Prograde and retrograde evolution in high-temperature corundum granulites (FMAS and KFMASH systems) from In Ouzzal terrane (NW Hoggar, Algeria)

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 850degreesC to more than 1100degreesC 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 Type D 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, K MASH, FMAS and KFMASH petrogenetic grids that connect quartz-present 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.