METHODS AND UTILIZATION OF T-EFFECTIVE BULK COMPOSITION FORWARD MODELING TO INVESTIGATE THE ROLE OF PARTIAL MELTING IN GARNET GROWTH, ZONING AND RESORPTION

Pseudosections produced by programs like THERIAK-DOMINO are commonly used to predict mineral assemblages and P-T paths for constant bulk rock compositions in which equilibrium is attained and maintained throughout a metamorphic event. Here we expand this usage to systems undergoing partial melting by modeling variation in effective bulk composition (EFB) and comparing predictions to garnet occurring across lithological domains in migmatite. Samples collected in Fiordland, NZ contain garnet selvages adjacent to monzodioritic to granitic veins in host monzodiorite to diorite gneiss. Reaction zones bordering the garnet selvages show early dehydration of the host and local, late rehydration.

Melt migration changes the EFB by subtraction or addition of the melt composition within host and vein, respectively. Modeling these changing compositions requires pressure or temperature to be known and held constant for construction in two-dimensional space. In our models, P and/or T are estimated from published results, P-T pseudosections for rocks with little/no changes in EFB and thermobarometry.

Bulk compositions were defined using petrographic analysis. Water content was chosen for each EFB. T-H2O models were constructed and water content was selected for each peak mineral assemblage. Selection of an appropriate pressure is critical to determination of water content.

High-Grs garnet rims are observed throughout much of the mid- to lower-crustal Western Fiordland Orthogneiss and are widely attributed to loading. In samples from the Misty Pluton, garnet is restricted to selvages where zoning and resorption textures are observed. Here we show that high-Grs rims can also result from the changing EFB produced during partial melting.

Preliminary results show correlation between predicted and observed garnet growth, zoning and resorption. The importance of water content is clearly shown in this sample and emphasizes the need to consider water in traditional pseudosections as well.