Chemistry of chromium spinel in high-Mg rocks from the Morungava Intrusion, Cretaceous Paraná Igneous Province, southernmost Brazil

Mafic-ultramafic intrusions within continental flood basalt terrains are frequently associated with Cu-Ni-PGE mineralization. This study aims to constrain the petrogenesis of early crystal phases in a promising exploration target, the Morungava mafic-ultramafic intrusion that is associated with the Paraná flood basalts. Therefore, we analyzed the composition of chromiferous spinel and associated olivine of ultramafic rocks from the sill-like intrusion. Samples were selected from different drill core intervals with high MgO and Cr contents. Two generations of Cr-spinel and olivine were identified in olivine gabbronorite and wehrlite rocks from the intrusion. The first type (Spl 1 ) is enclosed in the core of high-Mg idiomorphic olivine crystals (Ol 1 ) and has higher Cr 2 O 3 (28–35 wt.%), Al 2 O 3 (13–33 wt.%), MgO (12.6–14.6 wt.%), and lower TiO 2 (0.5–0.8 wt.%) compared to the second type of spinel (Spl 2 ). Spl 2 occurs in the interstitial space between olivine and clinopyroxene and contains higher TiO 2 (2.0–15.8 wt.%) and lower Al 2 O 3 (5.2–10.5 wt.%), Cr 2 O 3 (10–25 wt.%), and MgO (2.8–7.6 wt.%) contents. Geothermometric calculations using high-Mg idiomorphic Ol 1 - Spl 1 pairs resulted in temperatures below the corresponding solidus, indicating subsolidus exchange between these minerals and accounting for the Cr-Al trend in Spl 1 . The high Mg-contents, normal zoning and high but decreasing Cr contents in Ol 1 are consistent with crystallization at an early stage during progressive fractional crystallization. Fractionation of olivine, Cr-spinel, clinopyroxene, and plagioclase deceased Mg, Al, and Cr, and increased Fe and Ti in the residual melt. The compositional hiatus with the low-Mg olivine (Ol 2 ) and especially with Spl 2 crystals most likely reflects crystallization from an evolved interstitial liquid at a time where Ol 1 -Spl 1 were relatively isolated from diffusional interaction, in addition to solvus processes in spinel. The Fe-Ti- trend in Spl 2 likely reflects varying degrees of evolution of the interstitial melts. The Morungava intrusion thus records a complex history of extensive reactions that started with fractionating Ol 1 -Spl 1 in a magma chamber that experienced periodic magma addition, and finished in trapped, interstitial, in situ intercumulus liquids (Ol 2 -Spl 2 ). The setting, host rocks, and geochemical characteristics of the Morungava intrusion are reminiscent of the Cu-Ni-PGE mineralized Noril’sk-Talnakh ultramafic comp