Formation of manganoan ilmenite in Archean tonalite-trondhjemite-granodiorite (TTG) gneisses inferred from re-equilibration of biotite and Fe-Ti oxide assemblage: a case study from the Aravalli Craton, northwest India

Abstract

Ilmenite is a widespread accessory mineral in tonalite-trondhjemite-granodiorite (TTG) gneisses of the Banded Gneissic Complex (BGC-I) from the Aravalli Craton. Studied ilmenites are found as an inclusion in micas, magnetite and has low Mg (~0.00-0.21 wt%) and high Mn contents (3.94-10.29 wt%) in contrast to the ilmenites from igneous rocks. Studies that record the transformation of Fe-Ti oxides under low- to medium-grade metamorphic conditions are very restricted. The rocks of BGC-I of the Aravalli Craton have experienced amphibolite-facies metamorphism under medium grade metamorphic conditions and the effects of this metamorphism can be studied in the Fe-Ti oxides. Petrographic, bulk rock and mineral chemical data reveal that the primary magmatic composition of ilmenite has been diversely modified during metamorphism by diffusion re-equilibration with a co-existing silicate mineral. Biotite being the dominant mafic mineral in the TTG gneisses, has re-equilibrated during metamorphism and the Ti-in-biotite geothermometer indicates that re-equilibrated primary biotite crystallized at temperatures between 530° and 680 °C. Therefore, enhanced Mn diffusion out of biotite into ilmenite under oxidizing conditions provides an explanation for high Mn contents in ilmenite. This further infers short-range equilibration during medium-grade metamorphism of the BGC-I from the Aravalli Craton. © 2023. E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany