Structural and metamorphic evolution during tectonic mixing: is the Rocca Canavese Thrust Sheet (Italian Western Alps) a subduction-related mélange?

Abstract

In the Sesia-Lanzo Zone (SLZ), the subunit Rocca Canavese Thrust Sheet (RCT) is characterised by a mixture of mantle- and crust-derived lithologies and can be a good candidate to be a former subduction-related melange of the Austroalpine domain. The unit consists of metapelites, metagranitoids, metabasics, metagabbro and serpentinised lherzolite lenses from metre to hundred-metre size.According to the literature, PT peak conditions for all lithologies are 0.8-1 GPa at 300-400°C, in lawsonite-blueschist facies conditions recorded during the Alpine subduction. However recent work describes different mineral assemblages for the metamorphic peak, separating rocks with lawsonite from those with jadeite. Therefore, we refined the meso and microstructural analysis of the tectonic slices of RCT and we performed a detailed thermo-barometry of different metamorphicstages in order to quantify Alpine peak conditions and P-T-d-t paths, and to test whether the RCT represents a subduction-related melange. We focus on metagabbros, Jd-bearing and Lws-bearing glaucophanites since they have the most suitable chemistry allowing to reconstruct the complex evolution of the mixing. Metagabbros and Jd-bearing glaucophanites experienced a D1a metamorphic stage characterised by a pressure of 1.3-1.8 GPa and temperature of 450-550°C, in eclogite facies condition. Lws-bearing glaucophanites experienced a D1b metamorphic stage at a temperature