2013

Аrchean alkaline magmatism of OIB-type on the Baltic Shield

Zozulya D. ¹, Eby N. ², Bayanova T. ¹, Lyalina L. ¹

¹Geological Institute Kola Science Center, Apatity, Russia

²University of Massachusetts Lowell, Lowell, USA

Archean alkaline complexes (and carbonatites) are rare worldwide. Only a few tens complexes in the Canadian Shield (Superior Province), Greenland, and Australia (Yilgarn craton) are known to date. They are composed of alkali and nepheline syenites, foidolites, carbonatites, peralkaline granites, lamprophyres, and potassic volcanic rocks. The age of the oldest alkaline rocks is estimated at 2.70-2.65 Ga. The Archaean alkaline complexes belong to the potassic series, the saturated and oversaturated rock series show sanukitoid affinities (i.e. they share geochemical characteristics with high-Mg andesites found in modern arc settings). The most of rocks are depleted in high field strength elements (HFSE) and enriched in some large ion lithophile (LILE) and compatible (Ba, Sr, Ni, Co, Cr) elements. Therefore, the geodynamic setting of the Archean alkaline rocks is interpreted as a subduction-related environment and the depleted mantle is thought to be their source. The Archean subduction-related alkaline complexes presumably formed at the final stages of the evolution of greenstone belts, while the depleted source is accounted for by the absence of metasomatic processes in the Archean mantle (Blichert-Toft et al. 1996).

Sakharjok alkaline massif is located in the western part of the Keivy Terrane (NE Baltic Shield). Keivy Terrane is composed mainly of Neoarchean metavolcanics and metasediments (2.8-2.9 Ga), vast peralkaline granite intrusions of A-type and gabbro-anorthosite bodies (2.65-2.67 Ga). The Sakharjok (5-6 km²) fault-type alkaline intrusion occurs in the central part of the West Keivy peralkaline granite massif and consists of nepheline-bearing phlogopite gabbro, ferrohastingsite-lepidomelane syenite, and aegirine-lepidomelane nepheline syenite. The alkali gabbro occurs as large (80 x 200 m) patches in the nepheline syenites. The most abundant magmatic phase is alkali syenite. The nepheline syenite intrudes the alkali syenite and peralkaline granite. The Zr-Y-REE deposit is confined to nepheline syenite body and composed of several lenses of zircon and britholite ores (Zozulya et al., 2012).

The U–Pb zircon ages of nepheline syenites from the Sakharjok massif is 2613 ± 35 Ma and of alkali syenites from the Sakharjok massif is 2682 ± 10 Ma (Bayanova, 2004; Zozulya et al., 2007). The K-Ar and Rb-Sr isotope systems in syenites have recorded the several metamorphic events at time span 1.76-1.81 Ga, which is relevant to the Svekofenian regional metamorphism. Mineralogical features of zircon demonstrate its long multistage crystallization on magmatic, postmagmatic (hydrothermal) and metamorphic stages. Judging from geochemical data (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm  Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of protracted fractional crystallization of primary alkali basaltic melt (Zozulya et al., 2012).

The Sakharjok alkali gabbro is the massive medium-grained rock composed of pyroxene (30-40 vol. %), plagioclase (30-45 %), mica (10-20 %), nepheline (5-10 %) and amphibole (3-5  %, up to 20 % for contact facies). Pyroxene is essential diopside with high components of hedenbergite and jadeite (Di_62-63Hed_17-20Jad_14-17Acm_2-5). Amphibole is of pargasite-ferrohastingsite composition. Mica is represented by phlogopite and high-Mg biotite. Accessory minerals are apatite, fluorite, magnetite, and zircon. The chemical composition of the rock is characterized by following values (wt. %): SiO₂ (47.3-48.7); TiO₂ (0.25-0.30); Al₂O₃ (14.4-15.3); Fe₂O_3t (7.0-8.1); MgO (8.4-8.8); CaO (10.6-13.3); Na₂O (3.0-6.0); K₂O (1.3-2.6); F (0.3-0.7) and rare elements (ppm): Sr (280-560); Rb (140-350), La (8.1-13.7); Ce (13.4-19.7), Yb (1.0-3.6); Y (4-30); Nb (9-12); Ta (0.15-0.82); Zr (36-70); Hf (1.0-1.6). Broad variation of alkalis is explained by the influence of nepheline syenite. The rock belongs to alkaline series on the plot SiO₂ - Na₂O+K₂O. Normative composition is characterized by the presence of ne (7-15 vol. %) and fo (7-8 vol. %). The rock has elevated Zr/Hf and Nb/Ta (> 40) , that is characteristics of alkaline rocks. The alkaline gabbro has the unfractionated REE distribution and shows no Eu anomaly.

The U–Pb zircon age of Sakharjok alkali gabbro is 2666 ± 4 Ma, which is at whole in geological and geochronological accordance with other rocks of the massif. Morphology and inner structure of zircon dated suggest its magmatic origin, which refines the age of formation of the Sakharjok massif.

Geochemical methods were used for elucidation of tectonic setting and source of Sakharjok alkali gabbro. It is known that OIB-type rocks are distinguished from IAB- and MORB-type by elevated Nb content; moreover IAB-rocks are characterized by elevated Ba content. That is why the tectonic discrimination diagrams on the base of ratios Y/Nb, Ce/Nb, Nb/Zr, Ba/Zr can be useful to solve the problem. The Sakharjok alkali gabbro plot well into the OIB field on these diagrams. Of interest, the Superoir alkaline rocks plot into (or closed) IAB fields on the same diagrams. The enriched mantle source for Sakharjok alkali gabbro is confirmed by Nd-Sr isotopic studies. It is shown that the mantle source for gabbro was metasomatized and represented by EM2-type. Hence the Archean alkaline rocks of the Sakharjok massif from the Baltic Shield are unique by their similarity to OIB-type magma and formation during hot-spot (or continental rifting) evolution.

The study is supported by the Earth Science department of Russian Academy of Sciences (Program 9).

References

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Zozulya D.R., Lyalina L.M., Eby N., Savchenko E.E. Ore geochemistry, zircon mineralogy, and genesis of the Sakharjok Y-Zr deposit, Kola Peninsula, Russia // Geology of Ore Deposits. 2012. V.54. N2. P. 81-98.