USING PSEUDOSECTIONS TO EVALUATE CHANGING EFFECTIVE BULK COMPOSITION DURING GARNET GROWTH AND PARTIAL MELTING, EXAMPLES FROM FIORDLAND, NZ

P-T pseudosections provide the basis for defining a P-T path. Garnet compositions can constrain a P-T path and provide Sm-Nd and/or Lu-Hf dates. Construction of pseudosections is often complicated by possible changes in effective bulk composition (EBC) as elements move into or out of relatively inert phases and/or the rock (e.g., by melt movement in or out of the rock). This second type of variable EBC is difficult to model because the exact proportion of mobile components is generally unknown. In the case of partial melting, both aqueous and silicate melt components may be mobile during metamorphism and water content influences melting and phase stability. The fluid component, critical to constructing a pseudosection, may be poorly constrained by the observed mineral assemblage. Here, we present methods to estimate water activity in granulite gneiss by using garnet compositions plotted on isobaric T-X_H2O pseudosections, and then followed by T-X_EBC pseudosections to construct P-T paths for garnet growth.

Fiordland, New Zealand presents an ideal location for this study. The Western Fiordland Orthogneiss (WFO) contains planar trondhjemite leucosomes cross-cutting 2-pyroxene diorite gneiss. Garnet-clinopyroxene granulite (garnet reaction zones -GRZ), and coarse garnet selvages separate leucosomes from host diorite. Several studies have inferred tectonic/magma loading from well-documented garnet zoning with high Ca rims.

Following partial melting or melt injection WFO rocks contain three EBCs: leucosome, GRZ, and host. Garnet growth modeled in equilibrium with a continuous range of these 3 compositions allows construction of P-T paths. The observed garnet zoning is compatible with near isothermal and isobaric garnet growth. These findings suggest introduction of fluid along fractures initiated partial melting, dehydrating the host followed by late fluid release into the host during crystallization of the leucosome resulting in garnet growth within the GRZ.