Assembly of continental lower crust: constraints from garnet Lu-Hf geochronology of metapelites from the Ivrea-Verbano Zone, Italy

Characterizing the formation and evolution of continental lower crust is vital to understanding the Earths structure, chemistry, and dynamics. The Ivrea-Verbano Zone (IVZ), northwestern Italy, is considered an archetypal section of continental lower crust, comprising kilometer-scale tracts of metasediments, interspersed with mafic and ultramafic rocks. To better constrain the timescales and timing of emplacement of these lower crustal metasediments, we present results of a comprehensive garnet Lu-Hf geochronological investigation. Garnet-whole-rock Lu-Hf dates from five metapelites, collectively spanning the amphibolite-granulite transition, range from ~239 Ma to 291 Ma. Sample dates do not vary systematically with structural depth in the section: amphibolite facies samples yield ages of 276.62.4 Ma (MSWD = 4.3; IVZ18MW36) and 291.07.7 Ma (MSWD = 16; M91003H01); granulite facies samples yield dates of 238.67.8 Ma (MSWD = 0.3; M91005D01), 263.91.3 Ma (MSWD = 0.6; M91005E02), and 27410 Ma (MSWD = 9.1; M91002B01) These dates are significantly younger than published zircon U-Pb constraints on the age of peak high-temperature metamorphism, ~316 Ma (Ewing et al., 2013), implying that either the Lu-Hf system in garnet records cooling of the crustal section following regional high-temperature metamorphism, or the age of the thermal peakand attendant garnet growthis younger than previously thought. The absence of systematic age variation with structural depth goes against the former interpretation. We incorporate EPMA and LA-ICPMS constraints on the distribution of major, minor and trace elements in garnet to discriminate between these competing hypotheses.