FLUID TRANSFER IN ANORTHOSITES AND RELATED ROCKS FROM ROGALAND (SOUTHWEST NORWAY) - EVIDENCE FROM STABLE ISOTOPES

C and H contents and C, H and 0 isotopic compositions have been measur ed in sixteen samples of the Ana-Sira massif-type anorthosite (Rogalan d, Southwest Norway) and in 30 surrounding rocks, including the metamo rphic envelope and magmatic intrusions related to the anorthosite empl acement. Oxygen isotopic compositions of the Ana-Sira anorthositic mas sif (deltaO-18 = 6.1 +/- 0.3 parts per thousand) and the Tellnes orebo dy (deltaO-18 = 5.2 +/- 0.20 parts per thousand) are consistent with a pure mantle origin and show no evidence for exchange with hydrotherma l fluids. The associated rocks (norites to charnockites) have deltaO-1 8 values (5.2 +/- 0.9 parts per thousand) similar to uncontaminated ma ntle-derived magmas. The H2O content of the rocks (magmatic or metamor phic) is lower than 0.54%, suggesting the preservation of original gra nulite facies conditions during cooling and uplift. Due to their cumul ate 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 int rusions. Anorthosites and gneisses close to the contacts (less than 50 0 m) with later intrusions show an increase in their carbon content (u p to 700 ppm C), and this carbon is mainly in the form of interstitial carbonate. The carbon distribution observed at the contact between la ter intrusions and. anorthosites as well as gneisses indicates that so me CO2 has fluxed from the later magmas over a distance of a few hundr ed metres. This suggests only minimal lateral propagation of the fluid s by diffusion or infiltration. Tentative carbon budgets suggest that most of the volatiles were transferred upwards into the overlying cont inental crust.