GEDRITE-GARNET-SILLIMANITE-BEARING GRANULITES FROM AMESSMESSA AREA, SOUTH IN OUZZAL, HOGGAR, ALGERIA

Some granulites from the Amessmessa area (south In Ouzzal unit, Hoggar ) contain the peak assemblage gedrite + garnet + sillimanite + quartz that was used to estimate the P-T conditions of metamorphism. The rock s developed symplectites and corona textures by the breakdown of the p rimary paragenesis to orthopyroxene, cordierite and spinel. The succes sive parageneses formed in separate microdomains according to a clockw ise P-T path. Geothermometry, geobarometry and phase diagram calculati ons indicate that the textures formed by decompression and cooling fro m 7-9 kbar and 850-900 degrees C to 3.5-4.5 kbar and 700-800 degrees C . This P-T evolution is consistent with low to medium a(H2O), between 0.4 and 0.7, and is similar to the metamorphic conditions deduced in A l-Mg granulites from the north of In Ouzzal.