Geochemistry, geochronology and isotope geology of Nakfa intrusive rocks, northern Eritrea: products of a tectonically thickened Neoproterozoic arc crust

Abstract

The north-south-trending Neoproterozoic volcano-sedimentary plutonic associations in northern Eritrea are part of the Nubian Shield. The Nakfa intrusive rocks range in composition from gabbro to syeno-diorite to granite and alkaline syenite and intrude supracrustal rocks of volcanic and sedimentary origin. All granitoid rocks are metaluminous or slightly peraluminous and have typical I-type chemical signatures. The calc-alkaline intrusive rocks and the alkaline syenites have geochemical characteristics (e.g. low Nb values) typical of arc intrusives and plot as volcanic arc granites on various discriminant diagrams. Single zircon evaporation Pb-Pb ages and conventional multigrain U-Pb ages on zircons and titanites yielded emplacement ages of ∼620–640 Ma. These are comparable to those of adjacent juvenile terranes in the Nubian Shield. No pre-Pan-African rocks have so far been found in northern Eritrea. Isotopic data show a limited range, with initial ε Nd values ranging from 3.5 to 5.6 and initial Sr ratios from 0.7018 to 0.7037. The high positive initial ε Nd values and low initial Sr ratios indicate that the granitoid rocks were derived from a mantle and/or juvenile crustal source with no, or only insignificant, contribution from an older continental component. This is further supported by the absence of inherited zircons and the lack of rocks of continental affinity. Leached K-feldspars from Nakfa intrusive rocks have Pb isotope ratios ( 206Pb/ 204Pb = 17.60–17.88, 207Pb/ 204Pb = 15.49–15.53 and 208Pb/ 204Pb = 37.12–37.37) similar to those for ‘oceanic leads’ from Saudi Arabia, which are interpreted as manifesting a mantle source. Hence, the Pb isotope ratios, in agreement with the Sr and Nd isotopic data, indicate an insignificant involvement of older crustal components in the generation of Pan-African crust in northern Eritrea. The ages and isotopic characteristics of the Nakfa intrusive rocks are comparable to those of adjacent juvenile terranes in the Arabian-Nubian Shield and strengthen models for the generation of large volumes of juvenile continental crust during Neoproterozoic times. A substantial number of Nakfa granitoid rocks show geochemical characteristics (low Y, high Sr/Y and steeper REE pattern), indicating that crust formation involved the fractional crystallisation of mafic magma and the melting of lower crust.