SMALL-VOLUME LA-MC-ICP-MS GEOCHRONOLOGY AND EVIDENCE FOR ARCHEAN SOURCE TERRANES IN KRUMMEDAL SEQUENCE PELITES, MILNE LAND, EAST GREENLAND

Detrital zircon geochronology is a widely-used tool for determining sedimentary provenance, although because this method relies on microbeam geochronology with spot sizes typically > 25 μm, it is limited to relatively coarse-grained sediments. In this study we use small-volume LA-MC-ICP-MS with a spot size of ~10 μm to determine U–Pb zircon ages on metamorphic rims and detrital cores from high-grade pelites in the Krummedal Sequence from Milne Land, East Greenland. Our results include a sample dominated by Archean detritus, a signature that has not been previously observed in the Krummedal, and have implications for Rodinian paleogeography.

The Krummedal Sequence consists of high-grade metasediments composed of primarily Proterozoic detritus in metapsammites. Despite a lack of grains derived from Archean basement underlying the Krummedal, this detrital signature suggests a Laurentian source, and deposition on, or near Laurentian basement. Grenvillian plutonism at 955–910 Ma provides a minimum depositional age for the Krummedal, which was ultimately incorporated into the Greenland Caledonides during the Silurian. For this study, zircons from four Milne Land pelites were analyzed, and yield Caledonian metamorphic rims that range from 454–427 Ma. Detrital cores in three of the four samples are dominated by Proterozoic inheritance, and contrast with a fourth sample composed of 80% Archean detritus and minor Paleoproterozoic components. A fifth sample, an orthogneiss, yields Caledonian rims of 441 +6.7/-8.5 Ma with an upper-intercept of 950 ± 22 Ma, and confirms the presence of Grenvillian plutonism in Milne Land.

Archean detritus in the Krummedal provides a direct link to underlying Laurentian basement, and supports the correlation of sediments dominated by Archean detritus in Liverpool Land with the Krummedal. In addition, this Archean signature may indicate that the Krummedal is similar to the Torridonian of NW Scotland consisting of several serial basins with different depositional ages and source terranes, or alternatively, that significant changes in source terrane associated with varying sedimentary facies. Finally, this study underscores the utility of the small-volume LA-MC-ICP-MS and the necessity to analyze sediments of variable grain size when characterizing sedimentary provenance.