SHOOTING AT A MOVING TARGET: A MULTI-SCALAR APPROACH TO THERMODYNAMIC MODELING OF HIGH-TEMPERATURE METAMORPHISM

A primary objective of metamorphic petrology is to accurately quantify the pressure-temperature (P-T) paths recorded by rocks. To achieve this, petrologists commonly employ thermodynamic modeling and the construction of P-T pseudosections. A key assumption involved in interpreting a P-T pseudosection is that the bulk-rock composition used is representative of the effective bulk composition (EBC) from which apparently equilibrated mineral assemblages grew. Choosing a representative EBC is straightforward for many rocks, but can become extremely difficult for cases in which the rock’s composition evolved significantly throughout its P-T history (e.g. partial melting and melt loss at granulite-facies conditions). In addition to changing a rock’s composition, combined processes of melt/fluid extraction and/or consumption at high-T can modify equilibration volumes: domains of rock that were once equilibrated with each other at one P-T and in the presence of grain boundary fluids may depart from equilibrium as intergranular mobility is slowed greatly by fluid loss.

Here, we address how consideration of evolving EBC’s at multiple scales of observation can be used to resolve the history of complex high-grade rocks. We decipher the evolution of a single, mineralogically heterogeneous and texturally complex hand sample of UHT granulite. We show that at the hand-specimen scale an EBC can be identified and used to constrain the P-T conditions at which the ‘whole rock’ was last in mutual equilibrium, in the presence of intergranular melt that has subsequently been lost. Smaller macrodomains (~cm scale) and microdomains (~mm scale) can be identified to represent subsequent evolution during and after melt channelization and loss, and P-T pseudosections can be calculated for the compositions of these domains. Using this approach, we confirm that the sample, from the Gruf Complex (Central Alps), experienced a clockwise P-T path marked by decompressional heating to peak UHT conditions (~960 ˚C, 8.5 kbar) followed by near-isothermal decompression by at least 0.5 kbar. The approach places tight constraints on the prograde P-T path and enables construction of post-peak histories in rocks for which P-T pseudosections are otherwise difficult to interpret.