Basic and Associated Granitoid Magmatism and Geodynamic Evolution of the Altai Accretion–Collision System (Eastern Kazakhstan)

Magmatism of accretion–collision systems is the most important indicator of the processes of formation of the continental crust. As shown by recent studies, basic igneous rocks formed at the orogeny stage are significant members of accretion–collision systems (Izokh et al., 1998; Dilek and Altunkaynak, 2009; Vladimirov et al., 2013, 2017; Yar-molyuk et al., 2013; Kozlovsky et al., 2015; Moritz et al., 2016; Shelepaev et al., 2018). The manifestations of mantle magmatism in accretion–collisional orogens were originally explained by the extension of the lithosphere of mountain-folded structures and the excitation of the underlying asthenosphere (Dewey, 1988; Elkins-Tanton, 2007). In the late 20th century, a new model was proposed to explain mantle magmatism in accretion–collision zones by the detachment (or rupture) of the subducted oceanic plate (slab), the so-called “slab break-off model” (Davies and von Blancken-burg, 1995; Khain et al., 1996), which became widely used to explain basic magmatism in accretion–collision systems of different ages (Atherton and Ghani, 2002; Gorring et al., 2003; Niu, 2017). Another variant of this model was proposed for accretion–collisional transform orogens; it regards a rupture as a result of shears at the uneven boundaries of colliding plates (Martynov et al., 2016). Deeply penetrating shear zones also play a significant role in the active occurrence of mantle magmatism (Wang et al., 2014; Konopelko et al., 2018).

There is also a hypothesis of the major role of mantle plumes as independent sources of energy in magmatism of accretion–collision systems. In recent years, several large igneous provinces (LIPs) resulted from the interaction of mantle plumes and the fold belt lithosphere have been established in Central Asia: early Paleozoic Altai–Mongolia–Sayan LIP (Dobretsov, 2011; Vrublevskii et al., 2012; Vladimirov et al., 2013), middle Paleozoic LIPs in the Minusa basin (Vorontsov et al., 2013) and Vilyui rift (Kiselev et al., 2014), and late Paleozoic–early Mesozoic Tarim, Barguzin, Hangayn, and Henteyn LIPs in Mongolia, China, and Transbaikalia (Yarmolyuk et al., 2013, 2014, 2016; Xu et al., 2014). Their specific feature is the areal occurrence of magmatism, the intimate spatiotemporal association of gabbroid and granitoid rocks (or basalts and rhyolites), and active mantle–crust interactions expressed as synplutonic small basic intrusions and numerous composite dikes with intricate relationships between contrasting magmas. The role of mantle plumes and the significance of LIPs in the Earth’s history are the subject of active scientific discussion. The above concepts, however, are not unambiguously prevailing; many researchers deny the influence of mantle plumes in these regions and explain the mantle activity by plate tectonics (Xiao et al., 2010, 2018; Wang et al., 2014; Konopelko et al., 2018). The diversity of the used models and the absence of consensus on the causes of the mantle activity emphasize the great number of factors determining the regularities of magmatism in accretion–collision settings. Obviously, there might be a combination of different factors, namely, the plate tectonics and the plume activity, in the same systems (Biske et al., 2013; Kozlovsky et al., 2015).

To answer the question about the causes and regularities of magmatism in a particular region, it is necessary to reconstruct the evolution of endogenous activity in detail by elucidating the correlation between basic and granitoid magmatism. This paper presents results of such an approach to the late Paleozoic (Carboniferous–Permian) igneous associations of the Altai accretion–collision system in Eastern Kazakhstan.

The East Kazakhstan area is located in the west of the Central Asian Orogenic Belt and is part of the Hercynian Ob’–Zaisan folded system. It extends from northwest to southeast for ~700 km and is bounded by the Chingiz–Tarbagatai regional fault separating it from the middle Paleozoic structures of the Kazakhstan continent (in the south-west) and by the Irtysh shear zone separating it from the middle Paleozoic structures of the Rudny Altai active margin of the Siberian continent (in the northeast). The folded system formed during the convergence of the Siberian and Kazakhstan continents and the closure of the Ob’–Zaisan ocean basin in the late Paleozoic (Zonenshain et al., 1990; Vladimirov et al., 2003, 2008; Xiao et al., 2010). Geological research performed in Eastern Kazakhstan in the second half of the 20th century (Kuzebnyi, 1975; Shcherba et al., 1976, 1998; Ermolov et al., 1977, 1983; Lopatnikov et al., 1982; D’yachkov et al., 1994) gave an insight into the structure, composition, and age of sedimentary and volcanic formations. Based on these characteristics, the following structure-formational zones were recognized in Eastern Kazakhstan: (1) Zharma–Saur, (2) Charsk (West Kalba), and (3) Kalba-Narym. The sequence of geotectonic events is described below and shown in Fig. 1.

The oldest lithotectonic units within the folded system are of paleo-oceanic nature and apparently formed within the Paleoasian Ocean in the early–middle Paleozoic (Dobretsov et al., 1979; Volkova et al., 2008). The size and shape of the ocean basin obviously changed over millions of years from Ordovician to Devonian, which is confirmed by the successive change of oceanic formations: oceanic floor basalt–siliceous, terrigenous limestone–siliceous, and reef limestone.

The Early–Middle Devonian volcanic belts in Rudny Altai and the Chingiz–Tarbagatai zones are indicators of the interaction of the oceanic lithosphere with the Siberian and Kazakhstan continents in at least two subduction zones. Late Devonian volcanic rocks are absent from the Chingiz–Tarbagatai zone and are of limited occurrence in the Rudny Altai zone: This was the time of the termination of direct subduction and, probably, of the general restructuring of the continent–ocean systems. The Late Devonian–early Carboniferous was the time of the origin of the Irtysh shear zone and the Rudnyi Altai transform margin. Apparently, it was at the Devonian–Carboniferous boundary that the evolution of the folded system (formation of the first deep faults, deformations, and reduction in the Ob’–Zaisan ocean basin) began.

In the Late Devonian and early Carboniferous, there was a sea-marginal trough near Rudny Altai, which inherited the structure of the deep-water trench accumulating terrigenous sediments resulted from the destruction of the Siberian continent margin; the geodynamic setting corresponded to a passive margin. In the Ob’–Zaisan paleobasin, there was marine sedimentation at that time, which gave way to island arc andesitic volcanism in the Visean. Visean volcanic strata are widespread in the Zharma–Saur and, less, Charsk zones; their appearance indicates the resumption of subduction of the oceanic lithosphere beneath the margin of the Kazakhstan continent and, probably, the appearance of intraoceanic arcs.

The Serpukhovian deposits in the study area are shallow-water terrigenous sediments. Polymict sandstones (gray-wackes), gritstones, siltstones, and widespread olistostromes are indicators of the begun orogenic processes, namely, a significant convergence of continental blocks. By Serpukhovian time, the ocean basin ceased to exist: There are no deep-water siliceous sediments and limestones. Thus, Ser-pukhovian time can be considered the beginning of orogenic processes.

The peak of orogenic processes with intense nappe formation, folding, and a significant thickening of sedimentary sections was at the end of the Serpukhovian. This is confirmed by the strong structural unconformity between the early and middle Carboniferous deposits. The latter are a continental molasse; the base of the section often contains conglomerates and gritstones. Molasses are widespread in small isolated troughs accumulating material drifted from the formed orogen. Marine sedimentation completely stopped in the study area by the middle Carboniferous. Obviously, the orogenic structure formed mainly at the Serpukhovian–Bashkirian boundary (~323 Ma).

The formed orogen has specific features. In contrast to the typical Pamir–Himalayan collisional orogen resulted from the collision of thick lithospheric blocks, the formation of the Altai orogen was caused by the interaction between the relatively young (middle Paleozoic) margins of the Siberian and Kazakhstan continents and the island arc and oceanic terranes within the Ob’–Zaysan ocean basin. Such settings correspond to accretionary orogens (Windley, 1992) and are typical of the entire Central Asian Orogenic Belt. During the compression of thin lithospheric plates under intense shearing, the relief uplifting was probably no more than 2–4 km. The term “soft collision” was introduced for such orogens (Khain et al., 1996; Vladimirov et al., 2003). The general sequence of geodynamic events in the study area of the Ob’–Zaisan folded system is as follows: subduction → convergence of continental blocks and reduction in the ocean basin → termination of marine sedimentation and basin crowding → orogeny with tectonic growth of the sedimentary section and subsequent appearance of continental molasses → extension and orogen collapse. This gives grounds to classify the set of formed lithotectonic units as an accretion–collision system.

At the end of the middle Carboniferous, sedimentation ceased in the study area. Stratified formations of this age are only lavas and tuffs that erupted in the continental setting. At all stages of evolution of the Altai accretion–collision system, shearing of blocks caused by the collision of continents played a crucial role in the formation of structures (Zonenshain et al., 1990; Vladimirov et al., 2003; Buslov et al., 2003). It was motions along the main faults that controlled volcanism and intrusive magmatism in the late Carboniferous and early Permian (Ermolov et al., 1983; Vladimirov et al., 2003, 2008; Buslov et al., 2003).

The most comprehensive research into magmatism in the Ob’–Zaisan folded area was performed in the 1960–1970s. It gave an insight into the internal structure of intrusive plutons and volcanic structures and the main regularities of interactions among different rock associations. Based on the geological, stratigraphic, and geochronological (K–Ar method) data, the first information about the age and duration of magmatism was obtained. Also, the first magmatism correlation schemes were compiled (Kuzebnyi, 1975; Shcherba et al., 1976; Ermolov et al., 1977, 1983; Lopatnikov et al., 1982; Navozov et al., 2011), which showed that magmatism lasted from the early Carboniferous (Visean–Serpukhovian) to the Early Triassic. Ultrabasic–basic complexes were recognized in all structure-formational zones; the generalization of data on their geologic location and chemical composition made it possible to divide them into types and elucidate their petrogenesis (Ermolov et al., 1977, 1983; Kuzebnyi et al., 1979).

In recent years, we have studied basic and granitoid complexes using modern methods and obtained a large amount of geochemical, isotope, and geochronological data (Khromykh et al., 2011, 2013, 2016, 2018, 2019a, b, c; Kotler et al., 2015, 2021). Data on the geologic position and age of the igneous complexes are presented in Fig. 2. New data made it possible to revise early concepts, refine the scale of some earlier identified igneous complexes, and estimate the total duration of magmatism (from the early Carboniferous (Visean) to the end of the early Permian). The correlation scheme for magmatism of the Altai accretion–collision system, compiled from the new petrological and geochronological data, is shown in Fig. 3.

The Saur gabbro-diorite complex (C₁s) is the main structural unit of the Saur gabbro–granitoid series (Ermolov et al., 1977). There are many plutons of this complex in the Zharma–Saur zone, where they form a belt extending from northwest to southeast. Most of the plutons (the largest ones) are located in the south of the zone. The rocks of this complex form a differentiated series from troctolites to gab-bro, amphibole gabbro, diorites, and quartz diorites. Basic rocks are scarce; diorites and quartz diorites are predominant in most of the plutons. The petrographic varieties, chemical composition, and age of the rocks were studied earlier (Khromykh et al., 2019c). The age of the Saur complex was estimated by U–Pb dating of zircons from quartz diorites, 330 ± 2 Ma, which corresponds to the Visean-Serpukhovian boundary.

The Bugaz dike complex (C₂). All basic dikes (mostly of NE and N–S strikes) of the Zharma–Saur zone were earlier assigned to this complex and were regarded as the youngest igneous rocks (Shcherba et al., 1976, 1998). Study of the Bugaz gabbro-granitoid pluton revealed a dike complex that was then included into the early Carboniferous Saur Group (Ermolov et al., 1977). More than 20 basic dikes tens to hundreds of meters long and 0.5–1.0 to 3–4 m thick were found within the pluton. They are of two types: (1) dolerites and dolerite porphyrites and (2) lamprophyres (spessartites). The basic dikes are enriched in incompatible elements (alkalies, LREE, Rb, Ba, and Zr) relative to the rocks of the Saur complex (Khromykh et al., 2019c). The age of the dikes within the Bugaz pluton was estimated by U–Pb dating of zircons from spessartites, 315 ± 4 Ma, which corresponds to the middle Carboniferous. The certain geologic position, composition, and age of the dikes give grounds to regard them as the middle Carboniferous Bugaz basic-dike complex. There is one more, younger, complex of basic dikes intruding early Permian granitoids in the Zharma–Saur zone (Shcherba et al., 1976; Ermolov et al., 1977). The composition and age of these dikes call for further study.

The Irtysh peridotite–gabbro complex (C₂). Its plutons are localized in the Kalba–Narym zone, extending for more than 200 km from northwest to southeast along the Irtysh shear zone. They are intruded by the early Permian granitoids of the Kalba batholith. The central part of the batholith lacks plutons of this complex; they have been preserved among granites only in few roof sags. Most of the plutons are small bodies hundreds of meters in size; the largest Surovka (~4 × 15 km) and Talovka (~3 × 7 km) plutons expose south of Ust’-Kamenogorsk. The plutons intrude Middle–Late Devonian shales and siltstones to form hornfels. Study of the host-rock contacts showed that the plutons formed in the shearing setting. Comprehensive petrological and geochemical research (Khromykh et al., 2019b) made it possible to identify three types of rocks in the Irtysh complex: (1) gabbro-dolerites predominant in most plutons; (2) subalkalic gabbronorites and gabbro-diorites with typomorphic magmatic biotite (up to 5 vol.%); and (3) stratified low-alkali peridotite–troctolite–gabbronorite–gabbro series composing the largest Surovka pluton. These studies also revealed two types of magmas generated from the same mantle source and fractionated in deep-seated chambers. The age of the Irtysh complex rocks was established by U–Pb dating of zircons from subalkalic gabbronorites and gabbro-diorites and low-alkali troctolites. The ages of the three rocks are 313–312 Ma and correspond to the middle Carboniferous.

Basalt-andesite-dacite associations (C₂; $P11$⁠). accretion–collision processes in the Charsk zone resulted in orogenic depressions, which were then filled with continental molassa deposits. These deposits are most widespread northwest of Kokpekty Village, where several troughs filled with volcanic and volcanosedimentary rocks are superposed on the continental molasse. The stratigraphy and paleonto-logic findings point to surface volcanism here (Mossa-kovskii, 1975; Ermolov et al., 1983). The lower section is made up of basaltic and andesitic lavas and forms the Daubai area, the middle section is composed of polymictic sandstones, and the upper section is made up of andesitic and dacitic lavas and tuffs and forms the Saryzhal area; the section ends with red-colored sandstones and gritstones. East of the Saryzhal–Daubai depression, two extrusive domes composed of andesitic porphyrites expose. West of this depression, there is the Tyureshoke trough filled with subalkalic basalts and andesites. The trough basalts lie over red sand-stones, which gave grounds to date them at the late Carboniferous (Ermolov et al., 1983). Petrogeochemical studies showed that the basalts and andesites of the Tyureshoke trough are enriched in incompatible elements (alkalies, P, LREE, Rb, Ba, Zr, Hf, and Nb) relative to the basalts and andesites of the Daubai area (Khromykh et al., 2020). Geochronological studies showed two stages of volcanism. Zircon U–Pb dating of andesites from the Daubai area and extrusive domes yielded an age of 315–311 Ma. The U–Pb age of dacites from the Saryzhal area was estimated at 297 ± 1 Ma. The age of basalts and andesites from the Tyureshoke trough was not determined, but the age of extrusive dacite porphyry intruding the volcanic section was estimated by zircon U–Pb dating at 290 ± 4 Ma. Accordingly, the age of the Tyureshoke trough is estimated at 297–290 Ma, which corresponds to the early Permian. Based on the differences in chemical composition and age, two volcanic associations were identified: middle Carboniferous Daubai and early Permian Tyureshoke.

The Argimbai syenite–gabbro complex (⁠$P11$⁠). Its rocks compose about twenty small plutons in several areas of the Charsk zone. The Argimbai area is the largest; it includes the largest (~3 × 10 km) Argimbai pluton and several smaller (hundreds of meters) ones. The complex is composed mostly of subalkalic gabbro with typomorphic late magmatic K-feldspar. The most differentiated gabbro-essexites contain up to 5 vol.% K-feldspar. Syenites (a leucocratic variety with the predominance of plagioclase) are the vein phase of the complex and are of minor occurrence. In composition the Argimbai gabbro are similar to the subalkalic basalts of the Tyureshoke association. The age of the Argimbai complex estimated by U–Pb dating of zircon from the gabbro is 293 ± 2 Ma (Khromykh et al., 2013), which corresponds to the first half of the early Permian.

Gabbro of multiphase gabbro–monzonite–granitoid intrusions (P₁). There are several exposed large (up to 200 km²) multiphase plutons in the central part of the Charsk zone. They are characterized by highly diverse rocks (from olivine gabbronorites to leucogranites) and mingling between basic and granitoid rocks. The relationships between the rocks were most comprehensively examined in the Preo-brazhenka pluton (Khromykh et al., 2017). The detailed petrographic, mineralogical, and geochemical studies showed that the main rocks of the pluton are olivine dolerites, gabbro, monzogabbro, and diorites and that they resulted from the differentiation (fractionation and contamination) of sub-alkalic basic magma. Granitoids composing most of the pluton at the recent erosional-truncation level are the product of melting of crustal substrates under the impact of basic magma. We proposed a model for the pluton formation as a result of the synchronous interaction of subalkalic basic magma with crustal substrates and granitoid magmas at different hypsometric levels (Khromykh et al., 2018). The age of the Preobrazhenka pluton estimated by U–Pb dating of zircons from monzonites and granites is 291 ± 2 Ma (the dates obtained for three samples are in agreement).

The Maksut gabbro-picrite complex (⁠$P12$⁠). Its rocks compose small plutons in several areas of the Charsk zone, often together with the gabbro plutons of the Argimbai complex (many previous researchers regarded all these rocks as a single complex). The petrotypical Maksut pluton in the northwest of the Charsk zone is the most representative. The first phase of the complex is olivine dolerites and picrites, and the second phase is subalkalic olivine gabbronorites with typomorphic late magmatic biotite and accessory sulfide Cu–Ni mineralization. The age of the Maksut complex estimated by Ar–Ar dating of late magmatic amphibole and biotite is 280 ± 3 Ma (the dates obtained for four samples from different plutons of the Charsk zone are in agreement) (Khromykh et al., 2013), which corresponds to the second half of the early Permian.

The Mirolyubovka dike complex (P₁–P₂) is formed by a number of dikes of NE and E–W strikes intruding the early Permian granitoids of the Kalba batholith. According to geological data, it is the youngest igneous complex in the Altai accretion–collision system, and the discordant position of dikes relative to regional geologic structures of NW strike gave grounds to date this complex at the late Permian or Mesozoic (Lopatnikov et al., 1982; Shcherba et al., 1998; Navozov et al., 2011). The scarcer dikes intruding granitoids in the Zharma–Saur zone (Shcherba et al., 1976, 1998) are analogues of the Mirolyubovka complex dikes. In the Kalba–Narym zone, the Mirolyubovka complex comprises five large and several small dike belts; the largest one is formed by hundreds of dikes, extends through the Mirolyubovka granite pluton for more than 15 km, and is about 3 km in width. The largest basic dikes of the complex are up to 3–4 km in length and up to 10–15 m in width. The rocks of the complex are dolerites (~10 vol.% (Lopatnikov et al., 1982)), diorites (~60 vol.%), granodiorites and granite-porphyry (~30 vol.%). Some dike belts include large (up to 20 m thick) long (5–10 km) composite dikes. Comprehensive study of these composite dikes revealed mingling structures typical of the interaction of unconsolidated magmas of contrasting compositions. According to the known models (Sklyarov and Fedorovskii, 2006; Burmakina et al., 2018), basic magmas probably penetrated into granitic-magma chambers and formed rounded nodules and hybrid rocks. The resulted heterogeneous mixture intruded along fractures into above-located consolidated granites to form composite dikes. There is geologic evidence that the basic magmatism that contributed to the formation of dikes occurred subsynchronously with the formation of the Kalba batholith granitoids. This hypothesis was confirmed by geochronological studies: U–Pb dating of zircons from a large diorite dike intruding granites (286 Ma (Khromykh et al., 2016)) in the central part of the Kalba batholith yielded the time of the dike intrusion, 279 ± 3 Ma. U–Pb dating of zircons from another diorite dike intruding the Monastery pluton leucogranites (276 Ma (Kotler, 2017)) showed its age of 267 ± 1 Ma, which corresponds to the beginning of the early Permian.

The data obtained demonstrate three stages of basic magmatism during the evolution of the Altai accretion–collision system: early Carboniferous (~330 Ma), middle Carboniferous (315–311 Ma), and early Permian (297–267 Ma). The total magmatism duration is estimated at ~60 Ma, with the peak of the magmatic activity falling in the interval 300–275 Ma (Fig. 3).

The Semeitau volcanic structure composed of subalkalic basalts and rhyolites (249 ± 2 Ma (Lyons et al., 2002), i.e., the Permian–Triassic boundary) is the youngest product of endogenous activity in Eastern Kazakhstan. Its formation was caused by large-scale endogenous events in the Siberian LIP (Dobretsov et al., 2005; Reichow et al., 2009) and was not related to the evolution of the Altai accretion–collision system.

To understand the causes and mechanisms of endogenous activity during the evolution of the Altai accretion–collision system, it is important to compare the petrologic compositions of basic complexes, which will give an insight into the conditions of generation of mantle magmas. The petrologic composition of almost all basic complexes varies from peridotites to diorites. Unfortunately, there are no sufficient petrological information about the composition of rock-forming minerals and estimates of the P–T crystallization conditions for all of the studied ultrabasic–basic associations. Therefore, there are no reliable criteria for the correspondence of certain types of rocks to primary magmas. Nevertheless, we can try to compare the petrologic composition of associations of different ages, taking into account the general physicochemical principles and regularities of crystallization of ultrabasic–basic magmas:

• (1) In most cases, ultrabasic rocks are cumulates of solid crystals separated from basic magmas. This is also confirmed by petrographic observations of such rocks in the Saur, Irtysh, and Maksut complexes, i.e., the composition of ultrabasic and melanocratic rocks cannot be correctly compared with the composition of the parental magmas.

• (2) Almost each of the studied basic associations contains intermediate rocks depleted in pyroxenes and enriched in acid plagioclase, amphibole, and biotite. This indicates their formation from residual melts, whose composition was determined by changes in the composition of primary magmas either during fractional crystallization or during the contamination by the material of the host rocks or anatectic melts. These assumptions are confirmed by the major-element trends and trace element and REE patterns of almost all basic complexes.

Taking into account these restrictions, we assume that the rocks corresponding in chemical composition to basalts will be most similar in composition to the parental magmas. Based on this hypothesis, the petrologic characteristics of different complexes of the Altai accretion–collision system were compared for rocks with a massive structure, magmatic texture (ophitic, poikilophytic, and dolerite), and a SiO₂ content from 45 to 53 wt.%. The final sample included 38 analyses presented in Table 1.

Figure 4 shows the contents of some major and trace elements in the rocks versus MgO contents (the latter significantly reflect the degree of melting of mantle sources).

The gabbro of the Saur complex are characterized by low Mg# values, high contents of CaO and Al₂O₃, reduced TiO₂ contents, and low contents of K₂O, P₂O₅, Rb, Ba, Zr, Hf, Nb, and Ta. The gabbro of the Irtysh complex show elevated Mg# values and CaO contents and slightly reduced Al₂O₃ contents as compared with most of the studied rocks. They are also depleted in K₂O, P₂O₅, TiO₂, Ba, Zr, Nb, and Ta. The middle Carboniferous dikes of the Bugaz complex and the basalts of the Daubai association are similar in composition: low Mg# values, reduced (relative to other Carboniferous complexes) CaO contents, and elevated contents of K₂O, Rb, Ba, Zr, Hf, and Nb. The early Permian rocks are most enriched in Ti and incompatible components (K₂O, P₂O₅, Rb, Ba, Zr, Hf, Nb, and Ta). The highly magnesian rocks of the Maksut complex are also enriched in P₂O₅, Ba, and Nb (Fig. 4).

The compositional difference between the basic complexes is also seen in their REE and trace element patterns (Fig. 5). The gabbro of the Saur complex demonstrate gentle chondrite-normalized REE patterns with element contents close to ten chondrite values and with negative Rb, Th, Ta, Nb, Hf, and Zr and positive Sr anomalies. The gabbro of the Irtysh complex show similar REE patterns but higher contents of lithophile elements and Zr. The middle Carboniferous dikes of the Bugaz complex and the basalts of the Daubai association are characterized by domination of LREE over HREE and negative Th, Ta, Nb, and Ti anomalies, but the total content of the latter elements is much higher than that of LREE. The early Permian rocks are the REE-richest; the REE patterns of the volcanics of the Tyureshoke trough, gabbroids of the Argimbai complex and the Preobrazhenka intrusion, and dikes of the Mirolyubovka complex are similar. The gabbro-picrites of the Maksut complex have somewhat lower REE contents varying from 50 (for LREE) to 7–8 (for HREE) chondrite values. The multielement patterns of the early Permian basites show positive Ba, K, La, Ce, Sr, Zr, and Ti anomalies.

As evidenced from the variations in the contents of major components (SiO₂, Al₂O₃, FeO*, and CaO) versus MgO content, the differentiation of rocks in all basic complexes proceeded by similar mechanisms. Thus, the difference in the contents of incompatible elements is due to the different compositions of mantle sources or different conditions of their melting.

According to geological data, the Altai accretion–collision system is a collage of middle–late Paleozoic volcanic arcs, fragments of the Ob’–Zaisan paleo-oceanic basin, and fragments of the active margins of the Siberian and Kazakhstan continents. The lower lithosphere of this collage might have preserved paleo-oceanic or paleosubductional mantle sources. The orogenic structure is underlain by the weakly depleted lithospheric mantle, which existed beneath continental blocks before the collision. Thus, both depleted (harzburgites) and enriched (lherzolites) peridotites might have been sources for basic-ultrabasic magmas.

To estimate the conditions of magma melting-out and substantiate possible geochemical mantle reservoirs, I compared the rock compositions with the results of geochemical modeling of melting of various mantle substrates, using the ratios of indicator elements in two systems: (1) Sm–Yb (Aldanmaz et al., 2000) and (2) Nb–Yb (Yang et al., 2014). The results for these systems are generally in agreement (Fig. 6).

The Sm–Sm/Yb correlations in the rocks of the early Carboniferous Saur complex (Fig. 6 a, d) point to the genesis of magma through the melting of a mixture of garnet and spinel lherzolites, with the degree of melting of 20–30%. The Nb–Nb/Yb correlations suggest the genesis of magmas as a result of the considerable melting (~30%) of spinel lherzolites or harzburgites of the depleted mantle. The high contents of CaO and Al₂O₃, the ubiquitous presence of horn-blende in the gabbro of the Saur complex, and the predominance of diorites among the Saur complex rocks indicate the melting-out of parental magmas in the presence of volatiles (water fluid) in large amounts. In these conditions, the haplobasalt system has a reduced eutectic temperature and is shifted toward anorthite. This leads to the formation of high-alumina magmas with minor MgO contents, which is consistent with the chemical composition of the Saur complex rocks. Probably, the hydrated peridotites of the mantle wedge which were located above the subduction zone were the source of basic magmas for the Saur complex.

The Sm–Sm/Yb correlations in the gabbro and gabronorites of the Irtysh complex suggest the genesis of their magmas through the melting of a mixture of garnet and spinel lherzolites; the Nb–Nb/Yb correlations point to the melting of harzburgites (degree of melting of 10–20%) (Fig. 6,b, e). The high degrees of melting are consistent with the high Mg# values of the rocks. The low Nb/Yb ratio indicates a relative depletion of the magma source of the Irtysh complex, which suggests that the restite rocks preserved in the mantle wedge (harzburgites) were involved in the melting. This is confirmed by the similar behavior of most of major components (except for Ca and Al) and trace elements in the basic rocks of the Saur and Irtysh complexes (Figs. 4 and 5). At the same time, the rocks of the Irtysh complex are relatively enriched in K₂O, Rb, La, Zr, Hf, and MgO, which indicates that the generation of the parental basic magmas was contributed by a deeper-seated mantle reservoir enriched in these components.

The basic dikes of the Bugaz complex in the Zharma–Saur zone and the basalts of the Daubai association in the Charsk zone formed synchronously (in the middle Carboniferous) and have similar petrogeochemical compositions. The ratios of indicator elements (Fig. 6,b, e) show a more enriched composition of the mantle source, similar to that of the primitive mantle. These magmas might have been generated through the partial melting of garnet peridotites (garnet content of ~5%, Fig. 6 e) at greater depths, with the degree of melting of 5–15%.

The early Permian basites are also evident of the geochemically enriched composition of the mantle sources, similar to the composition of the enriched mantle. The differences in the Sm/Yb and Nb/Yb values (Fig. 6 c, f) reflect variations in the composition and degree of melting of the sources. The early basalts of the Tyureshoke trough and the gabbro of the Argimbai complex (297–293 Ma) resulted from garnet peridotites (with the content of garnet of 2–4%) at the degree of melting of 2–5%. The later intruded (280 Ma) gabbro and picrites of the Maksut complex formed at the higher degree of melting of the same source. These geochemical indicators confirm the earlier hypothesis of the comagmatic nature of the volcanic rocks of the Tyureshoke trough and basic rocks of the Argimbai and Maksut complexes (Ermolov et al., 1983). The basic magmas that formed gabbro–granitoid intrusions (Preobrazhenka pluton, 291 Ma) might have been generated from similar sources, namely, garnet peridotites (with the content of garnet of 1%) or spinel lherzolites, at the degree of melting of 5–10%. The basites of the Mirolyubovka dike complex (280–267 Ma), with the degree of melting of ~5%, formed from the same source.

Thus, the chemical composition of the ultrabasic-basic associations changed with a successive increase in the contents of K₂O, P₂O₅, TiO₂, LREE, Rb, Ba, Zr, Hf, Nb, and Ta. The variations in magma composition were due to the different compositions of mantle sources (harzburgites, spinel lherzolites, and garnet lherzolites) and the different degrees of their melting. The early Permian ultrabasic–basic associations are the most enriched in TiO₂ and incompatible components (P₂O₅, Zr, Hf, Nb, and Ta), which indicates that the partial melting involved geochemically enriched mantle sources.

The isotope characteristics of all the studied basic rocks point to their formation from depleted sources; the ε_Nd(T) values vary from +13.5 to +4.5 (Fig. 7). The early Carboniferous gabbroids of the Saur complex are the most enriched in radiogenic Nd. This isotope composition might be due to the previous (Late Devonian–early Carboniferous) partial melting of the mantle wedge material above the subduction zone located beneath the Kazakhstan margin; hence, the melting of mantle sources producing the Saur complex magmas involved restite material. The middle Carboniferous and early Permian basic associations almost do not differ in ε_Nd(T) values (+4.5 to +7.8). This implies the same model age of the mantle sources; the depleted mantle formed synchronously with the oceanic crust of this segment of the Paleoasian Ocean in the early–middle Paleozoic.

The results of comparison of the geological and geochronological data unambiguously prove that basic magmatism was accompanied by silicic magmatism at all stages of evolution of the Altai accretion–collision system. The localization of granitoid complexes and their ages are presented in Figs. 2 and 3.

In the Zharma–Saur zone, there are plutons of the Bugaz granitoid complex (327–326 Ma) associated with the gabbro and diorite plutons of the Saur complex. The most probable mechanism of their formation is the partial melting of crustal substrates, such as the middle–late Paleozoic volcanosedimentary rocks of the active margin of the Kazakhstan continent. This is confirmed by the geochemical characteristics of granites, which correspond to granitoid series of volcanic arcs or active continental margins (Khromykh et al., 2019c).

In the middle Carboniferous, basic volcanism was synchronous with silicic volcanism. In the southeast of the Kalba–Narym zone, two large volcanic structures, Aktobe and Kalguty, have been preserved (Fig. 2); their age was estimated by zircon U–Pb dating at 311 ± 3 Ma (Khromykh et al., 2020). Both structures are composed of dacites, rhyodacites, and rhyolites formed in a homodrome sequence. A petrological study of dacite-porphyry from the Aktobe trough showed that it resulted from the partial melting of metapelitic substrates in the lower crust under basic-magma impact (Khromykh et al., 2011). The Kalba–Narym zone also includes few late Carboniferous granitoid plutons of the Kalguty and Kunush complexes (308–300 Ma). According to a geochemical research (Kuibida et al., 2019), these plutons are the result of the partial melting of metapelitic and metabasic crustal substrates under thermal impact of basic magmas.

Global granite formation took place in all structure-formational zones in the early Permian. The most striking example is the large intrusive plutons in the Kalba–Narym (297–276 Ma) and Zharma–Saur (289–280 Ma) zones (Kotler et al., 2015, 2020, 2021; Khromykh et al., 2016; Kotler, 2017). Most of the plutons lack intermediate or basic intrusive rocks, and the contents of most components of the Kalba batholith granitoids are consistent with the modeling data on the partial melting of metasedimentary substrates (Kotler, 2017; Kotler et al., 2021). At the same time, the synchronous large-scale formation of basic associations unambiguously indicates the contribution of mantle magmas to the petrogenesis of granitoids. No signs of hybridization and mingling of the granitoids indicate normal segregation of anatectic melts. Mantle magmas did not interact directly with melting metamorphic substrates. However, study of the petrogenetic mechanisms for the granitoids of the Monastery complex showed that their composition was influenced by some components supplied with juvenile fluids to the melting chambers and that the mantle fluids might have been transported from a sublithospheric magma reservoir of mantle origin (Kotler, 2017; Kotler et al., 2021).

Special attention should be paid to the volumes of granitoid magmatism and silicic volcanism at different stages of evolution of the accretion–collision system. Using the geological scheme (Fig. 2), it is possible to compare the volumes of igneous complexes. The early Carboniferous granitoids of the Bugaz complex, present only in the Zharma–Saur zone, have a total exposure area of 500–700 km². These are mostly small plutons 1–3 km in average thickness; thus, the volume of the early Carboniferous granitoid magmatism is estimated at 1000–1500 km³. The middle Carboniferous silicic magmatism is manifested as the large Aktobe (~80 km², with lava and tuff deposits ~2000 m in thickness) and Kalguty (~180 km², with lava and tuff deposits ~3000 m in thickness) troughs and several small rhyolite occurrences in the Daubai and Maitobe formations; the total volume of magmatic products is estimated at 500–600 km³. The late Carboniferous granitoid magmatism (Kalguty and Kunush complexes) is manifested as small plutons and belts of NW striking dikes; its total volume is estimated at 300–400 km³.

The largest volume of granitoid magmatism was manifested in the early Permian. Several large gabbro–granitoid intrusions formed in the Charsk zone (Fig. 2); each of them is 200–300 km² in area and about 3–5 km in thickness (Ermolov et al., 1983). The total volume of the early Permian granitoid magmatism in the Charsk zone is estimated at 3000–4000 km³. In the Zharma–Saur zone, the early Permian intrusive belt formed from several (no less than ten) large granitoid plutons, each being 300–400 km² in area. Such granitoid plutons are usually 5–10 km in thickness; accordingly, the volume of the early Permian granitoids in the Zharma intrusive belt is estimated at 20,000–30,000 km³. The largest Kalba batholith with granitoid exposure no less than 10,000 km² in area formed in the Kalba–Narym zone. According to geophysical data, the thickness of the granitoid plutons varies from 2 to 12 km, being predominantly 7–10 km (Lopatnikov et al., 1982). The volume of the Kalba batholith granitoids is estimated at 70,000–100,000 km³. Thus, the total volume of early Permian granitoid magmatism might have reached 150,000 km³; this is tens (or almost a hundred) times more than the volumes of the preceding Carboniferous granitoids.

A significant increase in the volumes of granitoid magmatism in the early Permian implies an increase in temperature gradients in the lithosphere because of the mantle activity. It is impossible to estimate the volume of early Permian basic magmas reliably, because most of them did not reach the recent erosional-truncation level. According to existing models, most magmas of mantle origin do not penetrate above the Moho because of their higher density but propagate along it, forming subcrustal magma chambers. Judging from the volumes of granitoid magmatism, the volume of such magma chambers was small in the early and middle Carboniferous but an order of magnitude larger in the early Permian.

The comparison of the geological and petrogeochemical data made it possible to establish two types of interaction between basic magmas and crustal substrates:

• (1) Basic magmas did not reach the level of granite-forming substrates, because they stopped in subcrustal chambers. Thermal impact on crustal substrates led to their partial melting. The melts had an anchieutectic composition; their segregation gave rise to large volumes of granitic magmas and their migration to higher levels. The fluids separated from basic-magma chambers might have affected the composition of granites.

• (2) Basic magmas penetrated to the levels of graniteforming substrates and directly interacted with anatectic melts. The presence of basic magmas might have increased the degree of melting of substrates and led to the enrichment of anatectic melts with hard-melting components relative to the eutectic composition. The direct interaction of basic and acid magmas resulted in hybrid diorite–monzonite rocks (at greater depths) or specific mingling structures (at shallower depths).

Despite the same mechanisms of granite formation as a reaction of crustal substrates to the impact of basic magmas, the volumes of the early Permian basic and granitoid magmatism are different in structure-formational zones. This is obviously due to the different structures of the lithosphere.

The lithosphere in the Zharma–Saur zone is composed of middle and late Paleozoic volcanosedimentary complexes formed on the Kazakhstan active margin during the subduction of the oceanic lithosphere of the Ob’–Zaisan ocean. Suprasubductional volcanism was of large-scale occurrence here, as evidenced by the area occupied by Devonian and early Carboniferous (Tournaisian–Visean) volcanic rocks. By the early Permian, this lithosphere was obviously thick and more consolidated. Under thermal impact of basic magmas upon crustal substrates, the most low-melting ones underwent partial melting to produce predominantly granites and leucogranites. The absence of monzonitoids from most of the intrusive plutons of the Zharma–Saur zone indicates that basic magmas hardly penetrated into the crust.

The lithosphere of the Charsk zone is formed mainly by relics of the Ob’–Zaisan ocean basin with juvenile volcano-terrigenous crust. The small volume of granitoids in the Charsk zone is apparently due to the metabasic composition of the substrates. The thin lithosphere in the Charsk zone favored the penetration of basic magmas into the middle and upper crust. Deep shear–expansion faults played a crucial role in this process, as they controlled the localization of all igneous rocks in the Charsk zone (Fig. 2). The extension setting facilitated the rapid penetration of basic magmas, which either erupted onto the surface (basaltic and andesitic lavas) or formed extrusive and hypabyssal bodies (plutons of the Argimbai and Maksut complexes). In the case when basic magmas were retained in the lower or middle crust, they intensely interacted with crustal substrates and anatectic melts to form multiphase gabbro-monzonite–granitoid intrusions.

The lithosphere of the Kalba–Narym zone began to form as a sea-marginal basin in the passive-margin setting in the Devonian, after the termination of subduction beneath the Rudnyi Altai margin of the Siberian continent. In the period from Middle Devonian to early Carboniferous (C₁V), thick strata of terrigenous sediments (mainly siltstones and argillaceous shales) accumulated in this basin. Under reduction of the ocean basin at the end of the early Carboniferous (C₁S), these strata were intensely dislocated and folded, and the thickness of the sedimentary section increased several times. The lower parts of the section underwent metamorphism up to the amphibolite facies; fragments of these rocks are found along deep faults in the Irtysh shear zone. Under the large-scale impact of basic magmas in the early Permian, interbeds and lenses of partly melted plastic rocks (diatectites and migmatites) might have formed in the metamorphic strata of the lower–middle crust. Basic magmas could not penetrate to the levels of magma generation because of the higher density and plastic state of crustal substrates. However, thermal and fluid impact on crustal substrates led to large-scale melting. The basic dikes of the Mirolyubovka complex which intrude the granitoids of the Kalba batholith are indicators of subcrustal basic reservoirs. Intrusion of dikes into the middle and upper crust became possible after the formation and cooling of intrusive granitoids at the end of the early Permian.

Comparison of all information about the geologic position, chemical composition, and age of igneous basic and granitoid associations makes it possible to consider geodynamic scenarios and apply particular models for mantle magmatism in accretion–collision fold belts.

The early Carboniferous orogeny took place only in the southwest of the Altai accretion–collision system, in the Zharma–Saur zone. By the end of the early Carboniferous, sedimentation and volcanism stopped in this area, which indicates the termination of subduction and the beginning of orogenic processes. It is in this setting that gabbro–granitoid magmatism of the Saur series occurred. The current geodynamic concepts permit explaining this magmatism in the framework of the model of the detachment of subducted oceanic slab. The linear arrangement of the Saur series intrusions and the geochemical and geochronological data argue for this model. At the early stage of orogeny, a fragment of the subducted lithosphere of the Ob’–Zaisan ocean basin was detached beneath the margin of the Kazakhstan continent. This led to the asthenosphere activation, an increase in temperature gradient in the mantle wedge and melting of the latter, and the formation of hydrated basic magmas, which were then differentiated to dioritic ones.

At the early–middle Carboniferous boundary, an orogen formed. In the middle Carboniferous, mantle magmatism occurred in all structure-formational zones. The melting of the lithospheric mantle material beneath collisional orogens can be explained within the framework of different geodynamic models. The slab detachment model is poorly applicable to explain such magmatism proceeding over the entire area of the collision system: The synchronous detachment of at least three slabs subsiding at different places was hardly possible. The model implying the influence of a mantle plume can hardly be used as well, judging from the geochemical features of mantle rocks and the small volumes of magmatism. Geodynamic models of delamination of the lower lithosphere are more likely in this case. They explain the above phenomenon mostly by extension motions combined with large-amplitude shearing (also known as trans-tension). The main shear zones can act as conductors for basic melts resulting from the melting of mantle material. During shearing in orogen because of the inhomogeneities of the boundaries between different blocks, sites undergoing compression or extension can alternate. Compression sites probably had conditions favorable for high-pressure metamorphism, whereas extension might have caused faults, which were then filled with the ascended hot asthenospheric material. This asthenosphere process is currently called mantle upwelling (Parmentier, 2007).

Volcanic troughs and depressions appeared at the sites with the greatest extension; small intrusions and dike belts are specific to other sites. The middle Carboniferous basic magmatism can be considered late collisional magmatism occurring at the later stages of the orogen lifetime. In other words, the middle Carboniferous magmatism is an indicator of the orogen collapse. We can state that the orogeny stage of evolution of the Altai collision system terminated in the late Carboniferous. Thus, the lifetime of the orogen in the Ob’–Zaisan folded system can be estimated at ~20–30 Ma (the interval from 330 ± 5 to 310 ± 5 Ma).

In the early Permian, diverse basic and large-scale granitoid magmatism took place in all structure-formational zones. The early Permian basic complexes are enriched in incompatible elements; they resulted from the melting of geochemically enriched upper-mantle garnet and spinel peridotites. The partial melting of these sources, like the previous middle Carboniferous events, might have been caused by tectonic processes. However, the occurrence of basic magmatism throughout the study area and, most importantly, the tens of times greater volume of granitoids testify to a large thermal anomaly.

Similar early Permian basic and granitoid magmatism took place over a vast area in the west of the Central Asian Orogenic Belt: in Central and Southern Kazakhstan, Kyrgyzstan, and Uzbekistan (Biske et al., 2013; Konopelko et al., 2018), in the Xinjiang–Uyghur region of China (Pirajno et al., 2009; Xu et al., 2014), and in Southern Mongolia (Kozlovsky et al., 2015; Yarmolyuk et al., 2017). The extensive early Permian magmatism was united into the early Permian

Tarim LIP, which formed as a result of the activity of the Tarim mantle plume (Borisenko et al., 2006; Dobretsov et al., 2010; Ernst, 2014; Xu et al., 2014; Yarmolyuk et al., 2014). Since the early Permian magmatism in Eastern Kazakhstan had similar characteristics, it must also be included into the Tarim LIP.

The expansion of the Tarim LIP to the northwest, up to the territory of Eastern Kazakhstan, was, most likely, due to the postorogenic lithosphere extension. The early Permian magmatism in the area was the result of a combination of plate-tectonic and plume-tectonic factors. The positive ε_Nd(T) values for the early Permian ultrabasic–basic rocks indicate that they formed from the depleted lithospheric-mantle material that underwent partial melting under mantle plume impact.

In recent years, several models describing the mantle plume-lithosphere interaction have been proposed. All models imply several stages of this process: (1) the early stage of interaction of the plume head with the lithosphere; (2) the stage of spreading of the plume “cap” beneath the lithosphere and its maximum heating; and (3) the stage of relaxation, i.e., cooling of both the upper mantle and the lithosphere (Dobretsov, 2008; Dobretsov et al., 2010; Xu et al., 2014). These stages can be traced when studying the early Permian magmatism in Eastern Kazakhstan.

The first geochemically enriched basic magmas were generated ~297 Ma; this time can be regarded as the beginning of the plume–lithosphere interaction.

The early stage (297–293 Ma) was characterized by low degrees of melting (2–3%) of mantle substrates. According to geochemical data (Fig. 6), garnet (2–4%) peridotites melted. The generated subalkalic basic magmas of the Ty-ureshoke association and Argimbai complex intruded along local extension zones and appeared in several local areas.

The maximum interaction stage (290–285 Ma) was characterized by melting (to a degree of 5–10%) of garnet (1%) peridotites and of spinel peridotites (Fig. 6). Subalkalic basic magmas were generated, which were then involved in the formation of gabbro-monzonite–granite intrusions and dikes of the Mirolyubovka complex. These basic magmas appeared over a vast area but reached the crust only in the Charsk zone, where they actively interacted with crustal substrates and anatectic melts. In the Kalba–Naiym and Zharma–Saur zones, basic magmas stayed in the subcrustal magma chambers, but their thermal and fluid impact caused large-scale melting of crustal substrates and formation of the bulk of granites of the Kalba and Zharma intrusive belts.

The relaxation stage (283–267 Ma) began after the mass granite formation and the formation and cooling of granitoids. The cooled crust underwent brittle deformations, which favored the intrusion of the Mirolyubovka complex dikes (279 ± 3 Ma) from the subcrustal reservoir. Some dike belts in the Zharma–Saur zone might have formed in the same way. Hypabyssal bodies of the magnesian basites of the Maksut complex appeared in the Charsk zone. Like the gabbro of the Argimbai complex, they resulted from garnet (2–4%) peridotites but at the higher degree of melting (up to 10%) of the latter. Accordingly, the basic magmas of the Maksut complex intruded from the deepest mantle chambers. Brittle deformations in the cooled lithosphere permitted these magmas to rise to the lower and middle crust, which caused the second, smaller-scale, melting of crustal substrates with the formation of several intrusive granite–leucogranite plutons (283–276 Ma).

Ultrabasic-basic igneous complexes have been examined in many accretion–collision systems of the Central Asian Orogenic Belt. They formed at different stages of the system evolution and were accompanied by granitoid magmatism (Izokh et al., 1998; Rudnev et al., 2009; Fedorovsky and Sklyarov, 2010; Vladimirov et al., 2013, 2017; Yarmolyuk et al., 2013; Kozlovsky et al., 2015; Shelepaev et al., 2018; Shapovalova et al., 2019). In some CAOB regions, there are also areas of extremely large-scale endogenous activity, expressed primarily as huge volumes of granitoid magmatism. These areas are regarded as large igneous provinces: Angara–Vitim batholith in Transbaikalia (Tsygankov et al., 2010), Hangayn and Henteyn batholiths in Mongolia (Yarmolyuk et al., 2013, 2016), and Kaa-Khem batholith in Eastern Tuva (Rudnev et al., 2006, 2020; Vladimirov et al., 2013). In all cases, the active role of the mantle in the lithosphere transformation is obvious. It might be related both to plate-tectonic processes and to the impact of mantle plumes on the lithosphere of accretion–collision systems.

The performed studies have shown that mantle magmatism and associated crustal magmatism reflect a successive change in geodynamic regimes and the types of mantle–lithosphere interaction during the evolution of the Altai accretion–collision system (Eastern Kazakhstan):

• (1) The early Carboniferous (C₁s) magmatism of the early orogeny stage was the result of the detachment of the subducted lithosphere (slab) beneath the margin of the Kazakhstan continent.

• (2) The middle Carboniferous (C₂m) magmatism of the late orogeny stage resulted from active shear–expansion motions along large faults and reflected the orogen collapse.

• (3) The early Permian (300–270 Ma) large-scale magmatism was the result of a global thermal disturbance in the upper mantle under impact of the Tarim mantle plume. The regularities of occurrence of this magmatism reflect different stages of the mantle plume–lithosphere interaction (initial and maximum stages and relaxation). The large-scale occurrence of the early Permian magmatism over the studied area was due to the combination of the thermal disturbance in the upper mantle and the lithosphere extension.

I thank E.M. Sapargaliev for assistance in the research in the East Kazakhstan area; P.D. Kotler, E.I. Mikheev, and O.P. Gerasimov for help in the field works; D.V. Semenova and V.B. Khubanov for geochronological studies; and A.E. Isokh and O.M. Turkina for discussion and useful comments on the paper. I am also grateful to reviewers V.V. Vrublevskii and A. A. Tsygankov, whose comments helped to improve the manuscript.

The work was done on state assignment of IGM SB RAS with a financial support by grants 17-05-00825, 20-05-00346, and 20-35-70076 from the Russian Foundation for Basic Research.