Fluid channelling during ductile shearing: transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps

Abstract Ductile shearing in the core of the Tauern Window, Austria, transformed metagranodiorite into Si-undersaturated garnet-chlorite-staurolite schist at a depth of c. 35–40 km during the Alpine orogeny. Four distinct zones have been recognized extending from the wallrock into the centre of the shear zone: Zone I—unaltered metagranodiorite with subordinate amphibolite; Zone II—biotite-white mica-garnet schist; Zone III—biotite-phengite schist; Zone IV—quartz-absent, garnet-chlorite-staurolite schist with garnets up to 10 cm across. Whole-rock analyses show a dramatic decrease in SiO₂ from >65 wt% in Zone I to <35 wt% in Zone IV; Ca, Na, and Sr also decrease across the shear zone, whereas Al, Ti, Fe, Mg, P, Cr, Ni, Zn, and Rb all increase towards Zone IV. Mass-balance calculations indicate that shearing was accompanied by up to 60% volume loss near the centre of the shear zone. Comparison of the Tauern Window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in Mg, Fe, and Ti is typical for volume-loss shear zones, but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour. In the samples studied here, volume loss appears to have resulted from channellized fluid flow during shearing, producing time-integrated fluid fluxes of ± 10⁸ cm³ cm⁻² in Zone IV. This large volume of fluid may have originated, in part, from dehydration of flysch carried beneath the metagranodiorites during Eocene movement on the North Penninic subduction zone. Development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating.