Contrasting mineral parageneses in high‐temperature calc‐silicate granulites: examples from the Eastern Ghats, India

Abstract

Three types of mineral associations are described from calc-silicate granulites from the Eastern Ghats, India, where geothermobarometry in associated rocks suggests extremely high P-T conditions of metamorphism (c. 9 f 1 kbar, 950" C). These mineral associations are: (i) calcite + quartz + scapolite + plagioclase, (ii) calcite + scapolite + wollastonite + porphyroblastic garnet + coronal garnet and (iii) calcite + quartz + wollastonite + scapolite + porphyroblastic garnet + coronal garnet, all coexisting with K-feldspar, titanite and clinopyroxene. The first two associations evolved through nearly isobaric cooling retrograde paths, whereas the third evolved through a nearly isothermal decompression path followed by an isobaric cooling retrograde path. Textural and compositional characteristics suggest the following mineral reactions in the calc-silicate granulites: calcite + quartz = wollastonite + C02, calcite + plagioclase = scapolite, calcite + scapolite + wollastonite = porphyroblastic garnet f quartz + C02, CaTs + wollastonite = coronal garnet (association ii) and wollastonite + scapolite = coronal garnet (association iii)+quartz+CO,. Andradite content in garnet was buffered by the redox equilibria wollastonite + hedenbergite + O2 = andradite + quartz (association iii) and wollastonite + andradite + CaTs + scapolite = hedenbergite + calcite + grossular + O2 (association ii). The contrasting mineral parageneses have been ascribed to interplay of variables such as &,, f , , fHcl in the fluid, bulk Na content and the nature of the retrograde P-T-Xc, paths through which the rocks evolved.

Ultrametamorphism in Precambrian granulite terranes: Evidence from Mg‐AI granulites and calc‐silicate granulites of the Eastern Ghats, India

“…Experiments on biotite-bearing quartzo-feldspathic assemblages predict extensive melting even under vapour-absent conditions, leading to the formation and emplacement of voluminous granitic magma at shallow crustal depths, which leads to large-scale crustal differentiation. Along with some other regional granulite facies terrains of the world (Napier Complex, East Antarctica- Ellis 1980Ellis , 1983Harley 1985;Labwor Hill, Uganda-Sandiford et al 1987; the central zone of the Arunta Complex, Australia -Warren 1983;Goscombe 1992; Wilson Lake, Canada- Currie and Gittins 1988;Sipiwesk Lake, Canada-Arima and Barnett 1984;Ouzzal, AlgeriaBertrand et al 1992) among more than 90 well-studied terranes (Harley 1989), the Eastern Ghats Belt preserves unequivocal evidence of ultra-high temperature metamorphism (Sengupta et al 1990, 199 1;Dasgupta et al 1991). In this paper, we discuss the criteria of ultra-high temperature metamorphism from the Eastern Ghats Belt, citing evidence from the Mg-A1 granulites.…”

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

“…Consequently, the estimated temperature using carbon isotope thermometry will provide retrograde metamorphic temperature. Temperature and pressure of calc-silicate granulite rocks from the Eastern Ghats where three phases of metamorphism are recorded in the time range 1.6 Ga to $550 Ma have been determined by intersection of fluid-absent reaction equilibria for specific phase compositions (Dasgupta, 1993;Bhowmik et al, 1995;Shaw and Arima, 1996). However, determination of metamorphic temperature where metamorphism has occurred in presence of fluids is always challenging as the composition of the fluids are not always known.…”

Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system

“…Although less studied than metapelitic and basic lithologies, the importance of calc-silicate assemblages for the reconstruction of the pressure -temperature -fluid composition evolution (P-T-X fluid ) of metamorphic terrains is widely recognized (e.g. Warren et al 1987;Harley and Buick 1992;Dasgupta 1993;Fitzsimons and Harley 1994;Sengupta et al 1997;Stephenson and Cook 1997;Buick et al 1998). The majority of studied calc-silicate rocks come from granulite-facies terrains and are characterized by peak assemblages consisting of wollastonite + scapolite, with grossular-rich garnet often occurring as retrograde coronas (e.g.…”

“…This interpretation implicitly assumes that FeO and MgO components only have the effect of lowering the grossular activity in garnet, although many authors have provided evidence that clinopyroxene is an essential reactant consumed during garnet growth (e.g. Harley and Buick 1992;Dasgupta 1993;Fitzsimons and Harley 1994;Bhowmik et al 1995;Sengupta et al 1997). Recently, calc-silicates phase diagram topologies in the more complex CaO-FeO-Al 2 O 3 -SiO 2 -CO 2 (CFAS-CO 2 ) system have been considered.…”