Structure and petrography of the southwestern margin of the Biella pluton, Western Alps

This work presents a new form surface map of the southwestern margin of the Biella pluton at the scale 1:10,000. The Biella pluton is part of the Periadriatic intrusives of the Alps and is emplaced in the continental Sesia-Lanzo Zone of the western Austroalpine domain. The country rocks consist of metapelites and different metagranitoids. Pre-intrusive HP (high pressure) mineral assemblages are dominant in country rocks with the exception of metagranitoids dominated by HT (high temperature) assemblages. The plutonic rocks consist of monzonite with minor syenite. The ductile polyphasic deformation of the country rocks predates the pluton emplacement, with the exception of syn-intrusive folding and shearing, which were recorded in the country rocks of the deeper part of the pluton. Syn-intrusive deformation may be represented by brittle structures that bear mineralisation; however, the majority of the brittle deformation postdates the final emplacement of the pluton. The orientation of the pervasive foliation in the country rocks controls the space available for magma intrusion and possible magmatic flow during emplacement. The inferred diffusion of the thermal aureole in the country rocks is based on the variation in contact metamorphic minerals, which is described by microscopic analysis. The extent of the aureole appears to be controlled by the type of dominant mineral assemblages, rock permeability, and the orientation of the regional foliation in country rocks with respect to the pluton margin. The multiscale structural analysis reveals that the Biella pluton emplaced at a depth as shallow as the greenschist facies conditions or shallower.