Polyphase Deformation During Prolonged High-Temperature, Low-Pressure Metamorphism: An Example From the Namibfontein-Vergenoeg Migmatite Domes, Central Zone, Damara Belt, Namibia

Abstract

The Pan-African Damara Belt in Namibia is considered a polydeformed and polymetamorphic terrain, with a long history of magmatism (~100 m.y.). However, the timing, duration and tectonic significance of high-temperature, low-pressure metamorphic phases in relation to deformation in the Central Zone remain unclear. A combined pressure–temperature–time deformation (P–T–t-d) approach applied to rocks of the Namibfontein-Vergenoeg domes, two contiguous migmatitic domes in the southern Central Zone of the Damara Belt, suggests that these rocks underwent multiple deformation phases at suprasolidus HTLP conditions, over a prolonged period of time. Monazite petrochronology of structurally controlled leucogranite and leucosome reveals that early E–W shortening (D1) linked to the Kaoko Belt formation operated from < 558 to ~535 Ma. At ~535–532 Ma, a tectonic switch from E–W to NNW–SSE shortening and progressive D2 and D3 occurred, linked to the formation of the Damara Belt. Lastly, enigmatic localised NE–SW shortening (D4), parallel to the main structural trend of the Damara Belt, was active from ~523 to ~494 Ma. Monazite U–Pb geochronology records at least ~50 m.y. (~540 to ~494 Ma) of monazite crystallisation and recrystallisation at suprasolidus conditions. Petrography and pseudosection modelling of Damara Supergroup metapelite indicate that all deformation occurred at similar, anatectic, HTLP conditions of 740°C–780°C and ~5 kbar. This tectono-metamorphic evolution is recorded in a shallow dP/dT PT path where the three regional shortening events (D1, D2–D3 and D4) operated in a thermally driven system with little change in pressure. These results suggest that deformation is compatible with a model of distributed strain at the scale of the orogen and a lack of pronounced crustal thickening. Long-lasting, high-temperature suprasolidus conditions raise questions regarding possible heat sources for sustained HTLP metamorphism in the Central Zone. Furthermore, these data call into question the importance of crustal thickening and subsequent exhumation of the Central Zone.