Dyke-induced Contact Metamorphism and Metasomatism in Metapelites: a Study from the Paleoproterozoic Chaibasa Formation, India

Contact metamorphism and subsequent metasomatism processes provide a rare window to decipher the source and nature of fluids, and physicochemical conditions for alteration. Here, we characterize the contact metamorphism and metasomatism due to the intrusion of amphibolite dykes into the host metapelites that belong to the Paleoproterozoic Chaibasa Formation in the Singhbhum Craton of India. We have performed systematic petrography, mineral chemistry, and whole-rock geochemical study on dyke, contact zone and host rock samples to trace spatial mineralogical and geochemical changes in metapelites. The 6 m thick dyke intrusion has resulted in the formation of ~14 cm thick baked zone with two distinct layers (Zone 1 and 2) marked by colour variation running sub-parallel to the strike of the dyke in field scale. Zone 1 lies adjacent to the dyke, and Zone 2 lies away from dyke, adjacent to Zone 1. Zone 1 is composed of amphibole + epidote + chlorite + plagioclase + quartz ± Fe-Ti oxides assemblage. Zone 2 lacks amphibole and consists of epidote + chlorite + plagioclase + quartz ± biotite ± Fe-Ti oxides, indicating the albite-epidote hornfels facies of low P/T mafic assemblages. From the dyke to host rock, Fe content in epidote (Fe3+ = 0.69 to 0.74 apfu) increases, whereas Ca content in epidote (23.5 to 22.9 wt%) and plagioclase (6.42 to 5.26 wt%) decreases. The calcic amphiboles (Ca < 1.9 and Na > 0.1) formed along the dyke are pargasite to tschermakite in composition. Major oxide analysis reveals CaO and Na2O enrichment and TiO2, Al2O3, Fe2O3, MgO, and K2O depletion in baked zones relative to the host rock. Analysis of trace elements reveals depletion of REEs in both baked zones, highest in Zone 1 compared to Zone 2. These chemical changes or metasomatism are most probably driven by hydrothermal fluids. Fluids released due to thermal dehydration and mineral breakdown (e.g., muscovite and biotite) in the host rock combined with Ca-rich hydrothermal fluid derived from the mafic dyke, provide necessary nutrients (e.g., Ca, Al, Fe) to form amphibole and epidote in the baked zones. Therefore, increase in the density of epidote toward the dyke, and presence of calcic amphibole close to dyke contacts indicate Ca-metasomatism during dyke emplacement that controls the mineralogical as well as the chemical changes in the host metapelites.