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 similar 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 (similar to 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 degrees 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 similar to 440 and similar to 311 Ma, prior to the onset of regional Variscan metamorphism.