Significance of high-grade metasediments from the Neoproterozoic basement of Eritrea

High grade orthogneiss, amphibolitic dykes and paraschist to migmatites were studied in the Red Sea Lowlands of eastern Eritrea. Chemical data and a mineral assemblage of kyanite, staurolite, almandine garnet, biotite and quartz indicate that the schists were formed by metamorphism of pelitic metasediments derived by subaerial weathering, deposited in a marine environment. Thermobarometry based on element partitioning between coexisting minerals of schist, orthogneiss and amphibolite shows that peak metamorphic conditions were ca 700 degrees C and 8-10 kbar. An overall clockwise P-T loop is suggested by the thermobarometry results, and is consistent with a collision setting. The Eritrean high-grade metasediments can be correlated with similar rocks between Arabia and this defines a sedimentary basin that was at least 400 km from north to south, Low initial Sr-87/Sr-86 (0.7014-0.7028) and epsilon Nd-(800 Ma) of +2.5 to +4.7 along with a mean T-DM model age of 1.05 Ga indicates that juvenile Neoproterozoic crustal sources controlled the sedimentation of the pelites, and permits negligible involvement of older crustal sources. Rb/Sr geochronology suggests an age of ca 650 Ma for thermal resetting, consistent with a model whereby plate collision between east and west Gondwanaland was responsible for the metamorphism and exhumation. These metasediments reflect the transition between the northern and southern sectors of the East African Orogen, containing the juvenile isotopic signature similar to rocks of the Arabian-Nubian Shield but a chemical composition and metamorphic grade that are indistinguishable from assemblages characteristic of the Mozambique Belt. (C) 1997 Elsevier Science B.V.