Crystallisation of metamorphic garnet: mechanisms and rates from the inverted Barrovian sequence in the Sikkim Himalaya

Abstract

The compositional zonation, three-dimensional spatial distribution, and morphologies of a metamorphic garnet population represent an integrated record of the population's nucleation and growth history over changing metamorphic conditions. While previous inferences drawn from such datasets have proven diverse, their continued collection is key to better understanding the mechanisms and processes that control porphyroblast crystallisation, rates of intergranular element transfer, and the extent to which chemical equilibrium persists across a rock volume during metamorphism. This thesis utilises such datasets from the inverted Barrovian metamorphic sequence in the Sikkim Himalaya, India, where the record of recent orogenesis provides an ideal natural laboratory in which fundamental rock-forming processes can be investigated. Compositional analyses of a representative population of garnet porphyroblasts from a garnet-grade metapelite indicate a strong correlation between garnet crystal size and composition with respect to both major end-member and trace element components. Numerical simulation of progressive nucleation and growth using an equilibrium approach and multicomponent diffusion reproduces the sizes and major element zoning of the entire garnet population along a rapid heating trajectory (>100 C/Myr). Given the correspondingly rapid rates of garnet growth (~1.4 mm/My) and negligible departure from equilibrium, major element transport is inferred to have been non-limiting, with growth rates controlled by interfacial processes. In contrast, trace element zoning in the same population is indicative of persistent disequilibrium with respect to rare-earth elements, yttrium and chromium. Oscillatory zoning may reflect incorporation of chemical heterogeneities and minor fluctuations in the garnet growth rate not resolvable in major element zoning, and annuli are not demonstrably relatable to some rock-wide event. Elements that do not equilibrate across garnet surfaces document continuous spiral zoning, which permits estimation of syn-crystallisation strain-rates, on the order of 10^-11-10^-12 /s. Microstructural data from across the Barrovian sequence reveal garnet populations that are partially characterised by their grade, with changing crystal size and abundance interpreted to reflect the combined effects of bulk chemical variations and ripening with increasing pressures and temperatures. Deviations from this trend reflect populations in which garnet crystallisation was controlled by a highly heterogeneous spatial distribution of nucleation sites and transport-limited growth.