Petrographic Description and Structural History of the Megab-Hawzien Area, Tigray, Northern Ethiopia
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Date
2015-06-06
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Addis Ababa University
Abstract
The Megab - Hawzien area is a part of the Neoproterozoic basement rocks of
Northern Ethiopia. The Northern Ethiopia is one of the regions where the basement
units consisting of older, low-grade meta-volcanic rocks known as the Tsaliet Group
and younger, low grade meta-sedimentary rocks referred to the Tambien Group.
These low-grade meta-volcanic and meta-sedimentary rocks have long been
considered to be part of the Arabian-Nubian Shield. The study area contains
metavolcanic (i.e., meta-acidic) and metavolcanoclastic( i.e., felsic meta-volcanic
breccia) rocks which belong to the Tsaliet Group; Phanerozoic formations such as
Enticho sandstone, Adigrat sandstone and Edaga Arbi Glacials which are part of the
Tambien Group; felsic sub intrusive; granite, and phyllite(i.e., spotty micaceous and
quartzo-feldspathic) rock units. The largest part of the study area is covered by the
MV and MVC rock units followed by the granite rock unit. The phyllite unit of the
study area is not part of either the meta-volcanic or the meta-sedimentary units. It is
rather an intervening zone between these two main units.
Depending on the field structural data analysis, the rock out crops of the study area
has three phases of deformation. These are:- (i) the first or earlier phase of
deformation(D1), (ii) the Second phase of deformation(D2), and (iii) the third phase
of deformation(D3). D1 is associated with folding of primary bedding and appearance
of tight to isoclinal minor folds, persistent foliation (S1), and linear fabrics defined by
platy minerals. D2 is related to N-S trending upright, open folds and crenulation
lineations with sub horizontal fold axes without producing a pervasive foliation. D3 is
recognized by brittle structures like reverse faults, quartz veins and joints.
Based on the petrographic study, the out crop units of the study area are dominated by
index minerals such as chlorite, albite, epidote, muscovite, biotite, calcite and
hornblende. These minerals indicate low grade metamorphism and this particularly
belongs to the lower greenschist facies. The time relationship between metamorphism
and deformation of the rock units of the study area are formed between the two
tectonic events D1and D2 and hence they are called late or inter-tectonic.
Description
Keywords
Pan-African Orogeny, Neoproterozoic, Arabian Nubian Shield, Megab- Hawzien Area, Metavolcanic Rocks, Metavolcanoclastic Rocks, Phyllite, Petrography, Metamorphism, Deformation, Structures, Porphyroblasts