Tectonic Generation of Pseudotachylytes and Volcanic Rocks: Deep-Seated Magma Sources of Crust-Mantle Transition in the Baikal Rift System, Southern Siberia

Volcanic rocks from deep-seated sources of the crust-mantle transition (CMT) are geochemically distinguished from those of ocean island basalts (OIB). Here, we report geochemical data on tectonic pseudotachylytes from the Main Sayan Fault (MSF) and volcanic rocks from the Kamar-Stanovoy Zone of Hot Transtension (KSZHT) that represent the deep-seated CMT magmatic sources in the central part of the Baikal Rift System (BRS). The tectonic generation of the KSZHT magmas between 18.1 and 11.7 Ma is compared with present-day seismogenic deformations in the middle-upper crust of the South Baikal Basin and adjacent Tunka Valley, where strong earthquakes are distributed along the Main Sayan and Primorye sutures of the Siberian paleocontinent. From a detail seismic tomography model and geological evidence, we infer that the KSZHT crust–mantle magmatic processes were due to delamination and lamination of a thickened root part of the South Baikal Orogen existed in the Late Cretaceous and Paleogene. In addition, we identify similar deepseated CMT sources for melts erupted in the past 17 Ma from a delaminated root part of the East Hangay Orogen and adjacent Orkhon-Selenga Saddle in the southwestern BRS. We suggest that both in the central and in the southwestern BRS, the deep-seated CMT magma sources designate cooperative pull-to-axis and convergent effects created in the Japan-Baikal Geodynamic Corridor and in the Indo-Asian interactional region, respectively.


Introduction
In recent decades, intraplate volcanic eruptions were a priori interpreted as associated with deep mantle plumes, although alternative causes, such as abrupt lateral changes in stress at structural discontinuities of the lithosphere or the mantle global warming, were also advocated to explain the origin of some Large Igneous Provinces [1,2]. Less commonly hypothesized were tectonically generated melts as a result of decompression in the shallow lithospheric mantle or crust. Such interpretations were suggested for volcanic rocks from Cenozoic continental rifted areas of North America, Europe, Northeast Africa, and East Asia [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].
Volcanic rocks from the Baikal Rift System (BRS) have mainly OIB-like geochemical signatures, consistent with seismic tomography images that show low-velocity anomalies, possibly related to deep-seated magma sources occurring in the upper mantle and transition layer. These could provide migrated volcanic eruptions through the moving lithosphere [18]. Meanwhile, volcanic rocks from the East Hangay Orogen and adjacent Orkhon-Selenga Saddle of Central Mongolia (southwestern part of the BRS) show both mantle and crustal components geochemically different from OIB [19]. Volcanic rocks of some eruption phases in the Kamar-Stanovoy zone of Hot Transtension (KSZHT) (central part of the BRS) and in the Vitim Plateau (northeastern part of the BRS) also indicate their generation in shallow (garnet-free) sources. Such volcanic rocks of basaltic andesite compositions in the latter area have, however, 143 Nd/ 144 Nd isotope ratio similar to that in high-Mg basanites, indicating the secondary high-degrees shallow melting of an OIB-like material previously raised from the lower-middle part of the upper mantle. At the eastern end of the Tunka Valley, few high-Mg basanite flows also erupted, but basaltic ones, derived from shallow sources, distinctly show no OIB-type signatures [20,21]. In this case, it is reasonable to assume melting of a material in a region of the crust-mantle transition (CMT). In order to recognize crustal and shallow mantle components, it is necessary, first of all, to define the local geochemical signatures of the crust.
Tectonic motions of the crust are accompanied with heat release resulted from friction during incremental sliding along a fault zone, in which an adhesive wear mechanism dominates. Generated melts of silicate rocks are quenched in planes of strong fault displacements as pseudotachylytes [22][23][24][25][26]. Tectonic pseudotachylytes were described in mylonites of various metamorphic environments that developed in a regime of dominant plastic deformation of the lower crust [27][28][29][30] and in cataclasites typical of brittle deformations in the middle and upper crust [31,32]. Minerals of igneous rocks and glass, preserved in pseudotachylytes of the upper crust, are affected by metamorphic transformations in deeper conditions [33][34][35].
On the one hand, the similarity of bulk chemical compositions of pseudotachylyte veins and host rocks led to an assumption on melt generation by complete melting of host rocks [22,36,37]. On the other hand, the presence of relict mineral fragments of quartz and feldspar (but not amphibole and mica) in most pseudotachylyte veins from different parts of the world showed predominant melting of mafic minerals [38][39][40][41]. High-friction experiments and numerical simulations of pseudotachylyte generation demonstrated the formation of a gouge of finely crushed material later filled with a melt generated by friction. Such a melt could interact with small debris with further melting of the material and a partial change in the original size of frictionally molten rock fragments. Some pseudotachylytes could have purely crushed origin [31,[42][43][44][45][46][47]. In pseudotachylyte veins, a heterogeneous groundmass composition on a scale of microdomains was revealed, usually more mafic relative to a host rock bulk composition [39,48]. A composition of the pseudotachylyte matrix, enriched in mafic components, was explained by predominant crushing of softer mafic minerals during host rock cataclasis, followed by massive melting of a cataclasite [40,49].
A coefficient of volumetric expansion of a water is greater than in a host rock, therefore frictional heating causes an increase in pore pressure in a shear zone and, as a consequence, reduces the effective stresses, preventing melting that can only occur in a case of low permeability of a host rock and effective retention of a fluid under excessive pressure [50,51]. It remains unclear whether pseudotachylytes could be a result of melt penetration from a deeper root part of an active fault into its upper part.
In Southern Siberia, pseudotachylytes have been described so far only in the Primorye Fault that bounds the basement of the Siberian paleocontinent from terranes accreted in the southeast. It was inferred that this type of rocks inherited a chemical composition of a host diorite with relative depletion in SiO 2 and enrichments in FeO tot , MgO, and CaO [52]. A new pseudotachylyte site is found in the Main Sayan Fault (MSF) that separates the southwestern margin of the Siberian paleocontinent basement from accreted terranes and spatially corresponds to the South Baikal Basin and Tunka Valley of the central Baikal Rift System (BRS) [53,54]. In 2003-2019, seismicity focused within the South Baikal Basin without propagating into the Tunka Valley. Earthquakes distributions and temporal variations of 234 U/ 238 U activity ratio in groundwater indicated a future main seismic event of the 2020-2021 seismic reactivation in the South Baikal water area after an earthquake in the junction between the Tunka Valley and South Baikal Basin [55]. Respectively, the Bystraya earthquake (K = 14.5, M w = 5.4) occurred on 21 September 2020 [56] and then the Selenga one followed on 9 December 2021 (K = 13.9).
The aim of this paper is to highlight significance of the deep-seated CMT magmatic sources for the structural development of the South Baikal Basin and Tunka Valley, presently affected by strong seismogenic deformations, through geochemical studies of the MSF pseudotachylytes and the KSZHT volcanic rocks. First, we consider the structural setting of magma generation and present results of the comparative analysis of pseudotachylyte and host mylonite compositions and then report results of the comparative study of the MSF pseudotachylytes and the KSZHT volcanic rocks to designate complementary relationships of their deep-seated sources.

Methods and Materials
In each volcano of the KSZHT, a sequence of lava layers was sampled from a base to a top. Data on compositions and ages of volcanic rocks, as well as compositions of their crystalline inclusions, were presented earlier [20,[57][58][59][60]. For comparisons with the MSF pseudotachylytes, special attention was delivered to study compositions of lavas from the Kultuk and Meteo volcanoes. For understanding of volcanic activities in the entire hot transtension zone, rocks from the Karerny and Shirokiy volcanoes were additionally sampled. Pseudotachylytes were sampled in the MSF zone with their primary subdivision into rocks of basic, intermediate, and silicic geochemical/chemical compositions using a portable device to measure the magnetic susceptibility (produced by Geodevice Enterprise, Russia). In the KSZHT magma-controlling structure, syn-volcanic direction of extension was determined by the predominant orientation of dikes. Planes of main faults were reconstructed using the method of belts [61].
Major oxides of rock samples were determined by classical methods of analytical chemistry [62] and trace elements were analyzed by inductively-coupled plasma mass spectrometry (ICP-MS) using an Agilent 7500ce quadrupole mass spectrometer. Analytical results on major oxides and trace elements of the standard reference materials, obtained within the time interval of analyzing pseudotachylytes from the MSF and volcanic rocks from the KSZHT, are presented in Table S1 of the Supplementary Materials. Isotopic 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios were measured on a Finnigan MAT 262 mass spectrometer. Standard reference materials JNd-1 (Japan) and NBS SRM-987 (USA) were used. Obtained during the period of measurements were 143 Nd/ 144 Nd value of 0.512104 ± 0.000004 (2σ) for JNd-1 and 87 Sr/ 86 Sr value of 0.710233 ± 0.000012 (2σ) for NBS SRM-987. Measurement results of samples were recalculated to the recommended values of the reference materials: 0.512103 and 0.71025, respectively [63]. A secondary alteration effect, causing distortion of 87 Sr/ 86 Sr values, was removed by treatment of samples by a 0.75 M hydrochloric acid solution for 24 h with its further removal through repeated washing by ultrapure water from an Elix-3 Millipore system (France). The methods applied were described previously by Rasskazov et al. [19] and Yasnygina et al. [64]. Analysis of Pb isotopes was performed by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) using a Neptune Plus' mass spectrometer and the procedure of sample preparation described by Rasskazov et al. [11]. During the period of measurements, obtained values for NBS-981 were: 206   The geological scheme is modified after [65,73,74]. The spherogram is adopted from [54]. Density of points on the spherogram: 5%, 10%, and 15%.

Pseudotachylytes and Their Structural Setting
Pseudotachylytes are observed as a series of black and pink stripes and veins in solid-gray mylonites of an outcrop located 1 km west of the Kultuk village ( Figure 2a). The stripes are as thick as 10 cm (Figure 2b). Pink bands are cut by black veins (Figure 2c). Unidirectional black banded series are accompanied by wedge-shaped (upwardly tapering) bodies of dark-gray pseudotachylytes. This kind of bodies is up to 2.5 m wide at the base (Figure 2d). Different parts of the outcrop show horizontal fractures filled by pistachiogreen epidote-chlorite material. As a result of the most recent deformations, these fractures cut through all rocks.  In photograph a, dark saturated by injected rocks stripes dip under a slope of a quarry. In photograph d, a wedge-shaped fragment is cut at the base by a horizontal rupture.
Fracturing in a site of mylonites with black bands was studied to define inclinations of major fault planes using the method of belts [61]. Reconstructed are dips of two faults: 200°, angle of 74° and 274°, angle of 56°. The former has an azimuth of 290° that corresponds to the MSF strike; the latter designates the north-south direction of the KSZHT ( Figure 3). The MSF plane dips from the Siberian platform to the south-southwest, under the Khamardaban terrane and the KSZHT plane from the South Baikal Basin to the west, under the Tunka Valley. A series of injected bands, veins, and wedge-shaped bodies extends almost parallel to the MSF plane with a relative clockwise turn by six degrees. Pseudotachylyte melts injected into antithetical fissures of the MSF hanging wing. Fracturing in a site of mylonites with black bands was studied to define inclinations of major fault planes using the method of belts [61]. Reconstructed are dips of two faults: 200 • , angle of 74 • and 274 • , angle of 56 • . The former has an azimuth of 290 • that corresponds to the MSF strike; the latter designates the north-south direction of the KSZHT (Figure 3). The MSF plane dips from the Siberian platform to the south-southwest, under the Khamardaban terrane and the KSZHT plane from the South Baikal Basin to the west, under the Tunka Valley. A series of injected bands, veins, and wedge-shaped bodies extends almost parallel to the MSF plane with a relative clockwise turn by six degrees. Pseudotachylyte melts injected into antithetical fissures of the MSF hanging wing.

Compositions of Pseudotachylytes and Host Mylonites
For analytical studies of the MSF rocks, samples of injected wedge-shaped bodies, veins, and bands, host mylonites (aluminosilicate and carbonate-aluminosilicate), and an epidote-chlorite rock were selected (Table 1). In a study the injected magmatic rocks, on the one hand, it is important to follow an experience of the pseudotachylyte classification as derivatives of seismogenic melts in faults. Therefore, the injected rock compositions are compared with those of two types of pseudotachylytes identified in the Sarwar-Junia Fault Zone, India: Pt I, from gneiss, and Pt II, from a contact between gneiss and metadolerite [32]. On the other hand, the injected rock compositions should be considered in terms of the nomenclature of non-crystallized volcanic rocks [75].
Samples of black stripes that correspond to basic compositions in terms of a SiO2 content are subdivided into moderate-and low-Ti groups (TiO2 contents 1.43-2.18 wt% and 0.6-0.69 wt%, respectively). Sample KL-17 with the lowest MgO content (4.17 wt%) of the moderate-Ti group has a reduced (but not the lowest) TiO2 content (1.77 wt%). Specifically referred is sample KL-21, which shows TiO2 = 1.28 wt% and MgO = 5.18 wt% between contents of these oxides in the moderate-and low-Ti groups. In terms of SiO2, Al2O3, MgO, and TiO2 abundances, these rocks are comparable to the type II pseudotachylyte from the Sarwar-Junia Fault Zone, but differ from it by elevated Na2O and reduced FeOtot and K2O contents (Figure 4). In a thin section of the basic pseudotachylyte, shown in Figure 2b, a foliated groundmass composed of glaucophane, quartz, and magnetite is observed (Figure 5a). Obviously, this rock, originally solidified from an injected basaltic melt, was affected by metamorphism. Figure 3. Outputs of axes of tectonic fractures in the MSF mylonites with reconstructed dips of the MSF and KSZHT planes in the lower hemisphere. Fracture belts B1, B2, B3, B4, and B5 correspond to the outlets of axes A1, A2, A3, A4, and A5, respectively. Orientations of 100 crack poles are plotted with a density of 13%, 11% . . . 3%, and 1%. Injected bands extend along the MSF plane. Injected bands, extended along the MSF plane, correspond to the antithetical fissures of its south-southwestern hanging wing.

Compositions of Pseudotachylytes and Host Mylonites
For analytical studies of the MSF rocks, samples of injected wedge-shaped bodies, veins, and bands, host mylonites (aluminosilicate and carbonate-aluminosilicate), and an epidote-chlorite rock were selected (Table 1). In a study the injected magmatic rocks, on the one hand, it is important to follow an experience of the pseudotachylyte classification as derivatives of seismogenic melts in faults. Therefore, the injected rock compositions are compared with those of two types of pseudotachylytes identified in the Sarwar-Junia Fault Zone, India: Pt I, from gneiss, and Pt II, from a contact between gneiss and metadolerite [32]. On the other hand, the injected rock compositions should be considered in terms of the nomenclature of non-crystallized volcanic rocks [75]. Samples of black stripes that correspond to basic compositions in terms of a SiO 2 content are subdivided into moderate-and low-Ti groups (TiO 2 contents 1.43-2.18 wt% and 0.6-0.69 wt%, respectively). Sample KL-17 with the lowest MgO content (4.17 wt%) of the moderate-Ti group has a reduced (but not the lowest) TiO 2 content (1.77 wt%). Specifically referred is sample KL-21, which shows TiO 2 = 1.28 wt% and MgO = 5.18 wt% between contents of these oxides in the moderate-and low-Ti groups. In terms of SiO 2 , Al 2 O 3 , MgO, and TiO 2 abundances, these rocks are comparable to the type II pseudotachylyte from the Sarwar-Junia Fault Zone, but differ from it by elevated Na 2 O and reduced FeO tot and K 2 O contents (Figure 4). In a thin section of the basic pseudotachylyte, shown in Figure 2b, a foliated groundmass composed of glaucophane, quartz, and magnetite is observed (Figure 5a). Obviously, this rock, originally solidified from an injected basaltic melt, was affected by metamorphism.   [32]. In diagrams, measured major oxide values are used. Samples, taken from individual wedge-shaped bodies, show an SiO2 range of 61.69-66.43 wt% with total alkalis of 5.73-6.47 wt%. These rocks have intermediate (andesite and dacite) compositions. When recalculated to 100% without loss on ignition, all rocks are dacites. The MgO and FeOtot contents in these rocks (1.85-3.18 wt% and 4.77-6.72 wt%, respectively) are lower than in basic pseudotachylytes. In terms of SiO2, MgO, CaO, and FeOtot contents, these rocks are comparable to the type I pseudotachylyte from the Sarwar-Junia Fault Zone, but differ from it by elevated Na2O and reduced Al2O3 and K2O contents ( Figure 4). Thin sections of these rocks show large grains of amphibole, garnet, and microcline, surrounded by a fine-grained strongly deformed quartz-feldspar groundmass (Figure 5b-d).
Pink bands and veins have silicic (rhyolite) compositions with low MgO, FeOtot, CaO and elevated Na2O and K2O contents. The banding style of these rocks indicates structural conditions of their generations similar to those of basic pseudotachylytes. The pink injected rocks are compositionally distinguished from those of the types I and II in the Sarwar-Junia Fault Zone and are referred to the type III.
Mylonites are composed of a mixed aluminosilicate and carbonate rubbed material. Carbonate is present both in bulk mylonites and in veins. The aluminosilicate mylonite group contains 72.3-73.4 wt% SiO2. As SiO2 of this group increases, MgO, FeOtot, CaO, Al2O3, and K2O decrease, while Na2O increases. The carbonate-aluminosilicate mylonite group shows 18.0-61.8 wt% SiO2. In this group, increase of SiO2 content is also accompanied by decrease of MgO, FeOtot, and CaO abundances, but the K2O content consistently increases. A sample with the lowest SiO2 content (KL-7) has a negligible Na2O concentration (0.2 wt%) with Al2O3 content as low as 5.7 wt%. In other samples of the carbonate-aluminosilicate group, Na2O abundance is also relatively low (1.08 wt% and 2.14 wt%). An epidote-chlorite rock shows TiO2, FeOtot, and Mgo contents that lower than Samples, taken from individual wedge-shaped bodies, show an SiO 2 range of 61.69-66.43 wt% with total alkalis of 5.73-6.47 wt%. These rocks have intermediate (andesite and dacite) compositions. When recalculated to 100% without loss on ignition, all rocks are dacites. The MgO and FeO tot contents in these rocks (1.85-3.18 wt% and 4.77-6.72 wt%, respectively) are lower than in basic pseudotachylytes. In terms of SiO 2 , MgO, CaO, and FeO tot contents, these rocks are comparable to the type I pseudotachylyte from the Sarwar-Junia Fault Zone, but differ from it by elevated Na 2 O and reduced Al 2 O 3 and K 2 O contents ( Figure 4). Thin sections of these rocks show large grains of amphibole, garnet, and microcline, surrounded by a fine-grained strongly deformed quartz-feldspar groundmass (Figure 5b- In this group, increase of SiO 2 content is also accompanied by decrease of MgO, FeO tot , and CaO abundances, but the K 2 O content consistently increases. A sample with the lowest SiO 2 content (KL-7) has a negligible Na 2 O concentration (0.2 wt%) with Al 2 O 3 content as low as 5.7 wt%. In other samples of the carbonate-aluminosilicate group, Na 2 O abundance is also relatively low (1.08 wt% and 2.14 wt%). An epidote-chlorite rock shows TiO 2 , FeO tot , and Mgo contents that lower than values of basic pseudotachylytes but higher than those of mylonites. This could result from mechanical mixing between different materials in the fault zone. However, other oxides do not fit this simple mixing. For instance, K2O abundance of the epidote-chlorite rock, which is as high as 3.2 wt%, exceeds its values in both basic pseudotachylytes and mylonites ( Figure 4).
The subdivision of the MSF samples into mylonites and pseudotachylytes in terms of major oxide abundances is consistent with their grouping in terms of trace-element signatures. On a MgO -Th/U diagram ( Figure 6), illustrated are a high Th/U ratio (5)(6)(7)(8)(9)(10) in intermediate pseudotachylytes, its value about 5 in a silicic pseudotachylyte (sample KL-11), and interval of 2.5-6.0 in basic pseudotachylytes. The Th/U ratio of mylonites and epidote-chlorite rock varies from 1.5 to 3.5. values of basic pseudotachylytes but higher than those of mylonites. This could result from mechanical mixing between different materials in the fault zone. However, other oxides do not fit this simple mixing. For instance, K2O abundance of the epidote-chlorite rock, which is as high as 3.2 wt%, exceeds its values in both basic pseudotachylytes and mylonites ( Figure 4). The subdivision of the MSF samples into mylonites and pseudotachylytes in terms of major oxide abundances is consistent with their grouping in terms of trace-element signatures. On a MgO -Th/U diagram ( Figure 6), illustrated are a high Th/U ratio (5)(6)(7)(8)(9)(10) in intermediate pseudotachylytes, its value about 5 in a silicic pseudotachylyte (sample KL-11), and interval of 2.5-6.0 in basic pseudotachylytes. The Th/U ratio of mylonites and epidote-chlorite rock varies from 1.5 to 3.5.   (d) carbonate-aluminosilicate mylonites. Shown for comparison in panel c is the range of carbonate rocks of the Slyudyanka complex (without fine-crystalline marbles with lower REE abundances) [76]. For normalization, the chondrite composition was used after [77]. (d) carbonate-aluminosilicate mylonites. Shown for comparison in panel c is the range of carbonate rocks of the Slyudyanka complex (without fine-crystalline marbles with lower REE abundances) [76]. For normalization, the chondrite composition was used after [77].
Aluminosilicate mylonites have unenriched or slightly enriched REE spectra with and without Eu anomaly. A line of the epidote-chlorite rock extends above those of aluminosilicate mylonites (Figure 7b). Carbonate-aluminosilicate mylonites show a weak enrichment in light REE. The analyzed samples yield a range of normalized concentrations at the upper limit of the marbles from the Slyudyanka crystalline complex and slightly shift above them (Figure 7d).
Correlation between CaO/Al 2 O 3 and CO 2 ( Figure 8a) emphasizes a sufficient role of a carbonate mineral phase in the carbonate-aluminosilicate group and its decreasing in the aluminosilicate one. Common trends of mylonites on CaO/Al 2 O 3 -CO 2 , CaO/CO 2 -CO 2 , and Ba/Sr-CO 2 diagrams (Figure 8a,b,d) indicate a mechanical mixing of an aluminosilicate material (with low CaO content and high Ba/Sr ratio) and a carbonate one (with high CaO content and low Ba/Sr ratio). A deviation from the mixing trend of a single data point of mylonite KL-10 indicates an admixture of magmatic material processed into mylonites. All pseudotachylytes definitely deviate from the mylonite trend of carbonate admixture and incorporate variable proportions of carbonate phase. Correlation between CaO/Al2O3 and CO2 ( Figure 8a) emphasizes a sufficient role of a carbonate mineral phase in the carbonate-aluminosilicate group and its decreasing in the aluminosilicate one. Common trends of mylonites on CaO/Al2O3-CO2, CaO/CO2-CO2, and Ba/Sr-CO2 diagrams (Figure 8a,b,d) indicate a mechanical mixing of an aluminosilicate material (with low CaO content and high Ba/Sr ratio) and a carbonate one (with high CaO content and low Ba/Sr ratio). A deviation from the mixing trend of a single data point of mylonite KL-10 indicates an admixture of magmatic material processed into mylonites. All pseudotachylytes definitely deviate from the mylonite trend of carbonate admixture and incorporate variable proportions of carbonate phase.

Compositions of Basic Pseudotachylytes and Volcanic Rocks
The defined three types of pseudotachylytes in one outcrop and the difference of their compositions from those of host mylonites may indicate their generation in different sources of the MSF root and ascending of melts to an upper common level. These kinds of deep-seated sources for basic pseudotachylytes might have links with those for the KSZHT volcanic rocks.

Major Oxides
Volcanic rocks from the KSZHT vary from silica saturated basalts to strongly undersaturated basanites. Basaltic lavas erupted on the Kultuk and Meteo volcanoes. The former occurred directly in the MSF zone and was active both at the beginning and at the end of the KSZHT volcanism (about 18 and 13 Ma), the latter was the farthest one from the MSF to the south and was active only at the beginning of the KSZHT volcanism (18.1-17.6 Ma).

Compositions of Basic Pseudotachylytes and Volcanic Rocks
The defined three types of pseudotachylytes in one outcrop and the difference of their compositions from those of host mylonites may indicate their generation in different sources of the MSF root and ascending of melts to an upper common level. These kinds of deep-seated sources for basic pseudotachylytes might have links with those for the KSZHT volcanic rocks.

Major Oxides
Volcanic rocks from the KSZHT vary from silica saturated basalts to strongly undersaturated basanites. Basaltic lavas erupted on the Kultuk and Meteo volcanoes. The former occurred directly in the MSF zone and was active both at the beginning and at the end of the KSZHT volcanism (about 18 and 13 Ma), the latter was the farthest one from the MSF to the south and was active only at the beginning of the KSZHT volcanism (18.1-17.6 Ma).
Compared with volcanic rocks (Table 2), basic pseudotachylytes have a lower MgO content. Since abundance of this oxide in a melt is functionally dependent on temperature [78][79][80], the MSF pseudotachylyte melts had obviously lower temperatures than those of the KSZHT. On a MgO-SiO 2 diagram (Figure 9a), data points of pseudotachylyte samples KL-22 and KL-9 with the highest MgO content are plotted close to volcanic rocks with the lowest content of this oxide. Consequently, temperatures of melts quenched into pseudotachylytes KL-22 and KL-9 were comparable with minimal temperatures of erupted basaltic melts. Low-Ti pseudotachylyte KL-22 shows a lower SiO 2 content than moderate-Ti pseudotachylyte KL-9. Other low-and moderate-Ti pseudotachylytes show intersecting trends of increasing SiO 2 and decreasing MgO abundances  Compared with volcanic rocks (Table 2), basic pseudotachylytes have a lower MgO content. Since abundance of this oxide in a melt is functionally dependent on temperature [78][79][80], the MSF pseudotachylyte melts had obviously lower temperatures than those of the KSZHT. On a MgO-SiO2 diagram (Figure 9a), data points of pseudotachylyte samples KL-22 and KL-9 with the highest MgO content are plotted close to volcanic rocks with the lowest content of this oxide. Consequently, temperatures of melts quenched into pseudotachylytes KL-22 and KL-9 were comparable with minimal temperatures of erupted basaltic melts. Low-Ti pseudotachylyte KL-22 shows a lower SiO2 content than moderate-Ti pseudotachylyte KL-9. Other low-and moderate-Ti pseudotachylytes show intersecting trends of increasing SiO2 and decreasing MgO abundances  Compared with the KSZHT volcanic rocks, basic pseudotachylytes show broader SiO 2 and FeO tot variations. The low-and moderate-Ti groups indicate opposite trends of enrichment and depletion in SiO 2 and FeO tot relative to the prevalent FeO tot interval. Moderate-Ti sample KL-9 with elevated MgO content shows increase in FeO tot and decrease in SiO 2 contents relative to the MT18.1 data field. Similar trend was obtained experimentally with increasing pressure for melts from orthopyroxene-enriched lherzolite HK-66 [81]. Data point of low-Ti sample KL-9 with elevated MgO content shifts relative to the MT18.1 data field with a relative decrease in both FeO tot and SiO 2 contents. The KL18 data field falls on this trend. Moderate-Ti pseudotachylytes also show increasing FeO tot content as SiO 2 content increases from sample KL-9. At the lowest MgO content, sample KL-17 shows the FeO tot value in a range of the prevalent compositions.

La/Yb Ratio
In the Tunka Valley, basaltic and basanitic melts brought to the surface polycrystalline garnet-free cognate inclusions and xenoliths extracted from walls of magma conduits. The crystalline nodules from volcanic rocks incorporated olivine, ortho-and clinopyroxene, spinel, plagioclase, amphibole, and mica [57,60]. A garnet-bearing material, not represented by crystalline inclusions and shown only by erupted melts, are referred to as a deeper region [20,57]. Generally, basic pseudotachylytes and volcanic rocks from the Meteo and Kultuk volcanoes could be generated in the deep-seated CMT sources, while volcanic rocks from the Karerny and Shirokiy volcanoes with lower SiO 2 content than in rocks of the Meteo and Kultuk volcanoes could be produced in sources of the deeper lithospheric mantle.
Respectively, the transition from a garnet-bearing material to a garnet-free one can be considered as the crust-mantle boundary. A similar transition was suggested for Southeastern Australia, where the Moho boundary, determined from seismic data at a depth about 55 km, was compared with the same depth, estimated from a spinel-garnet transition in lherzolite xenoliths [82].
Garnet-free and garnet-bearing deep-seated sources of the KSZHT volcanic rocks were calculated through trace-element modeling of equilibrium melting [20]. It was shown that hypersthene-normative basaltic lavas, erupted on the Meteo Volcano 18.1-17.7 Ma, were generated in a garnet-free source with high degrees of melting (F = 0.15-0.23). Nephelinenormative basalts of this volcano, erupted 17.6 Ma, were modeled as derivatives of a garnet-bearing source with moderate degrees of melting (F~0.08). For 18 Ma and 13 Ma basalts from the Kultuk Volcano, garnet-bearing and garnet-free deep-seated sources were defined, respectively Therefore, the beginning of hot transtension was perceived by activities of separate garnet-free and garnet-bearing sources of the Meteo and Kultuk volcanoes, while the end of hot transtension was spotted by activities of a garnet-free one in the Kultuk Volcano.
From results of modeling, melts of garnet-bearing and garnet-free deep-seated sources were discriminated by a (La/Yb) N ratio of 9.5 (normalization to the pyrolite [77]). The values >9.5 and <9.5 were defined as indicators of garnet-bearing (mantle) and garnetfree (crustal) sources, respectively. The (La/Yb) N ratio is used further for a gradation of the KSZHT melts from relatively low degrees of melting in a deeper lithospheric mantle source (F~0.05, (La/Yb) N = 13-18), through elevated degrees of melting in a sub-crustal level (F~0.08, (La/Yb) N = 10-12) to higher degrees of melting at the base of the crust (F = 0.15-0.23, (La/Yb) N = 10-12) (Figure 10a).
Melting of the MSF basic pseudotachylytes also fits into the same gradation (without modeling, (La/Yb) N = <4). Comparison of basic pseudotachylytes from the MSF with the one from the contact between gneiss and metadolerite in the Sarvar-Juniya fault zone [32] indicates their comparable SiO 2 , Al 2 O 3 , MgO, and CaO, but different FeO tot , K 2 O, and Na 2 O contents ( Figure 4). The similarity of the former four components may indicate their distribution that obeys intensive (PT) parameters of tectonic generations of silicate melts, while the discrepancy between the latter three components may indicate their additive distribution, i.e., displaying of extensive (concentration) parameters of tectonic generation of silicate melts. Consequently, the FeO tot , K 2 O, and Na 2 O contents in tectonically generated melts could reflect complementary redistribution of these components in the lithospheric mantle and crust.   In panel a, model curves of equilibrium partial melting and compositions of garnet-bearing Cpx-poor and garnet-free Cpx-rich deep-seated sources were calculated for the KSZHT volcanic rocks by Rasskazov et al. [20] using equations of [83] and mineral-melt distribution coefficients given in [84]. In panel b, prevalent FeOtot interval is shown as in Figure 9b. For normalization of La/Yb and Yb, the pyrolite composition was used after [77].
Melting of the MSF basic pseudotachylytes also fits into the same gradation (without modeling, (La/Yb)N = <4). Comparison of basic pseudotachylytes from the MSF with the one from the contact between gneiss and metadolerite in the Sarvar-Juniya fault zone [32] indicates their comparable SiO2, Al2O3, MgO, and CaO, but different FeOtot, K2O, and Na2O contents (Figure 4). The similarity of the former four components may indicate their distribution that obeys intensive (PT) parameters of tectonic generations of silicate melts, while the discrepancy between the latter three components may indicate their ad-  Figure 9. In the Meteo Volcano, volcanic rocks of the initial eruption phase (MT18.1) belong to a garnet-free source, those of the intermediate phase (MT17.7) to both a garnet-free and a garnet-bearing deep-seated sources, and rocks of the final phase of this volcano (MT17.6) to a garnet-bearing source. In panel a, model curves of equilibrium partial melting and compositions of garnet-bearing Cpx-poor and garnet-free Cpx-rich deep-seated sources were calculated for the KSZHT volcanic rocks by Rasskazov et al. [20] using equations of [83] and mineral-melt distribution coefficients given in [84]. In panel b, prevalent FeO tot interval is shown as in Figure 9b. For normalization of La/Yb and Yb, the pyrolite composition was used after [77].
On a (La/Yb) N -FeO tot diagram (Figure 10b), data fields of eruptive phases MT18.1, MT17.7, KL18, and KL13 of the Meteo and Kultuk volcanoes are displaced relative to the one of the KSZHT 17.6-11.7 Ma age interval with a decrease in a (La/Yb) N ratio and with an increase in a FeO tot content, denoting a composite trend that extends from the KL18 initial lava generation through phases MT18.1 and MT17.7 to the KL13 final lava generation and moderate-Ti basic pseudotachylytes. Consequently, in terms of a FeO tot content, the garnet-free source of the MSF moderate-Ti pseudotachylytes was complementary to the garnet-bearing source that generated lavas of the Kultuk Volcano. Taking into account local occurrence of these complementary deep-seated sources in the MSF, one can assume that a deep-seated lava source of the Kultuk Volcano was depleted in an iron as a result of its removal from the garnet-bearing sub-crustal lithospheric mantle and its transfer to the crustal level. These additional portions were reflected in generation of moderate-Ti pseudotachylytes. Any complementary links between deep-seated sources of low-Ti pseudotachylytes and those of volcanic rocks are doubtful.
The KSZHT lavas erupted on the area between the Kultuk and Meteo volcanoes show a parallel complementary trend of rocks from garnet-bearing and garnet-free deepseated sources, shifted to the right and above the one of the Kultuk and Meteo volcanoes. The mantle (garnet-bearing) and crustal (garnet-free) sources of this trend yield higher FeO tot content than KL13, which corresponds to the prevailing FeO tot level. The KSZHT volcanic rocks with an elevated (La/Yb) N ratio and relatively narrow FeO tot range are also distinguished as low-degrees derivatives of the garnet-bearing mantle source.
On  1 and MT17.7). The Ce/Pb values for ocean basalts and continental crust are shown after [85], the Bulk Silicate Earth value is after [86]. Symbols are as in Figure  9.

Th/Yb and Ta/Yb Ratios
On the Th/Yb versus Ta/Yb diagram, rocks of the continental crust are plotted above the mantle array displayed by OIB and MORB [87]. In a garnet-bearing source, both ratios increase; while in absence of garnet, both ratios decrease. Points of initial lavas from a garnet-free source of the Meteo Volcano (MT18.1) are shifted below the ocean basalt array with a relative increase in a Ta/Yb ratio (Figure 12a). Points of lavas of the next eruption phase from a garnet-free source of the same volcano (MT17.7) are slightly shifted relative the data field of initial lavas toward the lower crust composition. A more ad-  1 and MT17.7). The Ce/Pb values for ocean basalts and continental crust are shown after [85], the Bulk Silicate Earth value is after [86]. Symbols are as in Figure 9.

Th/Yb and Ta/Yb Ratios
On the Th/Yb versus Ta/Yb diagram, rocks of the continental crust are plotted above the mantle array displayed by OIB and MORB [87]. In a garnet-bearing source, both ratios increase; while in absence of garnet, both ratios decrease. Points of initial lavas from a garnet-free source of the Meteo Volcano (MT18.1) are shifted below the ocean basalt array with a relative increase in a Ta/Yb ratio (Figure 12a). Points of lavas of the next eruption phase from a garnet-free source of the same volcano (MT17.7) are slightly shifted relative the data field of initial lavas toward the lower crust composition. A more advanced shift is shown by final basalts from the garnet-free source of the Kultuk Volcano (KL13), while lavas of the initial eruptions of this volcano (KL18) belong to a trend of garnet-bearing deep-seated sources that extends along the OIB and MORB array. Melts from the KSZHT deep-seated sources are complementary enriched in tantalum (with increasing Ta/Yb and decreasing Th/Ta) relative to OIB melts due to melting both garnet-free lower crust and underlying garnet-bearing mantle. Melts of basic pseudotachylytes from the garnet-free crust, on the contrary, are complementary depleted in tantalum (with a relative decrease in Ta/Yb) (Figure 12b). The KSZHT magmas erupted from sources of modified restite material that had lost the basic pseudotachylyte component.  (Figure 12b). The KSZHT magmas erupted from sources of modified restite material that had lost the basic pseudotachylyte component. . On the diagram b, a data field of peridotite nodules from trachybasalts of the Karerny volcano is shown as a possible source of the KSZHT melts after [60]. Additional abbreviations: N MORB and E MORB-normal and enriched basalts of the mid-ocean ridge, respectively [88], LC, MC, and UC-lower, middle, and upper crust, respectively [89]. Symbols are as in Figure 9.
In the MSF, pseudotachylytes of intermediate and silicic compositions are also associated with sources of the collisional crust. On the Th/Yb-Ta/Yb diagram (Figure 12a), points of these pseudotachylytes correspond to the end of a trend exhibited by crustal xenolith. The trend extends from the ocean basalt array to the middle crust composition. A common nature of intermediate and silicic pseudotachylyte compositions is emphasized by similar REE patterns (Figure 7a). At the same time, the middle crust differs from its lower part by oxidation conditions and occurrence of water-bearing mineral phases. This is characteristic of the source for pseudotachylytes of intermediate composition that lost uranium with notably increased Th/U ratio ( Figure 6).

Sr and Nd Isotope Ratios
On a 143 Nd/ 144 Nd-87 Sr/ 86 Sr diagram (Figure 13), a particular role of a garnet-free source for initial basalts from the Meteo Volcano is highlighted. Compared with the entire set of the KSZHT volcanic rocks, lavas of the MT18.1 eruption phase yield a separate group with the lowest 143 Nd/ 144 Nd and highest 87   . On the diagram b, a data field of peridotite nodules from trachybasalts of the Karerny volcano is shown as a possible source of the KSZHT melts after [60]. Additional abbreviations: N MORB and E MORB-normal and enriched basalts of the mid-ocean ridge, respectively [88], LC, MC, and UC-lower, middle, and upper crust, respectively [89]. Symbols are as in Figure 9.
In the MSF, pseudotachylytes of intermediate and silicic compositions are also associated with sources of the collisional crust. On the Th/Yb-Ta/Yb diagram (Figure 12a), points of these pseudotachylytes correspond to the end of a trend exhibited by crustal xenolith. The trend extends from the ocean basalt array to the middle crust composition. A common nature of intermediate and silicic pseudotachylyte compositions is emphasized by similar REE patterns (Figure 7a). At the same time, the middle crust differs from its lower part by oxidation conditions and occurrence of water-bearing mineral phases. This is characteristic of the source for pseudotachylytes of intermediate composition that lost uranium with notably increased Th/U ratio ( Figure 6).

Sr and Nd Isotope Ratios
On a 143 Nd/ 144 Nd-87 Sr/ 86 Sr diagram (Figure 13), a particular role of a garnet-free source for initial basalts from the Meteo Volcano is highlighted. Compared with the entire set of the KSZHT volcanic rocks, lavas of the MT18.1 eruption phase yield a separate group with the lowest 143 Nd/ 144 Nd and highest 87

Pb Isotope, Nb/U, and Th/U Ratios
A diagram of 207 Pb/ 204 Pb plotted versus 206 Pb/ 204 Pb incorporates only data on uranogenic Pb isotopes and yields timing of convective homogenization and modification of a source material. A diagram of 208 Pb/ 204 Pb plotted versus 206 Pb/ 204 Pb shows relationship between thorogenic 208 Pb and uranogenic 206 Pb. The former is a final decay product of the 232 Th series, the latter is a final one of a 238 U series that makes up 99.2743% of a total mass of uranium [90]. Consequently, distribution of points on the 208 Pb/ 204 Pb-206 Pb/ 204 Pb diagram designates variations of a time-integrated 232 Th/ 238 U ratio as a factor of the element concentrations. In ocean basalts, Th and U belong to a group of geochemically similar incompatible elements: Th > U~Nb [88]. Melts derived from mantle and crustal sources reveal markedly different Nb/U ratios: 47 ± 10 in OIB and ~10 in crust-derived volcanic rocks [85,91]. The Th/U value ~4 in mantle rocks increases to 6 in lower crustal ones [89]. This ratio may vary because of different garnet-melt and/or clinopyroxene-melt partitioning of U and Th [92,93]. Anomalously high or low Th/U values can also be related to input or removal of U, migrated under oxidizing conditions due to the formation of water-soluble uranyl UO compounds with hexavalent U.
On diagrams of Pb isotope ratios (Figure 14a,b), the KSZHT volcanic rocks are plotted both as slightly scattered fields and as discrete points. In Figure 14a  6.6. Pb Isotope, Nb/U, and Th/U Ratios A diagram of 207 Pb/ 204 Pb plotted versus 206 Pb/ 204 Pb incorporates only data on uranogenic Pb isotopes and yields timing of convective homogenization and modification of a source material. A diagram of 208 Pb/ 204 Pb plotted versus 206 Pb/ 204 Pb shows relationship between thorogenic 208 Pb and uranogenic 206 Pb. The former is a final decay product of the 232 Th series, the latter is a final one of a 238 U series that makes up 99.2743% of a total mass of uranium [90]. Consequently, distribution of points on the 208 Pb/ 204 Pb-206 Pb/ 204 Pb diagram designates variations of a time-integrated 232 Th/ 238 U ratio as a factor of the element concentrations. In ocean basalts, Th and U belong to a group of geochemically similar incompatible elements: Th > U~Nb [88]. Melts derived from mantle and crustal sources reveal markedly different Nb/U ratios: 47 ± 10 in OIB and~10 in crust-derived volcanic rocks [85,91]. The Th/U value~4 in mantle rocks increases to 6 in lower crustal ones [89]. This ratio may vary because of different garnet-melt and/or clinopyroxene-melt partitioning of U and Th [92,93]. Anomalously high or low Th/U values can also be related to input or removal of U, migrated under oxidizing conditions due to the formation of water-soluble uranyl UO 2+ 2 compounds with hexavalent U. On diagrams of Pb isotope ratios (Figure 14a,b), the KSZHT volcanic rocks are plotted both as slightly scattered fields and as discrete points. In Figure 14a [94]. Points of a lava subgroup from the Kultuk Volcano, designated as KL18 , yield a line with a slope corresponding to an age of about 2.22 Ga. The line relation to the Inner Asia common component assumes the initial source generation at that time. Points, shifted to the right and to the left in the middle and upper parts of the array, indicate its distortion. A lava subgroup KL18 designates a source processing, approximated with a line with a slope corresponding to an age of about 0.9 Ga. Separate data fields of the MT18.1 and MT17.7 eruptive phases assume a slope of points close to the 2.22 Ga array (or steeper than this), indicating coeval or older differentiation of a source material, while a shifted data field of the 17.6 Ma eruptive phase clearly designates a separate source material.   [94]. On diagram b, designated are the boundary of the DUPAL anomaly at Δ8/4Pb = 60 [95] and estimated maximal 232 Th/ 238 U ratio (explanation in the text). Symbols are as in Figure 9.  [94]. On diagram b, designated are the boundary of the DUPAL anomaly at ∆8/4Pb = 60 [95] and estimated maximal 232 Th/ 238 U ratio (explanation in the text). Symbols are as in Figure 9.
On a diagram of thorogenic and uranogenic Pb isotope ratios (Figure 14b), four points of the KL18 subgroup (source age of 2.2 Ga) show well correlated 238 U/ 204 Pb (µ) and 232 Th/ 204 Pb (ω) ratios. The only point of this subgroup, shifted to the left, indicates decreasing µ without changing ω. The KL18 subgroup (source age of 0.9 Ga) shows one point fitted to the KL18 correlation line and three points shifted to the left. On the one hand, increasing µ and ω values result in transition from the DUPAL to none-DUPAL signature, on the other hand, ω values do not vary under decreasing µ. Three phases of the Meteo Volcano show data points generally shifted to the left of the 2.2 Ma KL18 array of consistently varied µ and ω (but one point) and extend along the KL18 array of a source region affected by decreasing µ without changing ω.
On Figure 14b, the most radiogenic leads of the KSZHT volcanic rocks correspond to the Inner Asia common component. From this relationship, Pb isotope data can be used for estimation of a Th/U ratio in a source region. Since the common component represents the uniform deep-mantle material [94], the array of Figure 14b characterizes a differentiation event of 2.22 Ga. A 232 Th has a longer half-life than a 238 U (14.010 × 10 9 years versus 4.468 × 10 9 years) [90]. Hence, a 238 U produces more of 206 Pb than a 232 Th generates of 208 Pb in the same period of time. The Inner Asia common component yields the 208 Pb/ 206 Pb value of 2.10 that can correspond to the initial 232 Th/ 238 U ratio as high as 6.58. Respectively, radioactive decays of 232 Th and 238 U can result in increasing 232 Th/ 238 U ratio from 6.58 up to 20.6. This value is the upper limit of the present-day Th/U ratio.
The CMT deep-seated sources of the KSZHT volcanic rocks show significant deviations of Nb/U and Th/U ratios from the OIB values. On diagrams of Figure 15a,b, rare points are notably shifted to the right. Due to increased partial melting, the (La/Yb) N values in the left clusters of points are slightly decreased. Predominating data points with low Nb/U and Th/U ratios may indicate intensive magma generation processes in U-rich sources that are characteristic of both garnet-bearing and garnet-free layers of the CMT region.
A Nb/U ratio reveals a distinct contrast between melt generations in initial deepseated sources of the Kultuk and Meteo volcanoes; in the KL18 lavas from a garnet-bearing source of the former, the Nb/U ratio decreases (to 25) and in the MT18.1 lavas from the garnet-free source of the latter, on the contrary, increases (to 110). A similar contrast between the initial KL18 and MT18.1 lavas is displayed in the Th and U distributions; in the KL18 lavas from the garnet-bearing source of the Kultuk Volcano, the Th/U ratio decreases (to 1.8), while in the MT18.1 lavas from the garnet-free source of the Meteo Volcano, on the contrary, it increases (to 8). During the initial KL18 phase of the Kultuk Volcano, only the melts from a garnet-bearing source, enriched in uranium, took place. Displayed at the end of the KSZHT activities was a material of a garnet-free source of the Kultuk Volcano that, in terms of Th/U and Nb/U ratios, was partly similar to the one of the MT17.7 phase of the Meteo Volcano.
The KSZHT volcanic rocks from the deeper lithospheric mantle source show mainly Th/U ratio less than 4 with rare samples exceeded this value (Figure 15b), although data points of this group demonstrate almost symmetrical distribution relative to the OIB average Nb/U ratio (~47) (Figure 13). This difference indicates better sensitivity of a Th/U ratio than a Nb/U one to U-redistribution in a source region due to a Th abundance in volcanic rocks 1-2 orders of magnitude lower than a Nb one.
In contrast to the KSZHT volcanic rocks, the MSF basic pseudotachylytes show inconsistent variations of Nb/U and Th/U ratios. The Nb/U one in depleted rocks ((La/Yb) N < 1) approaches the ocean basalt value, decreasing to crustal one in rocks with chondritic slightly enriched REE distributions ((La/Yb) N = 1. 2-4.4). The Th/U ratio in the depleted rocks differs significantly from that of ocean basalts, overlapping the entire Th/U range of basic pseudotachylytes from 2.5 (MORB) to 6.0 (lower crust). Taking into account the better sensitivity of the Th/U ratio to uranium transfer, its low values in basic pseudotachylytes may be explained by local fluid enrichment in uranium of a source, similar to the ratio in the CMT deep-seated sources of volcanic rocks.  Figure 9. Green arrows indicate extraction of U from layers by oxidized fluids that result in decrease of (La/Yb)N due to locally increasing degrees of melting. The upper Th/U limit of 20.6 is shown for a source region from Pb isotope data. (La/Yb)N is calculated using the pyrolite composition after [77].

Long-Term CMT Evolution
On the one hand, volcanic rocks of the initial and final eruption phases in the Meteo and Kultuk volcanoes yield common trends with basic pseudotachylytes (Figures 8-10) that assumes searching for genetic relationships between the rock sources with a perspective of creating a comprehensive model for the development of seismotectonic processes in the crust and sub-crustal mantle of the Slyudyanka paleocollision zone, inherited by the South Baikal Basin and Tunka Valley. On the other hand, clearly expressed is the iron redistribution with its removal from the CMT garnet-bearing source of the KL18 lavas in the Kultuk Volcano to the crustal source of the MSF basic pseudotachylytes. From complimentarily redistributed Fe, U, and alkalis in deep-seated sources of the KSZHT volcanic rocks, the CMT processes are related to episodes from about 2.22 Ga and 0.9 Ga (Figure 14a).
The basement of the southern edge of the Siberian paleocontinent, accessible for observations, has a protolith age of 3.82 Ga estimated from the conjugated Holmes-Houtermans and Wasserburg model for J-type ore leads with μ as high as 20.1 [10]. The obtained Pb age estimate of the protolith is comparable with the Nd model age limit of 3.9 Ga for the Archean metamorphic rocks from the Irkutsk block of the paleocontinent [96].  Figure 9. Green arrows indicate extraction of U from layers by oxidized fluids that result in decrease of (La/Yb) N due to locally increasing degrees of melting. The upper Th/U limit of 20.6 is shown for a source region from Pb isotope data. (La/Yb) N is calculated using the pyrolite composition after [77].

Long-Term CMT Evolution
On the one hand, volcanic rocks of the initial and final eruption phases in the Meteo and Kultuk volcanoes yield common trends with basic pseudotachylytes (Figures 8-10) that assumes searching for genetic relationships between the rock sources with a perspective of creating a comprehensive model for the development of seismotectonic processes in the crust and sub-crustal mantle of the Slyudyanka paleocollision zone, inherited by the South Baikal Basin and Tunka Valley. On the other hand, clearly expressed is the iron redistribution with its removal from the CMT garnet-bearing source of the KL18 lavas in the Kultuk Volcano to the crustal source of the MSF basic pseudotachylytes. From complimentarily redistributed Fe, U, and alkalis in deep-seated sources of the KSZHT volcanic rocks, the CMT processes are related to episodes from about 2.22 Ga and 0.9 Ga (Figure 14a).
The basement of the southern edge of the Siberian paleocontinent, accessible for observations, has a protolith age of 3.82 Ga estimated from the conjugated Holmes-Houtermans and Wasserburg model for J-type ore leads with µ as high as 20.1 [10]. The obtained Pb age estimate of the protolith is comparable with the Nd model age limit of 3.9 Ga for the Archean metamorphic rocks from the Irkutsk block of the paleocontinent [96].
The Baikal granulite-gneiss terrane of the Irkutsk block was affected by collisional metamorphic events, dated by the U-Pb zircon method from about 2.8 Ga, 2.65-2.56 Ga, and 1.866-1.853 Ga. The Archean metamorphic episodes were designated in rocks of granulite facies, and the Early Proterozoic one in those of high-temperature amphibolite and partially granulite facies [97]. In addition, metamorphic events were recorded using the Rb-Sr isotope system. In the Onot greenstone belt, migmatites and granites of the basement are constrained between 2.64 and 2.24 Ga [98]. This interval also is shown by an age of 2.45 ± 0.38 Ma obtained for phlogopitization of the Krutaya Guba komatiite-like bodies [99]. These events were concurrent with the initial CMT complementary Fe-Ualkaline processes related to the Inner Asia common component.
The model calculations show that ore leads were extracted from crystalline rocks in time intervals from 1.8-1.5 Ga, 1.2-0.9 Ga, 0.6-0.5 Ga, and about 0.25 Ga [10]. The first episode of the Pb extraction in ore deposits began after the Siberian paleocontinent assembly, accompanied by A-type granite magmatism of the Primorsky complex from about 1.86 Ga [100][101][102][103]. The end of the second episode coincided with the CMT complementary Fe-U-alkaline processes. Two other episodes are also connected with tectonic and magmatic events spotted in the accessible crust.
The remarkable one is displayed along the southern edge of the Siberian paleocontinent in the Late Precambrian alkaline-ultrabasic magmatism with carbonatites [104,105]. Syn-collisional granulite metamorphism of the Slyudyanka Group is constrained by U-Pb zircon ages of 488.5 ± 0.6 Ma and 488.0 ± 0.5 Ma obtained for hypersthene-bearing biotite and two-pyroxene trondhjemites. Post-collisional gabbro-syenite magmatism is dated at 471 ± 2 Ma [106]. The latter age is consistent with the Rb-Sr one of 470 ± 25 Ma, obtained for syenites and related rocks of the Bystraya massif with an initial 87 Sr/ 86 Sr ratio of 0.7047 ± 0.0007 [107]. Numerous small bodies and relatively large gabbro-syenite massifs (Snezhnaya, Bystraya, and Bezymyany), cut through metamorphic rocks of the Slyudyanka paleocollision zone, are compositionally similar to basic pseudotachylytes from the MSF. A TiO 2 content of these rocks varies from 0.5 to 3.0 wt% [108].
In the Mesozoic, sufficient iron redistribution provided formation of phoscorite ore deposits in the Angara Province associated with alkaline carbonatite magmatism in the southern edge of the Siberian paleocontinent [109] that affected also accreted terranes [110,111].

U in CMT Processes
In terms of Pb isotope ratios, the KSZHT deep-seated magma sources show high Th/U ratio of the Inner Asia common material raised from the deep mantle in the Paleoproterozoic (Section 6.5). This material yielded the time-integrated DUPAL anomaly. Some KSZHT volcanic rocks show the Th/U ratio as high as 10, but basically, it ranges from 2 to 4. From the discrepancy between µ and ω signatures on the Pb isotope diagrams and the Th/U ratio of the KSZHT volcanic rocks, one can assume recent fluid redistribution of U relative to Th with a preferential concentration of U in melts of the CMT deep-seated sources.
Similar to the KSZHT volcanic rocks, strongly depleted and enriched by uranium with variable Th/U and Nb/U ratios with respect to the OIB values are Miocene lavas from the Primorye and Lesser Khingan and also Cretaceous-Paleogene lavas from the Tien Shan ( Figure 16). The Th/U ratio as high as 20 is characteristic of basalts and trachybasalts derived from the CMT garnet-free source in the Nakhodka volcanic field of the Primorye, Far East of the Russia. Associated with these rocks basaltic andesites, generated in a crustal source, show also elevated Th/U ratio (up to 10). The lower limit of the Th/U ratio in volcanic rocks from this area is 4 [9].
Uranium depletion with a Th/U ratio as high as 15.5 is characteristic of basanites and associated rocks from a garnet-free CMT source of the South Tien Shan (Tuyun Basin), in contrast to basalts and trachybasalts from a shallow (crustal) source of the Middle-North Tien Shan, which are anomalously enriched by U with a relative decrease in Th/U to 0.17. The deeper (garnet-bearing) mantle sources in the latter area do not show any signs of uranium redistribution, however. The crustal source could be enriched in uranium with its complementary depletion of the shallow garnet-free peridotite source due to the rise of oxidized fluids through the crust-mantle boundary during the Late Paleozoic closure of the Turkestan paleoocean. The Cretaceous-Paleogene disruption of the crust-mantle boundary under the South Tien Shan prevented an ascent of deeper mantle magmas and provided a long-term (122-46 Ma) generation of magmas in a shallow garnet-free peridotite source. A shielding effect of this rupture did not prevent, however, a short-term (61-53 Ma) upraise of magmas from the deeper mantle source accompanied by crustal melting in the Middle-North Tien Shan [112]. Uranium depletion with a Th/U ratio as high as 15.5 is characteristic of basanites and associated rocks from a garnet-free CMT source of the South Tien Shan (Tuyun Basin), in contrast to basalts and trachybasalts from a shallow (crustal) source of the Middle-North Tien Shan, which are anomalously enriched by U with a relative decrease in Th/U to 0.17. The deeper (garnet-bearing) mantle sources in the latter area do not show any signs of uranium redistribution, however. The crustal source could be enriched in uranium with its complementary depletion of the shallow garnet-free peridotite source due to the rise of oxidized fluids through the crust-mantle boundary during the Late Paleozoic closure of the Turkestan paleoocean. The Cretaceous-Paleogene disruption of the crust-mantle boundary under the South Tien Shan prevented an ascent of deeper mantle magmas and provided a long-term (122-46 Ma) generation of magmas in a shallow garnet-free peridotite source. A shielding effect of this rupture did not prevent, however, a short-term (61-53 Ma) upraise of magmas from the deeper mantle source accompanied by crustal melting in the Middle-North Tien Shan [112].
In the basalt-andesite assemblage from the Lesser Khingan, three groups of rocks are identified: (1) Moderate-K from a garnet-free source ((La/Yb)N = 5.3-9.8, Th/U = 3.7-15.9, Nb/U = 41-176); (2) moderate-K from a source of garnet-spinel transition ((La/Yb)N = 9.7-13.4, Th/U = 7.9-13.2, Nb/U = 84-179); and (3) high-K from a garnet-bearing source ((La/Yb)N = 15.7-33.5, Th/U = 4.6-5.9, Nb/U = 22-56). Anomalous distribution of U, relative to Nb and Th, in the two former rock groups is associated with processes of uranium redistribution by oxidized fluids in the CMT, in contrast to the OIB-like relationship between these elements in the third group. This is explained by the absence of this effect in reduced conditions of a deeper mantle source. Rocks of the two former groups are found in the vast area of the Lesser Khingan, whereas those of the third one are limited only in the central part of the Udurchukan volcanic field.
From data comparisons of the selected regions (Figure 16), one can see peculiarities of the KSZHT deep-seated magma sources that show a wide range of Th/U ratios covered the entire depth interval from the crust to the deeper lithospheric mantle. In other Asian regions, wide ranges of Th/U ratio are recorded in volcanic rocks from sources of a single level (South and Middle-North Tien Shan) or two levels (Nakhodka and Udurchukan volcanic fields). A deeper lithospheric mantle material of the KSZHT volcanic rocks differs from OIB. In those from other areas, it is either absent (Nakhodka volcanic field, South Tien Shan) or indicates OIB-like signatures (Middle-North Tien Shan, Udurchukan). . Anomalous distribution of U, relative to Nb and Th, in the two former rock groups is associated with processes of uranium redistribution by oxidized fluids in the CMT, in contrast to the OIB-like relationship between these elements in the third group. This is explained by the absence of this effect in reduced conditions of a deeper mantle source. Rocks of the two former groups are found in the vast area of the Lesser Khingan, whereas those of the third one are limited only in the central part of the Udurchukan volcanic field.
From data comparisons of the selected regions ( Figure 16), one can see peculiarities of the KSZHT deep-seated magma sources that show a wide range of Th/U ratios covered the entire depth interval from the crust to the deeper lithospheric mantle. In other Asian regions, wide ranges of Th/U ratio are recorded in volcanic rocks from sources of a single level (South and Middle-North Tien Shan) or two levels (Nakhodka and Udurchukan volcanic fields). A deeper lithospheric mantle material of the KSZHT volcanic rocks differs from OIB. In those from other areas, it is either absent (Nakhodka volcanic field, South Tien Shan) or indicates OIB-like signatures (Middle-North Tien Shan, Udurchukan).

CMT between South Baikal Basin and Tunka Valley
In the southern edge of the Siberian paleocontinent and adjacent Transbaikal areas, northwesterly oriented orogenic structures change laterally to the South Baikal Rift (Figure 17a). From deep seismic sounding, the crust of the Angara-Lena tectonic step as thick as 40 km decreases to 36 km beneath the Selenga saddle that corresponds to an antecedent breakthrough of the Selenga River between the Khamardaban and Ulan-Burgasy ranges of the Sayan-Baikal Upland. Along the southwestern edge of the Siberian paleocontinent, the crust thickness increases to 54 km and reaches 60 km under the Hangay [113]. tecedent breakthrough of the Selenga River between the Khamardaban and Ulan-Burgasy ranges of the Sayan-Baikal Upland. Along the southwestern edge of the Siberian paleocontinent, the crust thickness increases to 54 km and reaches 60 km under the Hangay [113].  [114] and Seminsky et al. [56]. The map on panel a is modified after Mats et al. [115]. The section on panel b is adopted from Mordvinova et al. [116].
Deep seismic sounding shows decrease of P-wave velocities in the upper (sub-crustal) part of the mantle under large basins of the BRS without any decrease under inter-basin uplifts [117]. Under the South Baikal Basin, a lens of low P-wave velocities  [114] and Seminsky et al. [56]. The map on panel a is modified after Mats et al. [115]. The section on panel b is adopted from Mordvinova et al. [116].
Deep seismic sounding shows decrease of P-wave velocities in the upper (sub-crustal) part of the mantle under large basins of the BRS without any decrease under inter-basin uplifts [117]. Under the South Baikal Basin, a lens of low P-wave velocities (7.7-7.8 km/s) is constrained at the mantle depths of 36-93 km. Under the central part of the Tunka Valley, a similar low-velocity lens is traced, unlike its eastern part, where a normal velocity of 8.1 km/s is obtained, comparable to that under inter-basin uplifts. A similar relative decrease of velocities is shown in S-wave seismic tomography models [118,119].
A structure of the crust with a change in its thickness from 54 to 36 km is deciphered in the west-east V S -section compiled along the Tunka Valley and South Baikal Basin with continuation to the southern coast of the lake (Figure 17b) Between stations TORI and TAL, the structure of the middle and upper parts of the crust is generally consistent with deformations displayed on the earth's surface. Under TORI station, alternating layers with lower and higher S-wave velocities occur approximately 5 km higher than under TAL one. A vertical displacement of layers below these stations corresponds to a subsidence of the South Baikal Basin bottom relative to the uplifted end of the Tunka Valley.
From sediment distributions in the southern coast of Lake Baikal, initial subsidence of the Central Baikal area is dated back to the Eocene [120,121] While assuming the main role of contrast vertical tectonic motions at the junction between the South Baikal Basin and Tunka Valley in reactivation and termination of crustmantle magmatism in the KSZHT, it is noteworthy that the former originated due to crustal deformations in the Primorye suture. In addition, a fragment of the paleocontinent basement was split off along the Obruchev fault and subsided beneath the Baikal. The northeastern ruptures were stretched along the Main Sayan suture of the Siberian paleocontinent, where the KSZHT volcanism displayed during the paleocontinental block isolation along the Obruchev fault, but became extinct after a structural disjunction between the South Baikal Basin and Tunka Valley with the subsidence of the former and uplift of the latter (Figure 18). initiated simultaneously with magma generation in the KSZHT crust-mantle sources since ca. 18 Ma.
While assuming the main role of contrast vertical tectonic motions at the junction between the South Baikal Basin and Tunka Valley in reactivation and termination of crust-mantle magmatism in the KSZHT, it is noteworthy that the former originated due to crustal deformations in the Primorye suture. In addition, a fragment of the paleocontinent basement was split off along the Obruchev fault and subsided beneath the Baikal. The northeastern ruptures were stretched along the Main Sayan suture of the Siberian paleocontinent, where the KSZHT volcanism displayed during the paleocontinental block isolation along the Obruchev fault, but became extinct after a structural disjunction between the South Baikal Basin and Tunka Valley with the subsidence of the former and uplift of the latter (Figure 18). Numerical simulations [122] indicate a dependence of magma intrusion on tectonic forces applied to the lithosphere and/or heat flow recorded on the earth's surface. Extensional forces are usually concentrated along a rheological boundary, so a lateral propagation of a fracture leads to intrusion of magmatic melts along it. Such control takes place in case of a significant application of tectonic stress to the lithosphere (>10 11 N/m). At a relatively low heat flux (<80 mW/m 2 ), the Moho boundary acts as a trap for rising mantle melts, while in case of a high heat flux (>80 mW/m 2 ), a stress in the mantle weakens, so the melts rise unhindered. The reported in our study control of U enrichment by oxidized fluids with progressed magma generation in the CMT layered structure suggests a propagation of low-viscosity layer at the depth of 37-38 km from the South Baikal Basin to the adjacent Tunka Valley that caused the cessation of ascends and eruptions of the KSZHT magmatic melts about 12 Ma. The volcanism extinction signified a temporal Numerical simulations [122] indicate a dependence of magma intrusion on tectonic forces applied to the lithosphere and/or heat flow recorded on the earth's surface. Extensional forces are usually concentrated along a rheological boundary, so a lateral propagation of a fracture leads to intrusion of magmatic melts along it. Such control takes place in case of a significant application of tectonic stress to the lithosphere (>10 11 N/m). At a relatively low heat flux (<80 mW/m 2 ), the Moho boundary acts as a trap for rising mantle melts, while in case of a high heat flux (>80 mW/m 2 ), a stress in the mantle weakens, so the melts rise unhindered. The reported in our study control of U enrichment by oxidized fluids with progressed magma generation in the CMT layered structure suggests a propagation of low-viscosity layer at the depth of 37-38 km from the South Baikal Basin to the adjacent Tunka Valley that caused the cessation of ascends and eruptions of the KSZHT magmatic melts about 12 Ma. The volcanism extinction signified a temporal transition from effective hot transtension of the crust and underlying mantle to strict control of magmas by the CMT layered structure.

Structural Control of Strong Earthquakes
The South Baikal Basin is infilled with sedimentary layers as thick as 4 km, lying on a seismically isotropic basement [70]. At depths of 6-14 km, the velocity section shows, however, a layer-like structure that was presumably interpreted by Suvorov and Mishen'kina [123] as Mesozoic and Paleozoic sedimentary units. Such interpretation contradicts a composition of the South Baikal oil, derived from a source with land angiosperm plants that cannot be older than Cretaceous [124]. In addition, the granulite metamorphic zone, recorded along the southern coast of Lake Baikal [73], suggests the MSF extension under the South Baikal from the Kultuk village to the Tankhoi tectonic step. Therefore, the layer-like structure of the middle and upper crust could result from seismogenic sub-horizontal tectonic move apart displacements at the suture zone of the Siberian paleocontinent.
In an area between Lake Baikal and Oka River (Eastern Sayans), the MSF plane dips to the southwest (angle 60-70 • ) with sub-vertical fragments [53,125,126]. Similar inclination of the MSF plane is reconstructed in the eastern part of the Tunka Valley from fractures in mylonites ( Figure 3) and is displayed in a nodal plane of the 2020 Bystraya earthquake mechanism solution. The latter coincides with the dip of a nodal plane obtained for the 2008 Kultuk earthquake, the epicenter of which was in the Baikal area (i.e., in the east-southeastern continuation of the MSF, about 40 km from the Kultuk village) ( Figure 19). Consequently, the Main Sayan suture zone played a major role in generating pseudotachylytes, the KSZHT volcanic rocks, and modern seismogenic deformations both in the South Baikal basin and in the adjacent Tunka Valley. transition from effective hot transtension of the crust and underlying mantle to strict control of magmas by the CMT layered structure.

Structural Control of Strong Earthquakes
The South Baikal Basin is infilled with sedimentary layers as thick as 4 km, lying on a seismically isotropic basement [70]. At depths of 6-14 km, the velocity section shows, however, a layer-like structure that was presumably interpreted by Suvorov and Mishen'kina [123] as Mesozoic and Paleozoic sedimentary units. Such interpretation contradicts a composition of the South Baikal oil, derived from a source with land angiosperm plants that cannot be older than Cretaceous [124]. In addition, the granulite metamorphic zone, recorded along the southern coast of Lake Baikal [73], suggests the MSF extension under the South Baikal from the Kultuk village to the Tankhoi tectonic step. Therefore, the layer-like structure of the middle and upper crust could result from seismogenic sub-horizontal tectonic move apart displacements at the suture zone of the Siberian paleocontinent.
In an area between Lake Baikal and Oka River (Eastern Sayans), the MSF plane dips to the southwest (angle 60-70°) with sub-vertical fragments [53,125,126]. Similar inclination of the MSF plane is reconstructed in the eastern part of the Tunka Valley from fractures in mylonites ( Figure 3) and is displayed in a nodal plane of the 2020 Bystraya earthquake mechanism solution. The latter coincides with the dip of a nodal plane obtained for the 2008 Kultuk earthquake, the epicenter of which was in the Baikal area (i.e., in the east-southeastern continuation of the MSF, about 40 km from the Kultuk village) ( Figure 19). Consequently, the Main Sayan suture zone played a major role in generating pseudotachylytes, the KSZHT volcanic rocks, and modern seismogenic deformations both in the South Baikal basin and in the adjacent Tunka Valley.  In the present-day structural development of the central BRS, elastic stresses accumulate with strong seismic shocks along the Primorye and Main Sayan suture boundaries of the Siberian paleocontinent. Along strike changes are displayed in both directions of the earth's surface motions, recorded by GPS geodesy, and earthquake mechanism solutions [56,130]. Deformations of the Primorye suture were implemented in the South Baikal earthquake on February 25, 1999 (Mw = 6.0). In a focal zone, a transverse rupture was reactivated, parallel to the Obruchev fault, with sub-vertical and sub-horizontal positions of the nodal planes. This indicates a control of extension by a sub-horizontal low-viscosity layer. The weaker aftershock on May 31, 2000 (Mw = 5.3) had a normal fault mechanism of slightly tilted nodal planes. In a seismic shock of the Kultuk earthquake on 27 August 2008 (Mw = 6.3) of the subsided (Baikal) part of the Main Sayan suture, a strike-slip mechanism of displacement of the transverse to it (northeastern) rupture, parallel to the Primorye suture, was implemented. In the 2020 Bystraya earthquake, lateral displacements The 2008 Kultuk earthquake focus as deep as 16 km corresponded to a low-velocity layer (Figure 17c). The seismic activity reflected its low viscosity. The deeper crustal leveling layer of 37-38 km is aseismic, but the one of 25-27 km with reduced velocities corresponded to the 2020 Bystraya earthquake. This low-viscosity seismically active layer is located under the Tunka Valley deeper than the one of the South Baikal Basin. Seismogenic deformations of this earthquake could be transferred to the underlying leveling layer of 37-38 km and trigger motions along it in the entire central part of the BRS. This sequence explains why the main earthquakes of seismic reactivations in the South Baikal water area are preceded by seismic events of the Kultuk area.

Geodynamic Control of CMT Magmatism
On a Th/Yb versus Ta/Yb plot, points of andesites and trachyandesites from the East Hangay Orogen correspond to a lower crustal source above the OIB and MORB array and basalts show a trend extended bellow this array [19]. A trend of volcanic rocks from the Orkhon-Selenga saddle is shifted below the OIB and MORB array, similar to the one of the KSZHT volcanic rocks shown in Figure 12. Both in the Tunka Valley and in Central Mongolia, the CMT signatures of volcanic rocks change to the OIB-type ones westward (Figure 20a).  [130,131]. The pull-to-axis force of the geodynamic corridor is weakly displayed along the South Baikal Basin in magmatic activities of crust-mantle sources in the KSZHT and is more distinct in magmatic activities of crust-mantle sources of the vast area in the Selenga River basin and East Hangay Orogen in the CMZHT.  [130,131]. The pull-to-axis force of the geodynamic corridor is weakly displayed along the South Baikal Basin in magmatic activities of crust-mantle sources in the KSZHT and is more distinct in magmatic activities of crust-mantle sources of the vast area in the Selenga River basin and East Hangay Orogen in the CMZHT.
From spatial-temporal distribution of volcanism and velocity structure of the mantle, it was inferred that the lithosphere of the Baikal and Hangay-Belaya mobile zones was affected by primary melting anomalies of the transition layer (West Transbaikal and Gobi, respectively) and by secondary melting anomalies of the upper mantle (Hangay, Vitim, Udokan, and others) [18,21,132]. The Vitim and Udokan melting anomalies and those of the Sea of Japan mobile system designated quasi-periodical responses of the Japan-Baikal geodynamic corridor to 2.5 Ma great eccentricity cycles of the Earth's rotation. Similar responses recorded melting anomalies of the vast area in Eastern Sayans and Central Mongolia, synchronized with those of the Tibetan-Himalayan Orogen, as constituents of the Indo-Asian convergent region. The Tunka Valley belongs to the western "hot" (Hangay-Belaya) part of the BRS, affected by Late Cenozoic volcanism (Figure 19a), whereas the South Baikal Basin belongs to its central-northeastern "cold" (Baikal) part, which reveals melting anomalies only at the ends of the Vitim-Udokan transtension zone (Figure 20b).
From various earthquake mechanism solutions and a crustal velocity structure, we propose that the crust and sub-crustal mantle of the South Baikal Basin was attenuated due to divergent forces of the Japan-Baikal geodynamic corridor, while the crust and sub-crustal mantle of the adjacent eastern part of the Tunka Valley, on the contrary, was flattened against the rigid edge of the Siberian paleocontinent basement in connection with the development of convergent processes in the Central Asian Orogenic System. From GPS data, it was inferred that blocks of the Siberian platform and Transbaikal diverge in the South Baikal basin at the NW-SE direction (130 • ) at a rate of 3.4 ± 0.7 mm/year [133]. This direction of motion corresponds to the axis of the Japan-Baikal geodynamic corridor. A maximal extension of the crust results in its thinning under the Middle Baikal up to 36 km (Figure 17a). Therefore, the crust of this part of the basin is about 2 km thinner than the leveling low-crustal layer. Respectively, the low-viscosity leveling layer likely provides a dynamic link between processes of the most extended and thinned crust of the Middle Baikal and the compressed crust of the eastern end of the Tunka Valley.
Lithospheric and sub-lithospheric materials moved along the axial part of the Japan-Baikal geodynamic corridor with a maximum rate, while the rate decreased in its NNE and SSW flanks. This change created pull-to-axis forces, caused alternating volcanic impulses in the Vitim and Udokan volcanic fields of the Vitim-Udokan transtension zone [132]. Pull-to-axis forces resulted also in the almost symmetrical development of the Vitim and Selenga basins. In addition, the crust of the latter basin was affected by the Indo-Asian convergence with suppressed pull-to-axis effect of the Japan-Baikal geodynamic corridor in its upper part, but with magma generation in the deeper crust and underlying mantle.
Hot processes in the eastern end of the Tunka Valley were contemporaneous with those in the Sea of Japan mobile system. The 22-18 Ma volcanic rocks from Northeast Japan showed no systematic variations in a 87 Sr/ 86 Sr at the ocean-continent direction. Along the continental margin (from the north to the south), this isotopic ratio of volcanic rocks ranged from 0.703890-0.704195 in the Okushiri Island to 0.704874-0.705165 in the Honyo area, indicating varying proportions of asthenospheric and lithospheric components [134]. In that time, the Sea of Japan was not opened yet. Since about 18 Ma, deep-seated sources of erupted lavas indicated zoning of a dynamically linked back and frontal sides of the Northeast Japan volcanic arc. The KSZHT volcanic interval corresponded to the one of the initial subduction of the Pacific plate beneath Northeast Japan, accompanied by back-arc spreading of the crust with the Sea of Japan opening in the time interval of 18-13 Ma [135,136]. The volcanic extinction in the KSZHT coincided with the structural reorganization in the Sea of Japan mobile system caused by the collision of the Izu-Bonin and Kuril island arcs with the Northeast Japan one in the time interval of 14-12 Ma [137,138].
We infer that both in the central and in the southwestern parts of the BRS, the CMT magmatism was controlled by pull-to-axis effects in the Japan-Baikal geodynamic corridor enhanced by forces of the Indo-Asian convergence. In addition to the CMT geochemical signatures for deep-seated sources of volcanic rocks in Central Mongolia, it is noteworthy that the seismotomographic profile MOBAL-2003 [118] clearly denotes a common leveling layer of the crust foot at a depth of about 40 km that extends from the Tunka Valley through the Orkhon-Selenga saddle to the East Hangay.

Conclusions
To recognize genetic links between pseudotachylytes and volcanic rocks from the central BRS, we present data on major oxides and trace elements of pseudotachylytes and host mylonites from the MSF and emphasize a magmatic origin of the former. We also characterize compositions of the MSF basic pseudotachylytes and the KSZHT volcanic rocks to show their geochemical differences from OIB that assume tectonically-derived melt generation in deep-seated sources of the Slyudyanka paleocollision zone.
Pseudotachylyte melts of basic, intermediate, and silicic compositions, injected into the Main Sayan Fault, were controlled by its junction with the Kamar-Stanovoy Zone of Hot Transtension that was active in the time interval of 18.1-11.7 Ma. From mylonite fracturing, we determine dips of these conjugated magma-controlling fault zones. We infer that the KSZHT volcanism accompanied a rift-related split off a rigid block of the Siberian paleocontinent basement along the Obruchev fault, but it became extinct when the South Baikal Basin and Tunka Valley turned out to be structurally separated from each other due to subsidence of the former and uplift of the latter.
The KSZHT volcanic rocks and the MSF basic pseudotachylytes show complementary variations of Fe, U, and alkalis in their sources occurred in the CMT and crust. From trace-element abundances of the KSZHT volcanic rocks, we distinguish garnet-bearing and garnet-free source materials and link the latter with garnet-free crystalline inclusions in lavas from the Tunka Valley. Meanwhile, lavas of the Kultuk and Meteo volcanoes erupted from both garnet-free and garnet-bearing deep-seated sources; the latter were not represented by a crystalline material. Respectively, we relate these sources to the sub-crustal garnet-bearing mantle of the CMT region. In addition, we show that lavas of the Karerny and Shirokiy volcanoes could generate in the deeper lithospheric mantle.
From uranogenic Pb isotope ratios of volcanic rocks, we define an initial generation of the CMT deep-seated sources of about 2.22 Ga from the Inner Asia common (deep mantle) material and its subsequent long-term evolution with distinct processing about 0.9 Ga. Comparisons between data points, distributed on the 208 Pb/ 204 Pb-206 Pb/ 204 Pb diagram, and those, showing variations of a Th/U ratio in the KSZHT volcanic rocks, indicate a high 232 Th/ 238 U ratio in sources and their recent uranium enrichment, associated with increasing degrees of melting. No mixing of components from garnet-free and garnetbearing sources emphasizes separate melt generations in different CMT levels during volcanic activities. We speculate that the progressively laminated CMT region could cause stricter control of melting by layers and could prevent the rising of melts, resulting in the extinction of volcanism.
To decipher a character of tectonics, responsible for the KSZHT magmatism, we use a high-resolution S-wave seismic tomography profile and interpret the CMT laminated structure of the central BRS as formed in the Neogene and Quaternary, when rift basins subsided after the Late Cretaceous and Paleogene development of the South Baikal Orogen. We suggest that vertical displacements both in the overlying rifted crust and in the underlying delaminated orogen root were compensated by lateral motions in the low-viscosity layer at the depths of 37-38 km. We explain the generation of the CMT magmatism both in the central and in the southwestern BRS as a result of cooperative pull-to-axis and convergent effects created in the Japan-Baikal Geodynamic Corridor and in the Indo-Asian interactional region, respectively. Present-day crustal deformations result in strong earthquakes distributed along the Main Sayan and Primorye sutures of the Siberian paleocontinent. Earthquake sources correspond to crustal low-velocity layers, located under the South Baikal Basin at shallower depths than under the adjacent Tunka Valley.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/min11050487/s1, Table S1. Analytical results on major oxides and trace elements of the standard reference materials obtained within the time interval of analyzing pseudotachylytes from the MSF and volcanic rocks from the KSZHT, Table S2. Analytical results on major oxides and trace elements of pseudotachylytes and mylonites from the Main Sayan fault, Table S3. Analytical results on major oxides and trace elements of volcanic rocks from the KSZHT, Table S4. Analytical results on Sr, Nd, and Pb isotope ratios of volcanic rocks from the KSZHT.