Carbonatites and lamprophyres of the Gardar Province – a ‘window’ to the sub-Gardar mantle?

Abstract

Carbonatite magmas are considered to be ultimately derived from mantle sources, which may include lithospheric and asthenospheric reservoirs. Isotopic studies of carbonatite magmatism around the globe have typically suggested that more than one source needs to be invoked for generation of the parental melts to carbonatites, often involving the interaction of asthenosphere and lithosphere.In the rift-related, Proterozoic Gardar Igneous Province of SW Greenland, carbonatite occurs as dykes within the Igaliko Nepheline Syenite Complex, as eruptive rocks and diatremes at Qassiarsuk, as a late plug associated with nepheline syenite at Grønnedal-Íka, and as small bodies associated with ultramafic lamprophyre dykes. The well-known cryolite deposit at Ivittuut was also rich in magmatic carbonate. The carbonatites are derived from the mantle with relatively little crustal contamination, and therefore should provide important information about the mantle sources of Gardar magmas. In particular, they are found intruded both into Archaean and Proterozoic crust, and hence provide a test for the involvement of lithospheric mantle.A synthesis of new and previously published major and trace element, Sr, Nd, C and O isotope data for carbonatites and associated lamprophyres from the Gardar Province is presented. The majority of Gardar carbonatites and lamprophyres have consistent geochemical and isotopic signatures that are similar to those typically found in ocean island basalts. The geochemical characteristics of the two suites of magmas are similar enough to suggest that they were derived from the same mantle source. C and O isotope data are also consistent with a mantle derivation for the carbonatite magmas, and support the theory of a cogenetic origin for the carbonatites and the lamprophyres. The differences between the carbonatites and lamprophyres are considered to represent differing degrees of partial melting of a similar source.We suggest that the ultimate source of these magmas is the asthenospheric mantle, since there is no geochemical or isotopic evidence for their having been derived directly from ancient, enriched sub-continental lithospheric mantle. However, it is likely that the magmas actually formed through a two-stage process, with small-degree volatile-rich partial melts rising from the asthenospheric mantle and being ‘frozen in’ as metasomites, which were then rapidly remobilized during Gardar rifting.

From Mantle to Motzfeldt: A genetic model for syenite-hosted Ta,Nb-mineralisation

“…5). The data of the present study are similar to results for other Igaliko carbonate (Pearce & Leng 1996;Coulson et al 2003). Mantlederived carbonate has δ 18 O and δ 13 C between +6 and +10 ‰ and -8 and -4 ‰, respectively (Keller & Hoefs, 1995;Taylor et al 1967, Fig.…”

“…5). Additional data from the FDC (Schönenberger & Markl 2008), the fields for Igaliko and Grønnedal-Íka carbonatite (Coulson et al, 2003;Goodenough, 1997;Leng, 1996 andPearce et al, 1997) and O and C isotope compositions of mantle-derived carbonatite (Keller & Hoefs, 1995;Taylor et al, 1967) are added for reference. Samples from the FDC and MSC plot within two discrete groups, each showing considerable range in both isotopes with several samples show high (>20 ‰) δ 18 O values.…”

“…Such variations have in the past been interpreted as a combination of fractionation, crustal assimilation and subsolidus re-equilibration (e.g. Coulson et al 2003).…”

Problematic but promising ponds? Palaeoenvironmental evidence from the Norse Eastern Settlement of Greenland

“…(2) There could be a reservoir effect resulting from an unanticipated source of carbonate within the pond catchment, such as carbonatite. This occurs as volcanic rocks around Qassiarsuk and as dykes within the Igaliku Nepheline Syenite Complex which outcrops 36 km west of Sissarluttoq (Coulson et al 2003). Dykes also occur in the Sissarluttoq catchment (Allaart, 1972; Garde and Chadwick, 1996) but none are reported as carbonatitic.…”

Petrogenesis of lamprophyres from Chhota Udepur area, Narmada rift zone, and its relation to Deccan magmatism

“…Earth's lithosphere (e.g., Coulson et al, 2003;Dessai et al, 2004;Nédli et al, 2010), (iii) diamond prospectivity (e.g., Lefebvre et al, 2005;Wyman et al, 2006) and (iv) temporal and spatial association (of the calc-alkaline lamprophyres) with mesothermal gold deposits (e.g., Rock and Groves, 1988;Duggan and Jaques, 1996;Huang et al, 2002).…”