Secondary siderite-oxide-sulphide and carbonate-andalusite assemblages in cordierite granulites from Sri Lanka: post-granulite facies fluid evolution during uplift

Abstract

We report here that some of the pelitic rocks from the Wanni and Highland Complexes of Sri Lanka reacted with CO₂-rich fluids to produce a wide range of unusual secondary carbonate-silicate-oxide-sulphide assemblages. These enable the depth, temperature and fluid compositions of CO₂ reactions to be calculated more rigorously than is generally possible for the patches of arrested charnockite that have been described from Sri Lanka. Magnesite-andalusite-quartz has partially replaced primary cordierite, and siderite-rutile replaced ilmenite. Paragenetic sequences involving primary pyrrhotite, ilmenite and magnetite and secondary pyrite-siderite-rutile-magnetite-(hematite) demonstrate the control which carbonate equilibria have upon evolving fluid compositions during cooling. Direct evidence for the role of graphite as a source of CO₂ is found in the Highland Complex where primary graphite partially reacted with silicates to form secondary siderite assemblages. It is proposed that following peak metamorphism, continued uplift along a clockwise P-T-t path was accompanied by a series of devolatilization reactions involving breakdown of graphite and the continuous production of secondary CO₂-rich fluids. The limited extent of disseminated secondary carbonate reflects the small amount of graphite inferred to have been present in the source rocks. These rocks demonstrate that CO₂-rich fluids, as found in disseminated fluid inclusions, need not form during peak granulite metamorphism but may be an inevitable consequence of continued uplift along a clockwise P-T-t path. The arrested charnockite which overprinted some of the hornblende-bearing felsic-intermediate composition rocks in Sri Lanka most likely formed by the same process.