The Raudfjellet ophiolite fragment, Central Norwegian Caledonides: principal lithological and structural features (original) (raw)

The Raudfjellet complex, Nord-Trøndelag, displays several of the features of a classical ophiolite pseudostratigraphy. At its base is a spectacular ultramafite mylonite. Elements represented include: (1) Ultramafic rocks, mostly dunite with minor harzburgitic and websteritic intrusions as well as sporadic dunite-chromitite cumulates. The dunite is interpreted to represent a single large body of cumulate dunite formed between the petrological and seismic Mohos. (2) A mafic unit consisting mostly of mafic cumulates and massive metagabbro, with alternating mafic and ultramafic cumulates near the base. (3) Possible dolerite dykes, in the upper part of the mafic block, with sheeted dykes observed in one small area. No tonalitic differentiates or basaltic lavas have been found at Raudfjellet. An unusual, hydrothermal alteration zone, up to 60 m thick, occurs at the interface between the ultramafic and mafic blocks. This zone consists of talc and listwaenite (magnesite-quartz rock). The ophiolite complex is unconformably overlain by a polymict conglomerate consisting mostly of mafic and ultramafic detritus. The basal part of the ophiolite complex is a 150 m-thick zone of ultramafic mylonites and ultramylonites with a WNW-ESE-trending mineral/stretching lineation.The olivine + pyroxene paragenesis of the mylonites is interpreted to relate to the original, high-T obduction upon rocks of the Skjøtingen (Seve) Nappe. Although, at present, there are no faunal or isotopic age constraints, based on regional correlations for the eastern Trondheim Region and into the Otta-Vågåmo district south of Dombås, it is argued that the Raudfjellet ophiolite is likely to be of Cambrian to earliest Ordovician age, and its obduction Early Ordovician. Subsequently, the complex and adjacent units were involved in the Siluro-Devonian Scandian orogeny and its ensuing extensional deformation.