Deformation, Metamorphim and Tectonics of the Precambrian Rocks of Atsbi Area, Tigray, N. Ethiopia

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Addis Ababa Universty


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.



Atsbi Area, Tigray