THE BALTICA-LAPETUS PASSIVE MARGIN DYKE COMPLEX IN THE SAREKTJAKKA-NAPPE, NORTHERN SWEDISH CALEDONIDES

The polymetaphoric Seve Nappe Complex (SNC) of the Scandinavian Caledo nides contains sequences representing the passive margin between conti nental Baltica and the Iapetus Ocean. The Sarektjakka Nappe in the SNC consists of sedimentary rocks intruded by mafic dykes that constitute 70-80 per cent of the nappe and frequently form sheeted dyke complexe s. The internal parts of the Sarektjakka Nappe escaped penetrative def ormation and metamorphism during Caledonian nappe transport and primar y igneous relations are preserved. PI sequence of nine dyke generation s have been identified in a sheeted dyke complex in one well exposed a rea (Favorithallen). The alteration in this dyke complex was triggered by heat in connection with the dyke emplacement; overprint from Caled onian regional metamorphism cannot be demonstrated. Neither the relati ve age of a dyke in the dyke complex, nor the contact metamorphism cau sed by the emplacement of subsequent dykes had any detectable effect o n the chemical composition of the dykes. The chemical composition of t he diabases thus reflects the composition of the magma. Xenoliths of d iabase and various types of gabbro are not uncommon and represent wall rock sampling during intrusion. Irregular patches of static alteration reveal the presence of fluids (brines), probably derived from metaeva porites in the wallrock, during the period of dyke intrusion. Scapolit e rich dioritic-quartz dioritic inclusions and veins formed by partial or complete melting of wallrock xenoliths. A Sm-Nd age determination indicates a crystallization age of 573 +/- 74 Ma for the Sarek diabase , coinciding with the Vendian-Early Cambrian period of global continen tal break-up. Geophysical data from continent-ocean transitions at mod ern passive margins agree well with the structure of the Sarektjakka N appe. It is suggested that the Sarek dyke complexes were intruded duri ng the initial stages of seafloor spreading in the Iapetus or pre-Iape tus Ocean.