The Suavjärvi impact structure, NW Russia

Authors


Corresponding author. E-mail: m_naumov@mail.ru

Abstract

Abstract– We present the geology and interpreted shock features of the Suavjärvi circular structure. Suavjärvi is a circular feature (illustrated by satellite imagery, topography, and magnetic data) located in the central part of the Karelian Craton (lat. 63°07′N, long. 33°23′E). To date, little information on the geologic and impact features of the Suavjärvi structure is available in the literature. The structure is characterized by gravity and magnetic lows and disruption of the regional magnetic fabric. In the northeastern and southwestern parts of the structure, several erosional remnants of highly disturbed rocks occur referred to as monomict and polymict megabreccia. These comprise blocks of both basement granitoids and supracrustal greenstone rocks. The impact origin of polymict megabreccia and therefore of the Suavjärvi structure is confirmed by observations of closely spaced planar microstructures at angles consistent with planes that have Miller indices indicative of impact shock effects, mostly of ω{10¯13}. The Suavjärvi is considered to be a remnant of a deeply eroded and metamorphosed impact structure, which has a diameter of 16 km and was formed during the Paleoproterozoic (older than 2.2 Ga); this is inferred from the age of the overlying volcanic-sedimentary Jatulian sequence. Suavjärvi underwent regional metamorphism that resulted in obliteration or transformation of shock metamorphic effects. Massive sulfides occur within megabreccia; originating probably from postimpact redeposition of pre-existing mineralization.

Introduction

The temporal distribution of known terrestrial impact structures is highly biased toward those of a younger age (Grieve 1997). To date, few Early Proterozoic astroblemes have been identified, namely the giant Sudbury (Canada) and Vredefort (South Africa) structures. The recently discovered Yarrabubba structure (Australia) is presumably also of Early Proterozoic age (McDonald et al. 2003). Therefore, the identification of a circular feature, described previously as “the Suavjärvi structure” and interpreted as an impact site of Early Proterozoic age (Mashchak and Orlova 1986, Mashchak and Naumov 1996), warrants further scrutiny. This structure was listed in Fennoscandian and world impact crater databases (e.g., Abels et al. 2002; Earth Impact Database 2012). However, the impact origin of the Suavjärvi structure was questioned due to both poor data on shock features and absence of peer-reviewed publications in the international literature (Reimold 2007).

In this article, we seek to address this shortcoming and present available geologic and mineralogical data on Suavjärvi. A main goal of this article is to provide a summary of the geology and deformation features of Suavjärvi to provide evidence of its impact origin and outline possible further study.

The Suavjärvi structure (lat. 63°07′ N, long. 33°23′ E) is located in the Republic of Karelia, NW Russia, approximately 50 km northwest of the town of Medvejegorsk, southwest of Lake Segozero. This area was the object of detailed geologic exploration as early as the 1930s and the beginning of the 1950s, due to the occurrence of sulfide mineralization. This included the Bergaul Mine, exploited in the 19th century. The exploration work was carried out by the North-Western Regional Geological Department (SevZapGeologija). These efforts did not reveal any economic resources in the area, but resulted in its geologic and petrographic description and detailed mapping (Belitsky 1935; Zil’ber and Bychkova 1954; Gromova 1955; and other unpublished reports of SevZapGeologija). The principal results of the exploration work were summarized on the State Geologic Map of 1:200,000 scale (Yakovleva and Savina 1965). Later, geophysical measurements, including airborne magnetic and gravity surveys, were made over Suavjärvi as part of the systematic mapping of Karelia. These data are available only as internal reports.

A circular feature southward of Lake Segozero had been noted on the Cosmogeological Map of the eastern part of the Baltic Shield (Berendeev 1981). However, no follow-up study or publications resulted. In 1983–1984, this circular feature was studied by geologists of the Karpinsky Geological Research Institute (St. Petersburg) among several dozens of enigmatic breccia occurrences in Karelia and Kola regions. The study resulted in the detection of interpreted shock metamorphic features (Mashchak and Orlova 1986). This provided evidence for the circular feature to be regarded as a deeply eroded impact structure (Mashchak and Naumov 1996); it was named as Suavjärvi after a lake located in the center of the feature.

Regional Geology

The Suavjärvi structure is situated within the Karelian craton, which is the oldest (Neoarchean) stable domain of the Fennoscandian Shield (Lobach-Zhuchenko et al. 1986; Rundquist and Mitrophanov 1993; Mints et al. 2009; and references therein). The basement of the craton is a late Archean (Lopian) granite-greenstone terrane consisting of granitoid-gneiss complexes and supracrustal rocks ranging in age between 3.14 and 2.62 Ga. Supracrustal metavolcanic-sedimentary rocks form north-trending greenstone belts, surrounded by spatially more extensive granitoids and higher grade gneiss domains (Fig. 1). Greenstone belts ranging in length between 2–3 km and 100–150 km are built by metasediments, mafic, komatiitic, and intermediate-felsic metavolcanics dated at 3.05–2.72 Ga. The Paleoproterozoic sedimentary-volcanic “proto-cover” (Sumian-Sariolian Group) that occurs preserved within the limits of rift structures blanketed the Karelian craton at 2.50–2.43 Ga while an overlying, widespread platform cover (Jatulian Group) is dated by 2.3–2.1 Ga.

Figure 1.

 Geologic map of the Karelian Craton (simplified after Lobach-Zhuchenko 1988) showing the locations of the Suavjarvi circular feature as well as of nearby confirmed impact structures (after Abels et al. 2002).

Geology of Suavjärvi Structure

As stated above, a circular structure of unclear origin near the southwestern corner of Lake Segozero has been noted for the first time by the processing of Meteor-2 satellite images (Berendeev 1981). The Suavjärvi structure is also evident on Landsat photographs (Fig. 2) as a near-circular feature of light colors of about 16 km in diameter, including a dark-colored inner ring of 7 km in diameter, displaced slightly toward the northeastern part of the circular feature. This inner ring is not evident in gravity, magnetic data, topography, or geologic patterns. Its nature remains obscure.

Figure 2.

 The Suavjärvi structure (indicated by arrows) on a satellite image. Landsat photograph ERTS E-2097-08243-7-01.

The circular imagery feature is emphasized by arciform lakes (Aeningalampi and Gormozero lakes) along its margins. The area is a hilly and swampy plain with elevations of 5–15 m and numerous lakes. On the north and west, this plain is surrounded by narrow stone ridges up to 80 m high (so-called “selgas”) that separate it from the large Seletsk and Segozero lakes.

The observed gravity field over Suavjärvi (Beda 1972) reaches a minimum of −20 mGal with respect to the general level of the regional field of about −10 to −16 mGal in the vicinity of the circular feature. This results in a gravity anomaly of about −4 to −10 mGal. This negative anomaly extends southward from the structure.

From airborne magnetic survey (Val’kov 1967), the Suavjärvi area is characterized by a slightly contrast magnetic low in which the regional patterns (positive linear anomalies trending NNW), caused by metavolcanics of greenstone belts, are partly subdued. Magnetic field strength varies from +0.5 to −3.0 mE (Fig. 3). In the western and northeastern sectors of the circular feature, some concentric patterns are observed, caused possibly by remnants of supracrustal volcanics within annular depressions in the granitic bedrock. Strongly magnetic linear anomalies to the north and west of Suavjärvi are caused by metadolerite sills of Jatulian age that postdate the circular structure.

Figure 3.

 Total aeromagnetic intensity map of the Suavjärvi area (from Val’kov 1967). White dashed line is a contour of the circular feature on satellite imagery. Linear anomalies to the north and west of Suavjärvi are caused by metadolerite sills of the Jatulian that postdate the circular structure.

Similar to the Karelian craton on the whole, more than 80% of the Suavjärvi area is composed of various granitoids and granitic gneiss of Archean and Early Proterozoic. Different stratigraphic charts for the Early Precambrian of Karelia have been suggested in the past. According to the recent State Geologic Map of the Russian Federation (Bogdanov 2000), four bedrock suites comprise the Suavjarvi area: (1) Saamian to Early Lopian (> 2.8 Ga) ultrametamorphic complexes; (2) Lopian (Upper Archean; 3.15–2.6 Ga) greenschist rocks; (3) Late Lopian porphyritic granite; (4) Lower Karelian (2.5–1.9 Ga) Jatulian Group.

The Suavjärvi structure is developed primarily within the oldest suite, which includes migmatitic gneiss, gneissic granitoids (mostly plagioclasic varieties), and orthoamphibolite (Fig. 4). Gneissic or gneissoid textures trend northerly or northwesterly with dips either westerly or easterly and under 60–80°. The rocks are commonly cataclastized and migmatized. The gneissic granitoids contain numerous shadow relict lenses of biotite and amphibole gneiss and plagioamphibolite presumably of Saamian age (Lower Archean; >3.15 Ga). These lenses measure up to 3–5 km long and 1.5–2 km wide.

Figure 4.

 Schematic geologic map of the Suavjärvi area compiled on the basis of detailed mapping (Zil’ber and Bychkova 1954; Gromova 1955), and State Geological Maps (Yakovleva and Savina 1965; Bogdanov 2000). Also shown are Bergaul mine and a continuous polymict breccia outcrop (see Fig. 6) where shock features in minerals were revealed.

Lopian units include metamorphosed volcanic-sedimentary sequences comprised of greenstone belts. Small areas of greenstone rocks, representing fragments of the nearby Vedlozero–Segozero greenstone belt, are observed at northwestern and central parts of the area. They consist of alternating quartz-sericite-chlorite schist (with interlayered magnetite quartzite and magnetite-amphibole schist), mica schist, phyllite, metamorphosed quartz porphyry, minor metabasalt and metasandstone. This sequence is related to the Upper Lopian (2.8–2.6 Ga) Himola Formation (Bogdanov 2000). A wedge-like area of greenstone rocks known as Bergaul Formation occurs in the northeastern part of the circular feature. It differs strongly from the above sequence in composition (mostly metamorphosed basic lithologies) and is attributed to the Parandov Group of Lower Lopian (Yakovleva and Savina 1965; Bogdanov 2000). However, we interpret this body differently, hence the Bergaul Formation is examined further below.

Late Lopian porphyritic granite occurs commonly close to the margins of the circular feature where they are associated with greenstone rocks. Furthermore, aplite and pegmatite veins intrude both granitic and metasedimentary Lopian rocks. Southeast of Suavjärvi similar granites have been dated at 2.70–2.68 Ga (Bogdanov 2000).

All units mentioned are overlain by the volcanic-metasedimenary Yangozero Formation related to the Paleoproterozoic Jatulian Group (provisionally dated at 2.3–2.1 Ga) that occurs in northern and western parts of the area. Jatulian lies ubiquitously on the weathering crust formed from granitoids and other basement rocks (Yakovleva and Savina 1965). At its base, basal boulderstone up to 10 m thick occurs, consisting mostly of pebbles of weathered basement rocks from 0.5 to 40 cm in diameter cemented by a sericite-quartz (in places, with chlorite and biotite) matrix. Upward in the section, the Yangozero Formation consists mostly of quartzite and quartzose sandstone intruded by two dolerite sills of 200 and 50 m thick (Yakovleva and Savina 1965; Kharitonov 1966). The total thickness of the formation, south of Lake Segozero, is estimated to be about 410 m. The regional metamorphic grade of the Jatulian varies from greenschist grade to epidote-amphibolite grade.

Most of the Suavjärvi area is covered with recent palustrine, lacustrine, and talus deposits up to 10–15 m thick.

In contrast to previously described regional stratigraphic units, the above-mentioned Bergaul Formation is unique to the Suavjärvi area. It was described in the vicinity of the abandoned Bergaul mine in 1938 (Kharitonov 1941) and encompassed small bodies of greenstone lithologies occurring within Proterozoic granites. Inner stratigraphy, layer succession, local composition, thickness, age, and areal extent of the Bergaul Formation were interpreted in different ways afterward. First, Kharitonov (1941) distinguished a lower schist-dominated thin-intercalated sequence (but including also augen gneiss, amphibolite, some skarns, and marble) of about 80 m thick, and an upper thick (>300 m) metabasite-greenschist sequence. This author placed the Bergaul Formation in the Karelian Supergroup of Mesoproterozoic. The geologists who carried out the detailed mapping in the 1950s (M. S. Zil’ber, T. Z. Gromova, and others) did not differentiate the Bergaul Formation, but noted its very diverse composition. Afterward, many authors including Kharitonov (1966) ascribed the Bergaul Formation to different greenstone lithologies occurring within the Vedlozero-Segozero greenstone belt, located northwest, southwest, and southeast of Bergaul (see Shurkin [1989] and references therein). The section southwest of Lake Segozero was regarded as a type section. The formation was commonly attributed to the Upper Lopian. On the State Geological Map of 1:200,000 scale (Yakovleva and Savina 1965), thin-layered phyllite and microschists (quartz-chlorite, quartz-chlorite-biotite, quartz-sericite-chlorite, etc.) are considered as a lower subformation while alternating metabasalt, metaporphyrite, greenschist, and sericite-quartz and other schists are considered as an upper subformation—in contract to Kharitonov’s scheme. In the most recent regional chart (Bogdanov 2000), the name “Bergaul Formation” is reserved only for a sequence composing a small (about 6 km2) wedge close to the Bergaul mine. For this modest area, three subformations, 1150 m in total thickness, have been distinguished: (1) a lower, composed of metabasalt (epidote-amphibole schist); (2) a middle, including metamorphosed dacite tuff (quartz-plagioclase-biotite schist), siliceous tuffite, quartz-sericite and graphite-bearing schists, carbonaceous sandstone, and skarns; and (3) an upper, consisting of foliated metabasalt. The stratigraphic position of the Bergaul Formation was revised again and placed to the Lower Lopian.

The metamorphic grade of the Bergaul Formation is also revised. It is believed either to correspond to greenschist facies (e.g., Yakovleva and Savina 1965) or to display a zonal metamorphism from greenschist grade to amphibolite grade (e.g., Shurkin 1989). It is commonly accepted that metamorphic overprinting was due to the Svecofennian tectonic event (approximately 1.8–1.9 Ga) (Bogdanov 2000).

The extremely mixed composition of the Bergaul Formation within a limited area as well as important differences in interpretations by many researchers of its inner stratigraphy, layer succession, thickness, and age point to the fact that the Bergaul Formation represents a mixture of fragments originating from various stratigraphic units. Our field observations combined with data from previous detailed mapping (Gromova 1955, and others) provide evidence for the so-called “greenstone sequence” occurring in the vicinity of Bergaul and Lake Gormozero as well as similar bodies in other segments of the Suavjärvi area, to be considered as megabreccias consisting of both greenstone metavolcanic and metasedimentary lithologies (of both Lower and Upper Lopian) and granitic gneiss, migmatite, and granitoids of pre-Lopian basement and Later Lopian granite.

Megabreccias

Three local areas where megabreccias occur within Suavjärvi are distinguished: (1) Bergaul, in the northeastern part of the Suavjärvi structure; (2) Sel’ga, in the southwestern part, along the eastern shore of Lake Aeningalampi; (3) Lizhma, 1–3 km east of the Sel’ga area. Two varieties of megabreccias could be distinguished as inferred from the contribution of various lithologies: polymict megabreccia and monomict granitic megabreccia. The term ‘‘polymict breccia’’ is employed herein to designate breccias that specifically include fragments from greenstone lithologies and from granitoids. Megabreccia consisting almost entirely of granitoid megablocks (originated from both Archean granitic gneiss/gneissic granite and Late Lopian porphyritic granite) with occasional greenstone fragments is herein termed as “monomict (granitic) megabreccia.”

In the Bergaul locality, there is a wedge-like, NNW-elongated body of polymict megabreccia of 1.5 × 4 km in size. It extends from Bergaul mine to the southern end of Lake Gormozero. The body occurs within cataclastized Later Lopian granite. In places (e.g., west of Lake Gormozero), xenoliths of greenschists are observed within deformed granite. Thus, this granite area may be regarded as monomict granitic megabreccia containing rare foreign clastics. For example, north of Bergaul, brecciated granite contains some irregularly shaped “xenoliths” reaching as much as 75 × 305 m in size and dipping randomly (from NE 60–70° to NW 330–340° under 40–70°). The “xenoliths” include phyllite, graphite-quartz-sericite schist, and epidote-actinolite and garnet-epidote-pyroxene schists. Northeastward, this granitic megabreccia changes gradually to brecciated granite.

Polymict megabreccia consists of blocks ranging from 0.5 × 7 m to 100 × 200 m in size oriented both northwesterly and (more rarely) northeasterly. This fragmental structure is well illustrated on large-scale maps and plans of the megabreccia field (see, e.g., Fig. 5). Lithologies comprising the blocks are schists (of amphibole-biotite, biotite-chlorite, epidote-actinolite, etc. composition), marble, skarns of so-called “Bergaul Formation” and gneissic granite or brecciated granite, the latter being predominant. Granite blocks are commonly cataclastized and brecciated. Blocks are separated by chlorite-actinolite and quartz-sericite schists that represent a matrix of megabreccia. Some fragments of greenschist occasionally exhibit relics of ophitic texture, which indicate their igneous origin. Linear textures in lithologies comprising both blocks and matrix are commonly conformable. This could be explained by the synchronous appearance of the actual linearity due to later metamorphic overprinting of the megabreccia together with country rocks during the Svecofennian tectonic event. In places, granitoid blocks are intruded by clastic dykes comprised of fine-crushed material resembling the matrix in composition. Numerous quartz veins cross both blocks and matrix. These veins are frequently discontinuous and disrupted.

Figure 5.

 Large-scale plan of a metamorphosed polymict megabreccia southwest of the Bergaul Mine (from Gromova 1955).

In the Lizhma locality, some brecciated metabasalt outliers within fractured gneissic granite and gneiss are referred to polymict megabreccia. At the Sel’ga segment, blocks of greenschists and metabasalt up to 150 × 350 m in size, and minor fragments of trondjemite and microcline granite, form a NW-elongated megabreccia field of 1 × 4 km in size, occurring between Later Lopian granite and Jatulian basal conglomerate. The bedrock is very poorly exposed in these two areas.

The most representative megabreccia outcrop is described in the northeastern part of the circular feature, on the left side of the Gormozerka River (500 m downstream of its source). In this outcrop, a transition from monomict granitic megabreccia to polymict megabreccia is observed. The outcrop is a horseshoe-shaped scarp up to 5 m high and 500 m in overall length from SE to NW (Fig. 6).

Figure 6.

 View in plane of a large megabreccia outcrop at the left side of the Gormozerka River. For location, see the geologic map (Fig. 4). 1—Quaternary loam, 2—greenschist, 3—metabasalt, 4—massive microcline granite, 5—massive trondjemite, 6—gneissic granite, 7—polymict blocky breccia, 8—polymict blocky breccia containing altered glass (?), 9—lithologies with “grieβ” structure.

Granitic megabreccia is developed at the eastern part of the outcrop. Blocks of granitic gneiss, trondjemite, and microcline granite reach 50 m in length. They are crushed in places to form a mortar (“Grieβ”) texture (Fig. 7) where the rock consists of angular fragments of several centimeters in size, cemented by a finely crushed (0.001–0.1 mm) matrix, which includes thin flakes of biotite, sericite, muscovite, epidote and calcite grains, and disseminated pyrite. At the western part of the outcrop, minor and smaller (no more than 5–8 m) blocks of greenschists and metabasalt occur.

Figure 7.

 Gneissic granite with mortar (“grieβ”) texture. Top—a hand specimen, bottom—thin sections (crossed polarizers). Within a heterograined recrystallized groundmass, quartz, feldspar, and granite clasts with corroded contours occur. Sample 1285/4, a central part of the polymict breccia outcrop (Fig. 6).

Megablocks are cemented by either thin zones of finely crushed matter of the same composition or blocky polymict breccia; the latter appears mostly in the NW part of the outcrop. This breccia consists of randomly distributed fragments ranging from 1–2 cm to 2–3 m in size; the fragments are gneissic granite, gneissic diorite, and, to a lesser degree, trondjemite, biotite gneiss, greenschist, and quartz. Granitoid fragments of mortar texture are common (Fig. 8). The rubble material from disintegrated granitic blocks frequently envelops large fragments or intrudes blocks, forming clastic veins up to 10 cm thick. The breccia matrix consists of lens-like or odd-shaped, granoblastic, fine-grained (0.02–0.5 mm) quartz-albite segregations with numerous hetero-grained inclusions of quartz, granite, and gneiss up to 5 mm in size. Quartz-albite aggregates are separated by fibrous biotite-chlorite bands and lenses; in parts where foliation occurs, epidote-biotite, and in places muscovite-biotite schists, comprise the matrix.

Figure 8.

 A part of the polymict breccia outcrop at the left side of the Gormozerka River (Fig. 6). Blocks of trondjemite (in center), granitic gneiss (at bottom) are cemented by crushed material of the same lithologies. Close to the top, a 10 cm inclusion of altered fluidal glass (?) is observed.

Some fragments in the above polymict breccia resemble altered fluidal glasses (according to shape and texture) (Fig. 9). They are comprised of flexural bands or uneven segregations with indistinct contours up to 30–40 cm in size and consisting mostly of fine-grained (0.01–0.15 mm) epidote (40–80 vol.%) plus quartz and albite (total 20–50 vol.%), and minor coarser (up to 0.4 mm) biotite. Epidote commonly forms thin, uneven bands resembling a fluid-like texture. Quartz-albite granoblastic aggregates are bordered frequently by biotite rims. Numerous quartz porphyroblasts up to 0.6 mm in size (conforming to a fluid-like texture) and shadow angular inclusions of intensely altered lithologies (possibly volcanite or pelite) also occur.

Figure 9.

 Recrystallized fluidal glass (?)—a fragment from polymict breccia. Top—a hand specimen, bottom—a thin section (crossed polarizers). Glass (?) consists of clasts and quartz segregations (with mosaic texture) cemented by a highly vesicular biotite-quartz-albite-epidote thin-grained matrix with fluid-like texture. Sample 1285 (see Fig. 8).

The glass-like inclusions are broken by fracture infilling of a dark-green micaceous matter. Such inclusions form oblong bands with corrugated texture in the host schist (breccia matrix). No shocked quartz grains are found in the glass-like inclusions. However, a study of their nonsoluble fraction (a residue from thermochemical decomposition) showed the presence of minerals that are characteristic for other lithologies or for different metamorphic grades—such as ilmenite, chromite, magnetite, kyanite, staurolite, zircon, rutile, and even schungite.

Shock Metamorphic Features

The occurrence of diagnostic shock metamorphic effects is the principal criterion for the recognition of terrestrial impact craters. Therefore, the identification and examination of shock effects should be given particular attention. More than 100 thin sections of granitoids and greenschists from polymict and monomict megabreccias (from both the above continuous outcrop—see Fig. 6—and brecciated gneissic granite in the vicinity of the Bergaul mine) were studied using an optical microscope with U-stage.

As a result, some deformation features have been noted in large granitoid blocks less affected by later metamorphic recrystallization. These deformation features are fracturing and mosaic structure in quartz grains, deformation bands, and planar microstructures in quartz, microcline, and graphite.

Fracturing and Mosaic Structure in Quartz

Fracturing is expressed as one or several sets of subparallel fractures spaced as close as approximately 1 mm apart crossing either a grain or aggregate of several quartz grains. Mosaic structure consists of disintegration of a grain into randomly orientated isometric blocks or near-parallel elongated domains no more than 0.01–0.5 mm in size. To produce such deformation features, a relatively low dynamic pressure, which is common for normal tectonic deformation, is required.

Deformation Bands in Quartz

Deformation bands are thin (no more than 0.005–0.008 mm) parallel lamellae with decreased birefringence, decreased index of refraction, and light-brown color while separated colorless domains are up to 0.01–0.05 mm wide (Fig. 10A). Optical orientations of deformation bands and host quartz are either the same or may be slightly different. Lower indexes of refraction and birefringence found to suppose that deformation bands are composed of any low-density silica modification. Some deformation bands are distinctive by their disorientation. These are sets of thin (from 0.003 to 0.025 mm) parallel lamellae rotated by small degrees (4–7°) to one another. In places, they are reminiscent of polysynthetic plagioclase twins (Fig. 10B). In some grains, two or three sets of deformation bands are observed.

Figure 10.

 Deformation features in quartz from trondjemite blocks from polymict megabreccia. A) Basal deformation bands (dark), parallel polarizers. B) Deformation bands in quartz resembling polysynthetic twins in plagioclase, crossed polarizers. C) Two sets of decorated planar fractures in quartz, crossed polarizers. D) Traces of planar deformation elements on {10¯13}, crossed polarizers.

Deformation bands are aligned close to the basal face of quartz crystals: angles between c-axis and poles of deformation bands usually do not exceed 20° while maximum values are at 5–10° (Fig. 11).

Figure 11.

 Frequency histogram plot of deformation bands (left) and PFs/PDFs orientations in thin sections of granitoids from blocks from the megabreccia outcrop at the left bank of Gormozerka River (see Fig. 4). After Mashchak and Orlova (1986). The measurements are binned in 5° increments of angle between the c-axis and poles of DB/PF/PDF planes. Dominant orientations are indicated. For PF/PDFs, 43 planes were measured on 31 quartz grains, for deformation bands, on 21 grains; all planes were indexed on these dominant orientations.

Although similar deformation bands in quartz from shocked gneiss were reported from Popigai, Terny, and some other impact structures, generally they are common in quartz that is deformed at the lithospheric brittle-plastic transition due to dislocation-glide controlled deformation (Trepmann 2009).

Planar Microstructures

This term (Stoffler and Langenhorst 1994) is used as a collective term for two types of microstructures: planar fractures (PFs) and planar deformation features (PDFs). Some quartz grains in granitoid blocks from the above polymict megabreccia outcrop and from brecciated gneissic granite in the vicinity of the Bergaul mine display sets of sharply defined parallel lamellae with specific crystallographic orientations, which can be attributed to PFs or PDFs. Planar microstructures are commonly decorated by irregularly shaped, transparent or opaque inclusions of about 0.003 mm in size. Due to extensive obliteration and deformation of planar features, it is not always possible to distinguish true PDFs from other lamellar features, so that PFs and PDFS were measured jointly. Individual grains commonly contain two, but very rarely, three PDF sets. Spacing between individual lamellae varies strongly, but is typically less than 10 μm (Figs. 10C and 10D). The frequency of planar microstructures reaches 200–250 per 1 mm.

Crystallographic orientations of PFs and PDFs were measured on an optical microscope fitted with a four-axis universal stage. The orientations were measured within 31 grains. Figure 11 shows a frequency histogram of the angle between the poles of PF/PDF planes and the c-axis of individual quartz grains, and displays diffuse maxima at angle interval of 20–25° corresponding to a rhombohedron with Miller index {10¯13} that is characterized by an angle of 23°. Besides planar features parallel to rhombohedon {10¯14}, {10¯13}, {101¯1}, {011¯1}, trigonal dipyramid {2¯131}, {5¯161}, and hexagonal prism {10¯10} planes are remarkable while other PDFs orientations, such as {10¯12}, {1¯122}, (1¯121}, are very rare. Planar microstructures parallel to ω {10¯13} were observed only in impact craters and nuclear explosions. They develop at about 10–15 GPa (Stoffler and Langenhorst 1994).

Deformation Features in Feldspars and Biotite

Deformation features in microcline include undulatory extinction, deformation and obliteration of tartan structure formed by intersection of two systems of polysynthetic twin laminae, and planar microstructures. Within disintegrated grains, the latter cross only separate subgrains, but not a whole grain (Fig. 12).

Figure 12.

 Deformation features in microcline. A) Three sets of closed planar fractures partly obliterated (parallel polarizers). B) Planar microstructures crossing the tartan structure of microcline. Original twinning in the feldspar formed by intersection of two polysynthetic twin laminae on {001} is cut by a single set of narrow, closely spaced planar features (north-northwest/south-southeast). Sample 1650 from a block of granitic gneiss from polymict megabreccia (crossed polarizers).

The most closed (40–120 per 1 mm) planar fractures are parallel to {11¯1}, {11¯1}, {001}, {010} and, more rarely, to {130} or {¯130} crystallographic planes. Planar fractures are decorated by fine (1–3 μm) opaque inclusions. As shown by Robertson (1975) for experimentally shock-loaded maximum microcline, lamellar features parallel to {11¯1} and {11¯1} appear under dynamic stress above 5 GPa in addition to preshock cleavage corresponding to {001}{010} orientations. With increasing pressure, planar microstructures parallel to {100} and {130} arise.

In plagioclase, the disappearance of twin structure, bending of twinning planes, and PDFs are observed (Figs. 13A and 13B). In biotite, kink bands are common. Angles of their external rotation, measured by Schneider’s (1972) method, vary between 30 and 40°, width of lamellae bending is 0.05–0.08 (Figs. 13C and 13D). The measured angles of external rotation differ slightly from their static counterparts, which range commonly from 40 to 60°.

Figure 13.

 Deformation features in plagioclase (A, B) and biotite (C, D) in granitic gneiss from a block of polymict breccia, sample 1650-12 and 1650-9, respectively. A) A kink band disturbing polysynthetic twin lamellae in an oligoclase grain. Crossed polarizers. B) Original polysynthetic twin lamellae in oligoclase are deformed and crosscut by straight continuous planar microstructures (northwest/southeast). Crossed polarizers. C, D) kink bands in biotite (the same grain, parallel and crossed polarizers)

Sulfide Mineralization

The occurrence of sulfide mineralization within megabreccias is an interesting feature of the Suavjärvi structure because some impact-related base metals deposits have been reported from the Serpent Mound, Crooked Creek, Decaturville, and Siljan Ring impact craters (for references see Grieve and Masaitis 1994; Reimold et al. 2005), and importantly, the presence of the world class Ni-Cu-PGE sulfide ores in Sudbury, which stimulated studies on sulfide distribution in impact craters.

As stated above, brecciated granitic blocks within polymict megabreccia contain disseminated cubic crystals of pyrite up to 2 mm in size. In places, pyrite contributes up to 0.5% of rock volume. Furthermore, rare disseminated pyrite occurs everywhere within crushed and altered material, which cements the megabreccia.

Sulfide mineralization within polymict megabreccia is the most abundant in the Bergaul where ore lodes were delineated in the 1930s and 1950s (Belitsky 1935; Zil’ber and Bychkova 1954; Gromova 1955). Sulfide lodes are confined to the contact zone between polymict greenstone-dominated megabreccia and monomict granitic megabreccia. Here, the target blocks are intensely brecciated and crushed while quartz-sericite, chlorite-epidote, and other schists cementing megabreccia are the most abundant. Sulfide accumulations occur within (1) quartz and quartz-plagioclase veins up to 30 m long, which cross both megablocks and cement matter and (2) within various schists.

At Bergaul, massive sulfide accumulations measuring 30 cm across consist mostly of coarse-grained, fractured pyrrhotite (80–90%). Abundant small chalcopyrite lens-like inclusions, hosted by pyrrhotite, contribute 1–3%. Microinclusions of pentlandite, galena, bornite, and sphalerite are observed. Pyrrhotite is occasionally replaced by spotty secondary crypto-grained melnikovite-marcasite that changes to pyrite (Fig. 14). Rare arsenopyrite and traces of native silver have been reported (Yakovleva and Savina 1965). Disseminated molybdenite occurs, separately to the other sulfides, within aplitic and quartz veins of 10–12 cm thick. This molybdenite occurs in both the Bergaul and Sel’ga localities (Zil’ber and Bychkova 1954).

Figure 14.

 Photographs of a massive sulfide ore occurring within megabreccia in Bergaul, the northwestern part of the Suavjärvi structure (polished section, oblique reflected light). At left—a general texture of the ore, at right—pyrite-marcasite veinlets replacing pyrrhotite.

From X-ray data, predominant pyrrhotite consists of an intimate association of hexagonal (4H) and monoclinic (1M) structural varieties. Ni and Co values in pyrrhotite are significantly lower than in secondary pyrite, the latter containing some Cu, Zn, and As. Ni and Cu tenors in the Suavjärvi massive sulfide lodes are estimated to be no more than 0.2 and 0.75% wt.%, respectively (Naumov 2010).

The sulfur isotope composition of pyrrhotite, which comprises the bulk of massive sulfides, was measured at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow. For the analysis, the sulfur of the minerals was converted into SO2 gas, which was analyzed by the decompensation method (Ustinov and Grinenko 1965) on a MI-1305 dicollector mass spectrometer. 1σ reproducibility for δ34S was better than ±0.3‰; δ34S data are expressed as per mil relative to the meteorite standard.

The pyrrhotite yields a mean δ34S value of 0.2‰ VCDT (n = 4), typical for basic mantle-derived igneous rocks (Seal 2006). This is in concordance with data from other impact craters formed in Precambrian crystalline targets, e.g., Kärdla, Popigai, Puchezh-Katunki, where δ34S values of postimpact sulfides are close to the meteorite standard (Naumov et al. 2004).

The sulfide mineralization in Bergaul is not economic due to the small size of the ore lodes (no more than 8.5 × 15 m), low tenors of metals and, therefore, insignificant resources (Yakovleva and Savina 1965).

Age

An older age limit of the Suavjärvi structure of about 2.7 Ga is set by age determinations on the regional Late Lopian granitoids (Bogdanov 2000) that occur as fragments within the megabreccias. The overlap of impact polymictic breccia by the Jatulian basal conglomerates as well as the occurrence of weathered breccia within Jatulian indicates clearly a pre-Jatulian age of the impact. The lower age limit of the Jatulian is not dated by off-line isotope techniques, but it is accepted tentatively to be approximately 2200–2300 Ma (Merilainen 1980; Shurkin 1989; Bogdanov 2000). In any event, it is not younger than 2150 ± 60 Ma (inferred from dating of dykes cutting the Jatulian rocks) or 2090 ± 70 Ma (Pb-Pb age of dolomite from the Tulomozero Formation comprising the upper part of the Jatulian sequence—Ovchinnikova et al. 2007). Therefore, a geologic age of the Suavjärvi structure fits into an age interval of between 2.7 and 2.2 Ga.

Discussion

The Suavjärvi structure is a circular feature located in the central Karelian Craton—a Late Archaean granite-greenstone terrane covered in places by Paleoproterozoic (Karelian) rocks. In the interior of the circular feature, highly disturbed rocks, referred to as polymict and monomict megabreccias, occur with a patchy distribution. The megabreccias consist of blocks of both basement granite-gneiss (early to later Archean), younger (later Archean) granite, and supracrustal greenstone rocks. Furthermore, some fragments resembling altered fluidal glass occur within the polymict breccia. The megabreccias are localized at certain positions within the circular feature. They form arcuate lens-like bodies, which coincide with the circular patterns on the satellite imagery. This is the position corresponding to annular depressions within other large impact craters where similar megabreccias occur at the base of impact rock sequences and, which pass gradually into autochthonous breccia. Thus, the megabreccias in Suavjärvi can be interpreted to represent remnants of the lowermost part of a typical impact sequence. Megabreccias contain, among others, some target lithologies (metabasic greenschists, etc.), which are absent elsewhere in the Suavjärvi area due to the deep erosion. In this respect, Suavjärvi resembles a deeply eroded Siljan Ring impact structure of 52 km in diameter where remnants of Paleozoic cover remain within the annular trough alone (Johansson 1984). The general geophysical characteristics of Suavjärvi—both gravity and magnetic lows—are typical signatures of impact structures (Pilkington and Grieve 1992). Moreover, a random sign-changing pattern of the magnetic field over Suavjärvi indicates a possible chaotic structure of the target as well as disruption of the regional magnetic fabric as observed.

Thus, one has a circular structure, some morphological, geophysical, and geologic attributes of which support a tentative extraterrestrial impact origin. However, the principal and invariable criterion for the recognition of terrestrial impact craters is the occurrence of shock metamorphic effects in target rocks (Grieve 1997; French 1998; French and Koeberl 2010). As stated above, some deformation features were observed in target lithologies comprising blocks within polymict megabreccia in Suavjärvi. Thus, the confirmation of the impact origin of Suavjärvi reverts to the recognition of shock metamorphic effects in the above deformation features.

Shock metamorphic features, per se, in Suavjärvi are sparse. Neither shatter cones, nor high-pressure mineral polymorphs nor diaplectic glasses, etc. were revealed. Such features as cleavage, mosaic structure, deformation bands in quartz, kink bands in biotite and plagioclase, although common in impact environments, can also be produced by static tectonic deformation. However, the distinctive planar microstructures in quartz are now generally accepted as unique criteria for shock waves and meteorite impact events (Grieve et al. 1996; French 1998; Koeberl 2002).

Many of the planar microstructures at Suavjärvi differ from those in other impact structures. In places, they are discontinuous, not completely straight, and extend only partly through grains. However, the planar microstructures are spaced commonly <10 μm apart; generally two PDFs sets are observed when seen under U-stage; crystallographic orientations of PFs and PDFs display a maxima at an angle interval of 20–25° corresponding to ω set {10¯13}, specifically for shock effects. In terms of patterns listed, these planar microstructures are clearly distinguished from features that are produced at lower strain rates, such as tectonic metamorphic deformation lamellae, which are not straight, occur only in one set, usually consist of bands that are >10 μm wide, and are spaced at distances of > 5–10 μm (Koeberl 2002; French and Koeberl 2010).

A definitive characteristic of shock-induced planar microstructures is their tendency to form along specific planes in the quartz crystal lattice. Some of the most frequently observed PDF orientations in quartz for all impact sites, in decreasing order of abundance, are ω {10¯13}, π {10¯12}, {10¯11}, and c {0001} (Grieve et al. 1996). Distribution of orientations of planar microstructures in quartz from Suavjarvi (Fig. 11) is similar to a high degree to that from some impact craters, e.g., Clearwater Lakes (French 1998) showing the distinctive concentration parallel to ω {10¯13} planes. However, the histogram of orientations from Suavjärvi is somewhat different, i.e., their comparative scattering, full absence of π {101¯2} orientations, and appearance of some features with high Miller indices such as {20¯23}. Nevertheless, this histogram does fit in with shock metamorphic effects from terrestrial craters. The predominance of ω {10¯13}; minor appearance of {2¯131}, {5¯161}, and {10¯10}; and absence of π {10¯12} planes can indicate the medium shock stage corresponding to shock pressures > 7–10, but <20 GPa for host quartz at moderate postshock temperature (Stoffler and Langenhorst 1994).

To date, it has been difficult to estimate a preservation grade of specific patterns of orientations of shock-induced planar microstructures in ancient impact structures, which underwent metamorphic overprinting. It is known, in the oldest of dated impact structures, Vredefort Dome of 2023 ± 4 Ma (Reimold and Gibson 1996), PDFs are anomalous with respect to their relative distribution, preservation, and orientations, when compared with those at other known impact structures. In particular, 90% of all PDFs studied exhibit basal orientations, which are mechanical twins of the Brazil type, while the indicative for shock effects {10¯13} and {10¯12} orientations have been observed much rarely; this has been attributed to postshock recrystallization of quartz (Grieve et al. 1996). This abnormal distribution in both extent and orientation of PDFs even was causing controversy regarding the origin of the Vredefort structure. Widespread obliteration and modification of PDFs have been reported, e.g., from the Puchezh-Katunki impact crater, with effects of higher shock levels being much less steady under postshock thermal influence (Masaitis and Pevzner 1999).

From our point of view, both the paucity and subdued appearance of shock metamorphic features and their distinction from those in many other impact structures can be attributed to later regional metamorphism. Both megabreccias and overlying rocks of the Jangozero Formation are considered to be metamorphosed to greenschist-epidote-amphibolite facies. The Karelian Craton underwent metamorphic overprinting during the Svecofennian orogeny at 1790–1890 Ma. The regional metamorphism was of high-T, moderate-P type (Bilibina 1980). It is suggested that this metamorphism is responsible for the transformation of cement matter in the megabreccias to epidote-actinolite, epidote-chlorite-albite, and other schists and possibly, for recrystallization of impact glass to quartz-albite-biotite-epidote aggregates and considerable obliteration of shock effects, primarily of indicators of higher shock grades including PDFs parallel π{10¯12}. In addition, both postshock and later tectono-metamorphic processes can result in the bending of shock-related planar microstructures or in producing tectonic deformation lamellae in quartz overprinted on PDF. However, some planar microstructures, with ω{10¯13} orientation being the most frequent, remained within certain target fragments in impact-derived megabreccias. Their presence confirms the impact origin of the polymict megabreccia and therefore of the Suavjärvi structure and is consistent with a crystalline target. As possible indicators of shock metamorphism, decorated planar microstructures in microcline along {1¯11}, {1¯11}, {100}, and {130} planes can be considered as well (Robertson 1975).

The presence of a variety of other deformation features (fractures, deformation lamellae and chessboard-like subgrains in quartz, kink bands in biotite and plagioclase, planar microstructures in plagioclase) cannot be used as definite evidence for an impact event. However, when combining distinctive shock indicators such as planar microstructures with specific crystallographic orientations in quartz and microcline, the above deformation features may be attributed to the impact event.

Thus, the combination of shock indicators and various other features (deformation features; hypothetical recrystallized glass; unique for the Suavjärvi area polymict megabreccias, located at the lowest part of the expected annular trough; circular patterns in topography and imagery; general geophysical characteristics), although not being direct evidence of impact, but characteristic of extraterrestrial impact structures, indicate Suavjärvi to be a remnant of a deeply eroded and metamorphosed impact structure. From geologic data, the age of the Suavjärvi impact is older than 2.2 Ga, i.e., Paleoproterozoic.

It is proposed, from both the occurrence of a thick weathering crust at the base of Jatulian and abundance of weathered granitoid breccia fragments within basal conglomerate of the Jangozero Formation, that the Suavjärvi structure has been eroded to a considerable degree as long ago as the pre-Jatulian epoch. As a result, some fragments of impact megabreccias remained only in the deepest segments of the annular trough. The megabreccias represent the base of an impact rock sequence, which was almost entirely removed as well as Lower Lopian supracrustal lithologies, which possibly overlied granite-gneiss basement in the Suavjärvi area during impact. These lithologies remained only as fragments and megablocks within megabreccia. Afterward, during Paleoproterozoic, both the impact structure and overlying volcanic-sedimentary sequence underwent regional metamorphism, which obliterated, reduced, or transformed the shock metamorphic effects of high shock grades. Thus, both deep erosion and overprinting tectono-metamorphic events resulted in the removal or transformation of the greater bulk of impact-generated features. Preservation of certain circular features of the impact structure outside the main tectonic zones is entirely possible. Preservation is also possible due to burial under the Jatulian volcanic-sedimentary cover, which reaches as much as 2000 m thick elsewhere in Karelian craton (Shurkin 1989). During the Neoproterozoic and Phanerozoic, the Suavjärvi area was part of the most stable region of the Fennoscandian Shield where a low-intensity uplifting is a prevailing tectonic feature until now.

If the impact origin of the Suavjärvi structure is accepted, the sulfide mineralization occurring in its northeastern part can be interpreted in the following way. It is obvious from geologic observations that massive sulfides in Bergaul appeared after breccia formation. The predominant pyrrhotite yielded a measurement of δ34S, which is close to the meteorite standard. Although there is no available sulfur isotope data from greenstone rocks from the Karelian craton, the assumption that sulfides in Bergaul originated from redeposition of primary sulfides from greenstone rocks (which remained only within megabreccias) is the most plausible. The ubiquitous occurrence of pyrite-pyrrhotite mineralization is typical for Later Archean metavolcanics from the Karelian greenstone belts (Bogdanov 2000). Two events point to redeposition and accumulation of sulfides within megabreccia (1) pre-Jatulian, impact-induced hydrothermal activity, which has been reported from many impact structures (e.g., Naumov 2002) or (2) post-Jatulian metamorphic event connected with the Svecofennian orogeny. The former appears most likely because no massive pyrite-pyrrhotite accumulations were reported for Jatulian lithologies (including metadolerite). The latter contain chalcopyrite-chalcocite mineralization both in the Suavjärvi area (south of the lake of Seletsk) and in adjacent regions (Yakovleva and Savina 1965) in contrast to pyrites in Bergaul. Therefore, it is a reasonable assumption that Suavjärvi presents an example of impact-generated epigenetic Ni-Cu-Zn mineralization. The position of sulfide lodes at the lowest part of the expected annular trough is similar to base metal locations in other impact structures (e.g., Siljan Ring—Johansson 1984). Because of deep erosion, only a small remnant of impact-related ore is preserved. Some boulders of schist containing massive pyrite-pyrrhotite ores like in Bergaul have been reported from a watershed area between lakes Segozero and Seletsk (Gromova 1955). The presence of massive sulfides in the Suavjärvi structure can be considered as additional evidence of ore-forming processes common in impact craters.

Concluding Remarks

The Suavjärvi has been proposed as an impact structure for more than 25 yr. Nevertheless, very little information on Suavjärvi has been available in international literature. This article serves to present, for the first time, the geology and mineralogical features of the Suavjärvi structure. These are preliminary findings only.

From our point of view, a combination of relict shock indicators and various other features, although not being direct evidence of impact, but rather characteristic for extraterrestrial impact structures, make it possible to consider Suavjärvi as a remnant of a deeply eroded and metamorphosed impact structure. It has an apparent diameter of 16 km and is considered to have formed in the Paleoproterozoic (older than 2.2–2.3 Ga). As very few impact structures have been described in Archean granite-greenstone terrains, the characteristics of Suavjärvi may help us elucidate the signatures of impact that survive in deeply eroded and metamorphosed settings, and refine the terrestrial cratering record in ancient times. Moreover, the massive sulfides in the megabreccias are of special interest in being a possible example of postimpact mineralization.

The Suavjärvi structure obviously needs to be studied in more detail to discover and measure more extensive and representative shock metamorphic features (using the transmission electron microscopy). Further study is necessary of geologic parameters, to reveal geochemical patterns of impact breccias—in particular, recrystallized tentative impact glasses). Lastly, a precision age determinations of Suavjärvi is strongly recommended.

Acknowledgments—  We are grateful to Victor Yezersky, Jeanna Orlova, and Irina Fedorova who took part in field and mineralogical study of Suavjärvi. Special thanks to Prof. Victor Masaitis for helpful comments. Heinz Fraenkel is acknowledged for his help in English. We greatly appreciate the constructive reviews by R. A. F. Grieve and C. A. Trepmann as well as editorial comments and suggestions by J. G. Spray.

Editorial Handling—  Dr. John Spray

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