The Malene metasedimentary rocks on Rypeø, and their relationship to Amîtsoq gneisses

Abstract

Two groups of Malene metasedimentary rocks on Rypeø, a small island south of Godthåb, have been studied in detail in order to evaluate their origin and relationship to older, adjacent Amîtsoq gneiss. Field observation shows that the metasediments range from massive to finely layered, and include an unusual pod-rock ('pseudoconglomerate'), which probably represents a series of deformed sedimentary layers (or lenses) of contrasting competence. Petrographic study indicates that the metasediments are quartz-rich, contain abundant plagioclase and biotite, and small but variable amounts ofsillimanite, muscovite, microcline and garnet. Rare earth elements (REE) in samples of four metasedimentary lithologies are similar to some published analyses of 'tonalitic' Amîtsoq gneiss. These observations suggest that the protolith of the Malene metasediments on Rypeø was dominated by sandstone type lithologies which evolved in a near-shore tidal to fiuviatile environment. These sediments were probably derived by weathering and erosion of Amitsoq gneiss and deposited unconformably on such a gneissic basement. Petrological study indicates that the Rypeø metasediments were metamorphosed to the beginning of Muse + Qtz breakdown (Musc-Sill-Kfsp transition zone), and locally underwent minor amounts of melting. The proposed original unconformable contact relationships, coupled with high-grade polymetamorphism - not only of the supracrustal rocks but also of Amitsoq gneiss - indicates substantial vertical crustal movement, perhaps as much as 75 km since the time of formation of Amitsoq gneiss at - 3750 Ma.