Mafic plutonic rocks in a continental-arc setting: geochemistry of 1.87-1.78 Ga rocks from south-central Sweden and models of their palaeotectonic setting

摘要

Mafic intrusive rocks (1.79-1.78 Ga) of the Transscandinavian Igneous Bell (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 gabbrosorites (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. epsilon(Nd)(t) ranges between +1.0 and +2.7. and Sr-87/Sr-86(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 Data Province (DP) tend to overlap with the c. 1.7Ga basic Data lavas of TIB at slightly elevated relative Sr/Nd ratios. With two exceptions, the epsilon(Nd)(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 epsilon(Nd)(t) +2.0 are scarce in the TIB. Mantle Sources represent depleted mantle wedge material that was enriched by fluids/melts not long before (T-DM c. 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.