Hf isotope systematics in granitoids from the central and southern Alps

Abstract

First initial-Hf isotopic compositions for samples from the Alpine domain are presented and discussed. The results are mainly based on zircons and a few whole rocks with ages between 30 and 450 Ma. Of those so far analyzed, the present-day Hf isotopic compositions of zircons from non-metamorphic and metamorphic granitoid rocks vary between 0.2824 and 0.2829. Zircon populations with concordant U-Pb ages have much higher initial 176Hf/177Hf than inversely discordant populations which have been contaminated with older zircons containing less radiogenic Hf. Correlated Nd-Hf crustal-residence ages have been found involving model parameters of eHf/eNd=f(Lu/Hf)/f(Sm/Nd) ∼1.6 for the depleted mantle and f(Lu/Hf)/f(Sm/Nd) ∼1.2 for elemental fractionations in the crust. The model implies 176Lu/177Hf of 0.017 for the bulk crust. It is suggested that the granitoid rocks are the result of mixing of subcontinental mantle-derived magmas with ∼1.7 Ga old recycled and partially molten crustal material. The continental/mantle component mass-ratio values for the granitoids range between 0.3 and 2.