Elemental mobilities produced by low-grade metamorphic events. A case study from the Proterozoic supracrustals of southern Norway

Abstract

Abstract Within the Sveconorwegian Province of southern Norway, the Telemark supracrustals represent the only low-grade metamorphic assemblages. These supracrustals are composed of metabasalts, acid metavolcanics and metasediments which have been subjected to three metamorphic events (an early “burial-type”, greenschist facies regional and thermal metamorphism). Within the supracrustal piles, each of the events is heterogeneously preserved and has caused variable degrees of elemental redistribution. Burial metamorphism is a localised phenomenon involving high fluid/rock ratios which has produced extremely anomalous compositions. In contrast, the regional metamorphism (greenschist facies) has completely obliterated the primary textures in the basaltic formations and has also produced a large degree of scatter in the more mobile elements. The late-thermal metamorphism is associated with the intrusion of late-kinematic granites (∼950 Ma). Within the thermal aureoles, the metabasalts contain randomly orientated porphyroblasts of biotite, hornblende and opaque phases. In these basalts the more immobile elements (i.e., Zr, Nb, Ti, Y) have been mobilised to varying degrees. This conclusion is of major significance since these are the elements which are frequently used to discriminate ancient tectonic environments. If these elements are used for this purpose in the present case spurious conclusions regarding the genesis and tectonic setting of the volcanics may be reached. From this study it is apparent that low-grade metamorphic events can have a profound effect upon whole-rock geochemical compositions and thus ancient volcanic suites must be effectively screened prior to their use in tectonic modelling.