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Journal of Petrology, Volume 40, Issue 8: August 1999.
Calcium-poor, magnesian granulites from two localities in central Sri Lanka contain a rich variety of assemblages composed of combinations of sapphirine (Spr), garnet, orthopyroxene, sillimanite, cordierite or spinel. Quartz is present in some assemblages at the Hakurutale locality, and corundum and rare gedrite are present in some assemblages at the Munwatte locality. Rocks from both localities display diverse sets of inclusion and sequential reaction textures. The inclusions suggest that the earliest assemblage at Munwatte was orthopyroxene + spinel + corundum. Garnet growth followed by garnet breakdown occurred via the reactions orthopyroxene + corundum = garnet and garnet + corundum + spinel = sapphirine. This produced coarse-grained coronas of sapphirine around the corundum and spinel. The earliest assemblage at Hakurutale was orthopyroxene + sillimanite + sapphirine. Garnet growth occurred via the reactions orthopyroxene + sillimanite + sapphirine = garnet and orthopyroxene + sillimanite = garnet + quartz. Locally, garnet was partially destroyed by a variety of corona-forming continuous reactions. The most important are garnet = orthopyroxene + sillimanite + sapphirine and garnet + sillimanite = sapphirine + cordierite (both localities) and, at Hakurutale, garnet + quartz = orthopyroxene + sillimanite, garnet + sillimanite + quartz = cordierite and garnet + quartz = orthopyroxene + cordierite. In places, orthopyroxene + sillimanite + quartz reacted to form cordierite, and orthopyroxene + sillimanite reacted to form cordierite + sapphirine coronas. At Hakurutale, inclusion textures indicate that sapphirine was present in the earliest recorded assemblages (prograde), but also formed later as various corona-forming reactions occurred. On the basis of calculated orientations of model univariant curves and continuous reaction slopes for the observed phases, the P-T path is roughly a `clockwise' (i.e. Pmax precedes Tmax) trajectory. The earlier, garnet-forming stage reflects heating possibly with a slight P increase. The final recorded leg suggests near-isothermal decompression starting at a peak P-T of about 9 kbar and 830°C and dropping to nearly 7·5 kbar at about 810°C. Sapphirine appears to have developed at several points along the P-T trajectory.
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Pages 1211-1239