DEVELOPMENT AND USE OF THE HIGHEST-RESOLUTION GARNET-BASED P-T PATHS

Garnet-based pressure-temperature (P-T) paths have long been used to constrain tectonic models. Many of these models commonly focus on higher T tectonic processes, as garnets record how distinct packages are displaced by large-scale fault systems, for example. However, Theriak-Domino-based thermodynamic modeling now offers new possibilities to increase the resolution of garnet-based P-T paths. These advances allow the estimation of specific P-T conditions at the scale of a compositional analysis, thereby tracking how a garnet grew from core to rim. Here we outline the approach, which only requires bulk rock and garnet compositions. The approach transforms the garnet into a model that shows how its composition and zoning evolves within a prescribed (effective) rock bulk composition. Predictions in garnet zoning suggested by the P-T path can be compared to original garnet compositions and predicted conditions can be compared those estimated by conventional thermobarometry or inclusion barometry. Not all garnets can be modeled. Those with significant changes in composition over short distances are problematic, as major changes in bulk rock composition and/or significant changes in P-T conditions are unable to be accommodated. Ultimately, the approach prescribes that each high-resolution P-T path is an estimate that approximates the reality of the sample. Here we report high-resolution garnet P-T paths from two field areas (the Himalayas and western Turkey) and indicate that these garnets record the removal of overburden (erosion, denudation). Garnets from these areas are growing at high T, but recording P processes associated with alteration of their overburden. The ability to increase the available resolution of garnet-based P-T paths has the potential to transform ideas regarding the development of field areas that contain them and expand the use of metamorphic petrology to fields outside of its traditional boundaries.