PETROLOGY AND CHEMICAL COMPOSITION OF GNEISSES OF NORTHWEST OBUDU PLATEAU, SOUTHEASTERN NIGERIA

Northwest Obudu area is dominantly underlain by basement complex rocks, while sedimentary rocks occur in a relatively smaller proportion. Gneisses which include migmatitic gneiss (MG), granite gneiss (GG), garnet biotite gneiss (GBG), garnet sillimanite gneiss (GSG), and hornblende gneiss (HG) constitute more than 70% of the basement rocks, while schists, amphibolites, meta-gabbros and metaquartzites constitute the rest of the basement rocks. These were intruded by granites, dolerites, aplites and pegmatites. The gneisses are composed of quartz (15-35%), plagioclase (20-28 %), K-feldspar (10-20 %), garnet (2-6%), biotite (1-6%) and accessories (1-5 %). The co-existence of garnet- orthoclase- oligoclase- kyanite- sillimanite shows that the migmatitic gneiss has attained the uppermost amphibolite facies metamorphism. Geochemically, the gneisses are enriched in Si02 (63.12 - 72.8 wt %), with average of 67.41 wt %, reflecting the granitic nature of the migmatitic and granite gneisses, Al203 averages (14.4 wt %), alkalis (6.3 wt %). MgO, CaO and Fe203 are depleted in the migmatitic and granite gneisses, but relatively enriched in the garnet biotite- and hornblende gneisses. The aluminum saturation index shows that all the analyzed gneiss samples are peraluminous with A > CNK or ASI >1. The plagioclase composition (An30) is corroborative of upper amphibolite facies metamorphism.