Tectonometamorphic evolution and U-Pb dating of the high-grade Hammar Domain (Southern Ethiopian Shield); implications for the East-African Orogeny
Abstrakt
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, D-1 to D-4, resulting in (a) relict compositional banding (S-1), (b) flat-lying migmatite foliation (S-2) defining the primary contacts of granulites and migmatites, (c) superimposed steeply dipping N-S trending compressional foliation (S-3) due to regional similar to E-W oriented compression and (d) later similar to NW-SE trending left-lateral transpressive fabric (S-4).
New geochmnological 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 Ma) where large volcanic arc construction as the source of rock photoliths (dated at ca. 770 Ma) was followed by crustal accretion and flat-lying fabrics origin (D-2 stage), intense migmatization and HT-MP metamorphism (T: 700-850 degrees C and P: 0.7-0.9 GPa) at depths of similar to 25-35 km (dated at ca. 720 and 715 Ma). (b) Late Cryogenian to early Ediacaran episode (ca. 650 to 620 Ma) as the key era of continental collision leading to the Greater Gondwana assembly. An early similar to E-W oriented compression (D-3 stage) resulted in similar to N-S trending fabrics that have been continuously changed to the left-lateral transpression (D-4 stage) forming similar to NW-SE oriented foliations.
The time-scale of D3 and D-4 events is inferred by syn-tectonic granitoid intrusions yielded at ca. 648 Ma and ca. 630 Ma respectively. Furthermore, the syn- to post-tectonic leucogranite dike, dated at ca. 630 Ma, marks the upper limit for the ductile or brittle-ductile deformation and regional metamorphic events.