Mesoproterozoic U-Pb and Pb-Pb ages of granulites in NW Namibia: reconstructing a complete orogenic cycle

Abstract

The chronology of the crustal evolution of the granulite facies fault-bounded Epembe Unit south of the anorthositic Kunene Intrusive Complex (KIC) in NW Namibia is presented. U-Pb zircon ages by ion microprobe (SHRIMP II) from a heterogeneous set of granulite facies rocks from the Epembe Unit reveal Mesoproterozoic ages between 1520 and 1510 Ma for prograde zircon growth. Cathodoluminescence (CL) imaging clearly shows metamorphic growth of zircon grains and zircon rims around igneous cores. Zircon core analyses obtained by SHRIMP yield ages between 1635 and 1810Ma. These ages are comparable to those of the Epupa basement and indicate derivation of the inherited zircon cores from Palaeoproterozoic protoliths. Pb-Pb stepwise leaching (PbSL) TIMS ages on peak-metamorphic garnet and retrograde sapphirine reflect granulite facies conditions at ca. 1490 to 1447 Ma. Our combined age data from the granulite facies Epembe Unit reveal a complete early Mesoproterozoic orogenic cycle between ca. 1640 and 1450 Ma. The cycle begins with erosion of the Palaeoproterozoic Epupa basement and involves sedimentation, burial down to the crust-mantle boundary leading to high-grade metamorphism and rapid exhumation. An orogenic cycle of this age matches in time with events dated in the Pinwarian (Grenville Province, Canada), in the Amazonian craton (Brazil) and in the Gothian (Baltic Shield, Sweden) but has not been reported in southwestern Africa before.