Garnet as a Proxy for Deciphering Polymetamorphic Histories: An Example From the Sanandaj-Sirjan Zone of Iran

Abstract

In the central sector of the Sanandaj-Sirjan zone of Iran, the Triassic-to-Jurassic ‘Hamedan Phyllites’ metamorphic complex shows a polymetamorphic evolution characterised by a main Barrovian- to Buchan-type garnet+staurolite+kyanite/andalusite regional sequence, locally overprinted by a low-pressure thermal event generated by vigorous Jurassic magmatic activity and the emplacement of voluminous composite plutonic complexes (e.g., Alvand). In this work, we explore the Hamedan phyllites enveloping the Jurassic Alvand granite batholith, particularly in the eastern sector at the transition from cordierite-bearing hornfels to staurolite-bearing schists. In this study, we present for the first time the textural characterisation and mineral chemistry of a previously undescribed minor population of post-kinematic Mn-rich garnet from the staurolite schists. We developed phase equilibria modelling to define the pressure-temperature condition of its genesis. We then integrated the existing literature on metapelites from the study area to develop new thermobarometric models with those data to propose a revised evolution of the metapelites enveloping the Alvand batholith and the extent of its contact aureole. On a wider scale, this study provides evidence for the relevance of garnet, as chemical-physical archive, in deciphering complex polymetamorphic histories where thermal events affect previously regional metamorphosed rocks under relatively hot gradients.