Characterization of Selen Wuha Mafic-Ultramafic Massif of Raya Marginal Graben, Northern Ethiopia
No Thumbnail Available
Date
2018-05-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
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.
Description
Keywords
Depleted Mantle Source, Porphyritic- Phaneritic Texture, Pyroxenite, Peridotite, Co-Genetic, Homogenous Source, Related by Fractional Crystallization