Spinel-bearing assemblages adn their metamorphic significance from the Central Zone of the Damara Orogen, Namibia.

PAM Nex, Grahame John Henderson Oliver, JA Kinnaird

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31 Citations (Scopus)

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 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.

Original languageEnglish
Pages (from-to)471-490
Number of pages19
JournalJournal of African Earth Sciences
Volume32
Publication statusPublished - Apr 2001

Keywords

  • GRADE METAMORPHIC ROCKS
  • PHASE-RELATIONS
  • PETROLOGY
  • HERCYNITE
  • BIOTITE
  • QUARTZ
  • PATHS
  • GRANULITES
  • STABILITY
  • HISTORY

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