Geochemistry and tectonic significance of silicic island-arc volcanic rocks from the Lower Koli Nappe, north-central Swedish Caledonides (Kultsjon area)
Grimmer, Jens Carsten; Greiling, Reinhard O.
The late Cambrian to early Ordovician Ankarede Volcanite Formation of the Lower K6li nappe (=Bj6rkvattnet Nappe) in northernmost Iiimtland and southwestern Vasterbotten is over- and underlain by phyllites, conglomerates, carbonates, and minor serpentinites. Geochemical analysis identifies the meta-volcanic rocks as low-K island-arc basalts, andesites, dacites, and rhyodacites. Sub-chondriticZr/Hf, Nb/Yb, Nb/Ta, low HFSE content and flat REE+Y patterns indicate a depleted mantle source and high degrees of partial melting. Low Mg#, Cr, Ni contents as well as compositional evolution imply at least two stages of fractionation for the origin of relatively large proportions of silicic volcanic rocks. Massive and detrital serpentinite bodies within the associated sedimentary rocks point to a forearc setting. It is thus suggested that these primitive island-arc volcanic rocks were emplaced in forearc sediments as submarine lava flows and shallow intrusions. The volcanic rocks pinch out towards east. Therefore, the easternmost occurrences could define the volcanic front of a largely submerged late Cambrian to early Ordovician intra-oceanic island arc above a westerly dipping subductionzone.