Petrography, Geochemical Features and Absolute Dating of the Mesozoic Igneous Rocks of Medvedev and Taezhniy Massifs (Southeast Russia, Aldan Shield)

: The paper presents the results of the petrographic and geochemical studies of igneous rocks of the Medvedev and Taezhniy massifs, including their ﬁrst absolute dating. The massifs are located in central Nimnyr block of the n shield within the Leglier ore cluster of the Evotinskiy ore district (Southeast Russia, Aldan Shield). For the ﬁrst time, the three-phase structure of the Medvedev massif has been deﬁned, as observed in our expedition and petrographic studies. Rocks from the three phases of the Medvedev massif include quartz syenites, syenites, and monzonites, and rocks from the two phases of the Taezhniy massif include quartz monzonites and syenites. Geochemically, the rocks are close to volcanic island arcs, the formation of which was related by subducted oceanic crust of the Mongol–Okhotsk Ocean. The deﬁned duality of the geochemical compositions of the igneous rocks of the massifs may be due to the presence of both mantle and crustal sources; however, it is most likely that these rocks resulted from the melting of a mixed mantle source or the latter was contaminated by the crust with further differentiation of melts in intermediate crust chambers. Additionally, geochemical characteristics suggest that the analyzed rocks are close to latite and shoshonite derivatives and can be considered as part of the monzonite–syenite formation type. The ﬁrst identiﬁed periods of formation of igneous rocks in the Medvedev massif are 122.0–118.0 Ma and Taezhniy 117.5–114.5 Ma, which correspond to the Early Cretaceous (Aptian).


Introduction
The study of the Mesozoic magmatism of the Aldan Shield began in the 20-30 s of the 20th century with the works of Yu. A. Bilibin, who described that the rocks in the Aldan complex formed in one volcanic cycle. In his works Yu. A. Bilibin refuted the hypothesis about the origin of the gold ore objects of the Aldan Shield in connection with Precambrian alaskite granites and proved their clear connection with Mesozoic alkaline rocks [1,2]. Recently, many authors have noted that the formation of alkaline rocks is accompanied by the appearance of the largest gold deposits [3][4][5][6][7][8][9]. Sillitoe [4] notes that about 20% of large gold deposits are associated with complexes of the shoshonitic and alkaline series. However, the rocks of these series are significantly inferior in volume to most of the main types of igneous rocks in terms of the scale of occurrence. Problems regarding the genesis and metallogenic specialization of multi-phase igneous formations in ore districts have been a topic of continuing debate [10][11][12][13][14][15]. One of the key points in solving these problems is a comprehensive study of each object, including their petrographic and geochemical features and age data in relation to the conditions of formation [12,13,16].
Alkaline rocks are formed in conditions of continental crust and island arcs associated with subduction processes and are known in many parts of the world. This paper presents the result of the study of the alkaline rocks of the Medvedev and Taezhniy massifs located in the central part of the Aldan Shield. In the opinion of the authors the formation of these  [17]. Terranes: WA-West Aldan; EBT-Batomga; ANM-Nimnyr; CG-Chogar; AST-Sutam; EUC-Uchur; TN-Tyndin; zones of tectonic mélange: am-Amga, kl-Kalar, td-Tyrkanda; faults: dj-Dzheltulaksky, ts-Taksakandin. (B) Geological scheme of location of studied area (according to [18,26] with changes and additions of the authors).
The Medvedev and Taezhniy massifs intrude a Precambrian crystalline sequence ( Figure 1B), which is a package of alternating, consonant, subparallel, compositionally contrasting linear sheet-like bodies composed of hypersthene and aluminous gneisses of the Nimnyr suite, rocks of the Medvedev Precambrian complex and subalkaline and normal biotite granites [17,27]. According to previous data [26], the massif is represented by augite-hornblende and hornblende syenite porphyry. During our expedition, we first uncovered that the massif has a three-phase structure that involves syenites, pyroxene-amphibole syenites and quartz monzonites.
The Taezhniy massif at the current level of the erosional cut has the shape of an irregular ellipsoid, with its long axis facing the northeast (Figure 2). Our expedition was the first to define its two-phase structure represented by quartz monzonite and syenites (Figure 2). The contacts found between rock differences are even and clear. Rocks of the massif intrude Precambrian granites ( Figure 1B) and are intruded by bostonite and vogesite dikes (for the detailed description of the geology of small bodies associated with the Taezhniy massif see [26]).
The Medvedev and Taezhniy massifs intrude a Precambrian crystalline sequence ( Figure 1B), which is a package of alternating, consonant, subparallel, compositionally contrasting linear sheet-like bodies composed of hypersthene and aluminous gneisses of the Nimnyr suite, rocks of the Medvedev Precambrian complex and subalkaline and normal biotite granites [17,27]. According to previous data [26], the massif is represented by augitehornblende and hornblende syenite porphyry. During our expedition, we first uncovered that the massif has a three-phase structure that involves syenites, pyroxene-amphibole syenites and quartz monzonites.
The Taezhniy massif at the current level of the erosional cut has the shape of an irregular ellipsoid, with its long axis facing the northeast (Figure 2). Our expedition was the first to define its two-phase structure represented by quartz monzonite and syenites ( Figure 2). The contacts found between rock differences are even and clear. Rocks of the massif intrude Precambrian granites ( Figure 1B) and are intruded by bostonite and vogesite dikes (for the detailed description of the geology of small bodies associated with the Taezhniy massif see [26]). feldspar and plagioclase ( Figure 2D), and the rocks also demonstrate elements of monzonite structure (idiomorphism of plagioclase relative to K-feldspar). The fine-grained groundmass lacks diagnostic evidence. The rocks of the phase show traces of secondary changes. The petrographic composition (hereafter, the composition is described in detail in [18]) demonstrates a predominance of K-feldspar 60% over plagioclase 30%, and in the dark-colored group of minerals, a predominance of amphibole 8% over pyroxene 1% is seen and the ore mineral content makes up to 1 %. The rocks of the second phase occupy most of the area of the massif ( Figure 1B) and are represented by gray, gray-pinkish pyroxene-amphibole syenites with hypidiomorphic granular and porphyry structure. The rocks have a massive texture. The formations of the phase have the numerous xenoliths of crystal schists, Archean microcline granites, as well as rocks of the previous phase ( Figure 2E). The rocks contain a significant number of miaroles of various sizes (from 20 cm to 1 m in diameter) ( Figure 2F). The composition is dominated by K-feldspar 60%, with 20% of plagioclase, an approximately equal amount

Materials and Methods
We collected samples for the study during the expedition in 2018 to 2020. Petrographic composition was studied using a polarized light microscope MIN The analysis of the Rb-Sr isotopic systems of rocks was performed using the isotopic dilution method to determine the concentrations of rubidium and strontium. To achieve this, weighed amounts of mixed indicators 85 Rb− 84 Sr were added to pre-ground sample portions. Prepared samples were then decomposed in a mixture of nitric and hydrofluoric acids. The isolation of rubidium and strontium for isotopic analysis was carried out by cation exchange chromatography on AG50W-X8 resin (Bio-Rad Laboratories Inc., Hercules, CA, USA). Isotope dilution analysis was applied to determine the quantity of rubidium and strontium in Rb-Sr isotopic systems of rocks using a multiple collector mass spectrometer TRITON (LTD Thermotest Finnigan MAT, Bremen, Germany) in static mode. The 88 Sr/ 86 Sr = 8.37521 ratios were normalized to correct for Sr-isotope fractionation. The ratios were normalized to 87 Sr/ 86 Sr = 0.71025 for NBS 987. Rb and Sr determinations were accurate to 0.5%. Blanks were 30pg for Rb and 30pg for Sr.
U-Pb zircon dating was performed using SIMS SHRIMP-IIe (LTD ASI, FYSHWICK, ACT, Australia) (Secondary Ion Mass-Spectrometry by a Sensitive High-Resolution Ion Micro Probe) at the Center for Isotope Studies of the A.P. Karpinsky Russian Geological Research Institute (FGBU "VSEGEI"). Next, the sequence of dating methods used is described.
Representative zircon grains were analyzed along with reference material TEMORA and 91500. Analyses were recorded in the scanning electron microscope CamScan MX2500 (CamScan Electron Optics Ltd, Cambridge, United Kingdom) with the CLI/QUA2 system to obtain CL and BSE images. The operating distance was 25-28 mm, accelerating voltage 20 kV, and the nearly full beam current into the Faraday cup was 4-6 nA. Current value was varied to obtain the maximum contrast of CL images and to minimize the surface corrosion of the disc resulting from local heating.
U-Pb ratios were measured following the method adapted in A.P. Karpinsky Russian Geological Research Institute (FGBU "VSEGEI") [28], as described in [29]. The intensity of the primary molecular oxygen beam was 4nA, and the size of the sampling crater was 20 × 25 µm with depth up to 2 µm. The data were processed using SQUID software (Berkeley Geochronology Center, Berkeley, CA, USA) [30]. The U-Pb ratios were normalized to the value 0.0668 of the standard zircon TEMORA, which corresponds to its age of 416.75 ± 0.24 Ma [31]. The zircon standard 91500 with U content 81.2 ppm and 206 Pb/ 238 U age of 1062 Ma [32] was used as a concentration standard. The Raster one-minute cleaning of a rectangular (50 × 65 µm) section of mineral prior to dating allowed the minimization of surface contamination.
Individual analyses (of ratios and ages) are reported with 1σ error, and the calculated ages, including concordant, indicate 2σ errors. Ahrens-Wetherill diagrams with Concordia [33] were plotted using ISOPLOT/EX (Berkeley Geochronology Center, Berkeley, CA, USA) [34]. Non-radiogenic plumbum correction was applied based on the measured 204 Pb and the modern plumbum isotopic composition of the Stacy-Kramers model [35].

Petrography
First phase syenites of the Medvedev massif intrusion (Figure 2A,B) are least common and come as light gray pinkish leucocratic varieties with a large-porphyry structure, with K-feldspar and plagioclase phenocrysts being up to 3-5 cm. The rocks show a massive texture. The rocks of the first phase of intrusion have xenoliths of host rocks ( Figure 2B). Numerous blocks of xenogenic crystalline schists were found in the central part. Contacts with subsequent phases are clear, sharp and without facies transitions ( Figure 2C). The crystal optical porphyry structure resulted from the presence of phenocrysts of K-feldspar and plagioclase ( Figure 2D), and the rocks also demonstrate elements of monzonite structure (idiomorphism of plagioclase relative to K-feldspar). The fine-grained groundmass lacks diagnostic evidence. The rocks of the phase show traces of secondary changes. The petrographic composition (hereafter, the composition is described in detail in [18]) demonstrates a predominance of K-feldspar 60% over plagioclase 30%, and in the dark-colored group of minerals, a predominance of amphibole 8% over pyroxene 1% is seen and the ore mineral content makes up to 1%.
The rocks of the second phase occupy most of the area of the massif ( Figure 1B) and are represented by gray, gray-pinkish pyroxene-amphibole syenites with hypidiomorphic granular and porphyry structure. The rocks have a massive texture. The formations of the phase have the numerous xenoliths of crystal schists, Archean microcline granites, as well as rocks of the previous phase ( Figure 2E). The rocks contain a significant number of miaroles of various sizes (from 20 cm to 1 m in diameter) ( Figure 2F). The composition is dominated by K-feldspar 60%, with 20% of plagioclase, an approximately equal amount of basic dark-colored minerals, pyroxene 6% and amphibole 8%, and the content of both biotite and quartz is significantly less than 1%, while the content of ore minerals is up to 4% ( Figure 2G). The geological age of the rocks was determined from clear contacts with other phases of the massif, as well as from the xenoliths of the first phase rocks.
The third phase rocks of the massif are represented by gray monzonites ( Figure 2H). The clear idiomorphism of plagioclase relative to K-feldspar as well as the presence of idiomorphic plagioclase crystals included in K-feldspar grains determines the monzonite structure ( Figure 2I). The rocks have a massive texture. The rocks of the phase as well as the pyroxene-amphibole syenites show miaroles, albeit in a smaller amount. The third phase rocks of the intrusion of the massif are characterized by the predominance of plagioclase up to 50% over K-feldspar with up to 25%. Mafic minerals are not distributed uniformly, with a significant predominance of 16% hornblende over 2% pyroxene. The main fine-grained mass consists of hard-to-diagnose feldspar and 2% fine biotite. The rocks also contain up to 2% of quartz. The amount of ore mineral is up to 3%.
The least abundant rocks of the Taezhniy massif ( Figures 1B and 3A) are dark gray monzonites ( Figure 3B), which form a small outcrop in its northeastern part and represent the first phase of intrusion. The porphyry structure results from the presence of K-feldspar and plagioclase phenocrysts up to 5 mm in size ( Figure 3B). The rocks have a massive texture. The groundmass is fine-grained ( Figure 3C). The mineral composition features a predominance of plagioclase 60% over KFS 20%, amphibole 8% over pyroxene 2%, quartz content at 6%, apatite and zircon content of 2% and 2% ore minerals. The rocks of the phase show traces of secondary changes (pelitization, seritization and chloritization).
The light gray essential syenites of the second phase ( Figure 3D) are the most widespread over the area of the massif ( Figure 1B). They are characterized by a hypidiomorphic granular structure and a massive texture ( Figure 3E). The petrographic composition of the least altered varieties defined the predominance of K-feldspar 60% over plagioclase 30%, amphibole 3% over pyroxene 1%, biotite content 1% and quartz up to 5%. The rocks of the phase are characterized by the presence of trace of secondary change-pelitization, sericitization and chloritization. In the southwestern part of the massif, the fragments of metasomatites along Archean granites contain single druses and brushes of quartz, the rocks of the phase are significantly quartz-flooded. zonites ( Figure 3B), which form a small outcrop in its northeastern part and represent the first phase of intrusion. The porphyry structure results from the presence of K-feldspar and plagioclase phenocrysts up to 5 mm in size ( Figure 3B). The rocks have a massive texture. The groundmass is fine-grained ( Figure 3C). The mineral composition features a predominance of plagioclase 60% over KFS 20%, amphibole 8% over pyroxene 2%, quartz content at 6%, apatite and zircon content of 2% and 2% ore minerals. The rocks of the phase show traces of secondary changes (pelitization, seritization and chloritization).
The third phase monzonites of the massif in terms of SiO 2 content (61.13%-64.43%) are close to syenites of the first phase but differ in relatively high content of Al 2 O 3 15.09%-16.81%, (Table A1) as well as low values of MnO 0.08%-0.13%, MgO 0.05%-3.33% and alkalinity (Na 2 O + K 2 O) = 7.87%-8.89%, where Na 2 O 4.25%-4.99% dominates over K 2 O 3.46%-4.25% (Table A1). The Na 2 O/K 2 O > 1 ratio corresponds with potassium-sodium alkaline, high-aluminous al' = 1.94 rocks with AG coefficient 0.72 (Table A1). According to the diagram (Na 2 O + K 2 O)-SiO 2 [37], the rocks of this phase are classified as monzonites ( Figure 4A). In the R 1 -R 2 ratio [38] the composition points almost fully come within the field of subalkaline quartz monzonites, with most altered residual partially falling into the field of monzodiorites and tonalities ( Figure 4B). Their K 2 O-SiO 2 ratio [39] corresponds to the high-potassic, calcareous-alkaline petrochemical series ( Figure 4C). The geochemical composition of monzonites, in contrast to rocks of earlier phases, is characterized by the lower content of chalcophile elements (Table A2). At the same time, in contrast to previous rocks, they demonstrate a somewhat higher content of lithophile elements and lower Cr, V, Ba and Rb (Table A2). In terms of Rb (61.20-91.80 ppm), Ba (1500-2000 ppm), Sr (780-1060 ppm), Zr (120-190 ppm), Nb (5.32-10.80 ppm) and Ni (7.99-14 ppm) content, they are closest to latite derivatives [40]. The REE pattern of quartz monzonites is close to that of the previous phases of the massif but differs in the lower concentration of both heavy and light elements (Table A3, Figure 4D). The ratios La/Yb = 13.97-17.10 and Ce/Yb = 27.16-34.61 (Table A3) in quartz monzonites are close to syenites of the second phase of the massif and belong to the latite series [40].
The least altered residual of syenites of the late phase of the Taezhniy massif, unlike the previous rocks, are characterized by a somewhat higher content of SiO 2 and Al 2 O 3 and a low content of TiO 2 , Fe 2 O 3 , FeO, MnO, MgO and CaO (Table A1). The rocks of the phase differ from the formations of the previous phase by high total alkalinity (Na 2 O + K 2 O = 10%-13.67%), which makes them subalkaline [36], where K 2 O (4.91%-8.12%) dominates over Na 2 O (5.09%-6.28%) ( Table A1). The rocks are potassium alkaline Na 2 O/K 2 O < 1, belong to the high-aluminous series al' = 2.51-5.70 with the AG coefficient 0.86-1.12 (Table A1). According to the diagram (Na 2 O + K 2 O)-SiO 2 [37], the rocks belong to syenites ( Figure 4A). In the R 1 -R 2 diagram [38], the composition points form a secant trend in the field of subalkaline rocks from syenites to quartz syenites ( Figure 4B). According to the K 2 O-SiO 2 ratio [39], syenites belong to the shoshonitic petrochemical series ( Figure 4C). Rocks of the phase are distinguished from the earlier syenites by the content of both chalcophile and lithophile elements (Table A2). By the content of Rb (90-126 ppm), Ba (450-3000 ppm), Sr (230-1000 ppm), Zr (80.2-190 ppm), Nb (5.9-11.8 ppm) and Co (3.2-7.8 ppm) content, they are closest to latite derivatives [40]. The REE pattern of syenites is close to that of the first phase syenites but with lower concentrations of all elements (Table A3, Figure 4D). The ratios La/Yb = 13.50-17.14 and Ce/Yb = 25.83-29.67 (Table A3) are close to those of the latite derivatives [40].

Geochronology
The obtained concordant U-Pb age of syenites (calculated from six zircon points) is 122.0 ± 5.2 Ma (MSWD = 5.9, Figure 5A). The studied zircons are characterized by the low values of the ratio 232 Th/ 238 U from 0.15 to 0.40 (average 0.27), and the U content ranges from 170 to 1052 (average 590) and Th 65-148 (124) ppm, respectively (Table A4). The obtained concordant U-Pb age of quartz monzonites (according to six zircon points) was 118.0 ± 1.6 Ma (MSWD = 0.016, Figure 5B). The studied zircons are characterized by significantly lower values of the 232 Th/ 238 U ratio from 0.01 to 0.02 (on average 0.01), as compared to those from the first phase rocks of the Medvedev massif. The U content is also much higher and ranges from 447 to 1237 (average 737) with low Th 4-20 (10) ppm (Table A4).
The obtained concordant U-Pb age of quartz monzonites of the Taezhniy massif based on six zircon points was 117.5 ± 1.5 Ma (MSWD = 0.58 Figure 5C). The studied zircons are characterized by higher values of the 232 Th/ 238 U ratio from 0.06 to 0.41 (0.25 on The obtained concordant U-Pb age of quartz monzonites (according to six zircon points) was 118.0 ± 1.6 Ma (MSWD = 0.016, Figure 5B). The studied zircons are characterized by significantly lower values of the 232 Th/ 238 U ratio from 0.01 to 0.02 (on average 0.01), as compared to those from the first phase rocks of the Medvedev massif. The U content is also much higher and ranges from 447 to 1237 (average 737) with low Th 4-20 (10) ppm (Table A4).
The obtained concordant U-Pb age of quartz monzonites of the Taezhniy massif based on six zircon points was 117.5 ± 1.5 Ma (MSWD = 0.58 Figure 5C). The studied zircons are characterized by higher values of the 232 Th/ 238 U ratio from 0.06 to 0.41 (0.25 on average) relative to zircons from the rocks of the Medvedev massif, and the content of U and Th is much higher compared to the latter at 304-1481 (average 825.20) and 38-345 (177.66) ppm (Table A4).
The obtained concordant U-Pb age of syenites (based on seven zircon points) was 120.8 ± 2.6 Ma (MSWD = 0.064, Figure 5D). The studied zircons are characterized by relatively low values of the 232 Th/ 238 U ratio from 0.005 to 0.33 (on average 0.17) in relation to the previous rocks (except for zircons of the 3rd phase of the Medvedev massif); the U content in them is also much higher, ranging from 486.29 to 1925.8 (average 1123.4), with Th 14.5-563.07 (255.17) ppm (Table A4). Despite the earlier values of the calculated concordant absolute age of the second phase rocks, the latter were identified as later formations according to geological relationships, and numerous xenoliths of quartz monzonites of the previous phase contained in the rocks ( Figure 3F,G) were determined.

Formation Conditions
According to the Rb/Sr-SiO 2 [43] and Al 2 O 3 /(Na 2 O + K 2 O) − Al 2 O 3 /(CaO + Na 2 O + K 2 O) [44] ratios, all rocks of the Medvedev massif correspond to rocks that form from a parent magma chamber of the oceanic crust ( Figure 6A,B). According to the discrimination diagram [45] ( Figure 6C-E) as well as ASI Al 2 O 3 /(CaO + Na 2 O + K 2 O) < 1.05 (Table A1), the geodynamic setting of the magma forming phases of the Medvedev massif defines them as volcanic arcs [46], while the ratio Zr/Al 2 O3-TiO 2 /Al 2 O 3 defines them as oceanic and continental arcs [47] (Figure 6F).   The ratio Sr-Rb/Sr [48] corresponds with the trend of differentiation in volcanic series of continental rift zones ( Figure 7A). The parameters of the primary strontium isotope ratio I 0 = 0.708 (Table A5) for the first phase of the intrusion of Medvedev massif are typical for mid-ocean ridge basalts and some volcanic ocean islands with a lower crust source [49]. It is noteworthy that this parameter decreases from earlier leucocratic to later rocks of the massif.  (Table A2) and K/Rb-Rb, Sr-Rb/Sr (Figure 6, 7) in quartz syenites of the first phase of Medvedev massif are characteristic to rocks formed from a mantle source [48,50]. The Nb/La = 0.25-0.27 ratio, as well as Th/U within the range of 3.20-3.93 (Table A3), shows the degree of magma contamination by continental crust [51,52]. A high Y/Nb = 2.33-2.43 > 1.2 ratio (Table A3) (in general, for all rocks of the massif) may indicate the contribution of both crustal and mantle sources [53]. The first phase rocks are characterized by a weak negative Eu-anomaly Eu/Eu* = 0.90-0.91 (Table A3), which may suggest the formation from primary mantle magmas during the fractional crystallization of dark-colored rock-forming minerals [53].
The ratios of К/Rb = 171.41-229,58, Ba/Rb = 4.81-29.63, Rb/Sr up to 0.12 (Table A2) and K/Rb-Rb, Sr-Rb/Sr (Figure 6, 7) in second phase syenites of the massif correspond to mantle rocks [48,50]. Whereas the parameters of the primary ratio of strontium isotopes I0 = 0.707 (Table A5) are for the rocks of the second stage of intrusion of the Medvedev massif, it is typical for rocks with a lower crustal source [49]. The values of Nb/La = 0.23-0.30 and Th/U = 4.31-5.89 (Table A3) may indicate the crust contamination of primary mantle magmas [52,54]. A significant increase in the ratio Eu/Eu* for 0.92-0.95 is close or equivalent to chondrite and indicates melt formation during fractional crystallization of only dark-colored rock-forming minerals. It also suggests deep differentiation [53].
The ratios of К/Rb = 179.95-251.62, Ba/Rb = 18.52-27.78, Rb/Sr ratios up to 0.1 (Table  A2), K/Rb-Rb ratios and Sr-Rb/Sr (Figure 6, 7) in quartz monzonites of the third phase of the Medvedev massif have mantle marks [48,50]. On the contrary, the parameters of the primary ratio of strontium isotopes I0 = 0.706 (Table A5) are typical for rocks formed from a lower crustal source [49]. Low values of Nb/La = 0.24-0.33, as well as high values of Th/U = 3.69-4.43 in quartz monzonites (Table A3), as well as in other rocks of the massif, may indicate the crust contamination of primary magmas [52,54]. Relative europium content Eu/Eu* 0.88-0.95 is close to the mantle [53], which may also indicate melt formation during fractional crystallization of only dark-colored rock-forming minerals, suggesting deep differentiation [53].
According to the Rb/Sr-SiO2 ratios [43], the rocks of the Taezhniy massif correspond to formations from parent magma chambers of the oceanic crust. According to the ratio  (Table A2) and K/Rb-Rb, Sr-Rb/Sr (Figures 6 and 7) in quartz syenites of the first phase of Medvedev massif are characteristic to rocks formed from a mantle source [48,50]. The Nb/La = 0.25-0.27 ratio, as well as Th/U within the range of 3.20-3.93 (Table A3), shows the degree of magma contamination by continental crust [51,52]. A high Y/Nb = 2.33-2.43 > 1.2 ratio (Table A3) (in general, for all rocks of the massif) may indicate the contribution of both crustal and mantle sources [53]. The first phase rocks are characterized by a weak negative Eu-anomaly Eu/Eu* = 0.90-0.91 (Table A3), which may suggest the formation from primary mantle magmas during the fractional crystallization of dark-colored rock-forming minerals [53].
The ratios of K/Rb = 171.41-229,58, Ba/Rb = 4.81-29.63, Rb/Sr up to 0.12 (Table A2) and K/Rb-Rb, Sr-Rb/Sr (Figures 6 and 7) in second phase syenites of the massif correspond to mantle rocks [48,50]. Whereas the parameters of the primary ratio of strontium isotopes I 0 = 0.707 (Table A5) are for the rocks of the second stage of intrusion of the Medvedev massif, it is typical for rocks with a lower crustal source [49]. The values of Nb/La = 0.23-0.30 and Th/U = 4.31-5.89 (Table A3) may indicate the crust contamination of primary mantle magmas [52,54]. A significant increase in the ratio Eu/Eu* for 0.92-0.95 is close or equivalent to chondrite and indicates melt formation during fractional crystallization of only dark-colored rock-forming minerals. It also suggests deep differentiation [53].
The ratios of K/Rb = 179.95-251.62, Ba/Rb = 18.52-27.78, Rb/Sr ratios up to 0.1 (Table A2), K/Rb-Rb ratios and Sr-Rb/Sr (Figures 6 and 7) in quartz monzonites of the third phase of the Medvedev massif have mantle marks [48,50]. On the contrary, the parameters of the primary ratio of strontium isotopes I 0 = 0.706 (Table A5) are typical for rocks formed from a lower crustal source [49]. Low values of Nb/La = 0.24-0.33, as well as high values of Th/U = 3.69-4.43 in quartz monzonites (Table A3), as well as in other rocks of the massif, may indicate the crust contamination of primary magmas [52,54]. Relative europium content Eu/Eu* 0.88-0.95 is close to the mantle [53], which may also indicate melt formation during fractional crystallization of only dark-colored rock-forming minerals, suggesting deep differentiation [53].
According to the Rb/Sr-SiO 2 ratios [43], the rocks of the Taezhniy massif correspond to formations from parent magma chambers of the oceanic crust. According to the ratio Al 2 O 3 /(Na 2 O + K 2 O) − Al 2 O 3 /(CaO + Na 2 O + K 2 O) [44], the rocks of the first phase belong to oceanic plagiogranites, while the rocks of the second phase are related to rifts. The geodynamic position the same as for the rocks of the Medvedev massif, determined by the discrimination diagrams [45] (Figure 6), and the ASI Al 2 O 3 /(CaO + Na 2 O + K 2 O) < 1.05, defines them as formations of volcanic arcs. The ratio Zr/Al 2 O 3 -TiO 2 /Al 2 O 3 attributed them to oceanic and continental arcs [47] ( Figure 6F). The Sr-Rb/Sr ratio [48] corresponds to the trend of differentiation of volcanic series in the rift zones of the continents ( Figure 7A).
The ratios of K/Rb = 271.13-559.74, Ba/Rb up to 30.56 and Rb/Sr = 0.06-0.09 (Table A2) and the ratios K/Rb -Rb and Sr -Rb/Sr ( Figure 7A,B) in quartz monzonites of the Taezhniy massif are characteristic to rocks formed from a mantle source [48,50]. The parameters of the primary ratio of strontium isotopes I 0 = 0.706 (Table A5) are typical for rocks formed from a lower crustal source [49]. Low values of Nb/La = 0.27-0.32, as well as values of Th/U = 2.19-2.66 the rocks of the phase (Table A3) may indicate magma crust contamination [52,54]. High Y/Nb = 2.31-2.43 (Table A3) may indicate the contribution of both crustal and mantle sources [53]. The rocks of the massif phases are characterized by the presence of a weak negative Eu-anomaly Eu/Eu* = 0.88-0.91, which may indicate the formation of the parent melt during the fractional crystallization of dark-colored rock-forming minerals [53].
The values of ratios K/Rb = 359-734.3, Ba/Rb up to 23, Rb/Sr = 0.13-0.16 (Table A2) and ratios K/Rb -Rb, Sr -Rb /Sr ( Figure 7A,B) in syenites of the second phase of the massif correspond to mantle rocks [48,50]. The parameters of the primary ratio of strontium isotopes I 0 = 0.707 (Table A5) for the rocks of the second stage of intrusion of the Taezhniy massif are typical for rocks with a lower crustal source [49]. The values of Nb/La = 0.42-0.51 and Th/U = 3.66-4.69 (Table A3) may suggest the crust contamination of primary mantle magmas [52,54]. A significantly higher relative content of europium Eu/Eu* = 0.85-0.95 compared to rocks of the previous phase is close to mantle [53]. It may also indicate melt formation during the fractional crystallization of only dark-colored rock-forming minerals and point to deep differentiation [53].

Petrography of Rocks of Medvedev Massif
The three phases of intrusion of rocks with distinctive contacts between the latter, differing in composition and structure were identified for the first time at expedition work and the crystal optical studies of igneous rocks of the Medvedev massif: I phase quartz syenites, II phase pyroxene-amphibole syenites and III phase quartz monzonites.

Geochemical Composition of Rocks of Medvedev Massif
According to geochemical composition next groups of rocks defined in Medvedev massif: quartz syenites (I phase), syenites (II phase) of shoshonite series and quartz monzonites (III phase) of high potassium calc-alkaline petrochemical series. The first and second phase rocks of Medvedev massif (as almost all studied rocks) belonging to the shoshonite series has important metallogeny significance, since that series is sometimes associated with various types of mineralization: industrial molybdenum, polymetallic, gold-polymetallic and gold mineralization, as well as occurrences of arsenic, antimony and other metals [8,[55][56][57][58][59].
The total content of alkalis (Na 2 O + K 2 O) < 12 in the igneous formations of the Medvedev massif corresponds to subalkaline rocks (I) 8.15%-11.88%, (II) 7.72%-9. The rocks of the Medvedev massif are geochemically specialized to Rb, Ba, Sr, B, Cr, V, Nb and Sc (Table A2), i.e., to lithophile elements. The content of chalcophile Cu, Sn, Zn, Pb, Ge and siderophile Ni and Co elements is significantly lower. These indicators differ in the contents in phases of the massif. High content in all rocks of massif Rb, Ba, Sr, Zr, Nb and Co correspond to the latite series. REE distribution with smooth negative incline shows the enrichment of cerium (La, Ce, Nd, Pr, Sm, Eu) and a depletion of yttrium (Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) elements. It is worth noting that the highest concentration of REE in quartz syenites ∑REE 180.77 decreases to monzonites ∑114.01.

Geochronology of Rocks of Medvedev Massif
The concordant period of the evolution of the Medvedev massif, dated by the SHRIMP 2 method, is 122.0-118.0 Ma, which corresponds quite well with geological observations.

Formation Conditions of Rocks of Medvedev Massif
Opinions vary on the role of subducted oceanic crust of the Mongol-Okhotsk Ocean in the formation of parent magma chambers of the Aldan Mesozoic igneous complex [15,17,[60][61][62]. The potassium alkaline Mesozoic magmatism of the Aldan Shield is of interest in terms of the genesis of intratelluric magmas and their ore content [15,60,62,63]. E.P. Maximov et al. [15,60] suggested that the subducted oceanic crust of the Mongol-Okhotsk Ocean participated in the formation of all parent magma chambers of the Aldan Complex. This point of view is shared by A.Y. Kochetkov and Lazebnik [61], who emphasized the geochemical affinity of Mesozoic magmatic formations to the vulcanites of island arcs. V.A. Kononova et al. [62] hold a different viewpoint based on the geochemical analysis of potassium rocks of Central Aldan and certain similarities between them and volcanic products of continental margin and island arcs (Italy, Indonesia) [62] and their distinction from rocks of ancient platforms (Africa) [62]. The geochemical data obtained for Mesozoic igneous rocks of the Evotinskiy ore district by and large support this position.
The ratios of K/Rb, Rb/Sr and Ba/Rb and K/Rb-Rb and Sr-Rb/Sr elements designate all rocks of the Medvedev massif as close to mantle rocks. A high ratio of Y/Nb > 1.2 may, in general, indicate the contribution of both crustal and mantle sources. A lower crust source is attested by the ratios of Sr primary isotopes I 0 = 0.706-0.708 (Table A5) [49] as well as low values of Nb/La < 1 characteristic for all analyzed rocks that indicate a negative niobium (tantalum) anomaly and high values of Th/U > 2 as a an apparent sign of crust contamination [52,54]. In contrast, the relative content of europium Eu/Eu*≈1 suggests that the rocks of the massif are close to mantle and that melt formed during fractional crystallization of only dark-colored rock-forming minerals [53].
It should be noted that the increased values of the elemental U/Th ratio from the first to the third phase, with maximum values in the second phase of the Medvedev massif, and some chalcophile elements may also indicate an increased filling fluids in the rocks [65]. The latter is apparently associated with degassing of the second and third phases of the massif followed by the circulation of fluids in the system [20,66], which can also be indicated by a decrease in pressure during the crystallization of pyroxenes from the first phase to the last one [67], degassing can be indirectly confirmed by the presence of miarols in the rocks of the second and third phases.

Petrography of Rocks of Taezhniy Massif
In the Taezhny massif during field and crystal-optical studies, a two-phase structure was identified for the first time: phase I quartz monzonites and phase II syenites.

Geochemical Composition of Rocks of Taezhniy Massif
According to geochemical composition next groups of rocks defined in Taezhniy massif: quartz monzonites (phase I) with ambiguous geochemical characteristics, in which they are close to both high-potassium calc-alkaline and shoshonite petrochemical series, and syenites (phase II) of the shoshonite petrochemical series.
The total content of alkalis (Na 2 O + K 2 O) < 12 in the igneous formations of the Taezhny massif correspond to subalkaline rocks: the first phase is 8.64%-10.35% and the rocks of the second phase are 10%-13.67%. All rocks of the Taezhniy massif are characterized by the potassium type of alkalinity Na 2 O/K 2 O < 1.
Geochemically, the rocks of the Taezhniy massif are specialized for the same number of elements as the rocks of the Medvedev massif, i.e., both for lithophile Rb, Ba, Sr, B, Cr, V, Nb and Sc (Table A2)

Geochronology of Rocks of Taezhniy Massif
The dating of rocks of the Taezhniy massif is not quite clear. The rocks of the first phase (quartz monzonites) dated by the SHRIMP 2 method are younger, with a mark of a concordant age of 117.5 ± 1.5 Ma, while the rocks of the second phase (syenite), which were later geologically observed, have a concordant age of 120.8 ± 2.6 Ma. As previously noted, the presence of older points in zircons is probably associated with the presence of quartz monzonite xenoliths in these rocks, the zircons from which were dated and taken into account when compiling Concordia. The obtained minimum age value of 114.5 ± 1.9 Ma for zircons from rocks of the second phase (syenites) is the closest to reality, while in the rocks of the first phase (quartz monzonites) this parameter is 115.6 ± 1.8 Ma. Thus, the formation interval of rocks of the Taezhniy massif should be accepted within 117.5-114.5 Ma.

Formation Conditions of Rocks of Taezhniy Massif
The geodynamic setting that existed during the formation of the rocks of the Taezhniy massif is similar to those of the Medvedev massif-defining them as the formation of volcanic arcs [45][46][47][48] (Figures 6F and 7A). The dual nature of the geochemical composition of massif rocks perhaps shows both the presence of crust and mantle sources. According to the K/Rb, Rb/Sr and Ba/Rb ratios and the K/Rb-Rb and Sr-Rb/Sr ratios of the elements, all the rocks of the Taezhniy massif are close to rocks of a mantle nature. A high Y/Nb > 1.2 ratio may indicate the contribution of both crustal and mantle sources. A lower crust source is attested by the ratios of Sr primary isotopes I 0 = 0.706-0.707 (Table A5) [49] as well as low values of the Nb/La < 1 characteristic, which indicates a negative niobium (tantalum) anomaly and high values of Th/U >2 as a an apparent sign of crust contamination [52,54]. In contrast, the relative content of europium Eu/Eu*≈1 suggests that the rocks of the massif are close to mantle derivates and that the melt formed during the fractional crystallization of only dark-colored rock-forming minerals [53].

Conclusions
The presented work is the final stage of many years of field and analytical work. The authors are the first who performed petrographic, geochemical, isotope and geochronological studies of the Medvedev and Taezhniy massifs and came to the following conclusions: 1.
According to field, petrographic and geochemical observations, it was revealed that the formation of the Medvedev massif occurred in three phases of rock intrusion and are presented in the following sequence: quartz syenites, pyroxene-amphibole syenites, quartz monzonites. According to the results of the same studies, two phases of rocks are defined in the structure of the Taezhniy massif: quartz monzonites and syenites.

2.
In terms of geochemical characteristics, the igneous formations of the Medvedev and Taezhniy massifs are similar to rocks derived from the latite and shoshonite series and can be considered as part of the monzonite-syenite formation type.

3.
The periods of formation of igneous rocks of the massifs were identified (Medvedev-122.0-118.0 Ma and Taezhniy-117.5-114.5 Ma) which correspond to the Early Cretaceous age of the Aptian.

4.
The opinion expressed previously regarding the participation of the subducted oceanic crust of the Mongolian-Okhotsk basin in the formation of all parent igneous chambers of the Aldan Complex, as well as the geochemical proximity of the rocks of the Mesozoic igneous formations with island arc volcanics, in general, is confirmed by our obtained geochemical data of the rocks of the Medvedev and Taezhniy massifs.

5.
The defined duality of the geochemical compositions of the igneous rocks of the massifs may be due to the presence of both mantle and crustal sources; however, it is most likely that these rocks resulted from the melting of a mixed mantle source or the latter contaminated by the crust with further the differentiation of melts in intermediate crust chambers. Funding: Work is done on state assignment of DPMGI SB RAS and partially funded by contract with PJSC Seligdar.
Data Availability Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflict of interest.