Tectonometamorphic evolution and U–Pb dating of the high-grade Hammar Domain (Southern Ethiopian Shield); implications for the East-African Orogeny

Abstract

Abstract Based on new U–Pb zircon data, field structural analysis and a detailed petrological study of the plutonic and high-grade metamorphic rocks of the Hammar Domain (Southern Ethiopian Shield) overall geodynamic scenario was inferred, bringing broad implications for the East-African Orogeny. The structural evolution of the Hammar Domain can be summarized into four phases, D1 to D4, resulting in (a) relict compositional banding (S1), (b) flat-lying migmatite foliation (S2) defining the primary contacts of granulites and migmatites, (c) superimposed steeply dipping N–S trending compressional foliation (S3) due to regional\xa0~E–W oriented compression and (d) later ~NW–SE trending left-lateral transpressive fabric (S4). New geochronological data point to long-lasting orogenic convergence forming the East-African Orogeny which resulted in two main geodynamic events: (a) Late Tonian to late Cryogenian episode (ca. 770 to 650\xa0Ma) where large volcanic arc construction as the source of rock photoliths (dated at\xa0ca. 770\xa0Ma) was followed by crustal accretion and flat-lying fabrics origin (D2 stage), intense migmatization and HT-MP metamorphism (T: 700–850\xa0°C and P: 0.7–0.9 GPa) at depths of\xa0~25–35\xa0km (dated at\xa0ca. 720 and 715\xa0Ma). (b) Late Cryogenian to early Ediacaran episode (ca. 650 to 620\xa0Ma) as the key era of continental collision leading to the Greater Gondwana assembly. An early\xa0~E–W oriented compression (D3 stage) resulted in\xa0~N–S trending fabrics that have been continuously changed to the left-lateral transpression (D4 stage) forming ~NW–SE oriented foliations. The time-scale of D3 and D4 events is inferred by syn-tectonic granitoid intrusions yielded at\xa0ca. 648\xa0Ma and\xa0ca. 630\xa0Ma respectively. Furthermore, the syn- to post-tectonic leucogranite dike, dated at\xa0ca. 630\xa0Ma, marks the upper limit for the ductile or brittle-ductile deformation and regional metamorphic events.