GNGTS 2016 - Atti del 35° Convegno Nazionale

240 GNGTS 2016 S essione 1.2 The thermal state of Biella pluton country rocks as a tool to unravel the late orogenic tectonics of the Western Alps D. Zanoni, M. Roda Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Italy Introduction. Crustal level and timing of late-orogenic plutons represent powerful tools for constraining the late exhumation history of tectonic units, within collisional belts. This work aims to contribute to this topic by quantitatively determining the thermal state of Biella Periadriatic pluton country rocks during its emplacement within the continental HP metamorphic rocks of the Sesia-Lanzo Zone (SLZ), in the western Austroalpine domain (Zanoni et al., 2008 , 2010; Zanoni, 2016 and refs. therein). During the Alpine subduction and exhumation, the Sesia-Lanzo Zone records a polyphasic tectono-metamorphic evolution. Pluton cooling involved contact metamorphism overprinting eclogitic to greenschist facies assemblages in the country rocks. On the basis of the amount and type of contact metamorphic assemblages the aureole extent was mapped and the variation of recorded thermal peak validated by comparison with numerical modelling of the heat transfer during magma crystallisation. Thus, this work aims constraining the late exhumation history of the internal western Alps by determining the crustal level of pluton emplacement and the thermal gradient of its country rocks at the time of intrusion. Geological setting. The Alps developed during subduction and closure of the Mesozoic Tethys and subsequent continental collision over Cretaceous–Oligocene times (e.g. Dal Piaz, 2010; Handy et al., 2010; Spalla et al. , 2010). The Alpine convergent system involved the subduction of the European lithosphere underneath the Adria plate. The Periadriatic line is a main crustal break of the Alpine bounding the Southalpine continental crust acting as backstop wall of the orogenic wedge during the convergence ������� (Polino et al., 1990)� ���� ��������� ����� �� . This lineament (Fig. 1) separates tectonic units that during theAlpine convergence experimented intense deformation and Fig. 1 – Tectonic map of the Alps and northern Apennines. PF = Penninc front; PL = Periadriatic line; AF = Apennine front; SLZ = Sesia Lanzo Zone.