OXYGEN AND CARBON ISOTOPIC CONSTRAINTS ON THE DEVELOPMENT OF ECLOGITES, HOLSNOY, NORWAY

Eclogite formation on the island of Hoisnoy required the addition of w ater to anhydrous granulite-facies protoliths. In order to assess this process, oxygen and carbon isotope ratios of whole rock powders and m ineral separates from eclogites and granulites have been measured. Who le rock oxygen isotope ratios range from 7.3 to 6.0 parts per thousand SMOW in granulites (average = 6.38 parts per thousand) and 7.2 to 6.1 parts per thousand in eclogites (average = 6.55 parts per thousand). Field relations permit identification of the granulite protolith of ec logites, Oxygen isotope measurements show shifts of up to 0.5 parts pe r thousand between some eclogites compared to their corresponding gran ulite protoliths, indicating open system and locally heterogeneous flu id behavior. Mineral pair fractionations in the eclogites show disequi librium, are incompatible with slow cooling and diffusive exchange bet ween phases, and suggest that open system fluid movement continued aft er eclogite-facies metamorphism. Carbonate is also present in some of the eclogites as a primary mineral (dolomite) and as part of a retrogr ade assemblage (calcite), Textural evidence suggests that carbonate fo rmation occurred during and after eclogite formation, however all meas ured carbonate is out of isotopic equilibrium with eclogite facies min erals, due to the influx of retrograde fluids. Massive calcite marble pods, containing amphibolite facies calc-silicate minerals, have avera ge delta(18)O of 9.5 +/- 0.6 parts per thousand, while calcite in retr ograded eclogites has delta(18)O 17.7 +/- 2.7 parts per thousand. The delta(13)C (= -4 +/- 0.8 parts per thousand) is indistinguishable betw een these two groups, Both whole rock and carbonate stable isotope dat a are interpreted as indicating a continued history of fluid infiltrat ion during and after peak eclogite facies metamorphism. The most proba ble source of fluids are from dewatered sediments tectonically juxtapo sed during the Caledonian orogeny.