A model of tectonothermal evolution of the Zagan metamorphic core complex (MCC) based on the new data from 40Ar/39Ar dating of amphibole, mica, and apatite fission-track dating is discussed. A relationship with the long-range impact of processes from the collision of the North China (Amurian–North China) block with the Siberian continent in the Mesozoic era is proposed. The Zagan MСС was formed in the Cretaceous period on the southern flank of a high mountain uplift of Western Transbaikalia, composed of late Paleozoic granitoids of the Angara–Vitim batholith. According to 40Ar/39Ar dating of amphiboles and micas from the mylonite zone, the active development time of the Zagan MCC corresponds to the early Cretaceous epoch (131, 114 Ma). The tectonic exposure of the core from about 15 km to the depths of about 10 km occurred at a rate of tectonic erosion of 0.4–0.3 mm/year as a result of post-collisional extension of the Mongol–Okhotsk orogen. Apatite fission-track dating shows that further exhumation and cooling of the rocks to about 3 km occurred in the lower-upper Cretaceous epoch (112, 87 Ma). The erosional denudation rate was about 0.3 mm/year.MCC- metamorphic core complexes, AFT- apatite fission-track

Mesozoic metamorphic core complexes (MCCs) [1-3] are common in East Asia. They mark global intracontinental extensions along the folded borders of the Siberian craton in Western Transbaikalia and the North China craton [4-13]. The Zagan MCC is one of the more than ten identified ones on the southern border of the Siberian Craton [6-13], where Paleozoic magmatic complexes of the world’s largest Baikal–Vitim and Khentei batholithes and well-known Cenozoic Baikal rift zone occur. Currently, the tectonothermal history of the rocks of the region using apatite fission track dating has been published in a small number of papers [14, 15], partly in [16-19]. In the papers [14, 15], the analysis of geological and geophysical data and the results of track dating revealed the evolution of the relief and tectonic stages of the region formation along the NE-SW profiles from the Baikal-Patom Upland to the Barguzin Ridge, located, respectively, in the northwest and northeast of Lake Baikal. It was assumed, that the Baikal-Patom Upland was reactivated in the middle Jurassic–early Cretaceous epoch after the Mongol-Okhotsk orogeny, occurred in the vast convergence zone of the North Chinese (Amurian–North China block) and Siberian cratons. Apatite fission track dating of the Barguzin Ridge (block) indicates [15] that it intensively rose (rapid cooling phase) in the period of 65–50 Ma (Pliocene-early Eocene epoch) and in the last five Ma (Pliocene-Quaternary period).

Tectonothermal evolution of the late Paleozoic granitoids of the Angara–Vitim batholith has been reconstructed using complex thermochronology, including U/Pb, 40Ar/39Ar, and partly fission track dating methods [16-18]. Closure temperatures of the isotope systems of zircon and amphibole show that the rapid cooling of the Angara–Vitim batholith rocks occurred immediately after crystallization indicating an epoch of intense denudation of rocks up to 4–7 km thick, associated with extensive late Paleozoic orogeny of the territory of southern Siberia. After the tectonic stabilization epoch, characterized by the gradual closure of the biotite isotopic system in the early Permian–Jurassic period (295, 170 Ma), the feldspar isotopic system was closed in the middle Jurassic–early Cretaceous epoch (170, 140 Ma). This interval coincides with the beginning of the formation of the Mongolia-Okhotsk orogen and is characterized by a denudation of about 3 km in thickness. In the Paleogene-Miocene epoch (60, 5 Ma), a slow denudation took place on the territory of Western Transbaikalia. Then followed a period of relatively fast cooling of rocks for over 5 Ma and denudation of about 2–3 km of thickness associated with the reactivation of Western Transbaikalia territory as a result of the distant tectonic impact of the Indo-Eurasian collision.

Thus, the available geological and geochronological data allow us to assume that on the territory of Western Transbaikalia, in the area of granitoid occurrence of the Angara-Vitim batholith, there was an uplift up to 6–8 km high. There are metamorphic cores in the southern marginal part of the mountain uplift.

In the paper, a model of tectonic-thermal evolution of the Zagan MCC is discussed, based on the new data from 40Ar/39Ar dating of amphibole, mica, and apatite fission-track dating as a structure formed in the marginal part of a high mountain uplift.

2.1. Distribution of Mesozoic MCC in NE Asia

Mesozoic MCCs are common along the southern folded border of the Siberian craton in Western Transbaikalia near the Mongol-Okhotsk suture zone (Figure 1) and in folded belts along the borders of the North China craton [4-13] (Figure 2). The closure of the Mongol–Okhotsk Ocean and the collision complex between the North China continent (Amurian–North China block) and northern Eurasia [11, 20-30] gave rise to Mongol-Okhotsk orogeny in the early Mesozoic era and to the formation of the Mongol-Okhotsk suture zone, extending for more than 2000 km (Figure 3). In the foreland of the Mongol-Okhotsk orogeny (Figure 1), a sub-montane trough was formed, which, due to the progressive penetration of the deformation in the direction of the Siberian continent, was partially involved in the process of high mountain uplift formation. The relics of the middle Jurassic–early Cretaceous sub-montane trough of the Mongol-Okhotsk orogen occur at the southern margin of the Siberian continent within the Irkutsk, Kuznetsk, and Kansk–Achinsk sedimentary basins. Jurassic–early Cretaceous sedimentary basins are also common in the area of the eastern part of the Mongol–Okhotsk suture, framing the mountain uplift of Western Transbaikalia from east and south [20-24].

Metamorphic and magmatic processes of different tectonic natures including the formation of MCCs accompanied a long-term and complex history of the orogenic belt formation. The Transbaikalian MCCs occur in a broad zone of NE–SW-trending extension [6-13] in the immediate vicinity of the world’s largest Angara–Vitim batholith. Only the Selenga МСС is localized in the granitoids of the Angara–Vitim batholith, and most of them are localized in granitoids of the northern part of the Khentei batholith. They are generally NE trending, 20–30 km wide, and extending for 50–150 km along the strike (Figure 4). The cores are usually composed of the late Paleozoic granites and granite-gneisses. The mylonite zone is characterized by various tectonites, primarily after rocks of the core, less often after rocks of the hanging wall (tectonic nappe). The hanging wall consists of nonmetamorphosed, brittle deformed Permian, and Triassic volcanic and sedimentary rocks. The MCCs of Western Transbaikalia are characterized by similar synmetamorphic structural parageneses: gently dipping foliation, micro- and macrostructures (folds, lineation, boudinage, pressure shadows, C–S structures, and kinkbands). The kinematic analysis shows that their mechanism is a simple shear along the deep-penetrating fault zones gently dipping to the southeast. The extensional deformation was NW–SE trending, and the tectonic transport occurred in the same direction, NW–SE trending. Such movements gave rise to the formation of listric normal faults and rift basins bordering on the MCCs. The most intensive period of tectonic exposure of metamorphic cores is determined as 112–123 Ma, and the period of occurrence of metamorphic processes is 140–130 Ma. The rocks composing the detachment zone were metamorphosed under conditions of greenschist and low amphibolite facies (Т=350–640°С and Р=3.2–4.6 kbar) [7]. The pressure estimates indicate that the formation of the complex, including the zone of intense ductile deformations (detachment zone), occurred at depths of 10–15 km.

The structural, petrological, and isotopic data [6-13] show that most of the metamorphic rocks of Western Transbaikalia are of the late Mesozoic era. They were formed in an extensional regime due to the collapse of the late Mesozoic orogeny, which occurred during the early Mesozoic closure of the Mongol-Okhotsk Ocean and a complex collision between the North China continent (Amurian–North China block) and northern Eurasia [25-30]. It is believed [6-13] that the thickening of the continental crust due to thrusting caused heating of the crust and extensional ductile deformations. This predetermined the unstable orogeny and orogenic spreading, which gave rise to the occurrence of regional extension and lower- and mid-crustal detachment faulting. Crustal thinning was accompanied by isostatic uplift at the late-stage extension, which provided exposure to the mid-crustal rock complexes and was favorable to the formation of the MCCs.

2.2. Geological Structure of the Zagan MCC

The Zagan MCC comprises the northeast-trending Zagan range surrounded by the late Mesozoic volcanogenic-sedimentary formations of the Khilok basin in the south and the Tugnuy basin in the north (Figure 5). The structure of the Zagan MMC is distinguished by three bottom-up structural elements: (1) a core, which is composed of magmatic rocks showing partially plastic behavior; (2) a mylonite zone, which is composed of metamorphic rocks with features of tectonically plastic major detachment deformations; and (3) a tectonic nappe (hanging wall), represented by nonmetamorphosed formations [7, 9, 10]. The core is composed of granites, syenites and granosyenites, gneiss-granites, medium-grained granites, and granodiorites. Gneiss-granites and foliated granodiorites are confined to the marginal parts of the Zagan MCC and gradually change the undeformed granitoid rocks. Dark-colored minerals therein acquire a mutually parallel position and provide linear-planar rock texture. Geochronological dating is available for the rocks of the Zagan MMC [7, 9, 10]. The weakly foliated granites and granodiorites have a whole-rock 10-point Rb–Sr isochronal age of 289 ± 23 Ma. The age of granitoids from the central part of the core, determined by the U/Pb method using zircon, is about 260 Ma. They are intruded by the 151.6 ± 1 Ma (U/Pb method) granosyenites from the Margituy massif (complex), syntectonic in accordance with their development time and reflecting initial processes of the MCC formation.

1 – Quaternary sediments; 2 – Cenozoic basalts; 3 – lower Cretaceous sediments; 4 – lower Cretaceous volcanogenic-sedimentary formations; 5 – Permian-Triassic volcanogenic-sedimentary formations; 6 – upper Jurassic granosyenites, including those from the Margituy massif with the age of 151.6 ±1 Ma [7, 10]; 7 – upper Paleozoic granosyenites; 8 – Permian magmatic rocks of core; 9 – mylonite zone; 10 – foliation; 11 – linearity; 12 – detachment; 13 – subvertical faults; 14 – sampling sites for dating; 15 – U–Pb zircon (Zr) ages, Ar–Ar amphibole (Amp), biotite (Bt) and muscovite (Ms) ages, apatite (Ap), and fission-track ages. Names of minerals are abbreviated hereon [31].

On the northern and southern flanks of the Zagan MCC, foliated and mylonitized Permian-Triassic volcanic and sedimentary rocks are intruded by the granite of the upper Jurassic Margituy complex. Massive granitoids in the core gradually change to amphibole-biotite and biotite gneisses of the mylonite zone.

The mylonites comprise a flat-lying zone of metamorphic rocks developed both along the granitoids in the core and the late Paleozoic-Mesozoic volcanogenic-sedimentary formations of the hanging wall. The apparent thickness of the mylonite zone is estimated at 2.0–2.5 km. The mylonite zone is characterized by the up-section transition from low amphibolite to greenschist facies metamorphism. Based on the degree of metamorphism, they may be subdivided into protomylonites, mylonites, mylonitic schists, blastomylonites, and pseudotachylites. Among those are amphibolites whose thickness can reach 100 m, layered bodies of pegmatites up to a few meters thick. Thin sills of syntectonic garnosyenites also occur in the mylonite zone. Rb/Sr isochron dating of 134 ± 6 Ma was obtained [7] for whole-rock samples of crystalline schists and gneisses of the mylonite zone, as well as for conformal pegmatite bodies. A single 40Ar/39Ar geochronological dating of the rocks of the mylonite zone was carried out. The Ar–Ar dating is 127 ± 2 Ma for syntectonic hornblende and 119 ± 2 Ma for syntectonic biotite from amphibolite schists that occurred among mylonitic gneisses (Figure 5) [7]. The tectonic nappe largely consists of nonmetamorphosed, brittle-deformed Permian-Triassic volcanic and sedimentary rocks, as well as Permian granites and late Triassic granosyenites. The tectonites from the low part of tectonic nappe derived from the early Triassic conglomerates localized in the northern part of the Zagan MCC [7, 9, 10].

Geological studies [7, 9, 10] have shown that a clearly defined structural paragenesis has formed in the mylonite zone, represented by small forms: foliation, linearity, orientation of lamellar quartz in segregation bands, pattern of quartz joints, flow folds, slip lenses, and kink bands. These data allow us to conclude that the displacement on both flanks of the Zagan MCC occurred along gentle planes of schistosity in one direction to the southeast. The mylonites of the southeastern flank of the Zagan MCC, in terms of the degree of structural and material transformation, correspond to deeper levels in comparison with the northwestern flank. Mylonites are a fragment of a listric fault, which in its current structure has an arcuate shape.

For geochronological studies, samples were selected from various tectonic zones of the Zagan MCC (Figure 5). Diorite (SE1205B and SE1206A), syenite (SE1207), and granite (SE1207) were sampled from the core, pegmatite-aplite (SE1201 and SE1202D), amphibolite (SE1203), and syn-kinematic granosyenite sill (SE1212) from the mylonite zone, granosyenite (SE1208A), and metaconglomerate (SE1213) from tectonic nappe (hanging wall).

Thermochronological reconstructions of the Zagan MCC were performed based on U–Pb dating of zircon, 40Ar/39Ar dating on amphibole and micas, and fission-track dating on apatite. The uranium content for apatite fission-track ages was obtained by LA-ICP-MS at the Kazan Federal University. 40Ar/39Ar mineral ages were obtained by step heating at the Institute of Geology and Mineralogy SB RAS [32], and apatite fission-track lengths and densities were also measured therein.

Minerals for 40Ar/39Ar dating (amphibole, biotite, and muscovite), as well as apatite fission-track thermochronology, were performed by the conventional techniques of magnetic and density separation at the Analytical Center of Multi-element and Isotope Studies (Novosibirsk) as in [32].

3.1. 40Ar/39Ar Dating

Samples were wrapped in aluminum foil together with biotite MCA-11 and OCO 129-88 standard monitor samples, placed in quartz capsules, vacuumed, and sealed. The capsules were irradiated by fast neutrons in a Cd-lined tube of the IRT-T nuclear reactor at the Tomsk Polytechnical University, with a neutron flux gradient not more than 0.5% of the sample size. The samples were exposed to external stepwise heating in a quartz tube. The 40Ar and 36Ar blank runs (10 min at 1200°С) did not exceed 3 × 10–10 and 0.003 × 10–10 ncm3, respectively. Argon cleaning was performed using ZrAl-SAES getters. The argon isotope composition was measured on a Micromass Noble Gas 5400 mass spectrometer (UK) to an accuracy of ± 1σ. The contribution of interfering Ar isotopes formed together with 39Ca and 40K was estimated using the coefficients of (39Ar/37Ar)Ca = 0.001279 ± 0.000061, (36Ar/37Ar)Ca = 0.000613 ± 0.000084, and (40Ar/39Ar)K = 0.0191 ± 0.0018. The plateau ages were calculated in Isoplot 3.41d [33], as weighted average values over at least three successive temperature steps. The results were interpreted with a reference to the conventional criteria [34]: (i) age difference between any two steps not exceeding K = 1.96 * √ (σ12 + σ22); (ii) consistent Ca/K ratios (mineralogical criterion); and (iii) at least 60% cumulative 39Ar released at each step.

3.2. Apatite Fission-Track Dating

Sample and epoxy mount preparation and calculation of densities and lengths of hidden tracks were carried out at the Institute of Geology and Mineralogy SB RAS (Novosibirsk, Russia). Individual grains of apatite after density separation were manually selected from the heavy fraction using ZEISS Stemi DV4 binocular, then fixed in a mount with Struers epoxy resin, and polished. The epoxy mounts with apatite were etched in a solution of 5.5 n HNO3 for 20 seconds at a temperature of 21 ± 1°C to detect tracks of spontaneous fission of uranium nuclei. Before the study on the microscope, acid washing and drying of mounts were performed. The calculation of the density and length of spontaneous fission tracks was performed using an Olympus BX51 high-resolution optical microscope with an x1250 magnification equipped with an InfinityX photo-video camera.

The uranium content was carried out by the LA-ISP-MS method strictly according to the protocol published in [35] using zeta calibration. The uranium concentration was measured at the Scientific and Educational Center of Geothermochronology of the World-class Scientific Center of the Institute of Geology, Oil and Gas Technologies of Kazan Federal University (Kazan, Russia) using a quadrupole ThermoScientific iCAP Qc ICP mass spectrometer connected to Analyte Excite (Teledyne Cetac Technologies) excimer laser ablation system. The uranium detection was carried out strictly within the track counting area. The equipment parameters were optimized to obtain the maximum sensitivity of 238U and 43Ca and the minimum value of Th/ThO (≤ 0.2%) and Th/U ≈ 1 using NIST SRM 612.

The equipment and laser ablation parameters and the AFT data and operational equipment and laser ablation parameters are given in the Suppl. Materials (online supplementary Tables S1 and S2).

For the primary zeta calibration, 300 measurements of fragments of the Durango apatite standard placed in the equipment parallel to the c-axis were carried out. The resulting zeta factor (ζicp) was used in further sessions. The measurement of the Durango apatite standard was also performed in each session with samples to assess the effect of equipment settings drift. The analysis of the sample was carried out according to the following scheme: at the beginning and at the end of the measurement session, three measurements of the standard synthetic glass NIST SRM 612 (external standard) were performed, two measurements of the Durango standard, and then every ten measurements of the studied apatites, one measurement of each of them. Primary data processing was carried out using the Iolite 3.65 program built into Igor Pro [36], using the internal 43Ca standard according to the modified Trace Elements FTD procedure [35]. The track age of apatite was determined according to [35, 37]. Modeling of thermal histories based on track analysis data was carried out using the HeFTy Version 1.9.3 software [38]. When interpreting the thermal evolution graphs, the geothermal gradient value of 30–25°C/km was assumed.

Thermochronological reconstructions comprise a complex of geochronological methods characterized by different temperatures of closure of mineral isotopic systems: from zircon U–Pb dating (the temperature of closure Tc ~900°С) and amphibole 40Ar/39Ar dating (Tc ~550°С), biotite, muscovite (Tc ~330°С), feldspar/plagioclase (Tc ~250°С) to fission-track dating of apatite (Tc ~110°С) [39]. The comparison between the obtained ages of mineral isotopic systems and temperatures of their closure makes sequential estimation of rock occurrence depths (considering the average temperature gradient of 25–30°/km [40]) at different time intervals starting from their formation and ending with their surface exposure during tectonic events. Thermochronology measures the timing and rates of rocks reaching the surface and cooling as a result of exhumation. 40Ar/39Ar and fission-track thermochronometric systems have closure temperatures ranging from ∼550 to ∼60C⁰, making them sensitive to exhumation at crustal depths of about 1–10 km. Spatial-temporal patterns of thermochronometry determined erosion rates, flow of material through orogenic growth and decay cycles, paleo relief, and relationships with structural, geomorphic, or climatic features.

Thermochronological reconstructions of the Zagan MCC were carried out on the basis of samples from the central part of the footwall, detachment zone and hanging wall (Table 1 and Figure 5) using 40Ar/39Ar dating of amphibole and micas, and fission-track dating of apatite (Figures 6 and 7). Table 1 and thermochronological diagram (Figure 8) present results for the new and already published data that include isotopic and fission-track ages of minerals from the Zagan MCC rocks.

The 40Ar/39Ar ages obtained for hornblende and biotite (Table 1; Figures 5 and 6) from the detachment zone fall within the ranges of 131–125 and of 120–114 Ma, respectively. At the same time, 40Ar/39Ar amphibole dating is consistent within the error with Rb/Sr isochronous dating (134 ± 6 Ma, [7]) from whole-rock samples of gneiss and crystalline schist of the detachment zone. Amphibole and biotite separated from the detachment zone are syn-tectonic and correspond to the ductile deformations. In the Earth’s crust, as a rule, the transition zone from ductile to brittle deformations is located at depths of about 10 km. Therefore, the 40Ar/39Ar dates 120–114 Ma obtained for biotite correspond not to the age of deformations but to the time when the detachment zone rocks were uplifted to a depth less than the transition zone (Tc ~330°С for biotite). On the basis of this, it can be concluded that the amphibole dating corresponds to the stage of metamorphism and deformations of the detachment zone at the beginning of exhumation of the complex from the depths of its formation about 10–15 km. The recorded age difference between the amphibole and biotite dating should correspond to the difference in depth of the detachment zone rocks, which is about 5–7 km. This makes it possible to obtain an estimate of the denudation rate of about 0.3–0.4 mm/year for the period 131–114 Ma (lower Cretaceous epoch).

The Rb–Sr isochron whole-rock age and zircon U–Pb ages of rocks from footwall vary in the interval 270–250 Ma [7, 10, 12], corresponding to crystallization of granite. This interval is similar to the formation time of granitoids of the Khangai batholith (255 ± 10 Ma) [41, 42], located to the south from Zagan MCC (Figures 1 and 4).

Results of numerical simulation of thermal history based on AFT dating show that intense cooling of the rocks of the Zagan MCC occurred in the lower-upper Cretaceous epoch (112, 87 Ma) (Table 1, Figure 7 , online supplementary Table S1). In that period, different parts of the footwall crossed the Tc ~110°С isograd (depth of about 3 km) from which the apatite fission tracks are formed (AFT age). The denudation amplitude can be estimated using the sample SE1202 (Table 1). The difference between the ages of 40Ar/39Ar biotite (Tc ~ 330°С) and AFT (Tc ~110°С) is 26 Ma, and the change in the depth is about 7–8 km. Accordingly, the denudation rate is estimated at about 0.3 mm/year.

The described multisystem approach was used by [16, 17] to study tectonothermal evolution of the late Paleozoic granitoids of the Angara-Vitim batholith located in the central part of Western Transbaikalia near the Zagan MCC (Figure 4). It was concluded that during the Mongol-Okhotsk orogeny (170, 140 Ma), the rock uplift of the Angara-Vitim batholith from 7–10 to 3–4 km depths occurred, which may be due to intensive orogeny in Western Transbaikalia and denudation of about 4–7 km of the Earth’s crust. The authors consider the location of the Angara-Vitim batholith in the axial high mountain uplift part of Western Transbaikalia.

The Mongol-Okhotsk orogen collapse in Western Transbaikalia caused extension and the initiation of detachment fault—a large-amplitude normal fault dipping gently southeastwards (in recent coordinates) (Figure 9, stage 1). The existence of a regional gently dipping fault (detachment) is evidenced by thick, up to 2.0–2.5 km, mylonite zone [6, 7, 9, 10]. The kinematic analysis shows that it is a deep-penetrating fault zone gently dipping into the southeast. The extensional deformation was NW–SE trending. Such movements gave rise to the formation of listric normal faults and rift basins bordering on the MCCs. The rocks in the deep-seated fault zone (mylonite zone) were transformed under conditions of greenschist and low amphibolite facies metamorphism (Т=350–640°С and Р=3.2–4.6 KBar) [7], which corresponds to maximum depths of 10–15 km. The 40Ar/39Ar ages obtained for syn-tectonic hornblende and biotite from the mylonite zone fall within the ranges of 131–125 and 120–114 Ma (Figure 6), respectively, characterizing its formation age in the early Cretaceous epoch.

During the early Cretaceous epoch (131, 114 Ma) (Figure 9, stage 2), there was a significant decrease in upper crustal thickness that resulted from tectonic denudation. It was caused, in accordance with the Wernicke model [1], by two interrelated processes (Figure 9, stage 2): displacement of the upper plate to the southeast (in recent coordinates) and formation of a series of listric normal faults (tilting). In the period of tectonic denudation, about 5–7 km of overlying rocks were denudated. The tectonic denudation rate is estimated at about 0.3–0.4 mm/year.

The lower-upper Cretaceous epoch (112, 87 Ma)( Figure 9, stage 3) is characterized by intensive denudation rate of overlying rocks, which may be due to isostatic uplift of relatively light granite masses and erosional denudation of the upper crust. The erosional denudation rate at that period is estimated at 0.3 mm/year. It was at this stage of its evolution that the Zagan MCC took a dome-like shape (Figure 9, stage 3). Previously [18], based on preliminary track dating data, we estimated the denudation rate at more than 0.1 mm/year.

The studied samples of late Paleozoic granitoids of the Angara-Vitim batholith [16, 17] experienced cooling in the period 320–280 Ma to Tc ~330°C (biotite) and then remained in a stable state until the beginning of the Mongol-Okhotsk orogeny (Figure 9). During the orogeny (170, 140 Ma), they cooled to Tc ~110°C (apatite), and high mountain uplift, probably up to 6–8 km, was formed. The beginning of the detachment formation about 131 Ma coincides with the maximum mountain growth of the Western Transbaikalia, which makes it possible to determine the localization of the Zagan MCC on the southern flank of this mountain uplift.

After a long-term quiescence (Paleogene-Neogene period), reflected in the graphs of the AFT age (Figures 7 and 8), the territory of Western Transbaikalia including the Zagan MCC area was reactivated in the Pliocene–Quaternary period. In the mountain surroundings of the northern and central parts of Lake Baikal (the Baikal-Patom and Barguzin ridges), according to the AFT data [14, 15, 17], an intensive uplift of territories was noted. This period is also marked by a strong compressive deformation and mountain growth in the Altai, Sayan, and Tuva regions due to the northward propagation of the compressive stress generated by the India–Asia collision [43-47]. Pliocene-Quaternary period as well as late Mesozoic reactivation and orogeny of these areas were confirmed by numerous data of low-temperature geochronology [48-54].

Thus, the obtained results of tectonothermal study of the Zagan MCC rocks confirm and complement the previously obtained results in the area of Western Transbaikalia of two stages of orogenic events associated with the late Mesozoic evolution of the Mongol-Okhotsk orogen and its late Cenozoic reactivation. Late Mesozoic tectonic stages were accompanied by the mountain growth. Their destruction caused MCCs and sedimentary basins typical for northern Eurasia to form. It can be assumed that Mesozoic MCCs of East Asia mark the flanks of large mountain uplifts formed as a result of the Mongol-Okhostsk collision (Figures 1-4).

The goal of future research is to determine the relationships between orogenic events and the formation of Mesozoic sedimentary basins in Western Transbaikalia and surrounding areas based on thermochronological reconstructions and correlation with U–Pb dating of detrital zircons.

The review of published geological and geochronological data and the new data obtained for the Zagan MCC led to the following conclusions:

  1. The Zagan MСС was formed in the Cretaceous period on the southern flank of a large mountain uplift of Western Transbaikalia, composed of the late Paleozoic Angara-Vitim granitoid batholith.

  2. According to 40Ar/39Ar dating of amphibole and mica from the mylonite zone, the time of the active development of the Zagan MCC complex corresponds to the early Cretaceous epoch (131, 114 Ma). Tectonic exposure of the footwall complexes from about 15 km to the depths of about 10 km occurred at a high rate of tectonic erosion (about 0.4–0.3 mm/year) because of postcollisional extension of the Mongol-Okhotsk orogen.

  3. Apatite fission-track dating shows that further cooling of rocks in the Zagan MCC to Tc ~110°С (a depth of about 3 km) occurred in the early–upper Cretaceous epoch (112, 87 Ma). The erosional denudation rate was about 0.3 mm/year. Due to the active denudation rate, the Zagan MCC took a dome-like shape caused by floating of a relatively light granitoid substrate.

  4. In the Pliocene-Quaternary period (the last 5 Ma), the Zagan MCC, like all of Western Transbaikalia, underwent reactivation associated with the distant influence of the Indo-Eurasian collision.

All datasets generated for this study are included in the article/SupplementaryMaterial.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors thank the editors of the journal “Lithosphere” for the excellent organization in the preparation of the article, and the anonymous reviewers for their useful comments and suggestions.

Table 1. Equipment and laser ablation parameters and the AFT data.

Table 2. Operational equipment and laser ablation parameters.

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Supplementary data