Geochemistry and paleogeographic implications of Permo-Triassic metasedimentary cover from the Tauern Window (Eastern Alps)

Abstract

Abstract. The chemical composition of metasediments is a valuable\nsource of paleogeographic information about the protolith s sedimentary\nenvironment. Here, we compile major- and trace-element whole-rock data,\nincluding B contents, and 10/11B-isotope ratios from the Permo-Triassic\nmetasedimentary cover of the Pfitsch–Mörchner basin, overlying the\nVariscan basement in the western Tauern Window, Eastern Alps\n(Austria and Italy). The basement consists of orthogneiss (“Zentralgneis”,\nmetamorphosed Variscan granitoids with intrusion ages between 305 and 280\u2009Ma), and the roof pendant consists of granites (amphibolites, paragneiss, and\nminor serpentinites). The Zentralgneis is partly hydrothermally altered into\npyrite quartzite with high Al–S contents, low Na–Sr–Ca–Mg contents, and very strong\ndepletion of the light rare earth elements. Comparison with published\ndetailed mapping of this and other time-equivalent basins in the western\nTauern Window, with radiometric age data in the literature, and with\nunmetamorphosed basins in the South Alpine realm yields a late Permian to\nEarly Triassic age of sedimentation. Although during Alpine metamorphism all rocks were strongly deformed, the\nwhole-rock chemical compositions of the metasediments were not pervasively\nchanged during deformation. We show that the sediments were deposited in a\nsmall, probably lacustrine–fluviatile, intramontane basin, under arid to\nsemi-arid climatic conditions. The sequence starts with metaconglomerates,\nwhich can be interpreted as a mixture of the different basement rocks, based\non a combination of major-element ratios Na2O\u2009/\u2009(Na2O\u2009+\u2009K2O)\nand MgO\u2009/\u2009(MgO\u2009+\u2009Fe2O3) with concentrations of trace elements Cr, V,\nand Ni. The sequence is overlain by a fining-upwards sequence of clastic\nsediments, in which the behavior of K, Rb, and Sr allows the reconstruction\nof intense diagenetic K–B\xa0metasomatism, which raised the K2O\xa0contents\nup to ∼\u200910\u2009wt\u2009%. The average B content of 218\u2009µg\u2009g−1 is\nwell above the B content of common sediments, and the B-isotope composition\nreaches extremely low values of down to −33\u2009‰\u2009δ11B. The top of the sequence is a lazulite quartzite, interpreted as a\nformer conglomeratic phosphatic sandstone, which marks the transition from a\nclosed Permian basin to an open Triassic basin. Within the clastic sequence,\nthe presence of hydrothermal tourmalinite veins documents a hydrothermal\nevent after deposition but before the onset of Alpine metamorphism. A\nmetamorphosed mafic dike swarm in the orthogneiss indicates a post-Variscan\nevent of basaltic magmatism, and this event is tentatively correlated with\nincreased heat flow in the Triassic basin and hydrothermal activity. A\nconsistent conceptual model of this basin and its diagenetic modifications,\nbased on a combination of geochemical data with petrographical and field\ninformation, provides the geodynamic context of the European margin at the\nonset of the Alpine orogeny.\n