Abstract. The chemical composition of metasediments is a valuable
source of paleogeographic information about the protolith's sedimentary
environment. Here, we compile major- and trace-element whole-rock data,
including B contents, and 10/11B-isotope ratios from the Permo-Triassic
metasedimentary cover of the Pfitsch–Mörchner basin, overlying the
Variscan basement in the western Tauern Window, Eastern Alps
(Austria and Italy). The basement consists of orthogneiss (“Zentralgneis”,
metamorphosed Variscan granitoids with intrusion ages between 305 and 280 Ma), and the roof pendant consists of granites (amphibolites, paragneiss, and
minor serpentinites). The Zentralgneis is partly hydrothermally altered into
pyrite quartzite with high Al–S contents, low Na–Sr–Ca–Mg contents, and very strong
depletion of the light rare earth elements. Comparison with published
detailed mapping of this and other time-equivalent basins in the western
Tauern Window, with radiometric age data in the literature, and with
unmetamorphosed basins in the South Alpine realm yields a late Permian to
Early Triassic age of sedimentation. Although during Alpine metamorphism all rocks were strongly deformed, the
whole-rock chemical compositions of the metasediments were not pervasively
changed during deformation. We show that the sediments were deposited in a
small, probably lacustrine–fluviatile, intramontane basin, under arid to
semi-arid climatic conditions. The sequence starts with metaconglomerates,
which can be interpreted as a mixture of the different basement rocks, based
on a combination of major-element ratios Na2O / (Na2O + K2O)
and MgO / (MgO + Fe2O3) with concentrations of trace elements Cr, V,
and Ni. The sequence is overlain by a fining-upwards sequence of clastic
sediments, in which the behavior of K, Rb, and Sr allows the reconstruction
of intense diagenetic K–B metasomatism, which raised the K2O contents
up to ∼ 10 wt %. The average B content of 218 µg g−1 is
well above the B content of common sediments, and the B-isotope composition
reaches extremely low values of down to −33 ‰ δ11B. The top of the sequence is a lazulite quartzite, interpreted as a
former conglomeratic phosphatic sandstone, which marks the transition from a
closed Permian basin to an open Triassic basin. Within the clastic sequence,
the presence of hydrothermal tourmalinite veins documents a hydrothermal
event after deposition but before the onset of Alpine metamorphism. A
metamorphosed mafic dike swarm in the orthogneiss indicates a post-Variscan
event of basaltic magmatism, and this event is tentatively correlated with
increased heat flow in the Triassic basin and hydrothermal activity. A
consistent conceptual model of this basin and its diagenetic modifications,
based on a combination of geochemical data with petrographical and field
information, provides the geodynamic context of the European margin at the
onset of the Alpine orogeny.
Alpine metamorphism in the central segment of the Western Greywacke Zone (Eastern Alps)