Neoproterozoic Magmatism in the Southern Arabian-Nubian Shield: Implications for Petrogenesis and Tectonic Setting

Abstract

The metavolcano-sedimentary assemblages and the associated granitoid intrusions in Central Eritrea, part of the southern Arabian-Nubian Shield, were primarily formed by accretion of juvenile island arcs during the Neoproterozoic. The metavolcanic rocks comprise mafic to felsic rocks, which show tholeiitic affinities. They exhibit variable SiO2 (41.63–78.4 wt%), Al2O3 (8.51–23.41 wt%), MgO (0.05–13.98 wt%), Fe2O3 (3.11–15 wt%), Cr (2.83–834 ppm), Ni (1.63–244 ppm), and Zr (16.4–128 ppm) that form linear correlations. They have low ΣREE (20–144 ppm) and display flat chondrite-normalized REE patterns with negative and positive Eu anomalies (Eu/Eu* = 0.67–1.36). They are generally rich in LILE (Ba, Pb, and U) and depleted in HFSE (Ta, Nb, and Ti), and have low initial 87Sr/86Sr (0.70079–0.70525), positive eNd values (+ 4.0 to + 6.4) and moderate Pb isotope compositions (206Pb/204Pb = 17.598–19.446, 207Pb/204Pb = 15.518–15.639, and 208Pb/204Pb = 37.175–38.811). These lines of chemical evidence indicate that slab-derived fluids and melts significantly modified the mantle source. In contrast, the ca. 850 Ma Emba-Derho granites display restricted compositions which show high SiO2 (71.28–73.64 wt%), Al2O3 (14.93–15.89 wt%), Na2O (3.16–4.15 wt%), and low K2O (1.25–2.04 wt%). These samples exhibit highly fractionated REE patterns (e.g., La/Yb = 14–30), high Sr/Y (42–86), and low Y (3.75–6.03 ppm), as well as high eNd (+ 4.7 to + 7.8), suggesting that they were partial melts of the subducting oceanic crust. Both the metavolcanic rocks and the granitic intrusions are suggested to have been formed in a forearc setting.