GARNET GROWTH AND COMPOSITIONAL ZONING RESULTING FROM CHANGING EFFECTIVE BULK COMPOSITION DURING GRANULITE METAMORPHISM, INTRUSION, AND PARTIAL MELTING

P-T-t paths are important for understanding large-scale processes in magmatic arcs. These paths generally rely on understanding mineral growth and zoning in metamorphic rocks. Such grain-scale processes depend not only on P & T, but also local hydration/dehydration and melt processes. For example, dehydration associated with partial melting and/or magma injection producing granulite mineral assemblages from hydrous precursors force a change in effective bulk composition (EFB). This results because melt plus/minus a separate H₂O phase can migrate into or away from melt veins. Proportions of melt will vary with migration, thus changing the EFB. Forward models of mineral assemblages provide a quantitative test of these effects.

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.