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The polymetaphoric Seve Nappe Complex (SNC) of the Scandinavian Caledonides contains sequences representing the passive margin between continenral Baltica and the lapetus Ocean. The Sarektjikki 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 complexes. The internal parts of the Sarektjikki Nappe escaped pecetrative deformation and metamorphism during Caledonian nappe transport and primary igneous relations are preserved. A sequence of nine dyke generations have been identified in a sheeted dyke complex in one well exposed area (Favorithallen). The alteration in this dyke complex was triggered by heat in connection with the dyke emplacement; overprint from Caledonian regional metarnorphism cannot be demonstrated. Neither the relative age of a dyke in the dyke complex, nor the contact metamorphism caused by the emplacement of subsequent dykes had any detectable effect on the chemical composition of the dykes.The chemical composition of the diabases thus reflects the composition of the magma. Xenoliths of diabase and various types of gabbro are not uncommon and represent wallrock sampling during intrusion. Irregular patches of static alteration reveal the presence of fluids (brines), probably derived from metaevaporites in the wallrock, during the period of dyke intrusion. Scapolite 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 It 74 Ma for the Sarek diabase, coinciding with the Vendian-Early Cambrian period of global continental break-up. Geophysical data from continent-ocean transitions at modern passive margins agree well with the structure of the Sarektjikki Nappe. I t is suggested that the Sarek dyke complexes were intruded during the initial stages of seafloor spreading in the Iapetus or pre-Iapetus Ocean.

In the Caledonide orogen of northern Sweden, the Seve Nappe Complex is dominated by rift facies sedimentary and mafic rocks derived from the Late Proterozoic Baltoscandian miogeocline and offshore-continent-Iapetus transition. Metamorphic breaks and structural inversions characterize the nappe complex. Within the Sarek Mountains, the Sarektjikki Nappe is composed of c. 600-Ma-old dolerites with subordinate screens of sedimentary rocks. These lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels. The Sarektjlkki Nappe is situated between eclogite-bearing nappes (Mikka and Tsakkok nappes) which underwent high-P metamorphism at c. 500Ma during westward subduction of the Baltoscandian margin. 40Ar/39Ar mineral ages of c. 520-500 Ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the Sarektjikki Nappe. Plateau ages of 500 Ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction. Cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c. 510-490 Ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets. Hornblende cooling ages of 470-460 Ma are recorded from massive dyke protoliths within the Sarektjlkkl Nappe. These are similar to ages reported from the Seve Nappe Complex in the central Scandinavian Caledonides. Probably these date imbrication and uplift related to Early Ordovician arrival of outboard terranes (e.g. island-arc sequences represented by structurally lower horizons of the Koli Nappes).Metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various Seve structural units are themselves internally imbricated, and were individually tectonically uplifted through argon closure temperatures during assembly of the Seve Nappe Complex. The cooling ages of 520-500 Ma recorded within Seve terranes and along terrane boundaries of the Sarek Mountains provide evidence of significant accretionary activity in the northern Scandinavian Caledonides in the Late Cambrian-Early Ordovician.