Tectono-metamorphic history of Neoproterozoic rocks in eastern Eritrea

Abstract

The Neoproterozoic juvenile crust of Eritrea lies within the East African Orogen that marks a megasuture along which the East and West Gondwana converged. In eastern Eritrea, repeatedly deformed Neoproterozoic rocks of this orogen consist of two lithotectonic terranes, Ghedem and Nacfa. The Ghedem Terrane consists of amphibolite facies metamorphic rocks with dominantly subhorizontal to moderately dipping fabrics. The structurally overlying Nacfa Terrane consists of greenschist facies metamorphic rocks with steeper fabrics. Three deformation phases are distinguished that can be related to E–W lateral compression and E–W orogenic collapse of Neoproterozoic age. D1 structures are represented by an originally N–S/NNE–SSW steep and penetrative S1 foliation which is axial planar to F1 isoclinal upright folds. D1 structures were refolded by recumbent folds and nappes during D2, when deformation was dominated by subhorizontal shear zones accompanied by recumbent meso-microscopic NNW to NNE trending F2 folds. D3 deformation formed open upright F3 folds co-axial with F2 and a steep S3 spaced axial planar foliation accompanied by strike–slip shear zones. The D1–3 structures and fabrics are interpreted as phases of a long-lived Neoproterozoic lateral compression. Orogenic collapse during D2–3 was characterized by structures that were ductile at depth and semi-brittle at shallow levels. P– T conditions of metamorphism were determined from the mineral chemistry of pelitic–semipelitic rocks in the Ghedem Terrane. Mineral assemblages included within garnets and matrix assemblages in textural equilibrium with garnet rims define two P– T fields. Estimated P– T conditions of metamorphism for the inclusion assemblages in garnet cores range between 3.9 and 5.3 kbar and 516–584°C whereas those for the rim–matrix assemblages are between ∼5–11 kbar and 526–637°C. Microstructural inferences and the P– T conditions obtained from the mineral assemblages within garnets indicate that prograde M1 greenschist facies metamorphism was contemporaneous with D1 deformation. Similarly, the P– T equilibrium conditions constrained for the rim–matrix assemblages relate to late M2 amphibolite facies metamorphism during late D2 deformation. Textural relationships reveal that M3 metamorphism was retrogressive during D3. Inclusion and rim–matrix geothermobarometry depict a family of anticlockwise P– T paths indicating heating and contemporaneous loading during M1/D1 at intermediate crustal levels and nearly isothermal loading during early M2/D2 at deeper crustal levels followed by a return path during late D2 and D3 deformations. These rocks represent a segment of the laterally shortened and subsequently extended mid-lower continental crust of the East African Orogen that was exhumed from as deep as 48 km (12 kbar) during late Neoproterozoic interaction between orogenic collapse, erosion and deep isostatic root recovery. The preliminary interpretation is that the relationships between the P– T conditions of metamorphism and deformation in the Ghedem Terrane rocks of eastern Eritrea indicate a continental orogen related to the late Neoproterozoic terminal convergence between East and West Gondwana.