Geochemistry of eclogites from Western Norway: implications from high-precision whole-rock and rutile analyses
Wilkinson, Darren James
The Western Gneiss Region (WGR) in Norway is home to some of the world’s most spectacular exposures of high pressure (HP) and ultrahigh pressure (UHP) eclogites. Despite extensive petrological studies into their pressure, temperature and time (PTt) histories, relatively few have reported on their trace element compositions. Such data can be used to supplement our understanding of the provenance and history of Norwegian eclogites, as well as to further our understanding of trace element fluxes during HP to UHP metamorphism in subduction zone settings. In order to address this shortfall in data availability, the first step was to investigate and apply the best dissolution techniques for preparing eclogite samples for chemical analysis. Eclogites commonly contain up to a few weight percent rutile (TiO2), which is known to be an important host for a variety of major and trace elements (e.g. Ti, Nb and Ta). However, typical rock digestion procedures are incapable of dissolving rutile, and thus may lead to inaccurate measurements. It was found that that total dissolution of rutile can be achieved by dissolving samples in sealed pressure vessels at increased pressures and temperatures, ultimately leading to greatly increased data accuracy for analyses of any rutile-bearing lithology. The solutions were analysed by standard ICP-MS techniques and the results compared to analyses of powders by XRF spectrometry. Our high-accuracy and high-precision data were subjected to immobile trace element discriminant analyses that suggested eclogites belonged to three broad geochemical groups: eclogites with mid ocean ridge Basalt (MORB)-like composition; eclogites with arc-like composition; and eclogites with geochemical signatures significantly perturbed by metamorphism. The geochemistry of eclogites in the first two groups are shown to likely reflect protolith composition, and as such we used model protolith compositions to calculate estimated element mobilities (EMMs) for those elements considered relatively mobile during metamorphism. It was not possible to determine protoliths for eclogites in the third category using trace elements alone. Finally, the trace element geochemistry of a large number of separated eclogite-hosted rutiles was studied. The data collected were used to demonstrate that rutile contains significant amounts of the whole-rock’s high field strength element (HFSE) budget, and may exert significant control on the HFSE composition of passing hydrothermal fluids. Furthermore, Zr-in-rutile thermometry (ZRT) was applied to separated rutiles. This temperature information was used to better our understanding of the thermal history of the WGR, as well as to create a map of eclogite temperatures in the Nordfjord-Statlandet area. This high-resolution thermal map of arguably the most important area of the WGR, supports current interpretations that during the Caledonian Orogeny the leading edge of the Baltica plate was consumed in a northwest to north-northwest-dipping subduction zone under Laurentia. Furthermore, isotherms on this map mimic several major fold hinges in the region rather well, thus providing support to the hypothesis that such structures were most likely formed during the collapse of the Scandinavian Caledonian Orogen after the peak metamorphism of most eclogites.