Rb-Sr study of rapakivi granite and augen gneiss of the Risberget Nappe, Oppdal, Norway.

Abstract

Rb-Sr dating of rapakivi granite and associated augen gneiss of the Risberget Nappe suggests that the rocks are post-Svecokarelian intrusions that were strongly foliated during Caledonian orogeny. Microstructures of the augen indicate igneous crystallization followed by cataclastic and ductile deformation, and the augen are interpreted as porphyroclasts rather than metasomatic porphyroblasts. Rb-Sr data from 56 presumably co-magmatic rapakivi rocks yield an errorchron date of 1618\u00B144 Ma with low initial Sr ratio (.7021\u00B15). The least deformed rapakivi samples are isotopically disturbed and partially reset, while an isochron from an ultramylonite within the Risberget Nappe yields a date within the range of error of the suggested 1618 Ma intrusive date. The dating of a younger pegmatite intrusion (1163\u00B180 Ma) suggests that the rocks were subjected to a Sveconorwegian event. Minerals of unfoliated rapakivi granite are only partially reset to Sveconorwegian and Caledonian dates, while minerals of foliated rocks yield Caledonian isochrons.