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 y HENRIKKI RUTANEN 1 * and ULF B. ANDERSSON 2 1 Department of Geology and Mineralogy, A ˚ bo Akademi University, Turku, Finland 2 Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Sweden 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% SiO 2 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. e Nd (t) ranges between þ1.0 and þ2.7, and 87 Sr/ 86 Sr(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. 1.7Ga basic Dala lavas of TIB at slightly elevated relative Sr/Nd ratios. With two exceptions, the e Nd (t) of þ1 to þ2 conform to an isotopically ‘mildly depleted’ source, typical for mafic TIB rocks and many Svecofennian rocks in the region. Reported values above e Nd (t) þ2.0 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. Copyright # 2008 John Wiley & Sons, Ltd. Received 24 January 2008; accepted 3 June 2008 KEY WORDS Transscandinavian Igneous Belt; mafic plutonic rock; Sr and Nd isotopes; mantle enrichment; continental arc 1. INTRODUCTION Precambrian mafic rocks have developed in various plate tectonic settings and from variable mantle types, and have often been modified by metamorphism and deformation. The nature of the subcontinental mantle and the geodynamic environment that persisted during the creation of crustal tracts can be assessed through the study of geochemistry and isotope geochemistry of mafic rocks in large igneous suites (cf. e.g. Faure 2001). Modern mafic volcanic and plutonic rocks form at constructive margins (mid-ocean ridge basalts (MORB)), oceanic intra-plate settings (ocean island basalts), intra-continental settings (continental flood basalts) and in destructive margins (island and continental arcs) (e.g. Wilson 1989). Mafic rocks in arc settings are mostly tholeiitic to calc-alkaline, and enriched in large ion lithophile elements (LILE), but relatively depleted in high-field strength elements (HFSE), often more depleted than MORB (e.g. Pearce 1982). Rocks in continental arcs show normally GEOLOGICAL JOURNAL Geol. J. 44: 241–279 (2009) Published online 30 July 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/gj.1133 * Correspondence to: H. Rutanen, Department of Geology and Mineralogy, A ˚ bo Akademi University, Domkyrkotorget 1, FIN-20500 Turku, Finland. E-mail: hrutanen@abo.fi y This article was published online on 30 July 2008. An error was subsequently identified. This notice is included in the online version to indicate that this has been corrected 7 November 2008. Copyright # 2008 John Wiley & Sons, Ltd.