Zircon U‐Pb Dating of a Lower Crustal Shear Zone: A Case Study From the Northern Sector of the Ivrea‐Verbano Zone (Val Cannobina, Italy)

Abstract

A geochronological study was performed on zircon grains from a middle‐lower crustal shear zone exposed in the northern sector of the Ivrea‐Verbano Zone (Southern Alps, Italy) for the first time. The shear zone developed at the boundary between mafic rocks of the External Gabbro unit and ultramafic rocks of the Amphibole Peridotite unit. It is ~10–20 m wide, can be followed along a NE strike for several kilometers, and consists of an anastomosing network of mylonites and ultramylonites. Zircon grains were studied in thin sections and as separates from three representative outcrops along the shear zone. Zircon grains are more abundant in the shear zone compared to wall rocks and are generally equant, rounded to subrounded with dimensions up to 500 μm. U‐Pb data are mainly discordant, and the apparent 206Pb/238U dates show a large variation from Permian to Jurassic. Isotopic data, combined with microstructural, morphological, and internal features of zircon, reveal an inherited age component and suggest partial zircon recrystallization under high‐temperature conditions during Late Triassic to Early Jurassic. High‐temperature deformation in the shear zone, at lower crustal levels, was coeval with amphibolite to greenschist facies mylonitic deformation at upper crustal levels and is inferred to be related to Mesozoic rifting processes at the Adriatic margin.

Porphyroclasts: Source and Sink of Major and Trace Elements During Deformation-Induced Metasomatism (Finero, Ivrea-Verbano Zone, Italy)

“…Geochronological surveys suggest that these mylonites formed over a large time interval at granulite-facies conditions (ca. 230-200 Ma, [50]), with deformation likely continuing at amphibolite-facies conditions down to 180 Ma [50]. A similar age interval for deformation was proposed for the Pogallo Line (210-170 Ma, [51]), an intermediate-crust extensional shear zone interpreted in literature as the main structure accommodating necking at the Adria margin during the opening of the Alpine Tethys [52,53].…”

“…These conditions are not compatible with deformation events that occurred during the Alpine orogenic cycle, which formed low-T shear zones at depths of about 5-10 km or less [49]. Granulite-facies conditions have been also documented for mylonites in the crustal sequence of the Finero Complex at the boundary between gabbros and peridotites [50]. Geochronological surveys suggest that these mylonites formed over a large time interval at granulite-facies conditions (ca.…”

Application of Ti-in-zircon thermometry to granite studies: problems and possible solutions

“…This would suggest that Ti-in-zircon temperatures for granites have to be significantly upward corrected relative to the original TiO 2 - and SiO 2 -saturated calibration of the thermometer, in the order of + 70 °C. Looking through the literature, we observe that many applications of Ti-in-zircon thermometry to granites so far (including the most recent papers) involved no or only a minor / correction and could thus bear a significant systematic error (e.g., Sepidbar et al 2018 ; Roberts et al 2018 ; Chen et al 2018 ; Grajales-Nishimura et al 2018 ; Langone et al 2018 ; Steshenko et al 2017 ; Zhang et al 2018 ; Shabanian et al 2017 ).…”

Triassic magmatism in the European Southern Alps as an early phase of Pangea break-up

“…However, U-Pb zircon ages of the deep crustal intrusions from Finero (e.g. Zanetti et al 2013;Schaltegger et al 2015;Langone et al 2017Langone et al , 2018 show that, at least in some regions, the lower crust at the margins of Adria was affected by a long-lasting cycle of heating pulses from early Permian to Early Jurassic time. These pulses were coeval with the widespread extensional tectonics recorded all over Adria (e.g.…”