A hibonite-spinel-corundum-hematite assemblage in plagioclase-clinopyroxene pyrometamorphic rocks, Hatrurim Basin, Israel; mineral chemistry, genesis and formation temperatures

The intergrowths of Fe-rich corundum + Al-rich hematite + spinel + hibonite have been found as an assemblage in a plagioclase-clinopyroxene rock (paralava, former hornfels) at the Hatrurim Basin, Hatrurim combustion metamorphic Formation. Most spinels show oriented exsolution structures and vary from (Mg0.75Fe0.252+) (Al1.80Fe0.203+) O4 (with exsolutions) to (Mg0.77Fe0.232+) (Al1.95Fe0.053+) O4 (homogeneous) indicating a tendency towards magnesioferrite and magnetite, and enrichment in NiO (up to 1.9 wt.%) and ZnO (up to 1.4 wt.%). Hibonite is Ti rich (TiO2 > 8 wt.%) and close to CaAl9Fe3+(Mg,Fe2+)TiO19. Corundum varies in Fe2O3 (4.2-11.8 wt.%). Hematite is also inhomogeneous and contains oriented exsolution structures of corundum. It shows variable concentrations of TiO2 (0.7-5.6 wt.%), Al2O3 (0.7-8.6 wt.%), Cr2O3 (0.2-1.5 wt.%), V2O3 (0.1-1.0 wt.%) and MgO (0.3-2.0 wt.%). Crystallization of this specific assemblage is assumed to be at 1000-1200°C using evaluations for the corundum hematite pair with reference to published experimental data. The active role of superheated oxidised volatiles is suggested during both crystallisation of this corundum-bearing association and host-rock transformation (melting event for hornfels → paralava).