Regional Metamorphism of Non-Calcareous Manganiferous Sediments from India and the Related Petrogenetic Grid for a Part of the System Mn-Fe-Si-O

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Abstract
Mineralogical assemblages developed in the non-calcareous manganiferous sediments in India and subjected to regional metamorphism under chlorite to sillimanite grade conditions have been studied in detail. Based on a series of idealized reactions compatible with the recorded assemblages in the system Mn-Fe-Si-O, for many of which there is unambiguous textural evidence in the rocks, a combined schematic petrogenetic grid consistent with topological and thermodynamic considerations has been constructed. The inferred petrogenetic grid, coupled with the mineralogical and textural evidence present in the manganiferous assemblages and the enclosing rock formations, can be reconciled with the following: (1) The mineralogical reactions attending regional metamorphism of the manganiferous sediments buffered the composition of the coexisting fluid phase. (2) Due to the closed nature of the system as a whole and also due to lack of communication between the different parts thereof, local variations in the initial proportions of the non-volatile to volatile phases and their compositions led to the development of contrasting sequences of mineralogical reactions and, therefore, fo2-T gradients even within the same metamorphic grade. (3) Rhodonite, developed as a prograde reaction product in the garnet to sillimanite grade conditions, was converted to rhodonite-pyroxmangite mixture/intergrowth during cooling. Compositional variance, resulting from the substitution of Mn by Fe and incorporation of components such as Mg, Ca in the phases, would tend to shift the univariant reaction curves in the grid towards opposite directions and/or split them into multivariant intervals in fo2-T space without altering the general style of the topology or the principal deductions made therefrom.

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