Long-term memory of subduction processes in the lithospheric mantle: evidence from the geochemistry of basic dykes in the Gardar Province of South Greenland

Abstract

The rift-related magmas of the Proterozoic Gardar Igneous Province were emplaced across the contact between the South Greenland Archaean craton and the Palaeoproterozoic Ketilidian mobile belt. It has been suggested that the geochemistry of Gardar intrusive rocks in the two areas varies across the craton margin and that this reflects a lithospheric control. However, comparison of the geochemical and isotopic signatures of basic and ultrabasic dykes from across the area shows that there is no systematic variation related to the age of the country rock. All the Gardar basic rocks are inferred to have been derived from the mantle, with relatively little crustal contamination. We suggest that the lithospheric mantle beneath the Gardar Province was enriched by slab-derived fluids during the Ketilidian orogeny (c. 1800 Ma). Subsequent melting of this mantle source was promoted during Gardar rifting when volatile-rich, small-degree melts from the asthenosphere were introduced into the lithospheric mantle, forming enriched metasomites. Ultrabasic lamprophyre dykes in the Gardar Province represent melts derived largely from these metasomites, whereas basaltic magmas were formed by larger-scale melting of the lithospheric mantle, inheriting a subductionrelated signature. There is no evidence that the Gardar magmas were derived from a highly enriched lithospheric keel that had existed since craton formation.

Petrology and geochemistry of the Salma dike, Raniganj coalfield (Lower Gondwana), eastern India: linkage with Rajmahal or Deccan volcanic activity?

“…is somewhat similar to that proposed by Cousen et al (1995) for the Juan de Fuca Ridge in the NE Pacific and that proposed by Goodenough et al (2002) for the Gardar Province of South Greenland, although the geodynamic and geological environment in Rajmahal is different. Interestingly, Simonetti et al (1998) observed that high Nb/Y and CaO/Al 2 O 3 ratios in alkaline rocks of Barmer and Mundwara, western India, correlate with lower initial Nd isotopic compositions and suggested an origin of the alkaline rocks via small degrees of partial melting of a metasomatized mantle.…”

Europe's rare earth element resource potential: An overview of REE metallogenetic provinces and their geodynamic setting

“…This pattern of repeated enrichment of mantle sources appears to be fundamental to the formation of REE-enriched magmas. A clear example of this comes from Southern Greenland, where Gardar Province magmas were derived from lithospheric mantle that was metasomatised and fertilised some 600 million years earlier during subduction and formation of the Ketilidian belt (Goodenough et al, 2002). In Norway, the Fen carbonatite and associated dyke swarms, and the subsequent Oslo Rift, represent two episodes of alkaline magmatism (Dahlgren, 1994) localised along the margin of a Neoproterozoic subduction-related igneous province associated with the Sveconorwegian event (Slagstad et al, 2013).…”

Nd-, O-, and H-isotopic evidence for complex, closed-system fluid evolution of the peralkaline Ilı́maussaq intrusion, south Greenland

“…The occurrence of contaminated samples at the margins of the complex shows that significant assimilation of country rock occurred only on a local scale (Ferguson, 1964;Stevenson et al, 1997;Marks and Markl, 2001). Whereas Stevenson et al (1997) believe that the parental melt for the Ilímaussaq rocks was derived from a depleted mantle, we favor a model where the Ilímaussaq rocks are derived from a mantle source depleted in 147 Sm compared to normal mantle values, either because of mantle metasomatism before melting (Upton, 1987;Goodenough et al, 2002) or because the mantle source had been modified by plume material (Zindler and Hart, 1986;Nicholson and Shirey, 1990;Halama et al, 2003).…”