Deformation and Metamorphic History of Precambrian High-Grade Rocks of Key Afer Area, Southwestern Ethiopia

Abstract

The deformation and metamorphic history of the Precambrian high-grade rocks in the Key Afer area, southwestern Ethiopia within the Mozambique belt is described. It comprises poly-deformed, metamorphosed, and migmatized rocks with intrusion of granitoids and overlain by Quaternary sediments. A combination of field litho-structural mapping, metamorphic mineral assemblages, and microstructural analysis there are three metamorphic events and four phases of ductile deformation, and one Cenozoic brittle fracture (D5) are recognized. The development of the relatively steep NNW-SSE trending S1 relict gneissic banding and the rise of pyroxene and anhydrous minerals indicate that the peak metamorphism (M1) is synchronous with D1. Subsequently, the hydration of M1 assemblages leads to the formation of amphibolite facies (M2). This is followed by the development of amphibolite facies (M2) caused by the hydration of M1 assemblages synchronous with the D2 deformation. It is defined by the major regional fabric (S2) of the area trending NW-SE, tight to isoclinal upright F2 folds, and local L2 lineation. These D2 upright folds are orthogonally superimposed by another upright F3 folds during D3 resulting in a type-I fold interference pattern. The replacement and breakdown of hornblende to epidote, biotite to chlorite, and plagioclase to sericite give a retrogressive event to greenschist facies (M3) syn-D4. It gave rise to NNE-SSW-oriented S4 mylonitic foliations associated with F4 drag folds. Both sinistral and dextral shear sense is recorded but dextral shear sense appears dominant. The fifth phase of deformation (D5) is characterized by brittle fracture and joint structures of the area with varying orientations. The three metamorphic events with deformational episodes of the study show a clockwise P-T path loop from burial to uplift similar to the collision-parallel shearing orogenic setting.