Deformation signatures of mass transfer processes: Insights from metasomatic reaction zones in the Archean Singbhum craton in eastern India using geochemical and microstructural analysis

Open system metasomatic processes generally involve change of volume. Volumetric strain resulting from fluid-mediated mass transfer in rocks is commonly inferred from bulk-rock chemical mass-balance analysis. But the manner in which such volume change is accommodated in deeper crust remains enigmatic. The occurrence of geometrically well-defined reaction zones at the boundary between country rock (pelitic garnet-mica schist) and metamafic dikes (epidote-amphibolites) in the Archean Singbhum craton of eastern India provides an opportunity to address this problem.

The boundary between the two lithounits is marked by two metasomatic reaction zones (MRZs) in the sequence metamafic dike – Zone 1 (amphibole-epidote-sodic-plagioclase-quartz and chlorite) – Zone 2 (Zone 1 assemblage minus amphibole) – pelitic schist, with well-defined boundaries between each unit. We studied these using bulk-rock geochemistry, mineral chemistry, thermodynamic modelling and EBSD analysis.

The transition from Zone-1 to Zone-2 is gradational and marked by a gradual decrease in amphibole modal content. The discontinuity (~ 200 to 8) in variation of bulk-rock Ti/Cr ratio across the zones suggests that the original contact between pelite-dike was within MRZs. Bulk-rock mass balance suggests MRZs formed by Na-metasomatism (gain of ~325 g/100g of protolith), which facilitated the exchange between pelite and mafic-dike and removal of elements (Ca-K-Fe-Ti-Mg) from the MRZs to an external system. Pseudosection modeling shows that the region was cooled isobarically (at 5-6 kbar and 600 -> 300 °C) during the mass transfer process. Major oxide compositions of amphiboles and epidotes show systematic variations within the MRZs.

The euhedral and equant quartz and plagioclase grains exhibit polygonal mosaic texture with anhedral epidote grains at the grain boundaries and triple junctions in the MRZs. Quartz intragrain misorientation analysis from the two MRZs suggests deformation temperatures below 500°C. The absence of relict grains and low CPO strengths indicates that the recrystallization of quartz and plagioclase occurred under fluid-present conditions. EBSD-based crystallographic vorticity axis analysis shows that the quartz and plagioclase grains of the MRZs, amphiboles in the dike and Zone 1, and the retrograde minerals (chlorite, ilmenite, and magnetite) in the pelite record pure shear-dominated deformation signatures. In contrast, the quartz grains in the pelite and the dike were deformed under a simple shear-dominated regime.

Micro-structural analysis and mineral-chemistry variation indicate that quartz-plagioclase recrystallization and amphibole-epidote formation via mass transfer are interconnected processes. The presence of idioblastic-poikilitic amphiboles and epidotes (at the grain boundaries and triple-junctions) indicates that new grains of epidote and amphibole formed by dissolution re-precipitation processes during metasomatic transport by fluids in the MRZs. Evidence for such fluid-mediated exchanges are observed at the microscale as interconnected intergranular fluid pathways within the MRZs and at the outcrop scale as fracture-filled amphibole-bearing-quartz veins within the mafic dike. We propose that the fluid-mediated mass transfer (via dissolution-precipitation) helped to accommodate the volume changes during retrograde cooling reactions through the creation of localized stress fields.