Deciphering accurate P–T–t path of evolution of migmatitic terranes, where significant melt (leucosome) components are preserved in-situ, is a challenging task in metamorphic petrology. In a new contribution on the cordierite–orthopyroxene migmatitic gneisses of the Kerala Khondalite Belt (KKB), southern India, Sreejith and Ravindra Kumar provide novel understanding on the aspect of rheological (thermal and mechanical) modifications of the lower crust due to melt generation in collisional orogens at extremely low–P conditions. Their study follows a systematic recording of prograde and retrograde mineral assemblages of a melt-bearing segment

of lower crustal origin within Achankovil unit of the KKB. They numerically model the observed mineral assemblages and progressive mineral reactions using P–T–X pseudosections (see the figure), over a specific range of thermodynamical variables on particular bulk-rock compositions. The mineral-melt equilibria modeling is achieved by applying theoretical calculations and improved set of phase equilibria calculations in the MnNCKFMASH system using Perple_X thermodynamic computation program. The study presents new evidences for partial melting of continental crust at low–P levels within granulite-facies regime. The results obtained envisages, for the first-time, a multi-stage crustal evolution pattern in the KKB; (i) thickening of the crust in relation to a continental-arc setting, followed by (ii) exhumation along a high-temperature stable geotherm with sufficient pressure release associated with syn- to post-convergence transpression and transtension.

Full paper can be read in November 2012 issue of the Journal of the Geological Society of India.

Sreejith, C. and Ravindra Kumar, G.R., 2012. MnNCKFMASH phase relations in cordierite–orthopyroxene migmatitic gneisses, southern India: implications for low-pressure crustal melting under granulite-facies. Journal of the Geological Society of India, vol. 80, No. 5, pp. 613–627.

Figure P-T-path-of-KKB-migmatite

P–T–X pseudosection calculated for bulk rock composition in MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (MnNCKFMASH) system (with H2O in excess) for cordierite-orthopyroxene anatectic gneiss of the KKB.