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 3 I3 C and 3 18 0 compositions of primary (marbles) and secondary (veins) calcites and primary (fahlbands and metapelites) and secondary (veins) graphites sampled across the am­ phibolite to granulite facies transition in the Bamble Sector (southeast­ ern Norway), have been investigated. The primary calcites display a regional trend in a 3 I3C/3I80_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 3 I3 C values. The 3 18 0 composi­ tions of amphibolite grade prImary calcites and of some vein tlpes appear to be buffered by the O-reservoir of the country rock. The 3 1 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 model­ ling 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 3 I3 C composition of J?rimary graphite throughout the metamorphic zoning are best explamed 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 hydro­ thermal actIOn produced graphite-quartz veins and several types of calcite veins, without resetting stable isotope systematics on a regional scale.