwestern to the orogenic events revealed actual polar wander paths towards the equator during the assemblage of the Rodinia supercontinent. The Raman spectra of maghemite through estimation and observations of analogous wavenumbers of magnetite pseudomorphs revealed its atomic origin. The individual specimens of biotite granite gneiss, granite and charnockite have maghemite at strong peak spectra 519, 521, 522.00 cm−1 and 1285.5 and weak shoulder Raman spectra 398.8, 663, 710 and 717 cm−1 with 830 and 785 nm infra-red Raman spectroscopy. SEM revealed evidence of magnetite and titanomagnetite and dehydration of biotite in biotite granite gneiss. Study area dominated by plume tectonics and lithospheric delamination. Numerical models suggest that the transition occurred at mantle temperatures 175–250°C higher than present-day values triggered by stabilisation of rheologically strong plates of continental and oceanic type. Widespread development of modern-style (cold) collision on Earth started during Neoproterozoic at 600–800 Ma decoupled and is thus the onset of modern-style subduction. The cold collision created favourable conditions for the generation of ultrahigh-pressure (UHP)metamorphic complexes in southwestern Precambrian rocks.

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Written By

Cyril C. Okpoli, Michael A. Oladunjoye and Emilio Herrero-Bervera

Submitted: 18 September 2021 Reviewed: 24 March 2022 Published: 29 September 2022