Petrological study of high-temperature granulites from In Ouzzal, Algeria; some implications on the phase relationships in the FMASTOCr systems

These granulites preserve excellent indicators of high-T conditions; they display a series of reaction textures involving sapphirine, sillimanite, spinel, garnet and cordierite, in addition to orthopyroxene, quartz and rutile. Geothermometry and geobarometry indicate peak metamorphic conditions of 1000 degrees C and 10-11 kbar. As most of the minerals contain appreciable Fe (super 3+) and Cr, the parageneses have been studied within the system FeO-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -TiO (sub 2) -OMV2-Cr (sub 2) O (sub 3) ; however, the colinearity of sillimanite, sapphirine and orthopyroxene (Al (sub 2) O (sub 3) 2.54-9.73 wt.%) lead to a different compatibility diagram from that of previous authors. A new set of petrogenetic grids have therefore been developed for the FMASTO system with sapphirine, spinel, sillimanite, garnet, cordierite, magnetite, ilmenite, and rutile in the presence of quartz and orthopyroxene. The new grids account for the parageneses observed in the Khanfous area, N In Ouzzal, and for most of the recorded high-T granulites.