Element mobility during Sveconorwegian metamorphism in the Modum complex, South Norway.

Abstract

The Modum complex within the Kongsberg sector of southern Norway is dominated by Early- to Mid-Proterozoic quartz-rich metasedimentary rocks that have been metamorphosed to sillimanite grade during the Kongsbergian orogeny (ca. 1500 Ma). At the onset of the Sveconorwegian (Grenvillian) metamorphism, multiple gabbroic units were emplaced at mid-crustal levels (less than 8 kbar) along north-south deformation zones. Alteration of the metagabbros to amphibolite was nonisochemical, with increases in the concentration of Na, Fe, P and high field strength elements (HFSE) (i.e. Ti, rare earth elements (REE), U, Th, Nb), and decreases in the concentration of Al, Mg, and Ca. The consistent and significant increase in HFSE is not observed in other studies of amphibolitization from southwest Scandinavia. The HFSE were either mobile and enriched or alteration to amphibolite was accompanied by a loss of mass in the order of 60%. Hornblende and scapolite in metagabbros and amphibolites contain very high concentrations of Cl (ranging from 0.76 to 2.03 wt. % and 2.85 to 3.34 wt. % for the respective minerals). Unlike the amphibolites, amphiboles or biotite in the albitized rocks do not contain abundant Cl, which may be a result of the ionized character of Cl in fluids at greenschist grade temperatures. Albitites display relative increases in the concentration of major elements Na and Si and decreases in Mg, Fe and Ca. The trace element concentrations are similar to amphibolites, with the exception of Sr and Cl, which occur at lower concentrations in the albitites. North-south trending bands of disseminated Fe-sulphides, in the order of ten metres wide, occur within the quartz-rich metasedimentary rocks of the Kongsberg sector; historically known as fahlbands. Chemical similarities between the amphibolite and the albitized rocks suggest that one evolving fluid may be responsible for both. (Abstract shortened by UMI.)