oa South African Journal of Geology - Metamorphic zonation and thermal history of pelitic gneisses from western Namaqualand, South Africa

Volume 89 Number 2
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590



Assemblages in pelitic gneisses reveal a symmetrical pattern of east-west-trending zones in the western Namaqualand Metamorphic Complex. Within the granulite facies region a central zone of highest grade contained hercynite-rich spine! coexisting with quartz at the metamorphic peak over an area of about 10 000 km2, suggesting T ≥ 800°C at P ≤ 5 kb.

The metasedimentary rocks experienced constant or increasing pressure during both the prograde and retrograde high T history. Uplift, therefore, did not play a part in achieving the very high thermal gradients of the metamorphic peak. Low variance mineral assemblages and evidence for local gradients in a(H20) indicate that moving fluids were not responsible for the heat flux. The P-T evolution can be explained by the addition of voluminous acid magmas at or above the present erosion level. The metamorphic history rules out any tectonic model for the Namaqualand Metamorphic Complex which would be grossly out of isostatic equilibrium during the metamorphic climax.

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