Barometric studies on different rock types from the Adula Nappe (Central Alps) by Raman spectroscopy of quartz inclusions in garnet

Abstract

<p>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. However, the pressure data derived from classical thermobarometry from eclogite and garnet peridotite lenses cannot be correlated with the tectonic record without several difficulties. The pressure gradient is very high, the structural record for the often suggested extrusion model is missing and the directly surrounding nappes show consistently lower pressures. Furthermore, it was discovered that at least parts of the Adula nappe underwent eclogite-facies metamorphism during the Variscan and the Alpine orogenic cycles. These two cycles were distinguished by age dating and the chemical zonation patterns of garnet, although in some cases it can be ambiguous. Otherwise, the Variscan and Alpine parageneses are hardly, if at all, possible to tell apart. Therefore, existing pressure and temperature data that were obtained using classical geobarometers rely on mineral equilibria, which may not have yielded true Alpine metamorphic conditions.</p> <p>For this study, around fifty felsic and metabasic 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 (RSQI) in garnet was used as a geobarometer to measure minimum peak pressures. The advantages of this method are its independence of a chemical equilibrium and the ability to yield reliable pressure constraints even if the high-pressure mineral assemblage has been retrogressed. The Variscan and Alpine garnet domains were carefully identified using the Electron Microprobe (EMP) and the Scanning Electron Microscopy (SEM). Temperatures were determined by means of Zr-in-rutile thermometry by measuring the Zr content with EMP.</p> <p>As a result, the obtained temperatures exhibit a gradient increasing from the north at ca. 500-550 <sup>o</sup>C to the south at around 700 <sup>o</sup>C. The minimum peak pressures in the northern and central Adula nappe range between 2.09 GPa and 2.17 GPa for metasediments and 1.41 GPa and 2.02 GPa for metabasites. 1.53 GPa were determined for an orthogneiss from the central part of the nappe. Lower pressures between 1.14 GPa and 1.31 GPa in the southern Adula nappe were potentially caused by viscous relaxation of the quartz inclusions during the high-temperature Lepontine metamorphism. Our new pressure data imply a very weak pressure gradient. Therefore, it is in contrast to the results of previous works, in which barometers based on a chemical equilibrium were applied. Additionally, no systematic difference in minimum peak pressures is observable for the different lithologies.</p>