Extremely high-temperature calcareous granulites from the Eastern Ghats, India: Evidence for isobaric cooling, fluid buffering, and terminal channelized fluid flow
Bhowmik, Santanu Kumar; Dasgupta, Sοmnath; Hoernes, Stephan; Bhattacharva, Prasanta Kumar
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 689 - 704
published: May 19, 1995
manuscript accepted: Jan 6, 1995
manuscript received: Mar 28, 1994
ArtNo. ESP147050703019, Price: 29.00 €
Abstract A suite of calc-silicate and calc-magnesian silicate granulites from Borra, Eastern Ghats belt, India, displays six critical mineral assemblages that were produced from widely varying bulk compositions at different episodes of metamorphism. Phase equilibria considerations in the calcareous rocks and geothermobarometry in associated rocks attest to metamorphic temperatures in excess of 950 °C at moderate pressures (9kbar). Subsequently, the complex evolved through a near-isobaric cooling path (ΔP = 2kbar, ΔT = 250 °C) that intersected several vapour-absent equilibria. Fluid composition was regionally buffered by mineral equilibria both during peak and retrograde metamorphism, and as a result of varied bulk compositions XflCO2 varied between ~0.8 and ~0.4 in microscale during peak metamorphism. However, increased CO2 activity (promoted by channelized CO2 flux) along late narrow shear zones is locally indicated by mineral equilibria. Carbon and oxygen isotopic data reveal that the calcareous rocks preserved gradients presumably from the premetamorphic stage, which rule out any pervasive fluid flux during granulite facies metamorphism in Eastern Ghats.