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Cordierite-orthopyroxene migmatitic gneisses exposed in Achankovil unit of the Kerala Khondalite Belt, southern India show evidences of melting, melt extraction and in-situ crystallization of melt under granulite-facies conditions. The sequential mineral assemblages garnet + biotite + orthopyroxene + plagioclase + quartz (± melt) in the mesosomes and garnet + biotite + orthopyroxene + cordierite + plagioclase + K-feldspar + quartz + melt in the melanosomes makes the Achankovil cordierite-orthopyroxene migmatitic gneisses a good example of anatectic rocks, where substantial melt fractions remained in-situ during decompression and cooling. Therefore, the rocks provide an opportunity to investigate deep crustal processes and record of rheological (thermal and mechanical) reequilibration prevailed during the final stages of orogeny. The significance of cordierite formation and its possible relationship with melt formation are investigated applying theoretical calculations in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnNCKFMASH) system. Results of numerical modelling of the mineral assemblages in pressure–temperature–composition (P–T–X) pseudosections using Perple_X infer that the sequence of reactions involving formation of cordierite-orthopyroxene-melt assemblage is consistent with an isothermal decompression (with a pressure drop of >1.5 kbars) at high temperatures (>800 °C), forming leucosomes. Biotite dehydration melting reactions, occurring above 4.5 kbars constrain prograde arm of the P–T trajectory and is interpreted as a product of crustal thickening, which was followed by rapid decompression. The final stage of exhumation is characterized by rehydration of cordierites in the melanosome by melt-solid interactions at exceptionally low-pressure (~3.2 kbars) conditions. The high-temperature isothermal decompression inferred from the mineral reactions and P–T–X pseudosections constitute a clockwise P–T path for the exhumation of the lower crust. This clockwise P–T path is consistent with the common tectonic model accepted for the genesis of granulite-facies migmatites during crustal thickening and later unroofing, accompanied with arc-continent collision. Our conclusions indicate low–P metamorphism and anatexis can be traced to convergent setting, where melt buoyancy considerably decreases density of the lithosphere and modifies rheology leading to rapid exhumation of the lower crust. Therefore, the crustal evolution in the Kerala Khondalite Belt is correlated with two stage processes: (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.

This note reports new occurrences of syenite bodies around Marunthurkota area from the Kerala khondalite belt (KKB). Petrological and geochemical studies suggest that the syenites have a pronounced A-type affinity, metaluminous characteristics with high concentrations of alkalies, Rb, Sr, Zr, and high K 2 O/Na 2 O ratio. Miaskitic nature (agpaitic index<1) of syenite suggest involvement of CO 2 related phase in their genesis. The petrological characteristics signify crystallization of the rock at shallow levels within the crust. Geochemistry favours mantle origin of the magma and enrichment of Ba and Sr are indicative of involvement of carbonatite melt in the source region. The study envisages the presence of a juvenile CO 2 enriched upper mantle below the southern Indian continental crust during the Pan-African time.