Modeling Pressure–Temperature Conditions of Garnet Amphibolite from The Himalaya of Northwestern Bhutan
Date
2018-05
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Publisher
The Ohio State University
Abstract
Garnet amphibolite collected in northwestern Bhutan was studied to determine the metamorphic conditions the sample underwent. Other studies in the central Himalaya reported eclogite-facies assemblages in mafic rocks, based upon the discovery of omphacite relics, but this could not be confirmed within the sample due to the lack of clinopyroxene. This is likely the result of higher temperatures overprinting the high-pressure peak metamorphic conditions during the post-peak stage of metamorphism. Equilibrium modelling of the sample to produce an isochemical section of mineral assemblages at various pressure and temperature conditions was refined by comparison with the measured modal composition consisting of hornblende, quartz, plagioclase, garnet, sphene and ilmenite. This comparison suggests the current mineral assemblage formed at approximately 4-8 kilobars and 400-750 ºC. Compositions of core and rim domains in garnet were compared to garnet compositions derived from the isochemical model to refine pressure and temperature conditions. A large discrepancy exists between measured and modeled phase compositions, thus making refinement unreliable. Despite this, the maximum temperature predicted by the thermodynamic modelling suggests the mineral assemblage does not reflect a peak pressure paragenesis. The mineral assemblage is proposed to have either grown early or late during the metamorphic event.
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Keywords
Perpl_x, garnet amphibolite, Himalaya, Bhutan, granulitized eclogite