The Kopri type of subalkaline granitoids was recognized in 1990 in the course of geological mapping on a scale of 1 : 1 000 000 as an early subphase of the main phase of the Paleoproterozoic Tukuringra complex [1]. Since then, geologists have lost interest in them, although they possess a wide range of interesting features. The main one is that the massifs of these granitoids are located exclusively within the Dzheltulak suture zone (Figs. 1a, 1b), which separates the Dzhugdzhur–Stanovoi and Western Stanovoi superterranes of the Central Asian fold belt and is considered as one of the most mysterious global structural features in East Asia.

Fig. 1.
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(a) Schematic map of the Kopri-type granitoids in the Dzheltulak zone of the junction of the Dzhugdzhur–Stanovoi and the Western Stanovoi superterranes (Central Asian fold belts). (1–4) Early Cretaceous postcollisional Stanovoi volcanoplutonic belt: (1) volcanosedimentary complex of rift troughs; (2) granitoids of the Tynda–Bakaran and Irokan complexes; (3) granitoids of Amudzhikan and Amanan complexes, (4) Kopri-type granitoids; (5) Early Cretaceous collisional granites and migmatites; (6) Mesozoic metamorphic rocks of the Dzheltulak and Gilyui formations; (7–8) pre–Late Mesozoic metamorphic rocks and granitoids of (7) the Western Stanovoi superterrane and (8) the Dzhugdzhur–Stanovoi superterrane; (9) gneisses, schists, and granitoids of the Aldan Shield; (10) Mongolia–Okhotsk fold belt; (11) faults; (12) geochronological sampling sites. (b) Geological position of the postcollisional Stanovoi volcanoplutonic belt in the northeastern part of the Central Asian fold belt. (1–3) Fold belts: (1) Central Asian (YTTC, Yenisei–Transbaikalia tectonic collage; WSS, Western Stanovoi superterrane; DSS, Dzhugdzhur–Stanovoi superterrane; AM, Amur microplate), (2) Mongolia–Okhotsk; (3) Sikhote–Alin; (4–5) Late Mesozoic rift systems: (4) West Transbaikalian, (5) Stanovoi volcanoplutonic belt; (6) study area.

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In addition, a characteristic feature of the granitoids of this type, which distinguishes them from the rest of the associated granitoids of the Tukuringra complex, is a somewhat specific composition, elevated basicity and alkalinity, of the constituent rocks. Based on the general geological situation, there are grounds to believe that Kopri type granitoids are Mesozoic in age. This is evidenced by the Mesozoic formation age of the Dzheltulak zone [2] rather than Paleoproterozoic, as was accepted until recently [3], and by the absence of any signatures of foliation or other metamorphic transformations in the granitoids of this type. However, more precise data on their age are not available. This complicates the correlation of the magmatic complexes of the above-mentioned superterranes and, thereby, the development of an integrated geodynamic model of their formation. In this context, the results of U–Pb (ID-TIMS) geochronological, geochemical, and Sm–Nd isotope geochemical studies obtained in recent years for Kopri-type granitoids are discussed in this paper.

Kopri-type granitoids are represented by medium and coarse-grained porphyritic monzodiorites, quartz monzonites, and syenites that make up relatively small massifs, elongated northwestward (Fig. 1a). They are characterized by rather widely manifested mingling and frequent ovate inclusions of fine-grained gabbroids. All these rocks are cross-cut by lamprophyre dikes.

The content of the oxides of major petrogenic elements in the rocks was determined by the X-ray fluorescence method, and the contents of impurity elements, by the ICP-MS method with an uncertainty of 5–10% at the Central Analytical Laboratory of the Karpinskii Russian Geological Research Institute, St. Petersburg (VSEGEI).

In terms of chemical composition, Kopri-type granitoids vary from quartz monzonites to syenites (56.7–62.5% SiO2) and are characterized as moderately alkaline with varying but elevated alkali content (Na2O + K2O in the range of 8.0–9.45% and K2O/Na2O ratios in the range of 0.76–1.22). They are characterized by low and moderate alumina, agpaitic, and iron moduli (A/CNK = 0.80–0.95, NK/A = 0.66–0.75, f = 0.60–0.77). The points of their compositions are located predominantly in the field of rocks of the shoshonite series on the K2O–SiO2 diagram and in the field of alkaline rocks, on the (K2O + Na2O–CaO)–SiO2 diagram. Fine-grained gabbroids from magmatic inclusions and lamprophyres (51% and 55.6–56.1% SiO2, respectively) also belong to the moderately alkaline type and the shoshonite series.

Kopri-type granitoids are characterized by low or moderate contents of most impurity elements except for Ba (1310–2050 µg/g) and Sr (1170–1670 µg/g). The spider plots, normalized to the primitive mantle, demonstrate distinct positive Ba, K, and Sr anomalies; “deep” negative Nb, Ta, and Ti anomalies; and less pronounced negative P anomalies. A certain depletion of HFS elements on the right-hand part of the spectrum is also characteristic (down to 0.6–1.9 µg/g Yb, 0.11–0.24 µg/g Lu, and 12–23 µg/g Y). The REE distribution in granitoids is fractionated ([La/Yb]n in the range of 27–63) with approximately similar LREE and HREE fractionation levels ([La/Sm]n in the range of 3.7–4.4 and [Gd/Yb]n, 3.5–8.3). The concave pattern of the spectrum in the HREE region is also characteristic. The Eu anomaly is either absent or weakly negative (Eu/Eu* is 0.73–1.08). The extremely low Rb/Sr ratios in the discussed granitoids (0.03–0.07) indicate a low level of their differentiation. The contents of V, Cr, Co, and Ni in basic rocks from inclusions in granitoids and lamprophyres are comparable with those in intraplate tholeiites and traps (see [4]).

A characteristic feature of both granitoids and the associated basic rocks are the fairly high values of the La/Yb and Sr/Y ratios (38–93 and 50–115), which make these rocks similar to adakites. The discussed granitoids on the classification diagrams of S.D. Velikoslavinskii et al. [5] fall in the field of the postcollisional type of adakites. In composition they are closest to the Early Cretaceous granitoids of the Tynda–Bakaran complex of the Dzhugdzhur–Stanovoi superterrane and the Amudzhikan complex of the Western Stanovoi superterrane (see [6, 7]).

U–Pb (ID-TIMS) geochronological studies were carried out for two small massifs of the Kopri type. Sampling sites are shown in Fig. 1a.

Zircons were separated according to the standard procedure involving heavy liquids. The zircon crystals that were selected for the U–Pb geochronological studies underwent multistep removal of surface contamination in alcohol, acetone, and 1 M HNO3 solution. In addition, zircon grains (or their fragments) were rinsed in ultraclean water after each step. The chemical decomposition of zircons and U and Pb separation were carried out using a modified version of the method of T.E. Krogh [8]. Isotope analyses were performed on a Finnigan МАТ-261 multicollector mass spectrometer in both the static and dynamic modes (with the aid of an ion multiplier). The isotopic studies were performed using a 202Pb –235U isotope indicator. The uncertainty of the U/Pb ratios and the U and Pb content estimates was 0.5%. The blank contamination did not exceed 15 pg Pb and 1 pg U. Experimental data processing was carried out using the PbDAT [9] and ISOPLOT [10] software. The ages were calculated using conventional uranium decay constants [11]. Corrections for common lead were introduced in compliance with the model values [12]. All errors are at the level of 2σ.

The accessory zircons, extracted from quartz monzonite (Sample A-142), forms transparent euhedral and subhedral prismatic and acicular yellow crystals, the size of which varies from 50 to 150 µm (elongation factor in the range of 2.1–4.1). The main faceting elements of these crystals are {100}, {110} prisms and {111}, {101} bipyramids (Fig. 2, I–V). Acicular zircon is characterized by fine magmatic zoning (Fig. 2, VI–VIII). In prismatic zircon crystals, only fragments of zoning and relics of inherited cores were found (Fig. 2, IX).

Fig. 2.
figure 2

Photomicrographs of zircon crystals from Sample A-142 shot on an ABT 55 scanning electron microscope: I–V, in the BSE mode; VI–X, in the cathodoluminescence mode.

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U–Pb geochronological studies were carried out for acicular and prismatic zircon crystals from the size fractions of 50–85 and 85–100 µm. As is obvious from Table 1 and Fig. 3, acicular zircons (nos. 2 and 3) have a concordant age of 127 ± 1 Ma (MSWD = 0.28, probability, 0.60), whereas the prismatic zircon (no. 1) demonstrates an age discordance due to the presence of an inherited component of ancient lead. The age, determined by the lower intercept of the discordia, calculated for three zircon microgram aliquots, with concordia (Fig. 3), is 127 ± 1 Ma (MSWD = 1.7; upper intercept, 2708 ± 57 Ma). As an estimate of the crystallization age of the zircon studied, we take the concordant age of 127 ± 1 Ma.

Table 1. Results of U–Pb geochronological studies of zircons from Kopri-type granitoids
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Fig. 3.
figure 3

Diagram with concordia for zircons from samples A-142 and A-148. Point numbers correspond to sequence numbers in Table 1.

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Accessory zircon from quartz monzodiorite (Sample A-148) is represented by transparent, translucent, and less frequent metamictized euhedral and subhedral short prismatic and prismatic yellow crystals. The shapes of the crystals are determined by a combination of the {100} prism and the {101}, {111}, and {201} bipyramids (Fig. 4, I–IV). The size of zircon grains varies from 50 and 200 µm (elongation factor in the range of 1.5–3.0). They possess a pronounced magmatic zoning (Fig. 4, V–XII). In some zircon grains, inherited cores are present (Fig. 4, VIII, X).

Fig. 4.
figure 4

Photomicrographs of zircon crystals from Sample A-148 shot on the ABT 55 scanning electron microscope: I–IV, in the BSE mode; V–XII, in the cathodoluminescence mode.

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In the course of U–Pb geochronological studies, three microgram aliquots of zircons (10–40 grains) from the size fractions of 50–85 and 85–100 µm were analyzed. As can be seen in the diagram with concordia (Fig. 3), zircon isotope composition points nos. 4–6 are located on discordia, the lower intercept of which with concordia corresponds to an age of 126 ± 1 Ma (MSWD = 0.062, the upper intercept is 2950 ± 150 Ma). The ten “cleanest” zircon grains from the size fraction of 50–85 µm (no. 6) are characterized by a concordant age of 126 ± 1 Ma (MSWD = 0.01; probability, 0.92), which can be used as the most accurate estimate of the crystallization age of the zircons studied.

The isotope compositions of Sm and Nd were measured on a TRITON TI multicollector mass spectrometer in the static mode. The measured 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.7219 and reduced to 143Nd/144Nd = 0.511115 in the JNdi-1 Nd-standard. The weighted average value of the 143Nd/144Nd ratio in the JNdi-1 Nd-standard during the measurement period is 0.512108 ± 7 (n = 10). The uncertainty of the estimates of the Sm and Nd concentrations was ± 0.5%; the uncertainty of the isotope ratios was ± 0.5% for 147Sm/144Nd and ± 0.005% (2σ), for 143Nd/144Nd. The blank experiment level did not exceed 0.2 ng Sm and 0.5 ng Nd. The εNd(t) values and tNd(DM) model ages were estimated using the present-day values 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967 for the chondritic uniform reservoir (CHUR) according to [13] and the depleted mantle (DM), according to [14] (143Nd/144Nd = 0.513151, 147Sm/144Nd = 0.21365).

The results of the Sm–Nd isotope studies for Kopri-type granitoids (Table 2) indicate that they have a relatively low radiogenic Nd isotope composition (εNd(t) in the range of –7.0 to –9.0, tNd(DM) from 1.2 to 1.4 Ga, and tNd(C) from 1.5 to 1.7 Ga), indicating the essentially crustal source of these rocks, most probably formed as a result of the mixing of the Early Precambrian and a younger Phanerozoic or Neoproterozoic component. Their comparison in terms of Nd isotope composition with the granitoids of the Western Stanovoi superterrane that are similar in age and composition (εNd(t) from –7.1 to –11.3, tNd(DM) from 1.2 to 1.4 Ga, and tNd(C) from 1.5 to 1.9 Ga; unpublished data of the authors) and the Dzhugdzhur–Stanovoi superterrane (εNd(t) from –11.9 to –16.7, tNd(DM) from 1.5 to 1.9 Ga, and tNd(C) from 1.8 to 2.3 Ga; unpublished data of the authors) demonstrates their greatest similarity in Nd isotope composition with the granitoids of the Western Stanovoi superterrane.

Table 2. Sm–Nd isotope data for Kopri-type granitoids
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The obtained data suggest that Kopri-type granitoids are close in age and composition to the granitoids of the Tynda–Bakaran complex of the Dzhugdzhur–Stanovoi superterrane (122–127 Ma, [15]) and the granitoids of the Amudzhikan complex of the Western Stanovoi superterrane (130 Ma, [7, 16]), which are parts of the Late Mesozoic postcollisional Stanovoi volcanoplutonic belt. This belt extends more than 1000 km westward in the sublatitudinal direction subparallel to the Mongolia–Okhotsk suture zone from the Sea of Okhotsk into the continent and stitches the tectonic structures of the Dzhugdzhur–Stanovoi and Western Stanovoi superterranes [15]. In other words, they can be considered as parts of the Stanovoi volcanoplutonic belt. However, the structural position of the massifs of Kopri-type granitoids gives reason to believe that their emplacement was simultaneous with final stabilization of the Dzheltulak suture and thereby recorded the upper age limit of its formation.

The similarity in the Nd isotope composition between the Kopri-type granitoids and the granitoids of the Western Stanovoi superterrane that are similar to the former in composition and age most probably indicates the similarity or identity of their sources. Considering the localization of some Kopri-type granitoid bodies in the marginal part of the Dzhugdzhur–Stanovoi superterrane as well (Fig. 1a), it can be assumed that the Dzheltulak suture zone plunges northeastward beneath the structures of the Dzhugdzhur–Stanovoi superterrane. This is in full compliance with modern ideas about the specific features of the subsurface structure of the junction zone of the Eurasia and Amur lithospheric plates [17].