Alpine metamorphism in the south‐east Tauern Window, Austria: 1. P–T variations in space and time

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 thestaurolite+ biotiteisograd: T = 490 2 50°C. P = 7 2 1 kbar;(b) Within 0.5 km of the staurolite+biotite isograd: T = 560 2 30°C. P = 7 2 1 kbar;(c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+ biotite isograd: 7=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 = 6 3 0 t 4 O 0 C , P = 6 . 6 2 1.2 kbar. The pressure estimates imply that the total thickness of overburden above the basementcover 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 2 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 < 1 2 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 (Dh) 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 2 20°C.Early Alpine, late Alpine and vein-formation P-T constraints have been used to construct a f-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 d P l d T 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 (D;) 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 ...

40Ar/39Ar muscovite ages from the Penninic‐Austroalpine plate boundary, Eastern Alps

“…The peak conditions of this Barrovian-type metamorphism are 7-9 kbar and 490ø-560øC SW of the study area [Kruhl, 1993] and 6.6-7.6 kbar and 490ø-630øC in the SE Tauern Window [Droop, 1985] bSamples are single grains.…”

Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland (Published in Scottish Journal of Geology, 35(1), 1999, pp. 31–50)

“…As regards kinematic indicators in the Loch Maree area, we found none that could be unequivocally ascribed to the annealed prograde fabric ('D^ of Droop et al 1999;'Di' of Fernandes 1987). All the kinematic indicators we observed were associated with the intense mylonitic fabric ('D M ' of Droop et al 1999;'late D^ of Fernandes 1987) and all that could be interpreted with confidence indicate top-to-NW movement.…”

“…All the kinematic indicators we observed were associated with the intense mylonitic fabric ('D M ' of Droop et al 1999;'late D^ of Fernandes 1987) and all that could be interpreted with confidence indicate top-to-NW movement. Whether these reflect compression or extension depends on a series of factors such as the original attitude of the mylonitic foliation.…”

“…to heat up, and only if exhumation was sufficiently slow for there to have been a period when structurally deep rocks continued to heat up while high-level rocks cooled, leading to a PT array with a shallow dP/dT slope (England and Richardson 1977). 'Flat' PT arrays have been documented in orogenic belts formed by continental collision, such as the Eastern Alps, in which the amphibolite-facies metamorphism significantly post-dated, and was probably a consequence of, tectonic thickening (Droop 1985). Again, our data seem to point to an extended period of heating of the LMG within a 'mature' collisional orogen.…”

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write: Droop et al (1999) make a welcome addition to the Lewisian literature in providing the first rigorous published estimates of FT conditions at the Laxfordian metamorphic peak in the Loch Maree Group (530-630°C, 6.5 kbar, using various methods including calcite-dolomite and garnet-biotite geothermometers and garnet-plagioclase geobarometer). In their discussion of the implications of the relatively high pressure-temperature estimates, they note an apparent contradiction with the tectonic model for the early Laxfordian deformation presented by Coward and Park (1987) and Park et al (1987).

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