Metamorphic Control of the Origin of Secondary Fe-Ti Oxides in the Noritic Rocks Around Saltora, West Bengal

Sub-parallel bands of noritic rocks occurring as concordant inclusions within anorthosite are abundant around Saltora (23°31'12'' N and 86°56'20" E). At places they occur as concordant intrusions in the surrounding country rocks which comprise acid and basic gneisses, granulites and migmatites. Noritic rocks are the hosts for Fe-Ti oxides of two generations, viz. primary and secondary. Secondary Fe-Ti oxide is almost exclusively ilmenite, whose proportion increases with increasing alteration of pyroxene in the noritic rocks. This trend of variation supplemented by the petrographic variation of the noritic rocks suggests that the secondary ilmenite is a product of diapthoresis, a process that involved the redistribution of Fe⁺², Fe⁺³ and Ti⁺⁴ between the silicate and oxide phases under a low partial pressure of oxygen.