RETROGRADE FLUID INFILTRATION IN THE HIGH-GRADE MODUM COMPLEX, SOUTH NORWAY - EVIDENCE FOR AGE, SOURCE AND REE MOBILITY

The high-grade metamorphic basement of the Modum Complex, South Norway , exhibits retrogradation and alteration due to late stage fluid infil tration. Extensive alteration zones of albite- and calcite-rich veinin g occur especially within and around numerous metagabbros. The gabbros , intruded at 1224 +/- 15 Ma, are now partly altered to amphibolites d ue to the subsequent high-grade metamorphism. Two generations of albit e-rich rocks have been recognized: (1) a fine-grained, foliated type: (2) a coarse-grained, crosscutting type. Both types show a typical gre enschist facies mineral assemblage; albite +/- actinolite +/- chlorite +/- talc. The calcite veins/dykes represent a younger generation of v eins than both albite-rich types. U-Pb data for sphene of type (1) yie lded an age of 1080 +/- 3 Ma, determining a point on the retrograde P- T-t path of the Modum Complex. Increasing albitisation of the metagabb ros leads to a decrease in epsilon(Nd) and an increase in epsilon(Sr). Albite- and calcite-rich samples show negative epsilon(Nd) and positi ve epsilon(Sr), suggesting that fluids which interacted with the metag abbros originated from a crustal reservoir. The Nd and Sr isotopic dat a show disequilibrium at the microscale as well as at the macroscale. Negative Sm-Nd model ages of the albite-rich rocks demonstrate that ra re-earth elements (REEs) were mobile and fractionated during albitisat ion.