Kyanite as a Petrogenetic Archive of the Metamorphic Reactions and Fluid Metasomatism in Aluminous Rocks

Abstract Kyanite serves as a robust indicator of metamorphic P–T conditions, forming through diverse reactions across sub- to supra-solidus conditions. This study investigates kyanite’s potential to decipher metamorphic reaction histories and fluid metasomatism through integrated cathodoluminescence (CL) imaging, trace element analysis, and phase equilibria modeling of various metamorphic rocks. A suite of kyanite-bearing metamorphic rocks was sampled across the east–central Himalayan orogen, comprising pelitic gneisses, meta-graywackes, pelitic granulites, garnet amphibolite, and leucosomes. The metasedimentary rocks recorded elevated P–T conditions from ~7 kbar/650 °C in pelitic gneisses, to ~9 kbar/~685 °C in meta-graywackes, and then to 12–13 kbar/815–830 °C in the pelitic granulite spanning from sub- to supra-solidus conditions. In these rocks, kyanite formation primarily results from mica or staurolite breakdown reactions. Three petrogenetic kyanite types were identified in pelitic granulites and meta-graywacke, including metamorphic, peritectic, and magmatic types. In contrast, pelitic gneisses host sub-solidus metamorphic kyanite formed through staurolite breakdown. The sub-solidus kyanite exhibits core–mantle-rim zonings that record a complete metamorphic evolution from prograde to retrograde metamorphism. These findings demonstrate that kyanite shows the capacity to record complete P–T paths in both sub-solidus and supra-solidus aluminous rocks, as evidenced by CL and trace element zonings. These zoning patterns faithfully document metamorphic evolution from prograde heating to retrograde cooling. Notably, kyanite occurs in quartz-bearing millimeter-scale veins and shows tabular forms with crack-filling veinlets in garnet amphibolites, features consistent with metasomatic kyanite precipitated from externally infiltrated fluids. By contrast, kyanites in the leucosome are entrained metamorphic origin, exhibiting resorbed shapes and encircled by sillimanite. Our integrated approach reveals systematic relationships between kyanite properties and metamorphic conditions. Kyanite in granulite-facies rocks shows high CL intensity, elevated Cr–V–Ti–Ga, and low Mg, Fe contents. Amphibolite-facies kyanite exhibits reduced CL intensity and Cr–V–Ti–Ga contents, whereas metasomatic kyanite has weak luminescence with significantly high Fe and low Cr/V ratios. We demonstrate such an application through the case study of the entrained metamorphic kyanite from the leucosome and prove its utility in reconstructing metamorphic histories. This study establishes kyanite as a sensitive recorder of both metamorphic conditions and fluid metasomatism in aluminous systems.