Characterization of Selen Wuha Mafic-Ultramafic Massif of Raya Marginal Graben, Northern Ethiopia

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Addis Ababa University


Ultramafic intrusion is found in Selen Wuha area of Raya marginal graben. Selen Wuha area is located 600km away from Addis Ababa to the northern Ethiopia. Some of the previous researchers were considered it as basement low grade metamorphic unit while others have not conformed to this idea and considered is as young aged un-metamorphosed igneous intrusion. Thus, the target of this study is to resolve this gap by characterizing the intrusion using field observation, petrography and geochemical data. The study area covers about 20km2 and it has mapped with the scale of 1:25,000. Accordingly, ultramafic unit, plagioclase-olivine phyric basalt, pyroclastic material and alluvial sediment deposit are recognized major mappable units. Moreover, small intrusions (dykes), normal faults, lineaments, joints and veins are identified geologic structures of the area. Hence, the contact of ultramafic with adjacent units is controlled by tectonic contacts. Minerals in the ultramafic are coarse grained Cpx, Opx, Ol and Fe-Ti oxide. Similarly, minerals in gabbroic units are coarse grained plag, Cpx, Ol and Fe-Ti oxides. Texturally, both units are porphyritic-phaneritic in which the size of grains is range from medium pheneritic to coarse pheneritic. Plagioclase-olivine phyric basalt has porphyritic texture in which Ol and plag are the dominant phenocryst minerals. Based on the proportion of Ol, Pxn, and concentration of MgO, the studied ultramafic unit is classified into pyroxenite and peridotite. Since any sign of metamorphism is not found in the unit, it is an igneous intrusion which has intruded in early Cenozoic formations and Mesozoic sedimentary successions. Accordingly, the studied ultramafic unit is not Neoproterozoic in age however it is Cenozoic aged intrusion. From the geochemistry point of view, the ultramafic unit is derived from depleted mantle source since it shows high concentration of MgO (22.31 wt. % to 37.08 wt. %), Mg # (72.63 to 81.94), Cr (1690ppm to 3000ppm), Ni (1030ppm to 1980ppm) and Co (96ppm-143ppm). It shows also slight depleted in LREE and very low concentration of the highly incompatible trace elements. Furthermore, the samples are cogenetic, sourced from homogeneous source and related by fractional crystallization since they show good correlation in the selected major oxide and trace element variation diagrams.



Depleted Mantle Source, Porphyritic- Phaneritic Texture, Pyroxenite, Peridotite, Co-Genetic, Homogenous Source, Related by Fractional Crystallization