Deformation, Metamorphim and Tectonics of the Precambrian Rocks of Atsbi Area, Tigray, N. Ethiopia
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Date
2014-06
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Addis Ababa Universty
Abstract
The Tsaliet Group of Atsbi area is covered by widely distributed basement rocks of
both metavolcanic and metavolcaniclastics rocks and Phanerozoic sandstone. The
study area is represented by extensively exposed metavolcaniclastic rocks consisting
of metabreccia, tuffaceous layers and slate units and metavolcanic rocks consisting of
metabasaltic andesite and meta-andesitic units. These low grade rocks show N-S
regional trend and they are presumably correlate with similar Neoproterozoic rocks of
Arabian Nubian Shield.
The Atsbi area is one of the area where tectonic structures are recorded by the Tsaliet
Group that related to the collision history of ANS. Poly phase deformations in rocks
of the study area are marked by the existence of different types of structural element
such as regional pervasive foliation, superimposed folds, various lineation and brittle
structure such as joints. Two phases of ductile deformation (D1 and D2) and one
phase of brittle deformation (D3) are recognized. D1 deformation phase is related to
N-S compression that resulted in a pervasive foliation, sub vertical to vertical folds
and lineation whereas D2 deformation produced open, upright and harmonic low
plunging folds and lineation. The brittle structures such as joints that crosscut the
regional foliation represent the younger (D3) deformation phases of the area.
Field and petrographic observations show that the metavolcanic rocks are locally
affected by propylitic alteration and sericitization. The metavolcanic rocks locally
experienced pervasive silicification, sericitization, chloritization and epidotization.
The critical minerals such as tremolite/actinolite, epidote, chlorite, sericite, muscovite,
calcite demonstrate lower green schist facies of metamorphism of the study area.
The geochemical result from the metavolcanic rocks of Atsbi area show a small range
of composition indicating that they may have originated from a single,
chemically uniform parent melt. The metavolcanic rocks are principally identified
as low-silica andesite and only a few samples fall in the basaltic andesite field. The
mafic/intermediate rocks also show predominantly calc-alkaline affinity and The Zr-
Nb-Y diagram shows that all the metavolcanic rocks lie within plume tholeiites
volcanic arc basalt field.
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Keywords
Atsbi Area, Tigray