Interaction between fluid flow, fracturing and mineral growth during eclogitization, an example from the Sunnfjord area, Western Gneiss Region, Norway

Abstract

In Sunnfjord, Western Gneiss Region of south Norway, a Proterozoic layered gabbro complex displays various degrees of transformation to eclogite. In the unreacted parts, layering in the gabbro is defined by modal variations of plagioclase, olivine, pyroxenes and minor Fe–Ti oxide and spinel. Coronitic and foliated eclogite formed from the gabbro by hydration reactions at T=510–620°C, causing a volume decrease of c. 13%. In the coronitic eclogite, the mafic magmatic phases are replaced by aggregates of omphacite, barroisite, tremolite, talc and rutile, whereas the plagioclase domains are pseudomorphed by omphacite, barroisite, clinozoisite, kyanite, paragonite and garnet. The felsic and mafic domains are separated by a garnet rim up to 5-mm thick. Garnet was also formed along dilational veins connecting and radiating out of coronas, including the same eclogite facies minerals. In addition, microfractures filled by amphibole and omphacite cut through the corona and vein garnet, oriented perpendicular to the garnet layering. The transformation of dry gabbro to eclogite with hydrous minerals requires supply of water. The timing of metamorphic reactions is therefore dependent on the timing of fluid introduction. The inclusion pattern of garnet in the Holt–Tyssedalsvatnet metagabbro complex indicates that transformation started under eclogite facies conditions. Brittle deformation, in form of fractures allowing infiltration of fluids and mobilisation of elements, is shown to be the most important process initiating transformation. Brittle deformation is thereby active in deep crustal levels corresponding to eclogite facies conditions. Fracturing is interpreted as having been caused by a combination of high fluid pressure, volume changes during mineral transformations and external stresses. Ductile deformation started after the initial metamorphic transformation. Garnet chemistry and zoning pattern are controlled by the chemistry of the growth place, the fluid influx and element supply. Abrupt chemical variations across corona garnet may have been formed by an interaction between rapid growth and transport along microfractures between plagioclase and mafic domain. The metamorphic transition proceeded rapidly. The transformation of the deep crust to eclogite can be viewed as a dynamic process where the volume changes result in fracturing, and these fractures funnelling fluid and element enhancing mineral reaction. The densification causes further fracturing, indicating that the process is self-driven provided that fluid is available.