Agnete Steenfelt

Craton formation in Late Archean subduction zones revealed by first Greenland eclogites

It is now well established that the early continental crust was formed by melting of basaltic lithologies such as amphibolite and eclogite. However, considerable uncertainty surrounds the geologic environment in which melting took place. Commonly invoked options range between melting at the underside of oceanic plateaus above mantle plumes or melting of oceanic lithosphere during shallow subduction. Distinguishing between these scenarios has important implications for the early evolution of continents. We use the first eclogites discovered from the North Atlantic craton (NAC) to constrain the formation of the deep root to this continent. Late Archean eclogite xenoliths (2.7 ± 0.3 Ga) from a kimberlite in West Greenland are broadly coeval with a major regional episode of tonalite-trondhjemite-granodiorite (TTG) magmatism. Major and trace element systematics of the eclogites reveal a highly refractory character that is mirrored by NAC peridotites. Moreover, the refractory eclogites define a complementary relationship to the Late Archean TTG granitoids from the NAC, and their elevated garnet δ 18 O values along with negative Eu anomalies suggest seafloor-altered oceanic crust as the most viable eclogite protolith. These results from Greenland provide strong support for a model in which early continental crust grew by melting of basaltic slabs in subduction zones, where tectonic stacking of down-going oceanic lithosphere provided the mechanism that coupled formation of cratonic crust and mantle.

Crust-mantle interaction in the evolution of the Ilímaussaq Complex, South Greenland: Nd isotopic studies

Sm-Nd isotopic compositions were determined for the peralkaline Ilimaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial ɛNd(≈ 0), whereas a few samples trend towards ɛNd values as low as − 6 at the time of intrusion (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilimaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust.

Rare earth elements in Greenland: known and new targets identified and characterised by regional stream sediment data

Increasing demand for lanthanides encouraged inspection of stream sediment data for Greenland. The database of the Geological Survey of Denmark and Greenland contains analytical data for c. 10600 samples covering 70% of the ice-free margin of Greenland. Data for La, Eu and Yb are considered representative for the suite of seven rare earth elements (REEs) determined in the samples, and spatial distribution of background values as well as of anomalies (La>364 ppm, Eu>7.4 ppm, Yb>16 ppm) shows that certain provinces are enriched in one, two or all three elements. Stream sediment REE-anomalous samples from South Greenland derived from the most important rock associations hosting REE deposits, i.e. carbonatite, peralkaline syenite, alkaline granite or non-alkaline granite, show that the REE fractionation for samples derived from each of these rock associations is distinct and can form the basis for defining screens. When applied to data from the whole of Greenland it is possible to identify samples likely derived from the same associations. Samples from diverse lithologies also occupy distinct parts in diagrams of La-Yb, Yb-Eu and La/Yb-Eu/Yb variation. Stream sediment REE properties reflect those of source rocks because REE are preferably contained in minerals that survive in the stream sediment environment.