Dolomitic Marble in a Context of UHT Metamorphism: Possible Signs of Melting, Lenadora Quarry, Sri Lanka

Abstract

Abstract The Lenadora quarry, located 25 km north of Matale, in central Sri Lanka, exploits dolomitic marble on an artisanal scale. It is located in the Highland Complex but very close to its thrust contact with the Wanni Complex. The rocks have undergone metamorphism to the granulite facies; by analogy with metasedimentary rocks in a similar position south of Kandy, conditions of metamorphism may even have surpassed 900°C and 0.9 GPa. How did dolomitic marble fare in such an ultrahigh-temperature environment? High-resolution large-area image mosaics of imaged polished thin sections were acquired by using both light microscopy and scanning electron microscopy. We focus on fifty areas of interest. As expected, diopside and forsterite porphyroblasts appeared during prograde metamorphism. However, we did not anticipate to see subhedral porphyroblasts of forsterite mimetically replaced by diopside + dolomite. The conversion could have involved a fluid phase or a carbonate melt that had become locally enriched in silica. The presence of a melt accounts for composite calcite + dolomite globules trapped in forsterite. There is no evidence of breakdown products of dolomite, like periclase or brucite. The evidence that a carbonate melt was present is circumstantial; carbonate systems are notorious for erasing telltale evidence of melting. Other minerals encountered in the dolomitic matrix are phlogopite, pargasite, rutile, hydroxylapatite, zirconolite, lakargiite, a magnesiobeltrandoite-like phase, pyrrhotite, and calcite as a minor phase, some of which is of a second generation. Unreplaced relics of forsterite were partially serpentinized at a retrograde stage.