DATING THE ONSET OF A LOWER CRUSTAL SHEAR ZONE: A (LUCKY) CASE FROM THE NORTHERN SECTOR OF THE IVREA-VERBANO ZONE (VAL CANNOBINA, ITALY)

A detailed textural and U-Pb geochronological investigation has been performed on zircons from a ductile shear zone established in lower crustal mafie intrusives. The investigated shear zone is hosted in the Finero mafic-ultramafic complex of the northeastern sector of the Ivrea-Verbano Zone, Southern Alps. Il developed al the base of the lower lo middle continental crust section, where mafie lo ultramafic rocks intruded into a sequence of metapelites and metabasites (i.e., Kinzigite Formation) constituting the polymetamorphic basement of the Adria plate. The shear zone forms a braided belt that can be followed for several km, from the Cannobino River lo the MI. Gridone, and developed entirely within gabbroic/dioritic rocks, partly intruded during Triassic lime (- 232 Ma; Zanetti et al., 2013). The shear zone is characterized by a well-developed foliation, a lineation, and a compositional banding, where amphibole, clinopyroxene and game! occur as large rounded lo elliptical porphyroclasts, resulting embedded in a fine grained matrix mainly consisting of plagioclase, amphibole, pyroxenes and ilmenite. The shear zone was active during uplift and cooling of the wall rock, and recorded retrograde metamorphic conditions ranging from 650 lo 500"C al 0.6-0.4 GPa (Kenkmann, 2000). The mylonitic deformation is generally placed no earlier !han 230 Ma, lasting until about 170 Ma al the latest (e.g., Handy & Zingg, 1991). Numerous zircon grains were obtained from the minerai separation of one (- 2 kg weight) sample collected in the Cannobino River (northem Ivrea-Verbano Zone, Southern Alps). Zircon grains were examined also directly on petrographic lhin sections of samples collected throughout the (ultra)mylonitic belt. According to petrographic and microstructural analyses zircon occurs as rounded grains within the mylonitic matrix, easily recognisable due lo their dimensions up lo 110 micron. Cathodoluminescence (CL) study al the SEM revealed a well developed systematic zoning of zircon grains with dark cores surrounded by brighter domains, locally asymmetric, and with a thickness up to 30 micron. The dark inner core can show zoning features suggesting magmatic growlh. Preliminary LA-ICP-MS U-Pb results from zircon separates yielded mainly late Triassic concordant ages of about 235 Ma for the dark CL cores and about 204 Ma for the brighter CL (mylonitic) overgrowths. The sizes, distributions, shapes and CL features of zircon grains provide strong evidence of a metamorphic response of zircon during ductile deformation. The observed features can be interpreted as evidence of dissolution/recrystallization in response lo fluid influx during high-temperature shearing. Further microstructural investigations, minerai chemistry and trace element characterization of different zircon domains are in progress and will provide more evidence for the petrochronological evolution of such a lower crustal shear zone.