Petrology and tectonic setting of relatively low temperature eclogites and related rocks in the Dalsfjord area, Sunnfjord, West Norway.

Abstract

An area to the south of Dalsfjord has been mapped and four lithological units differentiated. These are (from the top downwards) the Vardheia unit, consisting of heterogeneous paragneisses; the Flekke unit, consisting of metabasic, ultrabasic and oxide-rich rocks, frequently with corona structures; the Gjorlanger unit, consisting of dioritic to granodioritic orthogneisses with common metabasic bodies and websterites, and the Basal Gneisses, with pink granodioritic to monzonitic orthogneisses, paragneisses and pegmatites. The latter is separated from the overlying unit by a zone of protomylonites. Eclogites are found in all the units. Gradational or intrusive relationships show that all the units are mutually autochthonous. Many eclogites had demonstrable low-pressure igneous protoliths and/or intrusive relationships with the gneisses, indicating a crustal, eclogite-facies metamorphism of all the lithologies mapped. Relics of early granulite-facies assemblages occur in most lithologies. Bulk-rock geochemistry of Gjorlanger unit lithologies indicates calc-alkaline affinities, resembling magmatic arc rocks but possibly modified during granulite-facies metamorphism. Associated eclogites have been metasomatically altered but retain some tholeiitic characteristics. Websterites are tentatively related to the orthopyroxene eclogites of More and Romsdal. The Flekke unit rocks have affinities with some layered basic intrusions typical of mid-Proterozoic anorthosite suites.