Metasedimentary rocks of the Helgeland Nappe Complex and S-type granitic rocks of the Bindal Batholith are exposed on the island of Ylvingen. The Ylvingen plutonic rocks are part of a larger system dominated by S-type granodiorite exposed within the Vega Archipelago. Petrographic evidence and previously published P–T estimates indicate that the Vega–Ylvingen intrusive complex is tilted to the south and west. The results of bulk-rock thermodynamic modeling suggest that the Ylvingen host rocks were metamorphosed to ~7–8 kbar and <550 °C along a clockwise P–T path. At ~475 Ma, these host rocks were intruded by S-type magmas and underwent minor contact metamorphism. The modeled P–T path for an intrusive diatexite sample suggests that decompression from ~7–8 kbar to <5 kbar was synmagmatic. These results indicate that crustal thickening, pluton emplacement, and exhumation occurred in rapid succession during the Ordovician Taconian orogeny. Petrographic observations and bulk rock compositions suggest that the granitic magmas are variably enriched in residual phases and accumulated plagioclase and biotite at the level of emplacement.
Geology of the island of Ylvingen, Nordland, Norway: Evidence for pre-Scandian (~475 Ma) exhumation in the Helgeland Nappe Complex