Granitic magmatism by melting of juvenile continental crust: new constraints on the source of Palaeoproterozoic granitoids in Fennoscandia from Hf isotopes in zircon
Zircons from nine Palaeoproterozoic granitoid intrusions within the southern part of the Fennoscandian Shield have been studied by laser ablation inductively coupled plasma source mass spectrometry to obtain U–Pb ages (in the range 1.88–1.68 Ga) and Hf isotope compositions. Six granitoids are from the 1.85–1.65 Ga Transscandinavian Igneous Belt; during that period more than 106 km3 of granitoid magma intruded the pre-existing crust. The large majority of magmatic zircons from the nine granitoids have a limited initial range, 176Hf/177Hf = 0.2816–0.2818, and define an evolutionary trend given by an initial value of εHf(1.88Ga) ≈ +2 ± 3 at an average 176Lu/177Hf = 0.015. These data show that a geographically extensive, long-lived, relatively homogeneous, and dominant magma source resided within 2.1–1.86 Ga Svecofennian juvenile crust between 1.88 and 1.68 Ga. Zircon xenocrysts (1.91–1.98 Ga) with initial εHf = +0 to +2.5 from one of the intrusions provide additional evidence for such a long-lived crustal source of granitic magmas in central Fennoscandia. The granitoids were emplaced during a period of active mafic underplating that supplied heat to the anatectic zone in the lower–middle crust, but little or no mantle-derived Hf to the granitic magmas, in contrast to many mixed intermediate rocks.