The Urik–Iya Graben is an intracratonic structure of northwestern strike at the southwestern margin of the Siberian Craton (Fig. 1). The formation of the graben is associated with several stages of extension of the continental lithosphere in the interval of 1.91–1.53 Ga, accompanied by sedimentation and basic and granitoid magmatism [1]. Massifs of ultramafic, alkaline rocks, and carbonatite (Beloziminskii, Sredneziminskii, and Bolshetagninskii), which are associated with large reserves of Nb, Ta, U, Th, TR, P, Pb, Zn, and fluorite [3] were formed within the Urik-Iya Graben in the period between 720 and 630 Ma, during the structural transformation associated with the breakup of the supercontinent Rodinia ([2] and others). Along with alkaline rock massifs, this area is widely represented by dykes and veins of allkite, mica picrite, kimberlite-like pyroxene-free picrite, lamproite, and rare explosion pipes. However, the U–Pb and 40Ar/39Ar isotope dates have been obtained only for the Beloziminskii Massif: from 645 to 643 Ma (ID–TIMS U–Pb analysis of garnet [4] and 40Ar/39Ar dating of phlogopite [5], TIMS U–Pb analysis of zircon [2]). Isotopic dating of dyke–vein rocks was carried out mainly by the K–Ar and Rb–Sr methods. The ages obtained by the K–Ar method for picrite range from 698 to 603 Ma [6]; the 40Ar/39Ar ages of 645–622 Ma were obtained for allkitic breccia of the Beloziminskii Massif and the Yuzhnaya pipe [7]. The ages of lamproite obtained by the 40Ar/39Ar, Rb–Sr, and U–Pb methods vary from 1481 to 300 Ma [8]. An isotope–geochemical study of rocks of the Bolshetagninskii Massif, which included aillikite and picrite dykes spatially associated with the massif (Fig. 1), was performed in order to detail the history of the geological evolution of the intracratonic shear zone, to clarify the sequence of the formation of mantle magmatic rocks, and to elucidate the genesis of alkaline melts.

Fig. 1.
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Schemes of the geological structure (a) of the southern part of the Siberian Сraton and (b) of the area studied. (1) Phanerozoic sedimentary cover; (2) Early Precambrian basement blocks: (Bk) Baikal, (B) Biryusinskii, (G) Golousten, (Sh) Sharyzhalgai; (3) Paleoproterozoic Urik-Iya Graben (U); (4) deposits of the Neoproterozoic craton margin; (5) Central Asian foldbelt; (6) location of the studied area; (7) Cenozoic deposits; (8) volcano–sedimentary deposits PR1 and RF1; (9) Ziminskii Formation of ultramafic alkaline rocks and carbonatite RF3; (10) granite (a) and gneiss–granite (b) PR1, (11) gabbro-diabase PR1; (12) dykes, (a) veins and (b) pipes of the ultramafic composition (out of scale); (13) faults; (14) sample numbers.

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The Bolshetagninskii Massif (Fig. 1) has a zonal ring structure due to the successive formation of ijolite–melteigite, nepheline and subalkaline syenites, and calcite and ankerite–calcite carbonatite. The massif is characterized by the wide abundance of K-feldspar syenite, which had a metasomatic effect on previously crystallized alkaline rocks [3]. Dykes of pyroxene-free phlogopite picrites intrude ijolite and syenite, but precede carbonatite or are intra-carbonatite.

Ultramafic dykes intrude sand–shale deposits PR1 and rocks of the massifs (Fig. 1b). The dykes have a steep dip, thickness from a few tens of centimeters to 10–20 m, and a length of up to hundreds of meters. Most of the dykes are represented by allkite. Among the phenocrysts in this rock are partially serpentinized olivine Fo82–88, phlogopite, and titanomagnetite. The groundmass is composed of olivine, phlogopite, calcite (10–40%), diopside, titanaugite; kaersutite, aegirine, microcline, and albite are also registered. Accessory minerals are represented by chrome spinels, titanomagnetite, perovskite, manganilmenite, apatite, etc.

One of the dykes, known as the Bushkanai Dyke (Fig. 1), is petrographically heterogeneous. The dyke is composed of picrite consisting of serpentinized olivine (15–20%) and minor phlogopite in the groundmass of serpentine, diopside, hornblende, phlogopite, andradite, chrome spinels, titanomagnetite, perovskite, apatite, calcite, etc. Picrite contains melanocratic inclusions with a size up to 20 cm composed of serpentinized olivine by 80–85%; minor minerals are represented by chromium diopside, chloritized phlogopite, calcite, serpentine, chrome spinels, titanomagnetite, apatite, and andraditic garnet. In addition, veinlets with indistinct boundaries are detected in picrite. They are enriched in clinopyroxene, namely chromium diopside, augite, and titanaugite (20–25%), and mica (10–15%), but depleted in olivine (~5%).

The dykes of the Bolshetagninskii Massif are represented by phlogopite pyroxene-free picrite. Phenocrysts in this rock include serpentinized olivine and phlogopite; the groundmass consists of serpentine, serpentinized olivine, phlogopite, calcite, chlorite, melanite, grossular–andraditic garnet, monticellite, accessory chrome spinels, titanomagnetite, perovskite, apatite, Fe, Ni, and Cu sulfides, etc.

The age of the formation of the Bolshetagninskii Massif was studied by the 147Sm–143Nd method (Table 1). An isochrone with an age of 640 ± 11 Ma was obtained for the samples of ijolite, nepheline syenite, K-feldspar syenite, calcite, and calcite–dolomite carbonatite (Fig. 2). The points for picrite from dykes and allikite plot on or near the isochrone (Fig. 2, inset), which indicates the genetic relationship of rocks with the general event of melting of the upper mantle.

Table 1. Sm–Nd and Rb–Sr data for rocks of the Bolshetagninskii Massif and ultramafic dykes
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Fig. 2.
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Sm–Nd isochrone for rocks of the Bolshetagninskii Massif. The inset shows the position of the isotopic composition points of the dykes on the isochrones. (1) Rocks of the massif; (2) dykes of lamprophyre and picrite. Sample numbers are the same as in Table 1. The isochrone was constructed using the IsoplotR software [12].

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40Ar/39Ar dating of phlogopite from allkite (Samples 53/8 and 111/8) and picrite (Sample 47/8) was performed in order to estimate the age of the dykes. The results are reported in Table 2; the age spectra are shown in Fig. 3; the measurement errors given in the text and in the figures correspond to the interval of ±1σ. To evaluate the “excess argon,” the authors calculated the ages using the isochrone regression method as well. It was not possible to obtain isochrones suitable for publication; at the same time, construction of isochrones did not show the presence of “excess argon.”

Table 2. Results of 40Ar/39Ar geochronological study of phlogopite from ultramafic dykes
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Fig. 3.
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40Ar/39Ar age spectra for phlogopite from ultramafic dykes.

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The 40Ar/39Ar age spectra of Samples 111/8 and 47/8 have well-defined age plateaus corresponding to 644.1 ± 8.6 Ma and 87.4% of the released 39Ar and 646.1 ± 8.6 Ma and 99.4% of the released 39Ar, respectively (Fig. 3). Sample 53/8 has a “saddle” shape of the age spectrum. This spectrum does not provide reliable geochronological information.

The values of εNd(Т) corrected for the age of 640 Ma range from +4.2 to +5.0 for rocks of the Bolshetagninskii Massif including the picrite dyke (Table 1). At the same time, the (87Sr/86Sr)t value shows the following variations: 0.7031–0.7033 in ijolite and carbonatite, 0.7044 in nepheline syenite, and 0.7025 in picrite. The values of εNd(T) and (87Sr/86Sr)t for allikite are close to those of ijolite and carbonatite of the massif (Table 1), while the rocks of the Bushkanai Dyke demonstrate a higher value of (87Sr/86Sr)t 0.7055–0.7059 and a wider range of the εNd(T) value (from +2.9 to +4.5, Table 1).

The previously obtained ages for the Beloziminskii Massif vary from 622 to 645 Ma [2, 4, 5, 7]. Our data on the age of the rocks of the Bolshetagninskii Massif, as well as allkite and picrite outside the massif, coincide with these ages within the error.

The values of εNd(T) and εSr(Т) in the rocks of the Bolshetagninskii Massif and allikite correspond to the values obtained by other authors for the alkaline–carbonatite massifs of the Urik–Iya Graben [14, 15] (Fig. 4). These rocks had a single mantle source close to OIB by the isotope characteristics. The enrichment of rocks in incompatible microelements suggests that the metasomatic alteration of the mantle substrate preceded melting [16].

Fig. 4.
figure 4

Diagram εNd(Т)–εSr(Т) for rocks of the Bolshetagninskii Massif and ultramafic dykes. (1) Rocks of the massif, data of the authors; (2) dykes of lamprophyre and picrite; (3) alkaline rocks and carbonatite of the Beloziminskii, Sredneziminskii, and Bolshetagninskii massifs, after [14, 15]. DMM and OIB fields and the EM I and EM II components are shown after [17].

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Ijolite and carbonatite of the Bolshetagninskii Massif have close εNd(Т) and εSr(Т) values (Table 1, Fig. 4) indicating separation of alkaline silicate and carbonate melts from the same parental magma. The high (87Sr/86Sr)t ratio in nepheline syenite (Sample 106/9) may be due to the metasomatic impact by feldspar syenite on this rock, which is expressed in microclinization accompanied by the introduction of Rb. An alternative is the contamination of the nepheline–syenite melt by the upper crustal material enriched in radiogenic Sr, but depleted in REEs.

(87Sr/86Sr)t variations in pyroxene-free picrite composing dykes (Sample 116/9) and allikite (Samples 531/9 and 97/8), most likely, indicate the different degrees of mantle substrate phlogopitization, which stimulates the increase in the Rb/Sr and 87Sr/86Sr ratios.

Samples 47/8, 49/8, and 51/8 from the Bushkanai Dyke occupy a separate position on the εNd(Т)–εSr(Т) diagram (Fig. 4). The high (87Sr/86Sr)t value may be due to the contamination by the upper crustal material with a high 87Sr/86Sr ratio. Unfortunately, there are no data on the Sr isotope composition in the rocks of the Urik-Iya Graben. The lower εNd(Т) value in olivinite (Sample 51/9) and mica picrite (Sample 49/9) compared to picrite (Sample 47/9), which composes most of the dyke (Table 1), suggests that the rocks combined in the dyke are not the products of crystallization differentiation of the same ultramafic melt, but are derivatives of different melts.

The spatial alignment and similar age of ultramafic dykes and alkaline carbonatite massifs indicate their genetic commonality, i.e., the relation of the same episode of melting of moderately depleted mantle that underwent preliminary metasomatic enrichment in incompatible trace elements. Variations in the εSr(Т) value in the rocks reflect both the heterogeneity of the isotopic composition of the mantle source or, possibly, the process of contamination by the rocks of the upper continental crust, which is especially pronounced for the rocks of the Bushkanai dyke.