The Verkhoyansk–Kolyma orogenic region is characterized by the intensive development of Late Mesozoic magmatism. There are small bodies and dike series of variegated composition and numerous bodies of granitoids forming longitudinal belts elongated parallel to the boundaries of the main tectonic structures (main and northern), and transverse ones, oriented across or at an angle to them. The main batholith belt stretches in a north-western direction for a distance of about 1000 km along the border of the Polousno-Debin terrane and the marginal terranes of the composite Kolyma-Omolon microcontinent (superterrane). It also includes the Cretaceous granitoids of the Selennyakh block of the Omulevka marginal terrane considered in the work (Figure 1
]. The Cretaceous granitoids of the Selennyakh river basin considered in this work also belong to it.
Despite the long-term study of the granitoid magmatism of the orogenic region, many questions of the genesis of granitoids remain controversial. This primarily concerns the role of deep sources in magmogeneration processes and the causes of the petrographic diversity of granitoids. For a long time, all the granitoids of the region were considered as crustal, while their diversity was explained by various reasons, and their formation was related to orogenic processes. In this article, the author has tried to approach the solution of these issues.
At the early stages of studying the territory, the granitoids of the Main belt were assigned to the Kolyma Late Jurassic–Early Cretaceous complex [2
], or the formation of calcium-poor granites of crustal origin [4
]. The similarity of the parent melts for them was assumed, and the petrographic diversity was explained by the processes of contamination of the host rocks of a different composition. Later, the following three associations of granitoids were identified as part of the Main belt: (1) Late Jurassic–Early Cretaceous granodiorite-granite association, (2) Late Jurassic–Early Cretaceous granite-leucogranite association, (3) Late Cretaceous association of high-aluminous granites. The specific nature of granitoids was associated with the values of geothermal gradients and the composition of the basement during magma formation [5
]. The question of the presence of not only crustal but also mantle-crustal formations in the granitoids of the Main belt, was not even raised. The author of the proposed article, based on many years of research, identifies the Late Jurassic suprasubduction granitoids of the M-type of gabbro-tonalite-granodiorite association within the Main belt; formed at the turn of the Jurassic and Cretaceous mantle-crustal granitoids of the I-type of diorite-granodiorite-granite association; the Early Cretaceous crustal granites of IS- and S-types of granodiorite-granite and granite-leucogranite associations; the Late Cretaceous alkaline-feldspar granites of A-type, successively replacing each other in the process of the evolution of the orogenic region [6
The territory of the Selennyakh tectonic block and its nearby area attracts by the fact that numerous outcrops of intrusive rocks of various composition and age are concentrated here on a relatively small territory in the Selennyakh River basin. The study of not only granitoids, but also the basic rocks preceding and following them, allows us to identify the evolution of geodynamic settings and general genetic aspects of the formation of magmatism.
The proposed work presents the results of studying the geological position, petrography, mineralogy and chemical composition of derivatives of pre-granitoid basic magmatism, granitoid magmatism and post-granitoid sub-alkaline magmatism of the Selennyakh block and the following genetic conclusions from them.
3. Geological Setting
The studied area is located within the Verkhoyansk–Kolyma orogenic region. According to the concept of plate tectonics, the region records the Verkhoyansk fold-thrust belt, adjacent to the Siberian craton from the west, the Kolyma-Omolon microcontinent (or composite superterrain) in the east and the Okhotsk terrane in the southeast [1
]. The Kolyma-Omolon microcontinent is a collage of terranes of various nature. It was attached to the North Asian craton at the end of the Late Jurassic—the beginning of the Early Cretaceous with the formation of extended granitoid belts—the main one with an isotopic model Sm-Nd age of 152–145 Ma and the northern one with an age of 135–127 Ma [1
]. Along the southwestern and northwestern margins of the Kolyma-Omolon microcontinent, the Omulevka continental regional terrain is extended (Figure 1
). The Omulevka terrane is divided into several tectonic blocks, one of the largest of which is the Selennyakhsky block, within which the studied Mesozoic magmatic formations are localized. The Selennyakh block is composed of Early–Middle Paleozoic carbonate and terrigenous-carbonate sediments, pre-Ordovician marbles, amphibolites, amphibole-biotite and two-mica schists, plagiogneisses and green schists. The Upper Paleozoic, Triassic and Lower–Middle Jurassic deposits are insignificant and represented by siltstones and mudstones with rare interlayers of sandstones, limestones, marls, andesite and basalt flows.
In the center of the Selennyakh block, the following three granitoid massifs are exposed: Syachan, Sakhanya and Kuturuk, with areas of 700 km2
, 400 km2
and 40 km2
, respectively (Figure 2
). Granodiorites predominate in the Sakhanya and Kuturuk massifs, and granites prevail in the Syachan massif. At contact with terrigenous rocks, fine-grained leucocratic granites are observed; at contact with carbonate rocks, there is a zone of endo- and exoskarns of monzonite, granosyenite and K-feldspar-pyroxene composition [6
]. The massifs are accompanied by aplite and leucogranite dikes. Dikes and small stocks of granodiorite-porphyry and granite-porphyry are confined to numerous fault zones. Table 1
shows the results of determining the age of rocks performed by A.I. Zaitsev based on the samples of the author. The 40
Ar isotopic age of granodiorites of the Sakhanya massif is 142–132 Ma, and that of granites—136–134 Ma [14
]. Granites along fault zones and granite-porphyry dikes are intensely greisenized and impregnated with cassiterite and wolframite. Calcareous boron-bearing skarns with Sn-W mineralization are developed after terrigenous-carbonate rocks. Quartz-chlorite-carbonate metasomatites have associated gold-base metal mineralization with accompanying tin.
In the southern part of the studied territory, mainly volcanogenic-sedimentary formations of the Uyandina-Yasachnaya ensialic paleo-island arc are exposed [16
] (Figure 3
At the base of the volcanogenic-sedimentary strata lies a substantially andesite formation with a subordinate number of basalts, tuffs and tuff lavas of medium and basic composition. The total thickness of the section is 700–900 m. Higher up lies the shale-rhyolite strata with a thickness of up to 500 m. Volcanites are represented by alternating rhyolites, dacites, tuffs and clastolaves of acidic composition. At the top of the section, there are ash tuffs, pyroclastics in the cement of terrigenous rocks, interbeds of carbonaceous claystones, marking the transition to subaerial conditions. The pre-Late Mesozoic base of the belt is a system of blocks and thrust sheets composed of igneous and sedimentary rocks. The volcanogenic-sedimentary strata are intruded by the Agdai massif.
The eastern part of the massif is composed of a complex of rocks from gabbro-diorites to granodiorites, and the western part is composed of leucocratic alkaline-feldspar granites. The isotopic Rb-Sr age of the massif diorites is 148 Ma, alkaline feldspar granites—92 ± 3 Ma. Earlier, the massif was considered as a multiphase structure with a composition of rocks from gabbro to leucogranites. The formation of the massif is preceded by small gabbro stocks and dolerite dikes with an isotopic K-Ar age of 154–158 ± 3 Ma [18
Simultaneously, with the western part of the Agdai massif, the formation of the subvolcanic Ust-Burgokhchan and Burgakhchan massifs of leucogranite composition took place. The vein facies are represented by single thin (several cm) veinlets of fine-grained alaskites and aplites. A zone of intensive crushing is traced through the western and central part of the granite outcrop in the north-eastern direction, where both the granites and the host rocks are intensely ferruginated and bear nests of a pitch-black fine-crystalline mineral surrounded by red ochres. The western contact of the massif is tectonic. The Rb-Sr isochronous age of granites is 91 ± 3 Ma—the beginning of the Late Cretaceous [17
]. The magmatism of the territory is completed by the intrusion of dikes and small rods of trachydolerites and trachyandesites with an isotopic K-Ar age of 79–69 Ma [18
4. Petrography and Mineralogy of Magmatic Rocks
The earliest intrusive formations of the Mesozoic in the Selennyakh River basin are small stocks, chonoliths, dikes, sills of gabbro, gabbro-dolerites, dolerites and gabbro-diorites, lying within volcanogenic sedimentary rocks of the Late Jurassic [18
]. Xenoliths of metamorphosed Jurassic andesites are frequently found in them. The rocks are massive and trachytoid, with diabase, poikilophytic structures, porphyric and glomeroporphyric structures in dikes and a microdiabasic, dolerite, occasionally intersertal basis. Usually, the rocks are intensively modified (chlorite-carbonate-epidote facies of propylitization). Near the contacts of granite plutons, they are schistosed and broken down, with the development of contact biotite or quartz-biotite aggregates. The main minerals of the rocks are plagioclase and pyroxene. In porphyric rocks, they form phenocrysts with mutual inclusions in each other, and in medium-grained rocks, they have either a close degree of idiomorphism, or pyroxene fills angular gaps between idiomorphic plagioclase grains. Plagioclase is non-zonal or with indistinct direct zoning (from 66% to 40% an). Labrador predominates (50–58% an) a medium and high degree of structural ordering (0.5–0.8), with the cores of the most ordered being bitownite (78–85% an). Pyroxene is a magnesian augite with a calculated crystallization temperature from 1196 °C to 980 °C [19
], which is lower than the liquidus temperature of the dry melt (Table S1
). Sporadically, forsterite relics are found (Fo 84.3, Fa 15.7; Fo 82.2, Fa 17.8). Inclusions of a chromium spinel are noted in olivine from the dolerites dike (Al2
, 17.99%; Cr2
Pyroxenes are replaced by hornblende (f = 45–66%), which crystallizes in the temperature range of 850–810 °C and at a pressure from 4.4 kbar to 2.2 kbar from a water-saturated melt with a water content of 5.2–5.8% (Table S2
]. In porphyric rocks, it is present only in the groundmass. The substitution of pyroxene for high-alumina amphibole, chermakite, probably restite, was noted. Biotite is found in the form of single grains developed by pyroxene or amphibole. It is represented by eastonite-annite varying from moderate to high ferruginousity (Table S3
]. The calculated crystallization temperatures were from 725 °С to 685 °С [22
]. The composition parameters are similar to those of biotites of the Early Cretaceous granitoids. This allows us to assume a post-magmatic, contact-metamorphic origin of biotites. The accessory fraction is dominated by high-temperature titanomagnetite (TiO2
to 15%, SiO2
to 7.3%) and Сl-apatite (to 1.76% Cl). In the post-magmatic stage, plagioclase deanorthitization occurs (up to albite), as well as the replacement of dark colors with a serpentine-like mineral, a fibrous actinolite and a chlorite of the delessite group.
At the beginning of the Early Cretaceous, the formation of the Agdai massif of complex composition began. The eastern part of the Agdai massif is composed of diorites, quartz diorites and granodiorites with gradual transitions between them. The xenoliths contain gabbro-diorites. The massif cover is inclined to the south-east, where a number of small satellites of mainly diorite composition can be traced. The vein facies is represented by fine-grained granites and aplites. The western part of the massif is composed of leucocratic alkaline-feldspar granites. Earlier, the massif was considered as a multiphase structure with a composition of rocks from gabbro to leucogranites. The formation of the massif is preceded by small gabbro stocks and dolerite dikes.
Rocks of the eastern part of the Agdai massif are shlirovo-taxite, with a combination of sites of different composition, structure and different grain sizes. In rocks of diorite composition, idiomorphic labradorite grains are the first to crystallize (50–58% an) and magnesioaugite (ferruginosity f = 22–24%, Table S1
) forms panidiomorphic-granular autoliths. The gaps between them are filled with subidiomorphic grains of amphibole, zonal andesine oligoclase (44–22% an) and quartz, or a fine-grained pegmatite aggregate. The amphibole, magnesio-hornblende (f = 45.4–47.8%), as a rule, is replaced by canniloite. It crystallizes in an oxidizing environment when the water content in the melt is 4.7% [20
]. Grains of restitic tschermakite are also found. Biotite is a moderately ferruginous, low-alumina eastonite-annite. It begins to crystallize together with amphibole and oligoclase when there is high water and chlorine activity and the water content in the melt is 3.5–4%, and it ends after quartz, to which interstitial grains are sometimes confined (Table S3
). The most high-temperature generation in terms of composition parameters correspond to biotites of mantle and crustal–mantle derivatives (Figure 4
]. A potassium-sodium feldspar is observed as part of a xenomorphic-granular or micropegmatite aggregate. It is represented by a crypto- and micropertite intermediate-to-low microcline with a content of ab up to 22%.
The main accessory minerals are titanomagnetite (7.4% TiO2, 4.4% SiO2), magnetite, allanite, magnesium ilmenite (2–5% MgO at 0.05–0.2% MnO), apatite, rich in chlorine and fluorine (0.6–0.8% Сl and 3.6–5.4% F), zircon (ZrO2/HfO2 = 56–66), single grains of pyrope-almandine (29% py).
Granites of dikes are porphyric, with autoliths of an amphibole-plagioclase composition, plunged in a micropegmatite matrix. Plagioclase of autoliths—zonal andesine-oligoclase and oligoclase (30–15% an), amphibole—magnesial-ferruginous hornblende (f = 47.8%).
The massifs of the Early Cretaceous granitoids exposed in the center of the Selennyakh block are composed of amphibole-biotite granodiorites and granites with gradual transitions between them. Granodiorites of endocontacts contain xenoliths of diorites and gabbro-diorites. Pyroxene is found only in endoscarns and has the composition of ferrodiopside, ferroaugite and ferropigeonite with ferruginousity f = 44–64% and the calculated crystallization temperature T = 966–877 °С [19
In granodiorites, the earliest mineral is andesine-labrador composition of 52% an, 39% ab, 9% or, with the crystallization temperature 950 °С [31
]. Plagioclase of the periphery (40% an, 53% ab, 7% ort, was formed at 800–780 °С. Together with the middle zones of plagioclase, the hornblende begins to crystallize. It is formed in the temperature range of 846–712 °C, with a pressure from 4.6 kbar to 1 kbar and a water content of 4.1–5.3% in the melt. Its ferruginousity decreases with a decrease in pressure from 55.2% to 43.7%. Then, acid plagioclase (32–20% an, 7–2% ort) and an allotriomorphic-grained biotite-quartz-feldspar aggregate are formed. K-feldspar is represented by an intermediate to low orthoclase and microcline. Magnesian-ferruginous biotite (f = 51–67%), with a decrease in temperature is replaced by ferruginous biotite (f = 77%) (Table S3
). In terms of composition, biotite corresponds to that of the crustal granodiorite-granites of the ilmenite series, and only the most highly magnesian varieties correspond to biotites of the mantle-crustal rocks of the ilmenite-magnetite series (Figure 4
). Granites differ from granodiorites in the almost complete absence of minerals of the early amphibole-plagioclase paragenesis and the predominance of microcline among K-feldspars.
The accessory minerals of granitoids include ilmenite, F-apatite, zircon, titanite, allanite, garnet and rare magnetite grains. Zircon is mainly represented by low-temperature crustal morphotypes P, with ZrO2
= 30–60 typical for granodiorites and granites. There are also single crystals of crustal–mantle morphotype D with ZrO2
= 68–88 [32
]. In an intergrowth with the amphibole, the lower crustal pyrope-almandine (up to 29% py) is observed. Later, post-magmatic garnet has the composition of grossular-almandine, which is characteristic of derivatives of water-saturated acid melts and aqueous fluids [33
]. Microinclusions of gold are found in amphiboles, and single grains of cassiterite occur in leucogranites at endocontacts.
Aplites and leucogranites are composed of oligoclase (15–22% an, 4–5% ort), cryptoperthitic potassium–sodium feldspar (70–75% ort, 25–30% ab, 0–0.5% an), and siderophyllite. The latter is enriched in fluorine (1.6%) and crystallized from the water-saturated melt (Table S3
, Figure 4
). These rocks often contain pockets of quartz-feldspar pegmatites of up to 15 cm in diameter. They contain large scales of ferruginously eastonite-annite or siderophyllite, nests of schorl and fluorite in association with muscovite and cassiterite.
Granite- and granodiorite-porphyries have a porphyritic and glomeroporphyritic texture. The phenocrysts in the porphyry granite are composed of quartz, biotite and, occasionally, hornblende. Biotite is completely replaced by fine siderophyllite flakes (f = 85%). The accessories include ilmenite, fluorapatite, zircon, fluorite and thorite. Porphyritic phenocrysts of granodiorite-porphyry are composed of andesine-labradorite (56–45% an), hornblende and biotite, similar to those of granodiorites. The accessories include ilmenite, magnetite, chlor-apatite and fluor-apatite, zircon of morphotypes D and P, titanite, allanite and pyrite, arsenopyrite. Quartz-chlorite-carbonate metasomatites also contain galena and sphalerite. The greisenized rocks are characterized by tourmaline, fluorite, cassiterite and wolframite.
At the turn of the Early and Late Cretaceous, the western part of the Agdai massif and the subvolcanic Burgokhchan and Ust-Burgokhchan massifs of alkaline-feldspar granites and leucogranites and a series of dikes of rhyolite and rhyodacite porphyry were formed [17
]. The vein facies of the massifs are represented by rare thin veinlets of fine-grained porphyry-like leucogranites and aplites. The western part of the Agdai massif is in contact with the eastern part through a system of faults. It is composed mainly of medium-grained granites of a hypidiomorphic granular texture. The marginal parts of the Burgokhchansky and Ust-Burgokhchansky massifs are porphyry and glomeroporphyry with microgranite, micropegmatite or felsite groundmass.
The rocks of all the massifs are characterized by a high-temperature early magmatic association of minerals: sanidine or anorthoclase with composition of 45–50 ab, 48–54 ort and 0.7–1.8 an, and ferrodiopside or ferroaugite located in the form of idiomorphic inclusions in it (f = 83–86%) with a stable presence of the aegirine molecule (0.7–2.7%). The calculated [19
] crystallization temperature was 1063–1017 °С at a pressure of 5.8–8.6 kbar. More often, relics of clinopyroxene are identified among the schlieren of amphibole, replacing it in interstices of large anorthoclase grains. Sporadically, rounded fayalite inclusions in sanidine (f = 80–90%) are also observed. In the second stage, mesoperthite potassium-sodium feldspar with close ratios of albite perthites and high or intermediate orthoclase and oligoclase-albite and isometric and irregular quartz grains in sharply minor amounts are crystallized. Calculations [34
] indicate the temperature of the melt at this stage—850–856 °С. In the third stage, xenomorphic quartz grains or fine-grained pegmatite are formed. Amphibole and biotite are late and post-magmatic. They fill interstices between grains of salic minerals or form thin veinlets. They contain numerous inclusions of ore minerals, monazite, allanite, xenotime and undiagnosed small inclusions of radioactive minerals. In porphyry rocks of subvolcanic facies, subeuhedral quartz grains in intergrowth and with inclusions of uniaxial sanidine are the first to crystallize. Groundmass is micropegmatite or microgranite.
Amphibole has a composition of hastingsite or ferriedenite (f = 74–100%) with a high total alkali content. A high concentration of halogens is its most typomorphic feature (Table S2
). The calculated parameters of crystallization are as follows [20
]: P = 3.9–2.1 kbar, T = 835–761 °С. Biotite begins to crystallize somewhat later than amphibole, replacing it on the periphery of the grains. In the granites of the Agdai massif and the most eroded horizons of the Burgokhchan and Ust-Burgokhchan massifs, biotite has mainly the composition of siderophyllite with a ferruginosity of 86–97%. It crystallizes at 692–641 °C and the water content in the melt is 6–8%. In subvolcanic rocks, its first generation is represented by magnesian and magnesian-ferruginous eastonite-siderophyllite with a ferruginosity of 39–66%. The calculated crystallization temperatures are 777–677 °C with a water content of 2.5–6% in the melt. All biotites are enriched with fluorine and were formed at high activity of water and fluorine (Table S3
). The ratio of ferruginosity and fluorine content is comparable to the biotites of late gabbro-granitic differentiates, and low values of OH/F are comparable with biotites of A-type (Figure 4
Granites of all the studied massifs contain single grains of magnesioaugite (f = 21–29%) and andesine-labradorite. In terms of low ferruginosity and abnormally high crystallization temperature, magnesioaugite is comparable to clinopyroxenes of picrite–basalt associations of the continents [35
]. Corroded grains of ferrygedrite are found here, characteristic of highly metamorphosed rocks.
The accessory fraction is strongly dominated by allanite, which forms small grains and their clusters. Titanomagnetite is also a characteristic accessory mineral of rocks (3–12% TiO2). In all the samples, small grains of zircon with widely varying ZrO2/HfO2 ratios were also found—from typical for granites to typical for basic rocks (29–128). Rare grains of apatite contain 3.3–6.2% F and 0.07–0.12% Cl. Sulfides—pyrite, arsenopyrite, millerite—are practically devoid of impurities. Single grains of grossular-almandine (54–64% gross) and pyrope-almandine (20.7–20.8% py) are probably restytic or xenogenic. Single grains of chromite were also found in the crushed sample (30% Cr2O3, 50% Fe2O3).
The latest magmatic formations of the territory are represented by single dikes of trachydolerites and trachyandesites. They cut granitoids of the Syachan massif and granite-porphyry of the Ust-Burgokhchan massif. Their thickness ranges from 10–15 cm to 3–4 m. The predominant trachydolerites are massive, porphyry, with diabase and dolerite microgranular groundmass. The rocks are composed of andesine-labrador lathes (50–52% an) and subprismatic grains of pyroxene and hornblende. In the center of the plagioclase grains, corroded bitownite cores with inclusions of idiomorphic pigeonite are sporadically observed. The crystallization temperature of labrador-bitownite (аn
10) is determined at 1200 °С [31
]. Dark-colored minerals are represented by pyroxene and amphibole, rarely by replaced ferruginous olivine with iddingsite (f = 56–59%) containing lamellar separations of chromium-containing magnetite (1.1% Cr2
). Pyroxene is represented by ferrodiopside (f = 37%) and more ferruginous clinoenstatite (f = 68%), crystallized at 997–1000 °C (Table S1
). Amphibole forms pseudomorphoses on pyroxene with the preservation of close ferruginosity. It has the composition of hornblende and ferroedenite with crystallization parameters: Т = 985–840 °С, Р = 2.2–0.86 kbar. It crystallized under oxidizing conditions (‒log fO2
= 10.5) when the water content in the melt is 3.5% (Table S2
). In turn, it is replaced by ferruginous eastonite-annite with f = 64.2–78.8%. Up to 5–7% of the rock volume is composed of titanomagnetite (6–19.8% TiO2
), forming large idiomorphic grains. Clusters of needle-like chlor-apatite with a chlorine content of up to 1.66% are characteristic.
5. Petrochemistry of Magmatic Rocks
The chemical composition of the earliest igneous rocks of the Late Mesozoic within the Selennyakh block varies from gabbro to diorites (Figure 5
, Table S4
]. They are characterized by a potassium-sodium type of alkalinity (Figure 6
and Figure 7
), moderate aluminousity (Figure 8
] and low ferruginousity (f = 48–58%). The main parameters of the composition correspond to those of the tholeiitic, less calcareous-alkaline series of island arcs (Figure 9
]. Gabbro and dolerites are olivine-diopside normative, with a sharp predominance of normative albite over orthoclase. Diorite porphyrites are hypersthene-diopside normative, with a small amount of normative quartz. The differentiation index Dl of rocks is low—24–27% for gabbro, and only in one case—55% (for diorite). According to the classification [44
], they belong to the family of tholeites and olivine tholeites. The calculated parameters of magmogeneration determined by different programs [36
] are close: P = 12–14 kbar, Т = 1180–1220 °С.
The composition of the following age group of rocks (the beginning of the Early Cretaceous) was studied on the example of the eastern part of the Agdai massif. It varies from gabbro-diorites to granodiorites. Rocks are metaaluminous (Figure 8
), of a low-potassium calc-alkali series (Figure 6
). Their ferruginousity increases with the growth of SiO2
from 54% to 83%. The normative composition is quartz-hypersthenes-diopside, with a noticeable predominance of normative albite over orthoclase. Standard corundum is not identified. Dl = 39–67%. According to the composition parameters, the rocks occupy an intermediate position between island-arc magmatic formations and magmatic formations of continental arcs and correspond to M-type suprasubduction granites [6
The following temperatures of the parent melt calculated by different authors are close: 1000–1150 °С [47
], 1110–1155 °C [37
], 1007–1093 °С [36
]. The calculated magma generation pressure for the least differentiated samples is 10–12 kbar [38
]. The temperature range of crystallization, determined by the ratios Al2
]—1100–800 °С. The ratio of Al/(Mg + Fe)–Ca/(Mg + Fe) in rocks (Figure 10
) indicates the generation of the melt during the selective melting of amphibolites (Figure 10
Granodiorites and granites of dikes are meta-aluminous (Schend index, 0.92–0.98), ferruginous (f = 66–84%), diopside-hypersthene normative, not containing normative corundum, with a predominance of normative albite (31–58%) over orthoclase (22–27%). Dl = 77–83%. The calculated magmogeneration temperatures are 907–931 °C [36
] at a pressure of 7–10 kbar, and the variations in crystallization temperatures—900–700 °С.
The igneous rocks of both associations considered above are metamorphosed near the contacts of large granitoid massifs. The chemical composition of the latter corresponds mainly to granodiorites and granites (Table S4
. Figure 5
). Rocks are diopside-hypersthene normative, with a slight predominance of normative albite over orthoclase (on average, 21–32% and 16–27%, respectively).
Alkaline granites and quartz monzonites are developed in contact with carbonate rocks. All rocks are meta-aluminous or slightly supersaturated with alumina (Figure 8
) and ferruginous (f = 56–85% in granitoids of the main facies and up to 100—in leucocratic rocks of endoconacts). They belong to the high-potassium calc-alkali series (Figure 6
) and, according to the composition parameters, correspond to granitoids of the I- or IS-type of continental arcs or granitoids of the calc-alkali series of active continental margins (Figure 8
and Figure 9
The calculated depth of magma generation [38
] corresponds to a pressure of 9–11 kbar. The Al/(Mg + Fe)–Ca/(Mg + Fe) ratios in the rocks (Figure 10
) correspond to the initiation of magma chambers at the boundary between dacite-tonalite and amphibolite substrates. The composition points of the least differentiated granitoids are localized within the melting field of amphibolite. Taking this into account, the maximum temperature of melts for the Kuturuk and Sakhanya massifs was determined at 1000–1020 °С, and for the Syachan massif at 960–980 °С [36
]. From the Al2
vs. MgO ratios [46
], the crystallization temperature of the first two massifs was estimated at 1000–650 °C and for the third massif at 900–630 °C, at a pressure of 9–2 kbar.
The aplites and leucogranites of dikes correspond in composition to alkali granites of the high-K calc-alkaline series (see Figure 5
and Figure 6
). They have a high ferruginousity (85–100%) and are metaluminous or weakly peraluminous (Figure 10
). Pegmatites are distinguished by higher K contents with a transition from the high-K to shoshonitic series. The crystallization temperature of rocks of this group is 820–472 °С at 5–2 kbar. According to the Sr-Rb/Sr ratios, they are classified as A-type granites (Figure 9
). The granite and granodiorite porphyries of the dikes differ from the rocks of the main facies of the massifs with corresponding SiO2
contents only in the higher values of volatiles (H2
O, F, S, P). The maximum crystallization temperatures are 960 °C and 990 °C, and the maximum pressures during magma generation are 7.5 kbar and 9.4 kbar.
Magmatic formations of the following age group (the boundary of the Early and Late Cretaceous) are represented by the Burgokhchan, Ust-Burgokhchan massifs and the western part of the Agdai massif. Their chemical compositions correspond to granites and alkaline granites of the high-potassium calc-alkali series, with a transition to the shoshonite series of alkalinity (Figure 5
and Figure 6
). In the diagram of L. S. Borodin (Figure 7
), the points of their compositions outline a trend that cuts through the main evolutionary trends. Granites are highly differentiated (Dl = 84–97%), highly ferruginous (f = 87–100%), hypersthene normative, with a slight predominance of normative albite in the average compositions (34.9%, 30.5% and 30.2%, for granites of the Burgokhchan, Ust-Burgokhchan and Agdai massifs, respectively) over the orthoclase (23.4%, 25.8% and 23.1%, respectively), with low values of a normative corundum (0–0.6%). The normative composition of plagioclase is 1–14% an. Granites of all the massifs of this group are characterized by a moderate or slightly increased luminosity (Schend index from 0.85 to 1.2) (Figure 8
). According to all the parameters of the composition, they are defined as postorogenic (intraplate) or rift-related granites of the A-type. The maximum calculated parameters of magmogeneration are close for the least differentiated differences of granites of all the massifs: 1032–996 °C [36
] at a pressure of 6–9 kbar [38
]. The temperature intervals of crystallization determined by the GCDkit program [7
] for a series of samples using different geothermometers (Zr, apatite and REE saturation) were, for granites of the main facies of the Burgokhchan, Ust-Burgokhchan and Agdai massifs, respectively, 880–830 °С, 820–748 °С and 736–706 °С, and for endocontact—986–927 °С, 876–836 °С and 836–807 °C.
The composition of the latest igneous rocks of the territory is characterized by increased alkalinity and varies from sub-alkaline gabbro to monzonites and syenites. The rocks are mainly olivine-diopside normative, sometimes with a small amount of normative nepheline (12%), and a moderately aluminous, ferruginous (f = 50–85%), potassium-sodium type of alkalinity. The main parameters of the composition correspond to those of the rift-related derivatives of the trachyte series (Figure 7
and Figure 9
). The calculated parameters of magmogeneration are determined by different programs [37
], close: P = 16–17 kbar, Т = 1175–1220 °С.
In the Late Jurassic period or at the turn of the Late Jurassic and Early Cretaceous period, the formation of small stocks and dikes of gabbro and dolerites occurred. The mineral and chemical features of the rocks (magnesian olivine with f = 16–18%, olivine-normative composition, low differentiation index Dl = 24–27%, high magnesia) correspond to the generation of the parent melt under mantle conditions. According to the classification [44
], they belong to the family of tholeites and olivine tholeites. According to the La/Yb–Yb ratios (5.4–2.4; 7.4–2.3), these rocks correspond to the derivatives of the mantle melt formed during the melting of slightly metasomatized spinel lherzolite [49
]. The ratios in the rocks Ce/Sm-Ce (2.2–46; 2.8–47) indicate a low degree of its melting—2.5–3.5% [50
]. According to the composition parameters, the geodynamic conditions of the formation of gabbroids are defined as close to the island-arc (suprasubduction) (Figure 6
and Figure 7
). The rocks contain large grains of tschermakite hornblende framed by clinopyroxene. In experiments, such an amphibole was obtained at a pressure of more than 10 kbar and a temperature of about 800 °C. [51
]. It is most typical for eclogites and garnet amphibolites and is practically not found in igneous rocks [52
]. This allows us to assume the contamination of mantle melts by the substance of lower-crust substrates during magmogeneration. The values of primary Sr ratios (I0
= 0.7067–7069), according to A.I. Zaitsev (17), also suggest the contamination of primary mantle melts by crustal matter.
The next stage of magmatic activity at the beginning of the Early Cretaceous was marked by the formation of massifs of complex composition, ranging from gabbro-diorites to granodiorites and similar M-type granitoids. The ratios in rocks Al/(Mg + Fe)–Ca/(Mg + Fe) (Figure 10
) and normalized values La/Yb (7.4, 5.7, 3.8)–Yb (8.5, 10, 10) [53
], correspond to the origin of the parent melt in the amphibolite substrates of the lower crust. This is also confirmed by the presence of ferruginous tchermakite in diorites—a characteristic mineral of amphibolites. The variations in the K/Rb-Rb (180–320) values also correspond to the generation of melts in substrates that were mantle matter or a mixture of crustal and mantle matter [54
]. Thus, the actual material obtained indicates the origin of the parent melt for the East Agdai due to the selective melting of the lower crust amphibolites when they are exposed to the mantle basic melt and the latter is partially mixed with the resulting crustal melt.
In the next stage, the Early Cretaceous, the formation of granodiorite-granite massifs took place. The data of isotope analyses and geochemical features of the granitoids make it possible to attribute them to long evolved granitoids of continental arcs, formed in the conditions of an active continental margin (Figure 8
and Figure 9
). Magma chambers were formed within the lower crustal amphibolites or at the boundary between the amphibolite and dacite-tonalite substrates (Figure 10
) at a pressure of 9–11 kbar and a temperature of 1020–990 °С. Low chondrite-normalized La/Yb values (2.9–5.6) also testify to magma generation in amphibolite substrates [53
]. The presence of basic rocks in magma-forming substrates is also indicated by the high content of pyrope minal in accessory garnet, restite zircons of morphotype D, and K/Rb values (200–400) in granitoids. According to experimental data [55
], melting amphibolites can lead to the formation of granitoid melts in the presence of a fluid phase (primarily H2
O and NaCl), which agrees well with the high water content (913–1025 mg/kg) in magmatogenic inclusions and the composition of water extracts from the quartz of granitoids of the studied massifs (Na+
= 12–16 mg-equ/l, Cl‒
= 13–27 mg-equ/l [57
]. According to the composition parameters, granitoids belong to the I- or IS-type.
Granodiorite- and granite-porphyries differ from the rocks of the main facies of the massifs with a corresponding SiO2
content only in higher contents of volatiles (H2
O, F, S, P) (Table S4
). The maximum temperatures and pressures of their magma generation (990 °C and 960 °C at 9.4 kbar and 7.5 kbar) are also comparable. The retention of high temperatures of melt formation to the latest derivatives requires the supply of juvenile heat and fluids to the root parts of magma chambers during the process of the formation of granitoid massifs.
At the turn of the Early and Late Cretaceous, the formation of the western part of the Agdai massif and the Burgokhchan and Ust-Burgokhchan massifs began. The alkali-feldspar granites and leucogranites composing them are defined by all the geochemical parameters as postorogenic or rift-related A-type granites. Calculated pressures during magmogeneration (6–9 kbar), features of Rb-Sr isotope systems I0 = 0.71053–0.72540) indicate the crust nature of the protoliths. At the same time, high melt temperatures (up to 1032 °C) at such low pressures could be achieved only when additional heat was supplied to the magmogeneration levels from an external source. Granites are characterized by a combination of high-temperature minerals characteristic of the basic rocks (clinopyroxene with parameters of clinopyroxenes of basite–hyperbasite and picrite-basalt associations, titanomagnetite, magnesian ilmenite, zircon of D morphotype) and low-temperature, typically granite, minerals (oligoclase-albite, manganous ilmenite, F-apatite, fluorite). Taken together, these data indicate a mixed (syntexis) origin of the parent melts.
In Figure 11
, the points of the granite compositions of the massifs of this group are localized in the field of A1 granites, the formation of which is related to plumes or hot spots and continental rifts [58
]. This is consistent with the opinion of the majority of researchers who have dealt with the problems of A-granites, who have shown that such high temperatures are usually not reached in the earth’s crust; that is, "the involvement of mafic magmas, or high mantle heat flows, is a necessity" for the generation of melts forming A-type granites [59
The latest intrusive formations of the territory are represented by a sub-alkaline gabbro, trachydolerites and trachyandesites of the trachyte series of rift-related continental geodynamic conditions. Compared with rocks of the Late Jurassic with similar SiO2
contents, they are characterized by an increased alkalinity, and higher deeper magmogeneration conditions (pressure up to 17.9 kbar). According to the La/Yb–Yb ratios (17–1.6 and 23–2.1), the rocks correspond to the derivatives of melts formed during the melting of metasomatically enriched lherzolite [49
Thus, in the course of the evolution of the Late Mesozoic magmatism of the Selennyakh block of the Omulevka terrane, there was a change of the Late Jurassic magmatism, close to the island-arc (suprasubduction) magmatism, by the Early Cretaceous magmatism of the continental arcs of active margins and at the turn of the Early and Late Cretaceous by continental rift-related magmatism. At the same time, there was a change basically of the main magmatism to more and more silicic acid and more alkaline (an increase in the total alkalinity—Figure 5
) with the movement of magmogeneration levels to higher and higher horizons of the lithosphere, but with the preservation of close high magmogeneration temperatures. The magmatic activity within the territory ends in the very end of the Late Cretaceous with the formation of dikes—derivatives of melts of the metasomatized mantle. At the same time, all the studied igneous rocks are either direct mantle derivatives, or bear signs of the participation of mantle matter in the generation of melts in crustal substrates. On the A–B diagram of F. Debon and P. Lе Fort (Figure 12
), the overwhelming number of points of their compositions is localized along the mixing trend [61
]. The article presents the results of the study of Late Mesozoic intrusive formations of the Omulevka terrane of the Verkhoyansk–Kolyma orogenic region. The research area covers the Selennyakh block of the Omulevka terrane and the territory adjacent to the south. The compositions of rock-forming, accessory and restitic minerals and geochemical features of intrusive rocks are considered. The methods of optical microscopy, microprobe, silicate and spectral analyses were used. There are several stages in the evolution of magmatism as follows: (1) the Late Jurassic supra-subduction (gabbro, dolerites), (2) the beginning of the Early Cretaceous-transitional from supra-subduction to marginal-continental (gabbro-diorites, diorites, granodiorites), (3) the Early Cretaceous of active continental margin (granodiorites, granites), (4) the Late Cretaceous postorogenic or continental-riftogenic (alkali-feldspar granites of A-type), (5) the Late Cretaceous continental riftogenic (subalkaline gabbroids and basaltoids). In the process of evolution from stage one to stage four, there was an increase in the silicic acid content, total alkalinity and ferruginousity of the rocks with the movement of magmogeneration levels to higher and higher horizons of the lithosphere (calculated pressure from 1.6–1.4 GPa to 0.6–0.9 GPa). At the same time, the preservation of high temperatures of magmogeneration (1000–1150 °C) and crystallization implies the supply of additional heat from an external (deep) source during the formation of granitoid melts. The magmatic activity is completed by the intrusion of subalkaline derivatives of a deep hearth, formed by metasomatized lherzolites. All the studied igneous rocks are either direct mantle fusions, or bear signs of the participation of mantle matter in the generation of parent melts in crustal substrates: the presence of tschermakite in gabbroid,; nonequilibrium structures, the composition of early generations of biotites corresponding to biotites of mantle and crust–mantle derivatives, the presence of pyroxenes and accessory minerals characteristic of mantle magmas—in granitoids. In the diagram of Al-Na-K-2Ca–Fe + Ti + Mg, the composition points of the studied intrusive rocks tend to the mixing trend. In general, the research results suggest that the evolution of the Late Mesozoic intrusive magmatism of the studied territory and the specific matter of rock compositions were caused by the crust-mantle interaction as a result of the rise of mantle diapirs into the crust from a long-existing deep hearth of the main melt.
The actual and analytical material obtained in the course of research allows us to assume the presence of buried foci of the main melts within the studied territory, the repeated activation of which caused the melting of crustal substrates in the late Mesozoic. These conclusions are consistent with the results of long-term geothermal and geophysical studies, on the basis of which G. A. Stogniy and V. V. Stogniy identified the Verkhoyansk–Kolyma regional anomaly of increased heat flow due to mantle diapirism or the presence of a plume [62
]. E.A. Korago adheres to a similar point of view, linking the Late Paleozoic–Mesozoic history of Northeast Asia with the influence of the mantle diapir centered in the Kolyma loop [63