Palaeopressures of South Indian two‐pyroxene garnet granulites from thermochemically calibrated CMAS barometers

AbstractThe enthalpy of reaction of plagioclase and pyroxene to produce garnet and quartz has been a major source of error in granulite geobarometry because of relatively uncertain enthalpy values available from high‐temperature solution calorimetry and compiled indirectly from experimental phase equilibria. Recent, improved calorimetric measurements of ΔHRare shown to yield palaeopressures which are internally consistent between orthopyroxene and clinopyroxene calibrations for many South Indian granulites from the Archaean high‐grade terranes of southern Karnataka and northern Tamil Nadu. This represents a considerable improvement over previous calibrations, which gave disparate results for the two independent barometers involving orthopyroxene and clinopyroxene, requiring a 2‐kbar ‘empirical adjustment’to force agreement. Palaeopressures thus calculated for 30 well‐documented two‐pyroxene garnet granulites from South India give internally consistent pressures with a mean of 8.1°1.1 kbar at 750°C, consistent with the presence of both kyanite and sillimanite in many areas. Those samples for which garnet–pyroxene exchange thermometers give plausible granulite‐range temperatures and whose minerals are minimally zoned give the best agreement of the two barometers. Samples which yield low palaeotemperatures and different rim and core compositions of minerals yield pressures for the orthopyroxene assemblage as much as 2 kbar lower than for the assemblage with clinopyroxene. This disparity probably represents post‐metamorphic‐peak re‐equilibration. We conclude that considerable confidence may be placed in geobarometry of two‐pyroxene granulites where apparent palaeotemperatures are in the granulite facies range (>700°C) and where mineral zonation is minimal. Of the several possible sets of activity–composition relations in use, those constructed from analysis of phase equilibria give slightly higher palaeopressures and appear more consistent with analytical data from the Nilgiri Hills uplift, where kyanite is the only aluminium silicate reported to be stable in peak‐metamorphic assemblages. The present results support a palaeopressure gradient, increasing generally from south to north, across the Nilgiri Hills as inferred by previous geobarometry.