Geology and Rare Earth Element Geochemistry of Magnesian Granitoids Within Proterozoic Schist Belt of Southwest Nigeria

The granitoids of Okeho do not show a wide range of silica concentration (55.04–63.70 wt. %) as presented in Table 3a, despite their petrological contrast. SiO₂ values ranged from 55.04–59.77 wt. % for the syenite and 61.65–63.70 wt. % for the granodiorite. The respective average MgO and CaO values observed were 3.43 wt. % and 3.37 wt. % for the granodiorite and 5.05 wt. % and 5.59 wt. % for the syenite. K₂O values were observed to be higher than Na₂O values in both rock types (Table 3). In Figure 4, the sample plots of SiO₂ vs. Na₂O and K₂O suggest slightly varied mineral fractionation with the granodiorite being less enriched in alkali-rich mineral phases. This is also consistent with the observed modal and normative mineralogy (Tables 1 and 2).

Figure 4:

SiO₂ and Na₂O+K₂O concentrations, from major oxides presented in Table 3a, when plotted in the TAS plot by Middlemost [19] (Figure 5), characterised samples from the syenite pluton as dominantly monzonitic and samples from the granodiorite as granodioritic/dioritic to quartz-monzonitic. From the Cross, Iddings, Pirsson, and Washington (CIPW) normative mineralogy, an adapted QAP diagram [20] showed that the syenite samples are quartz-syenitic to quartz-monzonitic in character while the samples from the granodiorite are granodioritic (Figure 6). Granite lineages [21, 22] on the Streckeisen triangle were distinguished by Bowden [23] as CAT – calcalkaline tonalitic or trondhjemitic, CAG – calcalkaline granodiorite, SAM – subalkaline monzonitic, ALK – aluminous potassic, and ANA – alkaline soda. The characters of the syenite and granodiorite rock samples of the study are consistent with some of the generalised fields of granite lineages. The syenites share the subalkaline monzogranite (SAM) nature while the granodiorites share that of the calc-alkaline granodiorites (CAG).

Figure 5:

Figure 6:

Figure 7a is a plot of SiO₂ vs. FeO_(tot)/MgO [24] illustrating all granitoid samples plotted in the calc-alkaline field, and the adapted plot of Al₂O₃/SiO₂ vs. MgO/SiO₂ [25] (Figure 7b) suggests that the calc-alkaline primary magma is derived, probably, from partial melting of either mantle-derived mafic underplate or lower crust with crust contribution [26]. At low pressures, partial melting of tonalitic to granodioritic crust yields alkali-calcic to calc-alkalic granitoids that are metaluminous while peraluminous rocks are produced at high pressures [27]. These granitoids are magnesian (Figure 8a) with calc-alkali, alkali-calcic to alkalic characters (Figure 8b).

Figure 7:

Figure 8:

From Figure 8c, the plot of SiO₂ vs. K₂O, the primitive magma that possibly formed the granitoids of the study typifies the High-K calc-alkaline–Shoshonitic magma series. This character is consistent with the K-feldspar porphyritic texture granodiorite (KCG) type described by Barbarin [26]. High-K calc-alkaline granitoids are typical products of subduction zone processes [26, 28, 29].

The Aluminium Saturation Index (ASI) [32] is the molecular ratio Al/(Ca − 1.67P + Na + K) and accounts for excess Al that can be accommodated within feldspar crystal lattices and therefore accounts for the formation of other mineral phases, such as aluminous biotite in weakly peraluminous or muscovite, cordierite, garnet, or kyanite/andalusite/sillimanite (Al₂SiO₅ polymorphs) in strongly peraluminous rocks. The ASI for peraluminous rocks is greater than 1. When ASI < 1 and molecular Na+K < Al the rock has a metaluminous character, where Al is used up in the formation of feldspars and excess Ca is used in the formation of calcic mineral phases such as augite and hornblende. If ASI < 1 and Na+K > Al the rock is per-alkaline and hornblende is formed in weakly peralkaline or sodic amphiboles and pyroxenes are formed in strongly peralkaline [11]. The ASI value for each of the syenites is less than 1 (Table 3) and illustrated in the plot of A/CNK vs. A/NK (Figure 9) as samples from a pluton with metaluminous character.

Figure 9:

Samples of the granodiorite plotted in the metaluminous – peraluminous fields. ASI values > 1 were observed for the samples that plotted in the peraluminous field while sample (Pg3) with ASI < 1 plotted in the metaluminous field (Figure 9). Samples of the granodiorite with peraluminous character revealed normative corundum (Table 2) and suggest possible mineralogy consisting of muscovite, cordierite, and presence of aluminosilicate polymorph(s) in addition to those observed from thin section (Table 1).

The syenite body with a metaluminous character suggests that the pyroxene observed from the thin sections is possibly augite occurring in association with hornblende with normative sphene (Tn) (Table 2). The ASI calculation takes into account the presence of apatite [11] while the metaluminous character for the primary magma that formed the syenite pluton, suggesting excess Ca that possibly accounted for the formation of augite (clinopyroxene) and hornblende, agree with the plots of SiO₂ vs. CaO, MgO, FeO_(tot) and P₂O₅ where the samples of the granitoid showed a negative correlation (Figure 4) reflecting possible fractionation of apatite, Ca-rich pyroxenes (augite) hornblende.

Concentrations of Zr, Hf, Nb, Ta, Th, U, and REE representing high field strength elements (HFSE) based on ionic potentials were observed to be high (Table 3). Zirconium (Zr) concentrations in the granodiorite and syenite ranged from 431.70 ppm to 612.20 ppm and 324.10 ppm to 731.60 ppm, respectively. Hafnium (Hf) concentrations varied similarly. Niobium (Nb) concentrations, observed to be higher in the syenite, gave average values of 19.57 ppm and 36.94 ppm, respectively, for the granodiorite and syenite. Tantalum (Ta) values ranged from 1.00 to 1.20 ppm for the granodiorite and 0.90 to 3.30 ppm for the syenite. The Zr/Hf ratio ranging from 34.85 to 40.90 and Nb/Ta ratio ranging from 0.90 to 3.30 depicted by the syenite samples is representative of the granitoid. The trilinear plot of La-Y-Nb of Cabanis and Lecolle [33] showed that the Okeho granitoids are of calc-alkaline orogenic domain (Figure 10). Trace element plot using HFSE (Ta/Yb vs. Th/Yb) of Pearce [34] was adapted for this study and shows samples plotted in the volcanic arc domain and supports subduction zone as the environment of emplacement for the granitoids of the study (Figure 11). Geotectonic discrimination diagram for granites adopted for this study showed most samples plotted in the syn-collision granite field in the plot of (Ta+Yb) vs. Rb [12] (Figure 12a) while in the plot of Yb vs. Ta (Figure 12b) samples plotted around the boundaries of syn-collision granite––volcanic arc granite––within plate granite. The geotectonic diagram of Batchelor and Bowden [35] discriminated the granitoids of the study as syn- to post-collision granites (Figure 13). Samples from the granodiorite fell close to the syn-collision field while samples from the syenite fell mainly in the post-collision uplift field.

Figure 10:

Figure 11:

Figure 12:

Figure 13:

Copper (Cu), lead (Pb), and tin (Sn) mineralisation are commonly associated with granitic ore bodies. The average Cu concentration (Table 3) observed for the granodiorite (Cu, 25.6 ppm) compared well with Taylor and McLennan's [36] Upper Continental Crust (UCC) values (Cu, 25 ppm). In the syenite, an average Cu concentration of 41.07 ppm suggests Cu enrichment. Average UCC values for Pb (17 ppm) and Sn (5.5 ppm) are higher than Pb (7.30 ppm) and Sn (3.31 ppm) values observed for the granodiorite. Average Pb (20.83 ppm) and Sn (6.86 ppm) values observed for the syenite are higher than UCC average values, respectively. Possible gold (Au) enrichment may be found in the syenite, with an average Au value of 3.50 being almost twice the average Au value (1.80 ppm) reported by [36] for the UCC. Uranium concentrations in both rock types, syenite (6.27 ppm) and granodiorite (5.23 ppm), are observed to be higher than the average UCC uranium value (2.80 ppm).

Average REE values for the granitoids are presented in Table 3. The syenite of the study is enriched in LREEs while the granodiorite is enriched in heavy rare earth elements (HREEs). LREE/HREE ratio shows that the more metaluminous the granitoid is the higher the LREE/HREE ratio. LREE/HREE ratio ranges from 7.91 to 29.49 and 17.49 to 39.03 for the peraluminous granodiorite and the metaluminous syenite, respectively. Sample Pg3 from the granodiorite, which revealed the highest concentrations in K₂O (5.84 wt. %) and Ba (1629 ppm) and plotted in the metaluminous field (Figure 9), showed the highest LREE/HREE (29.49) value for the granodiorite of the study. The average values of total REE concentrations show that the syenite (625.06 ppm) is richer in REE than the granodiorite (364.51 ppm). The average REE_(TOT) for the granitoid of the study is 546.89 ppm (Table 3). LREE/REE ratio and REE_(TOT) for the granodiorite of the study compared well with the High K calc-alkaline–Shoshonitic granites from Nanling Range, Jiangxi Province, Southern China [37] and peraluminous calc-alkaline porphyry granite from Sanjiang Region [38]. Total REE values for the Dingnan and the Wuliting granites from the Nanling Range, Jiangxi Province had REE_(TOT) reaching 428.54 ppm and 344.27 ppm respectively. The average LREE/HREE ratio for the study (27.71) exceeded the highest values from Dingnan (15.00) and Wuliting (17.70). The LREE-enriched metaluminous syenite, however, showed enrichment in REE concentrations over some of the granite studied from China. The chondrite normalised plot after Boynton [39] (Figure 14) revealed REE enrichment in the granitoids with a negative europium anomaly.

Figure 14:

Yang [40] demonstrated that LREE behaviour varies depending on the nature of the magmatic fluid while HREE is fluid-incompatible. The evolution of the granitoids of the study based on the partitioning of LREE in the syenite over HREE in the granodiorite, therefore, suggests varied influence of magma fluxing during cooling, decompression, and degassing. The lithological setting (Figure 1) showing a syenite core with a granodiorite partial ring and the HREE enrichment in the granodiorite may favour an earlier crystallisation of the granodiorite from a melt-rich magma while the syenite core crystallised sometime later from a fluid-rich magma. LREE enrichment can be demonstrated to be related to the fractionation of Ca-rich minerals such as clinopyroxenes and titanite. The plot of CaO vs. LREE/HREE (Figure 15a) showed the syenite, with modal compositions of diopside (Di) and sphene (Tn), having a negative correlation of LREE enrichment, as CaO concentration decreased. Figure 15a revealed an inflection during REE fractionation in the granitoids. With all the samples showing modal orthoclase, with the syenite having higher amounts, the plot of K₂O vs. LREE/HREE (Figure 15b) suggests LREE enrichment with increased fractionation of K-feldspar. (La/Yb)_N vs. (Eu/Eu*)_N plot discriminated the LREE enriched samples with metaluminous character from the HREE enriched peraluminous samples (Figure 15c).

Figure 15: