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.
Highlights
McGregor (1973) first recognized the regional lithostratigraphic units in this area and posed two alternative models: (a) the Amitsoq gneisses constitute a basement on which younger supracrustal rocks were deposited, with the original relationships obscured by later deformation; (b) the gneisses and supracrustals formed in separate regions of the Earth and were juxtaposed by later thrusting
The most simple interpretation of the Malene metasedimentary rocks on Rypeø is that they are dominated by material derived locally from Amitsoq gneiss, which is consistent with the geological observations presented previously
The data and observations presented in this paper lend credence to the hypothesis that Malene metasediments were deposited unconformably on a basement of older Amitsoq gneiss
Summary
One of the more intriguing and controversial problems remaining in the Archaean craton of West Greenland concerns the relationship between the c. 3750 Ma Amitsoq orthogneisses and the > 3000 Ma Malene supracrustals. McGregor (1973) first recognized the regional lithostratigraphic units in this area and posed two alternative models: (a) the Amitsoq gneisses constitute a basement on which younger supracrustal rocks were deposited, with the original relationships obscured by later deformation; (b) the gneisses and supracrustals formed in separate regions of the Earth and were juxtaposed by later thrusting. The pod-rock (Unit 6) alternates with a very paIe brown quartz-rich psammitic gneiss (Unit 7) The latter contains ~ 80% quartz, ~ 10% plagioclase, ~ 5% biotite, with traces of green muscovite, garnet, sulfide, ilmenite and rutile. 3), but there are good exposures of a major mylonitized dike inland and on the east shore, indicating a fault boundary between the southern group of metasediments and Amitsoq gneiss The metasediments in this region are dominantly massive quartz-rich types not unlike Unit 9 mentioned above. After an initial strip of dark arnphibolite containing garnet with plagioclase coronas, there is a section of very paie biotite amphibolites characterized by fine migmatitic layering and calc-silicate pods Such calc~silicate horizons suggest that some of the amphibolitcs may have evolved from a earbonate-rieh protolith rat her than mafie to intermediate vo!canie rocks. Reaction of the metasediments (siHimanite~garnet-biotite rock) with an intervening pinched~out ul~ trarnafic layer has creatcd a 10-20 cm skarn zone eontaining lavender corundum por~ phyroblasts (up to 15 cm Ion g), plconaste spine! and gedrite, which obscures contact relationships
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