Thermodynamic modelling of phosphate minerals and its implications for the development of P-T-t histories: A case study in garnet - monazite bearing metapelites

Accessory phosphate minerals, such as monazite and xenotime, are commonly used as geochronometers to provide timing constraints on tectonometamorphic events recorded by the host rocks within which they occur. However, the formation and recrystallization of accessory minerals, and their interaction with major mineral phases, are still poorly understood. As a consequence, linking ages obtained from accessory minerals to the metamorphic pressure-temperature (P-T) paths obtained primarily from major mineral phases, such as garnet, remains challenging. While there have been studies that have advanced our understanding of the behaviour of various accessory minerals through thermodynamic modelling, limited examples are available to test their reliability in natural metamorphic rocks. This study incorporates phosphate minerals into one of the most commonly used thermodynamic data sets for phase equilibria modelling. This refined methodology is tested by modelling the detailed P-T-time (t) paths for rocks from two different regions of the Himalaya, one subsolidus and one that experienced suprasolidus conditions. The results obtained from our integrated models yield direct information on the behaviour of monazite and xenotime growth/breakdown along the calculated P-T paths. This allows us to tie different age populations obtained from the accessory minerals directly to the P-T paths derived from major mineral phases and facilitates a refined understanding of the P-T-t histories of those rock specimens. (C) 2019 Elsevier B.V. All rights reserved.