Constraints on the duration of high‐pressure metamorphism in the Tauern Window from diffusion modelling of discontinuous growth zones in eclogite garnet

Abstract

An eclogite sample from the Grossgockner region of the Hohe Tauern, Austria contains garnet with a pronounced compositional discontinuity between a Mn‐rich core and an Fe‐rich rim. This jump in composition was caused by a garnet‐consuming reaction followed by growth of the garnet rim + omphacite and marks the prograde transition from epidote–amphibolite to eclogite facies metamorphism. Garnet growth ended at peak metamorphic conditions of 570 °C, 17 kbar, but intracrystalline diffusion continued until about 450 °C, 4 kbar on the retrograde path. This garnet overgrowth texture represents a natural diffusion couple and a time span of 1 Myr was calculated from the diffusion profile developing out of the original sharp compositional step. For typical crustal densities, this time corresponds to a minimum average velocity in the range 4.6–7.4 cm yr−1 (for vertical movement), which is one of the fastest exhumation rates reported. The diffusion of all divalent cations of four profiles was modelled, both analytically and numerically. Both approaches gave comparable results, but the times computed for each element were always discrepant up to a factor of 2. Variations of diffusion coefficients within 2 in analytical calculations remedied this and gave consistent upper time limits. Numerical modelling does not require the simplifications introduced in the analytical approach. On the other hand, error propagation was computationally unfeasible with this method.

Using white mica40Ar/39Ar data as a tracer for fluid flow and permeability under high‐Pconditions: Tauern Window, Eastern Alps

“…Dachs & Proyer, 2001), but in general these nappes preserve evidence for peak blueschist facies conditions at 1.0–1.4 GPa and 500 °C (Holland & Ray, 1985; Proyer et al. , 1999; Dachs & Proyer, 2001, 2002; Proyer, 2003), Fig. 5.…”

“…(1994); (5) Holland & Ray (1985); Proyer et al. (1999); Dachs & Proyer (2001, 2002); Proyer (2003); (6) Selverstone et al. (1984); Cliff et al.…”

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

“…, 1999). Kyanite eclogites in the EZ yield pressures of 1.7–2.3 GPa and 570–600 °C (Holland, 1979; Hoschek, 2001; Dachs & Proyer, 2002) based on mineral assemblage and calculations in the NCFMASH system. Analyses of the associated metasedimentary rocks yield similar pressure and temperature conditions (Franz & Spear, 1983; Spear & Franz, 1986; Dachs, 1990; Hoschek, 2001), but conditions as high as 2.5 GPa at 595 °C have also been reported (Stöckhert et al.…”

“…Rocks of the EZ record several stages in their metamorphic history: Eoalpine blueschist facies metamorphism associated with rapid subduction; highpressure eclogite facies metamorphism; a younger blueschist facies overprint; and finally overprinting by greenschist/amphibolite facies assemblages (ÔTauern eventÕ) during exhumation (Hoinkes et al, 1999;Kurz et al, 1999). Kyanite eclogites in the EZ yield pressures of 1.7-2.3 GPa and 570-600°C (Holland, 1979;Hoschek, 2001;Dachs & Proyer, 2002) based on mineral assemblage and calculations in the NCFMASH system. Analyses of the associated metasedimentary rocks yield similar pressure and temperature conditions (Franz & Spear, 1983;Spear & Franz, 1986;Dachs, 1990;Hoschek, 2001), but conditions as high as 2.5 GPa at 595°C have also been reported (Sto¨ckhert et al, 1997).…”

On the significance of short-duration regional metamorphism

“…As tools to interrogate durations of tectonism and associated regional metamorphism have improved, it has been repeatedly shown that time scales of thermal events recorded by regional metamorphism are too short to allow the crustal-scale heat conduction required by thermal relaxation (e.g. Oliver et al 2000;Baxter et al 2002;Dachs & Proyer 2002;Faryard & Chakraborty 2005;Ague & Baxter 2007;Smye et al 2011;Viete et al 2011aViete et al ,b, 2013Spear et al 2012;Spear 2014). Moreover, examples of temporal non-uniformity in rates of metamorphism have been related to extremely short-duration thermal instabilities (tectonometamorphic episodicity) within collisional orogens and subduction zones (e.g.…”