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

The Central Zone of the Damara Orogen in Namibia is dominated by intrusive granitoids and upper amphibolite to granulite-facies metasediments. High-te mperature low-pressure mineral assemblages near the Atlantic Coast have not been previously described with spinel symplectites. In semi-pelitic gneiss es, the symplectites are composed of hercynitic spinel and Fe-Ti oxides occ urring within the cores of zoned porphyroblasts of antiperthite + sillimani te. In pelitic schists, hercynite-sillimanite symplectites occur together w ith minor quartz in the core of Mg-cordierite porphyroblasts (X-Mg = 0.66-0 .72). Thermobarometry determines the P-T conditions of spine] symplectites to be 527 +/- 112 degreesC and 4 +/- 1 kbar. Textural relationships indicat e that the symplectite post-dates the metamorphic peak conditions dated by syn-metamorphic granite at 534 +/- 7 Me (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 degreesC and 4 +/- 1 kbar for this episode. This regional isobaric heating is equated wi th the emplacement of voluminous granitoid magmas, dated at post-534 +/- 7M a and pre-508 +/- 2 Ma (published U-Pb zircon and monazite ages) and genera ted during decompression caused by the exhumation of the Central Zone. The system then cooled through 550 +/- 25 degreesC 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 evide nced by spinal symplectites and garnet growth argues in favour of a two-sta ge metamorphic model. This study confirms the clockwise P-T path for the Ce ntral Zone of the Damara Orogen. (C) 2001 Elsevier Science Limited. All rig hts reserved.