Deciphering the exhumation path of granulite facies rocks based on mineral reaction textures and monazite geochronology

The formation of mineral reaction textures such as coronae or symplectites is commonly attributed to post metamorphic peak processes. Thus, they can be used to make implications about the retrograde exhumation history of high-grade metamorphic rocks. Here we present an example of aluminous paragneisses from the Loosdorf complex (southern Bohemian Massif, Austria) showing metamorphic peak conditions of ~1.0 GPa and ~800 °C with the peak assemblage garnet–biotite–sillimanite–plagioclase–K-feldspar–quartz–granitic melt ± ilmenite ± rutile. In their melanocratic layers, these rocks developed cordierite moats around garnet and cordierite + spinel ± corundum ± anorthite reaction textures at former garnet-sillimanite interfaces. Chemical potential relationships and phase diagram calculations using the local symplectite bulk composition are used to explain the process of texture formation and to estimate the corresponding P–T conditions. Our results indicate that the reaction textures formed in the course of a near-isothermal decompression path subsequent to metamorphic peak at conditions of ∼0.3 GPa and ∼750 °C. This relatively "hot" exhumation could be explained by the contemporaneous exhumation of large HP–UHT granulite bodies now underlying the Loosdorf complex. The timing of regional metamorphism affecting the granulites and the southern Bohemian Massif in general is well constrained and has its peak at ~340 Ma. However, monazite from Loosdorf paragneisses yielded a slightly younger age of ~335 Ma. Although the ages overlap within errors, they are interpreted to represent the age of the isothermal decompression and exhumation resulting in the formation of the observed reaction textures.