Tracking magma intrusion and cooling in the lower crust with garnet Sm-Nd geochronology and phase diagram sections, Fiordland New Zealand

Garnet in the lower crust may grow during sub-solidus and hyper-solidus heating, and in some cases during cooling. Equilibrium phase diagram sections for mafic orthogneiss predict that the greatest volume of garnet should grow over a narrow temperature range with heating above the solidus. This peritectic garnet can preserve a record of multiple magma injections into complex magma mushes that form plutons in continental magmatic arcs. The exhumed lower crustal section from Fiordland New Zealand provides an ideal laboratory for using garnet to track processes in the continental magmatic arc within the Gondwana margin during the Cretaceous.

The Western Fiordland Orthogneiss (WFO) dominates the inboard lower crustal section exposed in Fiordland. The three major plutons in the WFO preserve a range of igneous and metamorphic fabrics, with some of the oldest igneous foliations and magmatic contacts obvious in the central Misty Pluton and the youngest sub-solidus fabrics observed in the southernmost Malaspina Pluton which intruded c. 117 Ma based on zircon U-Pb ages. In the Malaspina Pluton, garnet Sm-Nd ages for ≥1 cm diameter peritectic garnet grains along cross-cutting trondhjemite veins range from 117.0±4.1 and 116.2±2.0 Ma for garnet cores (N=3), and from 115.8±2.6 to 108.0±2.0 Ma for bulk grains and rims (N=10). In contrast, garnet Sm-Nd ages for ≤0.5 cm diameter garnet grains from 3 locations In the Malaspina are 104.1±1.8, 106.2±2.1, and 103.6±2.2 Ma. Major and trace element zoning is compatible with insignificant volume diffusion in the large grains and significant compositional changes in the small grains. The Bloch et al. (2020) diffusion coefficients for Sm and Nd indicate that significant diffusion would cease at c. 700°C in small grains from the Malaspina Pluton; therefore, we calculate cooling rates of c. 21 (northeast) to c. 10°C (southwest) per m.y. The cooling ages combined with phase diagram sections and mineral thermobarometry indicate exhumation rates of 0.2 to 0.4 kbar per m.y. for the southwestern part of the Malaspina Pluton.

Our results indicate that initial garnet growth ranged from near-synchronous to c. 4 m.y. after construction of the Malaspina Pluton by injection of magma sheets. We speculate that faster cooling in the northeastern Malaspina may have been driven by extension during growth of sub-solidus L-S tectonites which are best developed in this area. Combining these data with structural analysis and detailed evaluation of magma source compositions for individual sheets in WFO plutons allows us to evaluate magma plumbing systems for the Fiordland Arc.