Alpine and Variscan peak pressures of different rock types from the Adula Nappe (Central Alps) determined by Raman spectroscopy of quartz inclusions in garnet

The Adula nappe in the Swiss-Italian Central Alps is a continental basement nappe from the former European margin that was subducted to depths indicating (ultra)-high-pressure conditions. Many studies were performed to understand the pressure-temperature-time evolution of the Adula nappe. The Adula nappe underwent eclogite-facies metamorphism during the Variscan and Alpine orogenic cycles but the Variscan and Alpine parageneses are hard to distinguish.

For this study, around fifty samples were collected from different lithologies on a N-S transect through the Adula nappe parallel to the direction of subduction. Raman spectroscopy on quartz inclusions in garnet was used as a geobarometer to measure minimum peak pressures. This method is independent of chemical equilibria and yields reliable pressure constraints even if the high-pressure mineral assemblage has been retrogressed. Variscan and Alpine garnet domains were identified using the Electron Microprobe and Scanning Electron Microscopy.

The Variscan peak pressure was at least 2.3 GPa. For the Alpine metamorphism, the Zr-in-rutile temperatures exhibit a gradient increasing from ca. 500-550 ^oC in the north to around 700 ^oC in the south. The minimum peak pressures in the northern and central Adula nappe are 2.1-2.2 GPa for metasediments, 1.4-2.0 GPa for metabasites, and 1.5 GPa for an orthogneiss. Lower pressures of 1.1-1.3 GPa in the southern Adula nappe were potentially caused by viscous relaxation of the quartz inclusions during the high-temperature Lepontine metamorphism.