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)

A Langone,N R Daczko,M Mazzucchelli,S Piazolo,A Zanetti,M Tiepolo

doi:10.1002/2017tc004638

Abstract

AbstractA 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.

Highlights

The Ivrea-Verbano zone (IVZ) represents an uplifted part of the pre-Alpine middle to lower continental crust located in the Southern Alps of northwest Italy (e.g., Fountain, 1976; Schmid, 1993)
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
Based on the U-Pb dating of rutile from the high-grade metamorphic rocks of the central and southern part of the IVZ, some authors suggest that discrete regional-scale thermal pulses affected the lower structural levels of the IVZ in response to the Mesozoic hyperextension associated with rift/post-rift processes at the Adriatic margin (Ewing et al, 2015; Smye & Stockli, 2014)

Summary

Introduction

The Ivrea-Verbano zone (IVZ) represents an uplifted part of the pre-Alpine middle to lower continental crust located in the Southern Alps of northwest Italy (e.g., Fountain, 1976; Schmid, 1993). Recent studies suggest that the cooling and exhumation history of the IVZ was interrupted by discrete Triassic-Jurassic heating events, as documented by zircon, monazite, and rutile U-Pb dating (Ewing et al, 2015; Langone et al, 2017; Schaltegger et al, 2015; Smye & Stockli, 2014; Vavra et al, 1999; Vavra & Schaltegger, 1999; Zanetti et al, 2013, 2016). Based on the U-Pb dating of rutile from the high-grade metamorphic rocks (granulites) of the central and southern part of the IVZ, some authors suggest that discrete regional-scale thermal pulses affected the lower structural levels of the IVZ in response to the Mesozoic hyperextension associated with rift/post-rift processes at the Adriatic margin (Ewing et al, 2015; Smye & Stockli, 2014).

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