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Petrological constraints on ultra-high pressure metamorphism and frictionite formation in a catastrophic rockslide: The Koefels event (Eastern Alps).

The P-T conditions in extremely-rapid gravity-driven rockslides are difficult to constrain from the descended rock mass itself. Here, we report mineralogical observations from the Koefels rockslide and their interpretation. The Koefels event – happened between 9527-9498 cal BP – comprises 3.9 km 3 mainly of muscovite + biotite-bearing orthogneiss, and is one of the few large rockslides in silicate-bearing rocks worldwide. Detached by collapse of a valley flank, the rockslide impacted the opposite valley flank: While the lower part of the mass was sharply stopped, the overriding part propagated farther. This led to shear localization along discrete planes and, in consequence, to transient melting by frictional heating. The resulting frictionites comprise thin glassy levels with floating crystal fragments. The bulk composition of the glassy melt corresponds to the composition of the orthogneiss.