Crustal evolution in the SW part of the baltic shield: The Hf isotope evidence

Abstract

The results of a laser ablation microprobe-inductively coupled plasma mass spectrometry Lu-Hf isotope study of zircons in 0·93-1·67 Ga rocks from south Norway indicate that early Proterozoic protoliths of the Baltic Shield have present-day 176Hf/ 177Hf ≤ 0·28190 [εHf(t) = 5-6], whereas 1·52-1·60 Ga juvenile additions to the continental margin have 176Hf/177Hf = 0·2820 [εHf(t) = 12-13]. Mid-to late Proterozoic felsic igneous rocks in the region are characterized by a range of Hf isotopic compositions suggesting mixing of material derived from Palaeoproterozoic crust from the Baltic Shield and/or mid-Proterozoic juvenile crust. New mantle-derived magmas were added to the crust at ~ 1·48 Ga and in Sveconorwegian time. Late Sveconorwegian granites from the area west of the Oslo Rift have inherited zircons with low 176Hf/177Hf (<0·28180), suggesting that a pre-1·7 Ga crustal source contributed to the magmas. The evolution of the continental crust in this region is thus a result of repeated interaction between mantle-derived magmas and mid- to early Proterozoic crustal rocks. The results of this study confirm the presence of early Proterozoic rocks in the deep crust west of the Oslo Rift, and support tectonic models in which the protolith of the western part of south Norway has been part of the Baltic Shield since the early Proterozoic.