THE ROLE OF EFFECTIVE BULK COMPOSITION ON GARNET COMPOSITIONS AND PREDICTION OF PRESSURE DURING HIGH-GRADE METAMORPHISM

We present new isochemical and T-X phase diagrams for migmatitic orthogneiss from the Malaspina Pluton, Fiordland, New Zealand. Prior work has shown the importance of considering changing effective bulk composition (EBC) during garnet growth and much of this has emphasized elemental sequestration (Mn) into zoned garnet. The EBC during garnet growth may be calculated with theoretical fractionation models, systematic removal of garnet using observed zoning, or by physical removal of garnet from the sample and chemical analysis of the remaining rock. The composition of peritectic garnet may be subject to elemental sequestration and changing effective bulk composition resulting from removal and/or addition of chemical components in partial melt.

Metamorphism of the Malaspina Pluton produced garnet+clinopyroxene+trondhjemite melt that replaced igneous 2 pyroxene assemblages in monzodiorite. Fine-grained garnet+clinopyroxene ‘garnet reaction zones’ and peritectic garnet selvages border trondhjemite veins. Peritectic garnet is ca. 1.5 cm in diameter and nearly unzoned with Alm = 44%, Prp = 38%, Grs = 18%, Sps = 1%. However, narrow (ca. 150 um) rims of high grossular Alm = 40%, Prp = 31%, Grs = 29%, Sps = 1% are locally present adjacent to vein plagioclase. High grossular rims observed throughout Fiordland and elsewhere have been interpreted to result from late stage pressure increases.

An isochemical P-T phase diagram for the garnet reaction zone bulk composition predicts a ca. 6 kbar pressure increase for late garnet growth. In contrast, isobaric T-bulk composition models, which incorporate changing EBC from host to garnet reaction zone to leucosome compositions, predict that garnet growth could be isobaric with a possible temperature decrease of <50°C. This indicates that prior tectonic models for loading may not be valid. However, the sensitivity of the models to fluctuations in pressure, temperature, effective bulk composition, and water content remains to be documented. Our new phase diagrams test this sensitivity and indicate the importance of EBC for constructing meaningful P-T paths and predicting tectonic models.