Rb-sr microchrons of synkinematic mica in mylonites

Earth and Planetary Science Letters 180 (2000) 385^397 www.elsevier.com/locate/epsl

Rb^Sr microchrons of synkinematic mica in mylonites: an example from the DAV fault of the Eastern Alps
Wolfgang Muller *, Neil S. Mancktelow, Martin Meier «
Department Erdwissenschaften, ETH Zurich, CH-8092 Zurich, Switzerland « « Received 4 December 1999; received in revised form 16 May 2000; accepted 22May 2000

Abstract A technique for texturally controlled microsampling from rock thick sections and subsequent Rb^Sr dating of Wgsized samples has been developed. It allows direct dating of minerals unequivocally related to deformation. The technique has been applied to strictly synkinematically grown white mica prepared from mylonites related to the Defereggen^Antholz^Vals (DAV) fault south ofthe Tauern window (Eastern Alps). Control of deformation ages is provided by field relationships with independently dated plutons and dykes dated here by single mineral Rb^Sr. The field observations and isotopic ages are fully consistent and demonstrate that this is a reliable method for sub-mm-scale geochronology. The ages obtained establish that sinistral strike slip shearing associated with theDAV fault lasted from at least 33 Ma to 30 Ma, followed by an abrupt change to dextral transpression related to the adjacent Periadriatic fault. ß 2000 Elsevier Science B.V. All rights reserved.
Keywords: Rb/Sr; mylonites; dates; deformation; shear zones; Eastern Alps

1. Introduction Deformation within the Earth's crust is often localized within shear zones characterized by speci¢cassemblages of fault rocks. These allow the pressure^temperature conditions of faulting, the relative sense of displacement and (occasionally) the amount of accumulated strain to be deduced. Establishing the temporal evolution of such shear zones, which is critical for the quanti¢cation of orogenic episodes, is still a major challenge for geochronology. The main di¤culty is that defor-

* Correspondingauthor. Tel.: +41-1-6324644; Fax: +41-1-6321179; E-mail: wolfgang.mueller@erdw.ethz.ch

mation under greenschist facies conditions never fully recrystallizes a pre-existing mineral paragenesis, resulting in a ¢nal mixture of both newly formed minerals and old porphyroclasts. Microsampling techniques such as UV laser ablation 40 Ar/39 Ar [1] or Rb^Sr microsampling [2] overcome the limitations ofconventional mineral separation techniques based on variations in grain size. They can be used to directly sample minerals unequivocally related to deformation, such as ¢brous minerals between stretched porphyroclasts or minerals grown within pressure shadows around rigid objects. Here we demonstrate the potential of Rb^Sr microsampling dating of Wg-sized mineral samples, which ^ due to thecritical reduction in sample size ^ can be directly extracted from rock thick

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 1 6 7 - 9

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W. Muller et al. / Earth and Planetary Science Letters 180 (2000) 385^397 «

Fig. 1. Simpli¢ed geological map of the area SW of the Tauern window (Eastern Alps) showing themain tectonic units and major faults (DAV = Defereggen^Antholz^Vals fault, KV = Kalkstein^Vallarga fault, Sp = Speikboden mylonite zone), based on [38] and modi¢ed after [8,9,11,12]. Sample locations are indicated by stars (*). Ages of plutons shown are from [11,19,20,39]. Sinistral shearing predates dextral, more discrete deformation. The lateral continuity of the KV fault is not well constrainedin the ¢eld and is therefore indicated by question marks.

sections under microscopic control. With this approach, the textural relationships of the samples dated are fully known, which is essential for reliable dating of deformation. Both the high concentrations of Rb and Sr in white mica (often v 50 ppm Sr) and a substantial parent/daughter element fractionation allow the determination of...