Multi-Stage Growth of Kyanite in Migmatites Interpreted by Integrating Forward Thermodynamic Modelling and Trace Element Signature

Abstract

Trace element zoning in kyanite can retain information about its growth history, particularly in anatectic metapelites. There, kyanite can grow (i) at sub-solidus conditions through metamorphic reactions involving other aluminous phases as reactants, (ii) through muscovite dehydration melting reactions, and (iii) during cooling and melt crystallisation either through back-reactions between melt and solid phases (e.g., garnet) or crystallising directly from the melt. Thermodynamic modelling successfully reproduces these reactions, allowing a more robust interpretation of the observed features based on predicted reactants and products. In this study, we interpret the kyanite trace element zoning (particularly of Cr, V, and partly of Fe) observed through cathodoluminescence and quantified through LA-ICP-MS maps, using the forward thermodynamic modelling approach. The studied samples are biotite + kyanite + garnet migmatites from the Lower-Greater Himalayan Sequence of eastern Nepal, which experienced muscovite and incipient biotite dehydration melting. Three main generations of kyanite revealed by trace element zoning have been identified (i.e., Ky1, Ky2, and Ky3), consistent with the three main kyanite-producing reactions predicted by forward thermodynamic modelling, also applying a melt reintegration approach. Ky1 (i.e., sub-solidus kyanite) integrated only minimum amounts of Cr, V and Fe. Ky2 (i.e., peritectic kyanite) incorporates Cr and V released from muscovite during its dehydration melting reaction. Ky3 (i.e., back-reaction overgrowth or magmatic kyanite) is particularly developed in samples where melt segregation has been absent or limited and incorporates lower amounts of Cr and V than Ky2, but is enriched in Fe. The major implications of this study concern the interpretation of the melt segregation processes in anatectic rocks and our understanding of the Cr and V partitioning between minerals and melt. Further methodological considerations are also provided, which could help guide similar studies in the future.