Serpentinites and low-K island arc meta-volcanic rocks in the Lower Köli Nappe of the central Scandinavian Caledonides: Late Cambrian–early Ordovician serpentinite mud volcanoes in a forearc basin?

Abstract

The late Cambrian to early Ordovician meta-volcanic rocks of the Caledonian Lower Köli Nappe consist mainly of tholeiitic low-K island arc basalts, andesites, and rhyodacites. The dominance of rhyodacites in this meta-volcanic succession raises the question on whether fractional crystallization or partial melting were involved in their origin. Low Mg#, low Cr and Ni contents and compositional trends imply at least two stages of fractional crystallization for the origin of the meta-volcanic rocks. Sedimentary-hosted serpentinites occur stratigraphically below and above the meta-volcanic rocks raising the question on their origin. Geochemical data indicate strongly depleted harzburgitic–dunitic peridotite as precursor rocks of the serpentinites. Unusually high contents of As, Sb, Pb in these serpentinites are not in agreement with a depleted mantle geochemistry, but indicate enrichment by fluids from the subducted slab during serpentinization in the mantle wedge. The massive, detrital, and in places fossiliferous serpentinite bodies within the sedimentary host-rocks point to former serpentinite mud volcanoes within a non-accretionary forearc. Therefore it is suggested that the highly fractionated volcanic rocks were emplaced as lava flows and shallow intrusions in sedimentary forearc successions implying that the Lower Köli Nappe was part of a much larger trench–arc complex involving at least the immediate hanging wall Middle Köli Stikke Nappe.