Professional paper Received: February 8, 2013 Accepted: February 18, 2013 Mineralogical and chemical characterization of major basement rocks in Ekiti State, SW-Nigeria Mineraloške in kemične značilnosti glavnih kamnin podlage v državi Ekiti v JZ Nigeriji Abel O. Talabi1' * 1Ekiti State University, Faculty of Science, Department of Geology, Ado-Ekiti, Nigeria Corresponding author. E-mail: soar_abel@yahoo.com Abstract Mineralogy and chemical composition of rocks constitute a reliable means of rocks' classification. Mig-matite which occur in association with quartz schist/ quartzite, Pan African Granites and charnockitic bodies around Ekiti State, southwestern Nigeria, were studied with a view to elucidate their mineralogical, compositional characteristics and their evolution. Mineralogical determinations from optical studies revealed a high proportion of granular quartz and accessory muscovite in the quartz schist/quartzite. The migmatite on the other hand comprises mainly of quartz and feldspar (volume fractions <70%) with minor muscovite, biotite and opaque minerals. Both Pan-African granite and charnockitic rocks have similar mineralogical composition with quartz and feldspar as dominant minerals. However, the charnockites have more mafic minerals compared to the granites. Chemical analysis of the rocks involving major elements revealed the silicieous nature of all the rock units (mig-matites, quartz-schist/quartzite, granites and the char-nockites) in the study area. Three oxides (SiO2, Al2O3 and Fe2O3) constitute about 70-75 % of the chemical composition in migmatites, granites and the charnock-ites while in quartz-schist/quartzite, it was over 80 %. The chemical compositions as well as different variation plots suggest sedimentary origin for the granite, migmatite and quartz-schist/quartzite rocks while the charnockite has a preference for igneous source. The (Na2O+K2O) - CaO versus SiO2 plot indicated that majority of the rock units (migmatite, granite and charnock-ite) are in alkali-calcic and calcic-alkali categories while quartz-schist/quartzite is of calcic affinity. Key words: mineralogy, Ekiti State, optical studies, major elements, Igneous source Izvleček Mineraloška in kemična sestava kamnin je zanesljiva osnova za njihovo klasifikacijo. Razvit je migmatit v združbi s kremenovim metamorfnim skrilavcem/ kvarcitom. V študiji so preiskovali panafriške granite in charnokitska telesa z namenom pojasniti značilnosti njihove mineraloške in kemične sestave ter njihov razvoj. Z mineraloško optično preiskavo so ugotovili v kremenovem metamorfnem skrilavcu/kvarcitu velik volumenski delež zrnatega kremena in akcesorni muskovit. Po drugi plati vsebuje migmatit predvsem (<70 %) kremen in glinenec s podrejenim muskovitom, biotitom in neprozornimi minerali. Panafriški graniti in charnokitske kamnine imajo podobno mineraloško sestavo s prevladujočima kremenom in glinencem, vendar vsebujejo charnokiti v primerjavi z graniti več mafičnih mineralov. Kemične analize glavnih elementov so razkrile silicij-sko naravo vseh kamninskih enot (migmatitov, kreme-novega metamorfnega skrilavca/kvarcita, granitov in charnokitov) na raziskovanem območju. Trije oksidi (SiO2, Al2O3 and Fe2O3) obsegajo kar 70-75 % kemične sestave migmatitov, granitov in charnokitov, medtem ko je ta delež pri kremenovem kristalastem skrilavcu/ kvarcitu več kot 80 %. Kemična sestava in različni vari-acijski diagrami nakazujejo sedimentno poreklo granitnih, migmatitnih in kremenovo metamorfno skrilavih/ kvarcitnih kamnin in prednostno magmatski vir char-nokita. Diagram (Na2O+K2O) - CaO s SiO2 prikazuje, da pripada večina kamnin (migmatit, granit in charnokit) alkalijsko-kalcijevim in kalcijsko-alkalijskim kategorijam, le kremenov kristalasti skrilavec/kvarcit kalcijski. Ključne besede: mineralogija, država Ekiti, optične preiskave, glavne prvine, magmatsko poreklo Ekiti State in Southwestern Nigeria is underlain primarily by the basement complex rocks of Pre-Cambrian age comprising gneisses and migmatites, quartz-schist/quartzite, Pan-African granites and charnockite with migmatites covering over 50 % of the whole area (Talabi & Tijani, 2011]. Most rocks are made up of major and trace elements. These rocks components are subjected to intensive weathering processes especially in the tropical area (like Ekiti State] with subsequent release of regolith products into the groundwater of the area. The weathering processes are charachterized by intensive de-silisification and ionic lost from the rock forming minerals. Crystalline rocks are formed by interlocking silicate minerals such as quartz, feldspars, micas, hornblende, pyroxenes, olivine and a host of minor accessories. Chemical weathering involves the dissolution of these minerals resulting in the formation of both soluble as well as solid phase products. History of the various transformations are documented in form of mineralogical and chemical dynaism of the rock. A deligent search revealed scanty literature on the subject matter regarding the study area. Many of the research works in the study area were on geophysical assessment of the groundwater potential and hydrochemistry. Owoade et al. (1989] worked on hydrogeology and water chemistry in the weathered crystalline rocks of southwestern Nigeria and concluded that kao-linite was found to be the stable clay weathering product and that groundwater resides in the weathered regolith in the area. Bolarinwa & Elueze (2005] also reviewed the geochemical trends in the weathered profiles above granitic gneiss and schist of Abeokuta area, southwestern Nigeria. The research indicated that the Fe2O3 rich laterite is low in Na2O, K2O, CaO and MgO due to the removal of alkaline and earth elements, through leaching of the topsoil and laterite. Emofurieta & Salami (1993] while looking on the geochemical dispersion patterns associated with laterization process at Ile-Ife reported that the soils derived from the melanocratic bands are SiO2 rich, compared to soils derived from the leucocratic bands. Based on their average SiO2/Al2O3 + Fe2O3 ratios, the soils derived from the melanocratic bands are lateritic whilst the leucocratic derivatives are non laterite. This study therefore was to characterise the major basement rocks in Ekiti state using mineralogy and chemistry of the rocks. Study area Ekiti State is located between latitudes 7° 15'-8°5' N and longitudes 4°44'-5°45' E covering an approximate area of about 6 353 km2 (Figure 1). The area lies entirely in the tropical climate with two distinct seasons; rainy and dry seasons. These two seasons have elastic boundaries in view of the recent global climatic variability. However, in general, on yearly basis, the rainy season commences in April and terminates in October while the dry season spans through November to March. The temperature is high throughout the year (mean annual temperature is 27 °C). The relative humidity is high (60-80 %) most months of the year while the mean annual rainfall is 1 500 mm. The study area is located in a hummocky terrain having a well pronounced undulating topography with prominent hills characterized by steep slope with elevation between 200 m and 500 m above mean sea level. Prominent hilly features include inselbergs, whalebacks and other categories of residual hills which are commonly associated with massive granite bodies. The inselbergs are striking feature of the Pan African granites occurring as picturesque prominent hills, rising sharply above their surrounding plains. Geology of the study area The study area is an Archaean-Early Proterozo-ic terrain underlain by the basement complex of southwestern Nigeria (Clark, 1985, Alagbe & Raji, 1990, Oyinloye & Ademilua, 2005). Furthermore, Oversby (1975) and Olarewaju (1981), indicated that the study area is composed of migmatite-gneiss-quarzite complex, with supracrustal rocks relics. In this study, geological appraisal through systematic mapping reveal the following distinguishable lith-ologies namely: Migmatite-gneiss, Quartzite/ Quartz-Schist, the Pan-African granite, Char-nockite, Aplite and Pegmatite (Figure 2). The 75 rocks are not evenly distributed but migmatites predominate, covering a greater proportion of the study area (Figure2). Material and method The principal aim of the study was to establish the basic mineralogy and fabric properties of major rock types in the study area. Twenty (20) fresh representative samples comprising of 5 samples each of migmatite, quartz-schist/ quartzite, granite and charnockite were subjected to thin section study. The nature of outcrop, colour, texture, mineralogy and structures were noted on the field. As for the geochemi-cal investigations, collected samples were dried at temperature of 60 oC, crushed using a jaw crasher and pulverized with the ball milling machine and sieved to 80 mesh. Elemental compositions of the rocks were determined using Atomic Absorption Spectrophotometer (Unicam 969 model). OSUN STATE Major Roads m Local government head quaters | State boundary Figure 1: Location map of Nigeria showing the study area (Talabi & Tijani, 2011). Ten (10) grams of each sample was weighed and put in a clean digestion bottle. Using a calibrated plastic syringe, 15 ml of 40 % hydrochloric acid was added with the help of an automatic pipette. Subsequently, ten (10 ml) of hydrofluoric acid was added. To avoid the escape of silicon fluoride (SiF4) gas during mixing of the two acids the digestion bottle was tightly closed. The digestion bottle was later put on a water bath and warmed up to 70 oC for about two hours and allowed to cool down to 25-30 oC. A 100 ml saturated boric acid was added to the solution and the bottle was closed tightened. The bottle was put on a water bath up to 70 oC until the milky solution became clear. Distilled water was added to it after cooling to make a solution of 250 ml; part of distilled sample was put in a sample container which was then analyzed with a dilution factor of 25. Major elemental oxides such as SiO2, Al2O3, K2O, Na2O, CaO, MgO, Fe2O3 and TiO2 were obtained using Atomic Absorption Spectrophotometer (Unicam 969 model) with a precision of +0.5. The geochemical results were subjected to variation plots to infer the petrogenesis of the rocks. L Ai/ 1! f TfcX.Tii' WMjdt Ei ¡J1 ■ 0 * IC-rn | _-iQiiartz vem | Porphyrinic granite | Medium coarse gr.ai , ' Granite I_| Undiff- migmatite-gneiss _jFine grained granite Charnockite I | Meta -sediment lUiidiff. Schisfphyffites | Massive Quartzite I Schist with, pegmatite ] Migmatite |-*'] Fracture Figure 2: Geologic Map of Study area (Talabi & Tijani, 2011). Results and discussion Field petrographic features Generally, geological appraisal through systematic mapping revealed outcrops of gneisses and migmatites representing highly denuded hills while quartz-schist/quartzite, Pan-African granites (porphyritic and fine-medium grained) and charnockite commonly form ridges, hills and whalebacks in the study area. The rocks are not evenly distributed but migmatites predominate, covering a greater proportion of the study area. The Pan-African granites comprise of felsic and mafic minerals. The felsic minerals contain quartz, orthoclase, plagioclase feldspar and muscovite. Quartz is colourless, white and occassionally grey in colour while orthoclase display white, pink or buff grey colour. Pla-gioclase feldspar is often white, pink, grey or dark grey coloured while muscovite is the flaky mineral of the mica groupdisplaying colourless colour. The mafic group comprise of the black coloured biotite andthe dark green to black hornblende. The biotite is differentiated from hornblende in terms of hardness. The former has a hardness range of 2-2.5 and can easily be scratched with a pen-knife while the later range of hardness is 5-6 on the Mohs scale of hardness. Gneisses and migmatites Gneisses and Migmatites cover over 65 % of the study area (Figure 2] into which the other successions of rocks have been emplaced. Typical outcrops of migmatites and gneisses in the study area are presented in Figures 3(a-d). Field observation revealed close structural relationship between quartzite and the intrusive granitic and charnockitic rocks. Migmatite rock exposures occur as highly denuded hills of essentially fine texture while the pegmatites are very coarse-grained with phenocrysts of feldspar over 2500 mm in length, usually of granitic composition and forming at a late stage of crystallization. In the study area, the migma-tite-gneiss rocks composed of a mafic portion, made up of biotite, hornblende and opaque minerals while the felsic portion is quartzo-feldspatic. Compositional variation in the rock outcrops are indicated by closely spaced alternating bands of leucocratic minerals (quartz and feldspars] and melanocratic minerals indicated by the preponderance of biotite minerals (Figure 3a]. The banded gneisses with alternating parallel light and dark coloured bands are common in the study area especially at Ado, Iworoko, Ikere, Ise and Emure. Figure 3 c shows drag folds of leucocratic veins and ptymatic structures while figure 3d represents a typical granite gneiss at Ado-Ekiti with alternating light and dark coloured bands. Quartz-schist/quartzite Quartz-schist/quartzite is a hard, non foliated metamorphic rock derived from sandstone during tectonic compression, a process where heat and pressure beneath the ground increases to form new rocks. Found on hills and mountains, quartzite endures little wear or decomposition based on its elevated locations. The quartz-schist/quartzite in the study area exhibits white to gray colour due to varied iron oxide content in the rock. A few good massive quartzite outcrops rising up to 100 mabove the surrounding terrain occur around Ado-Ekiti, while the ones at Ikogosi are highly schistose with muscovite flakes littering its environment. However, around the south western part of Il-awe-Ekiti, the quartzite rock exists as rubbles. Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. The nearly pure silica content of the rock provides little to soil formation and therefore the quartzite ridges are often bare or covered only with a very thin layer of soil and little vegetation. The Pan-African granites The Pan-African granites (ca. 600 Ma] occur as intrusions within the migmatite-gneiss-quartz-ite complex (Oyinloye, 2002, Oyinloye, 2011, Folorunso & Okonkwo, 2011, Omosanya et al., 2012 and Okonkwo & Folorunso, 2013]. The granitic rocks outcropped as domes and small hills in the area. The granites are distinguish-ably unique because of their visible minerals, lack of foliation, fine-medium grained texture (Figure, 3e] and compact interlocking crystals that developed during the crystallisation of magma. However, granitic rocks of porphyritic texture (Figure 3f) occur around Ado-Ekiti, Ikere-Ekiti and close to Ikole-Ekiti. Some of the outcrops occur as well-rounded boulders devoid of any preferred orientation of component minerals. The contact relationship of the Pan-African granites with the surrounding country rock are abrupt in few cases while most are gradational over very short distances. Generally, the porphyritic granite is light coloured with signs of having been fairly weathered. Charnockites The charnockitic rocks outcropped as pavement and oval or semi-circular hills of between five and ten meters (10 m) high with a lot of boulders at some outcrops. They are generally massive, dark-greenish in colour with medium to coarse grained texture. The fresh outcrops with little or no sign of weathering have a lot of quartz, aplite and pegmatite intrusions occurring in it. The general trend of the intrusions (e) (f) Figure 3: Typical rock outcrops from the study area [(a) Migmatite, (b) Migmitite outcrop with pegmatite intrusion, (c) and (d) Granite gneiss, (e) Fine-medium grained granite, (f) Porphyritic granite. fresh surfaces, it is difficult to distinguish the contact between the granite and charnockite because the feldspars of the granite have the same greenish colour as the feldspars in the charnockite. However, the colouration fades away with increased weathering of the charnockite. Megascopic examination with the aid of hand lens revealed the presence of quartz, alkali feldspar, plagioclase and biotite as major minerals in the charnockitic rocks in the study area. In summary, field observations highlight gneiss and migmatite, granite, quartz-schist/quartz-ite and Charnockite as major rock units in the study area with variable mineralogical composition. Granite contains more minerals that are suceptible to weathering i.e. high percentage of mafic minerals (biotite and hornblende] which got easily weathered because the iron (Fe) in their crystals structures can easily be oxidized. However, the quartz in such rocks will show mild resistance to weathering. The weathering of rocks and minerals are significant to mineral and chemical evolution of rocks. is N-S. The dominant trend of the joints that occur on the rock is N-S. The rock outcropped around Ado, Ikere, Otun, Ifaki, Itapa and Ikole areas. Two mode of charnockite occurrence have been revealed through field observation. The charnockites that occurr along the margins of Older Granites bodies especially the porphyritic granites as exemplified by the char-nockitic outcrops in Ado, Ikere and Igbara-Odo areas. The other mode of occurrence comprise of charnockites that aligned in a NW-SE direction as shown by the charnockitic rocks at Oye, Itapa and Ijelu areas. The contact relationship of the charnockites to the surrounding rocks is variable. In some places on the one hand, gradational contact was observed between the charnockites and the surrounding Older Granites while on the other hand the contact is abrupt from migmatitic and granitic gneisses to the charnockitic rocks. At Ifaki, the charnockites showed cross-cutting intrusive contacts with surrounding country rocks while at Ikere, the charnockites appear on slightly weathered surfaces said to have been broken up into xenolithic blocks by a foliated porphyritic granite. Furthermore, on the Microscopic evaluation of rocks in the study area Thin section petrography of the migmatite/ gneiss shows abundance of feldspar (micro-dine], quartz, muscovite, biotite and opaque minerals. Microcline occurs as large euhedral crystals exhibiting cross hatched twining, bio-tite content is low, occur in disoriented masses and mineral alignment is poorly developed. Other minor components include ferromagne-sian minerals like hornblende (Figures 4 & 5]. Quartz and feldspar alone constitute up to volume fractions 70 % of the rock in thin section (Table 1]. Feldspar is second to quartz in abundance while minerals such as garnet and magnetite constitute the opaque minerals. The quartzites/quartz-schist shows predominance of quartz, accessory muscovite and opaque minerals (Figures 6 & 7]. Quartz occurs as granoblastic and euhedral crystals with well-defined outlines. It exhibits weak birefringence, low relief with wavy extinction. Few grains however appear cloudy. Muscovite that forms supporting minerals occupy intergranular spaces of interlocking quartz crystals and often is the platy brightly coloured minerals. Quartz and muscovite constitute up to volume fractions 88 % of the rock in thin section (Table 2). Table 1: Modal composition of Migmatite (in volume fractions) & rH N M 4, "M 4) 4) 4) M ^ ^ ^ ^ rt aj a a a Si .5 S S S % « ta ta ta Quartz 43 45 44 44 Feldspar 25 26 27 26 Hornblende 8 10 9 9 Pyroxene - - - - Biotite 9 10 8 9 Muscovite 6 5 4 5 Opaque 9 4 8 7 Total 100 100 100 100 Figure 4: Photomicrograph of migmatite gneiss in trasmitted light showing quartz (1), plagioclase (2), hornblende (3), biotite (4) and opaque mineral (5). Bar scale is 2 mm crossed polars. Figure 5: Photomicrograph of migmatite gneiss in trasmitted light showing quartz (1), biotite (2), hornblende (3), microcline (4) and opaque mineral (5). Bar scale is 2 mm crossed polars. Figure 6: Photomicrograph of quartzite in trasmitted light showing quartz (1) and muscovite (2). Bar scale is 2 mm crossed polars. Figure 7: Photomicrograph of quartz-schist showing quartz (1), muscovite (2) and muscovite. Bar scale is 2 mm crossed polars. Table 2: Modal composition of Quartzite (in volume fractions) 45. Pettijohn, T. J. (1975]: Sedimentary Rocks. Harper and brothers, New York, 718 pp. Rahaman, M. A. & Ocan, O. (1978]: On the Relationship in the Precambrian Migmatite Gneiss of Nigeria. Jour of Mining Geo, Vol. 15 (1], pp. 23-32. Talabi, A. O. & Tijani, M. N. (2011]: Integrated remote sensing and GIS approach to groundwater potential assessment in the basement terrain of Ekiti area southwestern Nigeria. RMZ - Materials and Geoenvi-ronment, Vol. 58, No. 3, pp. 303-328.