Mafic dykes in the Lkesdal Nappe at Sørli, Central Norwegain Caledonides: geochemistry and palaeotetonic implications

Abstract

Mafic dykes intruding feldspathic metasandstones of the Leksdal (Sarv) Nappe in the Sørli district of Nord-Trøndelag, Central Norway, vary from weakly foliated, porphyritic types to more common, strongly schistose, sparsely phyric to aphyric biotite-amphibole or biotite schists. Analytical data, however, show that the dykes are chemically fairly homogeneous. The geochemical signature of the dykes is transitional, i.e.between tholeitic and alcalic, and points indis putedly to a within-plate setting. Rare-earth element data show clear LREE- enriched, midly alkaline tendencies. The geochemistry of the Sørli dykes, based on trace and rare-earth element abundances, is, in fact, quite similar to that of the \"mildly alkaline\" group of Metadolerites distinguished by earlier workers from diverse parts of the Sarv Nappe, and equivalent nappes, over wide areas of the Scandinavian Caledonides. Although the emplacement age of the Sørli dykes is not known it is inferred to be Vendian, based on data available from comparable dolerite dykes in other parts of the Sarv \u00E4nd immediately overlying Seve Nappes. Dolerite dykes of T- and N-MORB affinities in some of the higher thrust-sheets of the Baltoscandian margin successions farther north have yielded Mid to Late Vendian ages, and are inferred to have intruded immediately prior to the actual inception of lapetus sea-floor spreading. The LREE-enriched, mildly alkaline dykes from Sørli, cutting a fluvial metasedimentary succession at the Middle Allochthon Tectonostratigraphic level, are thus assumed to have intruded in a more inboard continental setting, and most likely as a just slightly earlier stage of latest Neoproterozoic crustal extension and rifting, perhaps in Early Vendian time.