The Central Taimyr accretionary belt of the Kara (Taimyr–Severnaya Zemlya) orogen includes terranes of island-arc, oceanic, and continental origins that combined into a single unit close to the margin of the Siberian paleocontinent at the end of the Neoproterozoic era [1–3]. The age of the island-arc rocks of the belt is Neoproterozoic – 967–961 and 755–730 Ma [2, 4, 5] according to geochronological data. They are tectonically joined with the Mamont-Shrenk and Faddey cratonic terranes composed of metaterrigenous biotite-sillimanite and garnet-biotite plagiogneiss, biotite-amphibole crystalline schist and amphibolite of supposedly Paleo-Mesoproterozoic age [6, 7]. Granitoid plutons aged 940–846 Ma and a volcanic-plutonic complex dated at 869–823 Ma intruding these metaterrigenous rocks indicate collisional events assumed to be of microcontinent vs. island arc type [1, 3, 6–8].

The paleotectonic position of the Faddey and Mamont-Shrenk terranes remains unclear. Here we present geostructural, petro-geochemical, U–Pb geochronological, and paleomagnetic data for a batch of samples from over 20 intrusions of the Severobyrranga and Yasnenskiy complexes of metagabbro-dolerite that intrude the metasedimentary and metavolcanic rocks of the Oktyabr and Zhdanov formations in the southwest of the Faddey terrane, in the Barkova– Leningradskaya rivers interfluve (Fig. 1).

Fig. 1.
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Geology of the study area modified from [3] and position of sampling sites. 1, Jurassic–Cenozoic deposits; 2, Triassic gabbro-dolerite dikes; 3, carbonate-terrigenous deposits (NP3–PZ1); 4, Kolosov Formation (NP3): dolomite, stromatolite, oolite, and clastic limestone, rare mudstone interbeds; 5, volcanoplutonic belt formations (869–823 Ma): а—sedimentary-volcanogenic sequence with metarhyolite, metabasalt, tuff and quartzite bed, b—weakly metamorphosed rhyolite-porphyry flow several to 20 m thick, c—Zhdanov (Snezhnin) complex granitoids; 6, Stanovskaya Formation (NP3): siltstone, polymict green sandstone, red-purple sandstone with limestone, conglomerate and quartz gravelstone interbeds; 7, Mesoproterozoic metagabbro sills of the Severobyrranga and Yasnenskiy complexes; 8, Oktyabr and Zhdanov groups—volcanogenic-terrigenous deposits (MP1–2); 9, main and secondary thrusts; 10, bedding attitude; 11, sampling sites and their numbers. Vertical scale of the AA' section is conventional. 1357 ± 9 is the U–Pb zircon age, ref. Table 2 and Fig. 4. Abbreviations on inset: CTB—Central Taimyr Block, P—Pyasina-Faddey suture, M—Main Taimyr suture, I—Mamont-Shrenk terrane, II—Faddey terrane. Rectangle shows the area of the figure.

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According to state geologic mapping data [9, 10], the Oktyabr and Zhdanov formations have a metaterrigenous-carbonate composition and unconformably overlap the granite-metamorphic formations of the Faddey terrane basement. Detrital zircon ages, the composition, thickness and maturity of the deposits possibly indicate that their accumulation took place in passive continental margin settings not earlier than 1900 [11] or 1760 [10] Ma.

Field observations show that the host metasedimentary rocks have multiple traces of syn-sedimentological dislocations in the form of microfaults and microfolds in individual or several layers. The gabbro-dolerite intrusions are usually subconformable stratiform bodies with a thickness varying from 0.5 to 200 m. The upper contacts of the bodies display evidence of active intrusion (brecciation, host rocks xenoliths, schistosity etc.), which unequivocally characterizes them as sills. Deformed together with the hosting sequence, they feature a system of three large and several smaller tectonic sheets (Fig. 1). The rocks are intensely foliated in the fault plane areas and usually are overturned. Secondary deformations of fold axial planes indicate that the fold-thrust structure was formed at least in two stages of NW–SE compression. Inclined fold hinges, crenulation cleavage, late deformations of quartz veins – indicate a later strike-slip component of deformation.

The petrographic composition of the rocks of the sills was determined using a Nikon Eclipse LV100N POL polarizing microscope (IPGG SB RAS, Novosibirsk). Geochemical and isotope studies were done in the Centre of Isotopic Research and the Central Analytical Laboratory of VSEGEI (St. Petersburg). Main elements contents were detected by X-ray fluorescence with a relative error of 1–5%. Rare earth elements and other trace elements were determined by emission spectrometry with inductively coupled plasma on an Optima-4300 spectrometer: ICP-AES for Со, Ni, Zn, Pb, Li, Sc, and С; ICP-MS for the other elements including REEs. U–Pb analysis of zircon from metagabbro was performed on a SHRIMP-II SIMS. Paleomagnetic investigations were performed following standard procedures in the Laboratory of Geodynamics and Paleomagnetism of IPGG SB RAS and the Laboratory of Geodynamics and Paleomagnetism of the Central and Eastern Arctic of NSU (Novosibirsk).

Petrographic studies of the sills show that they are composed of fine-grained metagabbro-dolerites with relict gabbro, porphyry, and ophitic textures transitioning to fibroblastic and blasto-prismatic texture. The metamorphism level does not exceed the greenschist facies. Plagioclase (50–60%) is replaced by albite, carbonate and minerals of the epidote group; clinopyroxene (up to 30%) is replaced by amphibole and chlorite; hornblende (up to 20%) – by actinolite. There are individual grains of olivine replaced by serpentine, talc and chlorite. Accessory minerals include apatite, magnetite, and ilmenite that develops leucoxene pseudomorphs.

Results of geochemical investigations of the metagabbrodolerites are given in Table 1. Their SiO2 – FeO*/MgO affinity [12] corresponds to tholeiitic rocks, and the SiO2–K2O relationship [13] ranges from the tholeiitic to, less often, the low-K calc-alkaline series. REE distributions and multielement spectra in the metagabbro show an intermediate position between OIB and E-MORB for trace-elements (Fig. 2).

Table 1. Chemical composition of the studied metagabbrodolerites
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Fig. 2.
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REE distributions and multielement spectra for the studied rocks. Elements contents are normalized for contents in Chondrite from [15] and Primitive mantle in [16].

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One sample (T10-05) from a metagabbro sill in bank outcrops of the Leningradskaya R., 1.5 km downstream of the Barkova R. mouth yielded 11 grains for U–Th–Pb investigation (Table 2, Fig. 3). The zircons have a prismatic habit, are weakly fractured and have a distinct magmatic growth zoning. Two discordant analyses (4.1 and 2.1) were excluded from calculation of the mean age. The remaining 9 analyses plot on concordia with an age 1357 ± 9 Ma (MSWD = 0.88), which is interpreted as the crystallization age of the rock. Close ages – 1365 ± 11 Ma (SIMS, baddeleyite), 1374 ± 10 Ma and 1348 ± 37 Ma (by 207Pb/206Pb ratios) have been published in [11, 14].

Table 2. Results of U–Th–Pb investigations of metagabbro from sample Т10-05
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Fig. 3.
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Concordia diagram and CL images for zircon from metagabbro sample T10-05 (uncertainties are 2σ). Numbered ovals on CL images correspond to spot numbers in Table 2.

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The performed rock magnetic and paleomagnetic experiments on the metagabbrodolerites and the host rocks have shown that the paleomagnetic signal was poorly preserved and rewritten multiple times. In many of the studied outcrops, a characteristic component could not be identified. However, using methods of combined analysis of directions and great circles, for eight sills and host rocks we were able to detect two metachronous components in the natural remanent magnetization (Table 3, Fig. 4).

Table 3. Paleomagnetic directions and paleopole coordinates from results of investigations of the metagabbrodolerites and host rocks
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Fig. 4.
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Top diagrams show typical results of incremental demagnetization of gabbro samples recording the directions of the first (left, stratigraphic coordinates) and second (right, geographic coordinates) vector groups. Closed symbols are projections on the lower hemisphere (positive vector inclination), open symbols are projections on the upper hemisphere (negative vector inclination). The bottom scheme shows the position of the paleomagnetic poles (data from Table 3) relative to Siberia’s APWP from [17]. Numbers close to arcs connecting poles indicate the angular distance.

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The first component is characterized by northern declination and positive inclination of the remanence vector. According to the fold test results, it was recorded by the rocks prior to deformation. The highest precision is achieved at 85.8 ± 12.2% untilting. Nonetheless, we consider this component a metachronous one. Paleomagnetic directions in stratigraphic coordinates and the paleomagnetic poles calculated for them coincide within the margin of error with those obtained for Neoproterozoic rhyolites in the same area [3]. Their formation reflects crust-forming processes ca. ~840 Ma related to the collision of a cratonic block that included the Faddey and Mamont-Shrenk terranes with an island arc [3]. The regional heating caused by this event probably led to the remagnetization that destroyed the primary Mesoproterozoic paleomagnetic record corresponding to the time the sills intruded. At the same time, the pole coordinates calculated for the mean direction of this component differ from the expected one according to the apparent polar wander path (APWP) of Siberia [17] by an angle of about 30° (Fig. 4). Combining the poles requires first of all a change in the latitude (Table 3). Correspondingly, the events reconstructed for 840 Ma should have taken place at a significant distance from the margin of the Siberian paleocontinent. The basin separating the Faddey terrane and the Taimyr passive continental margin of Siberia was hundreds of kilometers in size. This conclusion is supported by paleomagnetic determinations for rocks of the oldest (960 Ma) island-arc complex (Three Sisters) located close to the study area [2]. Therefore, there is a basis for assuming that during the intrusion of the sills ca. 1350 Ma, the Faddey terrane (or the cratonic block it was part of) was similarly located far from the Taimyr margin of Siberia.

The second characteristic component has a noticeably more compact distribution of vectors in geographic coordinates (Table 3), meaning it was recorded by the rocks after the formation of the current fold-thrust structure of the Kara orogen. Maximum precision is achieved at 16.5 ± 9.3% untilting. Although there is no complete coincidence of the corresponding paleomagnetic pole with Siberia’s APWP, it is located close to its late Paleozoic–early Mesozoic segment (Fig. 4). This indicates a probable connection of the regional remagnetization with thermal events caused by the collision with the Kara microcontinent or the subsequent mantle plume magmatism manifested as the Siberian large igneous province. The observed differences in position of the compared poles imply the rotation of the Faddey terrane without changing its actual distance from the craton (Table 3). Such kinematics can be realized only in strike-slip conditions. Based on our magnetotectonic model, according to which the collision with the Kara microcontinent resulted from a soft, oblique convergence of tectonic plates [18, 19], the strike-slip mode of transformation of the orogen’s structure at the Paleozoic– Mesozoic boundary is to be expected and confirmed by our new paleomagnetic data. The lowest angular distance between the observed pole and the APWP is at 200 Ma. The Faddey terrane should be rotated around its axis to no more than 36.1° ± 24.5° (Table 3, Fig. 4). However, other variants of comparison are not excluded. From the model [18], a rotation around a Euler pole located in the center of the Kara block (which is the main indenter that caused the deformation of the Siberian margin) would cause the best coincidence of the calculated pole with the Siberian APWP at the Carboniferous–Permian boundary, about 300–280 Ma. During this time, the lithosphere was strongly heated by the formation of the Kara orogen, which is confirmed by absolute ages of collisional granitoids and numerical modeling [19, 20].

In conclusion, we obtained new geostructural, petro-geochemical, U–Pb geochronological and paleomagnetic data for the metagabbrodolerites of the Severobyrranga/Yasnenskiy complex. The determined U–Th–Pb age of crystallization of the metagabbro-dolerite 1357 ± 9 Ma and their structural position indicate that the intrusion of the sills took place in the Mesoproterozoic rocks of the Oktyabr and Zhdanov formations, probably in a rifted basin setting of a passive continental margin or a marginal sea. This is not contradicted by the geochemical data obtained for these rocks. The deformation of both the metaterrigenous rocks and the intrusions occurred later, probably during the continent—island arc accretionary-collisional events. New results of paleomagnetic investigations of the metagabbro-dolerite sills revealed a record of two metachronous components. These reflect crust-forming processes at ca. 840 Ma at several hundred kilometers from the Siberian margin, as well as thermal events at the Paleozoic–Mesozoic boundary due to the Kara orogeny.