Sr, Nd isotopes and geochemistry of the Bayuda Desert high-grade metamorphic basement (Sudan): an early Pan-African oceanic convergent margin, not the edge of the East Saharan ghost craton?

Abstract

The high-grade metamorphic basement of the Bayuda Desert is situated at the inferred transition between the juvenile Neoproterozoic mainly greenschist facies Arabian–Nubian Shield (ANS) and the pre-Neoproterozoic mainly amphibolite facies domain of the East Saharan Ghost Craton. New geochemical and Sr–Nd isotope data reveal that this basement in Bayuda constituted a Neoproterozoic oceanic convergent margin succession with limited and probably late input of old material. Within this series, garnet amphibolites and epidote–biotite gneisses have geochemical characteristics of HFSE-depleted tholeiitic basalts and low- to medium-K dacites and rhyodacites, indicating magmatism in an oceanic island arc or back-arc basin environment. This magmatism occurred at 806±19 Ma (Sm–Nd 11 WR isochron), similar in age to arc magmatism in the ANS. Leucocratic gneisses, muscovite schists and garnet–biotite schists form the dominant meta-sedimentary rocks of the study area. They were primarily derived from two different sources: volcanogenic sediments from a Neoproterozoic island arc (T DM Nd model ages between 790 and 900 Ma) and terrigeneous sediments from an older continental source (T DM Nd model ages up to 2100 Ma). The volcanosedimentary succession was metamorphosed under amphibolite facies conditions prior to 670 Ma, probably at approximately 700 Ma. The high-grade metamorphism is related to a frontal collisional event that also produced syn-collisional peraluminous granites preserved as muscovite–biotite gneisses. Meta-igneous rocks from eastern Bayuda have ε Nd values at 806 Ma of +5.2±0.4, indicating a less depleted mantle source (crustal contamination can be excluded) than the neighbouring Gabgaba–Gebeit terrane ( ε Nd, ca. +7) from the Arabian–Nubian shield. Less depleted mantle source is also known at Jebel Moya to the south, and inside the Arabian–Nubian Shield to the southeast of Bayuda. Lithological and structural similarities (dominantly northeast striking foliation) with the Bayuda Desert succession occur in many parts of central and western Sudan, suggesting a comparable geodynamic evolution. It is proposed that a Neoproterozoic oceanic convergent margin (Bayuda type succession) collided at approximately 700 Ma to the northwest with the East Saharan ghost craton, whose easternmost limit must lie below or more to the west than previously thought.