A Back-arc Palaeotectonic Setting for the Augaro Neoproterozoic Magmatic Rocks of Western Eritrea

The Augaro volcano-sedimentary assemblages of western Eritrea are part of the Neoproterozoic, N-S trending belt of low-grade volcano-sedimentary and associated plutonic rocks. In contrast to the volcanic-dominated oceanic-arc assemblages in central Eritrea, the predominant rock types in the west are supracrustal sequences of sedimentary origin with subordinate volcanic rocks. These Augaro supracrustal rocks are overlain, unconformably, by a basin-fill metasedimentary succession known as the Gulgula Group. The Augaro metavolcanic rocks are tholeiitic and range in composition from basalt to basaltic andesite. Comparison of trace element characteristics and N-MORB-normalised spidergrams of these rocks with those of modern volcanic environments and age-comparable metavolcanic rocks of known tectonic association from the Arabian-Nubian Shield suggest that the volcanic assemblages from western Eritrea were generated in a back-arc tectonic setting.

Single zircon Pb-Pb evaporation and vapour-transfer U-Pb analyses of magmatic zircons from pre/syn-tectonic granites yield a mean ²⁰⁷Pb/²⁰⁶Pb age of 849±20 Ma and an upper concordia intercept age of 849±26 Ma. These ages are interpreted to represent the time of major magmatism in western Eritrea and are comparable to ages of early arc magmatism in central and northern Eritrea and in the southern Nubian Shield. Initial eNd values and initial Sr isotope ratios of whole-rock samples of magmatic rocks calculated for an age of 850 Ma range from +4.0 to +7.1 and 0.7026 to 0.7037, respectively. Single zircon ²⁰⁷Pb/²⁰⁶Pb ages, initial eNd value and Sr isotope ratio for a granitic clast in the Gulgula metaconglomerate suggest that the source area for the Gulgula metasedimentary rocks is similar to the surrounding Neoproterozoic rocks of western Eritrea.