Corona textures in Proterozoic olivine melanorites of the Equeefa Suite, Natal Metamorphic Province, South Africa

Olivine-plagioclase and phlogopite-plagioclase coronas have been identified from olivine melanorites of the Mid- to Late Proterozoic Equeefa Suite in southern Natal, South Africa. Olivine, in contact with plagioclase, is mantled by a shell of clear orthopyroxene, in turn rimmed by pale green (pargasitic) clinoamphibole. Locally a third rim, composed of a fine pargasite-spinel symplectite is developed adjacent to the plagioclase. The second corona reaction has produced greenish-brown pargasite at phlogopite-plagioclase interfaces. A third, less obvious reaction, between olivine and phlogopite is also noted. Analytical data of all the mineral phases present, along with the coronas, are given. Two-pyroxene thermometry yields magmatic core temperatures (∼ 1120°C), with rim compositions indicating equilibration at ∼ 850°C. Consistent with this, the modelled olivine-plagioclase reaction occurs between 830-1050°C with a_waterbetween 0.1 and 1.0 at 7 kbar. The three reactions took place during a prolonged history of cooling and partial hydration of the magmatic olivine melanorites from over 1000°C down to ∼ 600°C. The P-T conditions indicated by the reactions suggest this cooling process was essentially isobaric, indicating that the area was not subjected to rapid uplift or burial throughout this entire period.