Textually controlled (‘in-situ’) chronmetry
The Sonnblick Dome forms a large antiformal structure within the Pennine of the SE Tauern Window. The structural and metamorphic evolution of this area is relatively well constrained. However, mineral age determinations of the Alpine event within the Zentralgneis granitoid basement remain ambiguous due to a lack of isotopic equilibration during the Alpine event. In this paper we use a newly developed technique, namely Rb-Sr microsampling, in an attempt to place more reliable age constraints upon Alpine deformation. The new age determinations range between c. 22 Ma and 28 Ma. The oldest ages are interpreted to result from shearing related to the earliest stages of dome formation while the youngest ages probably represent late fabric development during uplift of the Zentralgneis complex. A clustering of six fabric ages around 25.5 ± 0.3 Ma is thought to represent the peak of deformation activity associated with formation of the dome. These new data can be fitted into a model for the metamorphic and structural evolution of the SE Tauern Window.
In most of the rocks studied isotopic disequilibrium is apparent between and within minerals. It is clear that large feldspar augen did not attain isotopic equilibrium despite amphibolite facies metamorphism. In fact, even where chemical equilibrium was achieved isotopic homogenization did not always occur. The Rb-Sr data from white mica suggest that isotopic equilibration may occur during isochemical rotation of grains suggesting differing mechanisms for chemical and isotopic movement within grains. The data presented herein suggest that small-scale, in-situ sampling methods may provide the most reliable method for deformation chronology in basement gneisses.
Figures & Tables
Isotope geochemistry has produced many technical developments in the past decade or so that have revolutionized the potential information available on the tectonics of metamorphic belts from geochronology. These include the ability to date minerals and rocks on small spatial scales, scales that at last approach those from which other types of information — structural and petrological — are obtained. However, interpreting the new data, and their integration with the other datasets available, is not straightforward and requires careful chemical and textural observations that go hand-inhand with the geochronology. The increasing realization of the importance of this approach has led to a number of symposia at international conferences devoted to this topic in recent years. The set of papers in this book emanates from one such symposium and describes recent progress in integrating this new information with other datasets from metamorphic petrology on a mineral and sub-mineral scale.