Short Note

Geologische Rundschau

, Volume 83, Issue 3, pp 660-664

First online:

Conodont colour alteration indices: Palaeotemperatures and metamorphism in the Northern Calcareous Alps — a general view

  • H. -J. GawlickAffiliated withInstitut für Geowissenschaften, Montanuniversität Leoben
  • , L. KrystynAffiliated withInstitut für Paläontologie, Universität Wien
  • , R. LeinAffiliated withInstitut für Geologie, Universität Wien

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The middle and eastern parts of the Northern Calcareous Alps (NCA) can be subdivided into two distinct units with a lateral boundary marked by abrupt changes in the conodont colour alteration index (CAI-values). The first of these is a northern unit (Tirolikum) with a relatively homogeneous distribution of no or low grade conodont alteration (CAI 1.0–2.0). The thermal overprint is thought to be relatively young and related to a heat flow from the Tauern crystallization. The second unit consists of the Juvavic nappe system (Juvavikum), which is distributed along the southern rim of the NCA but also covers some of the northern parts of the Tirolikum. With respect to its CAI-distribution the Juvavikum is more heterogeneous on a regional and local scale, with some local CAI-inversions. The Juvavikum additionally shows distinctly different sets of CAI-values one with weak (CAI 1.0–1.5) and another with strong alteration (CAI 5.5–7.0) — at present the highest known thermal overprint measured in the NCA. The metamorphism is relatively old as it predates the Late Jurassic—Early Cretaceous gravity tectonic emplacement of the Juvavikum onto the Tirolikum. The high CAI-values of parts of the Juvavic nappe system are though to be related to tectonic burial in an accretionary wedge formed parallel to the closure of the ‘Vardar’ Ocean. The low CAI values of the Tirolikum apparently exclude a direct juxtaposition of the two units at the time of metamorphism.

Key words

Northern Calcareous Alps Conodont colour alteration index Metamorphism