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Prograde–retrograde P–T–t –deformation path of Austroalpine micaschists during Variscan continental collision (Eastern Alps)
Author(s) -
SCHULZ B.
Publication year - 1990
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1990.tb00492.x
Subject(s) - geothermobarometry , geology , shearing (physics) , geochemistry , continental collision , tectonics , petrology , metamorphic rock , subduction , paleontology , geotechnical engineering
A complete prograde P–T path, defined by 10 calculated P–T fields in succession, is recognized from metapelites by using geothermobarometry on garnet‐bearing assemblages with microstructural control. Overstacking of several tectonic units during an early Variscan continental collision explains the complex prograde P–T history. Isostatic uplift and deformation controlled the retrograde P–T path. Deformation with changing character acted continuously during all stages of the evolution of the Austroalpine basement complex. After the intrusion of Caledonian granitoids, metapelites and magmatic rocks suffered a shearing deformation D 1 –D 2 , which produced sheath folds as well as the main foliation S 2 . Spessartine‐rich first‐generation garnets, situated in microlithons enclosed by S 2 , record the onset of shearing under increasing high‐pressure–low‐temperature conditions (7 kbar/380°C). Geothermobarometry on second‐generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar/500°C and subsequent recompression/heating during continuing shearing. This is explained by overthrusting of a tectonic unit (unit 2) from NE to SW upon the micaschist unit (unit 1), followed by isostatic uplift and further overstocking of a third unit (unit 3). The resulting P max of 12 kbar at 650°C and further increasing temperatures up to 680°C accompanied by decompression have been calculated using a third generation of garnets. These high‐pressure–high‐temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions. Staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde P–T evolution. Shortening deformation D 3 and simultaneous growth of typical amphibolite facies minerals (staurolite 2, kyanite 2, sillimanite, andalusite) occurred during the retrograde path. A final step of Variscan evolution was marked by an oppositely directed shearing D 4 (at T > 300°C and P > 3 kbar), possibly indicating backthrusting or extension. Apart from acid intrusions, no signs of a previous Caledonian thermotectonic history were found in the area to the south of the Defereggen–Antholz–Vals Line.