Structure, Lithology and Metamorphism of Phyllite Zone Around Hawzien, Tigray, Northern Ethiopia
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Date
2013-08-12
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Addis Ababa University
Abstract
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.
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Keywords
Structure, Lithology and Metamorphism, Phyllite Zone Around Hawzien, Tigray, Northern Ethiopia