The production and evolution of the continental crust throughout the history of the Earth have always been in the spotlight of geological research. In the Phanerozoic these processes were related to convergent geodynamic settings [1] and, therefore, to intracontinental orogenic belts developed after the closure of paleo-oceans. The Central Asian orogenic belt (CAOB) is an example of an accretionary orogen formed as a result of tectonic coupling (accretion) of arc complexes, accretionary prisms, and ophiolites, volcanogenic–sedimentary complexes of oceanic plateaus and back-arc basins [2]. All the rock complexes mentioned are of mantle origin and represent the juvenile crust, which is involved in the formation of granites during accretion. This conclusion is supported by the fact that the most distinctive feature of granitoids in Central Asia is the predominant positive ɛNd(t) and young model TNd(DM) ages [3–5]. It was suggested that the CAOB represents the largest structural and material “unit” of continental crust growth in the Phanerozoic [5]. However, it was shown later that a significant part of COAB arc complexes cannot be considered to be fully juvenile, because they formed upon the older continental crust or with an admixture of more ancient recycled sedimentary material [6]. Thus, the relationship between juvenile crust and more ancient recycled material is a key parameter to determine the history of crust formation in intracontinental orogenic belts.

The Mongol–Okhotsk orogenic belt (MOOB) is one of the important structures within the CAOB [7] (Fig. 1). In the Transbaikalian part of the MOOB, there are the most completely preserved fragments of arc complexes, as well as the entire spectrum of rocks from its accretionary prism, so that we can assess the relationship between juvenile material and recycled sedimentary material in the crust of this orogen. The most representative fragments of the juvenile crust in this part of the MOOB are rocks of the Kamenka (Kamenskii) arc terrane and those of the Urtui volcanogenic–sedimentary formation [8] (Fig. 1). The Kamenka terrane is an indicator of a subduction zone running along the present-day northwestern margin of the MOOB and dipping beneath the Siberian paleocontinent; sediments of the Urtui Formation indicate that subduction processes took place in the margin of the Argun superterrane that framed the MOOB on its southeastern side.

Fig. 1.
figure 1

Schematic position of the Mongol–Okhotsk orogenic belt in the structure of the Central Asian orogenic belt and locations of terranes in the central MOOB, after [7, 8]: (1) West Stanovoi terrane (WST); (2) Kamenka arc terrane (KM); (3) Onon terrane of the accretionary wedge (ON); (4) Argun superterrane (ARG); (5) sediments of the Urtui Formation; (6, 7) thrusts and faults confining the MOOB; (8) positions of the objects studied: (1) Kamenka Formation; (2) Urtui Formation.

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The Kamenka terrane includes intrusive bodies of the Bereya gabbro–diorite–tonalite complex and spatially close volcanogenic–sedimentary rocks of the Kamenka (Kamenskaya) Formation [9]. The Kamenka Formation is represented by interbedding of sedimentary rocks (volcanomictic breccia conglomerates and gravelites, as well as tuffaceous sandstones and tuffaceous argillites) and volcanic ones (those of the basalt–andesibasalt–andesite–dacite–rhyolite series). Volcanic rocks underwent deep greenstone alterations. The sections of this formation are characterized by sharp changes and demonstrate an inhomogeneous distribution of volcanic rocks. Gabbros and diorites of the Bereya complex form dikes, multiple apophyses and small intrusive bodies within the volcanogenic–sedimentary sequence. Resulting from SHRIMP-II U–Pb isotope dating of gabbro-diorites on zircons (conducted at the Center for Isotope Research, Karpinskii All-Russia Geological Research Institute, St. Petersburg), the age of 254.3 ± 5.1 Ma was obtained [10], which corresponds to the Late Permian, giving us grounds to assume the age of the hosting volcanogenic–sedimentary sequence to be Late Permian as well. Volcanic rocks of this formation refer to the series of normal total alkalinity and moderate potassium content. A small number of basalt compositions fall within the range of subalkaline ones. Normalized REE distribution spectra for basalts, andesibasalts, andesites, and rhyolites are moderately enriched: La/Yb(N) = 1.96–4.31. The multicomponent diagram for basalts of the Kamenka Formation (Fig. 2) demonstrates the characteristics typical of arc volcanics: high level of accumulation for K, Rb, Ba, Sr, and Ba, and a sharp predominance of LILE over HFSE, which is indicative of the rocks whose origin is related to subduction zones.

Fig. 2.
figure 2

Multicomponent plot for basalts of the (1) Kamenka and (2) Urtui formations. The compositions of basalts with (3) normal and (4) high potassium contents from Tolbachik volcano (after [11]) are shown as references of the arc basalts. Normalization was made to the primitive mantle composition, after [12].

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The Urtui Formation is composed of tuffaceous sandstones, tuffaceous siltstones, and by arkose and graywacke sandstones, often associated with jasperoids and siliceous rocks, limestone lenses, and, sometimes, mafic and acid volcanic rocks. The age of rocks is confirmed by biostratigraphic evidence and corresponds to the Early Carboniferous [13]. The sediments of the Urtui Formation are characterized by diverse types of sections and their lateral variability. Volcanic rocks of the Urtui Formation are represented by predominantly basalts that underwent greenstone alterations. More acid varieties of volcanic rocks are present in a sharply subordinated amount. Basalts refer not only to the series of normal total alkalinity, but also to the subalkaline series. Most of basalt compositions are moderately potassic at the subordinated amounts of high-K varieties. Normalized REE distribution spectra demonstrate weak and moderate enrichment: La/Yb(N) = 0.98–1.83. High-K varieties are characteristic of more differentiated distribution spectra for lanthanides: La/Yb(N) = 4.62–7.04. The multicomponent characteristics of basalts of the Urtui Formation fully correspond to the rocks formed in subduction settings (Fig. 2).

Thus, volcanic rocks of both the Kamenka and Urtui volcanogenic–sedimentary formations are derivatives of arc magmatism and represent a juvenile component of the continental crust within the MOOB.

The Sm–Nd isotope studies of volcanic and volcanogenic–sedimentary rocks of the Kamenka and Urtui formations were carried out at the Shared Use Center for Isotope–Geochemical Research (Vinogradov Institute of Geochemistry, Siberian Branch, Russian Academy of Sciences, Irkutsk) with the use of a NEPTUNE Plus multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) by the technique from [14]. The results obtained (Fig. 3) allow us to estimate the characteristics of the juvenile crust in the Transbaikalian sector of the MOOB, as well as the possible role of its material in the formation of the accretionary wedge sediments within the belt. To estimate the values of the TNd(DM) model age in rocks of volcanogenic–sedimentary sequences, we used rocks with 147Sm/144Nd < 0.14.

Fig. 3.
figure 3

εNd versus age plot for volcanogenic-sedimentary and intrusive rocks from the Transbaikalian part of the MOOB: (1) granites of the Urulyungui complex, Argun superterrane [15]; (2) metasedimentary rocks of the Daurian Series, Argun superterrane [16]; (3) metasedimentary rocks of the Onon Formation (Onon terrane, accretionary wedge); (4) metasedimentary rocks of the Chindat Formation (Onon terrane, accretionary wedge); (5) metasedimentary rocks of the Ust-Borzya Formation (Onon terrane, accretionary wedge); (6) volcanic rocks of the Urtui Formation; (7) sedimentary rocks of the Urtui Formation; (8) volcanic rocks of the Kamenka Formation; (9) sedimentary rocks of Kamenka Formation; (10) evolution trend of the Nd isotope composition for igneous and metamorphized terrigenous rocks of the Argun superterrane. The boundaries of the domains of crustal evolution of the Nd isotope composition are after [17].

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All volcanic rocks of the Kamenka Formation, from basalts to rhyolites, have positive ɛNd(254 Ma) values, +1.4 to +3.8, and TNd(DM) = 896–920 Ma. The values of εNd(350 Ma) in basalts of the Urtui Formation are also positive, +1.7 to +6.0, at TNd(DM) = 773–939 Ma. Thus, the values of εNd(t) in arc volcanics of most of the formations mentioned indicate the direct relationship of these units with an isotope-depleted mantle source. However, the TNd(DM) values of volcanics within the range of 773–939 Ma, which significantly exceed the estimated geological age, may indicate a certain influence of recycled sedimentary material, which was involved in magma generation processes during subduction.

The supply of juvenile volcanogenic material into the sedimentary basin should have been affected by the isotope characteristics of sediments that accumulated near island arcs and/or active continental margins. Tuffaceous siltstones of the Kamenka Formation have positive εNd(254 Ma) values, from +2.8 to +3.5, at TNd(DM) = 938–993 Ma, which are completely comparable to the isotope characteristics of volcanic rocks and indicate the juvenile character of the sediment source. Contrary to sedimentary rocks of the Kamenkaaya Formation, the studied tuffaceous siltstones of the Urtui Formation have lower values of ɛNd(350 Ma), from +1.0 to –3.4, at significantly older model Nd isotope ages: TNd(DM) = 1142–1408 Ma. This indicates a considerable fraction of more ancient recycled crustal material in the provenance of the Urtui Formation compared to the Kamenka Formation. Late Riphean rocks of the Argun terrane, represented by granitoids of the Urulyungui complex and sedimentary rocks of the Daurian Series, might be such a source. Granitoids are characterized by εNd(800 Ma) ranging from –0.4 to –1.7 at TNd(DM) = 1550–1720 Ma [15], whereas metasedimentary rocks have ɛNd(t) ranging from –2.0 to –7.0 at TNd(DM2) = 1657–2063 Ma [16] (Fig. 3).

The broader extent of recycled crustal material added to the sedimentary basin can be determined owing to the employment of the Nd isotope composition of metasedimentary rocks of the MOOB accretionary wedge (Fig. 3). The accretionary wedge is represented mainly by rocks of the Onon, Chindat, and Ust-Borzya formations, which are united within the Onon terrane [8]. The age of the first of these formations can be determined as Ordovician [18], while two others can be referred to the Devonian [19]. The Nd isotope composition of these rocks is characterized by chiefly negative values of εNd(t) (–0.3 to –7.0) at TNd(DM–2) = 1100–1770 Ma, indicating the presence of a significant amount of recycled crustal material, comparable to Late Riphean rocks of the Argun terrane. A certain part of metasedimentary rocks has weakly positive values of ɛNd(t) (from +0.7 to +2.0), which suggests the presence of a juvenile crustal component.

The Sm–Nd isotope studies of arc volcanics of the Kamenka and Urtui Formations from the Transbaikalian segment of the MOOB have allowed us to determine the parameters of the juvenile crust within the orogenic belt: the crust is characterized by positive ɛNd(t) and model ages TNd(DM) < 1000 Ma. This estimate completely agrees with the Sm–Nd isotope characteristics of the juvenile crust within the CAOB [20]. The composition of metasedimentary rocks from the MOOB accretionary wedge is considerably dominated by recycled crustal material, the source of which is likely Late Riphean rock complexes of the Argun superterrane.