Geothermometry and Geobarometry of High-Grade Rocks: A Case Study on Garnet-Pyroxene Granulites in Southern Sri Lanka

S. Faulhaber and M. Raith
Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloss, 5300 Bonn, FRG

Abstract: In the central granulite belt of Sri Lanka, garnet-pyroxene granulites of granitic and gabbro-noritic composition are the most abundant rock types. The micro-structures and mineral chemistry data prove complete attainment of textural and large-scale chemical equilibrium during and following a phase of extreme penetrative deformation at conditions of the granulite facies (800–850 °C, 5 to 9 kbar). On a local scale, especially along the intergranular system, continued cation exchange decoupling from the early ceasing net-transfer reactions destroyed the near-peak metamorphic equilibrium. The extreme compositional variation of the coexisting ferro-magnesian phases (Fe/(Fe + Mg): gar 0.98−0.65, opx 0.92−0.40, cpx 0.88−0.28) and the near-isothermal conditions of equilibration throughout the studied area enabled examination of the effects of non-ideal mixing in garnet and pyroxenes on the equilibrium constants of reactions used in geothermobarometry, and tests on the quality of commonly applied thermometers/barometers and the validity of activity models adopted in the calibrations. The Sri Lankan data set reveals more or less pronounced compositional dependences for all of the tested gar-opx/gar-cpx Fe-Mg exchange thermometers and the opx + plg ⇌ gar + qtz barometers. Evidently the recommended solution models do not adequately describe the mixing properties of the involved ferro-magnesium mineral phases (garnet, orthopyroxene and clinopyroxene).

Keywords: geothermometry • geobarometry • garnet • pyroxene • granulite • Sri Lanka

Mineralogical Magazine; March 1991 v. 55; no. 378; p. 33-56; DOI: 10.1180/minmag.1991.055.378.04
© 1991, The Mineralogical Society
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