Fluid transfer in anorthosites and related rocks from Rogaland (Southwest Norway): Evidence from stable isotopes

Abstract

C and H contents and C, H and O isotopic compositions have been measured in sixteen samples of the Åna-Sira massif-type anorthosite (Rogaland, Southwest Norway) and in 30 surrounding rocks, including the metamorphic envelope and magmatic intrusions related to the anorthosite emplacement. Oxygen isotopic compositions of the Åna-Sira anorthositic massif ( δ 18 O = 6.1 ± 0.3‰ ) and the Tellnes orebody ( δ 18 O = 5.2 ± 0.2‰ ) are consistent with a pure mantle origin and show no evidence for exchange with hydrothermal fluids. The associated rocks (norites to charnockites) have δ 18O values ( 5.2 ± 0.9‰) similar to uncontaminated mantle-derived magmas. The H 2O content of the rocks (magmatic or metamorphic) is lower than 0.54%, suggesting the preservation of original granulite facies conditions during cooling and uplift. Due to their cumulate origin, the anorthosites are very poor in carbon (bulk C < 50 ppm), the volatiles having been concentrated in the residual melt. Higher carbon concentrations (up to 1484 ppm) are found in the associated intrusions. Anorthosites and gneisses close to the contacts (less than 500 m) with later intrusions show an increase in their carbon content (up to 700 ppm C), and this carbon is mainly in the form of interstitial carbonate. The carbon distribution observed at the contact between later intrusions and anorthosites as well as gneisses indicates that some CO 2 has fluxed from the later magmas over a distance of a few hundred metres. This suggests only minimal lateral propagation of the fluids by diffusion or infiltration. Tentative carbon budgets suggest that most of the volatiles were transferred upwards into the overlying continental crust.