A corundum and sapphirine paragenesis from the Limpopo Mobile Belt, southern Africa

Abstract

An assemblage consisting of corundum, sapphirine, spinel, cordierite, garnet, biotite and bronzite is described from the Messina area of the Limpopo Mobile Belt, and consideration given to its petrogenesis. Various geothermometers and geobarometers have been applied in an attempt to determine the temperatures and pressures of metamorphism.A former coexistence of garnet and corundum is suggested to have developed during the earliest high pressure phase of the metamorphism, where temperatures exceeded 800°C and pressures as high as lOkbar may have been experienced. Subsequently, continuous retrograding reactions from medium pressure granulite facies at about 800°C and 8 kbar towards amphibolite facies generated spinel, cordierite, sapphirine and possibly also bronzite. The most notable reaction was probably of the form: garnet +corundum = cordierite + sapphirine + spinel.

Calculated phase equilibria in K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O for silica‐undersaturated sapphirine‐bearing mineral assemblages

“…Sapphirine typically occurs as a corona mantling spinel, isolating spinel from porphyroblastic/coarse‐grained garnet (Horrocks, 1983; Harley, 1986; Bertrand et al. , 1992), orthopyroxene (Horrocks, 1983; Warren, 1983; Windley et al. , 1984; Goscombe, 1992; Dunkley et al.…”

“…, 1986). A second corona comprising cordierite (Horrocks, 1983; Dunkley et al. , 1999), orthopyroxene (Bertrand et al.…”

Sapphirine parageneses from the Caraiba complex, Bahia, Brazil: the influence of Fe2+–Fe3+ distribution on the stability of sapphirine in natural assemblages

“…Such rocks are commonly developed in comparable Archaean high-grade terranes in close association with layered complexes, which contain prominent anorthosites, leucogabbros and meta-supracrustal rocks, e.g. Fiskenaesset, West Greenland (Herd, Windley & Ghisler, 1969;Herd, 1972;Herd & Windley, in press); the Limpopo belt of southern Africa (Horrocks, 1980(Horrocks, , 1983Ackermand, Herd & Windley, 1982;Windley, Ackermand & Herd, 1984); Sittampundi, southern India (Janardhan & h a k e , 1974); in the Arunta block of central Australia (Katz, 1981); and in Madagascar (Lacroix, 1941). There is an interesting comparison between the Curat$ valley sapphirinebearing and associated rocks and those from the Fiskenaesset region of West Greenland.…”

Age and isotopic geochemical characteristics of Archean carbonatites and alkaline rocks of the Baltic shield

“…These sediments contain detrital zircon grains as old as 3.9 Ga [Zeh et al, 2010]. A part of the metasedimentary rocks of the Beit Bridge Complex was intruded at ~3.36 Ga by the Messina gabbroanorthosite layered intrusion and Sand River TTG gneisses, all being affected by granulitefacies metamorphism (~10 kbars and >800 °C) at ~3.14 Ga [Horrocks, 1983;Van Reenen et al, 1992b;Barton, 1996;Kröner et al, 1999;Zeh et al, 2007;Chudy at al., 2008;Laurent et al, 2011]. Subsequent metamorphism within all three zones of the Limpopo Belt includes high-grade events between 2.74 and 2.57 Ga and between 2.03 Ga and 1.97 Ga (see below).…”