PETROGENESIS, MAJOR OXIDES AND TRACE ELEMENTS GEOCHEMISTRY OF MIGMATITE IN AJUBA, KWARA STATE, NIGERIA

Petrogenesis, major oxides and trace elements geochemical study was carried out on migmatite in Ajuba. The study area is located on Latitudes 8° 05'N and 8°13'N and Longitudes 5°23'E and 5°30'E. Five rock samples were taken from the migmatite outcrops and used for petrographic and geochemical analyses. The geochemical analysis was done using X-ray Fluorescence Spectrometer technique. The dominant rock type is migmatite; gneiss and granite outcrops were also found in sparse distribution.  Ptygmatic folds, which constitute the palaeosome, is the common structure observed on the migmatite rock. The petrographic analysis shows that the migmatite consists of quartz, biotite, plagioclase, hornblende and microcline. The major oxides analysis indicates SiO2 as the dominant oxide with concentration range values (70.71 wt. % - 79.32 wt. %) and average of 74.80 wt. %. Al2O3 (14.98 wt. % - 16.44 wt. %, average: 15.70 wt. %) and Fe2O3 (9.10 wt. % - 15.41 wt. %, average: 12.39 wt. %), K2O (6.67 wt. % - 8.86 wt. %, average: 7.50 wt. %) and CaO (0.49 Wt. % - 4.64 wt. %. average: 2.73 wt. %). P2O5, MnO and TiO2 are less than 1.0 wt. %. The trace elements analysis indicates the concentration distributions: Rb (0.11-0.15 ppm, average 0.13 ppm), Co (0.04-0.17 ppm, average 0.10 ppm). Trace elements ˂ 0.10 ppm are Zn, W, Ni, Cu, V and Pb. From the petrographic and geochemical assessment, the petrogenesis of the migmatite has silica-rich igneous parentage. Moreover, the plots of SiO2-CaO and K2O-SiO2 placed the migmatite on the “upper boundary field of Francisian Greywacke” protolith and Shoshonite series, respectively.


INTRODUCTION
Migmatite is a combination of metamorphic rock and igneous rock; it involves two or more different parts which are petrogenetically connected and to their protoliths (Sawyer, 2008). It is made when a gneiss, which is a metamorphic rock partially melts; and mixes with recrystallised igneous rock. They are also named diatexite. Types of migmatites include metatexites and diatexites. Metatexites have a lesser melt portion and preserved coherent pre-partial melting qualities in the palaeosome and residuum. Neosome exist dominantly in the diatexites and coherent pre-partial melting arrangements are absent (Pawley et al., 2015). The different phases of the rock are due to the transformation of some fraction that happens in the metamorphic palaeosome (Mehnert, 1971), when an originator rock is affected by considerable number of pressures and temperatures. Metamorphic rocks are aggregate of solid mass which are altered at extreme pressures and temperatures, though could form crystal again but not made through melting. Igneous rocks are solid masses that are made by cooling from entirely molten silicate material.
Migmatites are found beneath deformed metamorphic rocks that characterize the base of eroded mountain chains, frequently within Precambrian cratonic blocks (Sawyer, 2008). It is the oldest member of the Precambrian Basement complex in Nigeria (Oyawoye, 1964;Rahaman 1976;Ayodele, 2015). Migmatites are seen frequently as tightly, incoherently folded veins (ptygmatic folds). These create separations of leucosome, light-colored granitic components exsolved within melanosome (which is a dark coloured amphibole-and biotite-rich portion). Mesosome may also exist and its colour is intermediate among that of leucosome and melanosome.
According to Lyell (1837), migmatite is the penultimate part of an order of lithologic alterations. He stated that when there is a deposition of unconsolidated deposit (protolith for upcoming metamorphic rocks) and there is a rise in temperature and pressure with depth, a protolith passes through a diagenetic order from porous sedimentary rock through hard rocks and phyllites to metamorphic schists; the differentiation of the original sedimentary constituent is still possible. More so, the schists are re-formed as gneiss that folia of the remaining minerals substitute with quartzo-feldspathic portions; partial melting is still observed with sets of leucosome merge to produce separate strata in the neosome, and is seen as detectable migmatite. Goransen (1938) clarifies that the subsequent leucosome strata in stromatic migmatites hold water (H2O) and carbon (iv) oxide (CO2) content in an irregular reaction series from the paleosome which makes the leucosome exceptionally mobile. Holmquist (1916) stated that metamorphic rocks are converted into granulite; the procedure is termed anataxis. Imasuen and Onyeobi (2014) have conducted petrographic analyses of the rock samples which shows migmatites consist of quartz, feldspar (plagioclase, microcline and orthoclase) and also biotite.
Every rock is made due to different chemical, physical and biological conditions which lead to the creation of an extensive and distinct set of minerals and therefore result in submissions in various fields. Rocks are durable, soft, penetrable and impermeable; they are of several sizes and shapes. Migmatites have an attractive look, frequently being noticeable with irregular small areas of distinct shades fluctuating from nearly white to dark grey, and are extensively used as building stones, occasionally being refined for ornament. Migmatite in the construction industry comprises dimension stones and, road aggregate. It uses also include countertops, floor design, as facing stone, curbing, as artifacts, cemetery indicators, jewelry, tombstones, manufacturing of object stopping breeze from blowing papers and bookends. This study aims to determine the petrogenesis of migmatite from petrographic and geochemical analyses.

LOCATION, CLIMATE AND GEOLOGY OF THE STUDY AREA
Ajuba is one of the villages in Osi, Kwara State of Nigeria at about 4000m west of Osi on Latitude 8° 05'N and 8°13'N and Longitude 5°23'E and 5°30'E. It covers an area of 9km 2 (Figure 1). One of the major villages in Osi is Ajuba settlement and it is situated within two rock suites, namely: Osi migmatite-gneiss complex to the west and Egbe schist belt to the east (Bamigboye & Adekeye, 2011).
The study area has dry and rainy seasons as the two alternating climatic conditions. The annual rainfall is between 1270 mm and 1524 mm, spread over the month of April to October. The maximum rainfall is usually experienced around June/July. The monthly temperature is maximum in March at about 30°C and minimum (lowest) in August at about 25 0 C (Olasehinde, 1999). Conversely, the dry season starts in November and ends in March.
The geology of Nigeria involves older and younger meta-sediments, Older and Younger Granites, and volcanic intrusives which are overlain by younger sedimentary basins of Illumeden, Chad, Dahomey, Benue, Bida, and Niger-Delta (Rahaman, 1976). The most dominant rocks include gneiss, granite, gabbro, migmatite, amphibolite and pegmatite. The granites are porphyritic and occur as boulders beside the river channel. Enormous outcrop of porphyritic granite is sited about 8 x 10 2 m south of the area. A typical feature of the Basement Complex tectonics is the widespread existence of fractures (Akinrinmade et al., 2013). The varieties of surface essential features such as foliations, folds, faults, joints, fractures and fissures are noted. The presence of joints, faults and fractures in the subsurface are signs of a healthier and recommended aquifer (Olasehinde, 1999). It was observed that the rivers are constantly flooded by water at the peak of the rainy season; the flow rate drops to the average of about 2.7 ms -1 in the dry season and maintains the average of about 7.4 ms -1 during the wet season (Akinrinmade et al., 2013).

GENESIS OF MIGMATITE
A metamorphic rock is a rock that has been transformed inside the Earth unto a new type of rock in terms of its body (metasomatic), mineralogic and geochemical (petrologic) content. The source rock from which a metamorphic rock came is called the protolith. Metamorphism can be triggered by burial, tectonic stress, heating by magma or modification by intruding fluids. Protolith of a migmatite may suffer partial melting to become neosome. The unaltered protolith in migmatite is called palaeosome and it usually has conspicuous features such as foliations, folds and layering. Some rocks in a palaeosome are resistant to partial melting or microstructural alteration and they are called resisters (for example, quartzites, calc-silicates, and metamafic rocks). Neosome has two sections, namely: melt and magma. The melt is a silicate liquid without crystals, while magma is a silicate liquid that comprises crystals. The crystals may have either crystallized from the melt (which is liquidus phases) or they are the solid product of the melt response (also called peritectic products). Magmas are non-Newtonian fluids. Newtonian fluids are fluids that have a linear comparative connection among differential stress and shear strain (that is, the viscosity of the fluid will not be changed irrespective of any external stresses). Non-Newtonian ones are categorized by a non-linear relationship between the shear stress and the shear strain. As a consequence, the viscosity will vary and become thinner or thicker once external stresses are applied. The neosome comprises the solid (which are residuum and melanosome) and the liquid (which is leucosome) components of the melt creating metamorphic response (Sawyer, 2008;Pawley et al., 2013).
The leucosome results from segregated partial melt and it may comprise microstructures that show crystallisation from a melt or magma. Leucosomes may be in-situ (if a darker selvedge of mafic minerals and other minerals like garnet or cordierite are present which in turn characterizes the solid products of the melt reaction) or segregated and migrated from their source. In-situ leucosome results from the crystallisation of an anatectic melt or part of a melt, which has segregated from the residuum but has stayed at the site where the melt was formed. In-source, leucosome results from anatectic melt, which has moved away from the place where it was formed but is still around the boundaries of its source layer. The outcome of crystallisation of a felsic portion that has migrated from its source area entirely into non-metamorphosed rocks is granite dyke, sill, and pluton. Partial melting or anataxis yields metatexite and diatexite with mineralogical and chemical arrangements (Afolabi et al., 2019).

MATERIALS AND METHOD
The field materials used are topographic map, hammer, chisel, compass clinometers, sample bag, marker, safety gear, footwear, pencil, pen and notebook. The laboratory materials used are, petrographic microscope, rock cutting machine, X-ray Fluorescence spectrometer, jaw crusher, pulveriser, etc.
The fieldwork involved geological mapping on a scale of 1:12,500 resulting in the geological map ( Figure   2). The laboratory work involved sample preparation, petrographic study and geochemical analysis.
The petrographic study was undertaken with the aid of petrographic microscope at the Microscopic Laboratory,

RESULTS
The result of field studies reveals that the rocks in the study area are majorly migmatite, gneiss and granites [with migmatite about 60% of the total visible rocks] (Figure 2). Figure 3 presents the field photograph of migmatite with ptygmatic folds in Ajuba town. The photomicrograph of possible migmatite minerals is contained in Figure 4. The amounts of major oxides and trace elements detected are presented in Tables 1 and 2 [with their average results in Table 3]. The most abundant major oxides and trace elements are presented in Figures 5 and 8, respectively. Figure 9 presents the major oxides for sample locations.

DISCUSSION
The migmatite outcrops in the study area are medium-grained in texture and they occur in the southwestern and southeastern parts of the study area. Field evidence indicates that they were partially migmatised by the intrusion of acidic magma resulting in distinct felsic and mafic bands. Migmatite is very useful as dimension stones and aggregates in road, bridges and dam's construction. This outcrop covers about 60% of the study area ( Figure 2). Most of the outcrops are very visible along the river channels as weathered material. The common structure observed in this rock is ptygmatic folds which constitute the palaeosome (Figure 3).
In hand specimen, migmatites investigated are of alternating ensembles of light and dark coloured minerals (leucosome and melanosome minerals). The dark coloured are biotite and hornblende, while the light coloured are quartz and feldspar ( Figure 4). Thin section studies revealed that the melanosome consists of hornblende and biotite, while the leucosome consists of plagioclase, quartz and microcline. The partial melting was possible due to temperature and pressure greater than 650 0 C and 34Mpa, respectively.
Tables 1 and 2 accounts for the percentage weight of major oxides and the type of trace element detected, respectively. The average of major oxides was computed as seen in Table 3 to give an account of the mean percentage of these oxides in Ajuba. It also includes those of trace elements. Major oxides geochemical analysis indicates silicon (iv) oxide (SiO2) Figure 9) and K2O-SiO2 placed the migmatite on the "upper boundary field of Francisian Greywacke" protolith and Shoshonite series, respectively. K2O-SiO2 plot in Figure 8 was used in line with Peccerilo & Taylor (1976), Brako et al. (2020) & Morrison (1980] to classify the migmatite of the study area into shoshonite series. Besides, shoshonite series, other possible classification using the K2O-SiO2 plot for migmatite of the study area are tholeiite, cal-alkaline, high-K calc-alkaline series. K2O-SiO2 plot (Figure 8).
corroborated that the migmatite rock is mostly of igneous intrusion (which is a group of saturated in silica, low iron enrichment, low TiO2 as in the case of this research [it is the least amount], high but variable Al2O3 approximated to 16 wt. %.

CONCLUSION
Migmatite is the major rock type within the mapped area: Ajuba with 60% abundance when compared to other rocks noted during the survey. The common structure observed in this rock is ptygmatic folds which make up the palaeosome. Migmatite outcrops investigated are made up of alternating bands of light and dark coloured minerals (leucosome and melanosome minerals). Silicon (iv) oxide (SiO2)

CONFLICT-OF-INTEREST DISCLOSURE
No conflict of Interest.