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

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 AH, 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 f 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 ( > 7 W C ) and where mineral zonation is minimal. Of the several possible sets of activitycomposition 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.

High mantle heat flow in a Precambrian granulite province: Evidence from southern India

“…The DP shows a general increase in the grade of metamorphism from greenschist through amphibolite facies, from the north to the south. South of the orthopyroxene isograd, in the granulites, there is a well‐documented, systematic increase in metamorphic pressure in some sectors (e.g., Dharmapuri‐Namakkal), culminating in the highest pressures of 10–11 kbar just north of a major lineament designated as Palghat‐Cauvery lineament (PCL) [ Hansen et al , 1984, 1995; Drury et al , 1984; Raith et al , 1983, 1990; Raase et al , 1986; Rameshwar Rao et al , 1991a; Eckert and Newton , 1993; Bhaskar Rao et al , 1996]. The PCL with E‐W strike serves as a notional divide between two blocks in the SGP, namely, the northern block (NB) and the southern block (SB) [ Harris et al , 1994].…”

Geothermobarometer based on coexisting garnet-orthopyroxene-plagioclase-quartz equilibria

“…They used the margulus values for garnet activity as follows: Lal (1993) Lal (1993) gave an internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnetorthopyroxene-plagioclase-quartz assemblages. He calculated the activity of garnet through sub-regular solution mixing in garnet using equation of Ganguly and Saxena (1984), where he used the margulus values as follows: W FeMg = -1256 + 1.0 * T; W MgFe = 2880 -1.7 * T; W CaMg = 4047 -1.5* T; W MgCa = 1000 -1.5 * T; W CaFe = -723 + 0.332*T; W FeCa = 1090 and C 123 = -4498 +1.516* T, Activity of Mg and Fe in Opx (a Mg Opx and a Fe Opx ) calculated through regular solution model of orthopyroxene mixing model, Where; W MgFe = 948 -0.34 * T; W FeAl = -1950 and W MgAl = 0 and activity of anorthite in plagioclase (a An plag ) same as in Wells (1979) Holland and Powell (1985) calculated by Eckert et al (1993) Holland and Powell (1985) gave an internally consistent thermodynamic data set with uncertainties and correlations and Eckert et al (1993) Newton and Perkins (1982). Holland and Powell (1990) calculated by Eckert et al (1993) Holland and Powell (1990) gave an enlarged and updated internally consistent thermodynamic data set with uncertainties and correlations Opx same as in Newton and Perkins (1982).…”

“…He calculated the activity of garnet through sub-regular solution mixing in garnet using equation of Ganguly and Saxena (1984), where he used the margulus values as follows: W FeMg = -1256 + 1.0 * T; W MgFe = 2880 -1.7 * T; W CaMg = 4047 -1.5* T; W MgCa = 1000 -1.5 * T; W CaFe = -723 + 0.332*T; W FeCa = 1090 and C 123 = -4498 +1.516* T, Activity of Mg and Fe in Opx (a Mg Opx and a Fe Opx ) calculated through regular solution model of orthopyroxene mixing model, Where; W MgFe = 948 -0.34 * T; W FeAl = -1950 and W MgAl = 0 and activity of anorthite in plagioclase (a An plag ) same as in Wells (1979) Holland and Powell (1985) calculated by Eckert et al (1993) Holland and Powell (1985) gave an internally consistent thermodynamic data set with uncertainties and correlations and Eckert et al (1993) Newton and Perkins (1982). Holland and Powell (1990) calculated by Eckert et al (1993) Holland and Powell (1990) gave an enlarged and updated internally consistent thermodynamic data set with uncertainties and correlations Opx same as in Newton and Perkins (1982). Berman (1988) calculated by Eckert et al (1993) Berman (1988) gave an internally-consistent thermodynamic data for minerals in the system NaO 2 -K 2 O-CaO-MgO-FeO -Fe 2 O 3 -Al 2 O 3 -SiO 2 -H 2 O-CO 2 and also calculated the activity of garnet using Ca-Fe-Mg ternary sub-regular solution of mixing in garnet (Berman 1990) calculated at constant pressure of 7Kbar equation is given below, and Eckert et al (1993)…”

“…Holland and Powell (1990) calculated by Eckert et al (1993) Holland and Powell (1990) gave an enlarged and updated internally consistent thermodynamic data set with uncertainties and correlations Opx same as in Newton and Perkins (1982). Berman (1988) calculated by Eckert et al (1993) Berman (1988) gave an internally-consistent thermodynamic data for minerals in the system NaO 2 -K 2 O-CaO-MgO-FeO -Fe 2 O 3 -Al 2 O 3 -SiO 2 -H 2 O-CO 2 and also calculated the activity of garnet using Ca-Fe-Mg ternary sub-regular solution of mixing in garnet (Berman 1990) calculated at constant pressure of 7Kbar equation is given below, and Eckert et al (1993)…”

“…These mineral pairs are commonly found in the assemblages of basic granulites/ charnockite and hence are suitable for estimating equilibrium pressure of most metamorphic rocks. For this the authors have considered the equations of Newton (1978); Wells (1979); Newton and Perkins III (1982); Raith et al (1983); Bohlen et al (1983); Perkins and Chipera (1985); Moecher et al 1988); Eckert et al (1991); Bhattacharya et al (1991); Holland and Powell (1985); calculated by Eckert et al (1993); Berman (1988) calculated by Eckert et al (1993); Berman (1990); calculated by Eckert et al (1993) and Lal (1993). The equations of equilibrium are given below:…”