Geochemical signature of the Egersund basaltic dyke swarm, SW Norway, in the context of late-Neoproterozoic opening of the Iapetus Ocean

The Egersund basaltic dyke swarm is made up of 11 ESE-WNW-trending dykes in the Proterozoic crystalline basement of southwest Norway. Two types of dyk e occur: porphyritic dykes with plagioclase phenocrysts and aphyric dykes. Limited magmatic differentiation occurred in the dykes during shallow level intrusion. The porphyritic dykes and one of the aphyric dykes have subalka line to mildly alkaline compositions, whereas the other aphyric dykes have alkaline compositions. The most primitive magmas occur in the porphyritic d ykes; they have Mg numbers of 56-61, TiO2 contents of 2.0-2.1% and enriched incompatible trace element contents. Initial isotopic compositions, Sr-i a nd epsilon(Nd)(i), are 0.7034-0.7039 and +2.0+3.1 respectively. The most ev olved magmas in alkaline dykes are SiO2-poor (45-46%) ferrobasalts, very en riched in TiO2 (3.2-3.1%), P2O5 (2.0-2.5%) and incompatible trace elements. They have Sr-i of 0.7058-0.7060 and epsilon(Nd)(i) of +1.0. The compositio ns of the most primitive magmas and minerals suggest that the porphyritic d ykes tapped a magma chamber situated at the crust-mantle boundary, at ca. 1 0 kbar. It is shown that one of the aphyric dykes is possibly related to th e porphyritic dyke magma by a fractional crystallization process accompanie d by limited crustal assimilation at this pressure. The alkaline dykes repr esent a distinct suite with a distinct mantle source. The 616 +/- 3 Ma Eger sund Swarm is placed in the geotectonic context of late-Neoproterozoic rift ing leading to opening of the Iapetus Ocean. At the Baltoscandian passive m argin of Baltica. there is a first order 'oceanwards' decrease in incompati ble trace element contents and Sr-i and an increase of epsilon(Nd)(i) value s of basaltic dyke swarms, from the Egersund Swarm in the continental basem ent, to the sheeted dyke complexes situated at the continent-ocean transiti on. This geochemical trend is interpreted as a change from high-pressure pa rtial melting of garnet peridotite with residual garnet in a mildly deplete d mantle. to lower-pressure partial melting of a depleted mantle.