Petrochemistry of some gem-related metamorphic rocks in Sri Lanka

Abstract

Sri Lanka has long been known as the major source of gemstones supplying wide variety, high quality and high quantity to the world market. Gem occurrences in the country are generally related to regional metamorphism. Precambrian high-grade metamorphic rocks have been prospected for gem deposits which are common sources of corundum, tourmaline, chrysoberyl, spinel, zircon, garnet etc. Characteristics of some gem-related metamorphic rocks collected from this country were investigated under petrographic description, whole-rock geochemical analysis and mineral chemistry. Rock collections were divided, based on their textures and mineral assemblages, into 6 groups including garnet gneiss, graphitic gneiss, hornblende-biotite gneiss, granulite, mafic granulite and felsic granulite. Moreover, some crucial groups were collected from different localities for comparison such as hornblende-biotite gneiss from Bakamuna and Kadugannawa. In general, petrographic description indicates high grade metamorphic textures, particularly mosaic suture and triple junction. Various minerals are present in different rock types; however, common assemblage usually contains plagioclase, alkali feldspar, quartz, garnet, pyroxene, hornblende and biotite with different proportions; moreover, sillimanite, rutile and opaque minerals may be found as accessory in some groups. Whole-rock geochemistry shows that most of all samples are peraluminous composition, except only a sample of mafic granulite from Bakamuna is metaluminous. Mineral chemistry of some crucial minerals was analyzed for comparison and reconstruction of P-T history of these rocks. As a result, garnet composition is mostly almandine garnet. Pyroxene can be divided to orthopyroxene with significant ferosilite composition and clinopyroxene with composition varying from diopside to hedenbergite. For feldspar, estimated temperature of crystallization appears to be an above 445-550 °C based on compositions of two feldspar presented within the same sample.