Evolution of H₂O content in a polymetamorphic terrane: the Plattengneiss Shear Zone (Koralpe, Austria)

Abstract

The Koralpe of the Eastern European Alps experienced high-temperature/low-pressure metamorphism (650°C and 6.5 kbar) during the Permian and eclogite facies metamorphism (700°C and 14 kbar) during the Eo-Alpine (Cretaceous) metamorphic event. In the metapelitic Plattengneiss shear zone that constitutes much of the Koralpe, the second metamorphism caused only partial re-equilibration of the assemblages formed during the first metamorphism. It is shown here that the Eo-Alpine mineral assemblage, garnet + biotite + muscovite + plagioclase + quartz (with or without kyanite), formed under low water activity conditions that are consistent with the level of dehydration that occurred during the Permian event. This implies that the rocks were essentially closed-system from the peak of the Permian metamorphism through the Eo-Alpine event. The evolution of water content of the rocks is traced through time: that prograde dewatering during the Permian metamorphic event terminated at the metamorphic peak with a water content around 3-4 mol.%. The water content remained then constant and led to water-undersaturation during the subsequent Eo-Alpine metamorphism. From the water content and activity evolution a post-peak isothermal decompression path close to the solidus is inferred for the Eo-Alpine event.

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

“…In this regard the KMZ differs from most other exhumed and active continental retrograde shear zones for which high fluid/rock ratios have been suggested (Kerrich et al, 1977;Etheridge et al, 1983; or for which high fluid pressures are inferred (Rice, 1992;Ozacar and Zandt, 2009). It consequently appears that retrograde shearing can be sustained under a variety of fluid regimes, from dry and entirely fluid absent (Tenczer et al, 2006), through low-volume fluid presence such as described here, to examples where shear zones are inundated and dominated by fluid. There is also a range in cases from where high fluid pressures are sustained by a combination of high fluid volumes and low permeability, to shear zones where either low fluid volumes or high permeability prevent the build-up of high fluid pressures.…”

“…In the absence of rehydration, the rocks would have retained their (low) fluid content and granulite mineralogy from peak metamorphic conditions and experienced shearing and reworking under fluid-absent conditions (Guiraud et al, 2001;White and Powell, 2002;Tenczer et al, 2006;Diener et al, 2008). If this were the case, the samples would have consisted of orthopyroxene-bearing assemblages and would have grown garnet at the P -T conditions of shearing (450-480 • C and M H 2 O < 3 − 5 mol %; Fig.…”

Thermobarometric constraints on pressure variations across the Plattengneiss shear zone of the Eastern Alps: implications for exhumation models during Eoalpine subduction

“…The multivariant peak assemblage fields of samples T18 and T29 overlap with the 1 σ error ellipses of calculated av PT and the av P for the same samples consistent with peak metamorphic conditions of ~700 °C and 12–13 kbar, consistent with previous estimates (e.g. Stüwe, ; Stüwe & Powell, ; Gregurek et al ., ; Tenczer & Stüwe, ; Tenczer et al ., ).…”

Conflicting P–T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?

“…Numerous workers have demonstrated that is it an extremely powerful and useful method for providing detailed P-T-X information for a variety of metamorphic environments (e.g. Powell et al, 1998;Vance and Mahar, 1998;White et al, 2001;Cononolly and Petrini, 2002;White et al, 2003;Zeh et al, 2004;Harris et al, 2004;White et al, 2004;Evans, 2004;Johnson and Brown, 2004;Pelletier et al, 2005;White et al, 2005;Kelsey et al, 2005;Stipska and Powell, 2005;Zeh et al 2005a,b;Konrad-Schmolke et al, 2006;Tenczer et al, 2006;Dziggel et al, 2006;White et al, 2007;Diener et al, 2007;Powell and Holland, 2008;White et al, 2008;Millonig et al, 2008;Diener et al, 2008;Kelsey et al, 2008;Mahan et al, 2008). A recent paper by Powell and Holland (2008) discusses at the length the advantages and disadvantages of a pseudosection approach in comparison to a conventional thermobarometric approach.…”