SPINEL-GROUP MINERAL CHEMISTRY AS A RECORD OF MAGMATIC AND METAMORPHIC PROCESSES: EVIDENCE FROM THE MORRO DO ONÇA KOMATIITES, MINAS GERAIS (BRAZIL)

Komatiites are ultramafic lavas (> 18 wt. % MgO) erupted at extremely high temperatures and that often preserve distinctive spinifex texture­. They disproportionately formed during the Archean and therefore provide potential insights into mantle conditions, melting regimes and broad-scale geodynamic processes operational during this Eon. However, suites of ultramafic rock are highly susceptible to alteration, with the primary geochemistry overprinted and petrogenetic significance obscured.

The Morro do Onça suite is a < 5 km thick package of komatiites located in the southern São Francisco Craton, Brazil. Owing to the complete replacement of primary silicate minerals by chlorite, amphibole, talc and serpentine, assessment of the petrogenetic characteristics is highly challenging. This includes a scatter on the bulk-rock Al₂O₃ versus TiO₂ plot, which are two elements traditionally used to classify komatiites as they are generally considered immobile during low-T metamorphism.

Using quantitative chemical analyses of spinel-group minerals (n=517), alongside petrography, bulk-rock geochemistry and silicate mineral chemistry, we untangle the magmatic and metamorphic evolution of the Morro do Onça ultramafic suite. Progressive Al-enrichment and Cr-depletion (Al-chromite and picotite species), which is attributed to the pervasive effects of (amphibolite-facies) metamorphism, has completely overprinted most of the spinel grains analyzed. Subsequent retrograde metamorphism/hydrothermal alteration is recorded by Fe-enrichment and Cr- and Al-depletion, with such effects restricted to µm–cm-scale, cross cutting veins and rims (magnetite and Cr-magnetite species).

A small number of primary compositions (chromite species) are identified in rare spinel-group mineral cores (n=11; mean Cr-number = 79; mean Fe²⁺-number = 0.78). These data, alongside bulk-rock element ratios established here as relatively immobile ([Gd/Lu]_ch-norm = 0.6–2.0), classify the Morro do Onça komatiites as Al-enriched (Weltevreden-type). This research places a constraint on the geodynamic evolution of the southern São Francisco Craton and provides a framework for classifying highly altered komatiites globally.