11.
We report here that some of the pelitic rocks from the Wanni and Highland Complexes of Sri Lanka reacted with CO
2-rich fluids to produce a wide range of unusual secondary carbonate-silicate-oxide-sulphide assemblages. These enable the
depth, temperature and fluid compositions of CO
2 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 CO
2 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 CO
2-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 CO
2-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.
Received: 4 May 1994 / Accepted: 25 October 1996
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