Spinel-bearing assemblages adn their metamorphic significance from 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-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 spine] symplectites to be 527 +/- 112 degreesC and 4 +/- 1 kbar. Textural relationships indicate 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 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 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 evidenced by spinal 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. (C) 2001 Elsevier Science Limited. All rights reserved.