ARRESTED CHARNOCKITE FORMATION IN SRI LANKA : FIELD AND PETROGRAPHICAL EVIDENCE FOR LOW-PRESSURE CONDITIONS

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Field data of the arrested charnockite (orthopyroxene-bearing quartzo-feldspathic rock in a broad sense) formation are the striking evidence for CO_2 flushing and resultant subsolidus dehydration reactions and/or partial melting. Such a local phenomenon is, however, distinct from the preceding regional granulite-facies metamorphism, because it took place under much lower-pressure conditions in Sri Lanka. The orthopyroxene+cordierite+K-feldspar as well as orthopyroxene+garnet+K-feldspar±cordierite assemblages are stable in the charnockitized pelitic rocks of the Southwestern Group. In addition, secondary andalusite occurs commonly, though small in amount, together with siderite and cordierite in other pelitic rocks (gneiss and migmatite) of the Southwestern Group. Secondary andalusite has also been revealed to occur in the Highland Series rocks. This, along with the occurrence of relict kyanite in sillimanite-stable rocks of both the Highland Series and the Southwestern Group establishes that rocks, at least metasediments, of these geological units in Sri Lanka have similar pressure-temperature histories. The later low-pressure metamorphic overprint including arrested charnockite formation was, however, much more extensive in the Southwestern Group than in the Highland Series.
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