• The granitoids exhibit sub-alkaline and calc-alkaline properties. They belong to the S-type granites and display peraluminous characteristics

• The genesis of granitoids can be attributed to the process of partial melting of crustal rocks with input from greywacke and pelitic materials in a typical post-collisional tectonic setting.

• Granitoids show enrichment in Th, U, Nd, Pb, Sm, and depletion in Ba, K, P, Rb, and Ti. The REE pattern indicates an abundance of LREE over HREE, with a negative Eu anomaly.

A suite of Pan-African Orogenic belts composed of extensively reworked Archean and Proterozoic rocks extends across the African continent into the Braziliano orogens of South America (Castaing et al., 1994; Jacobs and Thomas, 2004; De Wit et al., 2008; Goodenough et al., 2014). These belts are mostly composed of granitoid plutons whose source have been linked to both mantle and crustal derived materials (Black and Liégeois, 1993; Küster and Harms, 1998; Liégeois et al., 1998; Bonin, 2004; Goodenough et al., 2014). The Nigerian Basement Complex forms part of the Pan-African mobile belt and lies between the west African Craton to the west and Congo Craton to the east and south of the Tuareg shield (Black et al.,1979; Garba, 2003). The Nigerian basement is intruded by several Pan- African and post-syn collisional plutons called Older Granites that are mostly found in the western part of the country while the eastern part contains fewer and the northern part includes a suite of Mesozoic alkaline plutons emplaced in an intra-plate setting known as Younger Granites (Victor et al., 2022). These granitoids are also associated with rare earth metals such as tin, tantalum, and niobium that are associated with pegmatites (Garba, 2003; Okunlola, 2005; Adetunji and Ocan, 2010; Melcher et al., 2013). U-Pb geochronological data on the Older Granites yielded ages of between 750-500 ma (Ekwueme and Kroner, 1998; Adetunji et al., 2016) while Rb-Sr and K-Ar isotopic measurements of younger granite yield ages ranging from 174 ± 5 ma. in the north to 154 ± 4 ma in the south while at the Jos Plateau region about 164 ± 4 ma was reported (Van Breemen et al.,1975; Adetunji et al., 2016).

The granitoid exposures in Kaiama form part of the southwestern Nigeria basement complex. Previous investigations conducted in the Kaiama region have primarily focused on reconnaissance geochemical studies, which aimed to identify the distribution of various elements and potential mineralization sites in rocks, soils, and stream sediments (Alepa et al., 2019a; 2019b). Additionally, Dada and Ajadi (2018) identified several promising minerals in the area, including gold, cassiterite, columbite, tantalite, rare earth metals, and gemstones. Despite being an essential component of the southwestern Nigeria basement complex, the granitoids in Kaiama lack adequate scientific exploration with regards to their petrology, mineralogy, geochemical properties, structural analysis and petrogenesis. The only available information on their geology is found in geological maps created by the Nigerian Geological Survey Agency (NGSA, 2004). Therefore, a new study is necessary to map and distinguish the various granitic rocks, conduct detailed petrographic and geochemical analysis to determine their classification and outline their petrogenesis and tectonic settings.

The study area lies within the southwestern basement complex of Nigeria (Figure 1a and 1b). The basement complex of Nigeria is made up of three litho-petrological components. 1) The Precambrian Basement Complex, 2) Cretaceous-Tertiary Sedimentary Basins, and 3) Jurassic Younger Granite. The Basement Complex is a part of the Pan-African Mobile Belt lying between the West African and Congo Cratons, and south of the Tuareg Shield (Figure 2) (Black, 1980). It is overlain unconformably by Cretaceous and younger sediments. The basement complex was affected by the Pan-African Orogeny (ca. 600 Ma) and occupies the reactivated region resulting from the collision of the passive continental margin of the West African Craton and the active Pharusian continental margin (Burke and Dewey, 1972; Dada, 2006). The Liberian (ca. 2700 Ma), Eburnean (ca. 2000 Ma), Kibaran (ca. 1100 Ma), and Pan-African (ca. 600 Ma) orogenic cycles of deformation, metamorphism, and remobilization have been proposed as events leading to the formation of the basement rocks (Rahman et al., 1983). The Pan-African deformation is believed to have been accompanied by regional metamorphism, migmatization, and extensive granitization, with its end marked by faulting and fracturing (Abaa, 1983; Gandu et al., 1986; Olayinka, 1992). Four major petrological units are recognized within the Basement Complex: Migmatite-Gneiss-Quartzite Complex, Schist Belt (comprising metasedimentary and metavolcanic lithologies), Older Granites, and undeformed Acid and Basic Dykes (Obaje, 2009).

Figure 1. (A) Geological sketch map of Nigeria, showing the different lithostratigraphic components, adapted from NGSA (2004), (B). Geological map of Kwara state (adapted from NGSA, 2004), the location of the study area is in a black rectangle, (C) Geological map of the study area (Kaiama) showing the granitoids (adapted from Umaru et al., 2022).

Figure 2. Generalized geological map of Nigeria within the framework of the Geology of west Africa (Modified from Wright, 1985).

The basement complex of southwestern Nigeria comprises three petrological units of pre-Cambrian to Cambrian age namely; the migmatite-gneiss complex which is made up of granite-gneiss/migmatitic granite-gneiss, banded gneiss, and biotite gneiss, low to medium grade metasediments mostly of amphibolite Facies and comprises of amphibolites, schist (quartz schist and mica schist), undifferentiated schists, amphibole schist, and marble. The schist belts are known for their complex geology and mineral resources endowment such as semi-precious metals, precious metals, base metals, and gemstones. This mineralization is mostly intrusions of ore-bearing veins of gold, and pegmatite veins which are also host to gemstones such as (beryl, garnets, tourmaline), tin, tantalite, and columbite. The Pan African granitoid (Older granites) are syn to late tectonic intrusions occurring as stocks or large batholithic bodies intruding into the basement gneisses and the supra crustal rocks. They are generally composed of granites, biotite hornblende granites, syenites, pyroxene diorites, and the late Pan African intrusive pegmatite. The basement complex rocks have been strongly deformed. Such deformation coupled with migmatisation processes tends to mix rocks of different ages and origins in such a manner that lithologic boundaries are not easily discernible. Some authors believed that the relicts metasedimentary and basic rocks are those that have restricted migmatisation and granitisation processes. McCurry (1971) suggested that some of the relict metasedimentary rocks may be lateral equivalents of the rocks making up the much older West African Craton. Ajibade (1976), believes that there are two generations of metasedimentary rocks, one belonging to the older migmatites-gneiss complex e.g., the quartzites, schist, and amphibolites of Ibadan and Ilesha area, and the other belonging to the younger metasedimentary sequence consisting of Effon Psammite formation and the Igarra formation which are composed of quartzites-schist, calcsilicates rocks, metaconglomerates and marble (Obaje, 2009). The migmatites-gneisses being the oldest rock in this part of the country is both litho and tectonostatigraphically basal to all subsequent superjacent lithologies. The rocks in the migmatites-gneiss complex are considered to be the oldest member of the Precambrian basement complex and they evolved through successive stages of sedimentation, deformation, metamorphism, and igneous intrusion over a vast period of geologic time (Obaje, 2009).

Detailed field mapping exercise was carried out to identify the rock units and relationships, as well as structures, while about 2-3kg of representative rock chips were obtained for petrological and geochemical studies. A total of eighteen (18) representative samples were collected from the study area comprising 5 K-feldspar rich granite, 7 porphyritic granite, and 6 granite gneiss. Thin sections were produced and analyzed at the Department of Geology, University of Ibadan, Nigeria purposely to study the mineralogical composition in both plane-polarized and cross-polarized light as well as their textural characteristics. For geochemical analysis, samples were crushed and pulverized to 70% passing 2mm after which a split of 250g was further pulverized to better than 85% passing 75 microns at ALS global services geochemical laboratory, Vancouver Canada. The pulverized samples were thoroughly mixed with lithium metaborate/tetraborate flux and ignited at 1000°C in a furnace. After cooling, the resultant melt was dissolved in 100 mL of a 4% (HNO₃)/2% (HCl) solution. ICP-MS (Agilent 7700) and ICP–AES (Agilent 5800) were used to examine the resultant solution for their major oxide, trace elements, and rare-earth elements (ALS, 2021). The Limit of Detection (LOD) is 0.01wt% for major oxides; 1ppm for Co, Ni, Cu, Zr, and Mo; 0.1ppm for Sr, Ta, Y, and Ga; 2ppm for Zn and Pb; 0.5ppm for Sn and Ba; 0.05ppm for Nb, Hf, and Th; 5ppm for As and V, 0.005ppm for Au and 0.01ppm for Cs. In order to control the quality of each method, analysis was also carried out on blanks and duplicates. Standards used were AMIS0085, AMIS0167, AMIS0304, AMIS0343, AMIS0461, AMIS0547, AMIS0571 and BCS-512 and were consistent with accepted values. The resultant data was processed and interpreted using software such as ArcGIS 10.8.1 for map production, GCD Tool kit, and Graph pad for producing geochemical discrimination plots.

Field Occurrence and Petrography of Kaiama Granitoids

The granitoids in the study area include; K-feldspar rich granite, porphyritic granite, and granite gneiss while quartz and aplite veins and dykes intrude the granitoids. A geological map showing the relationships between the rock units is presented in (Figure 1c). The descriptions of the individual lithological units, field relationships, and thin section microscopic description are given below.

Porphyritic Granite

The rock occurs mostly as domes and rarely as low-lying whaleback-shaped outcrops (Figure 3a). They are leucocratic and contain phenocryst of plagioclase feldspars in sizes of (2-5cm along a long axis and 0.5cm to 3cm of the short axis) in a fine to medium-grained groundmass of quartz and biotite mica (Figure 3b). The surface morphology of the outcrop shows several cross-cutting quartz veins oriented majorly NE-SW and a few NW-SE, with the former crosscutting the latter suggesting that it is older. Aplitic veins and dykes also oriented NE-SW constitute the surface morphology. Microscopic observation of porphyritic granite, reveals minerals such as plagioclase, quartz, sericite, muscovite, biotite, and alkali feldspar. Plagioclase shows a characteristic polysynthetic twinning with alternating dark and light-colored bands. Plagioclase commonly shows transformation into sericite, especially within fractured zones. Inclusions of muscovite are observed on some of the plagioclase. Quartz shows a wavy/undulose extinction which is characteristic of deformation. Sericite occurs as a secondary mineral due to the alteration of plagioclase feldspars forming a myrmekite texture within plagioclase (Figure 4). Biotite has a high 2^nd to 3^rd-order interference colour from dark brown to green and also undergoes extinction four times upon rotation of the microscope stage (Figure 4).

Figure 3. A) Low-lying outcrop of porphyritic granite B) Close view of porphyritic granite showing phenocryst of feldspar on fine groundmass of quartz and biotite (long axis=4cm; short axis= 2.5cm).

Figure 4. Photomicrograph of porphyritic granite in Cross Polarized Light (CPL) showing Qz=Quartz, Pl=Plagioclase, Se=Sericite, B=Biotite, Kfs=Alkali feldspar, and Ms=muscovite.

Granite Gneiss

The rock occurs low-lying and as domes in places. They are leucocratic due to the preponderance of felsic minerals with uniformly distributed euhedral porphyroblast of plagioclase feldspars on a fine grained groundmass of quartz and biotite mica (Figure 5a and 5b). The biotite micas show a flowing pattern around the boundaries of the feldspar porphyroblast probably due to ductile deformation. Several cross-cutting joints and quartz veins also constitute the outcrop. Petrographic examination reveals porphyroblast of plagioclase feldspars with clear polysynthetic twinning that is altered along their rims (Figure 6). Quartz occurs as polycrystalline minerals and undergoes wavy extinction at around 50⁰. Biotite undergoes extinction at about 60⁰ and shows second-order dark to light brown interference colours(Figure 6). Microcline shows a cross hatched twinning pattern with the two sets of lamellae at right angles and the inclusion of plagioclase, it also undergoes extinction at about 15-20⁰ and shows a first order gray to white interference colours.

Figure 5. A) Low lying outcrop of granite gneiss with porphyroblast of plagioclase feldspar, B) granite gneiss showing mafic enclave inclusion.

Figure 6. Photomicrograph of granite gneiss showing Pl=plagioclase, Qz=quartz, Mc=microcline and B=biotite.

K-Feldspar Rich Granites

The rock occurs low-lying with a unique pink colour due to the predominance of k-feldspars minerals (Figure 7). It is medium to coarse-grained and contains quartz, biotite, K-feldspars, and a few occurrences of plagioclase feldspars. Microscopic observation reveals minerals such as microcline, quartz, plagioclase, albite, sericite, and flakes of biotite (Figure 8). Microcline shows a tartan twinning pattern with a first order gray to white interference colour and extinction angle at about 15-20 degrees. Biotite showed a 2^nd to 3^rd-order pale brown to dark brown interference colours. A late tectonic quartz vein cut across earlier anhedral quartz grains (Figure 8). Plagioclase has a unique lamellar twinning with inclusions of biotite. Sericite occurs as an alteration product of plagioclase.

Figure 7. Low lying outcrop of K-feldspar rich granite around Kaiama settlement.

Figure 8. Photomicrograph of K-feldspar rich granite showing Pl=Plagioclase, Mc=Microcline, Qz=Quartz and B=Biotite

Geochemical Characteristics of Kaiama Granitoids

Porphyritic granite

SiO₂content in porphyritic granite range from (71.90-73.80 wt%), Al₂O₃ (13.10-14.00 wt%), Fe₂O₃ (1.84-2.18 wt%), CaO (0.83-1.30 wt%), Na₂O (3.48-3.85 wt%) and K₂O (4-4.53 wt%). Low concentrations of MgO, CaO, TiO₂, and MnO were recorded (Table 1). LOI range from 0.38-1.11 (Table 1). While porphyritic granites have an average total alkali ratio of 8.08% and an average K₂O/Na₂O value of 1.19%. The variations of major oxides with SiO₂ as fractionation index on a Harker diagram show that Na₂O, CaO, MgO, and FeOt decrease with increasing SiO₂ while Al₂O₃ and K₂O show a positive correlation with increasing SiO₂ (Figure 9). TiO₂ shows a scatter with increasing SiO₂ (Table 1). Trace elements including Ba, Sr, V, and Rb show a decrease with increasing SiO₂ (Figure 10) while values of K/Rb range from 13.70-15.22, Ba/Rb range from 2.46-3.14, and Rb/Sr range from 1.59-1.92 (Table 1). On a primitive mantle normalized multi-element variation plot (Sun and McDonough, 1989) porphyritic granite show enrichment in Th, U, Nd, Pb, Sm, and depletion in Ba, K, P, Rb, and Ti (Figure 11). On a chondrite-normalized REE diagram normalized using the values of Boynton (1984), the porphyritic granite shows enrichment in LREE relative to MREE and HREE, with a distinct negative Eu-anomaly (Eu/Eu* = 0.31–0.42). The fractionated (La/Yb) _N ratios in the porphyritic granites range from 4.87-8.33, (La/Sm) _N range from 3.36 – 4.4, total REE ranged from 159.15 – 309.17 (Table 1; Figure 12).

Table 1 . Major elements oxides (wt%), trace and rare earth elements (ppm) composition of granitoids in Kaiama area.

Sample No	K-feldspar rich granites					Porphyritic granites							Granite gneiss
Oxides (%)	P1	P2	P3	P4	P5	PG1	PG2	PG3	PG4	PG5	PG6	PG7	G1	G2	G3	G4	G5	G6
SiO2	78.8	76.6	79.7	77.3	76.8	72.4	73	73.1	71.9	72.2	72.5	73.8	68.2	69.2	67.4	69.2	63	64.7
Al2O3	12.25	13.3	11.85	12.4	12.35	13.15	13.7	13.8	13.3	14	13.1	13.8	15.4	14.2	14.75	15.15	16.1	16.15
Fe2O3	0.95	1.36	1.04	1.24	1.28	2.06	1.84	2.18	2.06	2	2.1	2.07	3.61	4.95	4.94	3.77	6.13	5.86
CaO	0.14	0.18	0.51	0.5	0.15	1.16	1.24	0.95	1.24	1.11	1.3	0.83	2.38	2.13	2.02	2.42	2.96	3.2
MgO	0.03	0.02	0.06	0.02	0.02	0.26	0.24	0.27	0.28	0.25	0.28	0.27	0.78	0.98	0.79	0.6	1.2	1.12
Na2O	3.82	3.96	4.06	4.19	4.12	3.48	3.75	3.72	3.64	3.85	3.76	3.53	3.85	3.11	3.15	3.21	3.31	3.43
K2O	4.99	5.15	3.74	4.43	4.54	4.49	4.5	4.5	4.31	4.54	4	4.53	3.39	4.28	4.85	4.77	4.54	3.93
TiO2	0.05	0.03	0.05	0.04	0.03	0.19	0.18	0.22	0.21	0.19	0.22	0.2	0.54	0.72	0.71	0.43	0.98	0.86
MnO	0.04	0.05	0.02	0.02	0.02	0.03	0.03	0.04	0.04	0.04	0.04	0.04	0.05	0.06	0.07	0.05	0.08	0.08
P2O5	BDL	0.01	BDL	BDL	BDL	0.04	0.04	0.04	0.06	0.05	0.05	0.06	0.2	0.27	0.29	0.1	0.35	0.35
LOI	0.51	0.51	0.48	0.46	0.44	0.81	0.83	0.54	0.93	0.38	1.11	0.82	1.06	0.5	0.32	0.44	0.47	0.55
Total	101.5	101.1	101.5	100.6	99.7	98.16	99.44	99.47	98.07	98.71	98.55	100.06	99.55	100.51	99.42	100.23	99.29	100.38
K/N	1.3	1.3	0.92	1.05	1.1	1.29	1.2	1.21	1.18	1.17	1.06	1.28	0.88	1.37	1.53	1.48	1.37	1.14
N+K	8.81	9.11	7.8	8.62	8.66	7.97	8.25	8.22	7.95	8.39	7.76	8.06	7.24	7.39	8	7.98	7.85	7.36
A/CNK	1.36	1.43	1.42	1.36	1.4	1.44	1.44	1.5	1.44	1.47	1.44	1.55	1.6	1.49	1.47	1.45	1.48	1.52
K	0.41	0.43	0.31	0.37	0.38	0.37	0.37	0.37	0.36	0.38	0.33	0.38	0.28	0.36	0.4	0.4	0.38	0.33
K/Rb	8.50	8.19	12.35	7.88	7.93	15.22	14.68	14.80	14.28	13.70	14.10	14.84	16.09	17.30	20.05	21.22	21.65	21.49
Ba/Rb	0.26	0.08	0.67	0.09	0.06	2.59	2.62	3.24	2.86	2.46	2.53	3.14	3.41	3.34	4.60	3.40	6.92	7.13
Rb/Sr	33.94	59.66	5.69	47.37	50.42	1.81	1.69	1.59	1.66	1.92	1.65	1.72	0.63	0.87	0.88	1.34	0.66	0.53
Trace element concentration (ppm)
Au	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL	BDL
Co	45	63	39	78	71	51	68	44	40	47	51	38	55	59	93	69	63	72
Ni	4	1	2	1	1	3	2	5	5	1	5	4	7	7	4	4	6	4
Cu	4	50	3	23	1	19	2	13	2	8	2	7	7	13	7	8	7	15
Zn	51	65	37	62	73	52	49	53	58	55	51	49	65	86	89	70	104	93
As	5	9	<5	5	<5	5	<5	<5	<5	9	<5	<5	<5	<5	<5	<5	<5	<5
Rb	482	525	251	469	479	243	252	250	252	276	234	256	174	208	199.5	188.5	175.5	153.5
Sr	14.2	8.8	44.1	9.9	9.5	134.5	149.5	157	151.5	144	141.5	149	275	240	227	140.5	267	287
Sn	11.1	7.9	8	9.3	10.3	6.1	6.4	6.6	7.6	10.3	6.4	5.7	2.9	2.6	3.7	3.1	3.2	2.7
Ba	123.5	42.5	167.5	40.7	30.8	630	661	810	720	680	592	805	594	694	917	641	1215	1095
Pb	61	68	45	60	58	34	32	32	33	38	32	32	23	25	34	23	26	23
Ta	7.3	6.2	7.6	6.5	7.2	3.3	4.1	4.2	3.3	4.5	4.1	3.4	1.1	1	1.6	1.6	2	1.9
Y	122.5	144	74.8	114	118.5	40.4	40.4	62.3	39.1	49.3	48.6	78.3	23.4	33	26.8	24.2	27.2	25.3
Nb	61.2	48	50.3	57.2	49.4	22.4	23	27.7	21.7	22.3	25.1	23.2	15.5	19.9	22.1	16.55	25.1	21.2
Hf	5.99	6.24	4.7	5.98	5.91	5.75	5.47	6.12	5.54	5.53	7.21	5.61	7.67	10.5	9.51	7.47	8.4	9.93
Zr	116	107	79	115	115	188	164	185	182	164	230	176	293	412	389	282	363	414
Th	48.8	39.4	29.1	43.6	44.6	17.4	19.1	21.4	21.2	26.8	23	19.75	13.15	22.5	6.16	22.5	6.15	2.03
V	<5	<5	<5	<5	<5	10	10	13	14	12	13	11	40	52	41	34	62	60
Ga	31.2	35.8	28.6	35.5	34	24.9	25.4	25.4	23.9	25.1	24.5	24.7	24.7	22.1	22.4	21	24.5	23.6
Mo	2	7	<1	1	4	<1	1	<1	1	<1	1	<1	1	1	1	<1	1	1
Cs	4.61	6.62	4.82	6.43	5.73	7.19	7.04	7.21	7.53	29.5	8.03	7.2	1.23	2.94	3.18	4.53	3.24	2.64
Rare earth elements concentration (ppm)
La	11.9	13.5	9.5	14.6	11.8	34.4	32.4	67.7	36.6	39.1	65.2	56.2	59	97.5	46.5	95.3	31.9	29.2
Ce	46	38.2	22.4	40.7	33.2	70.5	63.9	121	75.9	80.7	130	97.9	122	204	99.8	197	68.9	62.7
Pr	4.69	5.75	3.17	6	4.92	7.87	7.26	15	8.3	8.97	13.65	13.35	13.3	23	11.5	21.7	8.76	8.03
Nd	21	27.4	14	27.6	23.7	30.6	25.6	55	32.5	34.5	48	50.9	45.7	78.5	42.6	74.8	34.7	32
Sm	7.41	11.4	5.15	11.75	9.87	6.44	6.05	10.7	6.51	7.31	9.32	12.05	8.3	13.65	7.92	12.55	7.39	6.62
Eu	0.13	0.08	0.27	0.09	0.1	0.72	0.79	1.15	0.73	0.76	0.84	1.29	1.72	1.63	1.98	1.38	2.35	2.6
Gd	9.81	15.85	7.25	14.6	13.05	6.3	5.57	10.35	5.82	7.19	7.5	12.3	6.48	10.55	5.98	8.89	6.34	6.37
Tb	1.99	2.84	1.69	2.8	2.39	0.98	0.96	1.79	0.99	1.26	1.28	2.13	0.99	1.38	1.04	1.13	1.02	0.92
Dy	16.8	21.8	13.2	19.4	18.15	6.51	5.85	10.05	5.89	7.4	7.64	12.5	4.76	6.78	5.85	5.44	5.1	4.76
Ho	3.52	4.58	2.67	3.6	3.69	1.27	1.33	2.11	1.26	1.59	1.74	2.62	0.86	1.25	0.99	0.96	1.06	1.02
Er	12.15	14.85	8.56	11.55	12.25	4.23	4.01	6.01	3.83	4.98	4.84	7.93	2.49	3.37	2.54	2.33	2.83	2.54
Tm	2.09	2.44	1.39	1.73	1.93	0.61	0.66	0.98	0.57	0.77	0.84	1.28	0.29	0.42	0.35	0.31	0.33	0.3
Yb	14.9	17.4	9.4	11.5	13.5	4.23	4.12	6.42	3.78	5.22	5.28	7.78	1.76	2.43	2.1	1.68	2.03	1.95
Lu	2.03	2.33	1.16	1.73	1.96	0.51	0.65	0.91	0.62	0.73	0.86	1.11	0.24	0.37	0.34	0.27	0.28	0.28
Rb/Sr	33.94	59.66	5.69	47.37	50.42	1.81	1.69	1.59	1.66	1.92	1.65	1.72	0.63	0.87	0.88	1.34	0.66	0.53
Rb/Ba	3.9	12.35	1.5	11.52	15.55	0.39	0.38	0.31	0.35	0.41	0.4	0.32	0.29	0.3	0.22	0.29	0.14	0.14
Y/Nb	2	3	1.49	1.99	2.4	1.8	1.76	2.25	1.8	2.21	1.94	3.38	1.51	1.66	1.21	1.46	1.08	1.19
K/Rb	0.001	0.001	0.001	0.001	0.001	0.002	0.001	0.001	0.001	0.001	0.001	0.001	0.002	0.002	0.002	0.002	0.002	0.002
(Ce/Yb) N	0.8	0.57	0.62	0.92	0.64	4.31	4.01	4.88	5.19	4	6.37	3.25	17.93	21.17	12.29	30.33	8.78	8.32
(Ce/Sm) N	1.5	0.81	1.05	0.84	0.81	2.64	2.55	2.73	2.81	2.66	3.37	1.96	3.55	3.61	3.04	3.79	2.25	2.29
(Gd/Yb) N	0.53	0.74	0.62	1.02	0.78	1.2	1.09	1.3	1.24	1.11	1.15	1.28	2.97	3.5	2.3	4.27	2.52	2.64
(La/Yb) N	0.54	0.52	0.68	0.86	0.59	5.48	5.3	7.11	6.53	5.05	8.33	4.87	22.6	27.05	14.93	38.24	10.59	10.1
(La/Sm) N	1.01	0.74	1.16	0.78	0.75	3.36	3.37	3.98	3.54	3.36	4.4	3.93	4.47	4.49	3.69	4.78	2.72	2.77
Eu/Eu*	0.05	0.02	0.14	0.02	0.03	0.35	0.42	0.33	0.36	0.32	0.31	0.32	0.72	0.42	0.88	0.4	1.05	1.22
⅀REE	154.4	178.4	99.8	167.6	150.5	175.1	159.1	309.1	183.3	200.4	296.9	279.3	267.8	444.8	229.4	423.7	172.9	159.2

Figure 9. Harker diagram for K-feldspar rich granites, Porphyritic granites, and Granite gneiss showing the variation of SiO₂ with some selected major elements. (Note: red squares are K-feldspar rich granites, blue triangles are porphyritic granites and green diamonds are granite gneiss).

Figure 10. Harker variation diagram of K-feldspar rich granites, Porphyritic granites, and Granite gneiss showing SiO₂ variation with selected trace elements.

Figure 11. Primitive mantle normalized multi-element variation plot (Sun and McDonough, 1989) for the granitoids.

Figure 12. Chondrite-normalized rare earth elements (REE) plot of the granitoids with normalization values after Boynton, (1984).

Granite Gneiss

The SiO₂ content in granite gneiss range from (63-69.2 wt%), Al₂O₃ (14.2-16.15 wt%), Fe₂O₃ (3.61-6.13 wt%), CaO (2.02-3.2 wt%), Na₂O (3.11-3.85 wt%), MgO (0.6-1.2 wt%) and K₂O (3.39-4.85 wt%). Low concentrations of MnO, P₂O₅, and TiO₂ are recorded with concentrations <1% (Table 1). The average total alkali concentration is 7.63% and the average K₂O/Na₂O value is 1.29% (Table 1). The variations of major oxides with SiO₂ as fractionation index on a Harker diagram show that Na₂O, CaO, MgO, and FeOt also decrease with increasing SiO₂ while Al₂O₃ and K₂O show a positive correlation with increasing SiO₂ (Figure 9) while trace elements such as Ba, Sr, V, and Rb show a decrease with increasing SiO₂ (Figure 10) while values of K/Rb range from 16.09-21.65, Ba/Rb range from 3.34-6.92 and Rb/Sr range from 0.53-1.34 (Table 1). The primitive mantle normalized multi-element variation plot (Sun and McDonough, 1989) for granite gneiss show that they are enriched in Rb, Pb, U, and depleted in K, P, and Ti (Figure 11). The rare-earth elements pattern of granite gneiss normalized using the values of Boynton, (1984), shows enrichment of LREE relative to HREE, with no Ce anomaly and a negative Eu-anomaly (Eu/Eu* = 0.40–1.22), The fractionated (La/Yb) _N ratios range from 10.10-38.24, (La/Sm) _N range from 2.72-4.78 and total REE of 159.29 – 444.83 (Table 1; Figure 12).

K-Feldspar Rich Granites

SiO₂ content in K-feldspar rich granites ranges from (76.60-79.70 wt%), Al₂O₃ (11.85-13.30 wt%), K₂O (3.74-5.15 wt%), Na₂O (3.82-4.19 wt%) and Fe₂O₃ (0.95-1.36 wt%). Low concentrations of MgO, CaO, TiO₂, and MnO were measured. LOI range from 0.44-0.51 (Table 1). The concentration of P₂O₅ was below the detection limit (Table 1). The average total alkali is 8.60% and the average K₂O/Na₂O value is 1.14%. The variations of major oxides with SiO₂ as fractionation index on a Harker diagram show that Al₂O₃, MgO, Fe₂O₃, and K₂O decrease with increasing SiO₂ while CaO, TiO₂, and Na₂O show a positive correlation (Figure 9). Trace elements such as Ba, Sr, V, and Rb show a decrease with increasing SiO₂ (Figure 10) while values of K/Rb range from 7.88-12.35, Ba/Rb range from 0.06-0.67, and Rb/Sr range from 5.69-59.6 respectively (Table 1). The primitive mantle normalized multi-element variation plot (Sun and McDonough, 1989) show an enrichment in Th, U, Nd, Pb, Sm, and a depletion in Ba, K, P, Rb, and Ti (Figure 11). On a chondrite-normalized REE diagram normalized using the values of Boynton (1984), the Kfeldspar rich granite shows a slight enrichment of the HREE relative to MREE and LREE, with a distinct negative Eu-anomaly (Eu/Eu* = 0.02–0.41). The fractionated (La/Yb) N ratios in the k- feldspar rich granites range from 0.52 to 0.86, (La/Sm) N range from 0.74 – 1.16 and the total REE ranged from 99.81 – 178.42 (Table 1; Figure 12).

Petrochemistry and Petrogenesis of the Granitoids

Major, trace, and rare earth elements geochemical signatures of granitic rocks are vital in deciphering their petrochemistry and petrogenesis (Clemens et al., 2011; Chappell et al., 2012; Clemens and Steven, 2012; Ngatcha et al., 2019). The granitoids in Kaiama comprise porphyritic granites, K-feldspar rich granite, and porphyroblastic granite gneisses that define a NE-SW orientation with intrusions of quartz and aplite veins. Based on the petrochemistry of the rocks, the SiO₂ content is highest in K-feldspar rich granites (SiO₂=76.6-79.9%) while porphyritic granite and granite gneiss records SiO₂ values of between (71.9-73.8%) and (63-69.2%) respectively. The moderate to high silica and aluminum values of the granitoids correlates to the abundance of quartz, biotite, and feldspars. Fe₂O₃ and MgO content is higher in granite gneiss relative to K-feldspar rich granite and porphyritic granite this may be due to the higher amount of mafic minerals such as pyroxene, amphiboles, biotite, and opaques as well as the presence of abundance mafic enclaves observed on the rocks. Na₂O and K₂O are relatively higher than CaO in all the analyzed samples highlighting the felsic nature of the rocks. Harker’s variation diagram exclusively used to study the evolution of granitoids based on the variations of major oxides with SiO₂ as fractionation index shows that Al₂O₃, MgO, Fe₂O_t, CaO, and TiO₂ oxides decrease with increasing SiO₂ suggesting fractional crystallization of mafic minerals and accessory minerals phases such as ilmenite or titanite from the melt during magmatic differentiation (Rollinson, 1993). The relationship of Na₂O with SiO₂ shows a positive trend that may suggest plagioclase fractionation while K₂O versus SiO₂ show a scatter with no clear correlation which according to Khalaji, (2007), maybe due to contamination or assimilative reactions of the melt that brings about inhomogeneity in granitic rocks.

On a plot of total alkali (Na₂O+K₂O) versus silica (SiO₂) of Middlemost, (1994), used for the nomenclature of granitic rocks, both K-feldspar rich granites and porphyritic granites plot mainly in the field of granite while granite gneiss plots partly as granite but mostly in the field of Quartz monzonite (Figure 13). Further confirmation based on the normative mineral compositions of anorthoclase-albite-anorthite feldspar triangle of O’ Connor, (1965), also plots the rocks in the field of granites and quartz monzonite respectively (Figure 14). This is probably due to their high K₂O and Na₂O content (Table 1). The granitoids plot within the sub-alkaline field of Na₂O + K₂O vs SiO₂ diagram of (MacDonald and Katsura, 1964) and in the calc-alkaline series field on an AFM diagram of (Irvine and Barager, 1971) (Figures 15 and 16). Using the molecular ratio of alumina versus alkalis plot after Shand (1943), the granites were classified as S-type granite and of peraluminous affinity (Figure 17). According to (Chappell and White 1974), S-type granites, are orogenic granites, that are created as a result of continent-to-continent collisions and formed by melting or ultra-metamorphosing (Chukwu and Obiora, 2021). In order to determine the protolith of the S-type sedimentary/metasedimentary source, the plot of Alther et al., (2000) was used and a mixed source was identified that involves partial melting of metagraywacke and metapelitic sources (Figure 18). Chappell (1999) asserts that a metasedimentary source often generates peraluminous magmas that are high in alumina and low in alkali concentrations, which is also consistent with the findings of this study.

Figure 13. Na₂O+K₂O versus SiO₂ chemical classification diagram for the granitoids (after Middlemost, 1994).

Figure 14. Normative mineral composition of the granitoids after (O’Connor, 1965).

Figure 15. K₂O+Na₂O versus SiO₂ plot of MacDonald and Katsura (1964), showing the sub alkaline compositions of the rocks.

Figure 16. AFM classification plot after Irvine and Barager, (1971) Alphabets stands for (A: total alkalis, F: total Iron, and M: MgO).

Figure 17. A/NK against A/CNK diagram after Shand, (1943) discriminating the granitoids as S-type and of peraluminous compositions.

Figure 18. Molar CaO/FeOt+MgO vs Al₂O₃/FeOt+MgO of granites in the study area (after Alther et al.,2000).

Trace elements Ba, Sr, V, and Rb plotted against SiO₂ revealed a typical calc-alkaline trend characterized by decreasing Ba, Sr, and V with increasing SiO₂ while Rb increases with increasing SiO₂ (Chukwu and Obiora, 2021). The enrichement of trace elements such as Th, U, Nd, Pb, Sm in Kaiama granitoids is a common signatures associated with Pan-African granitic rocks in most parts of Nigeria (Goodenough et al., 2014; Akoh et al., 2015; Chukwu and Obiora, 2021), and considered as signatures of magma generated from crustal sources due to partial melting of crustal materials (Wedepohl 1995; Clemens and Stevens, 2012). The depletion of Ba, together with negative Eu anomalies may be attributed to the removal of plagioclase feldspar from the fractionating melt (Rollinson, 1993). This is because barium tends to remain in the solid phase during partial melting and would be most abundant in the higher melting temperature fractions, while liquid phases associated with magmatic activity have relatively low Ba concentrations (Imeokparia, 1981). The depletion of Nb, P, and Ti may be due to the fractionation of these elements in mineral phases, as ilmenite, rutile, or titanite, which may have persisted in the source (Ngatcha et al., 2019; Oljira et al., 2022). The depletion of Ti and Nb is controlled by titanium bearing minerals such as titanite, ilmenite, rutile, garnet, and also some amphiboles (Oljira et al., 2022). While the depletion in P reflects the segregation of accessory minerals such as apatite from the melt (Ngatcha et al., 2019). The binary plot of MgO versus FeOt after Zorpi et al. (1989) further reveals the partial melting of the crustal material (Figure 19). The rare-earth elements pattern of the granitoids is similar indicating that they are probably co-genetic. The moderate to high REE pattern and pronounced negative Eu-anomaly also indicate a moderate fractionation or preservation of plagioclase from the source magma (Rollinson, 1993; Ojira et al., 2022). According to Frost et al., (2001), granitic rocks related to crustal processes are characterized by moderate to high fractionated REE patterns and discernible negative Eu anomalies. This is in accordance with the REE pattern of the granitoids of the present study which further suggest a crustal source for granitoids.

Figure 19. FeO_t versus MgO diagram after Zorpi et al., (1989) for
the granitoids of the study area.

Tectonic Setting of the Granitoids

Several studies have shown that immobile trace elements such as Rb, Y, and Nb are very useful in determining the tectonic settings of granitoids magma due to their relative stability during alteration and metamorphic processes (Pearce et al. 1984; Harris et al. 1986; Saunders et al., 2014). Others, like Zhou et al. (2014), suggest that such immobile trace elements should be used carefully given that they might reflect instead the genesis of the protolith rather than the derived magma. Pearce et al., (1984) classified tectonic settings of granitoids through discriminative plots involving immobile trace elements such as Rb, Y, and Nb classifying them into fields of Volcanic arc granite (VAG), Syncollisional granitoids (Syn-COLG fields), Within plate granitoids (WPG fields) and Oceanic ridge granitoids (ORG fields). Applying discriminant plots of Pearce et al., (1984), the granites (porphyritic granites, K-feldspar rich granites, and granite gneiss) of Kaiama plots dominantly in the WPG and VAG fields (Figures 20a and 20b) which suggests that they were formed in a collisional tectonic setting (Ojira et al., 2022). A typical collision tectonic setting according to (Stammeier et al. 2015) is characterized by magmatism, compressional deformations, and extensive high-grade metamorphism. Pearce, (1996) was also of the view that such settings are characterized by subduction of continental crust, crustal thickening, late-post orogenic collapse, and premature within-plate setting. The granitoids of Kaiama are S-type, calc-alkaline, peraluminous, and enriched in Th and U, while they are depleted in Ba, Nb, Sr, P, and Ti. Such characteristic according to (Taylor and McLennan, 1995; Ngatacha et al., 2019 and Ojira et al., 2022) is typically associated with volcanic arc granites (VAG) and within plate granitoids (WPG) tectonic settings. Generally, the tectonic setting of the granitoids of the present study is similar to those identified by several other researchers within the basement complex of Nigeria (Ferré et al. 2002; Obiora and Ukaegbu, 2008 and Goodenough et al., 2014) which were emplaced during crustal aggregation (650 ± 100ma) between a passive west African craton and the Tuareg shield. Generally, the geochemical features of the granitoids of the study area show that they were derived by the partial melting of metasedimentary/ sedimentary materials in a typical collisional tectonic setting.

Figure 20. a). Tectonic setting discrimination of the granitoids on an Rb versus (Y + Nb) plot after Pearce et al., (1984). VAG stands for volcanic arc granites; ORG stands for oceanic ridge granites; WPG stands for within plate granites; Syn-COLG stands for syn-collision granites. b). Tectonic setting discrimination of the granitoids on an Nb versus Y plot after Pearce et al., (1984).

The field, petrographic and geochemical study of Kaiama granitoids show that they are predominantly composed of K-feldspar rich granites, porphyritic granites, and granite gneiss. The granitoids are of sub-alkaline composition, and show calc-alkaline affinity that is emplaced in a collisional tectonic setting. They are weakly peraluminous S-type granites produced by the partial melting of greywacke and pelitic materials. Their chondrite normalized rare earth element pattern shows moderate to high enrichment of LREE relative to HREE with a remarkable negative Eu anomaly suggesting plagioclase fractionation while the primitive mantle-normalized trace element patterns reveal an enrichment in Th, U, Nd, Pb, Sm, and depletion in Ba, K, P, Rb, and Ti which is characteristic of a crustal derived melt in a volcanic arc granite (VAG) or within plate granitoids (WPG) tectonic settings.

This publication is part of the Ph.D. thesis of the first author, funded by the African Union through the Pan African University Institute of life and Earth Sciences (including health and agriculture), PAULESI.

The author(s) declare no competing interests.