Abstract
The mineralogy and phase relationships of corundum-bearing granulites from In Ouzzal (Hoggar, Algeria) are analysed within the context of the present knowledge of In Ouzzal metamorphism, as independently deduced from quartz-bearing rocks: a clockwise evolution characterized by peak temperatures ranging from 850 C to more than 1100 C at about 10 kbar, followed by decompression and cooling to about 5 kbar and 750 C. The corundum-bearing rocks have been divided into three types (A±B, C and D) according to their mineralogy. Type A±B is characterized by the occurrence of sapphirine± sillimanite±orthopyroxene±phlogopite with or without garnet, Type C by the occurrence of spinel±sapphirine±garnet, and TypeD by the occurrence of garnet±spinel±sillimanite. Thermodynamic data for sapphirine have been adjusted from the current THERMOCALC dataset to fit in with available experimental data and current theoretical phase diagrams. MAS, KMASH, FMAS and KFMASH petrogenetic grids that connect quartzpresent and corundum-present grids are presented. Pseudosections derived from these grids account well for the observed textures. The three types of rock agree with the decompression path experienced by the quartz-bearing rocks. The occurrence of sapphirine corresponds to peak and decompression conditions, and trends of sapphirine and orthopyroxene compositions are consistent with this evolution.
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