In the region east of the south-western belt of Ceylon, garnet is stable in biotite-bearing pelitic rocks which belong to the hornblende-granulite subfacies. In the south-western belt, which includes Zone B of the eastern part of the Gampaha sheet, garnet is unstable in biotite-bearing pelitic rocks, garnet reacting with biotite to produce cordierite which has a widespread occurrence in the belt to which it is confined. To accommodate the stable cordierite-bearing pelitic assemblages the erection of a cordierite-granulite subfacies is necessary. Wollastonite-bearing rocks, which commonly occur in the belt and are confined to it, belong to the cordierite-granulite subfacies.
Three subfacies of the granulite facies are recognized: garnet-diopsidequartz subfacies; pyroxene-granulite subfacies; and hornblende-granulite subfacies The erection of a garnet-diopside-quartz subfacies follows De Waard's suggestion (1965) of a garnet-clinopyroxene subfacies. Two major divisions of the hornblendegranulite subfacies are now recognized, (i) garnet-biotite division, (ii) cordierite division. Three sub-divisions of the former are suggested on the basis of critical basic assemblages. Since wollastonite has been found to have a limited areal occurrence within the latter, a two-fold sub-division of the cordierite division is suggested on the basis of the assemblages, (a) calcite-quartz, (b) wollastonite. A seven-fold sub-division of the granulite facies is thus proposed for Ceylon rocks. An attempt is made to trace briefly the major events in the metamorphic history of the rocks supported by available geochronological evidence.
Hornblende and biotite occur very commonly in the granulite facies rocks of the areas mapped, and both appear to occur as stable phases. Thus, these hornblende-and biotite-bearing rocks find accommodation only in the hornblende-granulite subfacies. It appears that most of the rocks in the areas mapped would fall into this subfacies and the mineral assemblages of these rocks are recorded in this paper under the different chemical classes. Hornblende and biotite are very commonly present in the pelitic, quartzo-felspathic, basic and magnesian rocks ; phlogopite and sphene are commonly present in the calcareous rocks. A tentative ^CFA'tetrahedral phase diagram has been constructed for these rocks.THIS paper deals with rocks in the following areas which have been mapped : Ratnapura Sheet (Western half), Rakwana sheet (Western half). Gampaha sheet (Eastern part), the Northern part of the Kirindi Oya Basin and the Kataragama area.These areas are covered by charnockite-metasedimentary rocks (succession of metasediments and charnockites) consisting of the following types : The rocks are characterized by the presence of the following minerals : hypersthene, diopside, garnet, sillimanite, scapolite, calcite, forsterite, perthite, antiperthite, rutile, and these rocks therefore clearly belong to the granulite facies. In addition to these, minerals such as hornblende, biotite, phlogopite, and sphene are also present. But for the presence of hornblende, biotite, phlogopite, and sphene, these rocks would belong to the pyroxene-granulite subfacies. Hornblende and biotite, apparently stable, are very commonly present in the pelitic, quartzo-felspathic, basic, and magnesian rocks ; phlogopite and sphene are commonly present in the calcareous rocks. Thus, most of the rocks in the above-mentioned areas belong to the hornblendegranulite subfacies of Turner (Turner and Verhoogen, 1960). The indications are that this subfacies is a much needed one for Ceylon rocks.Associated in places with the hornblende-granulite subfacies rocks are rocks belonging to the pyroxene-granulite subfacies the mineral assemblages of which are listed at the end of the paper. Before we proceed to deal with the hornblende-granulite subfacies rocks, it is necessary to explain this close association of rocks of the two subfacies
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