J. Nig. Soc. Phys. Sci. 5 (2023) 999 Journal of the Nigerian Society of Physical Sciences Tectonic Setting of the Syenite Around Igarra, Southwestern Nigeria: Constraints from Geochemistry F. C. Ugbea, O. E. Ominigbo ID b,∗, A. O. Akanoa aDepartment of Geology, Delta State University, Abraka, Delta State, Nigeria bDepartment of Earth Sciences, Federal University of Petroleum Resources, Effurun, Delta State, Nigeria Abstract Syenites are relatively rare within the Nigerian Basement Complex. As a result of their rarity, these rocks have been given less research attention over time and are consequently poorly understood. The syenitic rocks at Igarra were studied to ascertain their tectonic evolution using geochem- istry. Sampling was carried out using the survey-type geological field mapping approach. A total of 10 samples of syenitic rocks were collected for laboratory analyses. Compositionally, the rocks are intermediate with regards to SiO2 content (58.02% – 60.58%), having Al2O3 and alkali (Na2O + K2O) compositional ranges of 15.34% – 15.52% and 8.99% – 9.7% respectively. The sampled rocks are similar and consistent in their trace and rare earth elements concentrations (the only exception being Zr with values ranging from 4 ppm to 79 ppm). The rocks are relatively enriched in Ba, K, TI, and Sr but depleted in Tc, Nb, U, Hf, Yb, Te and Ta. The syenites also show fairly high ratios of Rb/Nb and Rb/Sr with mean values of 488.627 and 0.171 respectively. As seen from the geochemical analyses, the syenites around Igarra are high-K calcalkaline, alkalic to alkalic-calcic. The rocks are peraluminous in character as shown by the bivariate plot of A/NK vs. A/CNK. Sedimentary protolith with continental crustal parent magma is inferred for these rocks. The similarity and consistency of the trends of major, trace and rare earth elements is indicative of cogenetic origin for the rocks. The geochemistry and discrimination plots for the rocks indicate geodynamic setting ranging from orogenic to post-orogenic. A volcanic arc geotectonic setting is interpreted for the Igarra syenites, with magma emplacement and evolution thought to have been initiated during the late stages of the Pan-African reactivation and continued into post-orogenic times. DOI:10.46481/jnsps.2023.999 Keywords: Syenite, Tectonic setting, Geodynamic evolution, Pan-African orogeny, Nigerian basement complex Article History : Received: 21 August 2022 Received in revised form: 04 October 2022 Accepted for publication: 05 November 2022 Published: 24 February 2023 © 2023 The Author(s). Published by the Nigerian Society of Physical Sciences under the terms of the Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Communicated by: O. J. Abimbola ∗Corresponding author tel. no: +234 8075961910 Email address: ominigboedafe@gmail.com (O. E. Ominigbo ID ) 1. Introduction Syenites are relatively scarce in Nigeria, making them to attract less research attention. As such, these rocks are often given less research attention and are consequently poorly un- derstood. However, the syenites within the Nigerian Basement Complex are widely distributed amongst the crystalline coun- 1 https://orcid.org/0000-0002-3132-8856 https://orcid.org/0000-0002-3132-8856 Ugbe et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 999 2 try rocks – granites, gneisses, and schists [1 – 3]. The rarity of syenites relative to other crystalline rocks is not peculiar to the Nigerian Basement Complex as similar occurrence and dis- tribution pattern has been recognized elsewhere [4]. The oc- currence of syenites in Nigeria is restricted to two contrasting petrographic settings with a group associated with the Older Granites whereas the other member restricted to the Younger Granite series. The Older Granites syenites are characterized by significantly high concentrations of CaO, MgO and K2O, with K2O typically exceeding Na2O. On the other hand, the syenitic rocks of the Younger Granites show broader composi- tional ranges, often with Na2O/K2O ratios close to unity [2]. As a result of episodicity and prolonged duration of geolog- ical events, different geological settings with diverse geochem- ical and mineralogical attributes have evolved in the Basement Complex of western Nigeria. To be able to accurately deci- pher such diverse episodes of geological events and tectonic set- tings, detailed geological and geochemical studies are required [5]. More so, alkaline rocks typically occur in diverse geody- namic settings – post-orogenic, rift or intraplate tectonic envi- ronments. The understanding of these rocks and their tectonic settings provide significant insights into understanding mag- matic processes within and around the continental lithosphere [6]. Furthermore, it has been observed that by determining the petrogenesis of a rock suite from a given geological regime, it is possible to infer limits on the geodynamic conditions which were responsible within the mantle for the tectonic or crustal activity during the time of formation of the suite of rocks [7]. Although some appreciable works have been done with re- gards to geological field mapping and geochemistry of the Nige- rian Basement Complex, there remains a glaring need for more studies aimed at enhancing the current understanding of the geodynamic evolution of the crystalline rocks within the re- gion [8]. Notwithstanding the series of researches carried on the Igarra Schist Belt, there is scarcity of detailed studies on tectonic setting of the syenites in the area. It has been argued that such paucity of reliable data or studies has led to “individ- ual prejudices” with regards to the interpretation of the evolu- tion of the Basement Complex in Nigeria [8 – 9]. In view of the apparent knowledge gap regarding the syenitic rocks in Nige- ria, the present study seeks to investigate the tectonic evolution of the syenites around Igarra in south western Nigeria using evidences from geochemistry. Geochemical analyses were car- ried out using data derived from geological field mapping of the area of the study. The use of geochemical data for deciphering and reconstructing the earth’s history, processes and products is a widely acceptable practice in the Earth Sciences research community [5, 10]. This study is expected to give further in- sights into the geochemistry of the syenites in Nigeria. It is also expected that findings from the present study will contribute to- wards the overall understanding of the geology and evolution of the Nigerian Basement Complex. Figure 1: Simplified geological map of West Africa showing the position of Nigeria and its Pan-African Basement Complex, the Congo-Gabon Craton, the West African Craton and the Tuareg Shield [modified after 11 and 14] 2. Regional Geological Setting and Previous Studies on the Igarra Syenite The present study was carried out around Igarra in south- western Nigeria. The Igarra crystalline rocks are part of the Nigerian Basement Complex generally believed to have been impacted by the Pan-African orogenic event (650 Ma ± 150). The Nigerian Basement Complex is an essential component of the Pan-African Mobile Belt occurring east of the West African Craton and south of the Tuareg Shield, northwest of the Congo Craton (Figure 1). The Nigerian Basement Complex is widely thought to have resulted from the collision of the passive conti- nental margin of the West African Craton and the active Tuareg Shield (the Pharusian Belt). This orogenesis often referred to as the Trans-Saharan Pan-African Orogen led to the develop- ment of high-grade metamorphism, thrust-nappe, massive gran- ite plutons and late orogen-parallel tectonics [7, 11 – 12]. The Pan-African orogeny was accompanied by the emplace- ment of syn-tectonic to post-tectonic granites, adamellites, tonal- ities and granodiorites which are commonly referred to as the Older Granites [13 – 14]. The Older Granites are typically weakly foliated and porphyroblastic in places whilst being non- foliated and equigranular in others. The Older Granites vary from leucocratic to mesocratic in colour with diverse miner- alogical compositions, ranging from biotite granites, hornblende- biotite-granites, hypersthene granites, adamellites, syenites to garnetiferous granites and muscovite-biotite-granites [12]. Al- though the Older Granites generally have calc-alkaline attributes, the syenites within the Nigerian Basement Complex are typi- cally peralkaline [3]. [15 – 16] have reported on the hydrogeo- logical properties of parts of the southwestern Nigeria. The Igarra Schist Belt is thought to occur as a broadly N – S trending zone of low – medium grades metasediments and metavolcanics ranging from basic to ultrabasic [17]. As a result of the ubiquity of the Pan-African thermotectonic reactivation, the protolith age of the Basement Complex in Nigeria remains a subject of debate [18]. Although there is a general consensus that the entire Basement Complex in Nigeria was affected by 2 Ugbe et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 999 3 the pervasive Pan African reactivation, there remains a debate on the status of older orogenies within the basement as some authors have opined that traces of all pre-Pan-African events must have been obliterated. The policyclicity of the Nigerian Basement Complex has made the determination of the true age of the rocks rather more difficult [8]. Two distinct compres- sive stresses (NE – SW and E – W) have been reported to have affected the Igarra Schist Belt during the Pan-African event, with the relatively younger E –W event being more pervasive as structures related to it are more dominant within the base- ment [19]. According to [19], the syenite and granite in the Igarra area are characterized by E – W structures whereas com- pared to other rock types within the belt, the NE – SW structure are fairly absent. The area of the present study is located within the Igarra Schist Belt in southwestern Nigeria. Generally, the Igarra area is made up of several intrusive and metasedimentary rocks. No- table petrological units in the area include schist, marble, gran- ite, calc-gneiss, quartzite and pegmatitic rocks [17, 20]. As reported by [17], the Igarra syenites intrude the marble/calcic gneiss and the quartz-biotite/mica schist, and are considered to be relatively younger than the pegmatites of the Igarra Schist Belt. Greenschist to amphibolite grades of metamorphism have been reported for the Igarra Schist Belt. The greenschist and amphibolite facies are characterized by the presence of chlo- rite + quartz + epidote ± actinolite and hornblende + biotite + plagioclase ± quartz respectively [17]. The syenites around Igarra and other associated crystalline rocks within the Igarra Schist Belt have been the subjects of some previous studies [e.g.12, 17, 20 – 21]. However, apart from [21], most of the existing works on the area are largely regional in scope without paying close attention to the syenites outcropping around the area. More so, the study by [21] only dwelt on the petrochemical and geotechnical attributes of the syenites. The general petrology and structural characteristics of the granitoids around the Igarra have been reported by [17]. Again, their study was restricted to geological field occurrence and petrographic attributes of the rocks without paying atten- tion to the geochemistry of the rocks. Thus, the present study seeks to use geochemical evidence to ascertain the tectonic set- ting and evolution of the syenites of the area with a view to situating the rocks within the larger context of the geodynamic evolution of the Nigerian Basement Complex. 3. Research Methods Geological field mapping was carried out during which ten (10) fresh rock samples were collected for laboratory analy- sis. Field mapping and sampling were carried out following the survey-type geological field mapping technique described by [22]. Rocks were observed in their in situ conditions, their field relationships and mappable mineralogical attributes noted and then, fresh rock samples were collected from unweathered surfaces for laboratory analyses. Only unaltered rock samples were collected for analyses in order to ensure that such samples give the true geochemical signatures of the rocks. Prior to lab- oratory analyses, the samples were crushed in hardened steel Figure 2: Simplified geological map of Nigeria showing the area of the study [modified after 17] jaw crusher before being pulverized into fine powder using a disc mill. Sample preparation and analyses were carried out at the Activation Laboratories in Vancouver, Canada. Major ele- ments (Na2O, MgO, Al2O3, K2O, CaO and FeO) compositions were determined using thermal desorption-mass spectrometry. SiO2 concentration was analyzed using fusion-inductively cou- pled plasma; whereas Ti was analyzed using total digestion- inductively coupled plasma (TD-ICP). Loss on ignition (LOI) was determined for each of the samples using the gravimetric method. The trace elements and rare earth elements (REEs) were analyzed using total digestion-inductively coupled plasma. Sc was analyzed using the TD-ICP. For most of the major ox- ides, detection limit was 0.01% except for TiO2, and P2O5 which had detection limits of 0.0005% and 0.001% respectively. The trace elements and RREs were analyzed using a detection limit of 0.01 ppm. Trace elements and REEs analyzed with different detection limits include: Li, Ni, Pb (0.5ppm), Ba, Zr, V, Sn, Cr, Sc (1 ppm), Ag, Cs, Mo, TI, Eu, Re (0.05 ppm), Bi (0.02 ppm), Zn, Rb, Sr, Cu (0.2 ppm). Although high sensitivity of ana- lytical methods to elemental concentrations in rocks (like the ones adopted for the present study) is highly desirable in geo- chemical analyses, an excellent detection limit in itself does not necessarily translate into accuracy of analytical outcomes. To check for, and to minimize possible sources of error(s) there- fore, internationally certified reference materials were used for instrument calibration, method validation, quality control and comparisons as suggested by [10]. Rare earth element (REE) and trace element normalizations were done using the standard chondrite values and procedures given by [23] and [24] respec- tively. Those values were then used in plotting discrimination diagrams to ascertain the REE and trace elements patterns of the syenites. 4. Results The results of the geochemical analyses (major, trace and REE) are shown in Tables 1, 2 and 3 below. Alongside the 3 Ugbe et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 999 4 major elements composition, some important major oxides’ pa- rameters and variables (Na2O + K2O, N+K-C, A/NK, CNK, A/CNK, FeO + MgO, FeO/(FeO + MgO) and K2O/Na2O) have been included in Table 1. Similarly, some important trace ele- ments ratios and parameters (Rb/Sr, Rb/Nb, Rb/Zr and Y + Nb) have been included in Table 2. These major and trace elements parameters and ratios alongside other elemental compositions were used in plotting discriminant plots for the present study as well as form the basis for many of the inferences made. The rocks are generally intermediate with SiO2 ranging from 58.02% - 60.58%. There are significant Al2O3 and alkali (Na2O + K2O) enrichments with compositional ranges of 15.34% - 15.52% and 8.99% - 9.70% respectively. The trace elements distribution patterns are fairly consistently in all the samples except in Zr (4 ppm – 79 ppm). Generally, there is relative depletion of Tc, Nb, U, Hf, Yb, Te and Ta whereas there is a significant enrichment of Ba, Rb, K, TI and Sr. (Table 2). 5. Discussion 5.1. Classification of the Igarra Syenites The rocks were classified using different major elements. The application of major elements for the classification and nomenclature of igneous rocks is a widely acceptable practice in geochemistry [10]. As shown in Figure 3, the sampled rocks are syenitic. The total alkali versus silica content (TAS) classi- fication scheme for igneous rocks was adopted for the present study. Recommended by the International Union of Geologi- cal Sciences (IUGS), the TAS scheme is suitable for classifying unaltered volcanic rocks, with the total alkalis (Na2O + K2O) plotted against the silica contents (Figure 3). This classification is premised on the fact that both alkalis and silica are largely mobile in a variety of deuteric processes which commonly lead to the formation of secondary and post-magmatic feldspar [25]. The Igarra syenites are generally intermediate in chemical composition with regards to silica content (SiO2 ranging be- tween 58.02% - 60.58%). Similarly, the rocks are fairly en- riched in FeO, Al2O3 and MgO with values in the ranges of 5.01% - 5.66%; 15.34% - 15.52% and 3.62% - 4.78% respec- tively, giving the sampled rocks a magnesian characteristic (Fig- ure 4). The compositionally high concentration of Al2O3 and the comparatively lower values of K2O and Na2O (6.50% - 6.90% and 2.49% - 2.94% respectively) broadly give the syen- ites overall alkalic to high-K calcalkaline and peraluminous sig- natures recorded in the bivariate plots (Figure 5a – 5d). Com- positionally, the major oxides data obtained from the present study compare closely with earlier ones reported for the Igarra syenites [21]. Meanwhile, compared to the nepheline syenites around the Oyioba-Uganga area in southern Benue Trough as reported by [26], the Igarra syenitic rocks are enriched in CaO and K2O but depleted in Na2O and Al2O3. However, the syen- ites in both areas are compositionally comparable in terms of TiO2, MnO, FeO and P2O5. The high-K calcalkaline characteristics of the Igarra syen- ites are compositionally different from those of the Kanoma and Sabon Gida plutons in northwestern Nigeria which [3] reported Figure 3: TAS classification of the Igarra syenites [boundaries modified after 11] Figure 4: FeO/(FeO + MgO) vs. SiO2 classification plot for the Igarra syenites [after 11] as peralkaline. The high-K calalcaline, relatively high CaO and MgO as well as K2O > Na2O shown by the Igarra syenitic rocks however, closely match the geochemical signatures typical of the syenites associated with the Older Granites in Nigeria as earlier reported by [2]. 5.2. Petrogenesis The correlation of trends of major oxides against SiO2 on the Hacker diagram has been used to infer homogeneity or het- erogeneity of rock protoliths [14, 27]. As shown in Figure 6, there is a strong negative correlation for all the major oxides plotted. The consistency in trend clearly suggests a common protolith or petrogenetic history for the Igarra syenite. Ac- cording to [27], a strong negative correlation of FeO3, Al2O3 and Na2O on the Hacker diagram is indicative of hornblende and pyroxene fractionation. As such, the significantly strong negative correlation of the major oxides against SiO2 could be inferred to be indicative that fractional crystallization played a vital role in the evolution of the syenite. The peraluminous 4 Ugbe et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 999 5 Table 1: Major elements compositions of syenites around Igarra Oxides (wt. %) Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7 Sample 8 Sample 9 Sample 10 S1O2 59.08 60.09 59.68 58.95 58.02 58.90 60.25 59.45 60.58 60.51 T1O2 0.82 0.70 0.68 0.76 0.98 0.80 0.76 0.84 0.74 0.88 Al2O3 15.42 15.36 15.38 15.50 15.45 15.49 15.40 15.37 15.52 15.34 FeO 5.45 5.27 5.23 5.40 5.66 5.20 5.01 5.31 5.09 5.15 MgO 4.78 4.05 3.98 4.21 4.42 3.70 3.68 3.74 3.62 3.64 Na2O 2.49 2.65 2.59 2.75 2.90 2.94 2.77 2.67 2.82 2.78 K2O 6.50 6.54 6.61 6.80 6.80 6.90 6.69 6.72 6.66 6.70 CaO 3.79 4.12 3.96 3.97 3.95 3.80 3.74 3.70 3.69 3.64 P2O5 0.30 0.35 0.33 0.34 0.51 0.44 0.36 0.40 0.38 0.42 MnO 0.08 0.06 0.05 0.15 0.28 0.09 0.07 0.12 0.09 0.10 LOI 0.84 1.16 1.01 0.97 0.90 1.18 1.03 0.93 0.98 1.04 Total 99.55 100.35 99.45 99.80 99.87 99.44 99.76 99.25 100.17 100.20 Na2O+ K2O 8.99 9.19 9.2 9.55 9.70 9.84 9.46 9.39 9.48 9.48 N + K – C 5.20 5.07 5.24 5.58 5.75 6.04 5.72 5.69 5.79 5.84 A/NK 1.72 1.67 1.67 1.62 1.59 1.57 1.63 1.64 1.64 1.62 CNK 12.78 13.31 13.16 13.52 13.65 13.64 13.20 13.09 13.17 13.12 A/CNK 1.21 1.15 1.17 1.15 1.13 1.14 1.17 1.17 1.18 1.17 FeO + MgO 10.23 9.32 9.21 9.61 10.08 8.90 8.69 9.05 8.71 8.79 FeO/(FeO + MgO) 0.53 0.57 0.57 0.56 0.56 0.58 0.58 0.59 0.58 0.59 K2O/Na2O 2.61 2.47 2.55 2.47 2.34 2.35 2.42 2.52 2.36 2.41 Table 2: Trace elements concentrations in (ppm) of syenites around Igarra Element/ Sample1 Sample2 Sample3 Sample4 Sample5 Sample6 Sample7 Sample8 Sample9 Sample10 Ratio ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm Sr 1200 904 950 1200 1100 1300 942 1200 1200 935 U 1.4 1.5 1.5 1.6 1.4 1.5 1.7 1.4 1.4 1.3 K 44500 23400 24600 25700 36300 30000 30000 30000 25500 26500 Rb 194 152 162 151 194 182 241 210V 167 185 Cs 5.1 5.26 5.44 5.26 5.31 4.94 6.69 4.88 4.64 4.45 Ba 4130 3250 3460 3360 3900 3420 3440 3530 3990 2990 Th 5.3 6.5 6.7 8.4 6 8.4 8.8 6.8 6.4 6.5 Ce 66.2 64.2 68.8 65.9 67 67.9 65.1 66.9 73.8 66.2 P 3490 2800 2320 2600 4360 4200 4120 4430 3240 3000 Ta 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Nb 0.9 0.3 0.1 0.1 1.3 2 1.5 2.9 1.7 0.4 Te ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 Tc ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 ¡0.1 Zr 43 60 4 6 61 56 58 79 28 10 Hf 0.6 0.8 0.1 0.1 1.1 1.2 1.4 1.8 0.6 0.1 TI 2570 1730 1070 1780 3760 4080 4080 4130 1840 1270 Y 19 18.4 19.2 17.8 19.7 19.8 18.3 19.2 20.7 17.3 Yb 1.8 1.8 1.9 1.8 1.9 1.9 1.9 1.9 2 1.8 Y + Nb 19.9 18.7 19.3 17.9 21 21.8 19.8 22.1 22.4 17.7 Rb/Sr 0.162 0.168 0.171 0.126 0.176 0.14 0.256 0.175 0.139 0.198 Rb/Nb 215.556 506.667 1620 1510 149.231 91 160.667 72.414 98.235 462.5 Rb/Zr 4.512 2.533 40.5 25.167 3.180 3.250 4.155 2.658 5.964 18.5 5 Ugbe et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 999 6 Table 3: Rare-earth element concentrations in (ppm) of syenites around Igarra Element Sample1 Sample2 Sample3 Sample4 Sample5 Sample6 Sample7 Sample8 Sample9 Sample10 ppm ppm ppm ppm ppm ppm ppm Ppm ppm ppm La 35.4 34.1 36.9 36 36.4 37.7 36.3 36.4 39.8 30 Ce 66.2 64.2 68.8 65.9 67 67.9 65.1 66.9 73.8 66.2 Pr 8.1 7.8 8.3 7.8 8.2 8 8.1 8.1 8.4 8.2 Nd 31.1 30.1 31.3 30.1 31 30.8 29.8 31.9 32.8 31.1 Sm 6.1 5.9 6.1 5.4 6.4 5.7 6 6.5 7 6.4 Eu 1.66 1.53 1.62 1.48 1.62 1.58 1.51 1.6 1.72 1.65 Gd 5.2 5 5.1 4.9 5.5 5.2 5 5.1 5.7 5.3 Tb 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.7 0.7 0.6 Dy 3.5 3.5 3.6 3.5 3.8 3.6 3.6 3.6 3.6 3.7 Ho 0.7 0.7 0.7 0.6 0.7 0.7 0.7 0.7 0.8 0.7 Er 1.9 1.8 2 1.9 1.8 1.9 1.8 1.9 1.9 1.9 Tm 0.3 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Yb 1.8 1.8 1.9 1.8 1.9 1.9 1.9 1.9 2 1.8 Lu 0.3 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.3 0.3 Figure 5a: Modified alkali-lime index, MALI vs. SiO2 classifi- cation of the Igarra syenites [boundaries modified after 11] signature of the rock suggests the melting of sedimentary pro- toliths for the Igarra syenites. Basement rocks derived from sedimentary sources are characterized by peraluminous chemi- cal attributes [11, 14, 28]. Trace elements and REEs have also been widely used as “fingerprints” in understanding the origin and evolution of ig- neous rocks [e.g. 2, 29 – 31]. Ideally, the compositional de- pletion of Tc, Nb and Ta alongside enrichment of large-ion lithophiles (Rb, K, Sr and Th) suggests a partial melting of crustal origin for the syenites [32]. Similarly, the moderate to high fractionations and pronounced negative Eu anomalies displayed in the REE patterns are typical of crust-derived gran- ites [28]. The negative Eu anomaly exhibited by the samples is suggestive of either plagioclase fractionation or its retention during the melting of the parent magma. More so, the relatively high values of Rb/Sr and Rb/Nb ratios further support the litho- spheric crustal origin for the syenite. Mantle-derived magmas Figure 5b: K2O vs. SiO2 classification of the Igarra syenites [boundaries modified after 11] are characterized by low Rb/Sr ratios (usually ≤ 0.023) just as high Rb/Nb ratios are indicative of crustal origin [33]. How- ever, the intermediate compositional concentration of SiO2 in the rock poses some doubts to the crustal origin assertion as crust-derived magmas typically record silica content in excess of 60% [28] unlike the sampled rock that gave ≤60%. Consid- ering the small sample size used for the present study, it is ad- visable that the crustal origin suggested for the syenitic rocks at Igarra should be taken with caution, albeit it is safe to conclude that the rocks had common source magma and evolutionary his- tory based on the consistency and similarities in composition and distribution pattern, both for the major and trace elements. 5.3. Tectonic Setting The syenitic rocks around Igarra are thought to have been emplaced in an arc environment following a major orogenic event based on the discrimination plots for the tectonic setting (Figures 8a – 8c). Whereas Figures 8a – 8c clearly indicate that 6 Ugbe et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 999 7 Figure 5c: Na2O + K2O classification plot for the Igarra syen- ites [boundaries modified after 11] Figure 5d: A/NK vs. A/ACNK plot for the Igarra syenites [boundaries after 11 and 14] the emplacement and evolution of the syenites occurred in an active orogenic belt, Figures 8d and 8e suggest that the develop- ment occurred in more or less transitional setting between oro- genic and anorogenic geodynamic settings. Given that the area is part of the Basement Complex in Nigeria generally believed to have been affected by the Pan-African reactivation event, it is our opinion that the emplacement and evolution of the Igarra syenite is related to the Pan-African orogeny which led to the collision of the West African Craton and the Tuareg Shield. [11, 34] opined that the collisional event was preceded by the subduction of the lithosphere beneath an ancient oceanic crust around the eastern flank of the West African Craton underneath the Tuareg Shield. It is believed that it is this ancient oceanic crust that gives rise to the series of basic and ultrabasic rocks of ophilitic complex and high positive gravity anomaly within the Basement Complex of the eastern axis of the West African Cra- ton [11]. Given the orogenic to post-orogenic signatures shown from the discrimination plots for the tectonic setting, we pro- Figure 6: Hacker diagram (major oxides vs. SiO2) the Igarra syenites Figure 7a: Primitive-mantle normalized trace elements pattern for Igarra syenites [chondrite normalization values after 24] pose that although the emplacement of the Igarra syenite com- menced during the active phase of the Pan-African reactivation (probably towards the end of the orogeny), the formation and evolution of the rock continued into the early post-orogenic pe- riod. Similarly, the emplacement and formation of the syenites in the area of the study have also been interpreted to have occurred in a tectonic arc as clearly revealed in Figures 8c – 8f. Given the significant negative Eu anomalies displayed by the REEs and the relatively high ratios of Rb/Nb, Rb/Sr and Rb/Zr, an active continental arc tectonic setting is inferred for the Igarra syen- ite. The active continental tectonic setting is further supported by the plot of K2O/NaO vs. SiO2 (Figure 8b). Generally, 3 7 Ugbe et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 999 8 Figure 7b: Chondrite-normalized REE diagram for the Igarra syenites [normalization values after 23] Figure 8a: FeO + MgO vs. SiO2 plot for the Igarra syenites [boundaries after 14 and 35] geotectonic models have been proposed for the formation and evolution of syenites gobally: those associated with attributes of mantle evolution; those linked to crustal origin; and those associated with mantle-crustal origins for parent magmas [4]. The crust-derived syenites model proposes melting of crustal substrates with the rocks showing geochemical imprints similar to those recorded in the present study [4]. The arc geodynamic setting inferred for the Igarra syen- ite is in agreement with existing knowledge on the Basement Complex rocks of south western Nigeria which are thought to have developed in a back-arc basin [37 – 39]. The initiation of magmatism during active orogeny and its continuation well into post-orogenic periods have also been reported for basement rocks elsewhere within Nigeria [e.g. 11, 14, 40]. It is hoped that findings from the present study will spur further research interests in the syenites in Nigeria just as integrated approach of combining geological and mechanical attributes is recom- mended for such studies. Figure 8b: K2O/Na2O vs. SiO2 plot for the Igarra syenites [after 14 and 35] Figure 8c: Al2O3 vs. SiO2 tectonic setting bivariate plot for the Igarra syenites [after 14 1nd 35] Figure 8d: Rb vs. Y + Nb bivariate for the Igarra syenites [after 11 and 14] 8 Ugbe et al. / J. Nig. Soc. Phys. Sci. 5 (2023) 999 9 Figure 8e: Rb vs. SiO2 discriminant plot for the tectonic setting of the Igarra syenites [boundaries modified after 14] Figure 8f: TiO2 vs. Al2O3 bivariate plot for the Igarra syenites [after 14 and 36] 6. Conclusions The syenites around Igarra in southwestern Nigeria were examined to ascertain their tectonic setting and petrogenetic history using evidences from geochemistry. The data and resul- tant bivariate plots for the Igarra syenites show that the rocks are high-K calcalkaline, alkali to alkali-calcic and peraluminous. Based on the peraluminous character of the rocks, the melt- ing of sedimentary protolith is inferred, with primary magma probably derived from the lithospheric crust. The syenites were emplaced and formed in an arc tectonic environment during the Pan-African reactivation event and continued into the early post-orogenic periods. 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