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Geothermometry and geobarometry of high-grade rocks: a case study on garnet-pyroxene granulites in southern Sri Lanka

Published online by Cambridge University Press:  05 July 2018

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

geothermometrygeobarometrygarnetpyroxenegranuliteSri Lanka
Type
Research Article
Information
Mineralogical Magazine , Volume 55 , Issue 378 , March 1991 , pp. 33 - 56
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1991

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