Petrological study of high-temperature granulites from In Ouzzal, Algeria: some implications on the phase relationships in the FMASTOCr system
Guiraud, Michel; Kienast, Jean-Robert; Rahmani, Amel
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1375 - 1390
published: Jan 8, 1997
manuscript accepted: Jul 10, 1996
manuscript received: Jun 28, 1996
ArtNo. ESP147050806015, Price: 29.00 €
Abstract Granulites from the In Ouzzal complex, Algeria, preserve excellent indicators of high-temperature conditions. They display a series of reaction textures involving sapphirine, sillimanite, spinel, garnet and cordierite in addition to orthopyroxene, quartz and rutile. The peak conditions are represented by five assemblages characterized by quartz+orthopyroxene+rutile with the possible addition of spinel or sillimanite+sapphirine. Geothermometry and geobarometry show that the peak metamorphism conditions were T = 1000°C and P = 10-1 l kbar. As most of the minerals contain a significant amount of Fe3+ and Cr, the parageneses have been studied within the FMASTOCr (FeO-MgO-Al2O3-SiO2-TiO2-O2-Cr2O3) system. Neither of the two simple systems FMAS nor FMASCr account for the observed phase relationships. The assemblages occurring in the rocks are also inconsistent with previous FMASTO grids. However, the observed chemical relationships are different from those used by former authors. In particular, the collinearity of sillimanite, sapphirine and orthopyroxene lead to a different compatibility diagram. Therefore, we have investigated a new set of petrogenetic grids based on our own observations and valid for the FMASTO system with sapphirine, spinel, sillimanite, garnet, cordierite, magnetite, ilmenite and rutile in presence of quartz and orthopyroxene. The grids account for the parageneses observed in the Khanfous area (North In Ouzzal) and for most of the recorded high-temperature granulites.