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The Baltica–lapetus passive margin dyke complex in the Sarektjåkkå Nappe, northern Swedish Caledonides
Author(s) -
Svenningsen Olaf M.
Publication year - 1994
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350290403
Subject(s) - nappe , geology , metamorphism , baltica , geochemistry , igneous rock , mafic , ordovician , paleontology , tectonics
The polymetaphoric Seve Nappe Complex (SNC) of the Scandinavian Caledonides contains sequences representing the passive margin between continental Baltica and the Iapetus Ocean. The Sarektjåkkå 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 Sarektjåkkå Nappe escaped penetrative 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 (Favorithällen). The alteration in this dyke complex was triggered by heat in connection with the dyke emplacement; overprint from Caledonian regional metamorphism 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 ± 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 Sarektjåkkå Nappe. It is suggested that the Sarek dyke complexes were intruded during the initial stages of seafloor spreading in the Iapetus or pre‐Iapetus Ocean.