The timing of mantle and crustal events in South Namibia, as defined by SHRIMP-dating of zircon domains from a garnet peridotite xenolith of the Gibeon Kimberlite Province

Abstract

The Gibeon Kimberlite Province, southern Namibia, comprises numerous, 70-80 Ma old kimberlite pipes and dykes commonly containing abundant mantle and crustal xenoliths. We dated by SHRIMP II zircons from a garnet peridotite xenolith (Pipe Hanaus 1). The xenolith consists of garnet, orthopyroxene, clinopyroxene and, almost completely serpentinised, olivine. Crystallisation P-T conditions range between ca. 3.3 GPa, 910 °C (core composition of minerals) and ca. 3.5 GPa, 1000 °C (rim composition). Fifteen ca. 150-40 μm long and 70-10 μm wide zircon crystals show more or less pronounced oscillatory zoning indicating precipitation in a melt or fluid. The SHRIMP results define three age groups: 1013 ± 22 Ma, 751 ± 28 Ma and 627 ± 24 Ma. These ages are clearly correlated with large-scale crustal events in the broad area of Namibia, namely with the granulite-facies metamorphism during the Namaqua collisional event and post-collisional magmatism (ca. 1000-1100 Ma), with the intense magmatic activity and initial rifting stages in the Damara belt (ca. 730-770 Ma) and with the Early Pan-African orogeny, responsible for the formation of the Damara orogen (ca. 650 Ma), respectively. Formation of zircon in the xenoliths is related to the generation and migration of fluids/melts in the mantle via e.g. asthenospheric mantle upwelling or dehydration of subducting lithosphere, associated with crustal large-scale events.