Hafnium isotopes and Zr/Hf of rutile and zircon from lower crustal metapelites (Ivrea-Verbano Zone, Italy): Implications for chemical differentiation of the crust

- Rutile robustly records Hf isotope composition from high temperature metamorphism.
- Dissolution of ancient detrital zircon exerts a strong control on the Hf isotope budget.
- In the IVZ, rutile has subchondritic Zr/Hf and is the main host of Zr and Hf in high-T metapelites.
- The relative proportion of garnet and rutile should control bulk rock Zr/Hf in UHT metapelites.
- Efficient chemical differentiation of crust occurs only at ultra-high T in the IVZ.

We have used granulite facies metapelites from the classic Ivrea-Verbano Zone (IVZ) lower crustal section, northern Italy, to investigate the behaviour of the hafnium (Hf) isotope system in rutile and zircon under conditions of high temperature (700-1050 °C) metamorphism and partial melting. The Hf isotope composition of rutile is shown to be robustly preserved from these conditions, and is also unaffected by retrogression, making it a powerful new tool for metamorphic petrology. Rutiles from our suite of IVZ metapelite samples have crustal Hf isotope signatures (εHf(i)=−6±1 to −13±1 at 288 Ma), with the most strongly crustal signature (εHf(i)=−9.2±0.3 to −12.9±1.0) recorded by narrow slivers of metapelite incorporated within a gabbroic body. The Hf isotope composition of rutile from these metapelite slivers is the direct result of dissolution of ancient detrital zircon with strongly unradiogenic 176Hf/177Hf at ultra-high temperatures associated with the emplacement of the gabbro. Rutile is an important host of Zr and Hf in high temperature metapelites, containing up to 11 000 ppm Zr and 400 ppm Hf. It has subchondritic Zr/Hf (13-32) that is significantly lower than the Zr/Hf of both zircon (35-52) and the starting bulk rock composition (36-40). Dissolution of zircon into extracted melts is expected to lead to a lowering of bulk rock Zr/Hf towards that of rutile. In the IVZ samples this effect is balanced by the strongly superchondritic Zr/Hf of garnet (40-70). The IVZ case study demonstrates the strong control that dissolution of detrital or inherited zircon can exert on the available Hf isotope budget, whether in a metamorphic or magmatic setting. The link between dissolution of accessory phases into melt and changes in trace element and Hf isotope chemistry points to important differences between high- and ultra-high temperature metamorphism in terms of chemical and isotopic differentiation of the crust.

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