Low-P / high-T metamorphism in the Okiep Copper District, western Namaqualand, South Africa

• Published in: Journal of metamorphic geology Vol. 16; no. 2; pp. 281 - 305
• Main Authors: RAITH, J. G., HARLEY, S. L.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01-03-1998
• Subjects: granulite; low-pressure high-temperature metamorphism; Namaqualand; Okiep Copper District; polymetamorphism; thermobarometry
• ISSN: 0263-4929; 1525-1314
• DOI: 10.1111/j.1525-1314.1998.00131.x

The Okiep Copper District, part of the 1.2–1.0 Ga high‐grade terrane in western Namaqualand, is composed of a mid‐Proterozoic supracrustal sequence and several pre‐ to post‐orogenic intrusive suites affected by two high‐grade events (M2a/M2b, M3) of Kibaran and one low‐grade event (M4) of Pan‐African age. Peak assemblages in quartz‐bearing pelites are characterized either by garnet+cordierite coexisting with sillimanite/biotite, or by biotite+sillimanite±garnet; a difference controlled by bulk composition and variation in water activities (0.1–0.7) during dehydration melting. Maximum P–T conditions were reached during M2a coevally with the major deformational event (D2a) and are estimated at 750–820 °C and 5–6 kbar. A counterclockwise P–T  path is indicated by regionally occurring pseudomorphs of sillimanite after andalusite and by prograde reaction textures preserved as relics in M2a porphyroblasts. Two stages of retrograde metamorphism are distinguished: M2a garnet+cordierite‐bearing assemblages were retrogressed to biotite+sillimanite+quartz (M2b) along discontinuous foliation planes and shear zones (D2b). Retrograde M3 corona assemblages formed at similar P–T  conditions (580–660 °C and 5.8±0.5 kbar) to the M2b assemblages but M3 crystallization postdates penetrative D2 deformation, intrusion of 1.06 Ga granitoids and formation of associated W–Mo deposits. It is concluded that: (a) Kibaran high‐grade metamorphism in the Okiep Copper District is thermally punctuated and (b) reaction textures documenting apparent isobaric cooling of this low‐P high‐T  terrane must be interpreted with caution.