Unraveling orogenic processes through U–Pb garnet petrochronology

Orogenic processes typically proceed through a number of tectonic stages in which rocks are subjected to burial, heating, exhumation and cooling. The rates and durations of the prograde stages are commonly poorly constrained, because the minerals typically used for age dating mostly form late in the process during decompression and the onset of cooling, or they possess closure temperatures that are lower than the metamorphic peak temperatures of metamorphism.

In-situ garnet U–Pb geochronology is an emerging technique that has the potential of unraveling the prograde- and peak-metamorphic history and may thus be the ideal petrochronometer that is needed to investigate mountain-building processes, where most other chronometers only record the collapse and exhumation of orogens.

At FIERCE, we perform garnet laser-ablation MC-ICPMS U–Pb geochronological analyses of metamorphic garnet with typically <0.1 µg/g U. Distinct growth zones of garnet from polymetamorphic granulites that experienced ultra-high temperature (UHT) metamorphic conditions above 900 ºC were analyzed.

We find that ages of older garnet growth zones are not reset during subsequent UHT-metamorphism, and conclude that the U–Pb system in garnet has a closure temperature at UHT conditions that may only be rivaled by zircon. Consequently, garnet U–Pb ages from crustal metamorphic rocks have to be interpreted as crystallization ages. LA-ICPMS garnet U–Pb dating thus complements the information obtained from other geochronometers, as it provides accurate and precise insight into the prograde to peak metamorphic P–T–t history of (poly)metamorphic terrains and the formation of orogens.