Geochemistry and Geochronology of a Neoproterozoic Low-K Tholeiite-Boninite Association in Central Eritrea

Abstract

The N-S trending Neoproterozoic volcano-sedimentary-plutonic associations in Eritrea are part of the Arabian-Nubian Shield. In Central Eritrea, the dominant rocks are low-grade supracrustal volcanic rocks with subordinate sedimentary rocks. They are intruded by pre- to syn-kinematic diorites and tonalites, as well as late- to post-kinematic granites. The low-K tholeiitic metavolcanic rocks range in composition from basalt to rhyolite. The low-K tholeiites (K 2O = 0.03-0.40 wt.%) represent parental magma (Mg# from 0.46 to 0.71, average 0.63) and are characterized by relatively high abundances of MgO (5-11 wt.%), Cr (174-874 ppm) and Ni (28-249 ppm), as well as low abundances of HFSE (TiO 2 = 0.43-0.69 wt.%, Zr = 15-36 ppm, and Y = 8-20 ppm). A chondrite-normalised REE pattern for a tholeiite is depleted (Ce/Yb N = 0.2). The low-K tholeiites have trace element characteristics and MORB-normalized geochemical patterns that are similar to those of modern arc lavas. Plotting of the intermediate to felsic metavolcanic rocks on various discriminant diagrams also indicates that the metavolcanic rocks are the result of plate-margin volcanism. The geochemical data, together with the low-K tholeiite-boninite association, indicate that the metavolcanic rocks were emplaced in an environment similar to a modern oceanic island-arc (fore-arc) setting. In comparison to the boninites from this region, the low-K tholeiites have low abundances of LILE and HFSE, lower ratios of the more to the less incompatible elements, and higher initial e Nd values. The low-K tholeiites are also characterized by higher total FeO contents and relatively lower contents of refractory elements than is observed for the boninites. This may suggest that the low-K tholeiitic rocks were derived by a relatively lower degree of melting from a more depleted mantle source than the boninites. SHRIMP U-Pb isotope analyses of zircons from a pre- or syn-kinematic tonalite yielded a concordant age of 813±9 Ma. This age is similar to the crystallization age of a pre-/syn-kinematic diorite from the same region and is interpreted to approximate the time of major magmatism in Central Eritrea. Magmatic zircons from a late to post-kinematic granite yielded a concordant U-Pb age of 585±6 Ma. These results indicate that the crust of Central Eritrea was formed between ca. 600 and 850 Ma. These ages are comparable to those of northern Eritrea and adjacent juvenile terranes in the Nubian Shield.