Weathering Trends Unveil K-Metasomatism in Bundelkhand Granitoids: Resolving the Pink–Grey Granitoid Paradox

Shiba Shankar Acharya, Arnab Sain, Pragnyasini Behera, Adri Basak, Gautam Kumar Deb

The petrogenetic puzzle of pink coloration of granitoid rocks is rooted on the debate between primary magmatic versus metasomatic origin of K-feldspar, which, in turn, has persisted due to difficulties in distinguishing these processes through conventional petrographic methods. This study introduces a novel approach using chemical weathering patterns in ACNK ternary diagrams (Al2O3, CaO + Na2O, and K2O) to identify K-metasomatic signatures in the Bundelkhand granitoids. The method exploits the fact that normal weathering results predictable trends sub-parallel to the A-CN axis, while K-metasomatism creates distinctive deviations toward the K-apex. Three weathered granite profiles spanning pink and grey granitoid varieties were analysed using integrated bulk geochemistry and SEM-EDS microanalysis. Chemical weathering trajectories revealed diagnostic K-metasomatic signatures in pink granitoids, confirming their secondary origin from grey protoliths. This finding reconciles the puzzling identical radiogenic ages of both the colour varieties of granitoids, reported in previous studies.
Petrography of fresh samples suggests biotite chloritization as the fundamental alteration mechanism, operating under shallow crustal conditions (200-350°C), deduced through chlorite thermometry and mineral assemblages. This process drove cascading reactions including albitization, sericitization, epidote formation, and ultimately hematite precipitation responsible for pink coloration of the granitoids. Micropore networks within altered plagioclase served as conduits for fluid migration, enabling pervasive K-feldspathization without requiring major structural controls.
The integrated methodology reveals link between surface weathering and deep crustal metasomatism, providing the surface process as a powerful diagnostic tool for identifying cryptic alteration in granitoids. These findings have significant implications for understanding Archean crustal evolution and the petrogenetic interpretation of ancient granitoid terranes.