Vendian age of igneous rocks of the Chamberlain valley area (Northern part of the Wedel Jarlsberg Land, Svalbard Archipelago)

Introduction

For many decades, the age of the formation of Svalbard folded basement has been considered Caledonian and linked to the time and processes of the Scandinavian Caledonian formation [1-3]. Nevertheless, continuously, since the last third of the twentieth century, some authors, based on different materials, have argued for an earlier consolidation of this crust block [4-6]. Lately [7, 8] they have shown that the structural paragenesis of the Wedel Jarlsberg Land (WJL) Upper Cambrian complexes and adjoining areas of the archipelago is very similar to that of the Protouralian-Tymanian complexes of the Polar Urals and southern part of the Novaya Zemlya Archipelago. The apparent discrepancy in the spatial orientation of this paragenesis with the strike of the assumed Scandinavian Caledonian front continuation on the Barents Sea shelf [9, 10] suggests that the rock complexes of Svalbard folded basement are not Caledonian but represent a northwestern extension of the Protouralian-Tymanian complexes. Thus, there are strong grounds to suggest that the structure of ancient Svalbard folded basement contains both different-aged Early Precambrian and Grenvillean structural-compositional complexes [11, 12] and later Baikalian (or Protouralian-Timanian).

At the same time, the absence of specific geological bodies of this age on the archipelago, primarily magmatic, does not allow to construct complete tectonic schemes. It should be also noted that there are conglomerate beds dating to the Vendian age and identified as metatillites in the main distribution areas of the Late Precambrian complexes in Svalbard [13-15]. The presence of this formation allowed B.P.Barkhatov [16] to describe for the first time Svalbard as an Epibaikal orogenic area.

Statement of the problem

The geological structure, petrological features, and age of the magmatic formations of the Chamberlain valley region in the northern part of WJL of the Svalbard Archipelago remain poorly understood to date. This work is a new attempt to determine the general sequence of Late Precambrian magmatism in the area and to clarify its geological structure.

The Wedel Jarlsberg Land is one of the key areas of the Svalbard Archipelago, which has been used as an example to form a modern view of the geological structure of the folded basement of the region. In the north of this area (Fig.1), Late Precambrian volcanogenic-sedimentary formations are exposed to the surface, hosting comagmatic intrusions, and overlapping them with a structural unconformity of Late Paleozoic and younger sediments [17-19]. The Late Precambrian complexes are dislocated into a series of large conjugate folded forms (Fig.2).

Within the folded basement, the Middle Riphean Verenscheldbreen complex (> 3 km thick), the Upper Riphean Sofiebogen series (> 3 km thick), and the Vendian Kapp Lee series (4 km thick) have been previously identified. There is a structural unconformity between the Middle and Upper Riphean stratones [20], and there is also an unconformity between the Upper Riphean and Vendian horizons. The strata age is confirmed by paleontological finds [21] and studies of detrital zircons [22-24]. An important specific feature of the area is that the structural-geological information obtained here allows us to identify mesostructural parageneseses in Upper Precambrian complexes oriented at a high angle to the structures of Precambrian blocks, which are known in the north of the archipelago and are defined as Caledonian. These materials have allowed several researchers to identify here the Late Precambrian stage of tectogenesis – Baikalian or Timanian-Protouralian [7, 17], but it has not been confirmed by isotopic dating of magmatic or metamorphic objects.

S.I.Turchenko and co-authors [17] have attributed the magmatic formations of the Chamberlain valley area to the single basalt-trachyandesitic formation of the Early Riphean. The geological map [19] shows these rocks as Late Proterozoic. A.M.Tebenkov [2] had the same opinion. Later, Middle Riphean isotopic dating was obtained for magmatic complexes of the southern part of WJL [25-27], and, accordingly, their formation period was attributed to the Middle Riphean [11]. Absolute ages of magmatic rocks in the northern part of WJL have not been determined by anyone until recently. At the same time, the dating of magmatic rocks in other areas of Svalbard should be noted: zircons in granites composing boulders in the Vendian conglomerates of the Nordenscheld Land (50 km to the north of the WJL), are dated at 656 Ma [28]; pegmatites from the Lower Riphean rocks in the southern part of the WJL contain zircons with ages of 615 Ma [24]. So far, this is the most reliable data proving the possibility of the Vendian endogenous processes on Svalbard. Importantly, the Polish geologists [29] suggested that the age of magmatic events in the northern part of the WJL could be about 600 Ma.

Our work resulted in the first isotopic dating of volcanic and intrusive rocks of the Chamberlain Valley and its surroundings, which provided the basis for a new scheme of stratigraphic dissection of the Precambrian complexes of the area.

The overall structure of the Late Precambrian complexes in the northern WJL is characterized by a combination of three major folded forms, the Nordbuchta and Antoniabreen anticlines, and the Kapp Lee syncline separating them (Fig.2); the structures have a NW hinge strike. This fact points to the formation of the Late Precambrian complex structure under the compression conditions in the SW-NE direction with the general direction of tectonic motion to the NE [7].

A trough with the Vendian conglomerates is located in the Kapp Lyell syncline. To the east of the trough, the Chemberlendalen valley reveals a sedimentary-volcanogenic complex with intrusions of the basic-hyperbasic formation. The area of the Chemberlendalen and the adjacent parts of the Rechurchbreen and Antoniabreen glaciers is a junction of the anticline and syncline (Fig.2). The oldest rocks here are the rocks of the Verenscheldbreen complex, represented in the north of the WJL by the Rechurchbreen (> 1.5 km) and Nordbucht (> 2 km) series. The outcrops of the Rechurchbreen series are observed in the west face of the glacier of the same name, as well as in the southern part of the Chamberlain valley. The series is composed of conglomerates, gravelites, Ser-Chl schists, and piles of greenstones at the bottom; at the top, marbles with interlayers of schists and metavolcanics. The sides of the Antoniabreen Glacier are composed of sediments of the Nordbucht series (phyllites, quartzites, microgneisses, greenstones, and dolomites). The age of rocks of the Verenscheldbreen complex in the north of the WJL is determined, among others, by the age of the metamagmatic rocks intruding them (1187-936 Ma) and, when using materials from the southern part of the WJL, by a younger age – 1150-950 Ma [26, 27].

The Sofiebogen Series (RF₃) is exposed at the base of the west face of the Chamberlain Valley. Here they are represented by the upper Goshamn Formation (~2 km), which is composed of phyllites with interlayers of quartzite; the lower formations of this series, the Slungfjellet (conglomerates, up to 600 m) and Heferpünten (marbles, 500 m – with angular unconformity, overlie the Nordbucht Series outcrop). There is a clear structural unconformity between the Vendian conglomerates of the Kapp Lee series and the underlying phyllites of the Goshamn Formation.

The northern part of the Chamberlain valley is composed of a thick sedimentary-volcanogenic sequence of the Chemberlendalen series (~2 km), comprising volcanogenic rocks, phyllites, quartzites, limestones with conformable bodies of peridotites and dolerites. The sequence forms a northwest-trending syncline tilted northeastward. It is separated by discontinuities from the rocks of the Rechurchbreen series to the east and south and from the Goshamn Formation to the west. The age of the magmatic rocks of the Chemberlendalen series, based on absolute dating, has been determined by the authors of the article as Late Vendian (593-559 Ma).

Methods

Magmatic formations in the Chamberlain Valley and adjacent areas have been studied. Ten geochronological samples were taken from these rocks (Fig.2), each weighing about 5 kg. After crushing and sieving the samples, the heavy non-magnetic fractions, which contained zircon, were separated using electromagnetic separation and heavy fluids. Zircon monofractions were sampled at VSEGEI Central Laboratory, and their isotopic composition was analyzed at the Isotope Centre at VSEGEI. The zircons were aged by the local U-Pb method using the SHRIMP II ion microprobe under the standard procedure [30]. Optical (transmitted and reflected light) and cathodoluminescence (CL) images of zircon grains were used to select zircon dating points. The concentrations of elements in rocks and minerals from the samples taken by the authors were obtained at the VSEGEI Central Laboratory by X-ray fluorescence, microprobe, and ICP-MS analyses. When constructing the spectra of REE distribution their quantity was normalized to the content in chondrite CI [31]. Symbols of rock-forming minerals according to [32] are used in this work.

Results

The magmatic rock compositions of the Chamberlain Valley, the western side of the Recherche Glacier, and the Martin Ridge have been studied (Fig. 2, Tables 1-6).

For the first time granite intrusion of Martin Ridge [21] on Mount Bienemi was discovered and mapped. It is a tectonic remnant of granitoids with a total area of about 2 km² and a thickness of 150-200 m, lying flat (15-20°) with a slope to the west and NW. The plate is pushed on a folded phyllite sequence with interlayers of quartzites and greenstones, which has a NW strike and rock dip angles up to 30-40°. In the contact zone, granites are mylonitized and represented by fine-grained schistose Bt-Ms-Qz-Kfs rocks with small pink porphyroblasts. Among the rocks of the mylonitization zone, granitoid relics in the form of lenses of various sizes are constantly found.

Table 1

Chemical composition of Middle Riphean igneous rocks, wt.%

Note. CI – contamination index; K_f – fractionation factor; F – iron oxidation factor; MgO* – magnesia index; SI – Cuno solidification index.

Table 2

Trace element composition of Middle Riphean igneous rocks, ppm

According to their characteristics (Tables 1-2) granitoids correspond to moderately alkaline granodiorites and diorites (Fig.3). Petrochemical coefficients allow us to attribute them to the group of moderately alkaline and partially high aluminous rocks. The amounts of REE are low, and the degree of differentiation of light REE is higher than that of heavy REE. A prominent Europium minimum is clearly visible. Such trends of REE distribution are typical for the parental magmas of the deep origin connected with magmatic sources in the lower crust. The composition of the rocks indicates a weak differentiation of initial magma, which is reflected in its enrichment with light REE and depletion of Eu. The moderately alkaline type of the melt and other characteristics suggest its formation in intraplate conditions in the interval of the lower crust boundary horizons with the mantle.

Sample 4247-1 is a cataclased granitoid on Mountain Bienhami. Nine of the twelve points obtained gave a concordant age of 937±7 Ma. A discordant age (upper intersection) of 936±13 Ma (Greenville Stage) was also obtained. This age can be attributed to the magmatic stage of intrusion formation.

The group of gabbroids (RF₂) was identified by the authors of this article based on petrographic characteristics [33, 34] and isotopic dating obtained. The rocks of this group compose concordant bodies of varying thickness (from 10-15 to 60 m) among the Ser-Chl schists and dolomites of the Rechurchbreen series (see Fig.2). They are all localized in the southern part of the Chamberlain valley and on the western side of the Rechurchbreen glacier. Two bodies were sampled on the slopes of the Mountain Fold. The first one in outcrop 4181 is more than 40 m thick and is conformably deposited among shales, with hardening zones up to 1-1.5 m in the lower exocontact. A clear differentiation of rock composition is observed: the lower half of the body is composed of gabbro (sample 4181-3) and in the center, gabbro-diorite (sample 4181-4), which is confirmed petrographically and by petrochemical data. The second gabbroid body (outcrop 4366) conformably lies among metamorphosed dolomites and is up to 30 m thick. It is a heavily altered basic rock with relics of a gabbroic structure.

These gabbroids have Act-Ab-Chl composition, Cpx relicts are present in amounts from 3-5 to 40-50 %, Pl up to 10-15 %. Bt and Kfs may present in small quantities, which makes it possible to define some varieties as monzogabbro. At a weak degree of alteration, the newly formed paragenesis is represented by Ab and Chl; at a higher stage of transformation, Act, Chl, Ab, Ep, and Spn appear.

The gabbroids correspond to picrobasalts, trachybasalts, and trachyandesibasalts in composition (Figure 3) and belong to tholeiitic rocks. They are sodic and potassic-sodic in alkalinity, moderately potassic, and moderately aluminous in potassium content. These rocks are typified by a lower iron oxidation factor (up to 0.17 %) and high K_f (up to 79.7), which indicates that the parental magma was formed at depth and is less water-impregnated. The value of the Cuno SI index (45.6-12.4) of gabbroids indicates deep differentiation of the parental magma.

Magma differentiation is reflected in the variability of trace element contents: Cr, Ni, V, Cu, and Sc successively decrease, and Zr, Hf, Ga, Nb, Sr, and Y increase. Total REE content in these rocks is low (135-458 ppm), with the lowest amount in metabasic of 4366 outcrop and the highest in gabbro-diorites. The nature of REE fractionation, determined by the values of (Gd/Yb)_n, (La/Sm)_n, (La/Yb)_n, is also maximum for gabbro-diorites, whereas in gabbro it does not change noticeably, being at the same level. The Eu/Eu* ratio is positive. The distribution of REE normalized to chondrite is uniform, the largest deviations are characteristic for gabbro-diorites by light REE content. According to the recommendations [33, 34], gabbroids are attributed to intraplate continental formations, formed in continental rifting conditions.

By analogy with the gabbroids known in the south of WJL [26, 27], the age of the magmatic rocks studied by the authors of this paper is considered to be Middle Riphean, which is confirmed by new data.

Sample 4181-4 is represented by gabbro-diorites. The rocks are weakly metamorphosed. The concordant age obtained by Zrn from these gabbro-diorites was 967±6 Ma (Fig.4, Table 6). The new result is in good agreement with the ideas about the Grenville tectonic-magmatic and tectonic-metamorphic events, which occurred on Svalbard 1150-950 million years ago [1, 35].

Sample 4366-5 is an altered basic rock with relicts of a gabbroic structure, allowing it to be classified as a metagabbroid. The Zrn sampled from the metabasites yielded a range of ages between 1611 and 858 Ma; most dates are grouped within the 1162-1026 Ma interval, and the concordant age of 1152±11 Ma is calculated from them. This corresponds to the age of the gabbroids in the southern part of the WJL – 1156-1154 Ma [26] and also provides information on the age of the Rechurchbreen series.

The Late Vendian hyperbasic-basic rocks have been encountered and studied in the northern part of the Chamberlain valley. Previously, rocks of this complex were described in detail and, together with the host volcanics, were classified as a single basalt-trachyandesitic series [17]. The thickness of intrusions can exceed 50-60 m. The intrusions are in constant association with metavolcanics of the basic composition. The metavolcanic rocks could be host rocks for the intrusives. Rocks of the complex have experienced regional metamorphism of the greenschist facies. The complex includes Bt wehrlites, metaperidotites, metapyroxenites, and metagabbroids. The host volcanogenic rocks of the Chemberlendalen series include metabasalts, metapicrites, metaandesites, and metatufs.

Table 3

Chemical composition of the Vendian intrusive rocks of the Chamberlain Valley, wt.%

Table 4

Trace elements composition of the Vendian intrusive rocks of the Chamberlain valley, ppm

Gabbroids are the most common rocks in this complex. They are medium-grained varieties consisting of Cpx and Pl, with Ilm and Mag, Py, and Ap present (up to 10 %). Cpx is represented by salite with 60-75 % Di minal; Aug, enriched in Fe and Ti, is also noted in these rocks. Primary minerals are partially replaced by metamorphosed minerals: Act, Chl, Ab (N 0-7), Ep, Ch, Spn. Among Amp, along with metamorphogenic Act, kaersutite is noted. Rock structures change from ophitic to gabbroic, therefore dolerites and gabbro-dolerites can be distinguished.

Peridotites are distinguished by their black color, fine-grained structure, and massive appearance. They are attributed to the wehrlite group: they consist of Cpx and Ol, with constantly associating chromspinel, brownish Hbl, and Bt. Peridotites are metamorphosed: Srp, Chl, Act, Cal, and Tlc are noted. Mag, Cr-Ti-Mag, Ilm, and sulphides Po, Py, Ccp, Pn are permanently present. Microprobe analysis indicates that Cpx is represented in these rocks by Di (ferruginicity within 10-23 %) with increased, up to 5-6 %, Al₂O₃ content and the constant presence of small amounts of Cr, Ti, Mn. Bt is present in two generations. The early, magmatic generation, is Phl with ferruginicity in the range of 21-30 %, as well as high Ti content (0.33-0.48 f.u.) and low Al^VI (0 to 0.19 f.u.). The late biotite generation is metamorphogenic and is represented by annite with ferruginicity 64-66 % and high Al^VI (0.41-0.46 f.u.) and low Ti (0.03-0.05 f.u.). Chromspinels are characterized by a variable amount of Cr₂ O₃ (from 5-13 to 40-41 %) and the constant presence of Fe, Al, Ti, and Mn; half of the studied grains lack Mg. The grains of this mineral are often zonal: the core is represented by chromspinel, and the periphery of the grain is pure Mag. Primary Amp is represented in metaperidotites by magnesio-hornblende with ferruginicity 25-30 %, where a part of Ca is substituted by K (up to 0.25 f.u.), and Si content is 6.15-6.25 f.u. This Amp is also characterized by a high content of Ti and Al. Metamorphogenic Amp is represented by Act with high magnesia.

Metapyroxenites differ from metaperidotites only petrochemically; in thin sections, they contain the same set of metamorphogenic minerals, and the primary minerals, Cpx and Bt, are preserved everywhere. The only difference is the appearance in metapyroxenites of Ab, evolving by Pl.

All the described rock types have been petrochemically characterized previously [17] as basic and ultrabasic varieties of normal alkalinity, as well as alkaline basic and alkaline-ultrabasic varieties. They, in turn, form a single comagmatic series with alkaline and normally alkaline ultrabasic and basic volcanics of the Chemberlendalen series. Tables 3-6 present the petrogenic and trace element compositions of the meta-intrusive and meta-effusive rocks from the Chamberlain valley area.

Table 5

Chemical composition of metavolcanic rocks of the Chemberlendalen series, wt.%

Table 6

Trace element composition of metavolcanic rocks of the Chemberlendalen series, ppm

In the SiO₂ – Na₂O + K₂O diagram (see Fig.3), figurative points of intrusive and volcanogenic rocks form a single cloud and fall into the fields of picrites, picrobasalts, alkaline basalts, trachybasalts, and trachyandesibasalts. According to their alkalinity, these rocks can be classified as potassic, sodic, and potassium-sodic. At the same time, in terms of potassium content, most of these rocks belong to the moderately potassic and high-potassic series. In terms of aluminosilicate nature, moderately aluminous and low aluminous rocks sharply predominate

Biotite wehrlites and metaperidotites are characterized by the lowest silica content (36.8-39.3 %) and the highest magnesia content (23.3-27.2 %); Al₂O₃ and Na₂O + K₂O at K₂O > Na₂O are characterized by low values. The iron oxidation coefficient F is 0.38-0.66; the fractionation coefficient K_f is 31.6-44.7. The rocks are characterized by the lowest CI (1.53-1.81), indicating a minimal presence of crustal material in the parent magma.

Metapyroxenites are characterized by SiO₂ (from 42.8-47.0 %), MgO (5.77-16.0 %), have an increased alkalinity (Na₂O + K₂O to 5 %) and titanium (to 5.79 %). The F oxidation coefficient is low (up to 0.16), and the contamination coefficient is significantly higher than that of peridotites (up to 7.34).

The most numerous group of rocks are metagabbroids, represented by dolerites and gabbro-dolerites, differing in the degree of metamorphism and the character of relict structures. These rocks are characterized by a low iron oxidation coefficient (up to 0.15) and high K_f (up to 77.2).

The compositions of the metavolcanics are identical to the intrusive varieties, which may indicate their genetic unity. At the same time, the CI contamination index in the volcanic rocks is significantly higher, indicating the presence of a significant portion of crustal material in the magmas, while the Cuno SI index is lower, which may be an indication of significant crystallization fractionation.

Peridotites are characterized by the maximum values of transit elements (Cr, Ni, Co) and minimum – Y, Zr, Hf, Ga, V, etc. Pyroxenites occupy in such cases an intermediate position between peridotites and gabbroids. The content of Cr, Ni, and Co is variable but generally higher for gabbroids and for peridotites – Zr, Hf, Ga, Nb, V, Ba, Sr, etc. However, gabbroids are characterized by low concentrations of Cr, Ni, and Co and much higher concentrations of Zr, Hf, Ga, Nb, V, Ba, and Sr. The REE distribution in these rocks is also important. Minimum values of the sum of REE are characteristic for peridotites and pyroxenites, maximum – for gabbroids. The fractionation character of LREE and HREE is remarkable in that for all rock types (Gd/Yb)_n > (La/Sm)_n, i.e. HREE spectrum is more fractionated. At the same time, the (La/Yb)_n ratio has the greatest values for peridotites (up to 28.09) and gabbroids – minimal, but with large variations (2.85-20.46). This indicates an uneven enrichment of light REE in rocks of the Chemberlendalen complex. Eu/Eu* shows a weak positive anomaly (to 1.24-1.27) or its absence (1.02-0.97) for all investigated intrusive rocks. At the same time, a significant overall enrichment of rocks with light REE – (La/Yb)_n up to 21.67-28.09 – brings the studied rocks closer to the basalts of oceanic islands and continental rifts, i.e. to the intraplate complexes.

The distribution of trace elements in metaeffusives of the Chemberlendalen series is largely similar to that of the meta-intrusive rocks. For several rocks, there is a noticeable Europium minimum – up to 0.51. They are also characterized by significant enrichment of light REE compared to heavy ones. In general, for REE from metavolcanic rocks, there is a high level of fractionation.

In previous publications [11, 36] the figurative points of meta-intrusive and meta-effusive rocks according to [17] were put on the diagram of L.S.Borodin [37]. Most of the points fall into the fields of moderately alkaline and alkaline-basalt series. Therefore, taking into account all the presented data, the Chemberlendalen basic-ultrabasic complex should be attributed to the moderately alkaline potassium-sodium series, trachybasalt-trachyandesite formation and with a high degree of confidence – to intraplate formations.

The isotopic age of zircons from these rocks, collected by the authors, made it possible for the first time to obtain data on their absolute age. Seven samples were collected and studied: five from metavolcanogenic rocks interbedded with metasedimentary rocks, and two from concordant bodies of gabbroids deposited in the sedimentary-volcanogenic sequence. The metasedimentary rocks were diagnosed by comparing thin sections and chemical analysis data.

Sample 4174-7 is represented by metabasalts. It is a greenish-grey platy rock deposited as a concordant horizon up to 30 m thick in the sedimentary-volcanogenic sequence. Microscopically, these are Ab-Chl-Cal schists with a porphyroblastic structure; phenocrysts are represented by large Cal grains or aggregates of Cal and Qz. The fine-grained main mass is composed of Cal, Chl, Ab, Qz, Spn, Mag. From the sample 10 Zrn grains were analyzed; all calculated points lie on the Concordia. The obtained figures belong to the same age range from 509.5 to 582 Ma. Concordant age was calculated from nine measurements and was 562±5 Ma (Fig.5).

Sample 4221-1 – metapicrite: metavolcanic layer up to 7 m thick, concordant embedded in dark grey phyllites. Rocks are greenish-grey and fine-grained; in thin sections, they are described as Cal-Ser-Chl shales with Ab and Cal phenocrysts. The main shale fabric is represented by Chl, Ser, Cal, Ab, Qz, Spn, and Opq. 13 isotopic ratios were obtained for Zrn from the sample, all measurements fit into two age ranges: 1674-1897 and 506-627 Ma. The concordant age for Zrn of the second group was calculated from six grains and made 570±6 Ma (Fig.6).

Sample 4371-1 – metaandesibasalts: a layer up to 4 m thick of greenish-grey platy fine-grained rocks, conformably deposited among dark grey phyllites. The thin sections show phenocrysts of Pl; the main mass is composed of Pl, Chl, Cal, Ser, Kfs, and Opq. 15 points of Zrn grains from this rock have been studied. The results obtained can be divided into two age groups: 678-836 and 483-622 Ma. The concordant age was calculated from 10 measurements and amounted to 559±4 Ma (Fig.6).

Sample 4372-1 is the basic metatuf: greenish-gray platy rock with a porphyritic structure. The main mass is represented by Chl, Bt, Cal, Ms, Qz, Pl, and Opq with lenses of Cal-Qz composition; Pl is present as small elongated prisms, indicating its primary magmatic nature. Eleven analyses were made for Zrn from this sample, and the obtained figures give four age groups: 2862±21; 1403±9; 845-975; 543-596 Ma. For Zrn of the last group (six measurements), a concordant age of 593±6 Ma was obtained.

Sample 4174-1 is dolerite: a concordant body over 60 m thick, which is composed of medium-plate, medium-grained rocks. Under the microscope, it is weakly altered dolerite. The main minerals are Cpx and Ab, Ilm and single grains of Kfs are present; Chl, Act, Cal, and Spn are also noted. Eleven measurements were made for Zrn from the sample, and the isotopic ages obtained are in the range of 546-607 Ma. The Concordant age of dolerite calculated from this sample was 590±3 Ma (Fig.7).

Sample 4210-2 is gabbro-dolerite (a layered body of thick, coarse-grained rocks with a clearly visible magmatic structure and up to 10 m thick). The thin sections show fragments of ophitic structure, weakly pronounced cataclasis, and chloritization. Mineral composition: Pl, Cpx, Opq, Chl, Act. Five Zrn grains were taken from the sample, and the results give two age groups: 1744-1868 and 950-1006 Ma, which allows us to consider these Zrn as xenogenic (?), trapped by magma from older underlying complexes.

Sample 4351-1 – metaandesite: a more than 20 m thick horizon of greenish-grey, indistinctly platy fine-grained rocks, conformably interbedded with phyllites. The rock is described as metaandesitic with relicts of porphyritic structure, phenocrysts represented by Pl. Themain fine-grained mass is composed of Pl, Cal, Chl, Opq, Qz, Ser, Spn. Eleven Zrn grains were isolated from the sample; the results are distributed in two groups: 2332-3309 and 1130-1436 Ma, i.e. they should be also considered magma-captured xenogenic crystals (?).

As a result of the geochronological studies, the majority of the studied rocks are dated between 559 and 593 Ma, corresponding to the Vendian, the formation period of the Chemberlendalen series sedimentary-volcanogenic section, and the comagmatic basic and ultrabasic intrusions of the Chemberlendalen complex. The occurrence of older dated Zrn in some of the samples allows them to be considered xenogenic, trapped by magma during upward movement through rocks of the underlying crust.

Discussion

The results of the presented studies of magmatic complexes in the north of the WJL allow us to confidently discern two intervals of magmatic activity: Middle Riphean (1157-936 Ma) and Vendian (593-559 Ma).

Events of the Middle Riphean (Grenvilleian) age are widely observed on Svalbard. Several works have proved that this is the time of consolidation of the ancient basement of the archipelago [6, 11, 22]. In particular, in the southern part of WJL, the formations of this tectonic stage include the Skolfjellet gabbro-granite complex with rocks aged 1156-1072 Ma [26] and metaryolites of the Wimsodden formation (Verenscheldbreen complex) aged 1198-933 Ma [27]; in the northern part of WJL, to the east of Antoniabreen glacier, zircons aged 950±5 Ma [38] were obtained from gneisses of uncertain genesis. Taken together, these materials indicate magmatic and metamorphic events in this area in the 1200-950 Ma age interval, which is consistent with data from other areas of the archipelago [3, 35].

Important data for understanding the age correlation of endogenous processes in the basement rocks of the archipelago were published in [39], where the material of crustal and mantle xenoliths from basanites of Quaternary volcanoes in the northern part of the archipelago was analysed using U-Pb, Pb-Pb, Sm-Nd, Re-Os analytical methods. More than 560 zircons from crustal xenoliths (granitoids, gneisses, granulites) and 300 grains of sulphides from mantle xenoliths (peridotites) resulted in the sharp peak in the interval 1200-900 Ma with a maximum 963±30 Ma, corresponding to the Grenville events. The second dating peak is dated to the interval 750-524 Ma, corresponding to the Baikal events. A slight peak is also obtained in the interval 1900-1600 Ma (Late Karelian time). Archean and especially Caledonian events are represented by sporadic dating.

Our new dating complements this picture, allowing us to identify a new straton of the Middle Riphean age within the Verenscheldbreen complex in the north of the WJL – the Rechurchbreen series, whose rocks contain consonant bodies of metagabbroids with ages of 1152±11 and 967±6 Ma. The age and petrological characteristics of these rocks (intraplate continental formations) allow them to be compared to gabbro and diorites of the Skolfjellet complex in the south of the WJL [10, 26]. At the same time, the data obtained by the authors on age (936±6 Ma) and composition of metagranitoids of the Martin Ridge allow us to correlate them with granitoids and metaryolites also known in the south of WJL. These results make it possible to supplement the modern understanding of the tectonic structure of the whole area and the leading role of the Grenville (Middle Riphean) events in the formation of the archipelago basement.

The most important result is the Vendian dating obtained for the first time for Svalbard for metavolcanic rocks of the Chemberlendalen series, and the gabbroid bodies that break through them. It should be noted that some authors have paid much attention [7, 36, 40] to the Vendian time on Svalbard. According to them, it is marked by rare dating (Rb-Sr, Ar-Ar, K-Ar, U-Pb) on metamorphogenic rocks, mainly in the southwest of the archipelago [10, 41]. The first group of dating (660-580 Ma) corresponds to the Vendian period marked on Svalbard by widespread tillites, including the conglomerate strata of the Kapp Lee series. The second group of dates (553-505 Ma) corresponds mainly to the Cambrian period, which is characterized on Svalbard by the widespread absence of Early-Middle Cambrian sedimentation and structural unconformity with the Late Precambrian sequence [10].

The Baikal stage, which coincides with the Late Proterozoic-Early Paleozoic boundary, remains largely debatable for Svalbard, although it is associated with a change in tectonic regime, formation series, and partial structural restructuring of areas. For this reason, the Vendian tillite-like formation is of special attention [13, 15]. Scientists have different opinions about the tectonic regimes in the Late Proterozoic and Early Paleozoic. They have been considered as both miogeosynclinal [13, 42] and platform [5, 16]. The sections of this period are characterized by regional faulting, low sedimentation rates, amagmatic nature of the Baikal interval of the Ny Friesland Peninsula section as a basement tectonotype of the archipelago, presence of the appreciable amount of metavolcanics and small intrusive bodies in the western sections of Svalbard, low metamorphism of Baikalian age sediments on Ny Friesland, where a low-temperature subfacies of the greenschist facies and prehnite-pumpellyite facies are observed, higher metamorphism, fixed in the high-temperature subfacies of greenschist facies and higher, in western areas, proceeding under the conditions of increased pressures [11, 41]. Note that in Svalbard, the Upper Precambrian and Lower Paleozoic layered complexes are separated by a hiatus in sedimentation and unconformity [10, 13, 17], which are quite comparable in character and duration to the Protouralian-Timanian unconformity [9, 10].

Conclusion

The occurrence of the Vendian dating of magmatic objects on Svalbard was expected and is not an accident. The dating interval of 559-593 Ma, determined by the authors of the article, corresponds to the Vendian period, which confirms the existence of geodynamic conditions generating intraplate volcanism on Svalbard at that time. The nature of the ultrabasic and basic magmatic rocks, as well as the predominantly pelitic composition of the metasedimentary rocks of the Chemberlendalen series, indicate that the sedimentary-volcanogenic sequence was formed during a single tectonic-magmatic cycle when moderately alkaline ultrabasic-basic volcanism was combined with a high degree of chemical weathering of rocks in the sources of the Vendian age material. Therefore, we can assume that the environments of sedimentary-volcanogenic series formation were similar to continental rifting.

The age correlation between two Vendian strata of the archipelago, the Kapp Lyell conglomerate series, and the Chemberlendalen sedimentary-volcanogenic series, allows to consider the former as an earlier one because boulders and pebbles of volcanic rocks are completely absent in the conglomerate horizons, widely represented in all formations of this series (up to 4 km in total thickness). Therefore, we propose to consider the Kapp Lyell series as Lower Vendian and the Chamberlandallen series as Upper Vendian (see Fig.2), which allows us to predict the identification of Upper Vendian analogues in other areas of the western part of the archipelago.

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