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

Abstract

AbstractThe 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${\rm Fe}_{{\rm 0}{\rm. 25}}^{2 +} $)(Al1.80${\rm Fe}_{{\rm 0}{\rm. 20}}^{3 +} $)O4 (with exsolutions) to (Mg0.77${\rm Fe}_{{\rm 0}{\rm. 23}}^{2 +} $)(Al1.95${\rm Fe}_{{\rm 0}{\rm. 05}}^{3 +} $)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).