Lower Ordovician migmatisation in the Ötztal 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 Ötztal crystalline complex, a polymetamorphic basement unit of the Eastern Alps in Austria. One of these migmatite areas, the "Winnebach"-migmatite, has been investigated by single zircon Pb-Pb evaporation and conventional U-Pb zircon analysis in order to establish the time of the migmatisation. To be able to distinguish the anatectic event from other pre-Variscan metamorphic events, the zircon populations of the migmatite were compared to those of the adjacent paragneisses. Except for one zircon type, all populations exhibit polyphase crystal growth and do not show any specific mode of occurrence. Measured by single grain evaporation these zircon populations document three metamorphic events with mean ²⁰⁷Pb/²⁰⁶Pb ages of 484 ± 6 Ma, ∼ 560 Ma, and ∼ 635 Ma. However, since these ages are found both within the migmatite and the paragneiss, none of them can be directly assigned to the migmatisation. One population of spheroidal, clear and colourless specimens is found to be characteristic for the migmatite only. These anatectically grown zircons yield a concordant U-Pb age of 490 ± 9 Ma, thus proving the Early Ordovician event to have caused 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 Proterozoic and/or Archaean sources.