GNGTS 2016 - Atti del 35° Convegno Nazionale

242 GNGTS 2016 S essione 1.2 exhumation stages (e.g. Spalla et al., 1991). The shallower levels of the exhumation path took place between 45 and 30 Ma ������ (Inger et al., 1996� �������� ; Cortiana et al., 1998� ������ ; Babist et al., 2006� ������ ; Zanoni et al., 2010). To the east the SLZ is delimited by the Periadriatic line and in its inner part hosts the Biella and Traversella plutons (Zanoni et al., 2008; Zanoni 2010� ������ ��� ������ , ������ ��� 2016) ������ . The Biella pluton shows concentric zoning with the outer part constituted by monzonite and the inner part by syenite and granite and a calc- alkaline composition ����������� (Bigioggero et al., 1994)� ��� ������ ��� ���� . The pluton has been dated at about 30 Ma ������ (Romer et al., 1996� ������ ; Berger et al., 2012) ��� and its country rocks belong to the Eclogitic Micaschists Complex (Fig. 2). Results. Along the northeaster and southwester margins of the Biella pluton country rocks consist of metapelites, meta-aplites and metagranitods with minor metabasites. During theAlpine convergence country rocks recorded up to six ductile deformation stages that predated the intrusion of the pluton. Only locally syn-intrusive folding and faulting are recorded in the country rocks. Generally brittle structures post-date pluton emplacement. The igneous rocks record a magmatic foliation that in place is parallel to the pluton margin. During the emplacement the country rocks recorded different type of contact metamorphism assemblages that vary with the country rock type and the distance from the pluton margin. The contact metamorphism parageneses form fine-grained coronitic structures that overprinted eclogite parageneses and local greenschist parageneses. With the distance from the pluton the amount of contact metamorphism parageneses decrease. Close to the pluton margin the contact metamorphic mineral assemblages are characterised by plagioclase, K-feldspar, cordierite, biotite, spinel, Al-silicate (sillimanite closer and andalusite farer from the pluton), and locally corundum, orthopyroxene and garnet. Locally, up to about 10 m far from the pluton margin, partial melting is recorded. With the distance from the pluton corundum, orthopyroxene, garnet, spinel, and sillimanite disappear. The contact metamorphic minerals are detected up to about 900 m far from the pluton in plain view (Fig. 2). In igneous rocks interstitial amphibole has an Al content compatible with intrusion depth variable between 4 and 7 km. Ti content in amphibole and equilibrium between amphibole and plagioclase are consistent with intrusion temperature between 670 and 720°C (Zanoni et al., 2010). In the country rocks temperature peak, reached during pluton crystallisation, vary between about 700°C at the pluton margin, and 550°C at about 600 m far from the pluton (Zanoni et al., 2010). The comparison of thermal estimates in the country rocks with a 2D conductive thermal model for pluton cooling shows that the best fit is for initial temperature in the country rocks between 430 and 530°C at 8 km depth (Fig. 3). That involves a thermal gradient of the country rock at time of intrusion ranging between 55 and 65°C/km. Conclusions. The crustal level of Biella pluton emplacement is as shallow as a few kilometres and this is consistent with magmatic rocks intersecting all ductile structures in the country rocks Fig. 3 – Spatial distribution of Tmax due to heat conduction from the pluton to host rock for different base temperatures (lines) and comparison with natural data (dots).