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Vol.2 , No. 3, Publication Date: May 5, 2015, Page: 39-52
, granite gneiss (GG), garnet biotite gneiss (GBG), hornblende gneiss (HG) and garnet sillimanite gneiss (GSG), underlie more than 70% of the area northwest Obudu Plateau in Southeastern Nigeria. These rocks are associated with schists, amphibolites and metagabbros and are intruded by granites, dolerites and pegmatites. The mineral assemblage consists of biotite – garnet - plagioclase (An32), orthoclase – kyanite – sillimanite – hypersthene, indicating that the GBG and GSG had attained the uppermost amphibolite facies, and the MG and GG possibly the granulite facies metamorphism. The rare earth element (REE) distribution shows that the gneisses are enriched in the lighter rare earth elements (LREE) La, Ce, Pr and Nd, with concentration ranges from 8.6-112.9 ppm, 18.5-264.7 ppm, 2.39-33.99 and 9.9-137.8 ppm respectively, and relatively depleted in the heavy rare earth elements (HREE) Lu, Tm, Tb and Ho, with values of 0.07-0.37 ppm, 0.08-0.59 ppm, 0.34-1.58 ppm, and 0.29-1.07 ppm respectively, and they exhibit pronounced negative EU anomaly, indicating that the rocks are highly fractionated. Combined geochemical signatures, REE-chondrite normalized spidergrams and plots in the chemical discrimination diagrams including the Y versus Nb plot, show that the protoliths of the MG and GG gneisses 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. The GSG and GBG gneisses were derived from peliticprotoliths.&picture=http://www.aascit.org/aascit/decorators/img/journal/899.jpg)
| [1] | Obioha Young Ezenwa, Geosciences Department, Federal University of Technology, Owerri, Nigeria. |
| [2] | Ekwueme Barth Nwoye, Department of Geology, University of Calabar, Calabar, Nigeria. |
| [3] | Ephraim Bassey, Department of Geology, University of Calabar, Calabar, Nigeria. |
Gneisses, which include migmatitic gneiss (MG), granite gneiss (GG), garnet biotite gneiss (GBG), hornblende gneiss (HG) and garnet sillimanite gneiss (GSG), underlie more than 70% of the area northwest Obudu Plateau in Southeastern Nigeria. These rocks are associated with schists, amphibolites and metagabbros and are intruded by granites, dolerites and pegmatites. The mineral assemblage consists of biotite – garnet - plagioclase (An32), orthoclase – kyanite – sillimanite – hypersthene, indicating that the GBG and GSG had attained the uppermost amphibolite facies, and the MG and GG possibly the granulite facies metamorphism. The rare earth element (REE) distribution shows that the gneisses are enriched in the lighter rare earth elements (LREE) La, Ce, Pr and Nd, with concentration ranges from 8.6-112.9 ppm, 18.5-264.7 ppm, 2.39-33.99 and 9.9-137.8 ppm respectively, and relatively depleted in the heavy rare earth elements (HREE) Lu, Tm, Tb and Ho, with values of 0.07-0.37 ppm, 0.08-0.59 ppm, 0.34-1.58 ppm, and 0.29-1.07 ppm respectively, and they exhibit pronounced negative EU anomaly, indicating that the rocks are highly fractionated. Combined geochemical signatures, REE-chondrite normalized spidergrams and plots in the chemical discrimination diagrams including the Y versus Nb plot, show that the protoliths of the MG and GG gneisses 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. The GSG and GBG gneisses were derived from peliticprotoliths.
Keywords
Geochemistry, Gneisses, Metamorphism, Obudu Plateau, Protoliths, Rare Earth Element
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