GARNET GROWTH AND COMPOSITIONAL ZONING RESULTING FROM CHANGING EFFECTIVE BULK COMPOSITION DURING GRANULITE METAMORPHISM, INTRUSION, AND PARTIAL MELTING
Pseudosection models for diorite to monzodiorite gneisses, common in lower crustal rocks of Fiordland NZ, were constructed using THERIAK-DOMINO. In these rocks, 2 pyroxene +hornblende assemblages are replaced by garnet +clinopyroxene +plagioclase ±clinozoisite ±kyanite in garnet reaction zones (GRZ) adjacent to trondhjemite veins. Garnet typically is weakly zoned, but locally has high Grs annuli and/or high Grs rims. The latter have been used to infer P increases during growth. Garnet compositions and growth were modeled in T vs. composition plots: host-GRZ-leucosome. Predicted garnet modes are highest at T above the solidus and at or near the GRZ compositions compatible with observed modes in vein-bearing samples. Weakly-zoned garnet with Grs=20% observed in many samples is predicted over a range of EFB and/or T increase during growth at temperatures of ca. 700-850°C, depending on the initial host rock composition. However, garnet could only grow over a narrow range of T unless diffusion erased earlier Grs>20% cores. The max predicted Grs contents are near isothermal around the solidus for all P and bulk compositions. High Grs rims are predicted with EFB changes toward the leucosome at high P, isochemical suprasolidus T increases, or P increases +/- changing EFB. However, EFB changes to the leucosome require consumption of garnet. High Grs rims at T >850°C are unlikely to be preserved because diffusion would rapidly erase zoning. Therefore, P increases, T increases below 850°C, and/or Ca diffusion into rims are possible causes of Grs-rich rims.