The Mongol-Okhotsk Orogenic Belt (MOOB) is recognized as the axial structure of the Central Asian Folded Belt (CAFB) [1]. The final formation of the MOOB in the Late Mesozoic is the final completion of the formation of CAFP as an orogen. But the tectonic, geodynamic and magmatic processes that took place in the post-Mesozoic influenced the evolution of these regional structures. MOOB stretched from Inner Mongolia to the Pacific coast for 3000 km. Its formation is associated with the convergence of the Siberian and North China cratons and the closure of the Mongol-Okhotsk basin in the Late Mesozoic. Post-Mesozoic tectonic events have changed its original appearance. In the region of the 120th meridian, two cratons approached each other as close as possible, “absorbing” the formations of the belt itself, and divided it into the western and eastern flanks (Figure 1(a)).

The article considers the eastern flank of the belt. In the frame of this flank, from the end of the Jurassic to the beginning of the Late Cretaceous, the magmatic complexes of various compositions were formed almost continuously. Long-term studies of this territory have not led to an unambiguous view of the evolution of the region. The precise geochronological, isotopic, geochemical, and geophysical data obtained in recent years allow us to take a fresh look at the development of this complex region.

The solution to this problem was based on the study of magmatites in the frame of the MOOB. The material characteristics of rocks reflect not only their composition but also the geodynamic conditions that they accompany during their formation.

To establish what geological events influenced the post-Mesozoic evolution of the MOOB, not only the original data of the author were analyzed, but also the results of recent geochronological, isotopic, geochemical studies of magmatic complexes accompanying the evolution of the MOOB in the late Mesozoic-Cenozoic were analyzed (eg, [4] - [11]). The results of geophysical research are also used-transects passing through the MOOB [12] [13] [14] [15].

As a result of studies [3] [7] [8] [9] [10] [16] [17], it was found that in the southern and northern framing of the eastern flank of the MOOB, volcano-plutonic and volcanogenic complexes were formed starting from the turn of the Late Jurassic and Early Cretaceous. They were accompanied by various geodynamic settings. The belonging of these complexes to certain structures was revealed. In the northern frame of the belt, the Selenga-Stanovoy (SSS) and Dzhugdzhur-Stanovoy (DSS) are distinguished—the southern frame of the Siberian craton—superterranes. The southern boundary of the belt is represented by the Argun and Bureya-Jiamusi superterranes and the South Mongolian–Khingan orogenic belt (Figure 1(b)). It has been established that within the southern framing of the Siberian craton, the Argun superterrane, and the South Mongol-Khingan orogenic belt, rocks of homogeneous composition and formation time were formed. Their formation took place at the following magmatic stages.

Stage I: Magmatism within the listed structures first appeared 145 - 138 Ma ago. During this period, adakite volcanoplutonic complexes begin to form [10] [16] [17]. The rocks of these complexes are not widespread. They are represented by subalkaline granites, leucogranites, granites, subalkaline leucogranites, granosyenites and their porphyry varieties. These are rocks of the normal or subalkaline series, high potassium, belong to the calc-alkaline series, with Na₂O + K₂O = 7.86 - 10.92 wt% and Na₂O/K₂O = 1.25 - 1.81. The rocks are magnesian and peraluminous at ASI (aluminum saturation index) = 1.06 - 0.86, which characterizes them as I-type formations [18] (Zen, 1986). Elevated concentrations of Sr (670 - 1110 ppm), Ba (510 - 2400 ppm) were found in the rocks; Rb (82 - 160 ppm), Th (8.4 - 13.1 ppm); at low contents of Nb (4.0 - 11.0 ppm), Ta (0.4 - 0.6 ppm) and at abnormally low concentrations of HREE (in ppm): Tb (0.18 - 0.22), Dy (0.66 - 1.45), Ho (0.10 - 0.22); Er (0.25 - 0.55); Tm (0.03 - 0.07); Lu (0.02 - 0.05), as well as Y (3 - 7) and Yb (0.17 - 0.42). Chondritic-normalized diagrams show a positive Eu anomaly or its absence: (Eu/Eu*)n = 0.77 - 1.23 at (La/Yb) n = 26.45 - 64.13 (Figure 2).

On the classification diagrams (La/Yb)n-Ybn [20] [21] and Sr/Y-Y [22], the figurative points of this complex rocks are projected onto the rock field of typical adakite series of the world. According to the isotope-geochemical characteristics, they belong to the negative εNd-type with εNd(T) = (−3.3) - (−4.6). The values of ⁸⁷Sr/⁸⁶Sr are 0.7069 - 0.7071.

By its formation time (145 - 138 Ma), the adakite series rocks preceded (and partly coincided) the formation of the Early Cretaceous calc-alkaline complex: 140 - 128 Ma.

The formation of the adakite complex rocks was replaced by the formation of the rocks of a differentiated granite-granodiorite complex (140 - 128 Ma) [4] [17] [23] [24] [25]. These granitoids compose both large batholiths with an area of up to 500 km², and small bodies of the complex and dyke-shaped forms (Figure 1(b)). They are widely spread in the northern frame of the belt. This can be

explained by the strong erosion of this part of the region. The complex contains granodiorites, quartz diorites, granites, plagiogranites, and leucogranites.

According to the SiO₂ content, granitoids belong to the formations of the calc-alkaline series with a ratio of Na₂O/K₂O = 0.9 - 1.6. The amount of alkali is almost constant for all varieties (6.1 - 7.1 wt%) With a content of K₂O = 2.3 - 3.3, and Na₂O = 3.1 - 4.1 wt%. These are mainly high-potassium rocks of the peraluminous series at ASI = 0.9 - 1.2. They are characterized by moderate to low titanium. The strontium isotopic ratios of the granitoids of the complex are: ⁸⁷Sr/⁸⁶Sr = 0.7076 - 0.7068 [25].

The formation of hypabyssal granitoids of monzodiorite-granodiorite composition is shifted in time of formation: 130 - 126 Ma [4] [23] [24] [25]. They form large area laccoliths and lopoliths (up to 200 km²) composed of porphyritic quartz diorites, monzonites, quartz monzonites, granodiorites. The rocks of the complex belong to the high potassium calc-alkaline series at Na₂O/K₂O = 0.9 - 2.2. They are characterized by an almost constant content of Al₂O₃ (15.1 - 16.1 wt%) at ASI = 1.1 - 1.3, belong to the potassium-sodium series, moderately magnesian, moderately titanic.

Granitoids with an age of 130 - 126 Ma are comagmatic of a volcanic complex with an age of 128 - 122 Ma. The rocks of this complex are composed of paleo volcanoes of the central type. They are represented by andesite basalts, andesites, trachyandesites, dacite andesites, dacites, their tuffs, tuff aleurolites, tuff sandstones. According to the petrochemical features, volcanic are the rocks with a predominantly sodium type of alkalinity: Na₂O/K₂O = 0.81 - 2.66, with a total of alkalis from 4.6 to 6.9 wt%. Volcanites from low to high potassium species, moderately to low titanium; moderate to high magnesian, are belonging to the calc-alkaline series. Alumina varies from moderate to high (ASI = 1.0 - 1.3).

For andesites of the volcanic complex, the ratios ⁸⁷Sr/⁸⁶Sr = 0.7063 - 0.7078 were established [24], which are comparable to earlier plutonic formations.

According to the REE content, all formations of the beginning of the Early Cretaceous are comparable among themselves, which is reflected in the diagrams (Figure 3(a)).

Their compositions on chondritic-normalized plots are characterized by a hollow oblique shape, with an almost absent Europium anomaly (Eu/Eu* = 0.7 - 0.91).

It can be stated that in the range of 140 - 122 Ma differentiated calc-alkaline volcano plutonic complexes are formed with common geochemical characteristics that indicate the unity of the geodynamic conditions of their formation. These conditions (Figure 3(b)) correspond to suprasubduction environments of the active continental margins of the Andean type [16] [17].

Stage II: The development fields of bimodal volcano-plutonic complexes (Figure 1(b)) are territorially combined with the distribution fields of differentiated plutonic and volcanic formations of the calcareous-alkaline series of the beginning of the Early Cretaceous [7] [8]. Their formation began almost immediately after the completion of the formation of differentiated complexes and lasted

more than 20 Ma: 119 - 97 Ma [7] [24] [25]. The formation of bimodal complexes accompanied the final formation of the MOOB [8] as an orogen.

Paleo volcanic structures composed with the rocks of bimodal complexes form more than 30 volcanic fields in both the northern and southern frames of the MOOB with an area of up to 200 km². In the structure of these fields the cover, vent and subvolcanic facies are distinguished. The percentage of rocks of the complex from its total volume is: the main composition—55%, the average—9%, sour—20%, tuff-sedimentary—16%.

The lavas of the primary-middle composition are represented by trachybasalts, trachyandesitic basalts, trachyandesites and andesites. The increased alkalinity of the lavas of basic to medium composition is due to the presence of potassium feldspar or red-brown biotite in the matrix of rocks, less commonly, sanidine in phenocrysts.

Acidic varieties are represented by rhyolites, rhyodacites, trachyriolites, with interbeds of perlites, tuffs and ignimbrites. The plutonic formations comagmatic to volcanic rocks correspond with subalkaline granites, subalkaline leucogranites, granodiorites, quartz diorites, and quartz monzonites. The bimodal composition of the rocks of the complex is due to two ranges of content SiO₂: 47 - 64 and 72 - 78 wt% at the absence of intermediate varieties. Volcanics with a SiO² content of 47 - 64 wt% are high alumina (Al₂O₃ = 15.22 - 17.30 wt%), moderately low magnesian, low titanium (TiO₂ < 2 wt%) formations. They belong mainly to the high potassium calc-alkaline series. Volcanics with a SiO₂ content of 72 - 78 wt% are characterized by normal, less often moderate alkalinity, with an increase in SiO₂ content, the total alkalinity decreases (from 9.13 to 7.13 wt%), low alumina content (Al₂O₃ = 11.15 - 13.96 wt%), low magnesian, low titanium They belong to the high potassium calc-alkaline series. Plutonic acidic formations belong to A-type granites, while granitoids comagmatic with medium-basic volcanics are comparable to I- and S-type formations [7].

All rocks of the bimodal complexes are enriched with light rare-earth elements (La/Yb)n = 5.5 - 33.0 (values of 10 - 20 prevail).

The Eu-minimum (Figure 4(a)) in basic-medium rocks is weakly expressed (Eu/Eu* = 0.70 - 0.86), and in acid formations it is deeper (Eu/Eu* = 0.33 - 0.70). The multielement spectra are characterized by stable negative anomalies of Nb (0.10 - 0.43), Ta (0.12 - 0.79) and Ti (0.01 - 0.09) for all rock varieties and a highly variable Sr anomaly: for granitoids and acidic volcanic rocks, it is negative, and for main-medium rocks - from weakly manifested negative to positive. Positive values indicate the contents of Ba, Rb, Th, K (Figure 4(b)).

The rocks of the bimodal complexes are characterized by sustained isotopic compositions with variations in the ⁸⁷Sr/⁸⁶Sr ratio (0.7057 - 0.7063, 0.7081 - 0.7084) and a wider range of εNd(Т) = (−0.6) - (−3.6) [24] [25] and authors data. Model Nd age - T_Nd (DM-2st) - is characterized by a relatively narrow interval of 975 - 1314 Ma, which may indicate the material uniformity of the melting substrate with the crustal component of the late Riphean. They are superimposed on

the northern edge of the Argun terrane and the southern border of the Siberian craton, where Precambrian formations are widely developed.

Summarizing the available geochronological dating of the isotope dating of the rock, carried out by the U-Pb method on zircons [24]; ⁴⁰Ar-³⁹Ar method [4] [7] [24] [25], we obtained the following time sequence of complex formation. Andesitic trachybasalts were formed from the lower part of the section—118.7 ± 0.9 million years; rhyolites, trachirhyolites—118.7; 118.4; 117 ± 1; 117.1; 117.6; 115.3 ± 1.5 Ma and trachyandesites—115 Ma, 114.7 ± 0.6 Ma (middle section). The upper part of the section is 105.9; 100; 97 Ma and rhyolites - 97 ± 5 Ma. Thus, we can assume an almost continuous stage of magmatic activity, which lasted in the interval 119 - 97 Ma ago and died out at the very beginning of the Late Cretaceous.

An analysis of the geological, petrochemical, and geochemical characteristics of these rocks suggests that they formed as intraplate formations and accompanied the final completion of the formation of the Mongol-Okhotsk orogen [8].

III stage. Magmatic formations, which replaced the rocks of the bimodal series, began to form 94 Ma. Since that time, intraplate formations of the riftogenic complex have been developing: trachyandesites-absarokites [26]. Trachiandesites belong to the potassium-sodium series (Na₂O/K₂O = 1.66 - 2.05), and absarokites (Na₂O/K₂O = 0.47) belong to potassium series; the amount of alkali varies from 6.35 to 9.01 wt%. Volcanites are characterized by low-moderate contents of MgO, TiO₂, with an Al₂O₃ content of 15.24 - 17.45 wt% (saturation index Al = 1.1 - 1.3).

The rocks are characterized by a differentiated REE spectrum (Figure 5(a)), with (La/Yb)n = 13.95 - 20.67. Europium anomaly is practically absent or reveals a weak positive anomaly: (Eu/Eu*)n = 0.79 - 1.03. They are characterized by moderate enrichment Ba (670 - 1540 ppm), Sr (440 - 950 ppm), K (14,600 - 28,400 ppm), Th (6.4 - 21.4 ppm), Rb (60 - 240 ppm) with elevated contents of Nb (до 22 ppm), Ta (1.68 ppm), Zr (330 ppm), Hf (10.4 ppm), Y (22.4 ppm), Yb (2.04 ppm) (Figure 5(b)).

Trachiandesites-absarokites are developed within the rift depressions in the northern and southern frames of the belt. They indicate the beginning of the destructive processes in the region.

The analysis of the manifestation of Late Mesozoic magmatic activity in the frame of the eastern flank of the MOOB showed that synchronous geodynamic processes which were accompanied by the formation of magmatic complexes of the same age and material composition took place in the region during Late Jurassic - Late Cretaceous.

1) Along the southern border of the MOOB, the Argun and Bureya-Jiamusi super terrains stand out, separated by the South Mongol-Khingan terrain (Figure 1(b)). The area distribution of all the above formations along the southern frame of the EFMOOB is cut off in the east by the structure of the Bureya-Jiamusi super terrains (Figure 1(b)). Within its limits, Late Mesozoic magmatites are asynchronous in time of formation of the rocks of the above complexes [3] [9]. Late Mesozoic magmatites within it are asynchronous in time of formation to the above-described rock complexes in the frame of the belt.

According to our data [3] [9], the Bureya-Jiamusi super terrain did not participate in the closure of the Mongol-Okhotsk basin. It is an independent structure that joined the Eurasian continent after the described geological events.

The distribution of the above-described rocks is cut off by the SSS structure in the west along the northern margin of the belt. Within this structure, magmatites known as the Selenga-Vitim volcano plutonic belt (SVVPB) [27] are widely developed.

A clear difference from the considered formations is established by the belt formation time, by the material composition of the composing magmatites. The rocks of calc-alkaline and bimodal volcano-plutonic series were formed in its early development stages (C₂ – P₂). The magmatites of the bimodal series were formed at the later stage (T) [27]. There are no analogues to these formations in the northern and southern frames of the MOOB. According to the data [12] [13], the northern boundary between the SSS and the DSS (Figure 6(a)) is represented by the Dzheltulak fault zone of the crustal bed. Within this zone, milonites, blastomilonites, blastocataclazites, sections of layer-by-layer schismation of rocks, and silicon-alkaline metasomatism are widely developed. The age of zircons from blastomilonites determined by the U-Pb method showed that there are formations of different ages: 1960 - 1930, 1750 - 1700, 1600 - 1500 Ma (determination by zircons), 2000 - 1350 Ma (by pyrochlore) [28]. Late Jurassic – Early Late Cretaceous volcanoplutonic complexes are widely manifested to the north-east of the Dzheltulak fault (Figure 6(a)). These formations are traceable to the east along the northern margin of the MOOB. These rocks are an analogue of the formations developed along the southern margin of the eastern flank belt [17] [23].

2) The eastern flank of the SSS borders with MOOB (Figure 1(b)) in the south through the North Tukuringra fault. A tectonic zone with a length of 800 km and a width of up to 50 km is distinguished along this boundary. This zone is composed with sedimentary and volcanogenic rocks metamorphosed in the amphibolite facies. Geochronological data defined by U-Pb method and isotope geochemical (Sm – Nd) studies of zone rocks [11] indicate the presence of metavolcanic rocks with an age of 193 ± 1 Ma; granitoids with an age of 370 Ma, which are characterized by minimal tNd (DM) = 1.1 BA. According to the values of tNd (DM), metamorphic rocks of the stratum decompose into two groups

with tNd (DM) = 1.1 - 1.9 and tNd (DM) = 2.5 - 3.1 Ga. The authors [11] note that a clear spatial distribution of formations with Late Archean and Proterozoic tNd (DM) values is not detected. All these data allowed the authors to draw the following conclusions [11]: a) rocks of different ages are combined within the zone; b) the zone is a tectonic melange composed of metamorphosed rocks of the Mesozoic, Paleozoic and Early Precambrian ages; c) the formation of the zone occurred in the Mesozoic, during the Late Jurassic - Early Cretaceous collision processes. The latter fact is refuted by the authors themselves [11]: findings of Mesozoic rocks within the zone. This indicates that the formation of the melange zone occurred much later.

3) Magmatic processes in the northern and southern rims of the MOOB occurred simultaneously and were associated with the closure of the Mongolian-Okhotsk basin. What was provoked by the convergence of the Siberian and North China cratons. This process was accompanied by synchronous subduction of the Mongolian-Okhotsk basin in both northern and southern directions. Therefore, it is reasonable to assume that by the end of the Late Mesozoic, all the described magmatites were located at an equal distance from the supposed subduction boundaries. This regularity was most clearly preserved in the northern frame of the MOOB (Figure 6(a)). In this case, the position of the CCC did not correspond to its current state at that time. Most likely, SSS “wedged” between the MOOB and the southern margin of the Siberian craton after the end of the Late Mesozoic magmatism, later than 88 Ma (Figure 6(b)).

According to the geophysics data, the heterogeneous layering of the lithosphere structure is established at the base of the SSS, which is a sign of horizontal movements in the earth’s crust and in the subcrustal space. It was found [12] [13] [14] that within the zone there are deep inclined interfaces between both modern and earlier foundations. The modern (Late Cenozoic) borders have a southern fall, and the paleo-borders have the northern one. Probably, the paleo-boundaries of the northern occurrence arose as a result of the Late Mesozoic subduction processes, when the oceanic bed of the Mongol-Okhotsk basin subducted under the continental margin of the southern margin of the Siberian craton. Late Cenozoic borders indicate the existence of tectonic rearrangements in this period of time.

Such global tectonic transformations could only be associated with global tectonic rearrangements in neighboring, interdependent territories. Such transformations include tectonic events occurring during the collision of the Indian Plate and the Eurasian continent in the Cenozoic. There are various assumptions about the age of this process. Some consider 65 Ma the most likely age [30] [31], others 45 - 55 Ma [32] [33] or 35 Ma [34]. But most researchers attribute these processes to the 55 - 50 Ma period [35] [36] [37]. These authors of the researches [38] [39] [40] [41] reached the same conclusion. They believe that initially the Indian plate was subducted under the Eurasian continent. This led to the formation of an accretionary prism. But in about 25 - 20 Ma, a continent-continent collision began, which led to the extrusion of the Great Himalayas along southern Tibet and to the beginning of the deformation of more remote territories in Central Asia: the deformation and the rise of the Tien Shan [42]. This conclusion agrees with the age of post-collisional magmatism in Southern Tibet, which began 26 Ma ago [43] [44].

According to [45], the collision influence zone of the of the Indian Plate and Eurasia consists of six tectonic domains, among which there is a Central Asian domain with a deformation region extending from the Tien Shan in the south to the Baikal rift zone in the north. The formation of a mountainous terrain in the Tien Shan was completed in the Pliocene (review in work [46]). The duration of the orogenesis stage is almost 15 Ma and corresponds to the interval of 25 - 10 Ma. Based on these considerations, it can be stated that the post-Mesozoic tectonic restructuring in the north-western frame of the MOOB occurred at the Oligocene—Miocene border. It was during this period, under the influence of the processes occurring between the Indian plate and the Eurasian continent, when the Indian plate moved in a northeast direction at an angle close to 20 degrees, the SSS was displaced and wedged between the MOOB and the southern frame of the Siberian craton—DSS. It can be assumed that the entire north- western frame of the MOOB was affected by this process. This conclusion has indirect evidences. Symmetrically located magmatic complexes of the Paleozoic - Early Mesozoic are developed in the frame of the western flank of the MOOB (Figure 7). The scheme clearly establishes the displacement of these complexes along the north-west framing of the belt relative to their location in the southeast framing.

Magmatic activity in the frame of the eastern flank of the MOOB ended at the beginning of the Late Cretaceous—no later than the Santonian. In fact, since that time, the territory has been in a state of rest: the formation of platform formations begins [48]. And this process continues to this day. But it was during this period that the tectonic restructuring of the region took place. It was not accompanied by any magmatic events. As a result of this restructuring, in the region of the 120th meridian, the SSS wedged itself between the southern framing of the Siberian Craton and MOOB. The belt was divided into two flanks: western and eastern.

What could be the “driving force” of global movements of such large geological objects? It has been established that the Central Asian region, extending from the Tien Shan in the south to the Baikal rift zone in the north, is a zone of influence of the collision of the Indian Plate and the Eurasian continent [45]. Hence it follows that the junction of the MOOB structures and the southern framing of the Siberian Platform in the Cenozoic was under the influence of distant collisional processes. Since the structures described are interdependent, this should have influenced the evolution of MOOB. It can be assumed that the “driving force” of the SSS movement was the collisional processes occurring between the Indian and Eurasian plates at the boundary of the Oligocene and Miocene. It is these processes that changed the contour of the MOOB in the post-Mesozoic time.

The author is grateful to the staff of the IGiP FEB RAS (Blagoveschensk, Amur region, Russia) Elena V. Ushakova; ITiG FEB RAS (Hbarovsk, Russia) Dmitry V. Avdeev, Anna V. Shtareva, Larisa S. Bokovenko, Anastasia Lushnikova and Valentina E. Zazulina; to the staff of IGEM RAS (Mosсоw, Russia) Andrey V. Chugaev and Tatyana I. Oleinikova for conducting out analytical studies of the rocks.

The author declares no conflicts of interest regarding the publication of this paper.

Derbeko, I. (2022) The Influence of an Interdependent Structures on the Post-Mesozoic Evolution of the Eastern Flank of the Mongol-Okhotsk Orogenic Belt. International Journal of Geosciences, 13, 464-482. https://doi.org/10.4236/ijg.2022.136025