INTRODUCTION

The Trans-Baikal parametric borehole (BPB) was drilled in the 1-SB-Vostochnyi profile included in the State Network of Reference Geological and Geophysical Profiles and Parametric and Ultra-Deep Boreholes. The profile crosses the fold framing of the Siberian Platform within the Kerulen–Argun terrane of the Amur superterrane (according to [5]) of the Central Asian fold belt, south of the Mongol–Okhotsk suture zone (Fig. 1).

Fig. 1.
figure 1

Location of metamorphic core complexes in the Trans-Baikal region according to [4, 8]. Major tectonic structures of the region according to [5].

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The objective of this borehole was to study the geological nature of the dome structure recorded in the geophysical sections along the reference profile and to obtain the parametric information on the section based on study of the sampled core and the results of geological and geophysical studies of the borehole in order to construct further the deep geological and geophysical model along the profile. In the profile section, the width of the dome structure is about 100 km, and its base is located at a depth of about 15 km. The borehole was drilled at the site where the dome roof was the closest to the daytime surface, according to the RSS–CDP data (Fig. 2).

Fig. 2.
figure 2

Geophysical sections along the reference geological and geophysical profile 1-SB Vostochnyi and their geological interpretation: (a) fragment of the RSS–CDP deep seismic section (the Earth’s crust is 40 km thick; the position of the trans-Baikal parametric borehole is plotted in the section); (b) geological interpretation of geophysical sections.

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On the state geological maps, BPB is located in the southeastern framework of the outcropping Borshchovochnyi granite–gneiss swell composed of the Early Proterozoic (?) Urulga metamorphic complex enriched in Middle–Late Jurassic granitoids [2].

The Urulga metamorphic complex occupies vast areas in this region and is considered on geological maps as the crystalline base of the fold belt. However, in recent decades, the ideas of the deep structure of fold belts underwent considerable changes. They were greatly influenced by the study of abnormally deformed metamorphic rocks (metamorphic core complex) in the American Cordillera in the 1970s [7].

Three elements are distinguished in the structure of metamorphic core complexes (MCCs): the upper plate or cover of slightly metamorphosed rocks, the gently sloping zone of detachment, and the lower plate composed of metamorphic and igneous rocks and containing a mylonite zone in its roof. The mylonite zone as an antiform is characterized by a gentle dipping of structural elements and a clear mineral linearity distinguished by a constant direction within each or even a range of metamorphic core complexes contiguous in space. In the rock complexes of the lower plate, plastic deformations are noted under P–T conditions of predominantly amphibolite facies, while the cover is characterized by predominantly brittle deformations the age of which is synchronous with plastic deformations and metamorphism conditions of no higher than the greenschist facies [4, 6].

According to the current concepts, the Borshchovochnyi granite–gneiss swell is considered by most researchers [4, 6, 8] as a metamorphic core complex. To date, a range of metamorphic core complexes traced at a distance of more than 900 km has been identified and studied in Northern Mongolia and the Trans-Baikal region (Fig. 1). The metamorphic core complexes were formed in the Early Cretaceous [3, 4, 6, 8] in the Trans-Baikal region and, simultaneously, over vast areas of East Asia. These processes recorded the global intracontinental extension in the Asian region [9, 10]. The Trans-Baikal and Northern Mongolia metamorphic core complexes were formed in the extension conditions caused by the collapse of the Late Mesozoic orogen, which resulted from the collisional events related to the closure of the Mongolian–Okhotsk Ocean [4, 6, 8]. The Trans-Baikal parametric borehole was supposed to have penetrated into the rocks of the Kerulen–Argun terrane of the Central Asian fold belt for the study purposes. The actual borehole section made it possible to verify the structure observed in deep sections as a metamorphic core complex. Taking into account a continuous core sampling, a wide range of geophysical surveys in the borehole shaft (logging), and analytical studies of the rock material, the parametric drilling in the reference profile made it possible to open up a unique opportunity to study the metamorphic core complex not in separate outcrops, but in a continuous reference section, including deep rocks of the Central Asian fold belt.

METHODS

The borehole drilling was started in 2017 with the drilling of an pilot hole (the distance from the pilot hole to the main hole was about 60 m) with core sampling throughout the entire drilling interval to a depth reaching 1000 m in 2018. The main hole of the Trans-Baikal borehole was constructed in the period from 2018 to 2021, which was temporarily suspended in 2019 at the level of 1289 m, and the design depth of 2600 m was reached after replacing the rotary drilling rig with a core drilling rig. During the staged drilling of the main hole to a depth of 2600 m, the rotary drilling rig, which was primarily focused on drilling with coring in a large diameter borehole (HC drilling), turned out to be inefficient. For this reason, starting from a depth of 1289 m, drilling was carried out by a VD-8000 core-type rig (drilling with core barrel). The VD-8000 drilling rig (Atelier Val d’Or) based on advanced diamond drilling technologies using a top drive and equipped with a triple core barrel with a removable core receiver proved to be highly effective in drilling hard crystalline rocks provided for by the design section and confirmed by the drilling results. Drilling operations to a depth of 1289 m were carried out by the Research and Production Center for Deep Drilling and Comprehensive Study of the Earth; and in the interval of 1289‒2600 m, by the contractor JSC Urantsvetmet Uranium Mining Company. The altitude of the main hole mouth was 517.5 m; coordinates: 52°05′04″ N, 117°40′45″ E. Drilling the main hole to a depth of 4000 m in accordance with the Program for Regional Geological Survey of the Subsoil and Special Purpose Work until 2025 (Order of the Federal Agency for Subsoil Use no. 237 dated June 4, 2021) is not expected to be continued.

Geophysical surveys of the pilot and main holes in the depth range of 0‒1290 m were carried out by AO Irkutskgeofizika by means of the standard equipment used in boreholes in the oil and gas industry. The full borehole logging complex included recording the parameters of gamma-ray logging, neutron gamma-ray logging, spectral gamma-ray logging, density gamma–gamma logging, and caliper-profile logging, recording by electrical logging methods, geophysical borehole logging, magnetic susceptibility logging, and magnetic field logging, acoustic logging and inclinometry, and temperature logging, as well as acoustic logging of casing string cement. In the depth interval of 1289–2600 m of the main hole, the logging complex was carried out by OOO Vezerford using the advanced Compact equipment designed to work in small-diameter boreholes. The laboratory and analytical research program included detailed petrographic, geochemical, mineralogical, and isotope–geochronological studies. We carried out petrophysical, gas–geochemical, and thermophysical studies of the core and hydrogeochemical studies of the drilling fluid taken in the process of drilling the main hole.

RESULTS

In the Trans-Baikal borehole section, to a depth of 2600 m, two main structures were distinguished under the conventional names “fold complex” and “crystalline complex” (Fig. 3). The fold complex is composed of serpentinites, serpentinized peridotites, and numerous cataclasites and breccias after them. The degree of metamorphism of this complex corresponds to lower stages of the greenschist facies. The fold complex is underlain by the crystalline complex composed of metamorphic and igneous rocks. It is subdivided into two subcomplexes such as granite–schist and granite–plagiogneiss. The mineral assemblages of schists and plagiogneisses of this complex correspond to the metamorphism amphibolite facies.

Fig. 3.
figure 3

Generalized section of the Trans-Baikal parametric well.

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FOLD COMPLEX

Serpentinite massif (0‒788 m). Serpentinites occupy the upper part of the section down to a depth of 788 m (in the main hole). According to the results of a detailed description of the core and geophysical studies in the borehole shaft (logging), the serpentinite massif was interpreted as a set of blocks (plates) of relatively massive serpentinites separated by zones comprising talc–chlorite schists. The data obtained make it possible to conclude that the serpentinite massif is a mélange where massive rock blocks are embedded in a plastic talc–serpentinite matrix. This feature is typical for the structure of serpentinite massifs in the fold areas.

Most massive serpentinite (serpentinized peridotite) blocks (plates) with a thickness of a few tens to 70–100 m are correlated between pilot and main boreholes. The boundaries of these blocks are identified based on talc–chlorite schists in the section, which are confidently traced in the logging data by a rapid decrease in the density, P-wave velocity, and electrical resistivity of the section. The mineral assemblage of massive serpentinites contains magnetite; therefore, intensive positive anomalies correspond to serpentinite plates on the magnetic susceptibility logging curve. Talcing and chloritization areas are characterized by depletion in the Mg and Fe transition from the trivalent form to the divalent one, resulting in the disappearance of magnetite [1]. Sulfides are predominant in the mineral assemblages of talc–chlorite schists characterized by substantial tectonic transformations, while magnetite is almost completely absent. As a result, the rock conductivity increases and the magnetic susceptibility decreases.

The talc and talc–chlorite schist zone is located at the serpentinite massif base. In the lower part of this zone, there is a horizon of breccias and cataclasites after metasomatic carbonate–chlorite rocks, contrastingly marked according to the data of most logging complex methods in both the pilot and main holes.

CRYSTALLINE COMPLEX

The roof of the crystalline complex is accepted to be along the upper boundary of the tectonized granite body at a depth of 788 m in the main hole (in the pilot hole, at a depth of 778 m). From the point of view of the material composition, the rocks of the exposed section are divided into two subcomplexes. The upper part is a granite–schist subcomplex composed of schists with amphibolites and quartzites which contain layered biotite granite veins. The lower part is a granite–plagiogneiss subcomplex represented by plagiogneisses interbedded with leucocratic biotite granites forming both numerous veins and veinlets, and larger bodies. Gneissic biotite granites sometimes interbedded with leucocratic granites are located at the granite–plagiogneiss subcomplex base.

Granite–crystalline schist subcomplex (788‒1486 m). The section of the granite–crystalline schist subcomplex penetrated by a borehole consists of three main parts that are drastically different in terms of the predominant rock type. The upper part is composed of a large body of metasomatized and foliated (mainly in the roof) biotite granites (interval 788‒1043 m). Granites, similar in composition, but as thin veins, make up the sequence of amphibole–plagioclase, biotite–plagioclase, and quartz–biotite schists in the middle part (interval 1044‒1284 m). In the lower part of this subcomplex, biotite granite veins, together with biotite leucogranite veins, interbed with quartz–biotite schists (interval 1284‒1486 m).

In the schist sequences, biotite granites form interbedding members characterized by alternation of mylonitized biotite granites and schist; granite interbeds have a thickness from a few centimeters to a few meters. Taking into account the contrast physical properties (density, magnetic susceptibility, total radioactivity, and Th content), granite veins are confidently distinguished from schists according to the borehole logging data. Alongside with that, the density values based on the density gamma–gamma logging in the interbedded basic granites and crystalline schists range from 2.58 to 2.86 g/cm3. They are defined as intervals with exceptionally high density and P-wave velocity dispersion. Precisely these fragments of the section are characterized by a high reflection coefficient and are responsible for the formation of reflector horizons with a variable phase sign in the RSS–CDP seismic section confirmed by the vertical seismic profiling data.

Based on the isotope–geochronological data on biotite granites (local U‒Pb zircon dating by SHRIMP), all zircons analyzed from this granite type form concordant clusters at the same age mark of about 130 Ma (unpublished data by the author).

Biotite–amphibole–plagioclase and biotite–plagioclase schists make up the main part of the granite–schist subcomplex section. In terms of the mineral composition and silica content (SiO2 = 48–60 wt %), schists correspond to gabbro and gabbrodiorites, while amphibolites (SiO2 = 44–52 wt %) correspond to gabbro. The second most common rock type is plagioclase–quartz–biotite and quartz–biotite schist, sometimes with amphibole, often with accessory garnet and cordierite. In terms of the silica content (SiO2 = 60–70 wt %), they correspond to intermediate rocks. Quartz–biotite schists contain quartzite interlayers, including magnetite-bearing varieties. The mineral and chemical compositions of quartz–biotite schists with intermediate composition and quartzites make it possible to consider them as metamorphic rocks with a sedimentary protolith.

Granite–plagiogneiss subcomplex (1486‒2600 m). The subcomplex section uncovered by the Trans-Baikal borehole consists of two main parts. The upper part is represented by interbedding of biotite plagiogneisses and leucocratic biotite granites (1486‒2349 m). In some areas, biotite granites form large bodies in the section, while plagiogneisses remain only in the form of thin inclusions. The lower part of the section is a gneissic biotite granite body containing numerous plagiogneiss fragments; boundaries between the rocks are indistinct, and gneisses pass gradually into granites. The granite–plagiogneiss sequence is dominated by white biotite leucogranites, while the overlying granite–schist subcomplex is dominated by gray biotite granites. A frequent accessory mineral in leucogranites is light pink garnet, which occurs in single euhedral grains of up to 1–2 mm in size. Plagiogneisses contain leucogranite veinlets and lenses from a few millimeters to a few centimeters thick with wavy edges, oriented along the gneissic banding; in some areas, they are crumpled into small disharmonious folds. Interlayers of finely banded biotite amphibolites with a thickness of tens of centimeters occasionally finely interbed with plagiogneisses. Rare separate biotite granite veins are similar to those found higher in the section; granites are often highly mylonitized. In terms of the mineral composition and silica content (SiO2 = 62–72 wt %), plagiogneisses correspond to granodiorites. Their chemical composition makes it possible to consider them as metamorphic rocks with a magmatic protolith.

According to the density gamma–gamma logging data, fluctuations in the sequence with interbedded granites and plagiogneisses range from 2.56 to 2.70 g/cm3. These values are much lower than the corresponding limits in the schist subcomplex. Single amphibolite bodies are noted based on the density gamma–gamma logging anomalies of up to 2.9‒2.95 g/cm3. The gneissic granite body (2349‒2600 m) is characterized by a heterogeneous internal structure with leucogranite areas, numerous plagiogneiss relics, and gradual transitions between gneisses and granites preserving an indistinctly banded gneiss structure. An area dominated by white leucocratic granites (2433‒2493 m) is located in the middle part of the gneissic granite section. The gradual transition from gneissic granites to leucogranites is reflected in a gradual decrease in density from 2.67 to 2.60 g/cm3 recorded in the density gamma–gamma logging data. Leucogranites are characterized by an exceptionally low magnetic susceptibility making them different from rocks of all other types, including plagiogneisses and gneissic granites.

The isotope–geochronological study of plagiogneisses and leucocratic and gneissic biotite granites of the granite–plagioclase subcomplex (local U‒Pb zircon dating by SHRIMP) made it possible to reveal two concordant clusters with an age of about 280 and 145 Ma; the first of them corresponds to the formation time of igneous protoliths of plagiogneisses and granites, while the second one records the rock metamorphic transformation time (unpublished data from the author).

DISCUSSION

From the standpoint of identification of the metamorphic core complex, the structure and composition of its main zones are of fundamental importance for its verification. The lower plate composed of highly metamorphosed rocks should be exposed in the complex section under a narrow detachment zone as chlorite breccias. The lower plate section should start from the mylonite zone containing rocks with deformation signs under the brittle–ductile transition conditions.

In the reference section of the Trans-Baikal borehole, the mylonite zone of about 700 m thick is represented by a fragment of a granite–schist subcomplex as interbedding of schists and conformable granite veins. This part of the section is distinguished by abundant tectonites as recrystallization and mylonitization of metamorphic rocks of the amphibolite facies. The maximum degree of mylonitization is noted in the area where thin biotite granite veins are the most widespread; they form, together with schists, interbedding sequences (interval 1072–1272 m). Quartz grains are the most susceptible to dynamic recrystallization and plastic deformations, whereas feldspars show only fractures leading to typical augen structures in mylonites. The formation of mylonite, as suggested by some researchers, is related to thermal pulses caused by the intrusions [6]. For this reason, the spatial conjugation of mylonites and granite veins is not accidental and is also observed in other metamorphic core complexes. Taking into account the occurrence of granite bodies, their structural features, and widespread mylonitization and recrystallization, they are probably of synkinematic nature. Hence, taking into account the above-stated isotopic geochronology data, the formation age of mylonites is the Early Cretaceous. Biotite granites were formed at this stage (130 Ma) fully corresponding to the most common age of synkinematic granites in the metamorphic cores of the Asian region [10].

The granite–plagiogneiss subcomplex dominated by leucogranites and gneissic granites is located under the mylonite zone in the reference section of the borehole. Plastic deformation signs as small flow folds are recorded in it. This part of the section is characterized by the absence of intensive reflections in the RSS–CDP seismic section. Gradual transitions from plagiogneisses to gneissic granites actually being shadow granites are indicative of a high degree of melting of the gneiss substrate in the course of metamorphism.

CONCLUSIONS

The section drilled by the Trans-Baikal parametric borehole is fully described with the help of the core material, a set of geophysical studies in the borehole shaft, and detailed petrophysical, petrographic, and geochemical study results. The data obtained made it possible to verify the geophysical sections along the reference profile and to determine the geological nature of the deep structure observed. The metamorphic core section was uncovered in the borehole section. In the course of the multistage evolution including the intracontinental extension at the postcollision stage, this complex was brought into the upper horizons of the Earth’s crust and became available for direct study. This section is considered as a reference for studying the structure of metamorphic core complexes. Our further study will make it possible to answer many questions about the formation of such structures and the evolution of the Central Asian fold belt.