Petrology of ferroan alkali-calcic granites: Synorogenic high-temperature melting of undepleted felsic lower crust (Damara orogen, Namibia)

Abstract

The 556±4Ma-old Bloedkoppie granite (Central Damara orogen, Namibia) is a metaluminous to slightly peraluminous, alkali-calcic to calc-alkalic and ferroan granite. Its composition implies high-temperature, reduced, and anhydrous conditions during granite formation. The granite is fractionated, but heterogeneous radiogenic isotope data (87Sr/86Sr(init.): 0.712 to 0.727; εNd(init.): −7.2 to −13.1; 206Pb/204Pb: 17.30–17.72; 207Pb/204Pb: 15.54–15.67; 208Pb/204Pb: 37.80–38.23) indicate also that combined assimilation–fractional crystallization processes played an important role in the generation of the granite. Major and trace element compositions and isotope data of the least evolved samples and U–Pb data from zircon cores demonstrate that the source rocks are dominated by ca. 1.95Ga old felsic orthogneisses from the underlying basement. Zircon saturation temperatures and normative Qz–Ab–Or compositions indicate minimum melting P–T conditions of ca. 860°C at >5kbar and <5wt.% H2O. The most likely petrogenetic model involves high temperature partial melting of a Paleoproterozoic felsic source in the lower crust ca. 10–20Ma before the first peak of regional high-temperature metamorphism. Underplating of the lower crust by magmas derived from the lithospheric mantle may have provided the heat for melting of the undepleted basement to produce reduced and anhydrous melts.