PSEUDOSECTION MODELS FOR EVALUATING THE GROWTH, CONSUMPTION AND ZONING OF METAMORPHIC GARNET DURING PARTIAL MELTING, FIORDLAND NEW ZEALAND

Phase diagrams, produced with programs like THERIAK-DOMINO, have increased our understanding of Earth’s interior and tectonic processes. Forward models in P-T space can be used to construct isochemical P-T paths for metamorphism. However, pseudosections incorporating variable effective bulk compositions (EBC) are needed to help explain some mineral zoning and complex processes like partial melting.

Partial melting introduces a new phase, dehydrates the protolith, and produces new minerals. The complication of partial melting requires investigation of both the protolith and melt. In this study, we first determine representative EBC critical to the construction of useful models. Second, we determine a reasonable water content for each EBC by producing T vs. H2O diagrams at isobaric conditions. To evaluate the effects of partial melting, we use bulk compositions for the unmelted host, the partially melted rock (GRZ) and melt. Changing bulk compositions allows only one other intensive variable and models are either isothermal or isobaric.

T vs. EBC phase diagrams for monzodiorite from the Misty Pluton of the Western Fiordland Orthogneiss with localized partial melting illustrate this approach. The sample contains a peritectic garnet selvage with subhedral oscillatory Grs and Prp zoning and anhedral resorbed grains with high Grs cores and low Grs rims which separates the GRZ from the leucosome. The pseudosection provides a means for evaluating the conditions required to nucleate garnet, produce Grs zoning, and resorb garnet proximal to the leucosome.

Single oscillations in Grs (low - high – low) can be explained by changing EBC from the host to the leucosome. In addition, the predicted garnet modes show resorption would occur as the EBC changed from the GRZ to the leucosome. This example shows that high Grs rims could be produced under isobaric conditions and requires re-evaluation of P increases based on this zoning.

Phase diagrams, produced with programs like THERIAK-DOMINO, have increased our understanding of Earth’s interior and tectonic processes. Forward models in P-T space can be used to construct isochemical P-T paths for metamorphism. However, pseudosections incorporating variable effective bulk compositions (EBC) are needed to help explain some mineral zoning and complex processes like partial melting.