Ultrahigh-temperature metamorphism (1150°C, 12 kbar) and multistage evolution of Mg-, Al-rich granulites from the central Highland Complex, Sri Lanka

Abstract

Mg- and Al-rich granulites of the central Highland Complex, Sri Lanka preserve a range of reaction textures indicative of a multistage P-T history following an ultrahigh-temperature metamorphic peak. The granulites contain a near-peak assemblage of sapphirine-garnet orthopyroxene-sillimanite-quartz K-feldspar, which was later overprinted by intergrowth, symplectite and corona textures involving orthopyroxene, sapphirine, cordierite and spinel. Biotite-rims, kornerupine and orthopyroxene-rims on biotite are considered to be late assemblages. Thermobarometric calculations yield an estimated P-T of at least 1100°C and 12 kbar for the near-peak metamorphism. Isopleths of Al2O3 in orthopyroxene are consistent with a peak temperature above 1150°C. The P-T path consists of four segments. Initial isobaric cooling after peak metamorphism (Segment A), which produced the garnet sapphirine-quartz assemblage, was followed by near-isothermal decompression at ultrahigh temperature (Segment B), which produced the multiphase symplectites. Further isobaric cooling (Segment C) resulted in the formation of biotite and kornerupine, and late isothermal decompression (Segment D) formed orthopyroxene rims on biotite. This evolution can be correlated with similar P-T paths elsewhere, but there are not yet sufficient geochronological and structural data available from the Highland Complex to allow the tectonic implications to be fully assessed. © Oxford University Press 2004; all rights reserved.