Neoproterozoic terrane assemblages in Eritrea: review and prospects

Four, perhaps five large terranes that are structurally and lithologically distinct constitute the Neoproterozoic of Eritrea. Two are composites of structurally bounded smaller blocks, many of which also show considerable lithological differences. Accretion along regional shear zones was dominated by sinistral strike-slip. The westernmost Barka Terrane is an upper amphibolite to granulite grade complex, characterised by polyphase ductile strains. It contains metasediments and undeformed felsic dykes that have no counterparts in the other terranes. The Hagar Terrane is dominated by immature supra-subduction zone volcanic rocks, but its western parts contain oceanic basalts and pelagic sediments, deformed basic-ultrabasic rocks and a major olistostrome. Parts of this tectonised ophiolite retain high P, low T assemblages, suggesting that the Barka Shear Zone (its western boundary) was initiated as a subduction zone. Other subduction zone metamorphic assemblages in the internal Hagar Terrane may represent tectonic underplating of an evolving volcanic arc. The Adobha Abiy Terrane is a highly deformed post-accretionary sedimentary basin, formed after the close of volcanism. The Nakfa Terrane is a collage of many lithologically different volcano-sedimentary blocks bounded by shear zones, in which metamorphism generally reached greenschist and lower grades. It also includes several mafic-ultramafic belts. Those blocks for which geochemical data are available suggest magmatism in supra-subduction zone environments. In its axial part, the Nakfa Terrane reveals a regional steep belt dominated by high-grade migmatites and gneisses that formed from volcanic rocks subjected to lower crustal conditions. Late syn-kinematic granitoid plutons are regionally concentrated along this steep belt. However, these high-grade rocks show a number of differences from isolated occurrences of gneisses in the Red Sea lowlands that might constitute a fifth entity, the Arag Terrane. Three major episodes of deformation probably correspond to protracted terrane accretion, shortening and thickening of new crustal materials, and crustal reworking dominated by sinistral transpression. The last affects post-accretion sedimentary sequences in the Adobha Abiy Terrane and similar strata in the Nakfa Terrane that show lower metamorphic grade than the earlier volcano-sedimentary associations. This sequence spans a~300 Ma period from about 850 to 550 Ma

Control of the morphology of waterborne nanocomposite films

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