1 Introduction

Pan-African tectonics and crustal evolution have been the subject of much discussion during the last 30 years. The Nigerian Precambrian Basement Complex, which resulted from the referred tectonics is made up of three major petrological units: (1) migmatitic banded gneisses and migmatites, (2) weakly migmatized to unmigmatized paraschists also referred to as “Younger Metasediments” or “Schist belts”, and (3) the Older Granite suite comprising mainly granites/granitoids, granodiorites, charnockites (hypersthene granites), syenites, as well as minor gabbroic and dioritic rocks. Unmetamorphosed dolerite and rhyolite porphyry dykes, pegmatite dykes, and numerous veins of quartzo-feldspathic composition are intrusions commonly found in the Basement Complex (Oyawoye 1964; Rahaman 1976; Makanjuola 1982; Ekwueme 1987, 1994; Olarewaju 1987; Obiora 2005, 2006).

The Precambrian Basement Complex rocks in the study area have received little attention from researchers as evidenced in the literature. Much of the information on the geology of the area is contained in geological maps of Nigeria produced by the Nigerian Geological Survey Agency where it is shown to be underlain by “Undifferentiated Basement,” “Granulites Terrain,” and “Granitoids” (NGSA 1994, 2004, 2011). The present study was therefore needed for the mapping and delineation of the different varieties of granitic rocks within the Precambrian Basement Complex in the area, as well as to perform detailed petrographic and geochemical studies (major-, trace-, and rare-earth elements, REEs) on the rocks for their proper classification and assessment of their petrogenesis and tectonic origin.

2 Regional geological setting

The study area is located within the extension of the Bamenda Highlands of Cameroun into southeastern Nigeria, otherwise referred to as the Bamenda Massif. The Bamenda massif constitutes the southeastern Nigerian Precambrian Basement Complex. The Precambrian Basement Complex of Nigeria belongs to the Pan-African trans-Saharan belt which is located east of the West African craton and northwest of the Congo-Gabon craton. Based on evidence from the eastern and northeastern margins of the West African craton, it has been observed by previous authors that the Pan-African trans-Saharan belt evolved by plate tectonic processes which involved the collision of the active margin of the Pharusian belt (Taureg shield) and the passive continental margin of the West African craton, about 600 Ma (Fig. 1, Burke and Dewey 1972; Leblanc 1981; Black et al. 1979; Caby et al. 1981). Subduction and consequent collision at the eastern margin of the West African craton (McCurry and Wright 1977) produced extensive melting of the older rock suites resulting in the emplacement of the mainly calc-alkaline granitoids and basaltic intrusions.

Fig. 1
figure 1

The location of the Precambrian Basement Complex in Nigeria between the West African craton, the Congo-Gabon craton and the southern part of the Tuareg shield (after Obiora 2012). a Western part of the Tuareg shield called the Pharusian belt; b central part (the Hoggar-Air segment) and c eastern part (the East-Saharan craton)

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High positive gravity anomaly, which occurs in a narrow zone within the Dahomeyide orogen located at the southeastern margin of the West African craton in Togo and Benin Republic, is evidence of evolution by plate tectonic processes involving the collision of the Pharusian belt and the West African craton (Obiora 2012). The collision at this plate margin is believed to have led to the reactivation and remobilization of the internal region of the Pan-African belt. The Nigerian Precambrian Basement Complex lies within the remobilized part of the belt.

The Nigerian Precambrian Basement Complex rocks are also believed to be the results of at least four major orogenic cycles of deformation, metamorphism, reactivation, and remobilization corresponding to the Liberian (2650 ± 150 Ma), the Eburnean (2000 ± 50 Ma), the Kibaran (1100 ± 200 Ma), and the Pan-African cycles (600 ± 150 Ma). Using the International Geologic Time Scale (2002), these ages can be referred to as, Paleoarchean to Mesoproterozoic (3600 to 1600 Ma) for Liberian and Eburnean, Mesoproterozoic to Neoproterozoic (1600 to 1000 Ma) for Kibaran, and Neoproterozoic to Early Paleozoic (1000 to 545 Ma) for Pan-African.

Three major lithological units recognized within the Nigerian Precambrian Basement Complex include:

  1. 1.

    Migmatite-gneiss complexes, which is comprised of biotite and biotite hornblende gneisses, quartzites and quartz schist, and small lenses of calc-silicate rocks.

  2. 2.

    Slightly migmatized to unmigmatized paraschists and metaigneous rocks which consist of pelitic schists, quartzites, amphibolites, talcose rocks, metaconglomerates, marbles, and calc silicate rocks.

  3. 3.

    Older Granites which is comprised of rocks varying in compositions from granodiorites to granites and potassic syenite (Akoh et al. 2015).

The Pan-African (Older) granite suite includes porphyritic/porphyroblastic muscovite granites, aplites, granodiorites, diorites, quartz-hypersthene diorites, and charnockites (Obiora 2005). They are often weakly foliated and described as foliated granites and gneissic granites. Contacts of these Older granites with other rocks of the Precambrian Basement Complex are characteristically gradational suggesting a non-magmatic origin, possibly emplaced during the last of the reactivation events that affected the Basement Complex, i.e. during deformation and metamorphism of the supracrustals (Pan-African orogeny) (Rahaman 1976). The Pan-African granites are medium to coarse-grained, containing both muscovite and biotite, plagioclase (An6–An15), and microcline. Petrographically and geochemically, the Older Granites are calc-alkaline and peraluminous with high K2O contents and numerous K-feldspar porphyroblasts, which relates them to widespread basement rejuvenation during the Pan-African thermotectonic event (Oyawoye 1970).

Neoproterozoic to Early Paleozoic ages (638–510 Ma) from U–Pb, Rb–Sr, K–Ar dating methods on zircon have been reported for granitoids within the Nigerian Schist belt areas (Grant 1978; Caen-Vachette and Umeji 1983). On the basis of mineralogy and texture as well as field relationships, Jones and Hockey (1964) have recognized three phases of the Older Granite suite; an early phase comprising closely intermingled gabbroic rock, granodiorites and quartz diorites, and dolerites, the main phase, which comprises coarse porphyritic hornblende granite, coarse porphyritic biotite granite, syenite, and a late phase comprising homogenous granite, pegmatite, and aplite dykes. Because of field and geochemical evidence, however, Rahaman (1989), Dada and Respaut (1989), Dada et al. (1995), Ferre et al. (1998, 2002) have refuted the retention of dolerite as integral part of the early phase granites as reported by Jones and Hockey (1964) (Table 1). These authors are of the opinion that the unmetamorphosed doleritic rock, which cross-cuts the granitoids (granites, granodiorites, charnockites), is the youngest of the Nigerian Basement Complex rocks (Akoh et al. 2015).

Table 1 Details of samples of the granitic intrusions and their locations
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3 Field occurrence and petrography of the granitoids

The granitoids in this study occur mainly in the Ukpe–Bawop, Idum–Aragban, and Katchuan–Okundi axes where they intruded migmatitic gneisses (Fig. 2). The details of the samples of the granitic intrusions and their locations are shown in Table 1 and the estimated modal compositions of the granitic rocks in the study area (values in %) is presented in Table 2.

Fig. 2
figure 2

Geological map of the study area

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Table 2 Estimated modal compositions of the granitic rocks in the study area (values in %)
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3.1 Garnetiferous biotite granite (GBG)

The rock is leucocratic, coarse-grained, and porphyritic, with phenocrysts of garnet, set in a medium to coarse-grained, equigranular groundmass with the ratio of phenocryst to groundmass at about 1:4. The groundmass consists of interlocking crystals of biotite, feldspar, and quartz. Lots of cross-cutting quartzo-feldspathic veins characterize the outcrop. Garnet measuring about 5 mm occurs in somewhat rounded crystals riddled with inclusions of quartz, biotite, and plagioclase optically identified as Oligoclase (An12).

3.2 Porphyritic aplitic granite (PAG)

The rock is fairly leucocratic, medium-grained, and porphyritic, with a sugary texture. It consists of phenocrysts of feldspar and quartz in a groundmass of the same minerals as well as biotite and muscovite. It consists of plagioclase megacrysts embedded in an aplitic groundmass. The plagioclase megacrysts which constitute about 24% volume and measure about 8 mm are characterized by combined albite-Carlsbad twinning and contains inclusions of muscovite, biotite, and opaques (Fig. 3c). The groundmass is fine-grained, containing an equigranular mosaic of quartz, albite, biotite, and secondary muscovite resulting from the alteration of biotite. The quartz crystals, 2 to 3 mm in diameter, are mostly anhedral showing strong undulatory extinction.

Fig. 3
figure 3

Plot of the average modal compositions of the granitic rocks on the Quartz-Alkali Feldspar-Plagioclase (QAP) classification diagram (after Strekeisen 1976)

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The subhedral plagioclase grains are sericitized, and for this reason, the optical measurements of the anorthite content are uncertain. Accessory minerals in the rock include apatite, ilmenite, zircon, and magnetite. Biotite occurs as laths and shows pleochroic haloes resulting from radioactive damage caused by zircon.

3.3 Weakly foliated leucogranodiorite (WFL)

The rock is leucocratic and medium grained. The minerals present in the rock include quartz, plagioclase, and biotite. The rock is weakly foliated as shown by weak alignment of biotite in a preferred direction. The phenocrysts are made up of subhedral to anhedral crystals of oligoclase, set in a medium-grained groundmass of quartz, biotite, and oligoclase. Some of the phenocrysts have been sericitized. They measure about 5 mm and make up about 22% of the total rock volume. The foliation is shown by the subparallel arrangement of biotite, stretched quartz and oligoclase. The quartz crystals measure about 1 to 2 mm. Some of the crystals which exhibit undulose extinction occur as inclusions in the plagioclase phenocrysts. Plagioclase, which is oligoclase (An13), occurs in rectangular shaped, subhedral crystals. The biotite is brown in color and strongly pleochroic from light brown to dark brown and lath-shaped. The accessory minerals include primary hematite, magnetite, and ilmenite.

3.4 Porphyritic biotite muscovite granite (PBMG)

The rock is leucocratic, coarse-grained, and porphyritic. The phenocrysts consist of plagioclase and K-feldspars in a coarse-grained groundmass of quartz, K-feldspars, and biotite. Dimensions of the phenocrysts range from 2.4 to 5.0 cm by 1.0 to 2.4 cm. Some occur within the migmatitic banded biotite gneiss, the porphyroblastic garnet cordierite sillimanite gneiss, and the garnet mica schist while others just occur isolated as boulders. The phenocrysts consist of microcline and quartz, which are set in a coarse-grained groundmass of muscovite, biotite, quartz, and plagioclase (Fig. 3b). The accessory minerals include chlorite which replaces biotite at the rim and along cleavages, magnetite, and hematite replacing primary magnetite.

The phenocrysts of microcline range from 2 to 5 mm long and consist of either a single crystal or a mosaic of crystals. They frequently contain inclusions of plagioclase and minor amounts of the accessory minerals. Some of the plagioclase inclusions (An10) are euhedral, some are sericitized. Muscovite which measures about 4 mm is platy and subhedral and shows some degree of alteration.

3.5 Porphyritic biotite hornblende granite (PBHG)

The rock is leucocratic, coarse grained, and shows the very weak alignment of the mafic minerals (biotite and hornblende). The rock is distinctly porphyritic, with phenocrysts of well-formed, plagioclase and K-feldspars embedded in a groundmass of quartz, biotite, and hornblende. The rock mainly occurs as in situ boulders. Microcline phenocrysts are characterized by cross-hatched twinning and measure up to 8 mm. Hornblende occurs in subhedral elongate sections. It measures up to 6 mm (Fig. 3a).

3.6 Simple pegmatites (SP)

The rock is leucocratic and extremely coarse-grained. Its constituent minerals are mainly plagioclase (4 cm), biotite (3 cm) and quartz (3 cm). The pegmatite outcrop is restricted to the WNW part of the study area where it occurs in association with the porphyritic aplitic granite and the porphyroblastic garnet cordierite sillimanite gneiss host rock. Its total length is about 0.5 km with a width of about 20 m.

4 Sampling and analytical method

Seventeen (17) fresh representative samples consisting of 2 garnetiferous biotite granite (GBG), 4 porphyritic muscovite biotite granite (PMBG), 3 porphyritic hornblende biotite granite (PHBG), 3 weakly foliated leucogranodiorite (WFL), 4 porphyritic aplitic granite (PAG), and 1 simple pegmatite (SP) were selected for geochemical analysis. The samples were crushed in a jaw crusher at the Inorganic Geochemistry Research Laboratory of the Department of Geology, University of Nigeria, Nsukka. The crushed samples were pulverized in a Vibrating Disc Mill. Final size reduction, mixing, and homogenization to < 75 μm were done with a Mixer Mill. One hundred gram (100 g) of each sample were thereafter packaged and dispatched to Bureau Veritas Minerals Pty Ltd., Canning Vale, Perth, Western Australia for major element oxides and trace element geochemistry using an X-ray fluorescence (XRF) and inductively coupled mass spectrometry (ICP-MS), respectively.

The samples were fused with sodium peroxide and subsequently, the melt was dissolved in dilute hydrochloric acid for analysis. Boron was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES).

The samples were cast using a 66:34 flux with 4% lithium nitrate added to form a glass bead. The major elements were determined by X-ray fluorescence spectrometry (XRF) except for FeO which was determined volumetrically.

Trace elements were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Loss on ignition (LOI) result was determined using a robotic thermogravimetric analyzer (TGA) system. Furnaces in the system were set to 110 and 1000 °C.

5 Results

5.1 Major elements

The major element oxides data, with CIPW norms on the rocks, are presented in Table 3. Some important variables, including Fe-number (Fe*), Aluminium Saturation Index (ASI or A/CNK), and Modified Alkali-Lime Index (MALI or N + K-Ca) proposed by Frost et al. (2001) have been computed and included in Table 3. The granitic rocks are generally hypersthene and corundum-normative. They have SiO2 (60.86% to 77.18%), Al2O3 (13.6% to 17.5%), Fe* (1.14% to 12.49%) and alkali (Na2O + K2O) (4.54% to 9.95%) contents.

Table 3 Major element oxide (wt%) compositions of granitic rocks from Katchuan Irruan and adjoining areas
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5.2 Trace and rare-earth elements

Trace element compositions (ppm) of the granitic rocks, which are of petrogenetic significance, are presented in Table 4. The data is plotted on the Harker variation diagram (Fig. 14) and the rocks show calc-alkaline trends with Sr, Ba, La, Zr, and Nb decreasing and Rb increasing but almost scattered with increasing SiO2. Trace element data on the rocks from the study area have been normalized to chondrites after Thompson (1982) and plotted as a spidergram (Fig. 4). The rocks show an overall enrichment of the large ion lithophile elements (LILE: K, Th, Ba, and Rb). HFS elements usually occur in accessory minerals such as rutile and zircon. The samples are relatively enriched in Zr, Hf, and Nb (Table 4) indicating the presence of accessory minerals in the rocks.

Table 4 Trace and rare-earth elements compositions (ppm) of the granitic rocks from Katchuan Irruan and adjoining areas
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Fig. 4
figure 4

a Photomicrograph of PHBG from location IBE-11 showing basal section of hornblende (H), microcline (Mc) phenocrysts, myrmekite (Myr), sericite (Ser), biotite (B) and quartz (Q) in crossed and plane polars (Crossed nicols; X4). b Photomicrograph of PMBG from location IBE-10 showing quartz (Q) and microcline (Mc) phenocrysts in cross and plane polars. c Photomicrograph of (PAG) from location IBE-8 showing plagioclase (P) megacryst exhibiting combined Albite-Carlsbad twinning in crossed and plane polars (Crossed nicols; ×4)

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They also show strong positive anomalies in Th, K, La, Ce, Nd, Sm, and Tb and negative anomalies in Ba, Nb, Ta, Sr, Zr, and Ti. It also shows a high concentration of W and Co which resulted from the vibratory tungsten carbide (WC) disc mill used in preparing the samples but there were no Ca, Ta, and Sc contaminations as expected.

The rare-earth elements compositions (ppm) of the rocks are presented in Table 4.

Figure 5 shows the chondrite-normalized REE (Rare Earth Elements) patterns using the values of Sun and McDonough (1991).

Fig. 5
figure 5

Chondrite-normalized Spidergram for the granitic rocks (values from Thompson 1982)

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The granitic rocks exhibit similar REE patterns exhibiting LREE enrichment relative to MREE and HREE, with distinct negative Eu-anomalies (Eu/Eu* = 0.23–0.71), inclined MREE and flat HREE. The fractionated, LaN/YbN ratios in the granitic rocks range from 3.04 to 28.4 (Fig. 6).

Fig. 6
figure 6

Chondrite-normalized REE diagram for the granitic rocks (values from Sun and McDonough 1991)

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6 Discussion

6.1 Petrographic characteristics of the granite rocks

The granitoids in the Precambrian Basement Complex rocks in Katchuan Irruan and adjoining areas are garnetiferous biotite granite, porphyritic aplitic granite, porphyritic biotite muscovite granite, porphyritic biotite hornblende granite, and weakly foliated leucogranodiorite as well as simple pegmatites, representing the Older or Pan-African granite.

The granitic rocks plot mainly in the field of alkali granite on the total alkali (Na2O + K2O) versus silica, (SiO2) (TAS) diagram of Cox et al. (1979), adapted by Wilson (1989) for the chemical classification and nomenclature of the granitic rocks (Fig. 7). This is apparently due to their high contents of K2O. The graphical presentations of the variation of the major oxides with SiO2 in the granitic intrusions (Harker diagrams) are shown in Fig. 8. It is observed that Al2O3, CaO, TiO2, FeO(t), MgO, MnO, and P2O5 decrease with increasing SiO2. K2O displays a positive correlation with SiO2, whereas Na2O does not show any correlation. The scatter in the plots of Na2O may be due to contamination and local assimilative reaction leading to inhomogeneity within the granitic rocks (Khalaji 2007).

Fig. 7
figure 7

Classification of the granitic rocks using the total alkalis versus silica (TAS) diagram of Cox et al. (1979) adapted by Wilson (1989) for plutonic rocks. The curved red solid line subdivides alkaline and subalkaline rocks

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Fig. 8
figure 8

Harker variation diagram for the granitic intrusions in the study area

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The rocks plot mostly in the shoshonitic field of the K2O versus SiO2 diagram of Le Maître et al. (1989) for discrimination of subalkaline rocks, with a few of the samples plotting in the high to medium-K-calc-alkaline field (Fig. 9). The sporadic abundance of K-feldspar phenocrysts provide ample evidence for K-metasomatism.

Fig. 9
figure 9

Subdivision of subalkaline rocks using the K2O versus SiO2 diagram after (Le Maître et al. 1989)

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The Al2O3/(Na2O + K2O) versus Al2O3/(CaO + Na2O + K2O) binary diagram (Fig. 10) has been used to discriminate peraluminous, metaluminous, and peralkaline magma series (Shand 1943). This diagram was adapted, and it reveals that the granitic rocks in this study are peraluminous.

Fig. 10
figure 10

Binary plot of molecular ratio of alumina to alkalis [Al2O3/(Na2O + K2O)] versus alumina to lime and alkalis Al2O3/(CaO + Na2O + K2O)] classifying the granitic rocks in the study area as peraluminous (after Shand 1943)

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The data on the granitic rocks were also plotted in the modified alkali-lime index (MALI) (Na2O + K2O–CaO) versus silica (SiO2) and the Fe* index [FeOtot/(FeOtot + MgO)] versus silica (SiO2) (wt%) diagrams (Figs. 10, 11) of Frost et al. (2001). In the MALI–SiO2 diagram, the data points lie in the alkali-calcic to the calcic region, lying mainly in the I & S-type field while in the Fe* index versus SiO2 diagram, the data plot mostly as ferroan, with a few as magnesian. The pegmatite is classified as LCT according to Cerny and Ercit (2005). They are enriched in lithium, cesium, and tantalum. Most of the pegmatites with the LCT signature have a compositional affinity with S-type granites (Chappell and White 2001).

Fig. 11
figure 11

Plots of the granitic rocks on the modified Alkali-Lime Index (MALI) versus Silica (SiO2) diagram, with SIAM Classification (after Frost et al. 2001)

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Fig. 12
figure 12

FeOt/(FeOt + MgO) versus SiO2 (wt%) (after Frost et al. 2001)

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According to Chappell (1999), a metasedimentary source will usually produce magmas with low alkali contents and high alumina, exhibiting peraluminous character.

Most of the granitic rocks plot in the field of mafic pelites, but two samples of the weakly foliated leucogranodiorite plot in the field of the felsic pelites on the Patiño-Douce and Beard (1995) diagram for discrimination of the sources of granitic rocks, showing affinity to the low pressure (< 4 Kb) conditions (Fig. 13).

Fig. 13
figure 13

Discrimination diagram of Al2O3/(FeO + MgO + TiO2) versus Al2O3 + FeO + MgO + TiO2 of Patiño-Douce (1999) for the different sources of granitic rocks

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The trace element data are also plotted on conventional Harker-type variation diagram (Fig. 14), and the granitic rocks show typical calc-alkaline trends with Nb, Zr, La, Ba, and Sr generally decreasing and Rb increasing with increasing SiO2.

Fig. 14
figure 14

Plot of Zr, Rb, Sr, Nb, La and Ba against SiO2

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From trace and rare earth elements data, the depletion in Ti, Nb, Ta, and Sr and the enrichment in the LILE (Rb, K and Th) of the rocks suggest a crustal source. According to Harris et al. (1986) and Chappell and White (1992), high Rb, K, and Th and low P, Sr, and Ti values are compatible with typical crustal melts and suggest evolution from partial melting of crustal materials. Plots of MgO versus FeOt in a diagram (Fig. 15) after Zorpi et al. (1989) further suggest that partial melting was a major mechanism for the formation of the granitoids.

Fig. 15
figure 15

FeO versus MgO diagram (after Zorpi et al. 1989)

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The moderate to high fractionations with pronounced negative Eu anomalies shown in the REE pattern for the granitic rocks is typical behavior of crustal-generated granites and suggest either the fractionation of plagioclase or its retention of the source in the case of partial melting (Frost et al. 2001). The similarity of the REE patterns of the granitic rocks suggests that they are co-genetic. The light REE enrichment relative to the heavy REE enrichment in the rocks in this study is considered as an indicator of varying degrees of partial melting. The negative Eu anomaly shown by the granitic rocks suggests either the fractionation of plagioclase or its retention in the source in the case of partial melting. The REE patterns for the granites show moderate to high fractionation with pronounced negative Eu anomaly, which is typical behavior of crustally-generated granites. The rocks are characterized by widespread contamination and preferential incorporation of low-melting components including K2O through assimilative reaction with the K-rich metasediment rocks. This could be responsible for K2O enrichment in the rocks.

The peraluminous character of the granitic rocks in this study is consistent with the findings of other researchers of the Nigerian Basement Complex. Rahman et al. (1988) reported that granitic rocks around Oban Massif, southeastern Nigeria are peraluminous. The granitic gneisses of Aderan area are calc-alkaline and peraluminous granites emplaced during the Pan-African orogeny (Okonkwo and Folorunso 2013). They are different from granitic rocks in Igbeti (SW Nigeria) (Rahman et al. 1988), Ityowanye, and Katsina-Ala areas (SE Nigeria) (Obiora 2012) that are metaluminous.

The Older granites in northern and southwestern Nigeria Basement contain a significant amount of alkali and are peraluminous with corundum appearing in the norm (Odeyemi 1977; Olanrewaju and Rahaman 1982). Also, granitic gneiss around Jebba area (SW Nigeria) are peraluminous (Okonkwo and Ganev 2012). The strongly peraluminous nature of the rocks is very likely to be a reflection of derivation from partial melting of pelitic or semi-pelitic gneisses during the high-grade regional metamorphism that affected the area. The possibility of derivation of peraluminous melts from hydrous melting of pelitic or semi-pelitic rocks is supported by Frost et al. (2001), as well as Obiora and Ukaegbu (2008). The shoshonitic, alkali-calcic and peraluminous character of the granitic rocks make them similar to the Caledonian granitoids of Ireland and Britain (Frost et al. 2001), more commonly referred to as post-orogenic granitoids, shoshonitic granitoids, and K-rich calc-alkaline granitoids.

6.2 Tectonic setting of the granitic rocks

In the FeO*/MgO versus SiO2 diagram of Chappell and White (1974) (Fig. 16), the granitic rocks fall within the I and S type fields correlating positively with the Himalayan-type granite. S-type granites originate by melting or ultrametamorphism of pre-existing metasedimentary or sedimentary rocks (containing Al, Na, and K oxides and are said to be peraluminous) observable in deeply eroded cores of fold-thrust mountain belts formed as a result of continent–continent collisions, and are thus orogenic granites (Chappell and White 1974).

Fig. 16
figure 16

SIAM Classification (FeO*/MgO vs. SiO2) (wt%) diagram for the granitic rocks (after Chappell and White 1974)

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Numerous studies had recommended that trace elements be used to discriminate between the different tectonic settings of granitoid magmas (Pearce et al. 1984; Harris et al. 1986). Zhou et al. (2014) were of the opinion that they must be used with caution, as they could represent the formation of the protoliths, rather than those of the derived magma.

The granitic rocks in the study area plot within the post-collisional (Post-COLG) field on the Rb versus Y + Nb tectonic discrimination diagram after Pearce et al. (1984) (Fig. 17), which suggests that they could be formed in a compressional setting. The granitic rocks in this study are quite similar to those of the Pan-African (Older) granitic rocks rather than the Jurassic (younger) granites in terms of their geochemical characteristics. From the analysis for tectonic setting of the rocks using geochemical discrimination diagrams, the rocks are seen to be related to Pan African granites, otherwise known as the Older granites which were emplaced towards the end of the Pan African orogeny (600 Ma), during the collision of the West African craton and the Tuareg shield Obiora (2012). Table 4 shows a compilation of age data of the Pan African (Older) granites in Nigeria.

Fig. 17
figure 17

a Post orogenic character of the granitoids shown on the Rb versus Y + Nb of Pearce et al. (1984). Syn-COLG syn-collision granites, post-COLG post-collision granites, WPG within plate granites, VAG volcanic arc granites, ORG ocean ridge granites. b Plots of the rocks in the fields of Syn-collisional to Late/post-collisional granitoids on the Rb/30 Hf (Ta ×3) diagram of Harris et al. (1986)

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The overall geochemical features of the granitic rocks indicate that they were most likely derived from partial melting of crustal materials in an orogenic (post-collisional) tectonic setting. They are therefore related to the Pan-African granites, otherwise known as the Older Granites which were emplaced during the Pan African orogenic event.