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

Abstract

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.