Ultrahigh‐Temperature Metamorphism in Madurai Granulites, Southern India: Evidence from Carbon Isotope Thermometry

Abstract

Ultrahigh-temperature (UHT) metamorphism in the Madurai Block of the southern Indian granulite terrain has been verified using the calcite-graphite isotope exchange thermometer. Carbon isotope thermometry has been applied to marbles from a locality near the reported occurrence of sapphirine granulites that have yielded temperature estimates of around 1000 degrees C. The delta(13)C and delta(18)O values of calcite are homogenous, implying equilibration of the isotopes during metamorphism. However, the delta(13)C values of single graphite crystals show variations in the order of 1 per thousand within a hand specimen. Detailed isotopic zonation studies indicate that graphite preserves either the time-integrated crystal growth history or reequilibrium fractionation during its cooling history. The graphite cores preserve higher delta(13)C values than the rims. The fractionation between calcite and graphite cores gives the highest metamorphic temperature of about 1060 degrees C, which matches the petrologically inferred temperature estimates in the high-magnesian pelites. The fractionation between graphite rims and calcite suggests a temperature of around 750 degrees C, which is interpreted to reflect retrograde cooling. This event is also observed in the sapphirine granulites. Calcite-graphite thermometry thus provides a useful tool to define UHT metamorphism in granulite terrains.

Proterozoic crustal evolution of southernmost India and Sri Lanka

“…High-temperature to UHT conditions during the Pan-African Orogeny have been deduced for both units. Temperatures of c. 1000~ derived from sapphirine granulites from the Kodaikanal Massif were recently confirmed by carbon isotope thermometry on marbles from an adjacent area (Raith et al 1997;Satish-Kumar 2000) and match the results of twofeldspar and garnet-orthopyroxene thermometry on rocks from the HC fairly well (Harley 1998;Raase 1998;Kriegsman pers. comm.).…”

“…Temperature estimates from charnockites, sapphirine-bearing granulites and calc-silicate rocks of the Kodaikanal Massif range from 900 to 1000~ providing compelling evidence for ultrahigh temperature (UHT) metamorphism in these parts of the Madurai Block (Brown & Raith 1996;Raith et al 1997;Satish-Kumar 2000). The breakdown of orthopyroxene-sillimanite assemblages and the replacement of kyanite by sillimanite in sapphirinegranulites indicates maximum pressures of 10-12kbar for the Kodaikanal Massif.…”

Calcite–graphite isotope thermometry in amphibolite facies marble, Bancroft, Ontario

“…Coarse flakes of graphite from granulite facies terranes sometimes exhibit isotopic zoning from core-torim (Wada, 1988;Arita & Wada, 1990;Santosh & Wada, 1993;Kitchen & Valley, 1995;Farquhar et al, 1999;Satish-Kumar, 2000;Satish-Kumar & Wada, 2000;Satish-Kumar et al, 2002). When present, zoning is most often consistent with formation of graphite overgrowths involving a fluid phase during retrograde conditions.…”

Origin of graphite, and temperature of metamorphism in Precambrian Eastern Ghats Mobile Belt, Orissa, India: A carbon isotope approach

“…It is also possible that the graphite in these places represent multiple precipitation events depending on the pegmatite intrusion during D 2 or D 3 deformation period (Acharya and Dash, 1984). Although multiple episodes of graphite precipitation can be documented by isotopic analysis of graphite flakes in micro scale (Satish-Kumar, 2000), such high resolution sampling was not possible in the current study, and the number of events cannot be resolved. Graphite can also precipitate from carbon bearing fluids during cooling (Dubessy, 1984;Mathez et al, 1989).…”