Geochemistry and Petrogenetic Features of Metasediments in Northern Part of Kushaka and Birnin Gwari Schist Belts NW Nigeria

The Kushaka and Birnin Gwari metasediments and associated banded iron formations constitute important lithological units within the Precambrian Basement Complex. They were studied to evaluate their compositional characteristics and petrogenesis in order to contribute further to the understanding of the geodynamic evolution of Nigeria’s Schist belts. The Kushaka metasediments comprise quartzite, graphite and sulphur bearing staurolite-muscovite quartz schist interbedded with Banded Iron Formations (BIFs) while the Birnin Gwari schist comprise staurolite-biotite quartz schists with lithic (angular to rounded clastic quartz, schistose, volcanic and quartzo-feldspathic) sandstones. These schists are associated with fissile and ferruginous quartzite, banded and granitic gneisses, basalts and amphibolites. Petrographic work revealed varying proportions of quartz, staurolite, biotite and muscovite with subordinate iron-oxide minerals. Geochemically the metasediments in the Kushaka are enriched in SiO 2 (61.23 to 65.99 wt %) with elevated values of Al 2 O 3 (16.53 – 20.93 wt %), Ba, V, W, La, Nb, Nd, Rb, Th and Zr; while the Birnin Gwari schists, even though enriched in SiO 2 (63.03 to 65.13 wt %), has moderately elevated Al 2 O 3 (15.4 – 15.16 wt %) values but is depleted these trace elements. Field and geochemical characterization of the Kushaka metasediments suggests peraluminous, tholeiite and calc-alkaline character; arkosic and shale-greywacke sedimentary protoliths derived from quartzose sedimentary and granite-quartz monzonite provenance. Calculated ICV values of 0.52 - 0.99 and occurrences of graphite and sulphur in the Kushaka metasediment suggests shallow stable shelf-type sediment of carbonate and iron formations in a reducing environment with matured sedimentary protolith. The Birnin Gwari metasediments on the other hand have a peraluminous and calc-alkaline character, inherited from shale-greywacke and quartzose sedimentary protoliths derived from granodioritic and granite-quartz monzonite provenance. ICV values of 1.12 – 1.18 and angular and volcanic clasts suggest rapid subsidence of basin during genesis and / or tectonic instability in the surrounding environment with immature sedimentary protolith. This is an indication of two contrasting environment in an arc setting with contribution from basaltic and andesitic detritus. Available geochronological data on granite and granitic gneisses have ascribed the Kushaka schist belt to Kibaran and the Birnin Gwari schist belt a Pan-African age.

representative portion. The samples were thereafter sent to laboratories for whole-rock geochemistry in the Department of Geological Sciences, University of Vienna, Austria.
Rocks were crushed and grounded to the finest powder possible in an agate (or tungsten carbide) swing mill for the whole X-ray-fluorescence analysis. Major elements were determined by the fused bead method on calcined rock powder fused with lithium tetraborate as flux to form a glass bead. Trace elements used the pressed pellet method directly on the crushed and milled rock powder mixed with polyvinyl alcohol as a binding agent. The element analyses are performed on a sequential X-Ray spectrometer PHILIPS PW2404 using a super-sharp endwindow tube with a Rh-anode and a programmable 4kW generator (60kV max., 125mA max.; iso-Watt-switching), with accompanying PANalytical software.

RESULTS AND DISCUSSIONS 4.1 Field Occurrence and Petrography
The Kushaka metasediments comprise staurolite, biotite and muscovite quartz schists interbedded with banded iron formations (BIFs). Two varieties are recognized in the field. (i) The fine grained grey to silvery grey staurolitemuscovite types confined to the river channels in Kugu area; (ii) The yellowish to grayish type around Sabo-Layi is graphite, sulphur bearing, (minerals in hand specimen) biotite schist. Graphite is silvery grey to black colour with greasy feel and smudges the hand when touched, but may have been transformed or altered to magnetite and haematite as opaque minerals as observed under the microscope ( Fig. 2 and 3).
The staurolite-muscovite schist in Kushaka around Kugu consists predominantly of staurolite (30%), ironoxide (5%), quartz (30%), K-feldspar (20%) and muscovite (10%) embedded in a quartz, feldspar and muscovite matrix (Table 1). Staurolite occurs as pale brown (in plain polars and with yellow, orange, purple and blue interference colours in cross polars), euhedral six-sided elongated crystals and rounded edges with high relief from the groundmass (Fig. 2 A and B). The staurolite is also characterized by quartz inclusions and dotted with iron-ore crystals. Muscovite occurs as fibrous and acicular crystals stacked between the staurolite minerals. Iron-ore occurs as euhedral and subhedral crystal disseminations. Medium grain quartz crystals are elongated and prismatic tightly joined together with wavy extinction. Accessory mineral is magnetite.
Quartzites: Consists of greyish white fissile and reddish brown ferruginous types. The grayish white and fissile rock occur as poorly exposed low lying ridges in Nasarawa Kwona area of Kushaka schist belt. Where exposed, they are aligned in the N-S foliation direction. The ferruginous quartzite occurs as iron-formation interbedded with quartzite. They are characterized by clear microfolds, crenulation cleavages, nearly vertical dips and well defined schistocity. They are well exposed in Maganda area (Plate I). In the adjoining Kusheriki, south west of the study area, they are interlayered with phyllites. Banded Iron Formation (BIF) is exposed as series of N -S trending ridges and as isolated hills with elevation of ≥ 600 m in Sabo-Layi, Kugu and Galadimawa area. Where they are interbedded with the schist, exposures show iron-formation occurrence with semi-pellitic schist and the iron concretions at the top. Concretionary ironstone at the top of the isolated hill is composed of cherts and iron-rich minerals which form alternating bands (Fig. 1). Amphibolites: Amphibolites occurs as dykes in the Kungwi area, showing characteristic brittle deformation with fragmented boulders. The major one extend for over 1 km in length and >6m in width trending N160 0 direction. It is a fine grained dark coloured rock with acicular quartz occurring as bands (1 -2 mm), trending in the general N -S foliation direction. There are others of smaller length and width. The Birnin Gwari metasediment occurs as an elongate, N-S trending metasedimentary outcrop of 430 m elevation, and as a whale back outcrop exposed along Kogi Kusheriki river channels, through Mando (Fig. 2C). It consists of staurolite and biotite quartz schists with lithic sandstone (clastic quartz, schistose, volcanic and quartzofeldspathic materials) (Fig. 2). Four types based on field occurrence have been observed: The bluish grey fine grained rock with clast (2mm to 5cm and making up to 25% of the constituent of the rock), aligned in the foliation direction, showing flow structure. (ii) Light grey, mediun grained rock with dark minerals evenly distributed, the clast making up about 40% is angular to rounded and flattened, and are poorly sorted with range of particle sizes from sand to cobbles. (iii) Dark grey colour, fine grained rock characterized by ripple marks cross bedding and cross lamination, typical of sedimentary structures. (iv) Dark fine grained with granular texture without clastic materials, close to the Mando granitic intrusion with granitic dyke occurring in the biotite and ironrich metasediment (Plates I).

Geochemistry
Whole-rock major and trace elements data of representative rock samples of the metasediments are represented in Table 2 and 3.

Trace Elements
The Kushaka metasediments has fairly high Ba with a range of 599 -974.8 ppm, but a low range in Birnin Gwari with 27.6 -32.2 ppm. Rb shows high concentration (116 -153.6.8 ppm) in Kushaka compared to Birnin Gwari (13.5 -14.4 ppm). Zr also revealed high concentration (187.3 -212.6 ppm) in Kushaka compared to Birnin Gwari metasediments (32.7 -41.9 ppm). The high Ba, Rb and Zr concentration in Kushaka are within the range of supracrustal rocks (Brown et al., 1979) and similar to the semi pellitic schist of Isanlu area and Ijero Ekiti metasediments (Olobaniyi, 2002;Akinola and Okunlola, 2012); The high Ba indicates K-feldspar rich source rocks in Kuskaka and low Ba in Birnin Gwari indicates K-feldspar poor source rocks. The low Ba, Rb and Sr values in Birnin Gwari is similar to the Okemesi quartzites (Okunlola and Okoroafor, 2009).
The concentration of Rb especially in the Kushaka metasediments is similar to their derivation from clay and shales which is a reflection of their origin. Rb/Sr ratio of >0.4 % in Birnin Gwari metasediments is typical for pelitic metasediments (Van De Kamp, 1968). High Zr concentration is a reflection of the presence of detrital zircon in the Kushaka metasediments and lack of it in the Birnin Gwari metasediments (Elueze, 1981). Sr content is in the range of 99.2 -127.9 ppm in the Kushaka metasediments and 134 -557.6 ppm in the Birnin Gwari schists. Zn concentration is 32.5 -109.1 ppm in the Kushaka metasediments and 32.7 -41.9 ppm in the Birnin Gwari schist. Generally, Ba, V, W, La, Nb, Nd, Rb, Th and Zr are more enhanced in Kushaka metasediments than in Birnin Gwari metasediments. Enrichment of incompatible high field strength elements (HFSE) and large ion lithophile elements (LILE) in the Kushaka metasediments compared to Birnin Gwari metasediments may have been responsible for the mineralization of the earlier.
The (Na2O + K2O) − Fe2O3 − MgO ternary plot (Fig. 5B) (after Irvine and Baragar, 1971) discriminates the metasediment of Birnin Gwari as having a calc-alkaline affinity and Kushaka as tholeiitic and calc-alkaline. This Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol.11, No.16, 2021 45 shows that metasediments of Birnin Gwari area have a petrogenetic character that is similar to those in Okemesi area, while Kushaka metasediment bear similarity with the Sepeteri amphibolitic schist (Okunlola et al., 2005). The composition of the metasediments (Table 2), when plotted in the ACF diagram of Winkler (1967) (Fig. 6A), shows ferruginous quartzite (KFQ1) and biotite-muscovite quartz schist (KKSc1) samples (around Sabo Layi area in the Kushaka metasediments) plot at the edge of Al-rich clay and shale and clay (1a) and shales free of carbonate (1b). Kugu biotite and staurolite-muscovite schist (KKSc 2 and KKSc 3) of Kushaka metasediment, and staurolitebiotite schists (KBGSc1 and KBGSc 2) of the Birnin Gwari metasediment both plot in the field 1b (between arrows) of Winkler (1967), indicating shales and clays containing about 30 % carbonates; no sample plots in the field 2 (Wyoming greywackes), while banded iron formation samples plot close to field 3 (basaltic and andesitic rocks) (Fig. 6A). This shows that metasediments of Kushaka and Birnin Gwari area are possibly derived from clays and shales which could have been contaminated by mantle derived rocks of basaltic composition from the Kushaka Gneiss Complex. Similarly the chemical composition of the Kushaka and Birnin Gwari metasediments plotted in the ACF diagram (after Miyashiro, 1973) (Fig. 4b) reveal a shale protolith. The data falls in the field of shalegraywackes, which suggest they originated from shale; clay and shale also shows mixed nature of these sediments. When the Kushaka and Birnin Gwari metasediments are plotted on the CaO-Na2O-K2O diagram (Fig. 7A), and the fields showing the compositional variations in acid to basic rocks are superimposed (Condie, 1967), Kushaka metasediments plot in the field of granite -quartz-monzonite, while Birnin Gwari metasediments plot at the edge of the field of granodiorite/ granite -quartz-monzonite indicating that their shale-arkose protoliths were derived essentially from a granitic-quartz-monzonitic source.
The Na2O versus K2O plot of metasediment (after Pettijohn, 1975) (Fig. 8A) further distinguish Kushaka schist into the arkosic and Birnin Gwari schist into the greywacke field. As confirmed by (Pettijohn, 1957), the average composition of shale show high abundances of Al2O3 and K2O which is generally in excess of Na2O. K2O>Na2O is used to indicate shale, and high proportion of K-bearing rock forming minerals such as mica, Kfeldspar, illite, compared to Na-feldspar which is unstable prior to metamorphism. On the Al2O3-CN−K2O plot, (Fig. 8B) the ferruginous quartzite, staurolite, biotite and muscovite quartz schists all of the Kushaka metasediment plot close to the illite fields while the staurolite and biotite quartz schist of Birnin Gwari metasediment plot close to the average shale. Banded iron formations straddle between kaolinite and smectite.
The Index of Compositional Variability (ICV) (Cox & Lowe, 1995) measures the abundance of alumina relative to other constituents of the rock, except SiO2. Compositionally immature pelitic rocks have high ICV, whereas mature pelititc rocks with very little non silicates or those rich in kaolinite group clay minerals possess low values (< 0.6) (Elueze & Okunlola, 2003). In the study area, Kushaka ferruginous quartzite; biotite, staurolitemuscovite quartz schist; Birnin Gwari staurolite and biotite schist and Kushaka banded iron formation shows 0.52, 0.59 -0.99, 1.12 -1.18 and 3.11 respectively ( Table 3). The calculated ICV value for the Kushaka ferruginous quartzite (0.52) and biotite, staurolite and muscovite quartz schist (0.59 -0.99) shows the matured nature of the sedimentary protolith prior to metamorphism; whereas calculated ICV value for the Birnin Gwari staurolite and biotite schist (1.12 -1.18) shows the immatured nature of the sedimentary protolith prior to metamorphism. Mature to moderately mature pelitic metasediments are characteristic of relatively stable cratonic environments, marked by moderate to very intense chemical weathering of first cycle material (Weaver, 1989;Bershad, 1966).

Provenance of the protolith and Tectonic Setting
Th/U ratio is also a useful parameter in determining the source characteristics of clastic sedimentary rocks (Roddaz et al., 2006). This ratio ranges between 4.25-4.30 in present day crust, while its values of 2.6 and 3.8 have been assigned to upper and lower mantle respectively (Cullers and Podkovyrov, 2002). Higher Th/U ratios can also increase in response to oxidative weathering and/or removal of U. Nevertheless, clastic sedimentary rocks derived from the upper crust are characterized by ratio ≥4, whereas ratio <4 has been related to a mantle contribution (Roddaz et al., 2006). In the Kushaka metasediments, Th/U ratios vary from 3.40 -6.14 in the staurolite and biotite schist, and 13.75 in the ferruginous quartzite. This suggests that there have been significant proportion of the felsic rocks with mantle contribution from the source region.
The TiO2-K2O-P2O5 plot (Fig. 7B) (after Pearce et al., 1975) of the biotite muscovite and staurolite schist and ferruginous quartzite confirms continental nature of the sediments while the banded iron confirms oceanic nature of the sediment. The dual continental and oceanic field is a confirmation of the nature of the sediments which underwent metamorphism. Roser and Korsch (1986) have shown that sandstone-mudstone suits from different tectonic settings can be distinguished on the basis of the K2O/Na2O and SiO2/Al2O3 values and SiO2 contents (Fig 9A). On the SiO2/Al2O3 versus K2O/Na2O plots, all samples of the metasediment, plot in the basaltic and andesitic arc setting, except for Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol.11, No.16, 2021 the Kugu staurolite and muscovite schist which plot in the evolved arc setting -plutonic detritus (Fig 9B). Thus, metasediments of the Kushaka and Birnin Gwari are of arc basaltic and andesitic setting. Basalts and amphibolites have been used as geochemical indicators of the tectonic setting in the Nigerian schist belts as of back-arc tectonic environment, near subduction zone setting (Elueze, 1992;Okunlola et al., 2006;Danbatta and Garba, 2007). Grant (1978) obtained Rb -Sr data from Kusheriki granite emplaced across the gneissic envelope in Birnin Gwari and augen gneiss in the Sabon Gayam area of Kushaka schist belt. The Kusheriki granite with low and uniform Rb/Sr ratios, and an isochron based on four whole-rock samples and two spatially associated aplite veins gives an age of 500 + 4 m. y., with an initial ratio of 0.7119 + 0.0002 (Fig. 7a), ascribing Pan-African age to the Birnin Gwari schist belt. The augen gneiss have high and uniform Rb/Sr ratios, and no isochron can be based on them (Fig. 10B). Model ages for the five analysed samples are insensitive to initial 87Sr/86Sr ratio assumptions, and that the ratio for these rocks lies within limits 0.725 + 0.02, the data centroid implies that their ages lie between 730 and 650 m. y., a possibility that rocks from the schist belt was derived from pre-Pan-African rocks.
Kushaka and Birnin Gwari metasediments are of sedimentary origin, hence a common origin for the protolith and dominantly of peraluminous character. Birnin Gwari metasediment has calc-alkaline and Kushaka has tholeiitic and calc-alkaline affinity. Their metasediments are possibly derived from clays and shales which could have been contaminated by other mantle derived rocks. Kushaka metasediment is further distinguished as arkosic while Birnin Gwari metasediment is distinguished as greywacke. Their geochemical composition revealed continental affinities with oceanic contribution and interpreted as reflecting contamination of crustal sources (Elueze, 1985). These belts are ensialic and evolved most probably in a rifted environment. Occurrences of graphite and sulphur in the Kushaka metasediment suggests shallow stable shelf-type sediment of carbonate and iron formations in a reducing environment and matured sedimentary protolith. Angular and volcanic clasts in the greywacke of Birnin Gwari metasediment suggest rapid subsidence of basin during genesis and / or tectonic instability in the surrounding environment and immatured sedimentary protolith prior to metamorphism. These are two contrasting environments in an arc setting with contribution from basaltic and andesitic detritus. This is similar to greywacke rocks of the Isanlu schist belt (Olobaniyi, 2003) and schistose rocks of Okemesi on the eastern side of the Ife-Ilesha schist belt (Okunlola and Okoroafor, 2009) where they suggests a rapidly subsiding basin accompanied contemporaneously with tectonic instability resulting in deformation and fracture in a rifted environment. The metasediments of the Kushaka and Birnin Gwari are of both continental and oceanic affinities. In terms of tectonic setting, the metasediments are of arc basaltic and andesitic setting which evolved most probably in a rifted environment.