EFFECT OF GARNET A–X SOLUTION MODELS ON THE THERMODYNAMIC MODELING OF HIGH-GRADE METABASITES USING THE TSO MORARI UHP ECLOGITE OF NW INDIA

Garnet is a key mineral in the thermodynamic modeling of high-grade metabasites as they record evidence of the metamorphic history (e.g., pressure, temperature, and composition) during porphyroblastic growth of garnet crystals. The availability of different versions of garnet activity-composition (a-X) solution models may lead to different results in the calculation of the metamorphic pressure-temperature (P-T) paths of metabasites. To test this hypothesis, we use the Theriak-Domino program, thermodynamic dataset (ds 62), and main mineral a-X solution models from Green et al. (2016) to evaluate the effect of the update of garnet a-X solution models from White et al. (2007) (W07) to White et al. (2014) (W14). In this study, eclogite from the Tso Morari UHP terrane, NW India, is used as a representative metabasite to compare the P-T predictions generated from using the two sets of garnet a-X relations (W07 vs. W14), while accounting for the effect of garnet fractionation during growth of porphyroblastic grains. The modeling protocol using W14 yields a peak metamorphic P-T of 34 ± 1.5 kbar at 551 ± 12 °C, while W07 yields a P-T of 28.5 ± 1.5 kbar at 563 ± 13 °C. Results indicate that the two modeling protocol choices result in consistent mineral evolution and temperature changes; however, the pressure generated using W14 is ~5.5 kbar higher than that predicted using W07. The difference in peak pressure results for the two modeling protocols are beyond the suggested uncertainty using mineral isopleth thermobarometry in pseudosections: ± 50 °C and ± 1 kbar at 2σ (Powell and Holland 2008). The model using W07 is more consistent with the peak pressure result calculated with the highest phengite Si (Si p.f.u. = 3.54) barometer (~ 26 kbar at ~ 570 °C) and results of multiple thermobarometers (~520–700 °C and ~20–26 kbar by garnet-omphacite, garnet-phengite, and garnet-omphacite-phengite). This study illustrates the importance of careful consideration of which garnet a-X relations one chooses for modeling high-grade metabasites, as well as the need of comparison with non-thermodynamic techniques (e.g., thermobarometry).

References:

Green et al. J Metamorph Geol 34, 845–869 (2016)

Powell and Holland J Metamorph Geol 26, 155–179 (2008)

White et al. J Metamorph Geol 25, 511–527 (2007)

White et al. J Metamorph Geol 32, 261–286 (2014)