CONTRASTING MAGMATIC ARCS IN THE PALAEOPROTEROZOIC OF THE SOUTH-WESTERN BALTIC SHIELD

Following the Svecofennian are accretionary growth and extensive grani toid magmatism in the Transscandinavian Igneous Belt (TIB), new crusta l growth occurred west of the TIE to form the Gothian orogen. An early stage is manifested by 1.69-1.65 Ga subduction-related magmatism in t he Atran Terrane. A second stage, forming an 140 km wide segment east of the Permian Oslo Rift, is recorded by three 1.66-1.59 Ga metamorpho sed volcano-sedimentary units, exposed in the Horred, Amal and Stora L e-Marstrand formations in the Idefjorden Terrane. The 1.66 Ga Horred F ormation is dominated by felsic volcanics and has geochemical signatur es indicative of formation in an island are setting. In contrast, the lithologically similar volcanic sequences in the 1.61 Ga Amal Formatio n have geochemical signatures consistent with a continental-margin set ting. The 1.60-1.59 Ga Stora Le-Marstrand Formation is dominated by gr eywacke-type metasediments with subordinate metabasalts. These volcani cs have markedly primitive trace element signatures and depleted Nd is otopic compositions, all consistent with their derivation in an oceani c island are setting. The sediments document two provenances: a distal continental source and one with Nd isotopic compositions similar to t he SLM volcanism. Many of the metasediments in the Stora Le-Marstrand formation have chemical signatures consistent with derivation from con tinental crust, suggesting that this volcanic are developed in the vic inity of a continental massif, possibly in a setting similar to the Ph ilippine Sea. Accretion of the Horred and Stora Le-Marstrand are syste ms occurred prior to 1.61 and 1.59 Ga, respectively, and was followed by voluminous, ca 1.59 Ga calc-alkaline magmatism. (C) 1998 Elsevier S cience B.V. All rights reserved.