Preferential embrittlement of graphitic schists during extensional unroofing in the Alps: the effect of fluid composition on rheology in low-permeability rocks

Interlayered graphitic and non‐graphitic schists from the Tauern Window, Eastern Alps, record contrasting mechanical behaviour during extensional exhumation. Graphitic schists contain mesoscale extension fractures, pervasive microcracks in garnet, and abundant secondary fluid inclusion planes; all three types of structures are oriented perpendicular to the stretching lineation. Crack spacings in garnet from graphitic samples are tightly clustered around a mean of 180 μm. Non‐graphitic schists have fewer and more randomly oriented microcracks and fluid inclusion planes and maintained strain compatibility via crystal plasticity. The presence or absence of graphite appears to have exerted a fundamental control on rheology during unroofing. Calculations for a model graphitic rock at 500 °C and f_O2 = 10^?24 MPa show that the equilibrium metamorphic fluid evolves from X_CO2 = 0.07 to 0.38 during decompression from 700 to 400 MPa, in agreement with microcrack fluid inclusion data that show a change from X_CO2 < 0.1 to 0.45 in graphitic samples over the same pressure interval. This compositional shift results in >60% expansion of the pore fluid during decompression. H₂O‐rich fluid in non‐graphitic rocks expands <15% over the same pressure interval. The greater pore fluid expansion in low‐permeability graphitic horizons likely promoted tensile failure during unroofing. These results suggest that microcracking should be an inevitable consequence of decompression in many graphitic schists, whereas rocks that lack graphite are less likely to undergo microcracking. Microseismicity is predicted to be more common in graphitic than non‐graphitic rocks during unroofing of mountain belts.     

Metamorphic P–T Paths from pelitic schists and greenstones from the south-west Tauern Window, Eastern Alps

Abstract Petrological data from intercalated pelitic schists and greenstones are used to construct a pressure–temperature path followed by the Upper Schieferhülle (USH) series during progressive metamorphism and uplift in the south-west Tauern Window, Italy. Pseudomorphs of Al–epidote + Fe-epidote + albite + oligoclase + chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite-albite-chlorite subfacies of the blueschist facies at P ± 7–8 kbar. Geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475±25°C, 5–6 kbar; calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of ∼ 7.5 kbar. Garnet zoning patterns are complex and reversals in zoning can be correlated between samples. Thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity. Fluid inclusion data from coexisting immiscible H₂O–CO₂–NaCl fluids constrain the uplift path to have passed through temperatures of 380 + 30°C at 1.3 + 0.2 kbar.
There is no evidence for metamorphism of USH at pressures greater than ∼ 7.5 kbar in this area of the Tauern Window. This is in contrast to pressures of ± 10 kbar recorded in the Lower Schieferhülle only 2–3 km across strike. A history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the P–T data obtained from these adjacent tectonic units.     

Origin and mechanical significance of honeycomb garnet in high-pressure metasedimentary rocks from the Tauern Window, Eastern Alps

High-pressure schists (2–2.5 GPa) from the Eclogite Zone in the Tauern Window contain honeycomb garnet in which fine webs of garnet surround strain-free quartz ± carbonate grains. High-resolution X-ray computed tomography shows that the garnet webs form a cellular structure that coats all surfaces of the inclusions. Electron backscatter diffraction analysis shows that the garnet cells are crystallographically continuous with more massive garnet regions, and that the quartz ± carbonate inclusions have random orientations; in contrast, matrix quartz exhibits a prominent crystallographic preferred orientation (CPO). High-resolution transmission electron microscopy shows few dislocations in either the garnet or the inclusion quartz. Most honeycomb garnet is chemically homogeneous, but some displays asymmetric core–rim zoning. Taken together, these observations are most consistent with formation of the garnet sheets via precipitation from a wetting fluid along quartz–quartz grain boundaries, or possibly via wholesale precipitation of garnet + quartz ± carbonate from a fluid. In either case, a silicate-rich aqueous fluid must have been present. The likelihood that a fully wetting fluid existed at high pressure has important implications for rheology during subduction of metasedimentary rocks: strain may be accommodated by grain rotation and sliding in an aqueous silicate slurry, rather than via dislocation creep mechanisms at high pressures. The absence of a CPO in early quartz may thus point to involvement of a pervasive grain-boundary fluid rather than requiring low differential stresses during subduction.     

High-Pressure Metamorphism in the SW Tauern Window, Austria: P-T Paths from Hornblende-Kyanite-Staurolite Schists

The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O-CO₂.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.     

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.     

Stable isotopic and trace element evidence for restricted fluid migration in 2 GPa eclogites

Mineral stable isotopic and trace element studies in 2 GPa banded eclogites of the Tauern Window, eastern Alps, record mm- to cm-scale heterogeneities that reflect compositional variations in the accompanying metamorphic fluids. A close correlation between dolomite mode and dolomite δ¹⁸O is consistent with equilibrium partitioning among coexisting minerals and fluids. Small variations in dolomite δ¹³C values correspond with δ¹⁸O variations, but an overall decrease in dolomite δ¹³C by c. 1%o across a 12-cm sample is a relict feature that pre-dates eclogite equilibration. Garnet, omphacite, and clinozoisite rims show little systematic mineral-mineral partitioning behaviour for Ti, V, Cr, Y, Sr, or Zr; major elements, however, are well equilibrated among these same minerals. Despite the apparent lack of mineral-mineral trace element equilibration, most of the trace elements vary systematically with water activity calculated in each layer. Trace element behaviour during the eclogite metamorphism thus appears to have been controlled largely by mineral-fluid interactions along grain boundaries. Shallow structural levels in other subduction complexes (c. 10-45 km) typically exhibit fracture-controlled permeability and extensive metasomatism, but there is no field or geochemical evidence for extensive fluid advection during high-pressure metamorphism in the Tauern eclogites. Because most dewatering and devolatilization during tectonic burial occurs prior to eclogite conditions, the volumetric fluid/rock ratio in eclogites should generally be low. Low fluid/rock ratios, coupled with the possible non-wetting nature of the fluids, permits the production and preservation of fine-scale chemical heterogeneities in deeply subducted eclogites and associated fluids. However, the eventual breakdown at greater depth of volatile-bearing dolomite, phengite, clinozoisite, zoisite, or amphibole could lead to renewed fracture-controlled fluid release from the subducted rocks to regions appropriate for arc magma generation.