Corona textures in Proterozoic olivine melanorites of the equeefa suite, Natal metamorphic province, South-Africa

Abstract

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 (approximately 1120-degrees-C), with rim compositions indicating equilibration at approximately 850-degrees-C. Consistent with this, the modelled olivine-plagioclase reaction occurs between 830-1050-degrees-C with a(water) between 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-degrees-C down to approximately 600-degrees-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.