Low-pressure, high-temperature metamorphism in a compressional tectonic setting: Mary Kathleen Fold Belt, northeastern Australia

Abstract The Mary Kathleen Fold Belt in northeastern Australia consists of highly deformed, Mid-Proterozoic sedimentary and volcanic sequences as well as intrusives, which were metamorphosed under low-pressure, high-temperature conditions. In the light of current controversy on tectono-thermal settings of low-pressure metamorphic terrains, the interrelations of progressive deformation and metamorphism have been closely examined. Remarkably, there is no direct evidence for syn-metamorphic extensional deformation nor is any significant intrusive activity recorded. Syn-metamorphic structures indicate lateral, bulk coaxial shortening of at least 50–60%. Tight upright folds, pervasive axial planar fabrics, undulating fold axes, and a vertical mineral lineation characterize this deformation. The metamorphic textures, particularly those in andalusite- and/or cordierite-bearing schists, reveal the sequential growth of metamorphic minerals that was synchronous with progressively increasing bulk rock strain. The corresponding metamorphic reactions constrain a prograde P–T path segment that crossed the andalusite and sillimanite stability fields while temperature and pressure increased. After reaching the metamorphic peak, the region cooled down near-isobarically, before major decompression occurred. The prograde–retrograde P–T path forms a complete anticlockwise loop. Due to the lack of evidence for crustal thinning and large-scale magmatism in the upper crust, alternative models are discussed in order to explain the transient high geothermal gradient. These are in particular convective thinning of the lithospheric mantle and fast decompression of crustal sections, possibly linked to tectonic processes preceeding the low-pressure/high-temperature orogenic event.