Role of changing effective bulk composition in the formation of grossular zoning during garnet granulite metamorphism in the Western Fiordland Orthogneiss, NZ.

摘要

Garnet compositions and zoning, which are important records of crustal processes, are often used to constrain P-T-t paths for deep crustal rocks now exposed at the surface. These P-T-t paths require knowledge of the stable minerals and effective bulk composition during each increment of garnet growth, because effective bulk composition may change due to compositional fractionation by mineral growth and/or mass transfer. The importance of changing effective bulk composition as the result of partial melting is shown for two samples from the Western Fiordland Orthogneiss, New Zealand. Both samples experienced partial melting during garnet granulite metamorphism. High grossular garnet rims in granulite from Fiordland and elsewhere have been interpreted to result from pressure increases during garnet growth. Pseudosection models presented here indicate that changes in effective bulk composition from host, to garnet reaction zone, and then to leucosome, could produce peritectic garnet with high-grossular rims which grew along an isobaric path during melting and/or melt injection.