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

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 (similar to 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 degrees and 680 degrees 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.