Petrology and isotope geology of the Hunnedalen Monzonoritic Dyke Swarm, SW Norway: a possible late expression of Egersund Anorthosite magmatism

Abstract

The widest and northernmost dyke of the post-tectonic Late Proterozoic Hunnedalen Monzonoritic Dyke Swarm has been studied petrographically, chemically (major and trace element) and isotopically (Rb-Sr; Sm-Nd; K-Ar) toghether with its country rocks. Samples from the core of the dyke appear fresh and have a monzonoritic composition. Samples from within 1 m of the contacts appear severely altered, petrographically, chemically and isotopically, most propably during a very low-grade regional phase of metamorphism, M4, related to Early Palaeozoic burial and to Caledonian orogenesis. The unaltered central part the dyke is slightly inhomogeneous with some concentration of earlier formed minerals in the centre of the dyke (higher plagioclase content, higher Mg-ratio) due to flow differentiation. Trace element discrimination diagrams indicate a continental, within-plate setting. In altered samples the change in chemistry involved variable shifts in the contents of Rb and Sr, as well as in Sr-isotope ratios.