Abstract

Two-feldspar geothermometry is applied to granulite-facies metapelites from the Highland Complex of Sri Lanka, for which very high peak temperatures of metamorphism have been inferred. Two-feldspar thermometry can be applied, even where only one feldspar has preserved its original bulk-composition in terms of Al-Si content, in spite of or because of unmixing to perthite or antiperthite. Different methods of integration of the unmixed feldspars to get the original bulk-composition are evaluated. Several types of feldspar assemblages are distinguished: (1) disequilibrium and near-equilibrium perthite - plagioclase pairs (several generations), (2) disequilibrium antiperthite - perthite pairs, (3) single mesoperthite, (4) near-equilibrium antiperthite-mesoperthite pairs. The highest temperatures, above 900 degrees C, are derived from antiperthite porphyroclasts with evenly spaced exsolution lamellae and rods, from antiperthite-mesoperthite pairs, and from perthite inclusions in garnet porphyroclasts. The causes for the preservation of ternary feldspar compositions are discussed. Feldspar recrystallization in response to strong deformation is related to thrusting of the Highland Complex onto the Vijayan Complex. It occurred at about 830-900 degrees C. In the main part of the Highland Complex, the feldspars recrystallized at ca. 680-760 degrees C. Mylonitization along the Digana shear zone took place at about 710 degrees C. Late retrograde recrystallization or growth of feldspar rims occurred in the range 460-590 degrees C. Two-feldspar geothermometry combined with published geochronological data reveal new insights in the metamorphic evolution of the Highland Complex of Sri Lanka.

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