Spinel-bearing assemblages and P-T-t evolution of the Central Zone of the Damara Orogen, Namibia

Abstract

The Central Zone of the Damara Orogen in Namibia is dominated by intrusive granitoids and upper amphibolite to granulite-facies metasediments. High-temperature low-pressure mineral assemblages near the Atlantic Coast have not been previously described with spinel symplectites. In semi-pelitic gneisses, the symplectites are composed of hercynitic spinel and Fe-Ti oxides occurring within the cores of zoned porphyroblasts of antiperthite+sillimanite. In pelitic schists, hercynite-sillimanite symplectites occur together with minor quartz in the core of Mg-cordierite porphyroblasts (X Mg = 0.66–0.72). Thermobarometry determines the P-T conditions of spinel symplectites to be 527 ± 112°C and 4 ± 1 kbar. Textural relationships indicate that the symplectite post-dates the metamorphic peak conditions dated by syn-metamorphic granite at 534 ± 7 Ma (published U-Pb zircon age) but are still relatively early within the metamorphic history. Subsequent isobaric regional heating and partial melting occurred in association with biotite breakdown and garnet growth. Thermobarometry gives 700 ± 50°C and 4 ± 1 kbar for this episode. This regional isobaric heating is equated with the emplacement of voluminous granitoid magmas, dated at post-534 ± 7Ma and pre-508 ± 2 Ma (published U-Pb zircon and monazite ages) and generated during decompression caused by the exhumation of the Central Zone. The system then cooled through 550 ± 25°C and 2 ± 1 kbar at 465 ± 5 Ma (published Rb-Sr and Ar 40/Ar 39 biotite-muscovite ages) on its way to the surface. The occurrence of a later separate thermal overprint as evidenced by spinel symplectites and garnet growth argues in favour of a two-stage metamorphic model. This study confirms the clockwise P-T path for the Central Zone of the Damara Orogen.