Geochemical Characterization and Protolith of the Migmatite-Gneisses of Tandama Area, Katsina State, NW Nigeria

Migmatite-gneisses, which include migmatite, granite gneiss, and augen gneiss, underlie more than 70% of Tandama area, in North-western Nigeria. They are associated with schists, and are intruded by granites and pegmatites. These rocks are thought to have undergone a reworking during the Pan-African Orogeny. The aim of this research is to present results of geochemical investigation of Migmatite-gneiss Complex in the study area with a view to determine their geochemical characteristics and petrogenesis. Whole rock geochemical analyses have been used to evaluate the characteristics, petrogenesis and mode of emplacement of the protoliths. Geochemically, these rocks show granitic affinities. They are metaluminous to weakly peraluminous I-type, with S-type characteristic, magnesian to ferroan and alkali calcic and calcic. The protoliths could have been derived from the partial melting of tonalitic to granodioritic crustal rocks at low pressure, thus, producing metaluminous to slightly peraluminous high-silica, ferroan, alkali-calcic to calc-alkali melts, which is why it has some S-type character. These varying features are an indication that the protoliths are derived from mainly crustal melt mingled with mantle-derived component. The varying REEs and trace elements pattern displayed by the rocks is typical signature of arc rocks or continental crustal materials: the LREEs and LILE enrichment along with Rb, K, Pb, and negative Nb, Ta, Ti are evident of this signature. The incompatible trace elements show similarity to those of continental crustal rocks as indicated by the ratios in Th/U (2.21 12.4), Th/Yb (2.60 – 90.95), Ta/Yb (0.03 - 1.43), Ce/Pb (mainly 0.30 – 29.23) and high Ba/Nb (8.56 – 2402), the values of Sr/Y are generally<100, which is an indication of subduction-related rocks,the trend in Sr/Y ratio relative to Y contents in the rocks reflects essentially two types of felsic protolith namely crustal melts and slab melts. Similarly, the magnesian characteristic indicates close affinity to relatively hydrous, oxidizing melts, which is broadly typical of settings related to subduction. The high-K nature is characteristics of crustal rocks derived from remelting and differentiation of arc-accretionary complex crust. The rare earth element (REE) distribution shows that the migmatite-gneisses are enriched in the lighter rare earth elements (LREE) Sm, Pr, Nd, La and Ce, in that order of increasing abundance, with average values of 4.81 ppm, 7.90 ppm, 27.50 ppm, 38.44 ppm, 68.22ppm, respectively; and relatively depleted in the heavy rare earth elements (HREE) Lu, Tm, Tb, Ho, Yb and Er, with average values ranging 0.28 ppm, 0.30 ppm, 0.58 ppm, 0.65ppm, 1.91 ppm and 1.88 ppm respectively, and they exhibit negative EU anomaly, indicating that the rocks are highly fractionated. REE-chondrite normalized spider plot and plots in the chemical discrimination diagrams including the Y versus Nb plot, show that the protoliths were derived from partial melting and differentiation of granitic magma of hybrid origin which were emplaced in volcanic arc (VAG) to Syn-collision granite (Syn-COLG) tectonic setting. Variations thus, suggest igneous precursors for the migmatite-gneisses of this area, were derived from differing sources and depths.


INTRODUCTION
The Migmatite-gneiss complex of the study area is one of the four major petro-lithological units that constitute the Basement complex of Nigeria, which lies within the Pan-African mobile belt and sandwiched between the West African Craton and Congo Craton. It has intrusions of Younger Granites and coverings of Cretaceous and younger sediments. The Migmatite-Gneiss Complex is the most widespread of the component units in the Nigerian basement. It has a heterogeneous assemblage comprising migmatites, orthogneises, paragneisses, and a series of basic and ultrabasic metamorphosed rocks. Petrographic evidence indicates that the Pan-African reworking led to recrystallization of many of the constituent minerals of the Migmatite-Gneiss Complex by partial melting with majority of the rock types displaying medium to upper amphibolite facies metamorphism [1].
The Migmatite-Gneiss Complex of the study area is located in north-western part of Nigeria and has heterogenous assemblages of migmatites, gneiss and augen gneiss. Some authors have attributed the Migmatite-Gneiss Complex of Nigeria to be of sedimentary origin while others view them as rocks of igneous origin. According to Rahaman [2], the geochemical data available were insufficient to equivocally distinguish between sedimentary and igneous gneisses. Grant [3] on the basis of 87 Sr/ 86 Sr studies of the Ibadan granite gneiss supported an igneous origin while Burke et al. [4] argued that the granite gneiss were derived from isochemical metamorphism of a shale-graywacke sequence (a sedimentary origin). Onyeagocha [5] on the basis of field and geochemical evidence proposed an igneous origin by partial melting of crustal rocks for the granite gneisses of northcentral Nigeria. Supportive of the igneous origin, are also works of Ekwere and Ekwueme [6]; Imeokparia and Emofurieta [7] while Freeth [8] support a sedimentary origin. Elueze and Bolarinwa [9], studies of the granite gneiss of Abeokuta area gives a wide range of Ba and Zr concentration, concluded that the gneisses were of igneous parentage but with sedimentary input. This made it difficult to propose a single mode of origin for the granite gneisses in the Nigerian basement due to their variable compositions from location to location [9].
Works done on the genetics of migmatitegneisses are restricted to the south-western basement of Nigeria [9][10][11][12][13][14][15]. However, the gneisses of the Malumfashi schist belt where the study area is situated were earlier identified by McCurry [16] on the basis of petrography and not geochemistry, and were on a regional scale. These paper therefore present results of geochemical investigation of Migmatite-Gneiss Complex, around Tandaman area of northwestern Nigeria, with a view to determine their geochemical characteristics and petrogenesis.

METHODOLOGY
Rock samples that were obtained from different locations were taken to the sample preparation laboratory at the Department of Geology and Mining, University of Jos, to prepare the samples for whole-rock analysis and rock mineral analysis. The samples were broken into smaller pieces using geologic hammer so that they can be grinded easily in the pulveriser. The smaller fragments were crushed and reduced to powder using FRITSCH Pulverisette 7. The equipment has two grinding bowls and each of them has grinding steel balls which crush and rip apart the rock fragments while rotating for 30 minutes at 400 revolutions per minute (400 rpm). The powdered samples, pulverised to 85% passing 200 mesh, and measured to 5 g were transferred into labelled dispersing gloves for transportation to Acme laboratory at Vancouver, Canada, where the lithogeochemical analysis were carried out using X-ray fluorescence (XRF) and Inductively coupled plasma mass spectrometry (ICP-MS).
The major oxides concentrations (wt. %) in the samples were determined using X-ray fluorescence (XRF) method, following fusion with Lithium tetraborate (LiBO 2 ). In order to determine the loss on ignition (LOI), a predetermined amount of each sample was roasted. The major oxides concentrations were determined using the roasted samples by fusing them in a platinumgold crucible with a commercial lithium tetraborate flux. The molten materials are then cast in a platinum mold to form glass discs which were then analysed by X-ray fluorescence (XRF) to determine the major constituents. These oxides include SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, MgO, Na 2 O, K 2 O, MnO, TiO 2 , P 2 O 5 , and Cr 2 O 3 .
The trace elements were determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) method. The powdered samples were mixed with Lithium metaborate/tetraborate (LiBO 2 /Li 2 B 4 O 7 ) flux in platinum crucibles. These were then fused in a furnace. The resulting beads were dissolved in ACS grade nitric acid and analysed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in parts per million (ppm).14 elements which include gold (Au) and some volatile elements which do not report in lithium metaborate/tetraborate digestion were analysed after digestion with modified Aqua Regia Solution of equal parts concentrated Hydrochloric acid (HCl), trioxonitrate (V) acid (HNO 3 ) and dihydrogen monoxide (DIH 2 O) for one hour in a heating block or hot water bath. Each sample were made up to volume with dilute HCl and then split into 15 g or 30 g digestion for Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis in parts per million (ppm).

Geology of the Area
The study area lies between latitude 11° 22ʹ to 11° 30ʹN and longitude 07° 23ʹ to 07° 30ʹE and covers a total area of 195 km 2 and falls within the north western zone of the Nigerian basement complex. The Migmatite-Gneiss Complex of the study area has heterogenous assemblages of migmatites, granite gneiss and augen gneiss (Fig. 1).
The migmatites occurs as low-lying outcrops and boulders that are medium-grained with bandings, and they are associated with granite gneiss. The bandings are regular in form and consists of palaeosome and neosome, with stromatic and ptygmatic structures. The Granite gneiss comprises the main body of the palaeosome which consist of medium grained gneissic fabric of feldspar, quartz and biotite distributed into darker laminae. The neosome is consists of light grey medium grained granitic constituents that were introduced into the pre-existing gneissic parent's rock. They include quartz, feldspar, and little biotite and muscovite. The migmatitic gneiss outcrops at the centre of the western portion of the area, especially around Tudun Ibate, Dan Zaki and Dan Kauye as low lying outcrops and boulders. The gneissic palaeosome is highly foliated with simple fold structure. It is marked by alternating light leucosome band that is rich in quartzo-feldspathic minerals and dark melanosome band, which is composed of mafic minerals. The bands are generally discontinuous, thin and somewhat weak. The neosomes which are marked by veins of pegmatitic constituents of quartz and feldspars form a ptygmatitic structure. They form 30% of the migmatite-gneiss rock. The pegmatite veins are common and may be concordant or cross cutting and contain mostly pinkish feldspar, quartz and mica.
The granite gneiss occurs in association with migmatites, especially in the central area of the western portion of the study area. It outcrops as more resistant low-lying rock bodies and boulders along the north-western and southwestern portion of the study area. Those along the river channels are highly weathered. The rock is medium-gained light grey to grey in colour and it is well foliated. The foliation is marked by segregation of quartz-feldspar and biotite-rich successions in a north-south direction. The ferromagnesian minerals tending to concentrate in parallel alignment of darker bands, the coloured minerals consists of biotite and hornblende, while the light coloured bands are quartz and feldspar rich. The granite gneiss around Ungwan Balarabe in the north has green amphibole along the darker bands. The rocks are jointed with one set of jointing system trending north-south and the others trending east-west. The quartz-feldspar veins are concordant and sometimes develop pinch and swell structures. The pegmatite veins run parallel to the foliation of these rocks, some few are discordant. There are portions of the granite gneiss that has folded banding, somewhat complexly, within the migmatite, and there are areas where the granite gneiss consists of weakly foliated granite rock which gives the granite gneiss a migmatitic feature, thus called migmatitic granite gneiss. The folding gives the lecucocratic portion ptygmatitic structure. It is a light grey leucocratic rock with the banding varying in thickness and continuity. The individual bandings vary in thickness from fraction of a centimetre to several centimetres, the banding in some places are split into smaller units.

Fig. 1. Geological map and cross section of the study area
The Augen gneiss occupies about 20% of the area, and it occurs as boulders and low lying outcrops in the north-east and extends towards the centre of the study area. They are prominent around part of Rafin Gora, Ungwan Sambo, and Tandaman area as boulders and low-lying outcrops. The rock is dark grey in colour and weakly foliated, and bounded by mica schist. It is characterized by feldspathic augens enclosed in a finer grained groundmass of feldspar, quartz and ferromagnesian minerals which are weakly foliated. Generally, the shape of the feldspar porphyroblasts is irregular, the colour of the porphyroblasts is pale pink and they are embedded in a groundmass of flattened quartz crystals, chlorite and biotite. The feldspar augen has inclusion of wisps and small belbs of the groundmass, especially quartz. The grain size of the rock is finer, more foliated and less porphyroblastic in the north, while those in the central area are more porphyroblastic and less foliated. This shows that the rock experienced more shearing in the north than in the central area.
The other rocks in the study area include the biotite granite and mica schist. This granitic rock is located in the southern part of the study area; it occurs as grey coloured boulders. They are massivesly fractured plutons with irregular shapes that intruded the schist belt. The mica schists are generally soft, weathered and poorly exposed; most exposures occur mostly along river channels and road cuttings. They are extensive and constitute about 40% of the entire study area; they are in contact with migmatitegneiss, augen gneiss, and biotite granite.

Geochemistry
Major and trace elements distribution of the migmatite-gneisses of the area are represented in Table 1.
Major element concentrations in the studied rocks fully reflect their felsic granitic character. The granitic composition and the igneous nature of these rocks is supported by their position in various diagrams such as Na 2 O/Al 2 O 3 vs K 2 O/Al 2 O 3 (after Garrells and Mackenzie [17]) and TiO 2 vs SiO 2 discrimination diagram (after Tarney [18], as shown in Fig. 2. AFM plots of the all the rocks enable classification of the rocks into calk-alkaline and tholeiite series based on the proportion of their Na 2 (Fig. 4).
According to the classification of granitoid rocks by Frost et al. [19] using SiO 2 (wt.%) vs. Fe*number [molar Fe 2 O 3 /(Fe 2 O 3 + MgO)], most of the plots of migmatite-gniesses lie in the compositional field for Caledonian post-collisional plutons (Fig. 5a). This diagram shows that the augen gneiss belong to the magnesian group (only 1 sample of augen gneiss belong to ferroan group) while migmatite and granite gneiss belong to the ferroan group (only 1 sample of migmatite and granite gneiss belong to magnesian group).
In the migmatite-gneiss rocks, Al 2 O 3 , CaO, MgO, TiO 2 , Fe 2 O 3 , P 2 O 5 , Ba and Sr display a linear negative correlation with SiO 2 in the Harker plot suggesting fractional crystallization. The variations of Na 2 O correlate positively with SiO 2 up to 70% SiO 2 and then correlate negatively, K 2 O is broadly positively correlated with SiO 2 (Fig. 6).
In the incompatible trace-element concentrations normalized by Chondrite, after Thompson [20] ( Fig. 7a), the migmatite-gneisses display a near negative linear trend characterised by decreasing abundance from large ion lithophile elements (LILE) down to high field strength elements (HFSE).
The characteristics of the trace elements is a reflection of the various sources of the derived protolith of migmatite-gneisses, with Nb, Ta, and Ti depletion along with the enrichment in K and Pb indicating arc rocks or continental crustal signatures. High oxygen (O 2 ) fugacity is required to allow HFSE fractionation which produces Eu anomaly [21], with negative Eu anomaly more probable to occur [22].  [23,24], and those at active continental margins [25]. , the values of Sr/Y are less than 100 (<100) which indicate that the rocks are subductionrelated, thus, crustal-derived or subductionmodified component in mantle source is evident. Geochemical differences are apparent with regards to Sr/Y ratios in relation to Y; part of the granite gneisses (samples ALA15 and ALA15B) and migmatites (samples ALA28B and ALA31) tend to have elevated Sr/Y ratios at low Y contents, which is a feature observed in subduction-related activities where young and hot slabs are being subducted leading to differences in subduction geochemical fluxes.
The results of the abundance and distribution of the rare earth elements (REE) in the migmatitegneisses of the study area are presented in

Fig. 6. Harker diagrams of major elements for the migmatite-gneisses
The total rare earth elements abundance {TREE = Σ(La + Ce + . . . + Lu)} varies from 508.88ppm in the migmatite, to 1586.21 ppm in the granite gneiss, through 1114.05 ppm in the augen gneiss.

Classification
The migmaitie-gneisses can be classified according to the presumed origin of the parent rocks. The rocks plot in the igneous field which indicates an igneous protolith (Fig. 2). On the classification scheme of Frost et al. [19] in Fig. 5, the migmatite-gneisses display magnesian to ferroan characteristics, and alkalicalcic to calc-alkali characteristics. They are metaluminous to weakly peraluminous, and I-type in nature, with S-type characteristics in some augen gneiss samples (ALA17, ALA18, and ALA19) and granite gneiss samples (ALA13, and ALA14).
Results indicate that the rocks are I-type, metaluminous to weakly peraluminous [ASI= molar Al/(Ca + Na + K] contents which range from 0.7-1.22, and having wide range of silica contents are product of the partial melting of mafic to felsic meta-igneous source, possibly by partial melting of tonalitic to granodioritic crustal rocks at low pressure, with some S-type character [26].  [17]), and TiO 2 versus SiO 2 plot (after Tarney [18]) further support I-type affinity for the migmatite-gneisses.

Petrogenesis
A study into the origin of the migmatite-gneisses in the study area indicates an evolutionary trend in the major element values. The relatively low Sr/Y (<100) ratio and Y contents characterize transitional compositions between Archean and an arc rocks [1], which allows us to suggest evolution of the composition of the primary melt as a result of assimilation of Archean crust material or mixing of the basic melt with acid melt formed during crust melting.
On the Sr/Y vs Y diagram (Fig. 8a), the rock suites are projected into the melts that are typical of arc rocks and Adakite [27]. Some samples of the granite gneisses (samples ALA15 and ALA15B) and migmatites (samples ALA28B and ALA31) tend to show elevated Sr/Y ratios at low Y contents, features typical of Adakite. The contents of Y in the rocks are generally higher relative to Sr/Y ratio, features typical of basaltandesite-dacite-rhyolite (BADR). The trend in Sr/Y ratio relative to Y contents in the rocks reflects the different components and processes of crystal fractionation, crustal melting and/or contamination that operated during magma ascent, thus, reflecting two distinct sources of the felsic protolith namely crustal melts and slab melts [28].
The geochemical signature of the migmatitegneisses is characterized by the enrichment of K, Rb, Ba, which are LILE and negative Nb and Ta, which are similar to those of Archean rocks, thus, probably formed by melting of basalt in a subduction setting [29]. Fractional crystallization is reflected in the differences in the samples which indicate melts formed by mixing of mafic melt with crustal components during ascent, and thus, the migmatitie-gneisses follow sub-parallel alkali-lime trends during differentiation [19]. The migmatite-gneisses generally belong to magnesian to ferroan, and alkali-calcic to calcalkali (Figs. 5a and 5b), metaluminous to weakly peraluminous (Fig. 4). The augen gneisses are more magnesian while the granite gneisses and migmatites are more ferroan, which implies that the source of the augen gneiss protolith is from a more oxidized source compared to a more reduced source of granite gneiss and migmtite protolith; thus, the occurrence of extensive fractionation from magnesian towards iron-rich alkali compositions.
The Fe-number (Fe*) is effective in separating granitoids into magnesian and ferroan, as the value of the indices gives information about the history of differentiation of the magma, particularly with high SiO 2 content between 50 wt% to 75 wt% which is a reflection of the rocks. The origin of the migmatite-gneisses is thus from metaigneous source rocks, as evident in the geochemical signature of metaluminous to weak peraluminous, proportionately high sodic values, with broad silica values.
The rocks have high-K affiliation with K 2 O values ranging from 2.36 wt% to 6.55 wt% (an average of 4.83 wt%), which is also a reflection of the protoliths, which are more differentiated granitoids. This signifies that the migmatitegneisses have their origin at the convergent margin setting where the process of remelting, mixing and differentiation of the crust occurred.  (Fig. 8b), volcanic arc granite (VAG) is corresponding to syn-collisional granite, which is associated with continental arc collision that gave rise to the metaluminous to weak peraluminous I-type granitoids.

Tectonic setting
The high-K nature of the migmatite-gneisses shows the nature of the protolith; the melt have relative depletion in Nb and Ta which is typical of subduction-related magmas. The source has been modified by lithospheric subduction as it was consumed during subduction, which led to the addition of volatiles, enrichment of other incompatible elements, making it more potassic and depleted in Nb and Tb [31,37]. This is characteristic of metasomatism in relation to the Pan-African subduction. Thus, corroborating a crustal source for the protoliths of the rocks which have been affected by the Pan-African tectono-thermal events. The trend of distribution also shows the migmatite gneisses are highly differentiated and fractionated, and generally conform to the characteristics of similar rocks from other tectonic settings.

CONCLUSION
Precambrian migmatite-gneisses of the study area constitute the basement rocks. The high-K, calc-alkaline affinity, and negative distribution trend of Nb, Ba, and Sr as well as the enrichment of Rb and K, in the rocks suggest crustal source of the protolith derived by partial melting. In addition, the high Th/U in the rock suites suggests continental crust affinity. A slightly peraluminous nature of the rocks can indicate fractional crystallization dominated by fractionation of plagioclase as fractionation of plagioclase is expected to decrease the concentration of Al 2 O 3 in the melt which will in turn decrease the aluminum saturation index, thus, the protoliths could have been derived from the partial melting of tonalitic to granodioritic crustal rocks at low pressure, thus, producing metaluminous to slightly peraluminous highsilica, ferroan, alkali-calcic to calc-alkali melts, which is why it has some S-type character. The pattern displayed is characteristic of well fractionated LREE and flat HREE, with Eu and Ba significantly negative. The trend in Sr/Y ratio relative to Y contents in the rocks reflects the different components and processes of crystal fractionation, crustal melting and/or contamination that operated during magma ascent, thus, reflecting two distinct sources of the felsic protolith namely crustal melts and slab melts [28].