Assembly of lower continental crust: A garnet Lu-Hf petrochronological investigation of the Ivrea-Verbano Zone, Italy

Abstract

Garnet from five metapelitic samples that span the archetypal Ivrea-Verbano Zone (IVZ) lower continental crust section were dated with the lutetium-hafnium (Lu-Hf) method. Dates systematically decrease from 311.27 ± 1.90 Ma in the sub-solidus amphibolite-facies domain to 263.81 ± 1.29 Ma in the granulite-facies section. Core-to-rim zonation in Lu and Ca in sub-solidus amphibolite-facies garnet grains constrains a phase of isothermal burial of 1–2 kbar to ∼311 Ma. Garnet trace-element zonation and Lu-Hf systematics of a sample adjacent to the Mafic Complex preserves evidence for garnet core growth at or prior to 300 Ma and garnet rim growth concomitant with emplacement of the Mafic Complex (∼286–282 Ma). Garnet grains in a high-grade amphibolite facies sample and granulite-facies metapelites exhibit no evidence of Lu remobilization, indicating that their Lu-Hf dates of between 275.30 ± 2.47 Ma and 263.81 ± 1.29 Ma record the timing of garnet growth prior to cooling of the section during exhumation. The >50 Myr period of garnet growth recorded in this dataset implies that the IVZ lower continental crust remained at elevated temperatures, >650 °C, over a protracted period prior to the initiation of Tethyan rifting. When combined with constraints on the maximum depositional age for the IVZ metasediments, the P-T-t data presented here are consistent with entrainment of sedimentary rocks into the lower crust by tectonic underplating between ∼440 and ∼311 Ma, prior to the onset of regional Variscan metamorphism.