Fluid-rock interactions in the high-grade terrain of south India: a mineralogic and thermodynamic study

Abstract

The formation of major rock types in the Kerala high-grade terrains, northern Tamil Nadu, and southern Karnataka Precambrian shield has been studied thermodynamically by focusing on: a) characterization of the fluid phase attending high-grade metamorphism, b) documenting fluid–rock interactions by characterizing channel volatiles of cordierite in metapelites, and c) assessing the relative importance of different fluid species in the gneiss-to-charnockite transformation.

In southern India, a gneiss–granite–greenstone terrain passes into a granulite–gneiss terrain to the south through a transitional zone spread over southern Karnataka and northern Tamil Nadu. The occurrence of charnockites as patches, blebs, and cross-cutting veins showing transgressive contacts with amphibolite facies gneisses is a characteristic feature of this transitional zone and the predominantly high-grade Kerala Khondalite Belt (KKB). Field, mineralogical, and fluid inclusion data indicate that high-grade metamorphism was fluid-present in these regions.

Cordierite Channel Volatiles: Infrared (IR) powder absorption spectra of cordierites from amphibolite and granulite facies metapelites in southern/eastern Karnataka, northern Tamil Nadu, and southern Kerala show CO? and H?O as channel volatiles, indicating metamorphic fluids of CO?–H?O composition. A correlation between the relative proportions of CO? and H?O and metamorphic grade is apparent. The mole fraction of H?O in the fluid phase, X(H?O)fl, was computed using thermodynamic water barometry with gravimetrically determined channel water content and available P–T data. A modified Redlich–Kwong equation of state was used to account for non-ideal mixing.

X(H?O)fl values correlate with metamorphic grade:

Amphibolite facies metapelites of eastern Karnataka: high values (0.665–0.90).

Bangalore Gneiss region: lower values (0.205–0.545).

Metapelites near charnockite terrains: low values (<0.22). This indicates a decrease in H?O proportion and increase in CO? southwards, parallel with increasing metamorphic grade.

K-Feldspar Structural State: Petrographic and X-ray diffraction studies show a correlation between K-feldspar polymorphs and metamorphic grade. Amphibolite facies gneisses and incipient charnockites contain ordered triclinic feldspars (Al occupancy ~0.949–0.971). Charnockites of Halagur–Sivasamudram and B.R. Hills, and metapelites of M.M. Hills, host variable amounts of disordered K-feldspars (Al occupancy ~0.375–0.401). In KKB, feldspars are predominantly disordered (0.334–0.461). Textural evidence shows microcline development from grain boundaries and fractures in disordered perthitic feldspars, reflecting fluid interactions and deformation.

Fluid–Rock Interactions: H?O-deficient granulite facies metamorphism produced disordered K-feldspars, while H?O-rich amphibolite facies metamorphism produced ordered feldspars. Incipient charnockites may reflect interaction with hydrous metasomatizing fluids. Low H?O activity during granulite metamorphism and gneiss-to-charnockite transformation produced disordered feldspars, later modified by fluids or deformation.

Thermodynamic Modeling: Free-energy minimization using SOLGASMIX was applied to compute P–T–X conditions of charnockite formation:

Kabbal-type charnockitization (C–O–H fluids): X(H?O) ~0.25 at 5.5 kbar, 750 °C.

Ponmudi-type charnockitization (C–O–H–N fluids): X(H?O) ~0.20 under similar conditions.

Both require low X(H?O) to stabilize orthopyroxene. CO? and H?O dominate the fluid phase, while CO, CH?, H?, and O? are minor. In Ponmudi-type systems, N? is abundant but NH? is negligible, indicating N? stability under granulite conditions. Computations show N? acts similarly to CO? as a diluent, reducing H?O activity and promoting charnockite formation.