ALBERT

Notizen: Abstract The 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 ΔHR are 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.

Notizen: Charnockitic alteration (arrested orthopyroxene formation in biotite- and amphibole-bearing rocks) occurs in high-grade terranes of all ages. Three criteria are used to show that this alteration was produced in many locations by a migrating fluid phase: (i) diffuseness of the alteration—the alteration zones are often quite unlike discrete migmatitic veins; (ii) relation to deformation—most occurrences show alteration closely associated with warping of foliation or dilation cracks; (iii) open-system alteration—whilst some occurrences represent nearly isochemical alteration, slight changes in bulk composition, often loss of mafic constituents and gain of Na and Si, are evident in detailed mass-balance analysis. Y and sometimes Rb are characteristically depleted. Partial melting sometimes accompanied volatile infiltration, as evidenced by more discrete veins and euhedral orthopyroxene. It is quite unlikely, however, that open-system alteration was produced by escape of viscous quartzo-feldspathic melts. Pervasive migration of low-T lamprophyric (mafic–alkaline, CO2-charged) interstitial liquids is a possibility by virtue of their extreme fluidity, but CO2 infiltration was needed to generate these liquids. Vapour-deficient dehydration melting is another feasible mechanism of orthopyroxene formation which may have operated in conjunction with CO2 infiltration.Characteristic development of charnockitic alteration in some prograde amphibolite to granulite facies transitions, as in the Dharwar Craton of South India, suggests that the alteration is a fundamental feature of the granulite facies metamorphism, implying active and causal participation of migrating fluids. In other high-grade terranes like the Adirondack Mountains of New York, this kind of alteration is rare, and fluid action does not seem to have been important in the metamorphism.A vapour phase participating in charnockitic metamorphism was necessarily one of relatively low H2O, therefore presumably rich in CO2. Consideration of possible large CO2 sources leads to the conclusion that emanations from volatile-rich basalts emplaced in the lower crust are the most probable source of charnockitizing fluids. The ultimate source would therefore be enriched subcontinental lithosphere or asthenosphere. The Rb-depleted pyroxene gneiss (charnockitic) terranes may be characteristic of zones of large-scale transcurrent or oblique-motion faults which tap such great depths.