• Based on modal composition, the granitoids within the high grade migmatite gneiss - quartzite complex were identified as quartz syenite, porphyritic granodiorite, tonalite-trondjhemite-granodiorite, and charnockite.

• Major oxide geochemical data revealed high-K to shonshonitic calc-alkaline, metaluminous, magnesian, and alkali to alkali-calcic magmatic character.

• Higher concentrations of Ba, La, Ce, and La/Yb_(N) and values of Th, and U lower than the average upper crustal values suggest partial melting of andesitic to dioritic crustal rocks as the dominant magmatic differentiation process.

The Precambrian basement complex of Nigeria is characterised by diverse intrusive rocks with distinct mineralogical and geochemical features (Onyeagocha, 1983; Rahaman, 1988; Rahman et al., 1988; Dada, 2008; Oyinloye, 2011; Ogunyele et al., 2020; Okunola et al., 2023). Among these, granitoids are common occurrences that have been extensively studied for their geochemical characters and petrogenetic significance. Granitoids are coarse-grain felsic rocks with mineralogical compositions ranging from tonalitic to syenitic (Winter, 2010). These intrusives are derived from a wide range of sources, mantle evolution processes, and tectonic settings (Bonin et al., 2019), leading to variations in their chemistry, mineralogy, and environments of emplacement.

Despite various classification schemes proposed to identify different granite types (White and Chappell, 1988; Barbarin, 1990; Le Bas and Streckeisen, 1991; Pitcher, 1993; Le Maitre, 2002), no universally adopted system exists due to the subtle variations in their modal and chemical compositions. The iron-magnesium (Fe-Mg) index has demonstrated that these granitoid intrusives can display either ferroan or magnesian character. Combining the Fe- Mg ratio index with the Modified Alkali Lime Index (MALI) yielded additional geochemical features. Frost and Frost (2011) described eight kinds of ferroan granitoids: alkalic, alkali-calcic, calc-alkalic, and rare calcic (Bonin et al., 2019; Chappell and White, 2001; Frost et al., 2001; , 2013; García-Arias, 2020).

Frost and Frost (2013) demonstrated that granitoids with ferroan character, which can range in mineralogical compositions from granitic to quartz syenitic compositions, are widely distributed throughout the Proterozoic eon. El Bahariya (2021) described Ferroan A-type granites and a few magnesian granitoids from Egypt, while Frost and Frost (2011) reported the Younger granite from central Nigeria as A-type granite with ferroan character. Several studies on the granitoids of Nigeria attempting to classify the Older granites (Oyawoye, 1967; Rahaman et al., 1983; Bowden and Kinnaird, 1984; Rahaman et al., 1991; Ugbe et al., 2016) and ferroan granite (Ogunyele et al., 2020) did not use the Fe-Mg index. Only recently were some Pan-African Older granitoid occurrences described using the Fe-Mg index (Igonor and Abimbola, 2016; Ogunyele et al., 2020; Okunola et al., 2023). However, magnesian granitoids within Proterozoic terrain in Nigeria have not been adequately studied, and their geochemical characteristics remain poorly understood.

The observed diversity of granitoid suites is due to a number of geologic processes such as crustal anatexis (Sawyer et al., 2011; Martini et al., 2019; Singh et al., 2022), fractional crystallization, mantle-crust mixing (Jung et al., 1999), and disequilibrium melting (e.g., Barbero et al., 1995). These processes are capable of producing granitic melts, which have different elemental contents and hence define different positions in most elemental variation diagrams. These processes are commonly assisted by tectonic forces that drive the movement of melt out of the lower continental crust, giving rise to an irreversible chemical differentiation of the crust (Debon and Le Fort, 1988; Frost et al., 2001; Martini et al., 2019).

This study aims to understand the geochemical characteristics, source nature, and tectonic setting of the granitoid suite in the high-grade terrain underlying the north of Iwo area. By examining the modal composition and geochemical properties, the granitoids within the high grade Precambrian metamorphic terrain of southwest Nigeria are further characterised and constrained.

2.1. Basement Complex of Southwest Nigeria

The basement complex rocks of southwest Nigeria, within the Benin-Nigeria shield, are products of the collision between the eastern end of the West African craton (WAC) and the Congo craton (CC) (Ajibade et al., 1987; Black et al., 1979; Oyinloye, 2011; Tijani, 2023) (Fig. 1a). The collision between these cratons took place around 1.1 Ga to form part of the Rhodinia mega-continent. This complex is composed of high-grade migmatitic gneisses of the amphibolite to granulite facies, which are the oldest rock units. The gneisses are overlain by supracrustal rocks of predominant greenschist to lower amphibolite facies, forming linear Schist belts (Ibadan- Iseyin-Oyan, Okemesi-Ifewara, Igarra, and Egbe-Isanlu). The Neo-Proterozoic to Paleozoic time saw the break-up of the Rodinia supercontinent and the collision of the several crustal fragments such as the Benin-Nigerian shield and the Trans-Saharan belt. Multiple phases of deformation by several orogenies (Liberian, Eburnean and Pan-African orogenies) have imprinted folded and refolded structural styles in the Archean-Proterozoic migmatite gneiss from which the TTG were derived (Rahaman, 1988; Ajibade and Wright, 1989). Granitoid intrusions have overprinted these structural styles.

Figure 1. a) A part of the regional geology of West Africa where Nigeria is set and underlain dominantly by Mesozoic and younger, and Neo-Proterozoic basement (modified from Ajibade and Wright, 1989). b) Geological setting of southwest Nigeria showing lithological distribution (adapted after Oluwatoyin et al., 2021).

Granitoid intrusives, of Pan-African age, are of various compositions and appear as syn- to late-orogenic phases. Field study showed that they occur closely associated with the high-grade migmatite gneisses. These granitic rocks are predominantly calc-alkaline and likely suggestive of a volcanic arc and syn-collisional tectonic setting (Okonkwo and Folorunso, 2012). These plutons are syn-kinematic granitic plutons ranging from monzodiorite to charnockite and occur closely with granodioritic components of the gneisses derived mainly from anatexis (Dada and Respaut, 1989; Dada et al., 1989; 1993).

Members of the Older granite suite within the southwestern Precambrian basement complex (Fig. 1b) include granodioritic components within migmatite gneiss terrain, coarse-grained porphyritic granite, fine-grained biotite-hornblende granite, charnockite, syenite, and diorite (Fig. 1b). Charnockites and staurolite-bearing gneisses show evidence for hightemperature metamorphism during late-phase orogenesis (Rahaman, 1988; Olarewaju, 1988). Charnockites have been shown to outcrop in close relationship with other rocks such as granites, diorites, norites, and pyroxenites (Oyawoye, 1961; Rahaman, 1988).

Geochronological data indicated that the reworking of gneisses occurred during the Archean-Eburnean era (around 3040 +/- 60 Ma, Bruguier et al., 1994), while the schistose rocks formed during the Pan-African era (around 681 +/ - 36 Ma, Ajibade, 1980). The ages of the granitic intrusives range from 610 +/- 10 Ma to 586 +/- 5 Ma (Tubosun et al., 1984). Multiple episodes of emplacement of intrusive plutonic phases resulted in the migmatization and partial melting of pre-existing rocks (Odeyemi, 1988) and created complexity in the evolution of the Nigerian basement terrane. Lithostratigraphic and structural relationships of rocks have been used to understand the metamorphism, petrogenetic, and geodynamic setting of rocks within the basement complex of Nigeria (Odeyemi, 1988; Ogunyele et al., 2018).

The Pan-African orogeny, the last of the deformation episodes that shaped Africa, formed the Pan-African mobile belt with imprints that almost obliterated evidences from earlier orogenies (Wright et al., 1985; Rahaman et al., 1991). The belt is separated from the West African craton by a zone of Late Proterozoic to Lower Paleozoic sediments in the Volta basin and highly faulted rocks in the Togo belt. The Pan-African mobile belt is known for the basin and swell structure within the Togo-Benin- Nigeria shield and the Mesozoic-Tertiary sedimentary basin forming as a result of the failed arm of rifting of the pre-Mesozoic continent (Fitton, 1980; Wright et al., 1985; Obaje, 2009; Tijani, 2023).

2.2. Geology of the Iwo North Area

Migmatite gneiss with tonalite-trondhjemite-granodiorite (TTG) components, quartzite/quartz schist, charnockite, quartz syenite, and granodiorite with porphyritic texture are rocks that underlie the Ajawa-Pontela-Ikonnifin area, which is north of Iwo town (Fig. 2). About 40% of the total land cover is made up of the migmatite gneiss-quartzite complex. The migmatite gneiss is mainly tonalite-Trondhjemitegranodiorite (TTG) (Bruguier et al., 1994; Afolabi et al., 2019). Exposures are grayish and show evidence of anatexis (Fig. 3). The felsic and mafic bands are mostly aligned and curvy from intense anatexis. The felsic components are granitic and consist of quartz and alkali feldspar with fewer amounts of biotite, while the mafic components are richer in biotite and possibly hornblende. This mineralogical composition describes an upper amphibolite facies for the migmatite gneiss complex. Quartzitic bodies with quartz and muscovite dominantly show evidence for shearing with a north-trending shear sense. The gneisses and the quartzite form part of the migmatite gneiss-quartzite complex described by Rahaman (1988). Mica and quartz are the dominant minerals in the quartzite/quartz schist.

Figure 2. Geological map of the study area describing the main mappable units.

Figure 3. Field exposures of the granitoids in the area. a) migmatite gneiss, b) quartz syenite, c) charnockite d) porphyritic granodiorite, e) tonalite-trondjhemite-granodiorite (TTG) within the migmatite gneiss, and.f) hilly charnockite.

The late-phase intrusives observed within the migmatite gneiss of the study area include granitic components of the migmatite described as TTG, charnockite, quartz syenite, and porphyritic granodiorite. With the exclusion of the TTG, which were mostly encountered as low-lying units, these intrusives outcropped as localised inselbergs. The common mineral phases found in these intrusives, which are typical of granitoids, were quartz, biotite, and feldspar. Minerals observed from these intrusives are mostly medium size, except for the feldspar phenocrysts in porphyritic granodiorite, which are medium to coarse. Hand specimen samples of the quartz-syenite revealed feldspar as the dominant mineral phase, with variable amounts of quartz and biotite. The charnockite exposures were uniformly greyish, and the identified minerals include quartz, feldspar, biotite, and pyroxene. The granitic TTG were observed as bands of varying widths within the migmatite, from widths of about 3 cm to large areas covering several tens of square meters. They outcrop in some areas in close contact with the other intrusives. Pegmatite veins were observed to trend mostly in the north-south direction and cut through these outcrops. Depending on the width, the minerals in these veins are mostly medium- to coarse-grain quartz and feldspar.

Strong foliation was observed in the migmatite gneiss, with planar folds showing evidence of shearing. The foliation is weak in the porphyritic granodiorite. Weaker foliations were observed around the contact of the quartz syenite and porphyritic granodiorite exposures with the migmatite gneiss. Foliation, shearing, faulting, and folding were the observed structures in the area. A N-S trending foliation and the northerly axis of fold conformed to the Pan-African structural style. Field evidence showed that the area is faulted and sheared. Adetunji et al. (2018) gave 642 ± 6 Ma as the age of deformation in the Iwo area.

Collision events with attendant faulting and folding cum shearing, that characterised the Precambrian Eon, witnessed the emplacement of the basal migmatite gneiss - quartzite complex during the Archean to Early Proterozoic (2700– 2000 Ma), while the N-S infolded schistose rocks (composed of low-metamorphic grade, highly deformed, metasedimentary, and meta-volcanic rocks) formed within these gneisses as narrow shear zones during the Early Proterozoic to Neo- Proterozoic (1800–550 Ma). Neo-Proterozoic magmatism (780–770 Ma) related to the active subduction of the leading edge of the WAC saw the intrusion of Pan-African syn- to post-collisional plutons, which are known as the Older Granites (Goodenough et al., 2014 and all the references therein).

Field mapping was carried out to gather evidence for the lithostructural association and mode of emplacement of the rocks in Iwo Sheet 240, a topographical basemap on the scale of 1:50000. A total of thirty seven (37) fresh and representative rock samples were collected in the following order: five (5) samples from the porphyritic granodiorite, seven (7) samples from quartz syenite, twelve (12) representative samples of the granitic Tonalite-Trondhjemite- Granodiorite (TTG) within the migmatite gneiss, herein after is referred to as TTG samples, and thirteen (13) from the charnockite intrusive. Each rock type was examined for mineralogical identification both from hand specimen samples and thin sections. Five samples each were selected for thin section analysis. A total of twenty (20) slides of 0.03 mm thickness were prepared at the Obafemi Awolowo University, Ile-Ife, and viewed under the petrographical microscope at Ladoke Akintola University of Technology, Ogbomoso.

For the lithochemical analysis, samples were selected in this order: two (2) from the porphyritic granodiorite, three (3) samples from quartz syenite, four (4) from the TTG, and five (5) from the charnockite. These fourteen (14) samples were sent to the Bureau Veritas Laboratory in Canada, where samples were crushed and pulverised to <63 μm. Fifteen (15) grams of each sample were digested separately in the hot acids of HF, HCl, HNO₃, and HClO₄ in the ratio of 1:1:1:1 to achieve total digestion. The digested samples were fluxed with lithium borate (Li₂B₄O₇) and thoroughly mixed together to achieve a homogeneous portion. Each sample mixture was placed in a platinum crucible and placed in a furnace to cause fusion, where it was heated to temperatures of about 900oC. A sufficient amount of time was allowed for fusion, after which the sample in the crucible was brought out of the furnace and poured quickly into a dish to be cast as a glass disc. The glass disc is used to analyse for elemental concentrations using the Inductively Coupled Plasma–Mass Spectrometer (ICP-MS). Major elements are recorded as weight percentages (wt. %) while trace elements are recorded as parts per million (ppm).

4.1. Petrography of the Granitoid Intrusives

The mineralogy compositions of the rocks were observed from both hand specimen samples and their respective thin sections. Modal proportions of minerals identified in thin sections are presented in Table 1.

Table 1 . Modal composition (%) of the sampled granitoid intrusives.

Mineral in modal percentage	Quartz syenite	Porphyritic Granodiorite	TTG	Charnockite
Quartz	22	32	30	25
Plagioclase	4	21	27	26
Alkali Feldspar	22	15	26	12
Biotite	21	13	12	18
Muscovite	-	9	2	-
Pyroxene	8	-	-	14
Hornblende	15	7	2	-
Garnet	8	-	-	3
Zircon	-	3	0.5
Opaque	-	-	0.5	2

4.1.1. Quartz syenite

The massive greyish quartz syenite displayed an abundance of alkali feldspar grains (Mc, 22%), small books of biotite, and prismatic hornblende in hand specimen samples. Plagioclase crystals are much fewer (4%). Thin section analysis revealed subhedral grains of greenish hornblende (Hbl, 15%) occurring together with smaller elongate crystals of biotite (21%) displaying shades of brown to brownish yellow (Fig. 4a). Garnet (8%) grains were seen as either inclusion grains in some of the hornblende grains or as aggregates (Fig. 4b). Pyroxene crystals (8%) occur closely with the amphibole crystals.

Figure 4. Mineralogy of the quartz syenite shows abundance in hornblende (Hbl), microcline (Mc), and biotite (Bt) with garnet (Grt) and quartz (Qtz) occurring in accessory amounts. Magnification (x40).

4.1.2. Porphyritic Granodiorite

Grains observed from hand specimen in the porphyritic granodiorite are medium to coarse. Large grains of feldspar with the finer groundmass display porphyritic texture. Mineral composition observed from hand specimen and thin section slides revealed quartz crystals (32%), alkali-feldspar (microcline, 15%), plagioclase (21%), biotite (13%) and muscovite (9%) as the dominant phases. In some cases, the quartz grains appeared as vermicular quartz, forming blebs with feldspars and presenting as wormlike myrmekites (myr) (Fig. 5a). Some of the quartz crystals showed evidence for deformation. Figure 5b showed some of the quartz crystals displaying undulose extinction and recrystallised polygonisation of quartz subgrains suggestive of intense pressure. Also suggesting intense pressure is the annealed micro-fracture observed in a muscovite grain. Diagnostic twinning of cross hatching and albite twinning were used to identify microcline and plagioclase grains, respectively. Coarse grains of hornblende (7%) were observed in small amounts. Zircon grains occurred in accessory amounts (3%). The rock showed weak foliation in the weak alignment of feldspar phenocrysts.

Figure 5. Thin section mineralogy of the granodiorite under cross nicol, revealed polygonised quartz (p Qtz) suggesting recrystallisation, and myrmekite (Myr). Other minerals include biotite (Bt), microcline (Mc). Magnification (x40).

4.1.3. Tonalite-Trondhjemite-Granodiorite (TTG)

The TTG components within the migmatite appeared mostly as felsic units, while in some areas, where alkali feldspar (microcline) is in abundance, exposures appeared pinkish. Quartz (30%), biotite (12%), and microcline (26%) were the major mineral phases observed in the hand specimen samples. Other mineral phases revealed from the thin section study include plagioclase (27%), muscovite (2%), and hornblende (2%) with zircon and opaque minerals occurring in accessory amounts (Fig. 6).

Figure 6. Mineralogical composition of the TTG displaying quartz (Qtz), biotite (Bt), plagioclase (Plag), muscovite (Mus), and microcline (Mc) shows complete crystallisation of the feldspars suggesting a subsolvus condition. Accessory amounts of opaque minerals (Opq) were identified.

4.1.4. Charnockite

Plagioclase feldspar (Plag, 26%), pyroxene (Py, 14%), and biotite (Bt, 18%) crystals were observed as the abundant mineral phases (Fig. 7). Others include quartz (Qtz, 25%) and microcline (12%), while opaque minerals (2%) and garnet crystals (3%) were in accessory amounts. The grains are medium-sized and equant, displaying a granular texture.

Figure 7. Prevalence of plagioclase feldspar (plag) grains occurring closely with pyroxene (py) is revealed in the charnockite mineralogy. Garnet (Grt) microlites occurring around biotite flakes in 7b suggest high temperature granulite conditions. Other minerals observed include microcline (Mc) and quartz (qtz).

4.2. Geochemical Characters of the Iwo North Granitoid Intrusives

We present the major oxide (wt. %) concentrations for the four (4) petrologically distinct intrusive rocks in Table 2 and their averages in Table 3. The trace element concentrations, in parts per million (ppm), and some of their ratios are presented in Table 4.

Table 2 . Major element (wt. %) concentrations and normative mineralogy for the granitoid intrusives of the study.

	Granodiorite		Quartz syenite			Tonalite-Trondhjemite-Granodiorite				Charnockite
Sample ID/Elements	PorG-1	PorG-2	Sy-1	Sy-2	Sy-3	TTG-1	TTG-2	TTG-3	TTG-4	Ch-1	Ch-2	Ch-3	Ch-4	Ch-5
SiO2	58.46	59.14	58.16	58.46	61.23	60.70	63.29	52.90	59.38	53.63	53.54	52.86	53.61	53.51
Al2O3	16.71	16.47	12.88	12.72	14.24	14.05	14.36	17.59	16.22	17.91	17.99	18.35	17.85	18.20
Fe2O3	6.20	5.94	5.50	5.48	4.46	4.93	3.30	8.43	5.89	7.63	7.69	7.77	7.71	7.64
MgO	3.42	3.42	5.70	5.65	2.99	3.04	2.33	4.61	3.37	4.22	4.24	4.31	4.33	4.20
CaO	4.64	4.62	4.81	4.64	2.68	2.72	2.61	7.01	4.57	6.86	6.93	6.86	6.99	6.90
Na2O	3.69	3.64	2.24	2.25	3.33	3.31	3.12	3.95	3.62	4.01	3.95	4.08	3.97	4.01
K2O	4.18	4.07	7.29	7.34	7.49	7.49	8.40	2.38	4.17	2.54	2.50	2.65	2.53	2.56
TiO2	1.03	1.02	1.00	1.01	1.30	1.33	0.61	1.48	1.02	1.46	1.39	1.36	1.39	1.39
P2O5	0.42	0.41	0.78	0.74	0.50	0.53	0.46	0.61	0.40	0.58	0.57	0.57	0.57	0.57
MnO	0.09	0.09	0.09	0.09	0.08	0.08	0.05	0.13	0.09	0.12	0.12	0.12	0.12	0.12
Cr2O3	0.019	0.017	0.032	0.031	0.014	0.014	0.010	0.011	0.016	0.010	0.010	0.012	0.011	0.009
LOI	0.7	0.7	0.9	1.0	1.1	1.2	0.8	0.4	0.8	0.5	0.6	0.5	0.4	0.4
Sum	99.56	99.54	99.38	99.41	99.41	99.39	99.34	99.50	99.55	99.47	99.53	99.44	99.48	99.51
Norm	PorG-1	PorG-2	Sy-1	Sy-2	Sy-3	TTG-1	TTG-2	TTG-3	TTG-4	Ch-1	Ch-2	Ch-3	Ch-4	Ch-5
q	7.76	9.18	3.72	4.12	5.98	5.59	7.07	2.11	9.47	2.52	2.71	0.65	2.47	2.16
or	24.70	24.05	43.08	43.37	44.26	44.26	49.64	14.06	24.64	15.01	14.77	15.66	14.95	15.13
ab	31.22	30.08	18.95	19.04	28.18	28.01	26.40	33.42	30.63	33.93	33.42	34.52	33.59	33.93
an	16.69	16.58	3.56	2.93	1.79	1.36	0.37	23.24	15.69	23.37	23.97	23.93	23.41	24.10
di	0.76	0.85	9.98	9.96	3.83	4.08	6.74	2.84	1.37	2.37	2.40	2.27	3.07	2.20
hem	6.20	5.94	5.50	5.48	4.46	4.93	3.30	8.43	5.89	7.63	7.69	7.77	7.71	7.64

Table 3 . Summary of major oxide concentrations (wt. %) for the intrusives of this study and averages from ferroan granitoids from Igarra (Igarra* - Ogunleye et al., 2020).

This study							Igarra*
Elements	Av-PorG (2)	Av-Sy (3)	Av-TTG (4)	Av-Ch (5)	Min	Max	Por Gr (6)	Ch (6)
SiO2	58.8	59.28	59.07	53.43	52.86	63.29	73.14	68.02
Al2O3	16.59	13.28	15.56	18.06	12.72	18.35	13.15	14.03
Fe2O3	6.07	5.15	5.64	7.69	3.3	8.43	3.44	6.11
MgO	3.42	4.78	3.34	4.26	2.33	5.7	1.5	1.7
CaO	4.63	4.04	4.23	6.91	2.61	7.01	1.46	3.44
Na2O	3.67	2.61	3.5	4	2.24	4.08	2.64	2.2
K2O	4.13	7.37	5.61	2.56	2.38	8.4	3.64	2.34
TiO2	1.03	1.1	1.11	1.4	0.61	1.48	0.41	0.66
P2O5	0.42	0.67	0.5	0.57	0.4	0.78	0.03	0.02
MnO	0.09	0.09	0.09	0.12	0.05	0.13	0.14	0.16
Cr2O3	0.02	0.03	0.01	0.01	0.01	0.03
LOI	0.7	1	0.8	0.48	0.4	1.2	0.63	1.4
Sum	99.55	99.4	99.45	99.49	99.34	99.56

Table 4 . Trace elemental geochemical data (ppm) and ratios for the granitoid intrusives of the study.

	Granodiorite		Quartz syenite			Tonalite-Trondjhemite-Granodiorite				Charnockite
Sample ID/Elements	PorG-1	PorG-2	Sy-1	Sy-2	Sy-3	TTG-1	TTG-2	TTG-3	TTG-4	Ch-1	Ch-2	Ch-3	Ch-4	Ch-5
Ni	57	63	135	140	81	87	60	50	59	49	45	55	47	51
Co	44.4	45.9	27.8	25.9	20.8	22.2	19.8	29.6	42.3	31.8	30.8	30.8	30.6	31
Zr	366.3	392.2	531.2	515	328	209.2	181.2	150.5	357.9	414.8	145.7	458.9	167.1	146.7
Hf	9.4	10	12.5	12.4	8.1	5.9	4.7	3.5	8.9	9.1	3.5	9.9	4	3.5
Nb	13.2	13.3	22	21.3	34.8	36.5	17.2	10.2	12.4	10.5	10	9.6	9.3	9.6
Rb	98.9	96.1	291.5	297.6	285.5	285.8	346.3	46.6	100.9	47.6	46.4	47.1	46.2	46.1
W	115.8	119	78.5	77	66.2	65.4	135.3	79.7	113.8	111.4	112.9	101.7	111	103.8
Sn	2	2	7	6	9	9	6	1	2	1	1	1	1	1
Sr	687.6	691.8	726	691.3	816.8	825.7	1101	962.6	655.2	1026.6	1019.9	1009	971.4	1003.9
Ta	0.8	0.8	1.2	1.4	2.3	2.4	1	0.5	0.7	0.6	0.6	0.5	0.5	0.4
Th	6.6	6.1	28.4	19.6	25.8	25.6	28.3	1.4	5.6	1.6	1.6	1.5	1.4	1.5
U	1.1	1.2	4.1	3.9	5.1	4.8	11.1	0.2	0.9	0.3	0.3	0.3	0.2	0.3
Cu	16.1	16.8	26.5	24.6	24.3	24.8	30.6	29.8	16.9	22.3	20.2	21.3	22	22.2
Y	21.4	21.9	27.8	26.4	12.8	14.3	24	21.1	20.3	22.2	21.5	21.7	21.3	21
La	47.6	48.4	160.5	106.6	64.5	66.3	126	40.8	45.7	41.2	40.6	40.5	40.4	40.2
Ce	92.1	94.1	313.7	229.7	198.7	207.2	253.3	83	87.7	82.8	79.7	80.3	78.8	79.4
Pr	11.51	11.78	40.72	32.51	18.07	19.62	34.26	11.14	11.02	11.01	10.66	10.53	10.61	10.54
Nd	42.4	44.2	154.1	128.4	71.2	77.4	130.9	44.1	41.3	42.8	43.3	41.8	42.3	42.5
Sm	6.65	7.09	24.16	21.71	11.9	13.09	20.96	7.29	6.29	7.24	6.99	6.99	7.33	7.15
Eu	1.83	1.88	4.33	4.04	2.18	2.43	4.06	2.25	1.72	2.35	2.32	2.3	2.26	2.3
Gd	5.64	5.68	13.87	12.8	7.05	7.63	12.06	6.14	5.12	6.06	6.11	6	5.78	5.86
Tb	0.71	0.75	1.42	1.32	0.71	0.76	1.23	0.81	0.7	0.81	0.78	0.78	0.78	0.77
Dy	4.26	4.19	5.95	5.46	3.07	3.29	5.14	4.36	3.83	4.36	4.23	4.42	4.11	4.23
Ho	0.76	0.79	0.91	0.82	0.45	0.47	0.75	0.83	0.71	0.83	0.82	0.81	0.8	0.79
Er	2.15	2.24	2.18	2.02	1.14	1.11	1.72	2.24	1.97	2.3	2.14	2.16	2.07	2.15
Tm	0.3	0.32	0.31	0.3	0.16	0.17	0.25	0.3	0.29	0.29	0.28	0.3	0.28	0.28
Yb	1.94	2.1	1.91	1.84	1.07	1.15	1.57	1.87	1.88	1.91	1.77	1.79	1.73	1.7
Lu	0.29	0.32	0.27	0.26	0.14	0.16	0.22	0.27	0.28	0.29	0.25	0.27	0.25	0.25
ASI	0.88	0.88	0.63	0.63	0.77	0.76	0.76	0.81	0.86	0.82	0.83	0.83	0.81	0.83
Fe/Mg ratio	1.81	1.74	0.96	0.97	1.49	1.62	1.42	1.83	1.75	1.81	1.81	1.8	1.78	1.82
D.I	63.68	64.03	65.75	66.53	78.42	77.86	83.11	49.59	64.74	51.46	50.9	50.83	51.01	51.22
Total Alkali	7.87	7.71	9.53	9.59	10.82	10.8	11.52	6.33	7.79	6.55	6.45	6.73	6.5	6.57
Zr/Hf	38.97	39.22	42.5	41.53	40.49	35.46	38.55	43	40.21	45.58	41.63	46.35	41.78	41.91
Nb/Ta	16.5	16.63	18.33	15.21	15.13	15.21	17.2	20.4	17.71	17.5	16.67	19.2	18.6	24
Rb/Sr	0.14	0.14	0.4	0.43	0.35	0.35	0.31	0.05	0.15	0.05	0.05	0.05	0.05	0.05
Rb/Zr	0.3	0.2	0.5	0.6	0.9	1.4	1.9	0.3	0.3	0.1	0.3	0.1	0.3	0.3
Ba/Sr	2.8	2.83	3.25	3.24	3.75	3.68	2.94	2.09	2.81	2.13	2.09	2.25	2.25	2.22
K/Rb	350.88	351.6	207.62	204.76	217.8	217.57	201.38	424.01	343.11	443.01	447.31	467.1	454.63	461.02
Eu/Eu*	0.89	0.88	0.66	0.68	0.67	0.68	0.72	1	0.9	1.06	1.06	1.06	1.03	1.06
La/Yb(N)	14.87	13.97	50.93	35.11	36.53	34.94	48.64	13.22	14.73	13.07	13.9	13.71	14.15	14.33
La/Sm(N)	3.93	3.74	3.64	2.69	2.97	2.78	3.3	3.07	3.99	3.12	3.19	3.18	3.02	3.08
ΣLREE	207.7	213.1	711.4	535.8	373.6	393.7	581.5	194.7	198.9	193.5	189.7	188.4	187.5	188
ΣHREE	10.41	10.71	12.95	12.02	6.74	7.11	10.88	10.68	9.66	10.79	10.27	10.53	10.02	10.17
ΣLREE/ΣHREE	19.95	19.9	54.93	44.57	55.43	55.37	53.45	18.23	20.58	17.93	18.47	17.89	18.71	18.48
REEtot	218.14	223.84	724.33	547.78	380.34	400.78	592.42	205.4	208.51	204.25	199.95	198.95	197.5	198.12

4.2.1. Quartz syenite

In the quartz syenite, silica concentrations ranged from 58.16 wt. % to 61.23 wt. %, with an average of 59.28 wt. %. The alumina (Al₂O₃) concentrations averaged 13.28 wt. % from values ranging from 12.72 wt. % to 14.24 wt. %, while the average concentration of ferric oxide (Fe₂O₃) was 5.15 wt. % from values between 4.46 wt. % and 5.50 wt. %. The average values for lime (CaO) and magnesia (MgO) were 4.04 wt.% and 4.78 wt.%, respectively, with lime ranging from 2.68 wt.% to 4.81 wt.% and magnesia from 2.99 wt.% to 5.70 wt.%. Soda values range from 2.24 wt. % to 3.33 wt. %, while potash values range from 7.29 wt. % to 7.49 wt. %, with K₂O values remaining fairly consistent across all the quartz syenite samples. K₂O/Na₂O values were observed from 2.25 to 3.25, while the total alkali content was from 9.53 to 10.82 wt. %.

Barium (Ba) concentrations are high and range from 2240.00 ppm to 3065.00 ppm, while values of rubidium (Rb) and strontium (Sr) ranged from 285.50 ppm to 297.60 ppm and 691.30 ppm to 816.80 ppm, respectively. Average values of Ba, Rb, and Sr are 2556.00 ppm, 291.53 ppm, and 744.70 ppm. Zirconium (Zr) concentrations ranged from 328.00 to 531.20 ppm with an average value of 458.07 ppm, while the average value of Hf is 11.00 ppm from values ranging from 8.10 to 12.5 ppm. The calculated average values of Ta, Nb, and Y are 1.63 ppm, 26.03 ppm, and 22.33 ppm, respectively. Thorium (Th) concentration ranged from 19.60 to 28.40 ppm and averaged 24.60 ppm. Elemental ratios of large ion lithophiles and high field strength were calculated. Ba/Rb and Rb/Sr values were 8.79 and 0.39, while Na/Ta and Zr/Hf were 16.23 and 41.51. Total light rare earth elements (LREE) values for the quartz syenite ranged from 373.60 to 711.40 ppm, total heavy rare earth elements (HREE) values ranged from 6.74 to 12.95 ppm, and the total rare earth elements (REE_tot) values ranged from 380.34 to 724.33 ppm. The Eu/Eu* (av. 0.67) and K/Rb (av. 210.06) values for the quartz syenite were the lowest.

4.2.2. Porphyritic Granodiorite

The porphyritic granodiorite samples contain SiO₂ values of 58.46 wt. % and 59.14 wt. %. The alumina content (16.47 wt. % and 16.71 wt. %), Fe₂O₃ (5.94 wt. % and 6.20 wt. %), and CaO values (4.62 wt. % and 4.64 wt. %) were similar to those observed in the quartz syenite. The average MgO content (3.42 wt. %) was lower when compared to that observed for the quartz syenite (4.78 wt. %). The total alkali content (av. 7.79 wt. %) within the porphyritic granodiorite was lower than that found in the quartz syenite (av. 9.98 wt. %). P₂O₅ concentrations (0.41 wt. % and 0.42 wt. %) were lower, while TiO₂ values (1.02 wt. % and 1.03 wt. %) were similar to those observed in the quartz syenite.

Barium (Ba) concentrations for the porphyritic granodiorite were 1924.00 ppm and 1955.00 ppm, with an average value of 1939.50 ppm. The average Rb (97.50 ppm) value was observed to be lower than that obtained for the syenite (291.53 ppm) from concentrations of 98.9 ppm and 96.1 ppm. The average Sr (689.70 ppm) value was observed to be only slightly lower in comparison to that obtained for the quartz syenite (744.70 ppm). Niobium (13.20 ppm and 13.30 ppm) and tantalum (0.80 ppm) concentrations were not significantly varied. Ba/Rb values were 19.45 and 20.34, while the Rb/Sr value was 0.14 for both samples. Ba/Sr values were 2.80 and 2.83. The average Zr (379.25 ppm) and Hf (9.70 ppm) were obtained from concentrations of 366.30, 392.20 ppm and 9.40, 10.00 ppm, respectively. Yttrium (Y) values observed in the porphyritic granodiorite were 21.40 and 21.90 ppm, while thorium (Th) concentrations were 6.10 and 6.60 ppm. Total REE_tot were 218.14 and 223.84 ppm. The total LREE were 207.78 and 213.10 ppm, while the total HREE were 10.41 and 10.71 ppm.

4.2.3. TTG

The average SiO₂ value (59.07 wt. %) for the TTG from concentrations ranging from 52.90 wt. % to 63.29 wt. % compared well with the average SiO₂ concentration calculated for the syenite (59.28 wt. %). The minimum and maximum Al₂O₃ values obtained are 14.05 wt. % and 17.59 wt. %, respectively. Average alumina, ferric oxide, and magnesia values for the TTG are 15.56 wt. %, 5.64 wt. %, and 3.34 wt. %. Fe₂O₃ values ranged from 3.30 wt. % to 8.43 wt. % while concentration values for magnesia ranged from 2.33 wt. % to 4.61 wt. %. CaO concentration averaged 4.23 wt. % from values ranging from 2.61 wt. % to 7.01 wt. %. Sample Gr-3 gave a relatively high anomalous lime value of 7.01 wt. %. Alkali content showed varied concentrations of K₂O. Potash values ranged from 2.38 wt. % to 8.40 wt. %. Na₂O values ranged from 3.12 wt. % to 3.95 wt. %. Average K₂O and Na₂O values were 5.61 wt. % and 3.50 wt. %, respectively. The range of TiO₂ values (0.61 wt. % to 1.48 wt. %) compared well with values from the other rock types.

Concentrations of Ba, Rb, and Sr ranged from 1842.00 ppm to 3242.00 ppm; 46.60 ppm to 346.30 ppm, and 655.20 ppm to 1101.00 ppm, respectively, and their calculated average concentrations, respectively, were 2532.75 ppm, 194.90 ppm, and 886.13 ppm. Ba/Rb values ranged from 9.36 to 43.11, while Ba/Sr values were from 2.09 to 3.68, and Rb/Sr ranged from 0.05 to 0.35. Zirconium concentrations ranged from 150.50 ppm to 357.90 ppm and averaged at 224.70 ppm, while Hf ranged from 3.50 to 8.90 ppm with an average value of 5.75 ppm. Concentration values ranged from 10.20 to 36.50 ppm for Nb and 0.50 to 2.40 for Ta, with their respective average values of 19.08 ppm and 1.15 ppm. Zr/Hf values ranged from 40.49 to 42.50, while Nb/Ta values ranged from 15.13 to 18.33. Thorium values ranged from 1.40 to 28.30 ppm, with an average value of 15.23 ppm. The TTG samples yielded total LREE values that ranged from 194.70 to 581.50 ppm, while the HREE values, when summed for each sample, ranged from 7.11 to 10.88 ppm. Average LREE and HREE values were 342.20 and 9.58 ppm, respectively. REE_tot values ranged from 205.40 to 592.42 ppm and averaged 351.78 ppm. The Y values observed ranged from 14.30 to 24.00, with an average value of 19.93 ppm.

4.2.4. Charnockite

The silica and alumina concentrations for the charnockite samples ranged from 52.86 wt. % to 53.63 wt. % and 17.85 wt. % to 18.35 wt. %, respectively. Averaged silica and alumina concentrations were 53.43 wt. % and 18.06 wt. %, respectively. Averages of Fe₂O₃, MgO, and CaO concentrations were 7.69 wt. %; 4.26 wt. %, and 6.91 wt. %, respectively, from ranges of concentrations of 7.64 wt. % to 7.77 wt. %; 4.20 wt. % to 4.33 wt. %; and 6.86 wt. % to 6.99 wt. %, respectively. Na₂O values ranged from 3.95 wt. % to 4.08 wt. %. K₂O values were the lowest when compared with other rocks in the study and ranged from 2.50 wt. % to 2.65 wt. %. Average total alkali contents are 4.00 and 2.56 wt. % for Na₂O and K₂O, respectively. The TiO₂ and P₂O₅ concentrations did not show much variation, as values ranged from 1.36 to 1.46 wt. % and 0.57 to 0.58 wt. %, respectively.

K/Rb values ranged from 443.01 to 467.10, and Ba/Rb values were from 45.90 to 48.24, while Rb/Sr yielded the same value (0.05). Concentrations of Ba ranged from 2133 ppm to 2268 ppm, Rb from 46.10 ppm to 47.60 ppm, and Sr from 971.40 ppm to 1026.60 ppm. Barium, Rb, and Sr concentrations showed averages of 2199.40 ppm, 46.68 ppm, and 1006.16 ppm, respectively. The minimum concentrations of Zr and Hf were 145.70 and 3.50, respectively, and their maximum concentrations were 458.90 ppm and 9.90 ppm. Their respective average values of Zr (266.64 ppm) and Hf (6.00 ppm) were comparable to those observed for the granite data but much lower than those observed for the syenite. The values of Nb ranged from 9.30 ppm to 10.50 ppm, while those of Ta ranged from 0.40 to 0.60 ppm, and their respective average values were 9.80 ppm and 0.52 ppm. The average values for Zr/Hf (43.45) and Nb/Ta (19.19) were the highest for the granitoids in this study. The minimum value observed for Th in the charnockite was 1.40 ppm, and the maximum concentration was 1.60 ppm, with an average concentration of 1.52 ppm. Yttrium concentrations observed in the charnockite samples ranged from 21.00 ppm to 22.20 ppm. The average Y value was 21.54 ppm. Total LREE values ranged from 187.50 to 193.50 ppm, while total HREE values were observed from 10.02 to 10.79 ppm. The average values of LREE and HREE were 189.40 ppm and 10.36 ppm, respectively. Total REE values ranged from 197.50 to 204.25 ppm, and its average value is 199.75 ppm. Eu/Eu* values for the charnockite were the highest (>1).

5.1. Petrological and Whole Rock Characteristics

The Iwo magnesian granitoid intruded into the high grade migmatite gneiss as hilly exposures, and having little xenolithic inclusions indicating that magma mixing was not a dominant process. The major oxide geochemistry neither strongly supported fractional crystallisation from a common magma source (Fig. 8). The mineralogical compositions of the rocks in this study include alkali feldspar, plagioclase, quartz, hornblende, garnet, and biotite, and with the absence of sodic amphiboles, sodic pyroxenes, normative acmite, and nepheline and the absence of rocks such as nephelinite, kimberlite, anorthosite, or carbonatite, a strong implication for calc-alkaline rocks derived from postcollision events is presented. Post-collision magmatism, influenced by subducted crustal materials leading to crustal thickening and post-collision uplift, is responsible for the formation of calc-alkaline high-K to shoshonitic granitoids that range in character from peraluminous to metaluminous (Bonin et al., 1998; Eyal et al., 2014).

Figure 8. Bivariate plots of SiO₂ vs. major oxide. SiO₂ vs. MgO, CaO, and K₂O show linear trends suggesting possible co-magmatism. The SiO₂ vs. K₂O (Fig. 8f) showed the rocks are derived from high-K calc-alkaline to shonshonitic magma series.

Post-collision granitoids (a mix of syn-collison and volcanic arc granitoid intrusives) display biotite and/or hornblende as part of their major mineral phases and can range from metaluminous to peraluminous. Granitoids derived from post-orogenic collision represent the transitional phase of the continental crust undergoing stabilisation after the crustal collison, the end of the Pan-African orogeny, as is the case in this study (Maniar and Piccoli, 1989). Liegeois et al. (1998) described high K calc-alkaline magmatism with alkaline affinities associated with large movements along mega-shear zones (late Pan-African events, 650–550 Ma) due to oblique subduction of small oceanic basins in a post collision setting for the Tuareg - Trans-Saharan belt. The high K calc-alkaline to shoshonitic, metaluminous granitoid of the study, bounded within the Iseyin schist belt and the Ifewara shear zone, share similar geochemical features with these post-collision granites from Tuareg shield, which is north of the Benin-Nigerian shield within which the rocks of this study are emplaced.

Low modal quartz observed in the quartz-syenite implied membership of a saturated calc-alkaline rock series sourced from mixing of crustal melts from subducted arc slab with mantle derived magma. Silica concentrations in the TTG (59.07 w. %) were observed to be lesser than true granites (>66 wt. %) (Cox et al., 1979), an occurrence likely due to silica migration and transport in magmatic melt rendering SiO₂ to become less across broader ranges than that of Mg, Na, and K. This suggests that the TTG could have been contaminated by the intrusion of the quartz syenite and charnockite intrusions. When compared with older granites reported from elsewhere within Nigeria, especially those reported having ferroan and peraluminous character, the MgO, CaO and Sr values of the magnesian granitoids of the study were higher than those observed for the ferroan porphyritic granite of Igbeti (Rahaman et al., 1983) and the granitoids from Igarra (Ogunyele et al., 2020) while the Rb and Zr concentrations were similar. The silica values (52.86–63.29 wt. %) of these rocks, were below those reported for the ferroan granite and charnockite from the Igarra Schist belt in the north central area of Nigeria (Table 3), where silica values ranged from 63.40 to 76.60 wt. % (Ogunyele et al., 2020) and the porphyritic granite at Igbeti (66.00 − 74.41 wt. %) (Rahaman et al., 1983).

Harker’s plot of major oxides vs. SiO₂ (Fig. 8) showed positive trends in SiO₂ vs. CaO and K₂O, suggesting possible magmatic relationships one with another during magma generation and possibly through contamination or magma mixing during ascent. The plots of SiO₂ vs. Al₂O₃, Na₂O, and MgO (Fig. 8) showed two samples of the quartz syenite (samples Sy-1 and Sy-2) plotting differently from the trend described by the charnockite and porphyritic granodiorite rock samples. This observation suggests a possible variation in magma types. The quartz syenite may probably have been derived from the partial melting of a crustal source together with some mantle or basic magma composition, which is different in composition from those that produced the porphyritic granodiorite and the charnockite.

The Differentiation Index (DI) vs. K₂O and MgO plot (Fig. 9a and b) and the Irvine and Baragar (1971) AFM plot, suggest calc-alkaline magma of dual origin or varied crustal materials that contaminated the melts. Elueze et al. (2008) have made an argument for andesitic and dacitic magmas as source magmas for the Oke Iho syenite and Osuntedo charnockite, respectively. Figure 10a showed samples of the TTG, porphyritic granodiorite, and charnockite rocks, describing a trend that conformed with the fractionation of basaltic-andesitic/andesitic to dacitic magma within the calc-alkaline magma series. The quartz syenite samples, however, described a slightly deviated trend that may be associated with crustal contamination. An andesitic magma origin for the quartz syenite and a dacitic magma origin for the porphyritic granodiorite and charnockite rocks of the study are suggested.

Figure 9. Differentiation Index (D.I.) vs. a) K₂O and b) MgO showed different trends for the Iwo granitoids. c) Bivariate plot of Differentiation Index (D.I) vs Alumina Saturation Index (ASI) depicts two distinct probable primary magmas. The quartz syenite must have been formed from magmatic differentiation different from processes that formed the porphyritic granodiorite and the charnockite rocks.

Figure 10. AFM plot suggests varied magmatic differentiation processes for the high K calc alkaline intermediate granitoids (after Irvine and Baragar, 1971). The quartz syenite samples tend to follow a slightly deviated trend from the other samples suggesting crustal influence of a different composition. b) A/CNK vs A/NK plot (adapted after Shand, 1943. c) SiO₂ vs Fe/(Fe+Mg), and d) Modified Alkali Lime Index (MALI). (Figs. 10c and d are adapted after Frost and Frost, 2008).

The quartz syenite samples showed the least concentration of Al₂O₃ and Na₂O but were enriched in magnesia. The relationship between alkalis and alumina depends on whether a rock type is peraluminous, metaluminous, or peralkaline. The A/CNK vs. A/NK plot by Shand (1943) showed that all samples plotted in the metaluminous field (Fig. 10b), with values of the Alumina Saturation Index (ASI) ranging from 0.63 to 0.88. The ASI values were plotted against the differentiation index (D.I.) to examine magma character (Fig. 9c). Figures 9 and 10 implied that, although the primary magmas share a metaluminous character, they were not produced from the same partial melt. Samples TTG-3 and the charnockitic rocks recorded the lowest values for total alkali and the highest values for lime (Table 2). TTG-3, representing the boundary sample between the granitic and the charnockitic bodies, shares a very similar character with the charnockites, suggesting that the granite body was probably emplaced prior to the emplacement of the charnockitic pluton.

The average values of total alkali and lime for the charnockite were 6.56 and 6.91 wt. %, respectively. This may be due to the abundance of pyroxene and feldspar minerals in the charnockitic rock. The average total alkali for the syenite (9.98 wt. %), porphyritic granodiorite (7.79 wt. %) and TTG (9.11 wt. %) were observed to be higher than that obtained for the charnockite (6.56 wt) and agrees well with the predominance of alkali feldspar. The high K₂O values observed in the syenite of the study (7.37 wt. %) compared well with the potassic syenite from Okeho (6.45 wt. %) which is magnesian in character (Okunola et al., 2023) and were higher than the alkali syenite from Shaki (5.91 wt. %), which is the core of the syenite complex at Okeho (Oyawoye, 1961).

The Modified Alkali Lime Index (MALI) plot of SiO₂ vs. K₂O+Na₂O+CaO characterised the syenite as having alkalic magma character, while the porphyritic granodiorite and charnockite bodies showed alkali-calcic magma character (Fig. 10d). The TTG samples showed both alkali and alkali-calcic nature, suggesting contamination or magma mingling by both magmas that formed the quartz syenite and those that formed the charnockite and porphyritic granite. The plot of SiO₂ vs. Fe/(Fe+Mg) suggested that the intrusives of the study are all magnesian in character. This contrasts the charnockite of the study with the ferroan Igarra charnockitic rocks, which are largely peraluminous (Ogunyele, 2020).

The plot of Q’-A’ vs. ANOR (Fig. 11a) (Streckeisen and Le Maitre, 1979) and the trilinear plot of Enrique (2018) (Fig. 11b) classified the syenite as quartz syenite-alkali feldspar, which is consistent with the modal mineralogy with microcline-orthoclase crystals as the dominant alkali feldspar. The porphyritic granodiorite samples plotted in the quartz monzonite field; again, this is true due to the preponderance of modal plagioclase feldspar, while the charnockitic rock samples fell exclusively in the monzonite/ gabbro/diorite field and may hinge on magma source believed to originate from deeper levels of the continental crust rich in plagioclase or from basic wall-rock or partial melts from the upper mantle with possible middle to lower crustal contamination. Samples of the TTG fell in all three fields described for the other rock types, suggesting contamination during the late phase of the Pan African reworking of the migmatite gneiss c.a. 550 Ma (Adetunji et al., 2018). These re-workings have been described as a series of events from the collision between the Benin- Nigerian shield and the Trans-Saharan belt, a zone positive gravity anomalies, thrusting and folding, formation of lateorogenic extensional basins, to granite intrusion (Wright et al., 1985).

Figure 11. a) Discrimination diagram based on CIPW (Cross Iddings-Pearson-Washington) normative mineralogy classified the rocks as quartz-alkalifeldspar, quartz-monzonnite and monzo-gabbro-diorite (adapted after Streckeisen and Le Maitre, 1979). b) Trilinear plot using normative feldspar discriminated the rocks of the study as alkali-feldspar quartz syenite, quartz syenite, quartz monzodiorite and diorite (adapted after Enrique, 2018).

Rajesh’s (2012) classification of charnockites based on Sr values would place the charnockite of this study as high Sr-charnockites. K/Rb ratio could test for metamorphic imprint (Rollinson, 1996), although this has been advocated to be used with caution (Rajesh, 2012). K/Rb ratio values were typically high, with values exceeding 1000 when compared with the 250 reported for average continental crust. Condie and Allen (1984) demonstrated that the K/ Rb ratio has implications for pressure from their study on high-pressure charnockites within Archean terrains, which gave K/Rb values above 1000 and low to medium pressure values between 250 and 500. K/Rb values for the charnockite from the study ranged from 443.01 to 467.10, suggesting medium pressure while, K/Rb values for the quartz syenite (<220) (Table 4) indicated low pressure.

The presence of modal pyroxene in granitoids implies dry, high-pressure conditions. The quartz syenite and the charnockite contain both pyroxene and plagioclase. Strontium (Sr) substitutes for Ca in plagioclase but not pyroxene; hence, the plot of Sr vs. K/Rb (Fig. 12a) was drawn to discriminate between these magnesian intrusives. Figures 12a and 12b demonstrated good discrimination among these intrusives. A plot of Rb vs. Th/Cs (Fig. 12b) created to examine possible high pressure and temperature conditions showed a possibility for a granite dehydration process during partial melting. In the plot of K/Cs vs. REE_tot (Fig. 12c), samples of the charnockitic and porphyritic granodiorite plotted towards the origin, indicating a granite dehydration/dry melting trend. The quartz syenite showing higher enrichment in LREE above the charnockite and the porphyritic granodiorite suggests they are more fractionated (Fig. 13a).

Figure 12. a) Discrimination plot of Sr vs K/Rb suggested different trace element fractionation processes operated during the formation of these rocks. b and c) Plots demonstrated depletion in total REE with progressive dehydration of melt.

Figure 13. a) REE normalised plot (after Boynton, 1984) revealed LREE enrichment over HREE. b) Spider plot, adapted after McDonough and Sun (1995), revealing enrichment in large ion lithophile elements above primitive mantle.

5.2. Rare Earth Element Geochemistry and Petrogenetic Setting

Post-collision granitoids display a wide variety of mineralogical and geochemical compositions due to their formation from partial melting and fractional crystallisation processes that involve older crustal rocks and subductionrelated arc materials as pressure is released during uplift. Many of these rocks display complicated trace element signatures reflecting syn-collision and volcanic arc settings and rarely display characters typifying within-plate (extensional) settings. Scholars such as Pearce et al. (1984), Goodenough et al. (2014), and Omotunde et al. (2020) have utilised REE geochemistry to comprehend the intricate history of rock evolution. The REE plot, normalised after Boynton (1984), showed an upper continental crust pattern (Fig. 13a) for all samples, with the samples of quartz syenite and two of the TTG samples (TTG-1 and TTG-2) showing much elevated LREE values and a negative europium (Eu) anomaly. All samples showed enrichment in LREE, suggesting partial melting of deeply seated crustal material with a possible andesitic composition. The values of the LREE/HREE ratio were > 11 and higher than that reported for the pink granite gneiss (LREE/HREE = 11) in southwest Nigeria (Oyinloye, 2011).

All the samples showed similar La(N)/Sm(N) values, with the quartz syenite samples recording lower values, which is consistent with the highest modal hornblende composition. The enrichment in REE observed for the charnockite with concentrations above chondrite values favours a parental magma of andesitic-dacitic magma (Olarewaju, 1987) rather than a primitive magma of basaltic composition expected from deeper crustal levels. Enrichments in LREE (Fig. 13a) and large ion lithophile elements (LILE) observed in the primitive mantle normalised spider plot after McDonough and Sun (1995) (Fig. 13b) suggest partial melting processes of middle to lower crustal material having possible andesitic to dioritic composition. Granitic rocks are typically rich in K, Rb, Ba, Th, and Ce (LILE) relative to Ta, Nb, Zr, Hf, Y, and Yb (HFSE) (Pearce et al., 1984). The Taylor and MacLennan’s (1995) Upper and Bulk continental crust multi-element plots (Fig. 14) revealed higher enrichment in large ion lithophiles (LIL) for the quartz syenite and to lesser amounts for the porphyritic granodiorite and least for the charnockite in both plots. A model suggesting a small batch fraction from partial melting of the upper continental crust is given as the probable cause for the observed K, Rb, Th, and U enrichment in the quartz syenite and (porphyritic) granodiorite. A relatively larger fraction from partial melting of continental crust at deeper levels or contamination of mafic components within this deep crust may be responsible for the lesser concentrations of the mobile LIL elements observed for the charnockite. Similar Ba enrichment is observed for all the granitoid types, hence the abundance of feldspar and biotite in them. For both plots, depletions in Y and Yb were observed with implications for a possible enriched mantle as a magma source allowing for garnet fractionation in the quartz syenite and charnockite.

Figure 14. Multi-trace element plot of Taylor and McLennan (1995) is used to illustrate degrees of partial melting. The quartz syenite richest in large ion lithophiles might have been derived lesser degrees of partial melting of the upper crust.

Granitoids formed in collision arc environments by complicated processes such as partial melting and thrusting (Mitchell, 1985; Tamura, 2011) are diverse in mineralogy and elemental compositions (Maniar and Piccoli, 1989). Lameyre (1988) and Pearce (1996) underscore the importance of tectonic setting in the genesis of granitic rocks. Collision settings are complicated with granitoid intrusions at the end of subduction of oceanic crust (syn-collision granite) and at the end of orogenic processes (post-collision granite). Post-collision granites marking the transitional phase during crustal stabilisation show signatures of both the syn-collision and volcanic arc environments. The intrusive granitoids of the study, like those reported from the southwest of the Nigerian Precambrian basement complex, are from magmatic arc settings. A back-arc environment with post-magmatic volcanics has been described by Rahaman et al. (1988) and Oyinloye (2011). Obiora and Charan (2011) and Ogunyele et al. (2020) argued for the closure and opening of oceans in the eastern end of the southwest part of the Precambrian basement complex. The alkaline character and syn-collision magma type of the quartz syenite (Fig. 15a and c) may be related to magma contamination at the end of arc-related subduction since the study area lies on the western part of the extensional back arc environment proposed for the Ilesha area (Rahaman et al., 1988; Oyinloye, 1998; Oyinloye and Odeyemi, 2000).

Figure 15. Trace element plots after Pearce et al., (1984) revealing diverse sources and depths for the intrusive rocks. The quartz syenite and the TTG samples plotted in multiple fields except in the oceanic ridge granite (ORG) field, ranging syn-collision granite (syn- COLG), volcanic arc granite (VAG) to within plate granite (WPG), and suggesting crustal contamination in a post-collision setting. The porphyritic granodiorite and the charnockite consistently plotted in the VAG field.

The tectonic setting for the Precambrian granitoids of this study was inferred using the trace element plot of Pearce et al. (1984). Figure 15 shows samples plotted in the post-collision granite field (which is the region around the boundary between the syn-collision granite field and the volcanic arc granite field). Based on trace element geochemical data, Pearce et al. (1984) showed that granitic rocks within this field could be differentiated using the Rb/Zr ratio. The Rb/Zr ratio for the syenite (0.6–0.9) was observed to be higher than values observed for the porphyritic granodiorite (0.2–0.3) and the charnockite (0.1–0.3). Rb/Zr values for the TTG ranged from 0.3 to 1.9, which suggests that they are older and were affected by the intrusion of the other units. Field occurrences such as higher relief of syenite and charnockite exposures occurring closely with the migmatite gneiss and its TTG component, together with negative anomalies of Nb and Ta (Fig. 14), argue for syn- to post-orogenic granitoid intrusives rather than within plate magmatism, which is associated with alkaline rocks and A-type granites.

Magma derived from melting of lower crust arising from thermal relaxation caused by collision and from melting of upper mantle due to adiabatic decompression associated with post-collision uplift and erosion (Pearce et al., 1984) are dominant magma sources that form post-collision granitoids. This study suggests that the Pan African reworking of the migmatite gneiss might have produced melts of granitic composition (quartz, feldspar, and mica) that later cooled to form the TTG components within the migmatite gneiss. Field evidence showed that the TTG were widely exposed, allowing for their contamination by the intrusion and possible uplift of the quartz syenite and the charnockite. Figure 15a supports an Upper continental crust source material for the quartz syenite while the charnockitic and the porphyritic granodiorite rocks could have been sourced from deeper continental crust derived from a combined partial melting of an enriched magma of probable tonalitic composition and lower crust (Laurent et al., 2014).

High Ba values in igneous rocks are likely sourced from the mantle through the metasomatism of subducting slabs in arc-related or collision environments. This metasomatic fluid is rich in large ion lithophile elements (LILEs), such as Ba. Barium values, above average upper continental crust value (Fig. 14), agree with arc-related magmatism with possible crustal contamination. High values of Ba and Sr may be accounted for by the fractionation of modal hornblende and plagioclase in the rocks (Li et al., 2019). The relatively higher Rb, Ba, Th, Nb, Ta, Zr, Hf, and La/ Yb(N) values observed in the quartz syenite imply partial melting of an upper continental crust and crustal contamination. Partial melting rather than magma mixing is likely to be the dominant process for the formation of andesitic and dioritic melts (Lee and Bachmann, 2014). The formation of these melts must have taken place during Late-stage orogeny and post-collision uplift are illustrated for the intrusives in the Batchelor and Bowden (1985) plot as the Pan African orogeny waned leading to the stabilisation of the West African craton (Fig. 16).

Figure 16. R₁ = 4Si – 11(Na+K) – 2(Fe+Ti) vs R₂ = 6Ca + 2Mg – Al plot supports granitoid intrusion along with post collision uplift as late orogenic events that characterised the stabilisation of the WAC during the Pan African orogeny. (adapted after Batchelor and Bowden, 1985).

Field observations, petrography, and major oxide geochemistry revealed compositional variation among the intrusive granitoids found in the northern part of the Iwo area, southwest Nigeria. The granitoids were identified as quartz syenite, porphyritic granodiorite, Tonalite-Trondjhemite- Granodiorite (TTG), and charnockite.

Their silica concentrations, ASI, MALI, K₂O/Na₂O ratio, and normative diopside (Table 2) were used to characterise them as I-type metaluminous magnesian granitoids derived from high K calc-alkaline to shonshonitic magma series having alkali - alkali-calcic character. Charnockite and porphyritic granodiorite plutons displayed alkali-calcic character, while the quartz syenite displayed alkali magma type. The TTG shared both alkali and alkali-calcic characters, representing possible contamination by melts that formed these plutons. Concentrations of MgO, CaO, and K₂O were observed to be higher than many ferroan granitoids reported from Nigeria.

Granitoids derived from post-orogenic collisions share an alkali to calcic magma character. The REE data normalised using chondrite values supports partial melting as the dominant process that generated the granitoids of the study. This plot, together with the plot of Y+Nb vs. Rb, showed that the granitoids were likely derived from crustal source material of two different compositions. The quartz syenite (ΣLREE > 300) is believed to be derived from partial melting of a crustal composition (andesitic) different from the crustal material (dioritic) from which the porphyritic granodiorite (ΣLREE < 300) and charnockite (ΣLREE < 300) were derived.

Geodynamic models further identified these I-type magnesian plutons as syn-collision (quartz syenite) and volcanic arc (porphyritic granodiorite and charnockite) granitoids that were probable products of late orogenic to post-collision uplift processes associated with the Pan African orogeny.

We acknowledge Timilehin, Koleoso, and Idowu for helping with the mapping and sampling. The effort of Mr. Nasir in cutting the thin sections is appreciated. The effort and comments of the Editor and reviewers are sincerely appreciated.