Are stable isotopic trends in amphibolite to granulite facies transitions metamorphic or diagenetic?; an answer for the Arendal area (Bamble Sector, southeastern Norway) from mid-Proterozoic carbon-bearing rocks

Abstract

The isotopic delta(13)C and delta(18)O compositions of primary (marbles) and secondary (veins) calcites and primary (fahlbands and metapelites) and secondary (veins) graphites sampled across the amphibolite to granulite facies transition in the Bamble Sector (southeastern Norway), have been investigated. The primary calcites display a regional trend in a delta(13)C/delta(18)O-plot which is the reverse of the expected trend. The granulite grade primary calcites essentially preserved most of their initial sedimentary compositions, whereas the amphibolite grade calcites have increasingly lower delta(13)C values. The delta(18)O compositions of amphibolite grade primary calcites and of some vein types appear to be buffered by the O-reservoir of the country rock. The delta(13)C: of graphite of different types is specific for a certain lithology but not spatially related to the metamorphic zoning. Reaction progress calculations and Rayleigh fractionation modelling prove that prograde metamorphic decarbonation reactions alone cannot be held responsible for the isotopic trend in the marbles. The reversed isotopic trend in the marbles themselves combined with the lack of any trend in the primary graphites is not compatible with pervasive flushing with a homogeneous, mantle derived CO2-rich fluid at granulite facies metamorphism. The marble trend and the constant delta(13)C composition of primary graphite throughout the metamorphic zoning are best explained by low grade processes that did affect the stable isotopic compositions of the marbles but left the graphites unaffected. The processes may even occur under diagenetic conditions. Later amphibolite-granulite facies metamorphism preserved the trend despite the production of calcsilicates. Subsequent retrograde hydrothermal action produced graphite-quartz veins and several types of calcite veins, without resetting stable isotope systematics on a regional scale.