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 11-biotite-white mica-garnet schist; Zone 111-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 SiO, 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 210s cm3 cm-, in Zone IV. This large volume of, fluid may have originated, in part, from dehydration of flysch camed 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.

Quantification of mass transfers and mineralogical transformations in a thrust fault (Monte Perdido thrust unit, southern Pyrenees, Spain)

“…Demény et al, 1997;Dickin, 1988;Lonka et al, 1998;O'Hara, 1988), fault zones commonly show increasing contents of relatively immobile elements suggesting a volume loss that can be explained as the result of depletion of other more soluble elements during deformation, (2) volume gain during deformation: this case corresponds to a high activity of metamorphic fluids which change the rates of chemical and mechanical processes (e.g. Hippertt, 1998;Selverstone et al, 1991), (3) isochoric (constant volume) deformation: in this case, the activity of internal fluids may cause mineral reactions but no change in bulk volume nor chemistry (e.g. Steyrer and Sturm, 2002).…”

Sistema Sm-Nd em rocha-total aberto versus fechado: comportamento isotópico em zonas de alta deformação

“…Estas zonas podem se formar tanto em sistemas químicos e isotópicos fechados como abertos, envolvendo a entrada e mobilidade de grande quantidade de fluidos e causando mudanças nas características químicas das rochas deformadas durante os processos de milonitização. As questões relacionadas com a transferência de massa e mudanças químicas em zonas de cisalhamento procuram entender a perda ou ganho de volume dentro da zona de cisalhamento, onde fluidos podem ou não mudar a química total das rochas (O´Hara,1988;Selverstone, 1991;Hippert, 1988). Idade de eventos de metamorfismo pode ser determinada por meio de isócronas de minerais, que crescem ou são completamente recristalizados durante o metamorfismo (Gromet, 1991).…”

A stream sediment geochemical survey of the Ganga River headwaters in the Garhwal Himalaya

“…This would mean that REEs, U, and Th have been remobilized and concentrated into the fluid phase during the shearing and might have been emplaced as veins at a shallower depth along the fluid channels (thrust contacts). Zones of intense deformation and shearing in the MCTZ and PH become more porous and provide pathways for fluid channeling (Selverstone et al, 1991;Essaifi et al, 2004;Keller et al, 2004;and references therein) and their precipitation as ore minerals. Rengarajan and Sarin (2004).…”