Tertiary alkaline volcanics in the Nunatak Region, Northeast Greenland: new observations and comparison with Siberian maymechites

Abstract

Highly alkaline ultrabasic lavas which occur in the remote Nunatak Region of Northeast Greenland between latitudes 73°45′N and 74°25′N are approximately contemporaneous with the better known, voluminous Tertiary tholeiitic plateau basalts of the coastal region (68°N–71°N, 74°N–76°N). Recent reconnaissance mapping extended the known outcrop of the alkaline ultrabasic lavas southwards and northwards, at the same time revealing several new volcanic plugs, assumed to be feeders to the lavas. Rounded dunitic xenoliths were recovered from one of these plugs. The xenoliths have olivine with forsterite contents too low, and chrome–spinel too titaniferous, to be of mantle origin. Rather, these features are consistent with crystallisation from the picritic host magmas, and the xenoliths are thus regarded as cognate. The alkaline igneous rocks are all ultramafic with high MgO contents (up to 17 wt.%), high TiO 2 and FeO (up to 8.5 wt.%, and 18.1 wt.%, respectively) and very low silica and aluminium (generally 37–44 wt.% SiO 2 and 5–10 wt.% Al 2O 3). The lavas have phenocrysts of olivine, clinopyroxene and iron–titanium oxides. Some have discernable perovskite in the groundmass. Common mica in the groundmass testifies to the hydrous nature of the parental magma. Chemically, the Nunatak Region volcanics are rich in incompatible elements as well as in Ni and Cr, and range from melilitites to basanites. Compared to melilitites from most other settings, those from the Nunatak Region have lower Al 2O 3, and higher FeO and TiO 2 for a given MgO. There is, however, close chemical affinity with the strongly undersaturated, primitive lavas, so-called maymechites (formerly meimechites [Arndt, N., Chauvel, C., Czamanske, G., Fedorenko, V., 1998. Two mantle sources, two plumbing systems: tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province. Contrib. Mineral. Petrol. 133, 297–313]), described from the Maymecha region in the Siberian Traps. As recently proposed for the origin of the maymechites, it is suggested that volcanic rocks of the Nunatak Region formed from low degrees of mantle melting (<5%) at very high pressures (>5GPa) when a hydrated mantle transgressed its wet solidus during ascent beneath a thick cratonic lithosphere. Trace-element profiles of the nunatak lavas suggest recycled oceanic crust as an important component in the mantle source. The occurrences of both the Nunatak Region volcanic rocks and the Maymecha volcanics at the rims of ascending mantle plumes during continental rifting suggest that some mantle plumes may have a hydrous component.