LOWER ORDOVICIAN MIGMATIZATION IN THE OTZTAL CRYSTALLINE BASEMENT (EASTERN ALPS, AUSTRIA) - LINKING U-PB AND PB-PB DATING WITH ZIRCON MORPHOLOGY

Partial anatexis of bio-plag-paragneisses is known from several sites within the Otztal crystalline complex, a polymetamorphic basement unit of the Eastern Alps in Austria. One of these migmatite areas, the ''W innebach''-migmatite, has been investigated by single zircon Pb-Pb eva poration and conventional U-Pb zircon analysis in order to establish t he time of the migmatisation. To be able to distinguish the anatectic event from other pre-Variscan metamorphic events, the zircon populatio ns of the migmatite were compared to those of the adjacent paragneisse s. Except for one zircon type, all populations exhibit polyphase cryst al growth and do not show any specific mode of occurrence. Measured by single grain evaporation these zircon populations document three meta morphic events with mean Pb-207/Pb-206 ages of 485 +/- 6 Ma, similar t o 560 Ma, and similar to 635 Ma. However, since these ages are found b oth within the migmatite and the paragneiss, none of them can be direc tly assigned to the migmatisation.One population of spheroidal, clear and colourless specimens is found to be characteristic for the migmati te only. These anatectically grown zircons yield a concordant U-Pb age of 490 +/- 9 Ma, thus proving the Early Ordovician event to have caus ed the anatexis. The minimum age of some of the inherited zircon cores can be established around 2440 Ma using both methods, thus providing evidence for the assimilation of zircons derived from earliest Protero zoic and/or Archaean sources.