The Sapphirine granulites of the Usilampatti display evidence of metamorphic reactions involved in the formation of diverse mineral assemblages recognized in different types of reaction textures, symplectites and coronas. The analysis of symplectitic development is best studied through a combination of the maximum rate of energy dissipation and non-equilibrium thermodynamics. However, symplectite growth in rocks is mainly governed by any change in physical and/or chemical parameters. The evidence of melting and decompression reactions is seen in the petrographic studies of the symplectitic domains deciphering garnet, orthopyroxene and sillimanite resorption in various reaction textures and the development of orthopyroxenecordierite as well as sapphirine-cordierite-orthopyroxene symplectites characterised by the retrograde cooling stage of metamorphism. The P-T conditions obtained from the symplectite compositions of sapphirine granulites suggest such reaction was effectuated by a decrease in pressure resulting in decompression of the order of 6 Kbar and a non-significant temperature drop indicating nearly isothermal decompression.
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