Garnet Chemical Zoning Based Thermobarometry: Method Evaluation and Applications in the Menderes Massif, Western Turkey

Abstract

The garnet chemical zoning method (GZM) is a reliable thermodynamic approach for forward modeling pressure-temperature (P-T) paths using observed garnet and bulk rock compositions. However, intracrystalline diffusion is known to compromise the integrity of GZM modeled garnet-growth P-T paths. For this reason, extracting reliable metamorphic estimates from garnet-bearing schists in the Central Menderes Massif (CMM), western Turkey, has been difficult. To evaluate the impact of diffusion on GZM, we simulate garnet growth and diffusion for an average metapelite using the program Theria_G. Modeled garnet compositions from four simulations are used to estimate P-T conditions and paths by GZM, which are compared against Theria_G specified P-T-t trajectories. Factors influencing results are heating/cooling rate, grain size, and peak T. At a maximum T of 610 °C, both undiffused and diffused garnet compositions returned estimates comparable to prescribed conditions regardless of heating/cooling rate. Diffused profiles from simulations reaching a maximum T of 670 °C also reproduced prescribed P-T paths if tectonism occurred at high heating/cooling rates (50 °C/my). From these insights and additional Theria_G simulation-derived observations for CMM garnets, we deduce that metamorphism in the region exceeded 650 °C and achieved a maximum burial P between 8–10 kbar prior to Cenozoic exhumation.