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 (Mg0.75 Fe_ 0. 252 + )(Al1.80 Fe_ 0. 203 + )O4(with exsolutions) to (Mg0.77 Fe_ 0. 232 + )(Al1.95 Fe_ 0. 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).