EVALUATING THE PRESSURE, TEMPERATURE, AGE, AND EFFECTIVE BULK COMPOSITION DURING PERITECTIC GARNET GROWTH IN MIGMATITIC ORTHOGNEISS, FIORDLAND NEW ZEALAND

The P-T and timing of high-temperature metamorphism, peritectic garnet growth, partial melting, and melt migration are difficult to quantify due to changes in effective bulk composition (EBC) and intracrystalline diffusion. However, phase diagram sections (PhaDS), and garnet Sm-Nd and zircon U-Pb geochronology can link ages with igneous crystallization, metamorphism, melt migration, and cooling. Three granulite facies orthogneisses highlight the data preserved during garnet growth and re-equilibration. 09NZ22 and 13NZ34 (Malaspina Pluton) contain abundant leucosomes with cross-cutting veins, matrix leucosomes, and high leucosome to host rock ratios (L/H). In contrast, sample 12NZ39 (Misty Pluton) contains sparse <3 cm leucosome veins.

09NZ22: Zircon U-Pb ages of 117.5 to 116 Ma (CA-TIMS) date igneous emplacement at 117.5 Ma. Garnet ages of 113 (Sm-Nd) to 111 (Lu-Hf) Ma for ~1 cm grains date peritectic growth. Later garnet equilibration is recorded by <1 mm grains at c. 104 Ma. Garnet compositional zoning, PhaDS, and diffusion in ~1 cm and 1 mm garnet indicate cooling of ~20°C/m.y. Garnet grew in an EBC influenced by melt at 113 Ma and 13.8 kbar at 875°C. 13NZ34: Zircon U-Pb ages range from 117.2 to 115 Ma (CA-TIMS) and date emplacement at 117.2 Ma. Garnet Sm-Nd ages indicate that 9-7 mm garnet cores grew at c. 116 Ma, rims grew from 111 to 108 Ma during partial melting, and 0.6-0.1 mm grains equilibrated between 106 and 103 Ma. Isotopic variation and sharp oscillatory zoning of trace elements indicate partial preservation of growth zoning in 9-7 mm garnet grains. Garnet-pyroxene thermometry and a P-T PhaDS indicate decreasing T and P from 980°C at 14.5 kbar to 860°C at 12.5 kbar. 12NZ39: No CA-TIMS zircon ages are available; however, nearby zircon U-Pb SHRIMP-RG ages date emplacement at c. 115 Ma. A garnet (<1 mm) Sm-Nd age of 109 Ma, garnet zoning, and a P-T PhaDS indicate peak to post-peak metamorphism at 13.0 kbar 865°C.

We conclude that evaluating ages, P, and T for high-T rocks requires evaluation of water content, EBC, and intracrystalline diffusion. In samples with high L/H, inclusion minerals, zoning in large garnet, estimation of EBC-often not equal to bulk rock, and growth zoning or diffusion in small garnet grains allow construction of P-T-t. In samples with lower L/H, smaller garnet grains preserve internal zoning and EBC may equal bulk rock allowing reconstruction of P-T-t.