Geochemical discriminant study of the tectonic setting and source for lamproites, with emphasis on Fennoscandian, Greenland and Canada occurrences

 Zozulya D.*, Kullerud K.**

 *Geological Institute, Kola Science Centre, Apatity, Russia; **Geological department, University of Tromsø, Norway

zozulya@geoksc.apatity.ru

 

Lamproites have been found in both within-plate settings (anorogenic) and at destructive plate margins (orogenic,) (Foley et al., 1987; Mitchell and Bergman, 1991; Nelson, 1992). Within-plate lamproites are confined to Archaean and Proterozoic intracratonic areas and are commonly thought to be associated with upwelling mantle during rifting or plume activity. They are strongly enriched in both LILE and HFSE. The type localities of within-plate (anorogenic) lamproites are Leucite Hills, Prairie Creek and Smoky Butte (USA), West Kimberley (Australia), Gaussberg (Antarctica), Cuddapah and Krishna (India), and Sisimiut (West Greenland). Some of these anorogenic lamproites are derived from diamond-stable mantle. Orogenic type lamproites are emplaced in tectonically active zones such as island arcs, or in post-collisional settings. Such lamproites are often accompanied by calc-alkaline magmatism (shoshonites and lamprophyres). Although enriched in LILE, plate-margin (orogenic) lamproites are depleted in HFSE compared to within-plate lamproites. This group, which is represented by the type localities Murchia-Almeria Province (MAP, SE Spain), Sisco (Corsica), the West Alps and Tuscany (Italy), Serbia, Macedonia, East Sunda Arc (Indonesia), Andean Cordillera (Peru), is also called Mediterranean-type lamproites.

Major and trace element distributions have been used as tools for discussing the tectonic settings of lamproites from occurrences in Fennoscandia, Greenland and Canada: Kostomuksha (1230 Ma) and Porja Guba (1720 Ma) in NW Russia, Kvaløya in North Norway (330 Ma), Sunnfjord in West Norway (260 Ma), Sisimiut (1230 Ma) and Disco Bugt (1740 Ma) in West Greenland, Labrador (1.2-1.3 Ga), Baffin Island (1240 Ma), and Churchill province (1840 Ma) in Canada (Nikitina et al., 1999; Kullerud et al., 2011; Furnes et al., 1982; Scott, 1981; Larsen and Rex, 1992; Hogarth, 1997; Peterson et al., 1994).

Ti content is a very useful parameter for separating the two groups of lamproites according to tectonic settings. It is demonstrated that anorogenic lamproites are rich in Ti with Al/Ti and K/Ti ratios <4, while orogenic lamproites are Ti-poor with Al/Ti = 4-18 and K/Ti = 2-11. Both groups define separate fields on Ti-K/Ti and Ti-Al/Ti diagrams, and the Kostomuksha, Kvaløya, Sunnfjord, Sisimiut, Labrador and Baffin Island lamproites plot within the anorogenic field, while Porja Guba, Disco Bugt and Churchill plot within the orogenic field (Fig. 1).

Fig. 1. Lamproites from Fennoscandia, Greenland and Canada in the Ti-K/Ti and Ti-Al/Ti tectonic discrimination diagrams.

 

The Th/Nb ratio can also be used as a sensitive discriminator between orogenic and anorogenic magmas, because Nb is commonly immobile during the dehydration of subducted crust, while Th is among the most mobile elements. Therefore, the Th/Nb ratio will be higher for island-arc (orogenic) magmas than for anorogenic ones (~0.6 as discriminating value). Both anorogenic and orogenic lamproites from Fennoscandia, Greenland and Canada fall within the same tectonic field as defined on basis of the K-Ti-Al distribution (Fig. 2).

Further, Sr and Nd isotope data of the anorogenic lamproites from Fennoscandia, Greenland and Canada emphasize their anorogenic signature according to Nelson (1992) with moderately radiogenic ⁸⁷Sr/⁸⁶Sr and extremely non-radiogenic ¹⁴³Nd/¹⁴⁴Nd, indicating a mantle source with low Rb/Sr and Sm/Nd similar to the EM1 reservoir (Fig. 3). Orogenic lamproites from Fennoscandia, Greenland and Canada have different mantle sources with moderately radiogenic ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ranging from depleted to enriched mantle of EM2-type or with crustal contamination.

The strong Nb and Ta depletion, and the insignificant Ti depletion (relatively to primitive mantle) observed for the studied anorogenic lamproites indicates that subducted materials were involved during metasomatism of the mantle source of the lamproite melt. However, subduction-related melting and within-plate magmatism are two spatially separated processes that rarely operate together. A more valid explanation for the origin of a within-plate lamproite with subduction-process affinity is that the geochemical signature of subduction is inherited, e.g. is significantly older than the lamproite itself (> 1 Ga (Nelson, 1992)). The narrow span in Nd model ages (2.1-2.3 Ga) for the Mesoproterozoic and Palaezoic anorogenic lamproites from Fennoscandia, Greenland and Canada, indicates an old age of the subduction-related metasomatism of the mantle source of the lamproites. Possibly, this period might represent the first time for formation of an enriched mantle EM1 reservoir in the Lauro-Baltica lithosphere.

 

Fig. 1. Lamproites from Fennoscandia, Greenland and Canada in the Nb-Th/Nb tectonic discrimination diagram.

 

Fig. 3. εSr(T) vs. εNd(T) diagram for the studied lamproites and other type localities. Transparent fields - anorogenic lamproites, shaded fields - orogenic lamproites, dotted line - localities with contaminated sources.