Secondary siderite-oxide-sulphide and carbonate-andalusite assemblages in cordierite granulites from Sri Lanka: post-granulite facies fluid evolution during uplift |
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| Authors: | D J Ellis Yoshikuni Hiroi |
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| Institution: | (1) Key centre for the Geochemical Evolution and Metallogeny of Continents, Department of Geology, Faculty of Science, Australian National University, Canberra, A.C.T, 0200, Australia, AU;(2) Department of Earth Sciences, Faculty of Science, Chiba University, Yayoicho, Chiba 263, Japan, JP |
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| 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.
Received: 4 May 1994 / Accepted: 25 October 1996 |
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