The application of spinel-bearing metapelites to P/T determinations: An example from South India

Abstract

Metasedimentary migmatites from the Archean charnockitic terrain of South India contain the five phase equilibrium assemblage spinel-cordierite-garnet-corundum-sillimanite. The assemblages is a result of anatexis which has generated a silica-deficient anhydrous restite. Peak metamorphic conditions are defined by the intersection of two divariant reactions in the A1₂O₃-SiO₂-FeO-MgO system at which the five phases coexist. These reactions are univariant and their intersection invariant if the Fe/Mg ratio of at least one femic phase is fixed.

The location of the invariant point in P/T space is derived from extracting standard stage thermodynamic data from published equilibria experiments in the system Al₂O₃-SiO₂-FeO. Microprobe analyses of coexisting spinel, almandine and cordierite specify the Fe/Mg distributions between phases and allow the computation of the five phase invariant point for $P_{{\text{H}}_{\text{2}} {\text{O}}}$=P _total (770° C, 5.9 kb) and $P_{{\text{H}}_{\text{2}} {\text{O}}}$=O (740° C, 4.8 kb). A low $P_{{\text{H}}_{\text{2}} {\text{O}}}$, implied by evidence of extreme anatexis, indicates a P/T field of T=740±20° C and P_total=4.8±0.5 kb which is consistent with the field of equilibration of interlayered charnockites computed from garnet-hypersthene and garnet-plagioclase pairs.