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

In Sunnfjord, Western Gneiss Region of south Norway, a Proterozoic layered gabbro complex displays various degrees of transformation to eclogite. In t he unreacted parts, layering in the gabbro is defined by modal variations o f plagioclase, olivine, pyroxenes and minor Fe-Ti oxide and spinel. Coronit ic and foliated eclogite formed from the gabbro by hydration reactions at T = 510-620 degreesC, 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 a re pseudomorphed by omphacite, barroisite, clinozoisite, kyanite, paragonit e 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 an d 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 dependen t on the timing of fluid introduction. The inclusion pattern of garnet in t he Holt-Tyssedalsvatnet metagabbro complex indicates that transformation st arted under eclogite facies conditions. Brittle deformation, in form of fra ctures allowing infiltration of fluids and mobilisation of elements, is sho wn to be the most important process initiating transformation. Brittle defo rmation is thereby active in deep crustal levels corresponding to eclogite facies conditions. Fracturing is interpreted as having been caused by a com bination of high fluid pressure, volume changes during mineral transformati ons and external stresses. Ductile deformation started after the initial me tamorphic transformation. Garnet chemistry and zoning pattern are controlle d by the chemistry of the growth place, the fluid influx and element supply . Abrupt chemical variations across corona garnet may have been formed by a n interaction between rapid growth and transport along microfractures betwe en plagioclase and mafic domain. The metamorphic transition proceeded rapid ly. The transformation of the deep crust to eclogite can be viewed as a dyn amic process where the volume changes result in fracturing, and these fract ures funnelling fluid and element enhancing mineral reaction. The densifica tion causes further fracturing, indicating that the process is self-driven provided that fluid is available. (C) 2001 Elsevier Science B.V. All rights reserved.