INTRODUCTION

Paleotectonic reconstructions are one of the most important avenues of studying the evolution of the Earth’s crust. Such reconstructions are conventionally carried out with the use of extensive information on magmatic complexes, because characteristics of their sources and parameters under which their melts were derived may serve as informative indicators of the tectonic settings. This approach plays a determining role in studying Early Precambrian complexes, in which many features of rocks needed for tectonic reconstructions have been obliterated by later processes (e.g., Moyen and Laurent, 2018).

In this paper, we utilize recently acquired data on the petrology of magmatic rocks to discuss reconstructions of the conditions and circumstances under which the Paleoproterozoic Vorontsovka terrane was formed. This terrane extends over a vast area between the ancient Sarmatia and Volga–Ural cratons and plays a pivoting role for understanding the crustal evolution of the East European craton during the Rhyasian–Orosirian period. A key point for understanding the mechanisms of crustal growth at that time is the powerful pulse of magmatism at 2050–2090 Ma, which produced numerous mafic–ultramafic, diorite, and granitoid plutons. The isotope age and geochemical characteristics of intrusions in the Vorontsovka terrane were viewed as evidence that they were formed in a postcollisional setting (Savko et al., 2014; Savko and Terentiev, 2017; Terentiev and Savko, 2016; Terentiev et al., 2016, 2018).

Quite recently, we have found subvolcanic bodies of sodic dacite porphyry depleted in HREE, LILE, and HFSE in the western margin of the Vorontsovka terrane. These features make the rocks similar to Archean high-pressure TTG associations and modern adakites and may be used as indicators of tectonic processes in the lithosphere of the Vorontsovka terrane.

Our study was centered on determining the U–Pb zircon age of the dacitic porphyries, identifying the sources of their melts based on the elemental and isotope geochemistry of the rocks, and understanding the tectonic setting in which the felsic magmatism developed.

GEOLOGY

The Volga–Don orogen is an extensive Paleoproterozoic subductional–collisional area between the Archean cores of the Sarmatia and Volga–Ural cratons (Fig. 1). It includes the Losevo and Don terranes, which are adjacent to the Kursk block, the extensive Vorontsovka terrane, and the Tersinskii belt, which is bounded by Archean Volga–Ural complexes in the east.

Fig. 1.
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Schematic geological map of the Voronezh crystalline massif. The setting of the crustal segments of the East European craton is according to (Gorbatschev and Bogdanova, 1993).

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The Vorontsovka terrane is made up of folded metamorphosed Paleoproterozoic graywackes of the Vorontsovka Group 6–8 km thick (according to geophysical data). It extends for a distance over 600 km at a width of 100–300 km (Fig. 1). In the west, the terrane is bounded by the Losevo–Mamon deep fault, which separates this terrane from the Losevo and Don ones. In the south, it is terminated by the buried structures of the Azov block. In the east, the Vorontsovka terrane is bounded by the Paleoproterozoic Tersinskii belt of metavolcanic rocks and the South Volga supracrustal complex (Bibikova et al., 2009; Bogdanova et al., 2006).

The Vorontsovka terrane consists of a thick flyschoid sequence, which was accumulated in a tectonically active setting with weak chemical weathering of the rocks of different composition, from felsic to mafic (Savko et al., 2011). The sources of the terrigenous material were juvenile Paleoproterozoic arc complexes, as follows from Sm–Nd isotope data [ТNd(DM)) of 2100 to 2400 Ma (Shchipansky et al., 2007)] and from the age of the detrital zircon of 2100–2200 Ma (Terentiev and Santosh, 2016). The metasedimentary rocks were affected by zoned HT/LP metamorphism at 3–5 kbar and 450–700°С (Savko, 1990, 1994) and were transformed into schists, metasandstones and into garnet–cordierite gneisses in high-temperature zones (Gerasimov and Savko, 1995). The metamorphic processes were dated at about 2069 Ma (Savko et al., 2015).

The metasedimentary sequences of the Vorontsovka terrane are cut by mafic intrusions and bodies of geochemically contrasting S- and A-type granites, which were dated at 2.05–2.07 Ga and have juvenile isotope signatures [εNd(Т) = +2.0 to +2.6)] (Savko et al., 2014). The S-type granites are believed to be produced by melting metasediments of the Vorontsovka Group in the marginal part of the terrane. The A2 granites in the axial portion of the terrane were derived when the crustal thickness decreased. The A2-type granites are differentiation products of mafic magmas emplaced into the lower crust and extensively contaminated with crustal material. The melts were emplaced after collision-related metamorphism at 2.07 Ga (Savko et al., 2014, 2015) in relation to postcollisional tectonics. The later platform magmatism produced gabbro-dolerites of the trap association of the Novogol’skii complex (Savko and Bocharov, 1988), dated at 1805 ± 14 Ma (Chernyshov et al., 2001), and Cambrian syenite dikes of the Artyushki complex (Nosova et al., 2019; Skryabin et al., 2015).

The Vorontsovka terrane is separated from the Kursk block by the Don terrane in the south and Losevo terrane in the north. The latter consists of island-arc edifices on an active continental margin, which are made up mostly of volcanics of a bimodal series dated at 2170–2120 Ma (Terentiev et al., 2017). These rocks were later intruded by large batholiths of I-type granites of the Usman complex, dated at 2100–2075 Ma, in a postcollisional environment, after a collisional event at approximately 2100 Ma (Terentiev et al., 2016; Shchipanskii et al., 2007).

The Don terrane is dominated by batholiths of I-type granitoids of the Pavlovsk complex, which were dated at 2081–2063 Ma (Terentiev et al., 2020) and abound in amphibolite xenoliths as large as a few meters, whose protoliths were mafic rocks of the Losevso Group. The protoliths of the Pavlovsk granitoids already contained an Archean crustal component. Toward the Archean Kursk block, they have progressively less radiogenic Nd isotopic composition, εNd(T) of +0.3 to −4.2, and xenogenic 2.8-Ga zircon appears in them (Terentiev et al., 2020).

The dacite porphyries were recovered by Borehole 548-s in the western part of the Vorontsovka terrane, not far from the boundary with the Losevo terrane (Figs. 1, 2). They occur as an array of nearly vertical (5°−7° to the borehole core axis) subvolcanic bodies (dikes) hosted in metapelite schists and metasandstones of the Vorontsovka Group, which are conformable with the foliation of the host rocks. The apparent thicknesses of the dacite bodies (as seen in the core material) range from 0.8 to 14.5 m (Fig. 2). With increasing depth, the orientation of the foliation and contacts with the dacites becomes more gently sloping, 40°−45° to the core axis. The dacites and their host metaterrigenous rocks were metamorphosed and show schistose structures, with the foliation conformable to that of the host schists and metasandstones. The rocks are cut across by relatively thin (no thicker than 1 cm) quartz and quartz–carbonate veinlets, some of which are conformable with the foliations and others cut it.

Fig. 2.
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Geological section through Borehole 548-s and microphotographs of thin sections of the dacite porphyries: (a–e) altered plagioclase xenocrysts in fine-grained quartz–feldspathic matrix; (f) zoning of a plagioclase xenocryst in dacite. Sample numbers in this figure: borehole number/depth.

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ANALYTICAL TECHNIQUES

Analysis for major oxides. Rocks were analyzed for major components by XRF on a S8 Tiger spectrometer (Bruker AXS GmbH, Germany) at the Voronezh State University (VSU). The procedure of sample preparation to analysis involved fusing 0.5 g of the powdered sample material with 2 g Li tetraborate in a muffle and subsequent casting of the glassy pellet. The spectrometer was calibrated and the quality of the analyses was controlled using state-certified reference samples for the chemical composition of rocks (GSO №8871-2007, GSO № 3333-85, GSO № 3191-85). The analyses were accurate to 1–5 relative % for components whose concentrations were higher than 1–5 wt % and up to 12 relative % at concentrations below 0.5 wt %. The raw analytical data were processed by conventional techniques implemented in the Spectra Plus software (Bruker AXS GmbH, Germany).

Analysis for trace elements and REE. Trace elements and REE were analyzed by ICP-MS at the Analytical Certification Testing Center of the Institute of the Technological Problems of Microelectronics and Ultrapure Materials in Chernogolovka in the Moscow area. The rock samples were decomposed by acids in open and closed systems depending on the composition of the rocks. The detection limits were 0.02–0.03 ppm for REE, Hf, Ta, Th, and U; 0.03–0.05 ppm for Nb, Be, and Co; 0.1 ppm for Li, Ni, Ga, and Y; 0.2 ppm for Zr; 0.3 ppm for Rb, Sr, and Ba; and 1–2 ppm for Сu, Zn, V, and Cr. The accuracy of the analysis was controlled by replicate analysis of the internationally and nationally certified standards GSP-2, VN, SGD-1A, and ST-1. Most elements were analyzed accurate to 3 to 5%.

The isotope age of zircon was measured on a SHRIMP-II ion probe at the Center for Isotope Studies of the Karpinsky Russian Geological Research Institute (VSEGEI) in St. Petersburg by the conventional technique (Larionov et al., 2004; Williams, 1998), using the 91500 and Temora zircon standards. The calculations were done using the decay constants from (Steiger and Jäger, 1977), with corrections for radiogenic Pb according to (Stacey and Kramers, 1975) on the basis of measured 204Pb/206Pb. The results were processed with the SQUID v. 1.12 and ISOPLOT/Ex 3.22 software (Ludwig, 2005). Inasmuch as the results defined a discordia with a zero lower intercept, the age was evaluated by the upper intercept with the concordia. Following (Ludwig, 1998), concordance was understood as the fact that the concordia is overlapped by the 2σ error ellipse.

Sm–Nd whole-rock age. The Sm–Nd isotope studies were carried out at TsII VSEGEI. The Sm and Nd isotopic compositions were determined on a TRITON multicollector mass spectrometer in static mode. The measured 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.7219 and to 143Nd/144Nd = 0.512115 in the JNdi-1 Nd standard. The Sm and Nd concentrations were measured accurate to ±0.5%, and the 147Sm/144Nd ± 0.5% and 143Nd/144Nd isotope ratios were determined accurate to ±0.005% (2σ).

Microprobe analysis (EPMA). Spot analyses of minerals were conducted on a Jeol 6380LV microprobe equipped with an Inca EDS system for quantitative analysis at VSU. Analyses were done at an accelerating voltage of 20 kV, beam current of 1.2 mA, counting time of 90 s, and a beam diameter of 1–3 μm. ZAF corrections were introduced when oxide concentrations were calculated and the accuracy was assessed using the proprietary software of the microprobe. The accuracy of the analysis was controlled on a systematic basis by replicate analysis of reference samples of natural and synthetic minerals.

PETROGRAPHY AND MINERALOGY

The dacites are pale gray to pale brownish gray schistose blastoporphyric rocks consisting of a fine-grained groundmass and feldspar phenocrysts, which make up to 30% of the rock by volume (Fig. 2). The matrix is lepidogranoblastic.

The dacites are dominated by plagioclase (60–80 mod. %), which occurs in the matrix and as phenocrysts up to 2–5 mm across and is often replaced by secondary fine-grained muscovite aggregates. Plagioclase in the fine-grained rock groundmass is oligoclase (An10–23). The unaltered xenocrysts are concentrically zoned and have darker cores. The cores are more calcic (An49–55) than the margins (An31–35) (Fig. 2f). The groundmass contains minor amounts (no more than 5 mod. %) of potassic feldspar. The fine-grained matrix contains up to 20 modal % quartz.

Biotite (2−5 modal %) is the only mafic mineral and occurs as elongate brown laths up to 2 mm long, is Fe-rich (XFe = 0.72–0.81), and contains much TiO2 (2.4–2.9 wt %). The accessory minerals are apatite, titanite, and zircon, the ore minerals are magnetite and sphalerite, and the secondary ones are chlorite, epidote, and calcite.

GEOCHEMISTRY

The dacites contain 67.9 to 74.4 wt % SiO2, intermediate concentrations of alkalis (Na2O + K2O = 6.3–7.8 wt %), with Na obviously dominating over K (K2O/Na2O = 0.44–0.77), intermediate concentrations of TiO2 (0.1–0.4 wt %), and have XFe = 0.69−0.84 and A/CNK close to 1.0 (Fig. 3, Table 1). In the TAS classification diagram (Middlemost, 1994), the composition points of the rocks plot within the fields of dacites and rhyolites (Fig. 3). Binary diagrams show negative correlations of SiO2 with TiO2, CaO, Al2O3, MgO, and K2O (Fig. 4).

Fig. 3.
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Classification diagrams for the dacites of the Vorontsovka terrane: (a) (Na2O + K2O)–SiO2; (b) A/CNK–A/NK; (c) FeOtot/(FeOtot + MgO)–SiO2; (d) (Na2O + K2O – CaO)–SiO2.

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Table 1. Concentrations of major oxides, trace elements, and REE in the dacites of the Vorontsovka terrane
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Fig. 4.
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Diagrams showing the distributions of major oxides.

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The dacites are typically low in such “mafic elements” as Ni and Cr and in all HFSE (particularly Y) but are high in Sr (Sr/Y = 83–150), and contain moderate concentrations of Ba and Rb (Table 1). The rocks are also depleted in REE, whose normalized patterns are negatively sloped, with enrichment in LREE and with their very significant fractionation [(La/Yb)n = 56–177] and without clearly discernible Eu* anomaly (Table 1, Fig. 5), with very high (Gd/Yb)n = 10–19. The normalized multielemental patterns of the rocks show negative Nb and weak positive Ti anomalies (Fig. 5).

Fig. 5.
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(a) Chondrite- and (b) primitive mantle-normalized multielemental patterns.

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U−Pb ZIRCON GEOCHRONOLOGY

Zircon from a metadacite dike, sample 548-s/185.7 (Borehole 548-с, depth 185.7 m) was dated based on SHRIMP-II analyses. The zircon forms colorless euhedral and subhedral prismatic crystals, with numerous long-prismatic grains (150−200 μm long) that have acute pyramidal termination (Fig. 6). The crystals sometimes contain solid inclusions. The CL images of these crystals display their growth structures: thin concentric oscillatory zoning. No clearly seen inherited cores of the grains were found. Structural and morphological features of these grains suggest their magmatic nature and the almost absolute absence of any overprinted transformations.

Fig. 6.
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CL images of zircon grains from the metadacites of the Vorontsovka terrane. Small white circles with numerals are the spots of SHRIMP-II analyses. The numbers of the analytical spots correspond to the numbers of analyses in Table 2.

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We conducted 16 spot analyses of 15 grains (Table 2, Fig. 7). In a concordia plot, all data are approximated by a single regression line with closely similar concordant age values. No appreciable differences were found between the inner and outer portions of the crystals. The upper intercept of the regression line with the concordia for all analytical spots corresponds to an age of 2069 ± 20 Ma (N = 16, MSWD = 0.42). The concordant age (N = 12) is 2069 ± 13 Ma (MSWD = 0.0052, concordance probability = 0.94) (Fig. 7).

Table 2. U−Pb geochronologic data (SHRIMP-II) on zircon from the metadacites
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Fig. 7.
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U–Pb concordia plots for zircon from the metadacites of the Vorontsovka terrane: SIMS data.

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Sm−Nd ISOTOPE DATE

The isotope composition of Nd was analyzed in two whole-rock samples of the dacite: 548-s/185.7, whose zircon isotope age has been determined, and 548-s/237.6. The εNd(2069) = 2.6 and 3.4, and the model age TNd(DM) is 2258 and 2222 Ma (Table 3, Fig. 8), respectively, which indicates that the crustal prehistory of the juvenile mantle sources of the melts was brief.

Table 3. Sm−Nd isotope data on the dacite porphyries
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Fig. 8.
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Results of Sm–Nd study of the metadacites from the Vorontsovka terrane. Data on the granites of the Vorontsovka terrane are from (Savko et al., 2014).

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GEOCHEMICAL TYPES

According to their geochemical features, the dacites of the Vorontsovka terrane are iron-rich, mostly metaluminous rocks of the calc-alkaline series. In spite of their high Fe#, these rocks cannot be classed with A-type granites, which are characterized by potassic alkalinity, are typically enriched in HFSE and REE, and have negative Eu* anomalies. The relatively low Al# of the rocks and the fact that they do not contain muscovite (and/or garnet, cordierite), low LILE concentrations, and predominantly sodic alkalinity make the dacites different from S-type granitoids. These rocks differ from M-type granitoids in having strongly fractionated LREE and HREE patterns. In discriminant diagrams in terms of “alphabetic” classification, the dacites fall within the fields of I-, S-, and M-type granites (Whalen et al., 1987), and they plot within the field of arc granites in the Rb vs. (Y + Nb) diagram (Pearce et al., 1984) (Fig. 9). It is thus reasonable to believe that the dacites of the Vorontsovka terrane are I-type granitoids.

Fig. 9.
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Discriminant diagrams for the metadacites of the Vorontsovka terrane: (a) according to (Pearce, 1996); (b) according to (Whalen et al., 1987).

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According to their geochemical features, the dacites of the Vorontsovka terrane are close to high-pressure trondhjemites of Archean TTG associations (Moyen, 2011) (Fig. 10) but differ from them in being more strongly depleted in LILE and HFSE, having higher Fe#, and anomalously strongly fractionated LREE and HREE patterns. The dacites differ from typical adakites in containing low concentrations of the compatible elements Ni and Cr and those of Mg. In the latest geochemical review (Sotiriou et al., 2023), Archean TTG associations with (Gd/Yb)n > 10 are very sparse (a few instances). They are also very rare among Proterozoic and Phanerozoic trondhjemites and adakites.

Fig. 10.
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Discriminant diagrams (Drummond and Defant, 1990) for the dacites of the Vorontsovka terrane.

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PETROGENESIS

The strongly depleted HREE patterns of the rocks indicate that the dacite melts were derived by melting in equilibrium with a garnet-bearing residue. The sodic specifics of the rocks, their low concentrations of incompatible elements, and the high radiogenic Nd isotope composition provide grounds to think that the dacitic melts were derived from a depleted mafic source. Petrogenetic calculations indicate that this might have occurred at the partial melting of depleted mafic rocks of the N-MORB type in equilibrium with eclogitic residue (Fig. 11). This is consistent with experimental data, according to which dacitic melts can be derived via 20−40% melting of metabasalt at 1000–1100°C and 16–32 kbar in equilibrium with eclogite residue (Hastie et al., 2016; Rapp and Watson, 1995).

Fig. 11.
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Results of numerical simulations of the origin of the dacites and rhyolites. (1) Composition field of the natural rock samples; (2, 3) hypothetical sources of the melts: (2) MORB-type tholeiite from the bottom portion of the Losevo Group (sample 7782/422 from Shchipansky et al., 2007); (3) average N-MORB (White and Klein, 2014); (4) model dacitic melt produced by 20% equilibrium melting of tholeiite, with the residue consisting of clinopyroxene (50%), garnet (48%), and rutile (2%); (5) model rhyolitic melt produced by 30% differentiation of dacitic melt with the removal of the cumulus association of plagioclase plagioclase (90%) + apatite (9%) + zircon (1%). The calculations were carried out using formulas in (Arth, 1976) and partition coefficients in (Rollinson and Pease, 2021).

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Changes in the composition of dacite toward rhyolite composition may be explained by crystallization differentiation. The major cumulus phase was likely plagioclase, whose phenocrysts are widespread in these rocks (Fig. 2) and were likely entrained from an intermediate chamber. The massive crystallization of plagioclase may have been triggered by a pressure decrease during the ascent of the felsic magmas and resulted in noticeable decrease in concentrations of Al2O3, CaO, Sr, and Eu in the rhyolites. The radical decrease in the concentrations of all REE with the transition from the dacites to rhyolites may have been caused by the crystallization of a calcic accessory mineral that could concentrate REE, for example, titanite and/or apatite (Fig. 10).

The geochemical and isotope features of the dacite suggest that they could be generated by the melting of a depleted mafic sources of the N-MORB type at depths greater than 60 km, in equilibrium with an eclogite residue, and subsequent differentiation at shallow depths. This petrogenetic scenario might have been implemented in a subduction zone at the melting of a down-going oceanic slab (Martin and Moyen, 2002). However, this scenario is inconsistent with the low Mg# of the dacites and their low MgO, Cr, and Ni concentrations, which rules out that the felsic subductional melts could interact with rocks of the overlying mantle wedge (Martin, 1999; Smithies, 2000). Melts of such composition could be derived in tectonic settings of the following two types: (1) low-angle subduction and (2) crustal thickening at collision that had occurred before the dacites were derived.

In the situation of low-angle subduction, such magmas could be brought to the lower crust without any significant changes in their composition (Martin and Moyen, 2002). For example, derivatives of slab melting (adakite magmas in the Japanese island arc) retain low MgO, Cr, and Ni concentrations (Tsuchiya et al., 2007).

A likely more probable scenario is that the dacitic magmas were derived by the melting of the lower levels of strongly thickened mafic crust. Its thickening (the formation of an orogenic wedge) occurred before the 2.069-Ga dacite dikes were emplaced as a result of collisional processes during subduction (arc−continent collision), likely at about 2.1 Ga (Tsybulyaev et al., 2021; Korish et al., 2022). Collision and associated metamorphism reoccurred (or continued) at the collision of Sarmatia and Volga−Uralia (continent−continent collision) at about 2.07 Ga (Savko et al., 2015, 2018).

It has been demonstrated that the collision and metamorphism proceeded when the lithosphere of the young Vorontsovka terrane was “warm” and “wet” (Savko et al., 2015), and this was favorable for the eclogitization of the lower crustal mafic rocks. The partial melting of the eclogite protoliths was triggered by fluids that were released in the course of metamorphic reactions of dehydration and/or at delamination in the bottom part of the crust and magmatic underplating.

Scenarios of this type for the derivation of adakite magmas at the melting of thickened mafic lower crust were proposed for the Early Paleozoic collisional orogen Qinling in southern China (Qin et al., 2015) and the Miocene Himalayan−Tibetan collisional orogen (Chung et al., 2003; Hou et al, 2004; Wang et al., 2005). Note that the Volga−Don collisional orogen is thought to be a Paleoproterozoic analogue of the Himalayan−Tibetan orogen (Shchipansky and Kheraskova, 2023).

CONCLUSIONS

Metamorphosed dacite porphyry dikes (crystallization age 2069 ± 13 Ma) were first found among metaterrigenous flyschoid sequences of the Vorontsovka Group in the western part of the Vorontsovka terrane, at its boundary with the Losevo terrane. These are iron-rich, mostly metaluminous rocks of the calc-alkaline series and correspond to I-type granites. Such geochemical features and sodic alkalinity, depletion in LILE, HFSE, compatible elements, and REE, with the strong fractionation of the latter, and the absence of an Eu* anomaly, high Sr/Y ratios, and particularly (Gd/Yb)n > 10 and a radiogenic Nd isotope composition suggest that the melts were derived via the partial melting of juvenile mafic protoliths of the MORB type under high pressures, in equilibrium with garnet−pyroxene eclogitic residue. We believe that the dacites may have been formed either as a result of low-angle subduction of an oceanic slab beneath the Sarmatia craton or by the partial melting of highly thickened (>60 km) eclogitized mafic lower crust when the orogen was formed. The change in composition from dacite to rhyolite may have been caused by crystallization differentiation.