International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

SILURIAN–DEVONIAN 40AR/39AR MINERAL AGES FROM THE KAINTALECK NAPPE: EVIDENCE FOR MID-PALEOZOIC TECTONOTHERMAL ACTIVITY IN UPPER AUSTROALPINE BASEMENT UNITS OF THE EASTERN ALPS (AUSTRIA)

Pages: 229 - 239

Authors: ROBERT HANDLER, FRANZ NEUBAUER, SIEGFRIED HERMANN, DAVID DALLMEYER

Abstract: 40Ar/39Ar mineral dating has been carried out within the amphibolite-facies metamorphic basement and greenschist-facies metamorphic cover of various tectonic units within the Kaintaleck nappe, Eastern Alps, Austria, to evaluate the age of pre-Alpine metamorphism. Hornblendes display discordant 40Ar/39Ar apparent age spectra, minimum ages recorded in medium- to high-temperature gas release steps are ca. 430–405 Ma. White mica from micaschist record discordant 40Ar/39Ar age spectra with ages of ca. 350–379 Ma in medium- and high-temperature increments. White mica from discordant aplite and pegmatite record 40Ar/39Ar plateau ages of 375.4 ± 0.4 Ma and 364.0 ± 0.8 Ma respectively. The new isotopic ages indicate that (1) mid-Paleozoic (e.g. 430–380 Ma) tectonometamorphic activity is recorded within the basement of the Kaintaleck Nappe; (2) this basement cannot represent the metamorphic basement for Ordovician to Late-Cretaceous sedimentary sequences of the Noric-Tirolic nappe complex (within uppermost units of the Austroalpine nappe complex); (3) the tectonometamorphic evolution of this basement unit contrasts with that of other basement units exposed in the Eastern Alps, where predominantly "late Variscan" (e.g. 330–300 Ma) tectonometamorphic events are recorded in Silurian to Early Carboniferous passive continental margin sequences; and therefore (4) at least two contrasting terranes comprise the Austroalpine basement.

Keywords: Eastern Alps, Austroalpine, basement, mid-Paleozoic, terranes, 40Ar/39Ar, geochronology

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