SEARCH

SEARCH BY CITATION

Keywords:

  • Transscandinavian Igneous Belt;
  • mafic plutonic rock;
  • Sr and Nd isotopes;
  • mantle enrichment;
  • continental arc

Abstract

Mafic intrusive rocks (1.79–1.78 Ga) of the Transscandinavian Igneous Belt (TIB) and the c. 1.87 Ga Hedesunda Igneous Complex in the Fennoscandian Shield of south-central Sweden were studied using whole-rock and isotope geochemistry.

Rock types vary from gabbros/norites (and leucogabbros) to quartz diorites, with Mg# between 76 and 49, and wt% SiO2 between 43.6 and 59.7, indicating some variation in evolutionary levels and variable cumulus components. Geochemical signatures are calc-alkaline to shoshonitic, large ion lithophile elements and light rare earth elements enriched and high-field strength elements depleted of continental-arc type.

εNd(t) ranges between +1.0 and +2.7, and 87Sr/86Sr(t) between 0.7020 and 0.7038. There is no systematic correlation between chemical parameters and isotope ratios. These isotopic data overlap with other mafic plutonic TIB rocks; samples from the Dala Province (DP) tend to overlap with the c. 1.7 Ga basic Dala lavas of TIB at slightly elevated relative Sr/Nd ratios.

With two exceptions, the εNd(t) of +1 to +2 conform to an isotopically ‘mildly depleted’ source, typical for mafic TIB rocks and many Svecofennian rocks in the region. Reported values above εNd(t) +2.0 are scarce in the TIB. Mantle sources represent depleted mantle wedge material that was enriched by fluids/melts not long before (TDMc. 2.0 Ga), that is during subduction in the preceding Svecofennian (2.0–1.87 Ga) and/or during the TIB-0&1 event (1.85–1.78 Ga). The palaeotectonic settings inferred are active continental margins; N–S-directed convergence at 1.87 Ga and E–W-directed at 1.79–1.78 Ga. Copyright © 2008 John Wiley & Sons, Ltd.