Origin of Mesoarchaean arc-related rocks with boninite/komatiite affinities from southern West Greenland

• Published in: Lithos
• Main Authors: Szilas, Kristoffer, Naeraa, Tomas, Scherstén, Anders, Stendal, Henrik, Frei, Robert, van Hinsberg, Vincent J., Kokfelt, Thomas F., Rosing, Minik T.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2012
• Subjects: Geology; Archaean; Greenland; Nunatak 1390; Supracrustal belts; Ultramafic rocks; Karasjok
• Online Access: https://lup.lub.lu.se/record/2998095; https://doi.org/10.1016/j.lithos.2012.03.023

We report whole-rock elemental and Sm-Nd isotope geochemical data from mafic-ultramafic supracrustal rocks from the Nunatak 1390 area in southern West Greenland. Additionally, we report the metamorphic temperature history for these rocks as derived from tourmaline thermometry on a tourmalinite inlier, as well as in situ U-Pb, Hf and O isotopic data from zircons extracted from tonalite-trondhjemite-granodiorite (TTG) gneisses that intruded the mafic-ultramafic sequence. The supracrustal rocks from the Nunatak 1390 area have a minimum age of c. 2900 Ma defined by U-Pb zircon ages of cross-cutting aplite sheets of TTG composition. The supracrustal sequence comprises mafic rocks with pillow structures and ultramafic rocks with no evidence of their protolith. They all have amphibolite-facies mineral assemblages and a peak metamorphic temperature of approximately 550 degrees C. The mafic sequence has relatively flat trace element patterns (La-N/Sm-N of 0.70-2.4) and mostly negative Nb-anomalies (Nb/Nb* of 0.30-1.0) and resembles modern island arc tholeiites. The mafic sequence can be divided into a high- and low-Ti group, where the former group has lower MgO, and significantly higher contents of incompatible elements such as TiO2, P2O5, Zr, Nb and Th. The ultramafic rocks have major and trace element compositions similar to Ti-enriched/Karasjok-type komatiites described in the literature. However, there are no textural indications that the ultramafic rocks from Nunatak 1390 are komatiites sensu stricto. The low-Ti group of the mafic sequence appears to have been derived from a N-MORB source, whereas the high-Ti group and the ultramafic rocks appear to have been derived from a mantle source that is more enriched than the N-MORB source. However, there is no difference in the initial epsilon Nd of the mafic and ultramafic rocks. Additionally, assimilation-fractional-crystallisation (AFC) modelling is consistent with this enrichment being caused by introduction of juvenile low-silica adalcite (slab-melt) into the mantle ...