The Kola alkali province located in the northeastern part of the Fennoscandian Shield is one of the largest igneous provinces in the world. It consists of the multiphase Khibiny and Lovozero complexes, alkali–ultrabasic rock massifs with carbonatites, alkali and carbonate dikes, and, finally, kimberlite explosion pipes [1].

A large array of geological and geochronological data was accumulated in the course of the long-term study of the Kola alkali province. Most studies have been devoted to ultrabasic complexes and alkali–ultrabasic massifs of the province, whereas the age data on dike rocks are fragmentary. This fact is explained, firstly, by their geological position: along with dike bodies confined to intrusion complexes, there are also individual dike swarms which do not have crossing contacts with alkali–ultrabasic massifs. Another reason that makes it difficult to obtain the geochronological information is the specific chemical composition of these rocks and the absence of traditional geochronometer minerals in them. The available age data (360–380 Ma) were obtained mainly by K‒Ar and Ar‒Ar methods using mica, plagioclase, and amphibole [2–5].

This paper discusses the U‒Pb (ID-TIMS) geochronological data on Ca–Fe garnets from the aillikite dike of the Kandalaksha swarm (western part of Kandalaksha Bay) (Fig. 1). The rocks of this swarm are characterized by the prevailing ultrabasic lamprophyre dikes, whereas other swarms are dominated by alkali (monchiquite) and alkali–ultrabasic rocks (nephelinite and phonolite) [6].

Fig. 1.
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Geological map of the western part of Kandalaksha Bay showing outcrops of lamprophyre dikes (according to [1]) and sampling site 18KL-20 for geochronological studies.

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The specific features of the chemical composition of aillikites in the Kandalaksha dike swarm are indicative of a moderate melt fractionation with a general decrease in the SiO2 content [6]. Meanwhile, other alkali–ultrabasic rocks of the Kola Province are characterized by a high fractionation with an increase in SiO2 and a carbonatite component separation [1]. Understanding whether the generation stage of primitive melts reached the surface without experiencing significant fractionation is preceding, synchronous, or subsequent to the formation of large massifs will make it possible to take an important step in the igneous evolution reconstruction of the Kola alkali province.

The major aillikite minerals include olivine, clinopyroxene, and phlogopite. Garnet is a minor mineral (7–10%) observed as euhedral and subhedral crystals (no more than 200 µm) in the groundmass (Fig. 2) [7].

Fig. 2.
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BSE image of garnet (sample 18KL-20); (Px) pyroxene, (Grt) garnet. Composition of analyzed sections of the garnet grain. (А) schorlomite (Ca3Ti2(Fe\(_{2}^{{3 + }}\)Si)3O12), hutcheonite (Ca3Ti2(Al2Si)3O12); (В) morimotoite (Ca3(TiFe2+)2Si3O12), Mg-morimotoite (Ca3(TiMg)2Si3O12); (С) andradite (Ca3Fe\(_{2}^{{3 + }}\)Si3O12).

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The garnet composition corresponds to the andradite (5.83–61.0%)–morimotoite (20.0‒48.0%)–shorlomite (3.50–36.5%) range; Nb (up to 3.5 wt % Nb2O3) and Zr (up to 1.12 wt % ZrO2) impurities are present (Fig. 3a). Garnet grains have a zoned structure: the grain cores are enriched in Ti and Zr, while the margins are characterized by higher Fe. Solid-phase inclusions occur as diopside–hedenbergite–aegirine pyroxene, perovskite, and phlogopite (Fig. 2). REEs vary within 2128–5345 µg/g in garnet. Elements of this group are distributed in a zoned manner within individual grains (Fig. 3b). The MREE accumulation ([La/Sm]N = 0.86–1.01) with maximum Pr and HREE depletion ([Gd/Yb]N = 1.66–3.30) is typical (Fig. 3b). The correlation between U, REEs, and Ti is positive.

Fig. 3.
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(a) Garnet composition in aillikite of the Kandalaksha dike swarm (sample 18KL-20); (А) schorlomite (Ca3Ti2(Fe\(_{2}^{{3 + }}\)Si)3O12), hutcheonite (Ca3Ti2(Al2Si)3O12); (В) morimotoite (Ca3(TiFe2+)2Si3O12), Mg-morimotoite (Ca3(TiMg)2Si3O12); (С) andradite (Ca3Fe\(_{2}^{{3 + }}\)Si3O12). (b) REE distribution in garnet from aillikite of the Kandalaksha dike swarm (sample 18KL-20) and garnets from foidolite-range rocks of the Kola Province (according to [8]). Normalized according to [9].

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Garnet from aillikites of the Kandalaksha dike swarm is characterized by much higher REEs and their elevated fractionation compared to that of foidolite-range rocks from alkali massifs of the Kola Province [8] (Fig. 3b).

The U‒Pb (ID-TIMS)-geochronological studies of garnet were carried out in the Isotope Geology Laboratory, Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences (St. Petersburg), according to the method described in [8]. The Pb and U isotopic compositions were determined with a Triton TI multicollector mass spectrometer in the static or dynamic (using an ion counter) modes. The determination accuracy of the U/Pb ratios and U and Pb contents was 0.5%. The blank contamination did not exceed 15 pg for Pb and 1 pg for U. The experimental data were processed using the PbDat [10] and ISOPLOT [11] software. The generally accepted U decay constants were used to calculate the age values [12]. Corrections for common Pb were introduced in accordance with the model values [13]. All errors were given at the 2σ level.

Two weighted portions of garnet were used in the U‒Pb (ID-TIMS) geochronological studies. The U  content reached 44.89 μg/g in them (Table 1). Common lead was low (Pbc/Pbt = 0.19–0.45). As can be seen in Fig. 4, a concordant (374 ± 3 Ma, MSWD = 0.0013) age estimate was obtained for the garnet.

Table 1. U‒Pb (ID-TIMS) geochronological studies of garnet from aillikites of the Kandalaksha dike swarm (sample 18KL-20)
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Fig. 4.
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Garnet concordia in aillikite of the Kandalaksha dike swarm (sample 18KL-20). The numbers of points on the diagram correspond to the serial numbers in Table 1.

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The results of U‒Pb (ID-TIMS) geochronological studies of garnet from aillikites are consistent within the error with the K‒Ar age of alkali lamprophyres (360 ± 16 Ma and 368 ± 15 Ma; [2]), the U–Pb age of zircon from carbonatite dikes (380 ± 15 Ma; [14]), and the Ar–Ar age of phlogopite (375 ± 4–381 ± 4 Ma; [15]) from the Kandalaksha dikes.

The main stage of alkali–ultrabasic magmatism in the Kola Province, identified by U‒Pb geochronological studies of garnets from various rocks of five alkali–ultrabasic massifs of the province, was in the range of 373‒377 Ma. According to the data obtained, the rock formation in the dike complex of the Kandalaksha region was also related to this stage. Thus, the moderately differentiated rocks of the Kandalaksha dike swarm were formed simultaneously with the highly differentiated alkali–ultrabasic massifs of the Kola Province. The specific features of the chemical composition of aillikites were likely due to the rapid rise of the melt within the Kandalaksha–Dvina rift zone, which prevented melt fractionation.