Phase Relation Modelling of Mg‐Metasomatism: Application to a Granodiorite From the Tauern Window (Central Alps, Italy)

ABSTRACT

Fluid‐assisted Mg metasomatism profoundly alters crustal rocks, producing phyllosilicate‐rich assemblages, with major implications for rheology as well as petrophysical properties such as permeability. However, the effects such processes remain poorly constrained due to limitations in our ability to model the sequence of mineral reactions and bulk composition changes during fluid–rock interactions. Here we address this challenge through a phase equilibrium analysis of Mg–Ca/Na–Si metasomatism of a granodiorite infiltrated by an ultramafic‐derived fluid, using chemical potential phase projections and sections of mobile elements. We investigated a shear zone, in a granodiorite located in the Zillertal massif (Neves area, Tauern window in Eastern Alps, Italy), where Alpine amphibolite‐facies metasomatism and deformation (550°C—0.55 GPa) transformed the late‐Variscan granodiorite into amphibole‐bearing schists (garbenschiefer) and ultimately into a chlorite–muscovite schist. Mass balance calculations show strong MgO enrichment (up to 370%) and H ₂ O gain, coupled with substantial CaO, Na ₂ O and SiO ₂ loss (−92%, −86% and −13%, respectively). Chemical potential phase diagrams of these most mobile elements computed at constant pressure and temperature demonstrate that amphibole is a transient phase stabilized by MgO and CaO gain at the expense of quartz, before it breaks down into a CaO‐free chlorite–muscovite assemblage. Although absent in the studied area, our modelling indicates that greater MgO gain and SiO ₂ loss could stabilize that talc‐bearing assemblage: the so‐called whiteschist in the Alps. We infer that the metasomatizing fluid is externally derived from the dehydration of nearby ultramafic bodies, consistent with processes described in the Greiner shear zone, located to the north of the studied area. Finally, we demonstrate that our modelling approach can be applied to model metasomatic zones at contacts between chemically contrasting lithologies, such as ultramafic and sediments, like in subduction mélange zones.