Structure, Lithology and Metamorphism of Phyllite Zone Around Hawzien, Tigray, Northern Ethiopia

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Addis Ababa University


The Phyllite unit, which is found around Hawzien town, Tigray Northern Ethiopia, is known as the intervening zone between the two main groups of the Neoproterozoic basement rocks of Northern Ethiopia. The two groups, Tsaliet Group and Tambien Group, generally consist of metavolcanics and metasediments respectively. These lowgrade metamorphic rocks, which constitute the southern Arabian-Nubian Shield, are the least studied zone among Precambrian rocks of the country. As a result the Phyllite unit has been variously interpreted; as metasedimentary (Arkin et al., 1971), metavolcanic (Garland, 1972, 1980) and transitional lithology (Miller et al., 2003).Moreover, the unit has been depicted as a single lithology in various geological maps. However the detailed mapping, field investigation, petrographic study and data analysis revealed that the zone is not a single monotonous unit. It consists of various units and subunits; including different type of phyllites such as spotty qurtzo feldspathic phyllite, undifferentiated slate and phyllite, chloritic slate and phyllite, spotty micaceous phyllite, metagreywake, metasubintrusives, granitoids, graphite and quartz vein. The key mineral assemblages such as sericite muscovite, chlorite and albite, indicate the rocks in area experienced low-grade metamorphism that belongs to lower green schist facies. The occurrence of relict and preserved primary structure like bedding also imply the prevalence of the low-grade metamorphism. Structural data analysis, both mesoscopic and microscopic, discloses the study area has experienced at least three phases of deformation. D1 resulted in the development of the pervasive N-S trending and west dipping secondary foliation (S1). Whereas the later phase of deformation (D2) has caused the formation of incipient S2 cleavage and mesoscopic and regional scale overturned folds. The E-W trending and vertically dipping joints that cross cut bedding and foliations are related to D3.



Structure, Lithology and Metamorphism, Phyllite Zone Around Hawzien, Tigray, Northern Ethiopia