The Younger Granite province of Northern Nigeria occurred as an extension of the North-South trend of igneous activities which started during the Precambrian at Hoggar in Algeria [1]. It extended southwards during the Ordovician where it appeared within the Tuarreg region of the Niger Republic, being clearly emplaced within the Andrear Bous in Northern Niger Republic and Air Massive in the Southern Niger Republic [2]. It continued in its southwardly appearance into the Tibesti Massive in Northwest of Chad Republic and South of Libya where it was dated Ordovician to early Silurian [3]. Its continuous southerly extension intruded the Nigerian basement complex during the Jurassic age. The oldest of the Nigerian ring complexes is the Dutse complex in Kano State dated Early Jurassic while the youngest is the Afu ring complex located near Nasarawa Egon in Niger State and was dated Late Jurassic (Figure 1). The eroded volcanic complexes of the northern Nigeria are either isolated (e.g. Ririwai, Banke etc.) or coalesced into groups owing to the migration of the centre of magmatism e.g., Ningi-Burra [4], Tibchi [5]. Mineralogically and Geochemically, the Younger Granite ring complexes of Nigeria have been studied by the likes of Bowden [6-15]. Geophysically, the likes of Ajakaiye [16,17] Kogbe [18] and Etim [19] have made useful contributions in studying the ring complexes of Nigeria. Structurally, Ike, Abaa, Turner and Odia [5,11,20,21] have carried out worthwhile researches on the ring complexes. Subsurface resource potentials of individual ring complexes like Jos-Bukuru cassiterite mineralization and groundwater studies have been carried out by Ologun, Samaila and Solomon, Akanbi and Opara [15,22-24]. Ririwai, Zaranda, Banke, Jarawa, Kagoro and Ganawuri ring complexes have respectively been studied by researchers like Olasehinde and Ashano, Ibe, Adubok, Olasehinde, Alkali and Ibeneme [25-30].

Figure 1: Major Younger Granite Localities in Nigeria.

Geophysical methods have been variously applied in the study of diverse range of geological structures. Various techniques are employed in the estimation of such parameters like location, magnetization, depth and size of buried magnetic bodies. These interpretation techniques include curves matching [31-33], Fourier transform [34], Hilbert transforms [35], monograms [36], least squares minimization [37,38], characteristic points and distance approaches [39,40], correlation factors between successive least-squares residual anomalies [41], Henkel transform [42], linearized least squares [43], normalized local wave number [44], analytic signal derivatives [45-47], Euler deconvolution [48], Source Parameter Imaging [49], Fair function minimization [50], Deconvolution technique [51], second horizontal derivatives [52], Simplex algorithm [53], Simulated annealing [54], very fast simulated annealing [55-62], Global nonlinear optimization [63-65].

Previous geological and geophysical studies that were carried out within the Younger Granite province of Nigeria focused mainly on ring complexes with surface exposure with none taking into cognizance their possible subsurface geometry just as little is known about those without surface exposure, a lacunae which this study wants to close. In the current study, Horizontal Derivative, Analytic Signal, Euler Deconvolution and Source Parameter Imaging techniques were used to precisely map the Younger Granite ring complexes of Nigeria and estimate their surface and subsurface structural dispositions.

Location and Physiography of the Study Area

For this research, aeromagnetic sheets 103, 104, 105, 125, 126, 127, 146, 147, 148, 167, 168, 169, 188, 189 and 190 were used which were published on a scale of 1:100,000 and enclosed within the fifteen (15) aeromagnetic maps of the Nigerian Geological Survey Agency (NGSA) with important provinces which include Jemaa, kura, Pankshin, Maijuju, Naraguta, Kafanchan, Geshere, Lere, Toro, Kalatu, Ririwai, Dutsen Wai, Birnin Kudu, Burumburum and Ikara. The study area is part of the Younger Granite province of Northern Nigeria which lies between Latitudes 9°00´N to 11°30´N and Longitudes 8°00´E to 9°30´E covering an area of approximately 27,622.44 km². The elevation of the area ranges from 200 m (in low lying areas occupied by the weathered and undifferentiated basements) to about 1800 m above sea level (for high rising areas occupied majorly by the ring complexes) (Figures 2 and 3). The drainage pattern is dendritic with the major river sources originating from the base of the uplifted hills in the area (Figure 4).

Figure 2: Digital Elevation Model (DEM) map of the study area.

Figure 3: Topographic map of the study area showing some prominent towns.

Figure 4: Drainage map of the study area.

Geology of the Study Area

In Nigeria about fifty two separate Younger Granite ring complexes are recognized with a total area of 7511 km² (Figure 5). The origin and formation of the Younger Granite ring complexes in Nigeria and beyond were not related to any orogeny and thus are often referred to as being anorogenic in nature. The Younger Granites appear to have been subject to two distinct processes of differentiation resulting in divergent trends viz: a normal series and an alkaline series [66]. The Younger Granites exhibit a large variety of rock types but those that remain remarkably constant in character throughout the whole province are Rhyolites, Hornblende-fayalitegranites and porphyries, Hornblende-biotite granites, Biotite granites, Riebeckite and Riebeckite-biotite granites, Aegirinearfvedsonite granites, Syenites and Trachytes and Basic rocks (Gabbros, dolerites, basalts etc). Details of occurrence, character and petrography of the rock associations are discussed in MacLeod [66]. Some of the Younger Granite ring complexes within the study area are Ririwai, Banke, Sara fier, Jos Bukuru, Amo, Kigom, Tibchi etc (Figure 6).

Figure 5: Geologic map of Nigeria showing study area with some disseminated ring complexes

Figure 6: Geologic map of the study area.

Data source

The data used for this research forms part of the new high resolution digital airborne data acquired between 2005 and 2009. The entire data included 1,930,000 line km of magnetic survey flown at 500 m line spacing and 80 m terrain clearance. Data acquisition was carried out by Fugro Airborne Surveys while the processing and preliminary interpretation was contracted to Paterson, Grant & Watson Limited (PGW) [67]. A subset of the nationwide grid covering the study area (Lat 9o00’N-Lat 11o30’N and Long 8o00’E-Long 9o30’E) was made available in GDB (Geosoft Database) format by the Nigerian Geological Survey Agency (NGSA) [67]. The geomagnetic gradient was removed from the data using the International Geomagnetic Reference Field (IGRF) formula for 2005. This new data offers a lot of advantages in resolution and format over the old data in which most of the earlier interpretations were made. Table 1: compares 2011 with the 1974 data. Oasis Montaj. Ver 7.1 Mapping and Processing System with geosoft executable (GX) plugins, USGS Potential Field Software version 2.2, GM-SYS V. 5.0 and ArcGis Ver 10.2, were the softwares used for the interpretation. Figure 7 describe the methodology used for processing, analysis and interpretation.

Table 1: Old data versus New data.

Figure 7: Workflow for aeromagnetic data analysis and interpretation.

Analytic signal

The Analytic signal method is very useful for delineating magnetic source location [46]. The analytic signal image simplifies the interpretation by placing the anomaly peaks directly above the source. The magnitude of the analytic signal of the total magnetic field produces maxima over magnetic contacts regardless of the direction of magnetization and is always positive [46]. These magnetic contacts could result from differences in magnetic susceptibility between an intrusive rock and a country rock, between geologic contacts and according to Odia [21] across a fracture zone due to the oxidation of the magnetite or infilling of the fracture zone by magma thus forming intrusive bodies whose magnetic susceptibility is different from the host rock. This method is very useful at low magnetic latitudes as it is independent of the inclination of the magnetic field. This quantity is defined as a complex function that its real component is the horizontal gradient and its imaginary component is the vertical gradient. It can be proven that imaginary component is Hilbert transform of real component [45,68].

Nabighian [45,68] developed the concept of 2D analytic signal, or energy envelope, of magnetic anomalies. The amplitude of the 3D analytic magnetic field signal at location (x,y) can be expressed as:

(1)

where, A(x, y) is the amplitude of the analytic signal at (x,y); T is the observed magnetic field at (x,y). The purpose of the reduction to the pole is to take an observed total magnetic field map and produce a magnetic map that would result, had an area been surveyed at the magnetic pole.

Euler deconvolution

Thompson expressed the Euler homogeneity as

(2)

Where x_o, y_o, z_o are the positions of the magnetic body. T is total field measured at (x, y, z). N is the degree of homogeneity which can be interpreted as the Structural Index (SI). B is background value of the TMI. The standard 3D Euler method is based on Euler’s homogeneity equation, which relates the potential field and its gradient components to the location of the source, by the degree of homogeneity N. The method makes use of a structural index in addition to producing depth estimates [69]. The structural index combined with depth estimates have the potential to identify and compute depth estimate for a variety of geologic structures such as faults, magnetic contacts, dykes, and sills [70]. The algorithm uses a least squares method to solve Euler’s equation for each grid position within a sub-grid. A square window of predefined dimensions is moved over the grid along each row. At each point a system of predefined equation is solved, from which x, y and z and their uncertainties are obtained for a predefined Structural Index. A solution is only recorded if the depth uncertainty of the solution is within a limiting distance from the center of the data window [71]. Though Euler Deconvolution is applicable to all geologic models and is intensive to magnetic remanence and its initial assumption of the source geometry has to be made. This is done by selecting a structural index [72].

Source parameter imaging

The Source Parameter Imaging (SPI) technique (sometimes referred to as the local wave number method) was developed by Smith [73] and Thurston [74,75]. The technique is based on the principle of complex analytic signal and computes source parameters from gridded magnetic data. The SPI method requires first and second order derivatives and is thus susceptible to noise in the data and interference effect. The wave number theory is that for vertical contacts the poles of the local wave number define the inverse of depth. In other words depth is giving by the expression.

Where K_max is the peak value of the local wave number K over the step source? It can be shown that

(3)

(4)

(5)

Solution grids using the SPI technique show the edge locations, depths, dip and susceptibility contrasts. Hence the SPI map shows more close resemblance to geology than either the Total Magnetic Intensity or its derivatives. The technique works best for isolated 2D source such as contacts, thin sheet edges, or horizontal cylinder [45].

Result Presentation and Interpretation

The TMI image is characterized by regions of long wavelength and shows high amplitude around the axis of the Younger Granite ridges. The pockets of low magnetic values observed separating Younger Granite ridges (basement rocks) are due to the position of the study area on the globe (low magnetic latitude). Thus the anomalies are inverted, i.e. basement areas containing metamorphic and igneous rocks show low magnetic values. The TMI requires further processing in order to simplify interpretation.

As can be observed on the TMI image (Figure 8), the anomalies related to the source geometry are often not apparent at low magnetic latitudes (just as the study area), hence the need to reduce to pole or equator. One Reduction to Pole (RTP) image was generated with Amplitude Correction Inclination (ACI) of 40 referred to as RTP (40). The visual observation of its precursor (RTP 20) which was generated prior to generating the RTP (40) image displays a major limitation of the RTP procedure. North-South anomalies are highly distorted; consequently interpretations are obscured. The RTP (40) image appears better (Figure 9), despite the high amplitude correction applied. The RTP transformation has correctly placed the peaks over the source, magnetic highs are observed within the peripheries of the Younger Granite ridges with Jos-Bukuru, Ririwai, Kagaro, Kila-Wuji and Rishua displaying the highest. Prominent magnetic lows are observed within the Northern part of the study area. First and second vertical derivatives (1VD and 2VD) of the TMI were performed in order to enhance shallow sources. Figure 10 shows the distribution of basement rocks, volcanoes, hyperbysal rocks and intrusives in the study area (blue patterns). The 2VD image tends to reveal more features distinctively, but is noisier (Figure 11). The 2VD image shows the Younger Granite ring complexes as pockets of intrusive structures displayed in blue pattern. The horizontal gradient display is characteristically employed in the enhancement of linear features from aeromagnetic data. The magmatic induced patterns in the study area are observed and are accompanied by numerous linear features (Figure 12). Within the Younger Granite areas, there is high density of these linear structures indicating intense structural perturbation during emplacement [28]. Lineaments extracted from aeromagnetic data (Figure 13) are generally few (about 1529) and are deep seated structures. The persistence of the NE-SW dominant trend on the First Vertical Derivative (1VD) map (Figure 13) shows that this trend represents the dominant structural trend in the area. A closer look at these maps shows that there is high density of lineaments of the order of 50-100 km within the areas occupied by the Younger Granite ring complexes. This typifies intense deformation by faulting and fracturing of the country rock that permitted the emplacement of these Younger Granites.

Figure 8: Total Magnetic Intensity (TMI) Image of the study area.

Figure 9: RTP image of the study area with ACI = 40.

Figure 10: First Vertical Derivative image of the study area.

Figure 11: Second Vertical Derivative image of the study area.

Figure 12: First Horizontal Derivative image of the study area.

Figure 13: Lineament extracted from Aeromagnetic map draped on 1VD map of the area.

The Analytic Signal of the Total Intensity data is computed to understand the distribution of magnetic sources in the study region. Figure 14 shows the magnitude of the analytic signal of the residual magnetic intensity data (Figure 15) of the study area in the space domain. The maxima or peak of the magnitude of analytic signal represents the magnetic contacts between igneous intrusions (mainly the Younger Granite intrusions in the form of dykes and/or granite plutons) and the surrounding basement rocks within the area. The magnitude of analytic signal within the study area depends on the magnetic susceptibility contrast between Younger Granite intrusions and the Older Basement rocks (Older Granites, Migmatites and Gneisses). Basic igneous rocks have a higher magnetic susceptibility than acidic igneous rocks. In this study, the peaks in magnitude of analytic signal could result from magnetic susceptibility contrast between the rocks of the Younger Granite Complex (being mainly acidic in composition) and that of the Older Basement rocks mainly composed of Migmatites and Migmatitic gneisses which contain more of mafic minerals. Comparing Figures 13 and 15, it can also be observed that the maxima or peaks in the analytic signal are more within the Jos-Bukuru, Kila Wuji, Ningi Bura and Gamawa complexes when compared with other complexes in the study area. This implies that there may be more Younger Granite intrusions within the Older Basement rocks in these complexes than the other complexes within the study area. Areas with question marks on the Analytic Signal map (Figure 15) are locations with signatures and patterns similar to that of the Younger Granite ring complexes without a record of the presence of such in that localities.

Figure 14: Analytic Signal image of the study area.

Figure 15: Residual Magnetic map of the study area.

3D Euler map is displayed in Figure 16 with Structural Index (S.I) value of 2.0. The technique was applied to the upward continued aeromagnetic anomaly data to determine depth of ring complexes in the study area. Structural Index 2.0, which represents cylindrical shaped body, was chosen because magnetic field of alkaline intrusions (like ring complexes) can simply and accurately be inferred with cylindrical shaped body. Just like in the Analytic Signal map, those localities that display some signatures and patterns similar to that of the Younger Granite ring complexes wherein the occurrence of that in such an area has not been well documented are enclosed in white boxes with question marks on the 3D Euler Deconvolution map (Figure 16). One of such areas which fell within the Ririwai Sheet 126 (Figure 17), which has been studied by Raimi et al, revealed more information concerning these anomalies and as such referred to as unexposed ring complex, and was equally identified in this work and extensively studied.

Figure 16: 3D Euler Deconvolution on Analytic Signal image of the study area (SI=2.0).

Figure 17: Source Parameter Imaging (SPI) map of the study area.

The Source Parameter Imaging (SPI) map of the study area (Figure 17) shows that 85 to 90% of the study area contains sources that are relatively shallow. The shape, orientation and vertical extent of these ring complexes as mapped along profiles A-B, C-D and E-F on the Analytic Signal map are shown in Figures 18-21.

Figure 18: Analytic Signal map of the area showing Profile Directions.

Figure 19: Geologic profile along NE-SW direction passing through the unexposed ring complex and two other Complexes.

Figure 20: Geologic profile along NNW-SSE direction passing through the unexposed ring complex and two other Complexes.

Figure 21: Geologic profile along E-W direction passing through the unexposed ring complex and two other Complexes.

Table 2 summarizes the findings of the depth estimates using both the SPI and Euler techniques while Table 3 shows the approximate surface geometry of some notable ring complexes within the study area as extracted from the Analytic Signal map.

Table 2: Depth to Anomaly sources using Euler and SPI techniques.

Table 3: Surface Geometry of some Ring Complexes extracted from the Analytic Signal map.

Analysis and study of the magnetic data revealed that the Younger Granite province of Nigeria is punctuated by pockets of distinct magnetic anomalies with diverse shapes/plans. The first horizontal derivative map of the study area is characterized by majorly NE-SW trending structural pattern. This pattern is similar to that pointed out by Alkali Oluyide, Alkali and Gaiya [29,76,77] as relating to the Pan African structural trend of the basement complex. The distinct magnetic anomalies, clearly revealed in Figure 15, coincide spatially with outcropping ring complexes of the area and their surface geometry accurately measured (Table 3). Some anomalies with the same feature with the anomalies of the ring complexes were also revealed (indicated with question marks within white boxes) but without an associated surface manifestation (outcrop). These anomalies are believed to be intrusions, probably ring complexes without surface exposures, similar to that suggested by Ajakaiye [78] in the north eastern part of the province, Raimi [79] within the Ririwai sheet and by Mantovani, Dutra and Marangoni, Dutra, Louro and Mantovani in central Brazil [80-83]. The pattern/position of the unexposed ring complex and the approximate thickness of overburden covering it were clearly displayed on the 3D-Euler Deconvolution map. The results of the Euler solutions indicate that the depths of the ring complexes range from outcrop to 1800 m. For the inferred unexposed ring complex, the depth ranges from 5 to 1200 m. using the Source Parameter Imaging (SPI) technique, it was observed that the basement areas (undifferentiated basement and the older basement rocks) are generally exposed, where buried they rarely exceed depths of 124 m thereby accounting for the high magnetic responses observed within these areas. The Younger Granite ring complexes show low SPI depth values of about 4 km below the sea level indicating deeper magnetic sources (Figure 17). Three (3) profiles (AB, C-D and E-F) were respectively taken along the NE-SW, NNW-SSE and E-W directions on the Analytic Signal map (Figure 18) to estimate the possible depth of burial and subsurface structure of the unexposed ring complex and the nearby known Younger Granite ring complexes. From profile A-B, it was observed that overburden thickness of less than 20 m masked the unexposed ring complex with its root situated at approximately 200 m below the sea level (Figure 19). The Ririwai ring complex was observed to have an outcrop of approximately 1300 m above mean sea level with its mappable root situated at approximately more than 400 m below the sea level. Similarly, the Tibchi ring complex was observed to have an outcrop of approximately 950 m above sea level with its root resting at approximately 180 m below the sea level (Figure 19). Along profile C-D, the Banke ring complex was observed to have an outcrop of approximately 920 m above mean sea level with its root situated at approximately more than 400 m below the sea level whereas the Amo ring complex has an outcrop of approximately 1400 m above mean sea level with its root situated at approximately more than 800 m below the sea level (Figure 20). Profile E-F displayed Kudaru ring complex having a surface exposure of approximately 1150 m above mean sea level with its root situated at approximately 10 m below the sea level; just as the Zuku ring complex has an outcrop of approximately 1220 m above mean sea level with its mappable root situated at an approximate depth of 300 m below the sea level (Figure 21). Thus the two depth estimating techniques (3D Euler Deconvolution and Source Parameter Imaging) utilized in this research gave similar depth estimates for the studied outcropped Younger Granite ring complexes and the unexposed ring complex. On the subsurface orientation of some of the ring complexes, it was observed that along NE-SW direction (Figure 19), both the unexposed ring complex and the Tibchi complex were outward dipping with depth whereas the Ririwai ring complex was inward dipping with depth. Along the NNW-SSE direction (Figure 20), both Banke, Amo and the unexposed ring complexes were all outward dipping with depth. Similarly too, it was observed that along the E-W direction (Figure 21), Kudaru ring complex was inward dipping, the Zuku complex was outward dipping while the unexposed ring complex was near vertical to inward dipping with depth.

From the 2D model of the subsurface geometries and orientations of the studied ring complexes, it could be deduced that the likely reason for the unexposed nature of the newly proposed ring complex could be as a result of partial subsidence of the enclosed block on which the extrusive edifice of the central volcano originally rested probably occasioned by excess space (much subsurface accommodation) for continuously rising magma within the country rock. Its geometry is supportive of this assertion as it revealed majorly outward dipping trend with depth along different axis. Another reason could be due to incomplete fluidization along the already formed ring fracture and poor assimilation of the country rock by the fluidizing agent amidst a slow/near quiescent piece meal stoping process during the central granite intrusion phase which usually marks the end of ring complex formation [84-85].

This study area has demonstrated that the methodology supports shared cognitive responsibility for decision-making in structural interpretation. The Younger Granite ring complexes of Northern Nigeria are characterized by majorly NE-SW trending structural pattern. The surface shapes (plans) as well as the subsurface geometries of some of the Younger Granite ring complexes have been precisely mapped. The ring complexes appear in diverse shapes like elliptical, oval, circular, lenticular, oblong, polygonal and the fascinating cum admiring shape of Tongolo ring complex similar to the shape of a new moon (crescent). From this research, it was encapsulated that the ring complexes of Northern Nigeria exhibit both inward and outward dipping structural orientations with depth. Finally, it could be concluded that this research has demonstrated the efficacy of aeromagnetic data in delineating both outcropping ring complexes as well as those without surface exposures (outcrops).