Laboratory of Geodynamic and Petrogenesis, Institute of Geology and Nature Management, Far Eastern Branch, Academy of Science, Blagoveshchensk 675000, Russia
　　Received: December 26, 2011 / Accepted: February 8, 2012 / Published: February 20, 2012.
　　Abstract: Mongol-Okhotsk orogenic belt was formed during almost all the Phanerozoic period. The bimodal late Paleozoic-early Mesozoic formations were stated in the frames of the western section of Mongol-Okhotsk belt. Their formation is connected with the collision of north Asian and Sino-Korean continents. These collision processes were conjugated with the dimensioned intraplate processes in the region. The rocks of the bimodal volcano-plutonic complex were also stated in the frames of the eastern section of Mongol-Okhotsk orogenic belt. This should proof the identity of geological processes, which accompanied the collision of north Asian and Sino-Korean continents, during all the stage of the formation of the Mongol-Okhotsk belt.
　　Key words: Mongol-Okhotsk orogenic belt, bimodal volcano-plutonic complex, cretaceous period.
　　 1. Introduction
　　Mongol-Okhotsk orogenic belt is considered as the axis of central-Asian folded belt [1]. The belt was formed during almost all the Phanerozoic period. This process was conducted by the formation of the whole series of volcanic and volcano-plutonic complexes and by the closure of Mongol-Okhotsk basin in south-western north-eastern directions.
　　As the result of the following tectonic rebuilding the Mongol-Okhotsk orogenic belt was divided into two sections: eastern and western. The bimodal late Paleozoic-early Mesozoic formations were stated in the frames of the western section of Mongol-Okhotsk belt. The formation is connected with the collision of north Asian and Sino Korean continents. This collision processes were conjugated with the dimensioned intraplate processes, that were completed by the closure of the western part of Mongol-Okhotsk basin by the beginning of early Cretaceous - 190 Ma [2-8].
　　The rocks of the bimodal volcano-plutonic complex were also stated in the frames of the eastern section of Mongol-Okhotsk orogenic belt. The rocks of basalt-trachybasalt-rhyolitic composition that were formed in the end of Early Cretaceous (119-97 Ma) are present for a distance of 600 km along the southern border of eastern section of the belt [9-12]. The development of the rocks is framed by the structure of Bureja-Jiamusy superterrain (Fig. 1).
　　The volcano-plutonic formations of the northern framing of Mongol-Okhotsk belt are examined in the article. The presence of the rocks of the contrast series should proof the identity of geological processes, which took place at the collision of north Asian and Sino-Korean continents, during all the stage of the formation of the belt till the complete closure of Mongol-Okhotsk basin. Fig. 1 Structure-tectonic scheme: (a) Scheme of the dislocation of volcano-plutonic and plutonic complexes of late Paleozoic-Mesozoic in the frames of Mongol-Okhotsk orogenic belt; (b) Scheme of the dislocation of volcano-plutonic and plutonic complexes of Late Mesozoic in the frames of eastern link Mongol-Okhotsk orogenic belt [13]; 1—Mongol-Okhotsk orogenic belt: western section (WMO), eastern section (EMO). The scheme is made by the data of Refs. [13, 14]; 2—The region of spreading of the bimodal volcano-plutonic complexes in the end of early Cretaceous; 3—The western part of Mongol-Okhotsky orogenic belt on the west of 120 meridian, by Refs. [7, 15]: the region of spreading of the bimodal volcano-plutonic complexes and the collisional granites of late Carboniferous-early Mesozoic; 4—The age of the volcano-plutonic complexes. Letter symbols: superterrains and terrains: AR—Argunsky, SM—South Mongolian, BJ—Bureja-Jiamusi, BD—Badgialsky [14]; AS—Aldano-Stanovoy [16]; OK—Okhotsk-Koriakskian [10]; YZ—Yenisey-Zabaykalsk orogenic belt [14]; 5—Water basins; 6—The fields of the bimodal volcano-plutonic complexes at the end of early Cretaceous; 7—The fields of the volcano-plutonic complexes at the beginning of early Cretaceous; 8—Granites, comagmatic to the volcano-plutonic complexes of late Mesozoic (undividable); 9—Tectonic borders.
　　


　　 2. History of the Research
　　 2.1 The Geological Position
　　The volcano-plutonic complexes in the northern framing of Mongol-Okhotsk orogenic belt were stated in late 70 th, by the conduction of the geological survey[17]. The complexes are assembled with the volcanogenic-sedimentary layer, subvolcanic and plutonic bodies. The volcanic fields are occurred on the early Mesozoic granitoids and pre Mesozoic foundation of Aldan-Stanovoi [16].
　　The square of spreading of the paleovolcanic structures of the northern framing of Mongol-Okhotsk orogenic belt is less than 1,000 km2. But if one takes into consideration the plutonic formations that are comagmatic to the volcanites, the square of the development of the complex will increase in several times. Because of the spatial dissociation, the fields of their rocks are often separated as independent complexes. But the derived precision dissociation data of their substantial composition and age refute such dissociation. In the offered article this formations are combined in a single Bomnaksky volcano-plutonic complex. Each volcanic field has its own characteristics. Often the volcanic field is represented purely by tuff material (upper reaches of the river Zeya). The open-cast of stratifying component might have different thickness, percentage of lavas, tuffs and sediment rocks. The sequence of the formation of the open-casts of the covering facies is mainly kept. The base of the open-cast of the covering facies is presented by trachyandesitic basalts, andesitic basalts, andesites, trachyandesites, interstratifying with tuffsandstones, tuffgravelites and tuffconglamerates. The middle part of the open-crust is made by rhyolites, rhyolitic dacites, trachyrhiolites, dacites, perlites, tuffs and ignimbrites. In some of the volcanic fields the open-crast is completed by the covering of andesites, andesitic basalts. The percentage of the rocks of the complex to its general value is: main composition—60%-70%, acid—25%-30%, tufogenic-sediment—10%-15% and less.
　　The bodies of vent facies are stated in the frames of the volcanic fields and near them with the square of outcome 2 km. They are presented by aglomeratic, lapilli tuffs, psammitic tuffs, clastolavas of acid and main composition. Subvolcanic bodies, that are comagmatic to the covers, are forming not great stocks and dykes.
　　The plutonic formations that are comagmatic to vulcanites can make massifs (up to 400 km2), but more often they form not great stoks and dykes. Massifs have a complicated, zonal building, when the small bodies are mono rocks, as a rule.
　　2.2 Age
　　By the data of the geological survey of 80th-90th the age [17] of the rocks of volcano-plutonic complex in the northern framing of Mongol-Okhotsk belt makes 112 ± 8-99 ± 7 Ма for trachyrhyolites (by K/Ar method) and 98 ± 6 Ма for andesites (U/Pb method). The age of 101 ± 2 Ма was attained by U/Pb method for granosienites [18]. For quartz diorite and quartz monocite (Rb/Sr method):?117 ± 0.8, 109 Ма [19]; for the identification of the age of granites and leykogranites (U/Pb method): 108.6 ± 1.3 and 110.3 ±2.9 Ma [20].
　　As it was already mentioned, the rocks of the bimodal complex aged 117 ± 0.8-99 ± 7 Ма were revealed along the southern framing of the eastern section. It may be assumed that the formation of the volcano-plutonic complex rocks of the belt’s northern framing was synchronous to the bimodal complex rocks of the southern framing.
　　 3. Analytical Methods of the Research
　　The research of the gross geochemical composition of the rocks (petrogenic components, Sr, Zr, Nb) was made by using the X-ray fluorescence analysis methods in the following institutes: Institute of Geology, Nature Management of the Far Eastern Branch of Russian Academy of Science (RAS) (Blagoveschensk, Russia) and in the Institute of Geochemistry of the Siberian Branch of RAS (Irkutsk, Russia). The definition of the geochemical composition of the rocks by the mass-spectrometer inductively method (ICP-MS) was made in the Institute of Geochemistry Siberian Branch of RAS (Irkutsk, Russia) and the Institute of Tectonics and Geophysics of Far Eastern Branch of RAS (Khabarovsk, Russia). For the analysis of micro elements by the ICP-MS technology, a worn-out sample received an acid decomposition in HF and HNO3 in fluoroplastic containers.
　　Rb/Sr and Sm/Nd isotope research is made in IGGD RAS. Isotope compositions of Rb, Sr, Sm and Nd were measured by manycollectoral mass-spectrometers—Finnigan MAT-261 and TRITON TI in statistical rate. The accuracy of the determination of the concentrations of Rb, Sr, Sm, Nd was ? 0.05% (2δ).
　　 4. Substantial Characteristic of the Rocks
　　 4.1 Petrochemical Characteristics of the Rocks
　　The rocks of the main-middle and acid composition form the volcano-plutonic complex in the northern framing of Mongol-Okhotsk belt. The lavas of main-middle composition are represented by porphyritic differences with massive, rarely mandelstone textures. The lavas are presented by the occurrence of porphyric discharges with basalts, trachyandesite basalts and andesitic basalts, plagioclase, plagioclase-pyroxenes, pyroxenecorniferous, rarely olivine-pyroxene. Among trachyandesites and andesites following types are extracted: plagioclase, pyroxene, corniferous-pyroxene, corniferous. Porphyric impregnations (up to 25%-50%) are represented by clinopyroxene (augite) orthopyroxene (hypersthene, ferrohypersthene), plagioclase (An70-31), common or basaltic corniferous, biotite, in singular cases—olivine. Glomerporphyric joints of plagioclase or pyroxenes with titanmagnetite are often present.
　　The main mass (up to 75%) has got pilotoxite, doleritic, trachytoide or microphyte structure. It contains of volcanic glass with lath of plagioclase(An50-30), grains of ortho- and clinopyroxenes, corniferous, flakes of biotite. Accessory minerals are sphene, apatite, titan magnetite, magnetite.
　　Rhyolite dacites, rhyolites, trachytoide rhyolites, dacites, trachytoid rhyolites have fluidal and massive texture. They form flows with the interlays of perlites, tuffs and ignimbrites. The impregnations in the lavas with porphyritic structure are represented by zonal plagioclase (An10-28), potassic feldspar, quartz, biotite, rarely common corniferous (less than 5%), singular and really small granules of pyroxene. In some varieties of trachytoide rhyolites the phenol crystals of sanidine are present. The main mass (up to 90%) of quartz-feldspar composition with the flakes of biotite, rarely with small granules of corniferous, has got a perlitic micro poikilitic, micro felsitic or micro spherolitic(spherolites of potassic feldspar with albitic or quartz center) structure. Accessory minerals are zircon, sphene, apatite, titanomagnetite and magnetite.
　　Tuffs of acid, middle-acid composition are represented by cinereous psammitic varieties.
　　The bimodal composition of the rocks of Bomnaksky complex is stipulated by two diapasons of content of SiO2: 49 wt%-64 wt% and 69 wt%-78 wt%(Fig. 2).
　　Volcanites with the content of SiO2 49 wt%-64 wt%(Fig. 2а) are characterized with both moderate and normal alkalinity with a straight correlation between general alkalinity and the content of SiO2. The content of Nа2О is unsteady and the amount of K2О is irregularly changing from 1.7 wt% to 4.2 wt%. The rocks are high aluminous (Al2O3 = 15.65 wt%-18.86 wt%), mostly magnesial (moderate to low magnesial MgO < 2 wt%), lowtitanoferous (TiO2 < 2 wt%) formations. This rocks belong to low-highpotassic calk—alkali series. Singular samples get into the field of shoshonite series (Fig. 2b).
　　Volcanites with the content of SiO2 69 wt%-78 wt%
　　


　　 Fig. 2 Classification diagrams for the rocks of bimodal volcano-plutonic complex of the northern framing of Mongol-Okhotsk orogenic belt: (a) (Na2O + K2O)-SiO2 [21]; (b) K2O + SiO2 [22]. are characterized with the normal—moderate alkalinity(Fig. 2). By the content of SiO2 = 73 wt% the general alkalinity decreases. This is low alumina (Al2O3 = 10.55 wt%-14.99 wt%), low magnesial, low titanoferous formations. They belong to high potassic calk—alkali series (Fig. 2b).
　　The plutonic formations are presented by diorites, monzonites, quartz diorites, quartz monzonites, granitoids, quartz syenites, subalkali granites, subalkali leykogranites, leykogranites. Their mineral composition is identical to volcanites. Granitoids, that are comagmatic to volcanites of moderate-basic composition, correspond to the formations of I and S-types, and those that are comagmatic to the vulcanites of acid composition correspond to the rocks of S and A-types (Fig. 3).
　　The correlation analyze was made by the selection, that included all the varieties of rocks of the researched complex of the northern framing of the eastern section of Mongol-Okhotsk belt (Fig. 4).
　　


　　 Fig. 3 A diagram for the separation A-, S-, I-types of granitoids by petrochemical indications [23, 24]: 1—comagmatic to the rocks of moderate content, 2—comagmatic to the rocks of acid content.
　　


　　The two associations of the elements were marked out in the result of analyze: SiO2-K2O and TiO2-P2O5-FeO-CaO-Al2O3-MnO-MgO-Zr that were connected with each other by negative correlation dependence. Positive correlation dependence with silica is marked for Nb (r = 0.342 by r5% = 0.514) and Rb (r = 0.566 by r5% = 0.514) (Figs. 4h and 4i).
　　Singular trend of the ratio is attended almost in all of the examined correlations of petrogenic and a row of rare elements to SiO2 (Fig. 4). The trend is characteristic for all of the bimodal series.
　　4.2 Geochemical Characteristics
　　All the varieties of the rocks of volcano-plutonic complex of the northern framing of Mongol-Okhotsk belt are enriched with the light rare—earth elements(La/Yb)n = 8.4-33.6 (predominant values are 10-20). The accumulation of La relative to Sm (La/Sm)n = 3.2-10.5 (predominant values are 3-5) is stated in them, when the level of fractioning of the HREE is lower (Gd/Yb)n = 1.7-3.9 (Fig. 5а). On the diagram REE (Fig. 5) it is shown by the growth of the depth of Eu anomaly, which also reflects the degree of the fractioning of plagioclase: the correlation Eu/Eu* = 0.50-0.75 in the basic—moderate rocks and Eu/Eu* = 0.16-0.75 in acid formations. The many elements spectrums are characterized with the constant of negative anomalies Nb, Та and Ti for all the types of
　　


　　 Fig. 4 The correlations of the content of petrogenic and rare elements to the content of SiO2 in the rocks of bimodal volcano-plutonic complexes of the northern framing of Mongol-Okhotsk orogenic belt.
　　


　　 Fig. 5 Concentrations of the rear elements standardized to the composition of (a) chondrite and (b) primitive mantel. Compositions of chondrite C1 and primitive mantel are brought according to the data in Ref. [25].
　　the rocks and a rather changeable anomaly Sr: for granitoids and acid volcanoes it is negative, and for the basic-moderate rocks from poorly shown negative to almost positive (Fig. 5b). The content of Ba, Rb, Th, K(Fig. 5b) is marked by positive anomalies.
　　The outline shows the geochemical characteristics of the volcano-plutonic bimodal complexes of the southern framing of the belt. 4.3 Isotope Characteristics
　　The following correlations are characteristic for the formations of bimodal complex of the northern frames of eastern section-87Sr/86Sr = 0.70592-0.70620, 0.70648-0.70773. The values of εNd(Т) have a quite small interval-(-11.77)-(-12.20) [20]. The calculated age-TNd(DM-2st) also has a quite narrow interval from 1901 to 1937 Ma [20], this might show the substantial homogeneity of the substratum of melting with a crustal component of Early Proterozoic. This is comparable with the composition of the foundation of the territory of spreading of the researched complex rocks, which is represented by Aldan-Stanovoy terrain.
　　Consequently, the isotopes data of volkanites and plutonic formations correspond to the isotope
　　


　　


　　 Fig. 6 The varieties of the concentrations of incompatible microelements of the rocks of volcano-plutonic bimodal complex of Mongol-Okhotsk orogenic belt of the northern frame. N-MORB—normal middle-oceanic mountain ridges basalts, E-MORB—enriched middle-oceanic mountain ridges basalts, OIB—basalts of ocean islands, IAB—island arcs basalts, CC—continental crust. The outline shows the geochemical characteristics of the volcano-plutonic bimodal complexes of the southern framing of the belt.
　　characteristics of the blocks of the Earth crust. The rocks of the bimodal complex break and overlap the formations of Aldan-Stanovoy terrain (Southern framing of the Siberian platform). Early Proterozoic formations are widely developed there. It is shown by the characteristic of a crust source.
　　 5. Possible Mantel Source
　　The behavior of the couples of the incompatible elements was analyzed to establish the possible mantel sources that affected the formation of the rocks of the researched complex (Fig. 6).
　　The couples of this elements-compounding materials by the differentiation (or anatexis) of magmas, that are a product of the different sources, are reflected on the logarithmic diagrams as straight lines. The lines are crossing the compositions of the sours and the content of the elements-compounding materials that accumulate in the remand magmas by the degree of their differentiation. Analyze of the correlation of the concentrations of the couples of elements-compounding materials shows the proximity of the researched rocks to the corresponding correlations of the island-arc model source. The source is enriched with the OIB, CC sources in different extend and with the source MORB in a row of cases. The participation of the crystallization differentiation that affects the accumulation of the elements can’t be denied.
　　As it is shown on Fig. 6, almost all the types of the rocks of the bimodal complexes are situated in the region of high correlations Th/Ta = 9.7-41.4 by relatively constant values of the correlation Nb/U (Fig. 6a). The affection of the differentiation process to the meaning of the correlation Th/Ta can be proved with the magma’s differentiation index that is expressed with the content of Nb and SiO2. In that case the growth of the content of Nb is accompanied by the growth of the content of SiO2 (Fig. 4h). This correlates to the growth of the magma differentiation and decrease of the melting degree of the source. As the growth of the SiO2 content occurs by the correlation Th/Ta (Fig. 6f), that significantly exceed this values for the moderate composition CC [26], it can be stated that the process is more connected with the differentiation of magmas, that form the bimodal complexes. But with all that, it can not be excluded that the MORB, IAB, CC sources may be mixed together.
　　The correlation Nb/U (Fig. 6a) for the rocks fits in the limits of the field over subductional source IAB. The growth of the Nb and SiO2 content is accompanied with the reduction of the quantity of Eu/Eu* (from 0.86-0.95 to 0.16-0.23). Supposedly, there was a differentiation, which was conducted by the growth of SiO2 with the participation of fractioning of the field spars. This is very characteristic for the subduction calk-alkali magmatic associations. It can be stated that the formation of the bimodal complexes are a result of the differentiation of magmas or a result of assimilation of upper continental crust by magmas. For all the varieties of the rocks of the complex a straight correlation Nb and SiO2 (Fig. 4h) was mainly stated. The presence of the IAB, M-MORB, E-MORB sources, with the growth of the content of Nb from IAB to E-MORB is possible for the formations with the content of SiO2 < 57 wt%. At the same time the growth of the content of Nb occurs by relatively weak changing content of SiO2, which might confirm the affection of the sources on the composition of magmas and not influence the differentiation of magmas. The straight ratio of Nb and SiO2 is more shown by the growth of the content of SiO2 (57 wt%-64 wt%). Under such conditions, two variants are possible: the differentiation of magmas and the assimilation of the continental crust by magmas.
　　By the growth of the composition of SiO2 (> 64 wt%) and Nb (to 47 ppm), the straight line of the correlation between Nb and SiO2 forms a hyperbola. It moves away from the CC composition significantly. In that case, it can be proposed that the leading role in the formation of the acid rocks of the bimodal complexes belongs to the differentiation of magmas.
　　The high values of the Ce/Y and La/Nb [27] correlation, shape difference of fractioning degree of light and heavy lanthanoids show the crust material participation in the formation of the rocks of the bimodal complex. According to the data of the ratio of Ti/Y (380-162 and 225-43), Lu/Hf (0.04-0.10 and 0.05-0.32), Smn/Ybn (mostly 3.5-5 in main—moderate and 2-4 in acid types), it might be assumed that the component of the moderate-basic composition of the bimodal complexes was formed at more significant depths and is less enriched with the crust material.
　　It is confirmed by analyze of correlations Rb/Sr: 0.01-0.32 for moderate—main rocks and 0.14-3.98 for the acid varieties. The analogical rocks of the continental crust and granite -metamorphic stratum are characterized by the ratio Rb/Sr = 0.02 and 0.32 [26]. It is stated at the diagram of the correlation Ba/K-87Sr/86Sr (Fig. 7) that the implication of a continental sedimental material in the magmatic process starts already by the formation of the rocks of moderate—main composition. All this allows us to propose that the main role in the formation of acid varieties belongs to the magma material differentiation process.
　　The demonstration of the complex’s rocks on the diagram allows us to determinate the conditions of the mantel melting source [28-30]. It shows that the initial magmas were formed more likely as a result of garnet-spinel lerzolite (melting degree 1-5%), with a content of garnet less than 4%. Also the geochemical characteristics allow us to analyze the role of the processes of a partly melting of mantle substratum and fractioning crystallization of primary magmas. It is stated that by the fractioning of the depth magmas the content of La significantly varies and the correlation La/Yb is relatively weak changing. This reflects the location of the figurative points of volcanites of main-moderate composition of bimodal complexes on the diagram of the correlation of the elements (Fig. 8a).
　　It is stated that the composition of La significantly varies by the fractioning of the depth magmas and the correlation La/Yb slowly changes. This is reflected by the location of the figurative points of the vulcanites of basic-moderate composition of the bimodal complexes on the diagram of the correlation of the elements (Fig 8a). On the correlation La/Yb-Yb diagram (Fig. 8b) they are located in the region of the melt mixture, that were formed by a partly melt of both spinel and garnet containing mica lerzolites [31].
　　But a part of the figurative points approach to a fractioning trend. The interdependence of the normalized to chondrite [25] correlations (Tb/Yb)n on the values of the correlation K/Nb [32] indicates the participation of the of spienel and garnet peridotites in
　　


　　 Fig. 7 The location of the content of the rocks of bimodal volcano-plutonic complexes is shown on the diagram 87Sr/86Sr-Ba/K [28].
　　


　　 Fig. 8 The location of the compositions of the rocks of main—middle content of bimodal complexes on the diagrams: (a) La-La/Yb, (b) La/Yb-Yb. The fractioning and mixturing curves are shown by the data of Ref. [31].
　　the formation of the rocks of the bimodal complex. It can be assumed that the formation of the rocks of the given complex occurred both in a zone of stability of garnet and at smaller depths, where the melts are balanced with spinel mantel protolites.
　　 6. Geodynamical Reconstruction
　　Despite the aged researches of the region (the eastern flank of Mongol-Okhotsk belt), the problem of the Mesozoic tectonic rebuilding’s remains to be a one of the discussion problems till now. At the end of Mesozoic this part of the region underwent not just collisional processes that were provoked by the approaching of North-Asian and Chino-Korean cratons. This was conducted by the gradual closure of the eastern flank of Mongol-Okhotsk basin [1, 33], but also was underwent by the influence of subductional processes. The processes were conducted with the fault underthrust of Pacific oceanic platform under the continental margin. The combination of two superstructures—Central-Asian and Pacific folded belts took place here at the end of Mesozoic [34].
　　The bimodal complexes have a lineal spreading along the southern and northern borders of the eastern flank of Mongol-Okhotsk belt. Their spreading is framed by the structures of Bureja-Jiamusi superterrain along the southern border on the east. Along the northern framing at the same direction they are changing into younger calk-alkali formations of Okhotsk-Chukotsky orogenic belt (Fig. 1). The bimodal complexes get a wide development on the east, in the frames of the eastern section of the belt. They are one of the components of early Mesozoic North Mongolian—West trans-Baikal riftous zone. There is a geodynamic model worked out for the zone [4, 5, 8, 15]. The essence of the model is the simultaneously existing conditions of the collisional compression and the impact of the plume at the section that is under conditions of the collisional compression [35].
　　At the diagrams, that illustrate the tectonic situations(Fig. 9), the figurative points of the acid rocks of the researched bimodal complex are concentrated in the field of the collisional (in some cases-intraplate) (Fig. 9a) or on the border of collisional and intraplate formations(Fig. 9b). The field of the basaltic island arcs with the displacement and a partial location both in the field of basaltic continental rifts and trapps (Fig. 9c) is identified for the rocks of main—moderate compositions.
　　The obtained data show that the rocks of the bimodal volcano-plutonic complexes that were formed in the northern framing of the eastern section of Mongol-Okhotsk orogenic belt by the series of geochemical characteristics and isotope data are
　　


　　 Fig. 9 Discrimination diagrams for the formation of the tectonic situations: (a) Rb-Y+Nb [36] and (b) F(c-w)2/F(i-wc)1[37]; (F(c-w)2 = -752.3SiO2 - 6537.06TiO2 - 25.6Al2O3 -928.96FeOt + 1928.07MgO - 464.21CaO - 1808.19Na2O -1272.16K2O + 8675.33P2O5 + 71073.5; F(i-wc)1 = 2432.42SiO2+ 7900.33TiO2 + 2512.12Al2O3 + 1380.23FeOt + 2616.55MgO+ 3480.51CaO + 3045.39Na2O + 645.91K2O - 241285.5) for the salic formations; (c) Dx/Dy [38]; (Dx = 176.94SiO2 -1217.77TiO2 + 154.51Al2O3 - 63.1FeOt - 15.69MgO + 372.43CaO + 104.41Na2O - 19.96K2O - 873.69P2O5 -11721.488; Dy = 94.39SiO2 - 103.3TiO2 + 417.98Al2O3 -55.63FeOt + 57.61MgO + 118.42CaO + 502.02Na2O + 6.37K2O + 415.31P2O5 - 13724.66) for the main rocks. The fields of the basalts: I—island arcs, II—traps, III—continental rifts.
　　comparable with the intracontinental formations of central Asia. It was shown that the participation of the mantel source was possible in the formation of the in the formation of the rocks of Late Paleozoic—early Mesozoic bimodal complexes of the Central Asia [4, 6], its characteristic feature is a high value of the correlation Zr/Hf (38-50). For the rocks of the complex the value Zr/Hf = 25-66. This allows to assume that the formation of the bimodal volcano-plutonic complex of the northern framing of Mongol-Okhotsk belt is identical to the conditions, that are proposed in the geodynamical model for the bimodal complexes not only southern framing of the eastern section of the belt[11, 12, 35], but also for the western section of Mongol-Okhotsk orogenic belt [3, 5, 8, 15].
　　 7. Conclusion
　　The bimodal volcano-plutonic complex of subalkali—normal petrochemical series was formed along the northern border of the eastern flank of Mongol-Okhotsk orogenic belt in the interval of 119-97 Ma.
　　The protolith for basaltiform complexes were both spienel and garnet peridotites. The geochemical characteristics of basaltiforms of the complexes are following: low content of Ta and Nb and hightened content of K and Pb. They show the formation of the rocks in the situation of convergent margins of the plates. For all the varieties of the rocks of the bimodal complex the enrichment with inconsistent elements by the lowered content of Ta, Nb, Ti and higher presence of Ba, Sr, К and Pb is characteristic. As it was mentioned by A.A. Voroncov et al. [8], the features are characteristic to all late Paleosozoic—early Mesozoic intraplate rocks of Central Asian rifts system, which development is connected with the North Asian and Chino-Korean continents collision and an overlapping of the plume source with the continental lithosphere.
　　Therefore, it can be stated that the formation of the bimodal complex in the northern frame of the eastern section of Mongol-Okhotsk orogenic belt in the end of early Cretaceous accompanied the North Asian and North Chinese continents collision with a possible influence of the plume source.
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