Tourmaline’s record of fluid history in the Pfitscher Joch, Western Tauern Window

English Abstract:

The Pfitscher Joch is a petrologically well-investigated area of the Western Tauern Window in the Eastern Alps. Its basement comprises pre-Variscan amphibolite, serpentinite, and garnet-graphite schist and gneiss intruded by two late Variscan granitoids (Tux and Zillertal Zentralgneis). In the late-Permian to Mesozoic, a sequence of sediments were deposited, starting with conglomerates, fining upward to psammites-pelites, including carbonate-rich schists, possibly interrupted by a horizon of rhyolite. Together, they form part of the Subpenninic nappes, the former European distal margin. In the Alpine orogeny, this succession was folded between the Tux and the Zillertal granitoids, forming a WSW-ENE oriented syncline. During Alpine metamorphism, fluids were released and probably channelled in the Greiner Shear Zone, which runs subparallel to the folded sequence, leading to complex fluid flow in the whole Pfitscher Joch region. Whereas the geodynamics of the area have been well constrained, the fluid flow history is not yet understood.

A striking mineralogical feature in many lithologies of the Pfitsch formation is the presence of black, prismatic tourmaline (schorl-dravite solid solution). It is particularly pervasive in a ~ 25 m thick unit of tourmaline-feldspar-rich gneiss, which is associated with feldspar-dominated segregations. The largest and most abundant tourmalines are in the segregations, suggesting a link between tourmaline crystallization and their formation. These crystals record three growth stages, distinguished by major element chemistry and inclusion mineralogy. Outside the segregations, multi-stage compositional growth zoning is rare and individual grains match usually only one growth stage of the segregation-hosted tourmaline. Comparison of data from different horizons of the Pfitsch formation shows a bulk-rock compositional control on the Fe content of the hosted tourmaline. Conversely, the Ca/Na of the tourmaline is independent of the host rock, and is interpreted as a fluid signature.

The Pfitscher Joch is an example locality of tourmaline serving as a powerful petrogenetic indicator mineral. A combination of textural, isotopic, and major and trace element analysis of tourmaline has allowed part of the Pfitscher Joch’s fluid history to be revealed.