Textures and phase relationships in ferrous granulites from Tidjenouine (Hoggar, Algeria): fayalite-ferrossilite-quartz secondary assemblage
Ferrous granulites in the area of Tidjénouine (Central Hoggar) exhibit a remarkable mineralogical composition characterized by the association orthoferrossilite-fayalite-quartz. These granulites are metamorphosed mafic igneous rocks showing the juxtaposition of different metamorphic parageneses. Peak paragenesis with garnet-clinopyroxene-amphibole-plagioclase-quartz reach to assemblage with orthopyroxene-plagioclase 2. Secondary orthopyroxene reacted with garnet to produce symplectites with fayalite + plagioclase + quartz. The latest stage corresponds to an orthopyroxene-fayalite-quartz-plagioclase assemblage. The metamorphic history of the ferrous granulites is inferred by combining the study of phase relations with the construction of a petrogenetic grid and pseudosection in the CFMASH and CFAS systems using the Thermocalc program of [J. Metamorph. Geol. 6 (1988) 173]. The evolution of paragenetic minerals indicates a metamorphic P- T path through the following conditions: 7.1 ± 1 kbar at 880 °C, 4.9 ± 1.6 kbar at 750 °C and 3-4 kbar at 700 °C, which is consistent with a clockwise P- T path recorded throughout the area.