Alpine metamorphism in the south‐east Tauern Window, Austria: 1. P–T variations in space and time
Authors: DROOP G. T. R. 
Published: May 2007

Abstract
The Pennine rocks exposed in the south‐east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid‐Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are:

(a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd: T= 490.50°C, P= 7° 1 kbar;

(b) Within 0.5 km of the staurolite+biotite isograd: T= 560±300C, P=7.1 kbar;

(c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+biotite isograd: T= 610±30°C, P=7.6±1.2 kbar;

(d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd: T= 630±40°C, P= 6.6±1.2 kbar.

The pressure estimates imply that the total thickness of overburden above the basement‐cover interface in the mid‐Tertiary was c. 26.4 km. This overburden can only be accounted for by the Austro‐Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro‐Alpine’nappe) was itself buried beneath an ‘Upper Austro‐Alpine’nappe or nappe‐pile which was 7.4 km thick at that time.

The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south‐east Tauern experienced blueschist‐facies conditions (T<450°C, P<12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist‐facies imprint post‐dated the main phase of tectonic thickening (D1A) and was thus a direct consequence of continental collision.

Combined oxygen‐isotope and fluid‐inclusion studies on late‐stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C.

Early Alpine, late Alpine and vein‐formation P–T constraints have been used to construct a P–T path for the base of the Mesozoic cover sequence in the south‐east Tauern Window. The prograde part of the P–T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dT overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early‐ to mid‐Tertiary (D2A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary.

The peak metamorphic P–T estimates are compared with the solutions of England's (1978) one‐dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro‐Alpine nappes in the absence of extraneous heat sources, such as large intrusions, at depth.