Ductile-brittle deformation effects on crystal-chemistry and U-Pb ages of magmatic and metasomatic zircons from a dyke of the Finero Mafic Complex (Ivrea-Verbano Zone, Italian Alps)

Zircon grains/fragments (up to 2 mm in length) occur as porphyroclasts surrounded by fine-grained plagioclase within the leucocratic domains and may occur within the melanocratic layers completely or partially surrounded by biotite and spinets. Fractures are common within zircon, define networks and have associated displacements occasionally and/or they can be filled by secondary minerals.

A detailed microstructural, geochemical and geochronological study was performed on zircon grains from plagioclase-rich dioritic dykes discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex (Italian Alps). This unit is exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea-Verbano Zone, Italy). The leucocratic dykes consist mainly of plagioclase (An(18-24)Ab(79-82)Or(0.3-0.7)) with subordinate amounts of biotite and spinet defining melanocratic layers. Zircon and corundum are common accessory phases. Both the dykes and the surrounding meta-diorites/gabbros show evidence of ductile deformation under amphibolite-facies conditions.