Petrology and age of the Otjisazu Carbonatite Complex, Namibia: implications for the pre- and synorogenic Damaran evolution

The Otjisazu Carbonatite Complex intruded crustal rocks of the Damara Oroge n in central Namibia close to the Okahandja Lineament. The igneous complex consists of a suite of mela-syenites, syenites and alkali feldspar syenites , and a carbonatite suite comprising clinopyroxenites with calcite and calc ite-garnet lenses, feldspathic clinopyroxenites and sovites with wollastoni te-rich lenses. The two suites are considered independent magmatic sequence s. Thermodynamic calculations suggest that the syenite suite represents a c oherent fractionation sequence which evolved by predominantly clinopyroxene , and subordinate fluorapatite and magnetite fractionation at 5-6 kbar. The mid-crustal crystallisation level is supported by the supposed cumulative character of most lithologies of the carbonatite suite. Clinopyroxenites an d calcite +/- garnet lenses of the carbonatite suite are considered largely cumulative rocks and represent the lower portion of a carbonatite magma ch amber that crystallised clinopyroxene, calcite, andraditic garnet, titanite and fluorapatite. U-Pb dating of titanites suggests an intrusion age of 83 7 + 60/-49 Ma, and a metamorphic overprint at 558 +/- 5 Ma. These age const raints indicate a rift-related setting of the Otjisazu Carbonatite Complex within the Damaran Orogen. (C) 2001 Elsevier Science Limited. All rights re served.