An analysis of the peak-temperature recorded by the meta-sedimentary covers of the Central Alps using Raman spectroscopy

The Penninic nappes of the Swiss-Italian Central Alps are wrapped by layers of meta-sedimentary covers of Mesozoic to Cenozoic age that were involved in the Alpine orogeny. These meta-sedimentary rocks contain high amounts of carbonaceous material (CM). Its crystallinity has the peculiarity of recording the peak temperature to which CM has been subjected (e.g. French 1964). By analyzing this material through Raman spectroscopy it is possible to obtain such temperatures (e.g. Beyssac et al. 2002), which are then correlated with the geological evolution of the host rocks. With this focus, we collected and analyzed samples within the meta-sediments of the Swiss Misox Zone (Mesozoic sediments pinched between Adula and Tambò nappes) and of the Italian Splügen Zone (Tambò nappe autochthonous and para-autochthonous cover; Figure 1). This sector of the Alps underwent several deformation phases, ranging from subduction-related high pressure (HP) - low temperature (LT) conditions up to Barrovian conditions, followed by the subsequent more or less pervasive greenschist facies re-equilibration. The temperatures obtained were compared with the peak metamorphic conditions evaluated for the basement and for other portions of the nearby meta-sedimentary units (Wiederkehr et al. 2011), then validated through micro-structural and field observations for a better comprehension of the geological evolution of this portion of the Alps. Due to the complexity of the Alpine history, the recorded peak temperatures could relate to different stages of the orogeny, as already shown in the work of Wiederkehr et al. (2011) in nearby areas. Therefore, it is critical to compare these results with structural and petrographic analysis to attribute them to the correct geodynamic stage. An interesting observation that emerged during the Raman analyses concerns the response of CM material. In many samples from the two areas, two distinct spectra morphologies are often detected, apparently indicating two different recorded temperatures, even within the same centimetric sample. Although not yet clarified, this evidence leads to (at least) two possible interpretations: 1) CM grains recording lower temperatures originated in a different environment with respect to the higher temperature ones and were then mixed together in a later stage; 2) the same peak temperature could have induced different CM responses, reflecting an intrinsic difference in the source CM material, as already observed for various synthetic materials. Despite this work does not provide an unambiguous solution to this question, we believe that this situation may occur in different study areas and should be taken into consideration to avoid misinterpretation of recorded peak temperatures.