Tracing the exhumation of the Eclogite Zone (Tauern Window, Eastern Alps) by 40Ar/39Ar dating of white mica in eclogites

Abstract

New radiometric ages from the subpenninic nappes (Eclogite Zone and rote Wand -Modereck Nappe, tauern Window) show that phengites formed under eclogite-facies metamorphic conditions retain their initial isotopic signature, even when associated lithologies were overprinted by greenschist-to amphibolite-facies metamorphism. Different stages of the eclogite-facies evolution can be dated provided 40 Ar/ 39 Ar dating is combined with micro-structural analyses. An age of 39 Ma from the rote Wand -Modereck Nappe is interpreted to be close to the burial age of this unit. Eclogite deformation within the Eclogite Zone started at the pressure peak along distinct shear zones, and prevailed along the exhumation path. An age of ca. 38 Ma is only observed for eclogites not affected by subsequent deformation and is interpreted as maximum age due to the possible influence of homogenously distributed excess argon. During exhumation deformation was localised along distinct mylonitic shear zones. this stage is mainly characterised by the formation of dynamically recrystallized omphacite2 and phengite. Deformation resulted in the resetting of the Ar isotopic system within the recrystallized white mica. Flat argon release spectra showing ages of 32 Ma within mylonites record the timing of cooling along the exhumation path, and the emplacement onto the Venediger Nappe. Ar-release patterns and 36 Ar/ 40 Ar vs. 39 Ar/ 40 Ar isotope correlation analyses indicate no significant 40 Ar-loss after initial closure, and only a negligible incorporation of excess argon. From the pressure peak onwards, eclogitic conditions prevailed for almost 8-10 Ma.tracing the exhumation of the Eclogite Zone (tauern Window, Eastern Alps) by 40 Ar/ 39

U–Pb ages of apatite in the western Tauern Window (Eastern Alps): Tracing the onset of collision-related exhumation in the European plate

“…Some authors argue for Eocene timing ∼42-36 Ma (e.g., Zimmermann et al, 1994;Ratschbacher et al, 2004;Kurz et al, 2008;Smye et al, 2011, Fig. 1b) and others for Oligocene metamorphism (Glodny et al, 2005;Nagel et al, 2013).…”

HP–UHP metamorphism as an indicator of slab dip variations in the Alpine arc

“…Note that this 3-D slab geometry inferred for the western and central Alps does not continue toward the east because HP-LT metamorphism of the distal European margin (e.g., Eclogite Zone of the Tauern Window) occurred during Eocene time (45-39 Ma; Ratschbacher et al 2004;Kurz et al 2008). We infer a complex paleogeographic pattern of the European margin (Berger and Bousquet 2008;Handy et al 2010) including the possibility that the European slab was segmented somewhere between locations corresponding to the Adula nappe in the central Alps and the Eclogite Zone of the Tauern Window.…”

“…2b). Two strength gradients develop within the subducted lithosphere: (1) along the slab, the strength decreases with increasing burial depth, and (2) perpendicular to the slab, the strength increases with depth due to the (2007), Ratschbacher et al (2004), Kurz et al (2008) 23-34 45-50…”

Polyphase movement on the Lavanttal Fault Zone (Eastern Alps): reconciling the evidence from different geochronological indicators

“…Closure temperatures depend on grain-size, chemical composition and cooling rate. Moreover, several studies (e.g., Chopin and Maluski 1980;von Blanckenburg and Villa 1988;von Blanckenburg et al 1989;Dunlap 1997;Bossé et al 2005) have revealed that temperature is not the only controlling factor for setting or re-setting the isotopic system within minerals (see also Villa 1998, for a general discussion) and deformation can be of equal importance (e.g., Chopin and Maluski 1980;Müller et al 1999;Kurz et al 2008).…”