The Early to Late Ordovician rock record of the Oppdal area, Scandinavian Caledonides: Explosive volcanism, element recycling and basin infill during closure of the Iapetus Ocean

Abstract

This thesis describes the Ordovician rocks of the Oppdal area, central Norwegian Caledonides, which record the evolution of a section across the Laurentian margin. By fieldwork, geochronology and geochemistry a revised geological map has been made, and the processes active during rock formation have been examined. A ca 475-470 Ma succession of sandstone and MORB-like basalt coexist with pyroclastic rocks with a conflicting geochemical signature; extreme enrichment in elements associated with evolved continental crust combined with a high content of elements which point to a mantle source and limited fractionation. We suggest that subducted continental material partially melted and reacted with the mantle to form anomalous mantle domains which sourced the enriched rocks, whereas normal mantle sourced the MORB, all in an extensional setting possibly related to the death of a subduction zone close to Laurentia. These rocks are unconformably overlain by a volcanosedimentary unit formed in a continental arc setting >435 Ma, before the collision between Laurentia and Baltica when units described herein were thrusted onto Baltica along with the rest of the western Trondheim Nappe Complex.