Petrochemistry and Petrogenetic Features of Metasedimentary Rocks of Igangan Area, Southwestern Nigeria

The study was carried out to delineate the geochemistry of unstudied meta-sedimentary rocks in the Igangan sheet 240 NW. Geological field mapping was undertaken to delineate the different metasedimentary rocks; three different metasedimentary rocks were observed and 17 samples were analyzed using ICP-AES/MS to obtain elemental concentrations. Geochemical analytical results revealed average concentration (%) of 74.5, 12.66, 1.99, 4.23, and 3.59 for SiO2, Al2O3, Fe2O3, Na2O and K2O respectively. Trace elemental average concentrations (ppm) for selected trace elements in the metasediments are as: U (0.4 4.8 ppm, 2.39), Th (0.9 25.60 ppm, 10.38), Ni (0.1 17.70 ppm, 7.40), Sr (37 124 ppm, 81.75), Ba (55 1228 ppm, 620.31), Sn (1.4 4.8), Be (1 11 ppm, 3.31), Li (2.6 48.50 ppm, 23.28), Rb (32 376.3 ppm, 140.16), Ta (0.5 65.20 ppm, 12.48), Nb (7.76 47.5 ppm, 23.82) and Cs (0.8 33.30 ppm, 8.33). REE patterns of all samples are parallel to sub-parallel, LREE-enriched, with weak to distinct negative Eu anomalies and weakly fractionated HREE segments. Source rocks of the metsediments underwent low to moderate chemical weathering and metasediments are derivatives of felsic igneous sources.


Introduction
The study area is part of the Basement Complex of southwestern Nigeria; it is located in Igangan sheet 240 North West, Southwestern Nigeria; longitude 3˚00'' -3˚15'' and latitude 7˚45'' -8˚00''. The study area has an area extent of about 829 formation on the geology and geochemistry of rock units in the area.
The dominant rocks in the area are the meta-sediments which include quartzite, mica schist and phyllites underlain by variably migmatised tonalitic and granitic gneisses while members of Late-to Post-Pan African granitoids intruded and cut the fabrics of the meta-sediments outcropping as potassic porphyritic granites, which is a scenario that has also been reported from other parts of southwestern Nigerian schist belt [1]. [2] explained that the area is composed mainly of mica schists with quartzites and metamorphism is higher in grade than in most northern schist belts, with pelitic rocks containing biotite, garnet, staurolite and locally sillimanite.
Several studies have been carried out on metasedimentary rocks in different areas in Nigeria and around the world. For example, [3] studied the geochemical composition and petrogenesis of schists and amphibolites of parts of sheets 203 (Lafiagi) SW and 224 (Osi) NW. Petrographic and geochemical studies show that the metasediments are para-schists of arkosic to greywacke parentage which were sourced from a moderately weathered feldspathic igneous rock Derivation of the schists from K feldspar-rich protoliths is indicated by the enhancement of Ba over Rb while the presence of significant amount of mafic constituents is signified by relatively high amount of Cr and, sometimes, Ni. They suggested that the presence of quartzite intercalated with other metasediments is an indication of alternate flow regimes where heavy and light sediments were deposited at different times; such has also been described by [4] in some other parts of the Nigerian basement complex.  [6] also studied the geochemical characterization of the protoliths and the tectonic implications of the metasedimentary rocks of the northern portion of the Macururé Domain. Greywackes protoliths dominate with some subordinate shales and arkoses; major, trace and rare-earth element chemistry indicates a source with an average upper crustal composition for the protoliths of metasedimentary rocks, a mixed source detritus was also suggested for the rocks in the study area.
[7] examined the compositional characteristics and petrogenetic features of metasediments of Ijero-Ekiti area, southwestern Nigeria. They reported a sedimentary protolith for the investigated rocks based on variation plots and trace element concentrations in quartzite units of the study area suggests derivation of the sedimentary protolith from weathering of granitic rocks.
The present study is therefore based on delineation of the geochemistry, provenance and sediment recycling in the metasedimentary rocks in Iganagan sheet 240 NW.

Materials and Methods
The methods adopted in carrying out the research include desk study of related subjects, geological field mapping, sampling and geochemical analyses. Geological mapping of the study area was done on scale 1:50,000. Random samplings of big, fresh, representative rocks were carried out and samples for thin-section and geochemical analyses were obtained. 17 samples were selected and analyzed for their major, trace and rare earth elements compositions using ICP MS/AES. The samples were pulverized at the Department of Geology, University of Ibadan and analyses were carried out at the Bureau Veritas Mineral Commodities (BVM) Canada.

Local Geological Setting
In the study area, different types of metasediments were observed and mapped (  leucocratic ridge in the central part of the study area; the quartz biotite schist occur as melanocratic outcrops in the western flank of the study area while the quartzite occurs as intercalations with schists in different areas in the study area. The quartzites are fairly weathered in some areas, poorly jointed and they occur as boulders and pods. The metasediments have been intruded by fine to medium grained potassic porphyritic granites and biotite granites as well as pegmatite veins ( Figure 3).

Petrography
The

Major Elements
The metasediments reveals a wide range for the major elements; they are sili- ; the phyllites mostly plot within the field of greywackes while the quartz mica schist and the quartz biotite schists plots in the field of arkose suggesting different protoliths for the metasediments (  [10] in which the quartz biotite schists plots in the lithoarenite field while the quartz mica schists and phyllites plots in the arkose field ( Figure 6).
The phyllite and quartz mica schist fall within the arkose field while the quartz biotite schist falls within the litharenite field; discriminant function diagram for the provenance signatures [11] reveals that the metasediments are mainly of felsic igneous provenance (Figure 7).
Compared to the international standards (e.g., PAAS, NASC, UCC, Table 2), these metasediments are characterized by low contents of MgO (average 0.54) and Al 2 O 3 (average 12.66%), indicative of evolution from an igneous protolith and low clay content respectively. K 2 O/Na 2 O is generally low in the metasediments with the exception of a value of 20.97 which is possibly a reflective of secondary addition of potassium [12].

Trace Elements
The range and average value of selected trace elements in the metasediments are as follows (Table 3)

Provenance, Recycling and Maturity
Due to the ability of transition elements to resist dispersion by secondary processes and their ability to strongly reflect the chemistry of the original protolith, it is suggested that protoliths in which these elements are abundant and compatible had significant contribution to the source of the metasediments [12].
Th, Sc and the REEs are thought to be less affected by sorting during sediment transportation and deposition as well as diagenesis and metamorphism hence high field strength elements and rare earth elements as well as Sc, Th and Zr are very useful in determination of maturity of the sediments as well as intensity of sorting during transportation and deposition amongst other uses [16] and [17]. [19] Explained that ratios of incompatible elements with similar hydrodynamic behavior should remain invariant and this is exemplified by the Th/Sc vs.
Sc plots as well as the Zr/Sc vs. Sc plots (Figure 8 and Figure 9); which reveals normal to lower concentrations of Zr is an indication infers immaturity to semi-maturity for the metasediments studied [8].
Provenance discrimination diagram based on K/Rb ratios (    Th/U ratios above that of the upper crust while some have Th/U ratios approaching that of depleted mantle sources ( Figure 12).

Rare Earth Elements
The range and average concentration of selected rare earth elements in whole rock pegmatites is as follows (

Conclusions
Metasediments of parts of Igangan Sheet 240 NW have been studied; their geology and geochemistry have been delineated. The occurrence quartzite as layers within some of the weathered schists is an indication of sediment deposition under unstable flow regimes with heavy and light sediments deposited  Figure 13. (a) Condrite normalization plot for phyllites (values after [25]); (b) Condrite normalization plot for biotite quartz schist (values after [25]); (c) Condrite normalization plot for quartz muscovite schist (values after [25]).