Petrography, sedimentology and Neoalpine metamorphism of the Idalp ophiolite, Steinsberg and Idalp formations, Lower Engadine Window, Tyrol (Austria)

The Lower Engadine Window is the only tectonic window in the Eastern Alps in which all three Penninic nappes are exposed. The focus of this study lies on the sedimentologic investigation of the Jurassic Steinsberg and Idalp formations of the Middle Penninic Nappe system, the petrographic investigation of the Idalp ophiolite of the Upper Penninic Nappe system, and on the study of the Neogene metamorphism of these rocks. The Steinsberg Formation, composed of carbonate sediments rich in crinoid fragments (“encrinites”), accumulated in a shallow, normal marine depositional setting on the shelf of the Iberia-Briançonnais microcontinent. Overlying sediments of the Idalp Formation, represented by shale and intercalated sandstone, are interpreted to be of a deeper marine shelf setting. Rocks of the Idalp ophiolite represent fragments of the oceanic crust of the Piemont-Liguria Ocean. The obtained multi–equilibrium P-T estimates using the program THERMOCALC v.3.33 from two metagabbro samples (amphibole + chlorite + pumpellyite + albite + quartz ± muscovite ± clinopyroxene) yield pressures of 0.63 to 0.95 GPa and temperatures between 330 °C and 390 °C (Mode-1 invariant points), 0.70 to 0.80 GPa and temperatures between 340 °C and 360 °C (Mode-2 with and without clinopyroxene). These data are in good agreement with the data of Höck et al. (2004) which are 0.7–0.9 GPa at approx. 350 °C. For the first time T-data of 330 ± 25 °C were obtained from a calcareous sandstone of the Idalp Formation of the Middle Penninic Nappe system using calcite-dolomite geothermometry. The P-T results indicate that the Idalp ophiolite underwent the Neoalpine regional metamorphism at low-T/high-P at the transition from upper greenschist-to blueschist-facies and that the Idalp Formation was also characterized by this low-T metamorphism. This metamorphic overprint was then followed by uplift on the European shelf and the uplift and later exhumation of the rocks of the Idalp ophiolite, and sedimentary rocks of the Steinsberg- and Idalp formations.