High resolution Sm–Nd garnet geochronology reveals the uneven pace of tectonometamorphic processes

Chemically zoned garnets provide a record of the changing conditions within the tectonically evolving crust where metamorphic mineral growth occurs. Recent advances in Sm–Nd geochronology of zoned garnets now permit the measurement of a higher resolution temporal record than previously attainable. Here, we present precise (< ± 0.7 million years) Sm–Nd ages from 12 concentric growth zones within a single large garnet from the Austrian Alps. Within a total growth span of 7.55 ± 0.52 (2 SD) million years, the data reveal two brief distinct pulses of accelerated growth (at least 5 times faster than the background growth rate). These pulses correlate with compositional features in the zoned garnet that are indicative of growth in the presence of abundant fluids or abrupt P–T changes. These data show that tectonometamorphic processes leading to garnet growth (such as exhumation, fluid flow, and/or heating) were pulse-like in nature, rather than slow and steady, during the waning stages of the Alpine orogeny 28–20 million years ago.