Enrichment from plume interaction in the generation of Neoproterozoic arc rocks in northern Eritrea: implications for crustal accretion in the southern Arabian–Nubian Shield

Abstract

The Neoproterozoic volcano–sedimentary–plutonic associations in Eritrea are part of the Arabian–Nubian Shield (ANS). In the Nakfa region, northern Eritrea, low-grade metavolcanic rocks consist of calc-alkaline crystal tuffs, lapilli tuffs and basic and felsic lava flows. These are intruded by pre/syn- to late-tectonic calc-alkaline intrusive rocks ranging in composition from gabbro to granite. They have element characteristics and MORB-normalized geochemical patterns similar to modern arc lavas. These similarities include enrichment in large ion lithophile elements and depletion in high-field strength elements. SHRIMP U–Pb analyses of zircons from a metarhyolite yielded a concordant age of 854±3 Ma, suggesting that arc magmatism in Eritrea began at about 850 Ma. Initial ε Nd values range from 4.8 to 5.7 These values preclude significant contributions from a long-lived crustal source and indicate that the sources for these melts were juvenile. Furthermore, the remarkably restricted initial ε Nd values clearly suggest an intra-oceanic tectonic setting. These data support models that show the southern extension of the ANS in northern Eritrea as an oceanic island arc system. Compared to metavolcanic rocks of similar ages from southern Sudan, the metavolcanic rocks from Nakfa display lower initial ε Nd values at similar LILE/HFSE ratios. This is interpreted that the source of the Nakfa metavolcanic rocks was enriched prior to the onset of formation of subduction-related magmas. This enrichment process may be related to the presence of plume-related magmatism that affected the mantle source of the Nakfa metavolcanic rocks prior to subduction.