INTRODUCTION

The Vyatka Belt, located between the Archean blocks in the northeastern part of the Volgo–Uralian segment of the East European Craton (EEC) (Fig. 1), is a formation of Early Precambrian supracrustal rocks completely overlain by a thick sedimentary cover, and information about its structure and composition is based on the results of the study of several deep holes. According to the available data [1, 2], the Vyatka Belt is composed of metamorphosed volcanic–sedimentary deposits of the Uni Suite with an age of ~2.1 Ga, which are intruded by S-type granitoids of the Talitskiy Complex with an age of ~2.05 Ga. Based on the isotope–geochemical and geochronological study of the core samples, the Vyatka Belt is considered as a part of the Paleoproterozoic Orogen [2]. The results of the Uni Suite metamorphic rocks study, reported in this paper, provide additional independent assessment of the paleotectonic conditions for the formation of the Vyatka Belt.

Fig. 1.
figure 1

(a) Three segments of the East European Craton, after [3]; (b) major structural elements of the Volgo–Uralian segment of the EEC, modified after [3] with additions. OMMB, Osnitsko‒Mikashevichi‒Moscow Igneous belt.

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METHODOLOGY

This paper presents the results of the study of two samples of metapelitic rocks from the Uni-3 and Uni-50 holes. The mineral compositions were analyzed in transparent polished thin sections using a Tescan VEGA-II XMU scanning electron microscope with an INCA Energy 450 energy-dispersive spectrometer (Chernogolovka, Institute of Experimental Mineralogy, Russian Academy of Sciences).

The contents of petrogenic elements in the rocks were analyzed on a PW-2400 sequential spectrometer at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences. The accuracy of the analysis was 1–5 rel % for elements with concentrations of >0.5 wt % and up to 12 rel % for elements with concentrations of <0.5 wt %.

The pressure and temperature parameters of rock metamorphism were quantitatively characterized using classical methods of geothermobarometry based on exchange and biased equilibria and on the dependence of the limiting solubility of an element in a phase on temperature, and the methods of multiequilibrium thermobarometry (winTWQ 2.44, GeoPS 3.2.2.128). A detailed description of the research methodology, applied geothermobarometers, and software packages for modeling are given in Appendix 1. The compositions of rock-forming minerals are given in Appendix 2. The contents of the petrogenic elements of the studied rocks and pseudosection diagrams, made by GeoPs, are given in Appendix 3.

RESULTS

Within the Vyatka Belt, supracrustal rocks of the Uni Suite differ in their structural characteristics and mineral composition [2, 4]. In the eastern part of the belt, two nearby holes Uni-50 and Uni-3 revealed sillimanite–garnet–biotite gneiss and andalusite–two-mica schist, respectively, which were formed during the metamorphism of sedimentary rocks [4].

Sillimanite–garnet–biotite gneiss from hole Uni-50 has homogeneous textural and structural patterns and mineral composition throughout the entire interval of the hole section. The rock structure is banded due to the alternation of thin (3–5 mm) leucocratic quartz–feldspar bands and slightly wider (5–7 mm) melanocratic bands enriched in biotite. At the border of the melanocratic and leucocratic bands the content of biotite increases. The porphyroblastic texture of rocks is associated with the presence of garnet grains in a fine-grained rock matrix (Fig. 2a).

Fig. 2.
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BSE photomicrographs of rocks of the Uni Suite. (a‒c) Sillimanite–garnet–biotite gneiss, hole Uni-50: (a) Grt porphyroblasts in the melanocratic band, (b) Als-1 relics surrounded by Kfs, (c) Sil (Als-2) relics intersecting muscovitized biotite, quartz, and feldspar; (d) andalusite–two-mica schist from hole Uni-3: And porphyroblast surrounded by Ms-2 in the Bt-1–Qz–Ms-1 matrix.

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Garnet forms isometric grains 0.5–0.7 mm in diameter in the biotite–quartz–feldspar matrix and is most often intergrown with biotite. Garnet is characterized by a smoothly changing composition from the center (Grt-1Footnote 1: Grs5-7 Alm75-77 Sps6-7 Prp14-16) to the rim (Grt-2: Grs3-4 Alm77-79 Sps7-10 Prp8-13) of grains (Fig. 3e).

Fig. 3.
figure 3

(a–d) BSE images of garnet porphyroblasts and coexisting biotite in gneiss. The red arrows in the photo show the direction of the profile, the dots show the analysis sites and their numbers (Appendix 2). (e) Compositional profiles through garnets; and (f) coexisting biotites.

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Biotite forms flakes along the banding direction with an average length of 0.5–1 mm, occurs as individual grains and in the form of clusters, has “torn” edges at the ends of the flakes, and is closely adjacent to garnet grains. The composition of biotite varies slightly within each individual grain. In large grains the composition varies unevenly: Mg#Footnote 2 ranges within 46–49, the TiO2 content increases towards the grain center up to 2.7–3.2 wt % (Bt-1). In the contact zone with garnet, biotite has Mg# 49–51 and a TiO2 content of 1.7–2.7 wt % (Bt-2) (Fig. 3f). In some areas, chlorite occurs in the marginal parts of biotite and forms fine replacing aggregates. Some flakes of biotite contain muscovite intergrowths (Ms62 Pg5 Cel15 Fcel18). Muscovite flakes occur less frequently in the zones with sillimanite relics, almost completely replacing biotite (Ms81-83 Pg2-11 Cel4-6 Fcel4-7) (Fig. 2c). Small flakes of biotite in the rock matrix are similar in composition to Bt-1 (Mg# = 49, 2.7‒3.2 wt % TiO2).

Plagioclase forms grains with a diameter of 1–2 mm with uneven edges, numerous inclusions of quartz, relics of K-feldspar (Kfs), and relics of sillimanite. It is xenomorphic in relation to biotite, which overgrows it along the edges in melanocratic zones. It has a slight zoning from the center to the edge of grains. More basic composition (An26-28) is typical in the central parts (Pl-1). The edges of the grains, including those closer to the boundaries with garnet, are characterized by more sodic composition, An23-25 (Pl-2).

K-feldspar forms small poikiloblasts with uneven boundaries in plagioclase grains.

Aluminosilicate is represented by two polymorphs: andalusite (conditionally) and sillimanite. Andalusite (Als-1) forms grains of irregular shape, surrounded by a rim of K-feldspar (Fig. 2b). Sillimanite (Als-2) is registered in leucocratic bands in the form of needle-like grains with a length of 0.1–0.2 mm, which mostly intersect quartz or plagioclase (An25-28) grains, or scales of muscovitized biotite (with the composition of Bt-1) (Fig. 2c). Both polymorphs have low totals of compositions from 79 to 85 wt %, and the Al and Si atomic units in them are approximately equal, what does not correspond to the Al2SiO5 formula and, most likely, may refer to kaolinite pseudomorphs.

Thus, two assemblages may be distinguished: the earlier one, Grt-1 + Bt-1 + Pl-1 + Als-2 + Kfs + Qz, which most likely was formed under the peak conditions, and the later one (retrograde), Grt-2 + Bt-2 + Pl-2 + Qz + Ms + Chl. Most likely, Als-1 is a relic of the prograde stage of metamorphism; therefore, it is not included in the association of the peak and retrograde stages.

Fine-grained andalusite–two-mica schist from hole Uni-3 is composed of oriented fine flakes of biotite (Bt-1: Mg# 45‒47, 1.6‒2.4 wt % TiO2), muscovite (Ms-1: Ms75-81 Pg13-19 Cel3-4 Fcel3-10), and quartz grains. There are very few small grains of plagioclase (An15-17). K-feldspar occurs in plagioclase as poikiloblasts with indistinct boundaries. In the lower part of the hole section, there are relics of andalusite porphyroblasts “rotated” into a muscovite interlayer with X_Cel (Ms-2: Ms75-81 Pg15-20 Cel1-3 Fcel1-3) slightly lower than the matrix, framed by large flakes of biotite-2 (Mg# 42–47, 1.9–3.0 wt % TiO2), and located in a fine-grained matrix (Fig. 2d). Vast areas of muscovite-2 in the interlayer around andalusite are entirely replaced with chlorite.

DISCUSSION

The variety of rocks of the Uni Suite may be associated with a different protolith, as well as with the different P–T parameters of metamorphism of the original rocks with similar compositions. The variations in the degrees of metamorphism are the most pronounced in two nearby holes, the sections of which contain homogeneous sequences of rocks (gneiss in hole Uni-50; schist in hole Uni-3) with slight variations in the mineral and chemical composition (Appendix 3, Table 3.1).

Gneiss from hole Uni-50 is the most informative for the parameters of the peak and retrograde stages of metamorphism. A gradual decrease in the contents of Grs and Prp in garnets and an increase in the contents of Sps and Alm from the center (Grt-1) to the edge of the grains (Grt-2) (Fig. 3) provide evidence for the monotonic growth of margins in the retrograde stage under the conditions of uniformly decreasing pressure and temperature during one-stage metamorphism, most likely provoked by the influx of the fluid phase with decreasing pressure [6].

The Grt + Bt + Pl + Qz + Als-2 assemblage allowed us to estimate the parameters of different stages. To estimate the peak stage, we analyzed the core parts of garnet with high X_Prp, the central parts of large biotite flakes in contact with them with the lowest Mg# and the highest content of TiO2 and X_Al in the M2 site, and the central parts of plagioclase with the highest An content (Grt-1, Bt-1, Pl-1). The metamorphism peak parameters estimated by classical thermobarometers (Grt–Bt and Grt–Als–Pl–Qz (GASP)) have the following values: Т = 624‒644°C, P = 4.1–4.7 kbar. The compositions of contacting grains of garnet, biotite, and plagioclase (Grt-2, Bt-2, Pl-2) fix the following values at the retrograde trend of metamorphic evolution using the same methods: T = 539‒590°C, P = 2.1‒2.8 kbar. Modeling by winTWQ provided the following estimates for the peak stage: T = 621°C, P = 5.4 kbar; for the retrograde stage: T = 553°C, P = 2.9 kbar (Fig. 4), which are consistent with the data obtained by classical thermobarometry. Using the same compositions of biotite and Ti-in-Bt thermometer, we estimated the temperature for pressures of different stages: 623–659°C at 4–5 kbarFootnote 3 and 557–618°C at 2.2–4.4 kbar, which coincides with the estimates of the Grt–Bt exchange thermometer (Table 1).

Fig. 4.
figure 4

Diagrams (winTWQ-2.34) with intersecting reaction lines for the KCFMASH system and the Pl + Bt + Grt + Qz + Sil assemblage in gneiss of hole Uni-50, which determine the P–T conditions for (a) the peak (662°C, 5.59 kbar) and (b) the retrograde (553°C, 2.9 kbar) stages of metamorphism. The Thermodynamic Database (TDB) BA06 (Appendix 1) was used. Tav and Pav are the calculated average values; dT and dP are the bunch convergence parameters, the root-mean-square deviations of all considered pair crossings of reaction lines from the calculated average value along two axes. IR is the number of independent reactions; they are indicated in bold in the list of reactions and shown by bold lines in the graph.

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Table 1. Calculated P–T parameters of metamorphism of rocks of the Uni Suite
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The fine banding of gneiss with alternating bands with different proportions of leucocratic and melanocratic minerals suggests their formation during metamorphic segregation and, most likely, the beginning of partial melting. Many experimental studies ([11] and references therein) explain the initiation of melting in gneiss due to the decomposition of hydrous minerals in the absence of a free fluid (dehydration, higher temperature) or with the already present hydrous fluid (lower temperature). Based on the relationship of minerals in gneiss from hole Uni-50, we may assume different mechanisms. For example, relics of aluminum silicate (Als-1) surrounded by a Kfs rim (Fig. 2b) suggest its participation in the reaction of melting [12]: Bt + Pl + Als + Qz = Grt + Kfs + melt (1). The same reaction may be applied to interpret Kfs poikiloblasts in plagioclase grains and the appearance of garnet in leucosome. However, the temperatures that correspond to reaction (1) according to the experimental data [12] are somewhat higher (700–750°C) than the maximum values obtained in this gneiss (644°C) (Fig. 5), while garnet is much more common in melanosome. Relics of sillimanite (Als-2) are abundant in leucosome together with quartz, plagioclase-1, and K-feldspar, and as intergrowths with muscovitized biotite (composition Bt-1) (Fig. 2c). According to the experimental data [13], the formation of peritectic biotite, sillimanite, and K-feldspar is allowed through the decomposition of primary muscovite: Ms + Pl + Qz = Kfs + Sil + Bt + melt (2), the evidence for which in this rock is indirect only. In this case, the reverse reaction (2) may explain abundant muscovititization of biotite in zones with aluminosilicate upon retrograde hydration as well [14]. However, the temperatures of muscovite dehydration (700–750°C for 4–7 kbar) according to the data of [13] are higher than the obtained maximum P and T values as well.

Fig. 5.
figure 5

P–T estimates for metamorphism of rocks of the Uni Suite obtained using classical and multi-equilibrium thermobarometry and P–T trends of rock metamorphism. (1) Vyatka Belt; (2) Vorontsovka Terrane, after [17]; (3) Teya Complex, after [18], on the facies diagram for metapelite, after [19]. Black dotted lines with Roman numerals show the known reactions for granitoids and metapelite, respectively: (I) [12], (II) [13], (III) [15]. * Abbreviations of thermobarometers are given in Table 1.

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Considering these values, the structural patterns may be explained by hydrous melting of the leucocratic part [15]: Qz + Pl + Kfs + H2O = melt (3) (Fig. 5). Such melting occurs in the presence of a free aqueous fluid even at 650°C (4–5 kbar). This mechanism is supported by the presence of granitoid veins at the same depth as gneiss in hole Uni-50.

Using thermodynamic modeling (GeoPS software), we calculated the temperature of the peak stage of gneiss metamorphism, 670‒690°C, which exceeds the estimates by TWQ and the classical methods by 40–50°C, and the pressure range (4‒5.5 kbar) coincides with the calculated values (Appendix 3).

In contrast gneiss from hole Uni-50, schist from hole Uni-3 does not show evidence for melting, but contains andalusite porphyroblasts, which indicates metamorphism under lower pressure and lower temperature conditions. The lower Cel content in muscovite from schist in comparison to that in muscovite from gneiss provides evidence for lower pressures during metamorphism as well [16]. The temperature of the formation of the fine biotite grains (Bt-1) in the matrix obtained by the Ti-in-Bt thermometer at a pressure of 2–3 kbar, selected in accordance with the limitations of the andalusite facies series, was estimated as 540–580°C. An exchange Bt–Ms thermometer for matrix minerals at the same pressures showed the temperature of 520–580°C. Due to the fact that the average X_Cel in the muscovite matrix is 2% higher than that in muscovite surrounding andalusite, we may assume that the Bt-1 + Ms-1 + Qz assemblage is earlier, and the Ms-2 + Chl interlayer around andalusite is retrograde [16]. It can be assumed that coarse biotite (Bt-2), which overgrows the Ms-2 rim, is in equilibrium with andalusite. Both phases form the peak stage assemblage in the rock. The temperature of this stage estimated by the Ti-in-Bt thermometer at a pressure of 2–3 kbar was 570–593°C.

The area of schist association on the pseudo-section diagram plotted in GeoPS (Appendix 3) has a wide temperature range from 520 to 630°C, but is narrowly limited by the existing Mg#(Bt) isopleths from 600 to 630°C (at 2.6‒3.4 kbar), which exceeds the temperature estimated by classical methods for the peak stage by 10‒30°С, and is comparable to the errors of classical thermometers (Appendix 3).

This study allow us to consider the evolution of rock metamorphism for the Uni Suite. Estimation of the prograde stage begining parameters is difficult. Most likely, the parameters of the prograde stage for schist from hole Uni-3 may be taken as the P–T values determined for the paragenesis of the rock matrix (500°C, 2 kbar). The retrograde trend for schist and gneiss is well defined and has a slope of approximately 2.3 kbar/100°C (Fig. 5).

At the same time, the final stage of metamorphism for gneiss approximately coincides with the maximum values for schist, but the pressures and temperatures of the peak metamorphism, which formed the major paragenetic and structural–textural differences between rocks, are higher in gneiss by 100°C and 2–3 kbar (the peak conditions for schist and gneiss are 590°С, 2–3 kbar and 690°С, 5.4 kbar, respectively). Metamorphism of rocks is characterized by zoning and, according to the nature of such zoning metamorphism of the studied gneiss and schist, belongs to a relatively shallow type, to the andalusite–sillimanite facies series.

Such excesses of the P–T parameters could be associated with progressive metamorphism caused by local heating of rocks upon viscous deformations in a collision condition [20].

The closest analogue in terms of the age of metamorphism and the composition of the protolith is the Vorontsovka Terrane, which is part of the Volga–Don Orogen in the western flank of Volgo–Uralia [17]. It is characterized by the formation of metamorphic zones in metapelite in the temperature range of 430–750°C and pressures of 3–5 kbar. Garnet, staurolite, staurolite–sillimanite, and muscovite–sillimanite zones (560–600°C) and the most high-temperature (up to 750°C) sillimanite–K-feldspar–cordierite zone were distinguished in the terrane [17]. The author suggests an increase in heat flow during ductile deformations and folding in the hot lithosphere upon collisional processes as the most likely reason for metamorphism. In contrast to the Vorontsovka Terrane, the slope of the P–T trend in the Vyatka belt is steeper and is characterized by similar temperatures at lower pressures only, and there are no cordierite and staurolite zones (Fig. 5).

The slope of the P–T trend of the rock evolution in the central part of the Vyatka Belt is more comparable with the trend of the Teya Complex of the Yenisei Ridge (Fig. 5), where four zones of regional metamorphism associated with thickening of the Earth’s crust upon the collisional processes are distinguished: biotite, garnet, staurolite–andalusite, and sillimanite, as well as the zone of higher pressures (kyanite) in the fault area, associated with later local dynamometamorphism [18]. The mineral association of the first zone of the complex is similar to the And–Ms–Bt assemblage of schist from hole Uni-3 (however, it differs by the absence of andalusite), and the fourth zone is similar to the Sil–Grt–Bt assemblage of gneiss from hole Uni-50 (but differs by the presence of staurolite). The regional zones of the Teya Complex are characterized by an increase in the degree of metamorphism along the temperature axis towards the core of the anticline. An increase in the degree of metamorphism most likely occurred in the Vyatka Belt towards the junction zone with the Archean Tokmovo Megablock as well; however, this requires further study of the basement rocks from the available holes.

CONCLUSIONS

Zonal metamorphism of the andalusite–sillimanite facies series within the amphibolite facies of the HT–LP type has been identified for rocks of the Uni Suite of the Vyatka Belt. The peak stage of the formation of the Pl + Bt + Qz + Kfs + Grt + Sil assemblage in Sil–Grt–Bt gneiss was distinguished by the classical thermobarometers Grt–Bt and GASP: 624‒644°C, 4.1‒4.7 kbar; using winTWQ: 621°C, 5.4 kbar; and using the isopleth intersection method in GeoPS: 670‒690°С, 4‒5.5 kbar. The latter method in combination with the structural and textural patterns of gneiss confirmed partial melting under the water-saturated conditions. The final part of the retrograde stage was estimated using thermobarometers (539‒590°C, 2.1‒2.8 kbar) and winTWQ (553°C, 2.9 kbar). The area of the formation of the andalusite facies series Pl + Ms + Bt + Qz + Kfs + And ± Chl in And–Ms–Bt schist was estimated by the Ms–Bt and Ti-in-Bt thermometers as 520–590°C at 2–3 kbar, and this area was confirmed in the pseudo-section diagram using isopleths of Mg# in biotite and celadonite contents in muscovite.

Taking into account the anatexis in gneiss and temperature estimates from 520 to 690°C in the close zones of metasedimentary rocks of the Uni Suite, as well as the pressure difference from 2 to 5.4 kbar, we may suggest high-gradient metamorphism in the studied inner region of the Vyatka Belt. This is the most likely variant of heating as a result of ductile deformations at the collisional stage under the conditions of the “hot” lithosphere of the Paleoproterozoic Orogen, of which the belt is a fragment.