ISSN 08695938, Stratigraphy and Geological Correlation, 2014, Vol. 22, No. 2, pp. 123–146. © Pleiades Publishing, Ltd., 2014. Original Russian Text © R.A. Terentiev, 2014, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2014, Vol. 22, No. 2, pp. 7–31.
Paleoproterozoic Sequences and Magmatic Complexes of the Losevo Suture Zone of the Voronezh Crystalline Massif: Geological Position, Material Composition, Geochemistry, and Paleogeodynamics R. A. Terentiev Voronezh State University, Voronezh, Russia email:
[email protected] Received June 5, 2012; in final form, October 8, 2012
Abstract—In order to resolve the contradictions associated with uncertainty in the identification of the mate rial composition, subdivision, and conditions of formation of the Paleoproterozoic intrusive, metavolcano genic, and metasedimentary sequences of the Losevo suture zone of the Voronezh crystalline massif, this work presents geological, petrographic, petrochemical, and geochemical features of these sequences. The strati graphic and magmatic scheme of the central part of the Losevo suture zone is clarified. In particular, the Paleo proterozoic Losevo Series is divided into two sequences: Strelitsa (marginal sea) and Podgornoe (island arc). A new hypabyssal NovoVoronezh metagabbrodiabase complex, comagmatic to metatholeiites of the Podgor noe sequence, is distinguished. The isotope age of the Strelitsa sequence is assumed to be 2172 ± 17 Ma on the basis of the results of age dating of zircon cores from the Usman plagiogranites, intruding this sequence. The upper age boundary of the Strelitsa sequence corresponds to the age of premetamorphic gabbro of the Rozh destvenskoe complex, comagmatic to metavolcanites (2120 ± 11–2158 ± 43 Ma). The age of the Usman pla giogranite complex is clarified. On the basis of geologicalstructural and petrographicmineralogical analyses of metavolcanogenic rocks, lithological analysis of metasedimentary formations, and new geochemical data obtained from metavolcanites and metamorphosed deposits, the pattern of paleogeodynamic evolution of the Losevo suture zone in the first half of the Paleoproterozoic is proposed. The next stages are distinguished: (1) intrusion of tholeiites of transition TMORB type in spreading zones and deposition of terrigenous strata in the marginal sea basins; (2) intrusion of Nbdepleted tholeiites and plagiorhyolites, the geochemical char acteristics indicating their formation in the subduction setting; (3) intrusion of gabbroids of the Rozhdestven skoe complex; (4) formation of an island arc synchronously with stage 2, tholeiitic and calcalkaline (Podgor noe sequence) volcanism; (5) intrusions of gabbrodiabases, subsynchronous to volcanism, of the Novovoronezh complex and dioritegranitoides, crystallization of granitoides of the Usman complex; (6) a break in sedi mentation and formation of molasses of the Voronezh (Somovo) Formation. Keywords: Voronezh crystalline massif, Paleoproterozoic, magmatic complex, straton, Losevo Series, geochemistry, and isotope age DOI: 10.1134/S0869593814020087
INTRODUCTION The Losevo suture zone separates the Kursk and Voronezh terrains of the Voronezh crystalline massif (Fig. 1a), which, in turn, is a part of the East European Platform. The Voronezh crystalline massif represents a rise of the Precambrian basement, buried at the vary ing depth of 0–500 m under the cover of sedimentary rocks. Geological description and geodynamic inter pretation of the structural and compositional com plexes of the Losevo suture zone are of importance for deciphering the Precambrian history of the Voronezh crystalline massif. However, owing to insufficient geo logical knowledge of this massif, interpretation of many of the key characteristics of the Losevo suture zone is still controversial. In particular, contradictory
conclusions are often made even on the basis of the same data. The review of the published data on the Losevo suture zone is given below. At the initial stages of research, volcanites of the Losevo Series were assigned to the unified continuous basaltandesitedacite formation (Chernyshov and Egipko, 1974; Bocharov, 1988; Bocharov and Cherny shov, 1985; Bush et al., 2000). Later, the contrast basaltrhyolite formation was distinguished within the Losevo suture zone (Terentiev, 2002; etc.). This fact was confirmed by other scientists (Nenakhov et al., 2007; Shchipanskii et al., 2007). An uncertainty in the identification of formational assignment of metamorphosed volcanic rocks of the Losevo Series led to occurrence of different interpre
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TERENTIEV 39° E
40° (a)
Pac helm a au Kaluga lacog en Tula
ou gh
Smolensk
– –75 1000 0
Pri pya t tr
7720
Bryansk
AR1+2 Orel
Khlebnoe
Gomel’
AR1+2
–5 00
–30 0 –20 0
Lipetsk
Tambov –30 0
–100
Dn K16 7728
iep
Fig. 1b PR1
Do
100 km
K17
0
Kursk
er–
Voronezh
Sumy
n Belgorod au lac o
ge
0 –1 – 00 –3200 0 –5 0 00
n
1 2 3 4 5
Pre De Caspi pre ssio an n
К31
PR1
–75 –10 000
Usman
52° 7768 7767
7753 Ramon’
7718 7752
b12
52° N
b1
7755 7765
080 7796 7730 7781 7787а 074 0149 0182 Podgornoe Voronezh Strelitsa
7789 7809
7785
070 0150 069
7808
7818
7528 7529 7526 7527 054 053 041
7525
7782 0152
595с 589с 591с
7802 048
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051
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047
(b) 1
9
2
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а
3
7805
0181
11
b
4
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12
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7519 7517 454a 458a 599c 7518
Novovoronezh
Davydovka
7507 570с
553c
549с 0160 496c 494с 3c 0161 493с 449c 483c 492c 452c 454с
453с
7504
432с 439с 441с 7501 7503 460с 434с 438с 433с 455с 447с 462с 437с 436с 456с
6 51° 7
0
5
10
51°
15 km
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tations of the geodynamical history of the Losevo suture zone in the Paleoproterozoic. Kotorgin (2001) compared the Losevo zone with the Archean green stone belts of the Kursk magnetic anomaly and other regions. However, the similarity in the material com position and isotopegeochemical data was not revealed. Some scientists suggested that rocks of the Losevo Series formed in the subduction zones of the Pacific (Bocharov and Chernyshov, 1985) or Andean types (Bush et al., 2000; Shchipanskii et al., 2007). Other scientists suggested that rocks of the Losevo Series formed under the conditions of intraplate or continentalmargin rifting under the influence of pro cesses of plume tectonics (Kremenetskii et al., 2007; Mints et al., 2007). Finally, taking into account more complex polyformational composition of the Losevo Series than previously assumed, its formation was associated with early oceanic and late subduction stages (Chernyshov et al., 1997; Nenakhov et al., 2007; Chernyshov and Nenakhov, 2010; Nenakhov and Bondarenko, 2011). The variety of geodynamic interpretations of the Losevo suture zone at the early stages of its develop ment led to even more uncertainty in the interpreta tion of the geotectonic position of the Baygora volca noplutonic structure, which formed later than the Losevo Series. This structure formed as a result of intraplate volcanism (Schipanskii et al., 2007), taph rogenesis (Chernyshov et al., 1997), and subduction (Nenakhov et al., 2007; Chernyshov and Nenakhov, 2010). Even within the framework of one article, some authors could not determine the setting of its forma tion, suggesting both subduction and intraplate pat terns (Bondarenko et al., 2009). Data on the material composition of volcanites of the Losevo Series and the Baygoda volcanoplutonic structure, as borehole as their petrogeochemical fea tures are given in a few articles (Bocharov, 1988; Kotorgin, 2001; Terentiev, 2002; Nenakhov et al., 2007; Shchipanskii et al., 2007; Bondarenko et al., 2009; etc.). Except for Zaitsev et al. (1970), who first distinguished the Losevo Series, all listed authors ignored the high thickness of sedimentary sequences within this series and their significance for under standing the geotectonic position of the Losevo suture zone.
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Nowadays, the binomial structure of Paleoprotero zoic stratigraphic and magmatic complexes of the cen tral part of the Losevo suture zone is accepted: (1) more ancient Losevo Series in association with Rozhdestvenskoe gabbroic and Usman plagiogranite complexes and (2) young Voronezh Formation in association with the Ol’khovatka gabbronoritequartz monzonitegranite complex. These ideas, as will be shown below, need to be revised. During our work, on the basis of published and new data on petrogeochemistry of plutonic and metavolca nogenic rocks and isotope geochronology of plutonic formations of the Losevo suture zone, we made an attempt to resolve some of the contradictions associ ated with the material composition, subdivision, and the conditions of formation of the Paleoproterozoic intrusive, metavolcanogenic, and metasedimentary formations of this zone. MATERIALS AND METHODS In this work, the geological structure of the area of study is given on the basis of visual examination of core samples from more than 200 boreholes drilled during different geological exploration works, including depth geological mapping of the territory of the Vor onezh crystalline massif. The geological, geophysical, and petrogeochemical data of V.V. Bagdasarova, V.M. Bogdanov, Yu.S. Zaitsev, N.F. Kotorgin, V.I. Lositskii, Yu.N. Strik, V.Yu. Skryabin, K.A. Savko, A.P. Tarkov, A.A. Shchipanskii, etc., are used in this work. The locations of boreholes the core samples of which were used for petrographic, geochemical, and isotopic studies are shown in Fig. 1b. Metavolcanites of the Podgornoe (67 samples) and Strelitsa (43 sam ples) sequences of the Losevo Series, plagiogranites (1 sample and 3 samples after (Schipanskii et al., 2007)) of the Usman complex, gabbroids (12 samples) of the Rozhdestvenskoe complex, and metagabbro diabases (13 samples) within the field of the Podgor noe sequence of the Losevo suture zone were sampled. Crystal optical studies were carried out using an OLYMPUS BX51P polarizing microscope (analyst R.A. Terentiev).
Fig. 1. The scheme of structuraltectonic zonation of the Precambrian basement of the Voronezh crystalline massif (a) and sche matic geological map of the central part of Losevo suture zone (b). (a): (1) Formations of the Losevo suture zone; (2) Lower Proterozoic rocks of the Voronezh terrain; (3) Archean formations of the Kursk terrain; (4) synclinorial structures made of Proterozoic rocks; (5) absolute isohypses of the Precambrian basement sur face. (b): (1) Granitoides of the second phase of the Ol’khovatka complex; (2) plagiogranites of the Usman complex; (3) gabbroids and diorites of the first phase of the Ol’khovatka complex; (4) gabbro complex of the Rozhdestvenskoe complex; (5) Baygora volcan ogenic complex; (6) metaterrigenous formations of the Voronezh Formation; (7–9) Losevo Formation: (7) metavolcanogenic sedimentary rocks of continuously differentiated formation (Podgornoe sequence), (8) metasedimentary, metavolcanogenicsed imentary rocks (Strelitsa sequence) and amphibolite gneissic formation of uncertain stratigraphic position, (9) basic (predomi nant) and acid metavolcanites of the contrast formation (Strelitsa sequence); (10) geological boundaries; (11) tectonic faults: (a) regional LosevoMamon fault, (b) subordinate faults; (12) the localities and numbers of boreholes opening the stratified sequences; (13) localities and numbers of boreholes (reference sections). STRATIGRAPHY AND GEOLOGICAL CORRELATION
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The major petrogenic oxides were determined using a complex of methods at IMGRE (Moscow). Contents of SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5 were measured by the ICPOES method (optical emission spectrometry with inductively coupled plasma) using an Optima 3300 (analysts B.I. Volkov, Yu.I. Grosse, and I.A. Pichugin); FeO and CO2 contents were measured by the titrimetric and spectrophotometric dichromate method; H2O– and H2O+ were measured by the gravi metric standard method (analysts B.I. Volkov and Yu.I. Grosse); and the contents of F, CI, and S were measured by Xray fluorescence using an Axios Advanced device (analysts N.M. Kazakevich and M.S. Storozhenko). Measurement errors for all meth ods used did not exceed 5 rel. %. The contents of rare and scattered elements were determined by the ICPMS method at IMGRE (Mos cow) using an Elan 6100 DRC in standard mode (ana lysts T.N. Pavlova and N.V. Vasil’ev). Samples (50– 100 mg) were digested in a microwave oven. This method provides complete decomposition of most of igneous, metamorphic, and sedimentary rocks, including minerals which are difficult to decompose (zircon, monazite, etc.). The laboratory certified rock sample, prepared as studied samples, was used as the standard sample. The detection limits vary from 0.001–0.005 µg/g for elements of heavy and medium mass (U, Th, REE, etc.) to 0.02–0.05 µg/g for light elements (Ba, Rb, etc.). Accuracy of the analysis was 3–10 rel. %. For isotopegeochronological studies, 5 kg samples (Usman plagiogranites) and 20 kg samples (Rozh destvenskoe gabbro) were collected from the core of the least altered rocks. The samples were crushed to a particle size of 0.5 mm, washed, and separated in bro moform. Zircon crystals were selected under a binoc ular microscope from the fraction of density greater than 3.5 g/cm3. Selected zircons together with stan dards 91500 and TEMORA (Black et al., 2003) were mounted in Epofix resin in a 25 mm mold, which was then polished. Cathodoluminescent images of zircons were obtained using a CamScan MX2500 scanning electron microscope. The study of them allowed us to choose the coordinates of measurement points within zircon grains, appropriate from the point of view of methodology of U–Pb dating, for microprobe study. The prepared zircons were analyzed by a SHRIMP II highresolution multicollector secondary ion mass spectrometer (Center for Isotope Research, VSEGEI, St. Petersburg, analyst A.N. Larionov). The U–Pb ages and corresponding parameter values were calcu lated using Isoplot Ex ver. 3.6 (Ludwig, 2008). GEOLOGICAL POSITION On the basis of geological, geophysical, and petro physical data, the Losevo suture zone (Fig. 1b) is clearly distinguished among the surrounding segments
of the unified lithospheric plate at the present time (Tarkov, 1974). According to the classification of the Precambrian geostructural areas by Borukaev (1985), the Losevo suture zone is assigned to the boundary trough zones, which represent “predominantly Lower Proterozoic narrow, linear, and polygonal troughs, constrained by a fault from one side (in our case, the LosevoMamon regional fault). Troughs separate large blocks with different structure, which are usually characterized by a high volume of volcanic rocks of basic composition.” According to (Osnovy…, 1995), such geostructural regions are interpreted as oceanic or backarc basins and island arc or continentalmar gin systems. The Losevo suture zone developed on Archean formations of the Oboyan complex and the Mikhailovo series belonging to the Kursk terrain. Thus, it is probable that the basement of the Losevo Series is made of diorite gneisses, granodiorite gneisses, plagiogneisses (Don Association), and orthoamphibolites (Mikhailovka series). According to the presence of the attributive metavolcanogenic formation, the Losevo Series (Ter entiev and Chuvashina, 2003) is subdivided into the Strelitsa sequence, including contrast metabasaltspla giorhyolite formation, and the Podgornoe sequence. The latter includes two subformations (Terentiev, 2005): continuously differentiated metaplagiobasalt andesiteplagiodacite and contrast metaferrobasalt andesiteplagiorhyolite. Both of these subformations are normally interbedded, and, therefore, they are considered to be coeval. Strelitsa sequence is characterized by two types of sections (Terentiev, 2005), the features of which are given in Table 1. The firsttype section (effusivepyro clastic type), provided no overturned bedding, shows the general homodromous sequence: metabasites (effusive) compose the lower part of the section; the upper part of the section is represented by alternation of basic and acidic metavolcanites (Fig. 2). In the section of the reference borehole 7782 (from bottom to top) the volume of metapyroclastic rocks and, then, metavolca nogenicsedimentary rocks gradually increases (Teren tiev, 2005). The second type of sequence is represented by sedimentary rocks with bodies of acidic and basic subvolcanites. Metasedimentary rocks are often repre sented by pelitomorphic shales and also metapsam mites and metaaleurolites of diverse composition. All these rocks are constituents of rhythmically alternat ing horizons (Terentiev, 2002). The schistosity dipping angles in rocks of the Strelitsa sequence are 60°–90°. The thickness is not less than 1 km. Podgornoe sequence is represented by a wide vari ety of rocks. One of the most complete sections is opened by the borehole 0182, drilled on the north western outskirts of the city of Voronezh. The lower part is represented by alternation of metamorphosed plagiobasalts, andesite basalts, andesites, tuffs, and volcanogenicsedimentary rocks of andesite, pla giobasalt, andesite basalt, plagiodacite, and more acid
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Table 1. Features of two types of sections of the Strelitsa sequence First type of the section Characteristics (reference borehole 7782 + additional bore holes 7501, 7528, 7752, 7789, 549c, etc.) Volcanic facies
Effusive at the early stage; explosive at the late stage
Subvolcanic ± volcanogenicsedimentary
Metasedimentary Metaplagiorhyodacite Metabasites rocks plagiorhyolites Metabasites
Petrography
Actinolitites and quartzactinolite rocks, The most widespread rock green rocks, metaplagiorhyodaciteplagio rhyolites, sericitechloriteplagioclasequartz groups schists
Petrochemical features
Second type of the section (reference borehole 0150 + additional boreholes 570c, K16, K31, 7503, etc.)
Metaaleurolites, metasandstones of sericiteplagio clasequartz, biotite (chlorite)plagioclasequartz, and actinolite (±biotite)quartzplagioclase composition; actinolitites and quartzactinolite rocks, metaplagio rhyodacites
Porphyroblastic texture. Nematoblastic texture. Actinolite porphyroblasts (apophenocrysts) Large (3–5 mm) actinolite porphyroblasts are occured rare (up to 2% of the total volume (apophenocrysts) are typical of the rock) Relic phenocrystals (up to 2 mm) are repre sented by plagioclase and quartz. The volume of phenocrystals is up to 5–7% of the total volume of the rock. Phenocrystals are chaotically spread in the rock. The texture of the rock matrix is granoblastic and microhypidiomorphic
Relic phenocrystals (up to 5 mm; nearly 2–3 mm, on average) are represented only by plagioclase. The volume of phenocrystals reaches 30% (15%, on average) of the total volume of the rock. Phenocrystals are often concentrated as chains. The texture of the rock matrix is granoblastic
Subordinate significance relative to rocks of primarily volcanogenic genesis. Sericiteplagioclasequartz schists are typical. Finegrained texture
Predominance over rocks of primarily volcanogenic genesis. Metasandstones, metaaleurolites, and schists of seric iteplagioclasequartz, biotite (chlorite)plagioclase quartz, actinolite (±biotite)quartzplagioclase compo sition are typical. Relict psammitic textures are common
Relatively wide variation of SiO2 content (40.7–50.1 wt %). Lower Al2O3 content (11.2–14.5 wt %). Rela tively low MgO content (4.3–8.3 wt %); K2O content is up to 0.4 wt %
Relatively narrow variation of SiO2 content (42.9–49.1 wt %); Higher Al2O3 content (13.0–15.6 wt %). Higher MgO content (5.3–9.9 wt %). K2O content is up to 0.9 wt %
Metapla Relatively wide variation of SiO2 giorhyo content (65.9–76.0 wt %). litedac High Al2O3 content (10.5–15.0 wt %) ites
Narrow variation of SiO2 content (68.5–71.2 wt %). There no significant variations in Al2O3 (14.0–15.0 wt %)
Metasedi Occurrence frequency of “acid” (70%) mentary and “moderate” (30%) rocks rocks
Occurrence frequency of “acid” (30%), “moderate”(60%), and “basic” (10%) rocks.
The data of retrospective chemical analyses were used for petrochemical description of rocks.
compositions. In the depth interval of 838–1015 m, the sequence is intruded by a subvolcanic body of pla giodaciteandesidacite composition. This interval of the section is characterized by occurrence of interlay ers of metaferrobasaltandesites and their tuffs (7– 30 m). The upper part of the sequence is composed primarily of metavolcanogenicsedimentary deposits interbedded with thick (up to 20 m) metaplagioba salts, metaandesite basalts, and their tuffs, intruded by metagabbrodiabase dikes. STRATIGRAPHY AND GEOLOGICAL CORRELATION
Metavolcanogenicsedimentary rocks occur as sub layers among metavolcanogenic deposits or represent independent sections (borehole 054, the upper part of the sections in boreholes of 286c and 0182, etc.). The total proportion of these sublayers increases from bot tom to top. The schistosity dipping angles are 30°–60° (rarely higher); the thickness is nearly 1.5 km. Contact zones between Podgornoe and Strelitsa sequences of the Losevo Series are poorly understood. It is likely that the relationships between strata vary in Vol. 22
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(b) m 200
(c) m 200
400
400 400
600
600 555 1 2
800
800
3 4 5 6
1000
1000
7 8 9
1200
1200
Fig. 2. Schematic reference sections of the formation of the Losevo Series: (a) Podgornoe (borehole 0182), (b) Strelitsa (secondtype section, borehole 0150), (c) Strelitsa (firsttype section, borehole 7782). (1) Metasedimentary rocks, (2) metavolcanogenicsedi mentary rocks; (3) felsic metavolcanites; (4) basic metavolcanites; (5) plagiogranites; (6) metaplagiorhyo dacites, metaplagiorhyolites, metaplagiodacites, meta andesidacites, their metapyroclastic and metavolcano genicsedimentary analogues; (7) metaandesites and their metapyroclastic and metavolcanogenicsedimentary ana logues; (8) metaplagiobasalts, metaandesite basalts, and their metapyroclastic and metavolcanogenicsedimentary analogues; (9) metaferrobasalts, related metaandesite basalts, their metatuffs.
different parts. For example, as seen in the section of the 0160 borehole (the central part of the Losevo suture zone), metavolcanogenic and metavolcano genicsedimentary strata of the Podgornoe sequence lie on disintegrated (zone of tectonic disintegration)
gabbros of the Rozhdestvenskoe complex. This boundary can be interpreted as a tectonic contact. In the middle part of the section on the borehole, meta morphosed weathering crust, developed on basement rocks (Usman granites, Losevo metavolcanites, etc.) in basal polymictic metaconglomerates, is overlain by the metaterrigenous strata of the Voronezh Formation. The section of the 286s borehole (the southern part of the Losevo suture zone) presents a gradual transition (alternation) from rocks of the underlying strata of the Strelitsa sequence to the overlying Podgornoe sequence. Rozhdestvenskoe Complex. Gabbro intrusions of the Rozhdestvenskoe complex intrude metavolcano genicsedimentary rocks of the Strelitsa sequence and, in turn, they are intruded by granitoides of the Usman complex and dikes of plagioclase hornblendites, con sisting of brown hornblende and plagioclase. It is assumed that gabbroids of the Rozhdestvenskoe com plex and metabasalts of the Strelitsa sequence com prise the unified volcanoplutonic association, as evi denced by the similarity of their chemical composi tion. The intrusions of the Rozhdestvenskoe complex are homogenous in composition and have a sheetlike shape with steeply dipping contacts; the thickness is 30–150 m. The Rozhdestvenskoe complex is nearly 0.5–0.7 km2 in area. Usman complex. Predominantly, this complex is represented by large plagiogranite intrusions (up to 540 km2 in area) with zonal structures, representing regular changes both in structuraltextural and com positional features. In the marginal parts of intrusions, granites are finegrained and have much more distinct directional structure. Toward the central parts of intrusions from endocontacts, the grain size of gran ites increases gradually and granites become more massive. The contacts between plagiogranites and country rocks are sharp, distinct, often tectonized; distinct hornfelsification zones are not observed. In exocontact zones of intrusions, metavolcanogenic sedimentary rocks contain numerous apophyses of plagiogranites. The age of the Usman granitoides (2096.8 ± 3.3 Ma) was determined from zircons (three weights of different factions) using a Cameca TSN 206A singlecollector massspectrometer (Bibikova et al., 2009). This is the only currently reliable U–Pb age dating of Paleoproterozoic structuralcompositional complexes in the central part of the Losevo suture zone. Metavolcanogenicsedimentary rocks of the Podgornoe sequence contain thin (probably, dikes) intrusive metagabbro–diabase bodies, which are not taken into consideration in the modern scheme of tec tonomagmatic evolution of the Voronezh crystalline massif in the Precambrian. The thickness of dikes var ies from tens of centimeters to a few tens of meters. Their contacts are distinct, direct, sometimes indis tinct and baylike. Within the contact zone, rocks are most intensely foliated. We propose to unify the met
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agabbrodiabase bodies described into an independent Novovoronezh hypabyssal complex. The rocks of the Losevo Series are overlapped by metaterrigenous strata of the Voronezh Formation represented by metaconglomerates, metapsammites, metaaleurolites, and schists. Metaconglomerates contain pebbles of rocks of the Usman and Rozh destvenskoe complexes and both sequences of the Losevo Series. The rocks of the Voronezh Formation with gentle dipping relict layering were described in troughlike structures to the west and east of the LosevoMamon Fault. It is logical to assume that eruptions of stratovolcanoes of the Baygora structure occurred later than metamorphism of conglomerates (Terentiev and Chuvashina, 2003), since volcanic rocks are weakly metamorphosed and absent among pebbles of metaconglomerates. The rocks of the Voro nezh Formation and Baygora volcanogenic complex spread beyond the Losevo suture zone and are, in fact, imposed complexes. Because of this, these complexes are not discussed in this paper in detail. PETROGRAPHY Metavolcanogenic rocks (metabasalts, metaplagio rhyodacites, metaplagiorhyolites, their metapyroclas tics) of the Strelitsa sequence of the Losevo Series comprise a contrast metabasaltplagiorhyolite associ ation (Table 1). Metabasalts compose subvolcanic bodies (sills, dikes), flows and nappes (judging by the consistency of metavolcanites in area and in the sec tion), and layers of metapyroclastic rocks. Acid rocks are mostly of subvolcanic, pyroclastic, rarely effusive facies. Among metabasalts, two species are distin guished in mineral composition: quartzactinolite and (quartz, albite)carbonatechlorite (greenstones). There are transitional rocks between these two species. All metabasites contain a certain amount of epidote. Relict structures of basic rocks are not preserved, except metamorphosed subvolcanites, containing hornblende apophenocrystals. According to structural features, massive, banded, and foliated metabasites are distinguished. It is probable that structural variations in metabasites are determined by facies variation in volcanic bodies (effusion → pyroclastics → volca nosedimentary rock) (Terentiev, 2002). The matrix of metaplagiorhyodaciteplagiorhyolites is made of sericite, quartz, and plagioclase ± epidote ± chlorite. Relict phenocrysts are represented by distinctly zonal and azonal plagioclases, as borehole as quartz in high Si rocks. As borehole as basic rocks, massive, schis tose, and banded types of rocks can be distinguished. Metasedimentary and metavolcanogenic sedimen tary rocks of the Strelitsa sequence vary in mineral composition. The following types are distinguished: (1) sericiteplagioclasequartz; (2) biotiteplagio clasequartz, chloriteplagioclasequartz; (3) actino litequartzplagioclase, including biotitebearing STRATIGRAPHY AND GEOLOGICAL CORRELATION
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ones; (4) garnetquartz chlorite; and (5) sericite (±chlorite)quartzcarbonate rocks. According to granulometric composition of metasedimentary rocks of the secondtype section, in the lower part, the medium and finegrained metap sammites dominate; in the upper part, metaaleuroli tes. In general, higher in the section, the average size of relict detrital grains varies from 0.2 to 0.02 mm (Terentiev, 2004). This change is statistically signifi cant at a significance level of q = 0.01. The trend of decrease in the size of relict fragments is nonlinear: the boundaries of the three metaterrigenous units were drawn on the basis of relict grainsize distribution data (moving average method). These three units differ in particle size and mineral composition, the nature of rhythmicity, or its absence (Terentiev, 2004). Metavolcanogenic rocks of the Podgornoe sequence of the Losevo Series. On the AFM diagram, tholeiitic and calcalkaline series are distinctly distinguished. Metavolcanites of tholeiitic and calcalkaline series are normally interbedded in sections. Metatholeiites (metaferrobasaltsandesites) are confined to the middle and lower parts of the Podgornoe sequence. They are usually represented by metamorphosed volcanites, less commonly by pyroclastics of basaltic and andesibasaltic composition. The metabasites described are character ized by relict amygdaloidal structures. Amygdales are often filled with quartz, less commonly with epidote and chlorite. Aphyric and porphyritic varieties are dis tinguished. Porphyritic phenocrysts are represented by mafic minerals that developed on aggregates of epi dote, chlorite, and actinolite. The matrix of rocks is made of actinolite needles, chlorite flakes, grains of epidote and plagioclase, and small amounts of quartz, carbonates, and ore minerals. Sometimes it is possible to distinguish relict plagioclase blades. Owing to convergent metamorphic processes, metaplagiorhyolites correspond to acidic rocks of the Strelitsa sequence in mineral composition, but differ in geochemical parameters. These metaplagiorhyo lites are combined with metatholeiites into a unified association. Calcalkaline metavolcanites include a wide range of rocks—from metaplagiobasalts, metaandesite basalts, and metaandesites to metaplagiodacites of dif ferent facies: subvolcanic, effusive, pyroclastic, and volcanicsedimentary. Relict phenocrysts in basic and intermediate rocks are represented by plagioclase, less commonly by pseudomorphs of epidote, actinolite, and chlorite after mafic minerals. Relict quartz phe nocrysts appear in acidic metaeffusives. A wide facies variation of the calcalkaline volcanism is subject to the following pattern: from bottom to top of the sec tion, the volume of the metapyroclastics and then metavolcanogenicsedimentary rocks increases. The latter composes more than 50% of the total rock vol ume in the top of the generalized section of the com plex (Terentiev, 2005). Metabasalts–andesite basalts of the calcalkaline series are often characterized by an Vol. 22
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occurrence of relict amygdaloidal structure, but unlike metatholeiites, amygdales, made of epidote, chlorite, and carbonate, dominate. In the matrix of these rocks, quartz, epidote, albite, chlorite, carbonate, and very rarely actinolite are the most common minerals. Metavolcanogenicsedimentary rocks of the Podgornoe sequence of the Losevo Series. In mineral composition, these rocks correspond to metamor phosed pyroclastic and effusive analogues. The pre dominance of sedimentary material is confirmed by the presence of rounded detrital quartz and feldspar; lenses and sublayers filled with quartz, carbonate, and quartzcarbonate material; and fragments of isometric shape (products of sedimentation during volcanic eruptions). Most of the rocks are of primary volcanic sedimentary origin and have distinct banded struc tures, represented by alternation of sublayers com posed of different minerals, as borehole as alternation of beds containing differentsize relict quartz and feld spar fragments (graded bedding). A characteristic fea ture of metavolcanogenicsedimentary sections is the presence of finegrained, thinlayered albitequartz chlorite (±sericite)carbonate rocks. The content of carbonate minerals in these rocks sometimes reaches 80%. It should be noted that carbonate occurs as a rockforming and as a secondary mineral in intersect ing sublayers. Carbonate veins are common in all rocks of the Podgornoe sequence. On V.T. Frolov’s classifi cation plot, the metavolcanogenicsedimentary rocks correspond to feldspathic greywacke and greywacke feldspathic rocks (Terentiev, 2005). Such features as the periodic occurrence of metavolcanogenicsedi mentary rocks in the local sections and the change of metapyroclastics by metavolcanogenicsedimentary rocks testify in favor of the general trend of the evolu tion of volcanic structures that we used as an indirect sign of determining the upper and lower boundaries of the sequence distinguished and their positions in the generalized section of volcanic rocks. Rozhdestvenskoe complex. Massifs of this complex are composed of gabbroamphibolites, amphibolized gabbronorites, pegmatoid hornblende gabbro, and epidotized gabbrodiorites (Bagdasarova, 1983). The mediumgrained gabbroamphibolites are made of green compact and coarse fiber hornblende and pla gioclase (An45–60), partially replaced by epidote. Accessory minerals are pyrite, pyrrhotite, chalcopy rite, sphalerite, ilmenite, zircon, sphene, and apatite. Amphibolized gabbro occurs among gabbroamphi bolites as thin zones (15–30 m). Rocks are composed of partly amphibolized and chloritized hypersthene (Fs40) and clinopyroxene (En40Wo46Fs14), and also plagioclase (An60–58). Pegmatoid hornblende gabbro has gabbroophitic texture due to a combination of elongated coarse plagioclase (An60) and coarse fiber xenomorphic green hornblende. Secondary minerals are epidote, chlorite, and calcite; accessory minerals are sphene, apatite, magnetite, and ilmenite. For geochemical and isotopegeochronological studies,
the core samples of the least altered biotite and biotite bearing, leucocratic and mesocratic gabbro and gab bronorites (borehole 7785) were selected. Usman complex. According to the mineral compo sition, rocks of endocontact zones of massifs compris ing the Usman complex are considered to be typical plagiogranites. The main rockforming minerals are idiomorphic rhythmically zonal plagioclase (oligo clase) and quartz. Meshed microcline is present in very small quantities (tenths of a percent to a few per cent of the total rock volume). Biotite dominates among mafic minerals; hornblende is in a subordinate amount. Typical accessory minerals are apatite, sphene, orthite, zircon, and magnetite. At a distance from the contact zone, the content of plagioclase and biotite in intrusive rocks decreases; hornblende disap pears; in turn, the content of microcline increases. As a result, the central parts of intrusions are composed of submassive biotiteplagioclase microcline granites. This tendency is manifested both in lateral variation in mineral composition of rocks in the observed erosion section of intrusions and in the vertical sections of granite massifs. The latter is confirmed by statistically significant (at 95% probability) changes in the mineral composition of granites in sections of boreholes drilled to a depth of 1.2 km (V.Yu. Skryabin, personal com munication). Secondary transformations of granites are repre sented by structural and textural features and their mineral composition. Among the secondary minerals, epidote, muscovite (or sericite), and chlorite are in predominance; calcite and pyrite are less common. In total, the content of secondary minerals does not exceed a few percent of the total rock volume. How ever, in zones of late superimposed deformations in granites, the content of secondary minerals increases sharply. At the same time, chlorite almost completely replaces biotite; the albite content in plagioclase increases; sericite, epidote, and calcite intensely develop on plagioclase grains; and quartz becomes granulated, forming elongated lensshaped aggre gates. Large muscovite lepidoblasts appear in the interstitial space. The weakly altered biotite plagio granites were subjected to geochemical and isotopic geochronological studies (borehole 0152). Novovoronezh hypabyssal complex is composed of metagabbrodiabase bodies (tens of meters thick). In central part of the complex, rocks are relatively weakly metamorphosed and have gabbroophitic structure and consist of nearly equal amounts of hornblende and plagioclase. In other cases, the relic minerals in the matrix of rocks are represented by plagioclase (50%) and quartz (1–7%). Metamorphic transformations in rocks are as follows: the replacement of plagioclase by aggregate of epidote (3–20%) and carbonate (25%); the replacement of mafic minerals by chlorite (30%, on average) and actinolite (15–25%). Aggregates con sisting of sphene and titanomagnetite (3–7%) are con sidered to be typomorphic. A relatively large size of
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such grains (0.2–0.5 mm) is a characteristic feature of metagabbrodiabases of the Novovoronezh hypabyssal complex, compared to massive metaferrobasalts (grain size of 0.05 mm or less) of the Podgornoe sequence. The texture of metagabbrodiabases is usually relict porphyritic with plagioclase phenocrysts 1 mm or more in size. 1
PETROCHEMISTRY
Strelitsa sequence of the Losevo Series. According to petrogeochemical features, basic metavolcanites are poorly differentiated and are represented by metabasalts. All of them belong to the sodiumtype metabasalts with higher contents of TiO2, ΣFeO, and sometimes MgO (subvolcanic formations) and low contents of Al2O3 and K2O. On some diagrams (AFM, FeO/MgO–SiO2, etc.) figurative points of metabasalts lie in the field of the tholeiitic series (Fig. 3) and low alkaline rocks (al' < 0.87). Metaplagiorhyodaciteplagiorhyolites and metapla giograniteporphyries are extremely highSi formations containing from 66 to 73 wt % SiO2 and relatively high amounts of CaO, TiO2, and sometimes MgO. Accord ing to al' = Al2O3/(Fe2O3 + FeO + MgO), they belong to very highalumina varieties with low (Calcalkaline metaplagiobasalts
1
1 La Ce Pr NdSm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr NdSm Eu Gd Tb Dy Ho Er Tm Yb Lu
(e) Metavolcanites of the calcalkaline series 100
LSA CMA
(f) Acid metavolcanites of the Losevo Series 100
Metaplagiobasalts
Strelitsa sequence Continuous series of the Podgornoe sequence Contrast series of the Podgornoe sequence
Metaandesites
Rock/chondrite
Rock/chondrite
Metaplagiodacites
10 CAB (Jamaica)
10
NAS RIAT (Jamaica)
LSA 1
1 La Ce Pr NdSm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr NdSm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 4. Chondritenormalized plot of REE distribution (Sun and McDonough, 1989) in magmatic rocks of the central part of the Losevo suture zone. (a) Intrusive igneous complexes, (b) metabasaltoids of the Strelitsa sequence, (c) metabasaltoids of the Podgornoe sequence, (d) metavolcanites of the Strelitsa sequence, (e) calcalkaline metavolcanites of the Podgornoe sequence, (f) acidic metavolcanites of the Losevo Series. Volcanites of different geodynamic settings: NMORB and EMORB—normal and enriched basalts types of midocean ridges, respectively; OIB—ocean island basalts (Sun and McDonough, 1989); IAT (Jamaica), CAB (Jamaica), and RIAT (Jamaica)— tholeiites, calcalkaline basalts, and rhyolites of the tholeiitic series of the Jamaica island arc, respectively (Hastie et al., 2007); LSA and NAS—low and highSi adakites, respectively (Martin et al., 2005); CAB—highalumina island arc basalts (Balashov, 1976); CMA—andesites of continental active margins (Govindaraju, 1984). STRATIGRAPHY AND GEOLOGICAL CORRELATION
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100
(a) Metavolcanites of the Strelitsa sequence
100
(b) Metavolcanites of the Podgornoe sequence Metatholeiites Metaplagiobasalts Metaandesitebasalts Metaandesites Metaplagiodacites
Rock/primitive mantle
Rock/primitive mantle
Metatholeiites of the sequence top Metatholeiites of the sequence bottom Metaplagiorhyolitesdacites
10
1
10
1 Rb Ba K Nb La Ce Sr Nd Zr Eu Ti Y Yb
Rb Ba K Nb La Ce Sr Nd Zr Eu Ti Y Yb
Fig. 5. Primitive mantlenormalized distribution (Sun and McDonough, 1989) of the average contents of trace elements in vol canic rocks of the Strelitsa (a) and Podgornoe (b) sequences of the Losevo Series.
A high value of the La/Yb ratio in metaplagiorio dacitesplagiorhyolites indicates the LREE predomi nance over HREE, but the total sum of the absolute LREE values is significantly less than that in metaba sites. This is evidence that acid metavolcanites could not form owing to differentiation of basic magma in intermediate magmatic chambers of the upper hori zons of the crust. Many researchers studying magmatic rocks (Evolyutsiya…, 1983) noted the problem of attributing acidic volcanic rocks to the same or another petro chemical series. In turn, the same problem concerning intrusive granitoides was discussed in detail. However, the spatial relationship between acidic metavolcanites and metatholeiites (TMORB) allows us to assign the former to the tholeiitic geochemical type. REE spectra of acid metavolcanites of the subvolcanic facies occupy an intermediate position between Usman pla giogranites and effusive facies of metaplagioriodac itesplagiorhyolites. Shchipanskii et al. (2007) were first to note the so called slab (subduction) component (enrichment in largeion elements and Nb anomaly) in metarhyolites of the Losevo Series. It should also be noted that metabasalts in the basement of the Strelitsa sequence are characterized by 8 to 9fold enrichment in large ion elements relative to the primitive mantle with weak fluctuations in Ba and K contents (Fig. 5a). The slab component as a negative Nb anomaly appears in metabasites of the top part of the Strelitsa sequence and then becomes more contrast in metaplagiorhyo lites at the final stages of formation of the sequence. Metavolcanogenic rocks of the Podgornoe sequence of the Losevo Series. Metatholeiites (metaferrobasalts and genetically related metaandesitebasalts) are characterized by a low total REE content (31.8–
81.6 µg/g), a relatively low (La/Yb)N ratio (1.1–2.2), some enrichment in La relative to Ce and Sm ((La/Ce)N and (La/Sm)N > l), weakly contrast Eu anomalies (Eu/Eu* = 0.73–1.77), and enrichment in LREE (Figs. 4c, 4e). The results of comparative anal ysis of metaferrobasalts and basaltoids of typical geo dynamic settings show similarity of the former to ocean tholeiites (EMORB) and island arc tholeiites. Compared to the primitive mantle, metatholeiites are 10fold enriched in large ion elements and LREE and 8 to 9fold in Ti and HREE. Metaplagiorhyolites of the contrasting formation of the Podgornoe sequence differ from other silicate rocks of the Losevo suture zone in LREE enrichment and weak negative Eu anomaly (Fig. 4f). In calcalkaline metavolcanites (metaplagioba salts, metaandesites, metaplagiodacites), the total REE content is 2–3 times higher than in metatholei ites. In addition, 10 to 120fold enrichment in LREE and 2 to 25fold enrichment in HREE were estab lished (Fig. 4e). The positive Eu anomaly is weak or absent (Eu/Eu*= 0.6–1.3). REE have a differentiated distribution (Fig. 4e). In the series from basic to acidic metavolcanites, the total REE content and the total amounts of light and heavy lanthanides vary synchro nously. According to the REE distribution, calcalka line metavolcanites are similar to the island arc volca nites. When comparing the calcalkaline metavolca nites with the primitive mantle (Fig. 5b), the negative Nb anomaly and positive anomalies of largeion ele ments in all rocks of this series are distinct. This is evi dence that calcalkaline metavolcanites formed in a suprasubduction setting. Rozhdestvenskoe complex. Gabbroids of this com plex are similar to metabasalts of especially subvolca nic facies of the Strelitsa sequence, differing in lower
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contents of Ti and V and higher contents of Cr, Ni, Sr, and Ba. It should be noted that previous data showed an increase in TiO2 (up to 3 wt %) in the most melano cratic gabbros. This is one more confirmation of the similarity between Rozhdestvenskoe gabbroids and Strelitsa metabasites. The most important feature of gabbroids of the Rozhdestvenskoe complex is frac tionated REE spectra (Fig. 4a) with an enrichment in LREE and depletion in HREE. In general, the total REE amount in this complex is somewhat higher than that in plagiogranites of the Usman complex. Usman complex. According to the petrochemical data, plagiogranites of the Usman complex are ana logues of metaplagiorhyolites (especially subvolcanic) of the Strelitsa sequence of the Losevo Series, differing from the latter in higher concentrations of LREE, HREE, Sr, Th, U, and Nb. According to their geochemical features, plagiogranites are similar to collision granitoides (Osobennosti…, 1992). Novovoronezh hypabyssal complex. Metagabbro diabases of this complex are geochemically similar to metaferrobasalts of the Podgornoe sequence. The geochemical, petrographic, and petrochemical simi larities make it possible to combine both of these rocks into the unified volcanoplutonic association. How ever, metagabbrodiabases differ from metaferroba salts in Sshaped REE distribution patterns (Fig. 4a) with an overall enrichment in LREE and depletion in HREE. According to the REE distribution, metagab brodiabases of the Novovoronezh complex differ from gabbroids of the Rozhdestvenskoe complex, in spite of similarity in petrochemical parameters. ISOTOPEGEOCHRONOLOGICAL DATA Zircons from biotite gabbro (sample 7785/246.0 255.0; 28 zircon grains) of the Rozhdestvenskoe com plex belong to the same generation. All grains are opaque. Approximately 80% of zircon grains are rep resented by fragments of idiomorphic prismatic crys tals; about 20% are subidiomorphic grains up to 100 µm in size with elongation of 1 : 1.8–2. The cathodoluminescence intensity in zircons is extremely weak in gray and dark gray tones. In cathodolumines cence images, the zoning is weakly manifested only in single grains and is visible owing to the presence of a paler rim. Zircons from the Rozhdestvenskoe gab broids differ distinctly from zircons from other gab broids of the Voronezh crystalline massif by nontrans parency and an absence of zoning. The U and Th con tents are 517–1736 and 531–2604 µg/g, respectively. Zircons from plagiogranites of the Usman complex (sample 0152/1000.0; 66 grains) are mainly repre sented by transparent idiomorphic and subidiomor phic elongated crystals (Fig. 6), and also rare yellowish and pinkish fragments of zircon crystals. Inclusions in zircons are rare. The grain size is 50–250 µm, rarely larger. Cathodoluminescence intensity is weak in gray tone. As a rule, zircon crystals have bipyramidal short STRATIGRAPHY AND GEOLOGICAL CORRELATION
Homogenous rim 10.1
135 (core)
100 µm
(a) Homogenous rim 11.2
11.1
(core)
100 µm
(b)
Homogenous grain
6.1
100 µm
(c)
Fig. 6. Cathodoluminescence (CLE) images of zircon grains from plagiogranites of the Usman complex. (a, b) Grains with ancient core zones, (c) a homogeneous zonal grain. Numbers of analytical points correspond to those in Table 3.
prismatic shape and distinct thinrhythmic concen tration zoning characteristic of magmatic zircons. A few zircons are weakly zoned. The U and Th contents in homogeneous zonal zir con crystals and rimes are 93–238 and 17–163 µg/g, respectively. Seventeen euhedral grains (25%) have nonzonal spotty cores, which are pale gray in CLE images (Fig. 6). It is likely that we are dealing with xeno genic zircons of the Strelitsa sequence. Zircon cores vary from 30 × 50 to 60 × 110 µm in size; in one case, the size of a single zircon core reaches 180 × 260 µm. Cores are isometric and they differ distinctly from zonal cores in cathodoluminescence intensity (paler or darker). In addition, they have unevenly block or weakly banded, rarely zonal structure and are frac tured. The core boundaries are uneven and resorbed. Vol. 22
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Sometimes the contact zones include microinclu sions, which are isotropic in CLE images. The U and Th contents in the zircon cores are 66–216 and 21– 181 µg/g, respectively. The concordant U–Pb age of zircons from biotite gabbro of the Rozhdestvenskoe complex (borehole 7785, the depth interval of 246.0–255.0 m) is 2120 ± 11 Ma (two analytical points, MSWD = 0.89, probability of concordance is 0.35; Fig. 7a, Table 2). The discordant U–Pb age of eight zircons grains is 2158 ± 43 Ma. Four analytical points (2.1, 3.1, 4.1, 8.1) were used for calculation. If analytical points significantly deviating from the linear trend are excluded, then the age is 2124 ± 10 Ma, which is close to the concordant age. Taking into con sideration a small number of concordant analytical points and more ancient (2130 ± 26 Ma) 206Pb/238U age of the least altered grain no. 4, we can conclude that the crystallization of gabbro occurs in the interval of 2120 ± 11–2158 ± 43 Ma. Zircons from plagiogranites of the Usman complex (borehole 0152, the depth of 1000.0 m) form two clus ters that differ in the presence or absence of azonal cores and variations in the 206Pb/238U age interval for single crystals: 1842–2070 Ma in zonal homogeneous zircon grains and rims and 2123–2184 Ma in zircons containing cores at a low degree of discordance. Grain no. 3 has 206Pb/238U age of 2099 ± 14 Ma, which is due to probable modification of the original U–Pb isoto pic system. Because of this, it is excluded from consid eration. The concordant U–Pb age of homogeneous zircon grains and their rims is 2047 ± 11 Ma (seven analytical points, MSWD = 0.76, probability of con cordance is 0.38; Fig. 7b, Table 3). The discordant age of zonal segments of such zircons analyses is 2066 ± 28 Ma (11 analytical points). The oldest concordant 206Pb/238U age at point no. 1.1 is 2070 ± 14 Ma. The concordant U–Pb zircon age of zircon cores is 2172 ± 17 Ma (three analytical points, MSWD = 0.14, prob ability of concordance is 0.70; Fig. 7b). The discor dant age of zircon cores is close to the concordant age value and is 2176 ± 28 Ma (four analytical points). DISCUSSION OF RESULTS Characteristics and Division of Structural and Material Complexes of the Central Part of the Losevo Suture Zone Before engaging in the discussion of conditions for the formation of the Losevo suture zone as a struc turaltectonic unit, we need to understand the geolog ical structure of this zone itself. The above results of studying the structural and compositional complexes of the central part of the Losevo suture zone can sig nificantly revise the geological position and its stratifi cation into stratons and magmatic complexes (Fig. 8). It is probable that the metavolcanites of the Losevo Series overlie the gneissmigmatite Don Association
of the Oboyan series (Mints et al., 2007). Attention must be drawn to the fact that highgrade metamor phic rocks (gneisses and amphibolites) (they are not examined in detail in this paper) were initially included in the Losevo Series. According to petro graphic composition, these rocks are similar to rocks of the Don Association, including biotiteamphibole granodiorite gneisses, amphibolebiotite and biotite plagiogneisses, amphibole diorite gneisses, and amphibolites, as borehole as their migmatite varieties. To determine the belonging of rocks studied to the Losevo and Don series, it is necessary to examine their petrochemical, geochemical, and isotopic geochro nological characteristics. According to geological and structural position and petrographicmineralogical and petrographic geochemical features of metavolcanites, the Losevo Series is subdivided into two sequences: Strelitsa and Podgornoe. At the bottom, the sequence of the Losevo Series is represented by rocks of the Paleoproterozoic Strelitsa sequence. The sequence is intruded by gab broids and plagiogranites of the Rozhdestvenskoe and Usman complexes, respectively. On the basis of results of age dating of ancient cores of zircons from plagio granites of the Usman complex, the probable age of the Strelitsa sequence is 2172 ± 17 Ma. The Rozh destvenskoe gabbroids are considered to be premeta morphic and Usman plagiogranites are synmetamor phic relative to the main metamorphic event that affected stratified sequences of the Losevo Series. Consequently, the age of the Rozhdestvenskoe com plex must be somewhat younger than the Strelitsa sequence, which is confirmed by U–Pb dating of zir cons from the least altered biotite gabbro of the Rozh destvenskoe complex (2120 ± 11–2158 ± 43 Ma). The U–Pb zircon age datings of Usman plagiogranites are the following: 2047 ± 11 Ma (concordant), 2066 ± 28 Ma (discordant), and 2070 ± 14 Ma (the oldest age at concordant point no. 1.1). In the light of increased scientific knowledge, the previously established age of the Usman complex of 2096.8 ± 3.3 Ma (Bibikova et al., 2009) cannot be con sidered to be the reference age since the U–Pb age dating was made using weighed samples of zircons and, accordingly, the age obtained is the average. The samples could have included ancient zircon cores, which could have led to a significant increase in the age estimate. Thus, the beginning of intrusion of gra nitic melts occurred at the turn of 2047–2066 Ma. The widespread zones of secondary foliation and mineral transformations are a characteristic feature of granites of the Usman complex and are probably evidence of their participation in the later stages of orogenesis (V.Yu. Skryabin, personal communication). In this regard, it is assumed that Usman granitoides intruded younger metamorphosed formations of the Podgornoe sequence (Fig. 8). Metavolcanogenic and associated metasedimen tary rocks of the Podgornoe strata are different from
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(a)
2 3
N=2 Concordant age = 2120 ± 11 Ma (including 2σ errors in the decay constant) MSWD concordance = 0.89 Concordance probability = 0.35
0.41
137
1 2150 4.1
0.39 2.1 3.1
2050
7.1
0.37 Pb/238U
1.1 5.1
1950
N=8 Upper intersection is not fixed Intersection of discordia: 345 ± 700 and 2158 ± 43 Ma MSWD = 9.5
6.1
206
0.35
1
1850
0.33
0.31
N=8 Upper intersection is fixed Intersection of discordia: –1 ± 11 and 2120 ± 11 Ma MSWD = 13
8.1
N = 4 (points: 2.1, 3.1, 4.1, 8.1) Upper intersection is not fixed Intersection of discordia: 199 ± 160 and 2124 ± 10 Ma MSWD = 0.27
0.29 Error ellipses for ±2σ interval
0.27 4.8
5.2
5.6
6.0
6.4
7.2
6.8
207Pb/235U
2
3
8.0
7.6
(b) Homogenous grains and rims, N = 7 Concordant age = 2047 ± 11 Ma (including 2σ errors in the decay constant) MSWD concordance = 0.76 Concordance probability = 0.38
0.42
0.40
N = 11 Upper intersection is not fixed Intersection of the discordia: 152 ± 920 and 2066 ± 28 Ma MSWD = 0.58
0.38
2
206Pb/238U
0.36
7.1 10.1
13.1
2
2.1
Cores, N = 4 Upper intersection is not fixed Intersection of the discordia: 2176 ± 28 Ma MSWD = 0.017
0.32 Error ellipses for ±2σ interval
5
1.1
13.2 12.1
12.2
4
3.1 6.1 11.2
4.1
0.34
0.30
11.1
2100 1
1900
1700
3
8.1
9.1 5.1 N = 11 Upper intersection is fixed Intersection of the discordia: –374 ± 11 and 2047 ± 11 Ma MSWD = 0.76
1
3
Cores, N = 3 Concordant age = 2172 ± 17 Ma (including 2σ errors in the decay constant) MSWD concordance = 0.14 Concordance probability = 0.70
6
7
207Pb/235U
8
Fig. 7. U–Pb isotopic diagrams for zircons from (a) biotite gabbro of the Rozhdestvenskoe complex (borehole 7785, depth int. 246.0–255.0 m) and (b) biotite plagiogranites of the Usman complex (borehole 0152, depth of 1000.0 m). STRATIGRAPHY AND GEOLOGICAL CORRELATION
Vol. 22
No. 2
2014
138
TERENTIEV Sedimentary cover Weathering crust Dikes of spessartites Baygora sequence: and hornblend basalts, andesite gabbro basalts, andesites, and their tuffs Ol’khovatka gabbronorite Voronezh Formation quartz (Somovo sequence): monzonitesgranite metaconglomerates, complex metapsammites, 2050 ± 23 Ma metaaleurolites, schist (Chernyshov et al.1998)
nce que es e se eri rno vo S dgo ose Po the L of
Voronezh Formation
Weathering crust (?) Podgornoe sequence Novovoronezh of the Losevo Series: complex: metaferrobasalts, dikes of amphibole metaplagiorhyolites, metagabbrodiabase their tuffs of the contrast formation; metaplagiobasalts, metaandesites, metaplagiorhyolites, their metatuffs, ? metatuffites of the dif ferentiated formation Usman Strelitsa Formation plagiogranite of the Losevo Series, complex metabasalts, metaplagio rhyolites their metatuffs 2047 ± 11 Ma of the contrast formation 2066 ± 28 Ma metapsammites, Rozhdestvenskoe metaaleurolites, complex schists of gabbroids 2172 ± 17 Ma 2120 ± 11 Ma (zircon cores 2158 ± 43 Ma in Usman granites)
?
Strelitsa sequence of the Losevo Series
?
Gneissmigmatite complex, Don Association of the Archean (?) Oboyan series + amphibolites of the Mikhailovka series (AR2)
Losevo Series (?): gneisses, amphibolites
Gneissmigmatite complex, Don Association of the Archean (?) Oboyan series + amphibolite of the Mikhailovka series (AR2)
Fig. 8. Integrated section and stratigraphicmagmatic scheme of Paleoproterozoic formations of the central part of the Losevo suture zone of the Voronezh crystalline massif.
rocks of the Strelitsa strata in various ways: lower degree of metamorphism and gentlydipping folia tion, suggesting that the Podgorny sequence has a younger age. Nevertheless, in some cases, the transi tion between these sequences is gradual and repre sented by alternating layers. Plutonic analogues of tholeiitic metavolcanites of the Podgornoe sequence are represented by metagabbrodiabase dikes, which
are assigned to the new hypabyssal Novovoronezh complex. Gabbroporphyrites, diorite porphyrites, and granitoides intruding metavolcanites of the Podgornoe sequence, which are described in some borehole sections, are very poorly studied. Because of this, these rocks are not presented in the modern scheme of the Precambrian stratigraphy compiled for the Voronezh crystalline massif and our stratigraphic
STRATIGRAPHY AND GEOLOGICAL CORRELATION
Vol. 22
No. 2
2014
STRATIGRAPHY AND GEOLOGICAL CORRELATION
Vol. 22
7.1
5.1
3.1
3
4
5
No. 2
2014
4.1
8
0.05
0.00
0.08
0.09
0.03
0.03
0.58
0.35
Th, µg/g
1059
531
860
935
1145
1736 2604
1296 1702
517
926
1054 1213
876
1292 1873
Pbc, U, µg/g %
206
1.27
1.55
1.36
1.06
0.96
1.19
1.25
1.50
Th 238 U
232
315
568
427
169
300
340
271
340
±
Pb 206 Pb age, Ma
207
±
D, % U 206 Pb*
(1) 238
±, %
26
26
26
25
25
2155
2125
2204
2152
2164
2097
7
11
11
12
24
10
3
3
7
5
10
22
2.612
2.639
2.655
2.663
2.796
3.277
1.4
1.5
1.4
1.4
1.5
1.4
2130
2081
26
25
2129
2120
11
5
0
2
2.554
2.625
1.4
1.4
Zircons with nonviolated Pb–U isotope system
2089
2071
2060
2055
1968
22
Zircons with violated Pb–U isotope system
1716
206
Pb 238 U µg/g age, Ma
206Pb*,
0.132
0.132
0.134
0.132
0.138
0.134
0.135
0.130
Pb* 206 Pb*
(1) 207
0.64
0.31
0.41
0.64
0.63
0.67
1.40
0.58
±, %
7.140
6.910
7.090
6.900
7.170
6.940
6.650
5.463
Pb* 235 U
(1) 207
1.6
1.5
1.5
1.6
1.6
1.6
2.0
1.5
±, %
±, %
0.3915 1.4
0.3809 1.4
0.3828 1.4
0.3788 1.5
0.3766 1.4
0.3756 1.4
0.3571 1.5
0.3049 1.4
Pb* 238 U
(1) 206
0.915
0.978
0.962
0.914
0.917
0.905
0.724
0.927
Error corr.
Errors are given for ±1σ interval; Pbc and Pb*⎯nonradiogenic and radiogenic Pb, respectively, (1)⎯Pbc correction to the measured 204Pb; D, %⎯discordance 100 × [(207Pb/206Pb age)/(206Pb/238U age) – 1]. Calibration error is 0.39. Analyses were performed at the Center for Isotope Research (VSEGEI), analyst A.N. Larionov.
2.1
7
1.1
6.1
2
6
8.1
1
Number of grain, No. measure ment point
Table 2. U–Pb ages of zircons from gabbro of the Rozhdestvenskoe complex (borehole 7785, depth interval of 246.0–255.0 m)
PALEOPROTEROZOIC SEQUENCES AND MAGMATIC COMPLEXES 139
STRATIGRAPHY AND GEOLOGICAL CORRELATION
5.1
2.1
9.1
4.1
13.1
13.2
12.1
12.2
10.1
8.1
7.1
4
5
6
7
8
9
10
11
12
13
14
3.1
93
163
66
–
211
0.00019 114
0.00004 216
–
0.00004
0.00025 115
0.00015 185
0.00030 103
0.00028 107
0.00037 219
0.00038 119
0.00010 236
0.00040 197
0.00194
72
38
181
74
21
100
91
70
62
163
17
130
82
65
144
131
0.35
0.34
0.86
0.47
0.32
0.89
0.51
0.71
0.60
0.77
0.15
0.57
0.43
0.73
0.63
0.62
Th 238 U
232
69.7
38.2
74.4
56.0
22.8
32.9
57.0
32.0
33.6
69.7
38.7
75.6
63.9
30.8
76.3
71.3
±
Pb 206 Pb age, Ma
207
±
D, % U 206 Pb*
(1) 238
±, %
15
20
15
18
22
15
14
2014
1988
2064
2049
2060
2080
2084
41
51
30
56
63
35
25
0
–3
1
0
1
2
1
2.717
2.663
2.693
2.669
2.665
2.683
2.641
18
18
19
19
2049
2074
2068
2066
29
21
29
25
11
5
4
3
3.021
2.793
2.763
2.748
1.1
1.1
1.1
1.1
0.9
1.1
0.9
1.0
1.3
0.9
0.8
15
16
27
2175
2179
2184
24
29
42
0
0
1
2.499
2.496
2.501
0.8
0.9
1.5
20
2196
27
3
2.562
1.1
Zircon cores with violated PbU isotope system
2123
Zircon cores with violated PbU isotope system
2170
2172
2168
Zircon cores with nonviolated Pb–U isotope system
1842
1972
1990
1999
Zircons with violated Pb–U isotope system
2020
2055
2036
2051
2041
2042
2070
Zircons with nonviolated Pb–U isotope system
206
Pb Pb*, 238U µg/g age, Ma
206
0.1300
0.1375
0.1359
0.1361
0.1366
0.1264
0.1282
0.1278
0.1277
0.1240
0.1221
0.1275
0.1265
0.1272
0.1287
0.1290
Pb* 206 Pb*
(1) 207
1.3
1.5
1.4
1.6
2.4
1.6
1.2
1.7
1.4
2.3
2.9
1.7
3.2
3.6
2.0
1.4
±, %
±, %
6.8976 1.5
7.3900 1.9
7.4963 1.6
7.5194 1.9
7.5291 2.8
5.7700 2
6.3300 1.6
6.3700 2
6.4000 1.8
6.2926 2.5
6.3241 3.1
6.5309 1.9
6.5317 3.3
6.5300 3.8
6.6114 2.2
6.7352 1.6
Pb* 235 U
(1) 207
±, %
0.3849 0.8
0.3900 1.1
0.4002 0.8
0.4006 0.9
0.3998 1.5
0.3308 1.1
0.3578 1.1
0.3616 1.1
0.3635 1.1
0.3681 0.9
0.3755 1.1
0.3714 0.9
0.3746 1.0
0.3725 1.3
0.3727 0.9
0.3787 0.8
Pb* 238 U
(1) 207
0.522
0.582
0.507
0.475
0.519
0.558
0.670
0.559
0.624
0.347
0.366
0.444
0.311
0.334
0.392
0.475
Error corr.
2098
23
0
2.598
0.8
2014
14
No. 2
2099
Errors are given for ±1σ interval; Pbc and P*⎯nonradiogenic and radiogenic Pb, respectively, (1)⎯Pbc correction to the measured 204Pb; D,%⎯discordance 100 × [(207Pb/206Pb age)/(206Pb/238U age) – 1]. Calibration error is 0.58. Analyses were performed at the Center for Isotope Research (VSEGEI), analyst A.N. Larionov.
Vol. 22
16
11.1
11.2
3
15
6.1
2
0.00017 238
0.00009 219
1.1
1
U, Th, µg/g µg/g
Pb 206 Pb
204
Number of grain, mea No. sure ment point
Table 3. U–Pb ages of zircons from plagiogranite of the Usman complex (borehole 0152, depth of 1000.0 m)
140 TERENTIEV
PALEOPROTEROZOIC SEQUENCES AND MAGMATIC COMPLEXES
and magmatic scheme for the central part of the Losevo suture zone. The Usman granites and, presumably, the rocks of both sequences of the Losevo Series are overlain with a stratigraphic break by metaterrigenous deposits of the Somovo (Voronezh) Formation, beginning with basal metaconglomerates. The study of sections of metaterrigenous deposits revealed that conglomerates contain pebbles of rocks of the contrast formation of the Losevo Series and metavolcanites of the continu ously differentiated plagiobasaltandesiteplagiodac ite formation of the same series, which confirms their more ancient age relative to metaconglomerates. In the lower parts of metaterrigenous sections, rock frag ments of the Podgornoe sequence occur; in the upper parts, the rock fragments of the Strelitsa sequence. Such a relationship is explained by the gradual increase in the erosional level and destruction of young and, then, older rocks. In addition, metaconglomer ates contain pebbles of granitoids and gabbroids of the Usman and Rozhdestvenskoe intrusive magmatic complexes. Metaterrigenous rocks are intruded by intrusions, varying in composition from gabbro to granite, which are confined to the Ol’khovatka gab bronoritequartz monzonitegranite complex. The age of the first magmatic phase is 2050 ± 23 Ma (Chernyshov et al., 1998). The Ol’khovatka complex was distinguished in the petrotypical Ol’khovatka massif, located to the east of the Losevo suture zone behind the LosevoMamon fault. Nonmetamorphosed basalts, plagiobasalts, andes ite basalts, andesites of normal alkalinity, rocks of sub alkaline series, and their hypabyssal analogues (por phyrites) are unified into the Baygora sequence and volcanoplutonic association. The youngest intrusive formations are represented by cenotype dikes of spes sartites and hornblende gabbro, discordant to all for mations of the Losevo suture zone. Paleoproterozoic Volcanism and Sedimentogenesis in the Losevo Suture Zone The contrast volcanism of the Strelitsa time was manifested along two linear zones, which were proba ble spreading axes (Terentiev, 2005). The small volume of metapyroclastic rocks at the base of the Strelitsa sequence (Table 4) is evidence of their origin due to underwater (relatively deep) eruptions at the early stages of development of the paleobasin below the pressure compensation level (Fisher, 1987), which is not less than 500 m for basic magmas (McBirney, 1963) and 1000 m for acidic magmas (Fisher and Schminckle, 1984; Moore, 1970). This is confirmed by the absence of relict amygdaloidal textures (pri mary porosity). With decreasing depth level of erup tions, the vapor pressure in the magma began to exceed the external water pressure. At the shallow water level, volatiles contained in the magma could have been released with explosions (Moore, 1970). As STRATIGRAPHY AND GEOLOGICAL CORRELATION
141
a result, at the late stage of formation of the Strelitsa sequence, the accumulation of pyroclastic rocks occurred: the explosivity index (the ratio of the prod ucts of explosive volcanism to the total volume of vol canites (Rittman, 1962)) regularly varies from 0 to 90% from bottom to top (Terentiev, 2002) upon shal lowing of the basin. The association of volcanites and subvolcanic rocks with sedimentary rocks in the secondtype section of the Strelitsa sequence thickness is additional proof of volcanic eruptions in deepwater marine basins. Sedimentation in the Strelitsa time. The recon structed mineral composition of terrigenous sequences varies (from bottom to top) from quartz plagioclase rocks or arkoses and subarkoses after the classification by Shvanov (1987) (unit 1) and greywackeplagioclase rocks (unit 2) to subgreywacke and greywackequartz rocks (unit 3) (Terentiev, 2004). The graded bedding observed, an increase in unifor mity, distinct rhythmicity, finegrained clastic mate rial, and general “transgressive” section type (subse quent subsidence of the sedimentation basin) are evi dence of an extensive (but not oceanic) marine basin (Frolov, 1993). Thus, volcanically inactive areas of a paleobasin are composed of metaterrigenous and greywacketype rocks. They are immature clastic rocks with a low degree of sorting and significant vol ume of the matrix, which indicates avalanchelike deposition (Frolov, 1993). According to chemical composition, these rocks correspond to acidic, medium, and rarely basic igne ous rocks. If metavolcanites of a metabasaltplagio rhyolite formation had been the source of clastic material, then metaterrigenous rocks with SiO2 = 60 ± 3 wt % could not have been deposited in high volumes, given the immaturity of primary sludge. In addition, the magma did not reach the sea bottom surface but crystallized within the thick sedimentary sequence as subvolcanic bodies, which could not have been destroyed with formation of terrigenous deposits. Basaltplagiorhyolites of rift valleys also could not have served as a constant source of pyroclastic material (Obstanovki…, 1990). These considerations lead to the assumption that basement rocks of the Losevo Series, represented by amphibolites of the Mikhailovka series, biotiteamphibole gneisses, granodiorite gneisses, and amphibolites of the Oboyan series, could have been the source of clastic material in the Strelitsa time. In this case, metaterrigenous deposits of the Strel itsa sequence should be unified into the independent metaterrigenous formation. Supposing that a hypo thetical continent could have been a source of pyro clastic material (Terentiev, 2004), and the composi tional composition of deposits in the marginal sea basins was mostly terrigenous, we can assume the rel ative proximity of the paleobasin shoreline. This is another difference of marginalsea terrigenous com plexes from oceanic ones. Vol. 22
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2014
142
TERENTIEV
Stratum
Genetic types of volcanites
Strelitsa
Type of volca nism
Relationship of volcanites with terrigenous rocks
Structures
Setting
Contrast metaba saltplagiorhyolite
Lava flows and nappes, subvolcanoes, tephra and tuffs
Linear zones of submeri dional strike
Clastoliths comprise Shield Sub subvolcanic rocks volcanoes aquaeous
Differentiated metabasaltande siteplagiodacite
Lava flows, subvolcanoes, extrusions, pyroclastic flows, tephra turbidites, tephra and tuffs
Isometric, rarely linear zones
Spatial and genetic
Rocks
Surrounding environment
Character Thickness, of contacts m
Subvolcanoes (dikes) Rhythmic Metabasalts, sequences of metaplagior metaterrige Subvolcanoes phyodacites nous rocks (sills)
Podgornoe
Forms of distribution of volcanites
Fractured
Volcanogenic formation
Central
Podgornoe Strelitsa
Stratum
Table 4. The parameters of volcanism of the Losevo Series
Metabasalts, rarely meta plagiorphyo dacites Metatuffs of basalts and plagiorhyo dacites Metaplagio dacites, metaandes ites
Discordant
0.1–1
Structures (relict)
Stratovol Subaerial canoes subaqual
Structures
Finegrained, fine porphyric
Foliated, curved
Subconcor dant
2–40
Symmetrical zoning (from the contact zone to the center and back): aphyric → fine porphy ritic → coarse porphyritic
Symmetrical zoning (from the contact zone to the cen ter and back): aphyric → massive
Volcanogenic Lava flows and strata nappes
Concordant
1–20
Asymmetrical zoning (from top to bottom): clastic → aphyric → fine porphyritic
Asymmetrical zoning (from top to bottom): banded → foliated → massive
Zonal paleo Tephras and flows, tuffs sublayers
Concordant
0.5–10
Clastic
Banded, foliated
Discordant
up to 160
Porphyritic, finegrained, lavoclastic
Relict, fluidal, massive
Volcanogen icsedimen Extrusions tary sequences Volcanogen Metadiorites icsedimen (andesite) Subvolcanoes tary porphyrites sequences Metapla giobasalts, Volcanogenic Lava metaferro sequences flows basalts Rocks corre Volcanogen sponding to icsedimen moderate and tary Pyroclastic flows basic volca sequences nites Volcanogenic Tephra turbid sequences ites Acid, moder Zonal paleo ate, and basic flows, Tephra and tuffs sublayers
Discordant and subcon cordant
1–10
Concordant
5–30
Concordant
2–10
Concordant
0.1–3 0.5–10
Symmetrical zoning (from the contact zone to the center and back): aphyric → fine porphy Massive ritic → coarse porphyritic Asymmetrical zoning (from Asymmetrical zoning (from top to bottom): coarse top to bottom): clastic → amygdaloidal → fine aphyric → fine porphyritic amygdaloidal → massive Microclastic
Zoning (classical Bouma se quence) Crystalloclastic, lithoclastic
Directed
Relict layered Banded, foliated
Concordant
In the areas of ancient effusive and explosive erup tions, finegrained metasedimentary rocks of sericite plagioclasequartz and chloriteplagioclasequartz composition, which represent intraformational volca nogenicsedimentary formations, are in predomi nance. Assemblage of volcanites with sedimentary rocks and petrochemical similarity of metabasalts with oceanic and marginalsea basaltoids allow interpreting
them as products of volcanic activity in the underwater setting. Thus, at the early stage of development (Strelitsa time) within the Losevo paleobasin, processes of marine deposition and deepsea volcanic eruptions mainly occurred. Two types of sections distinguished are compared with rock assemblages that formed in morphostructures corresponding to modern basins
STRATIGRAPHY AND GEOLOGICAL CORRELATION
Vol. 22
No. 2
2014
PALEOPROTEROZOIC SEQUENCES AND MAGMATIC COMPLEXES
and marginal sea spreading zones under submarine conditions. Volcanism of the Podgornoe sequence is discordant to the volcanism axes in the Strelitsa time (Fig. 1b). On the basis of the results of formational analysis of metavolca nites, the Podgornoe sequence comprises elements of an island arc itself (continuously differentiated meta plagiobasaltandesiteplagiodacite assemblage) and elements of an interarc or forearc basin (contrast meta ferrobasaltandesiteplagiorhyolite assemblage). Sub volcanic bodies, lava flows, deposits of seismotectonic landslides, pyroclastics, and volcanogenicsedimentary rocks, which, apparently, composed large paleostrato volcanoes, play a significant role in formation of the Podgornoe sequence (Table 4). Extensive development of amygdaloidal textures in lavas (primary porosity) and their gas saturation (MacDonald, 1975) in volcanites of the Podgornoe sequence are evidence of their formation in shallow subaqueous and subaerial settings. These features, a high volcanic explosivity index equal to 70–75%, and petrogeochemical features are characteristic of depos its formed under island arc conditions. The Podgornoe sequence includes a large volume of volcanoclastic material that originated from large volcanic structures of central type with predominance of explosive erup tions (Carey and Sigurdson, 1987). Sedimentation in the Podgornoe sequence. In con trast to metaterrigenous rocks of the Strelitsa sequence, the main features of metavolcanogenic sedimentary rocks of the Podgornoe sequence are pre dominantly feldspar and quartzfeldspar composition, the presence of fragments of metaeffusive and metapy roclastic rocks, petrochemical similarity of effusive, pyroclastic, and volcanogenicsedimentary facies, and also their regular interbedding. This indicates that for mation of metavolcanogenicsedimentary rocks of the Podgornoe sequence occurred as a result of destruc tion of the island arc rocks, both synchronously and asynchronously to volcanic activity. In addition, the Podgornoe sequence includes deposits of ancient turbidity cones consisting of rewashed pyroclastic deposits and characterized by relict graded bedding. These paleoflows are interbed ded with metapyroclastic rocks and intruded by meta morphosed extrusions. The apparent thickness of the paleoturbidite layer varies from tens of centimeters to 2–3 m. Only in rare outcrops is it possible to observe an idealized section of a turbidite flow (classical Bouma sequence (Bouma, 1962)). The transition zone between one paleoflow to the other is a zone of disturbance of underlying ancient silts by overlying turbidites. Thus, periods of relative attenuation of effusive volcanic activity, intensively manifested at the early stages of formation of the Podgornoe sequence, were followed by periods of explosive volcanism in subaerial and subaqueous settings. Metamorphosed analogues of pyroclastic flows near the centers of vol canic activity contain rock fragments of the basement STRATIGRAPHY AND GEOLOGICAL CORRELATION
143
of a volcanic structure (borehole 7529). As noted by Maleev (1975), pyroclastic flows can move to a dis tance of over 50 km. It is probable that this process led to the deposition of pyroclastics, containing rare crys tal fragments, in association with volcanic and sedi mentary rocks at a considerable distance from strato volcanoes (borehole 054). In subaqueous settings, along with turbidites, chemogenicclastic rocks, such as siltstone, argillaceous rocks containing significant amount of carbonate material, and essentially carbon ate rocks, deposited on the slopes of paleovolcanoes. Recall that among rock fragments plagioclase crys talloclasts dominate; lithoclasts are rare. Metavolcano genicsedimentary rocks with high (greater than 50%) quartz content are rare. It was established that terrige nous rocks with a low SiO2 content, but with a high content of moderate plagioclase, are characteristic of active margins (Pacific and Andean types; Obstanovki…, 1990; etc.). In the diagram of quartz feldsparrock fragments (Yerino and Maynard, 1984), metavolcanogenicsedimentary rocks lie in the field of island arc sands (Terentiev, 2004). In the diagram (Bhatia, 1983), figurative points of compositions of metavolcanogenicsedimentary rocks predominantly lie in the field of island arc sandstones (Terentiev, 2004). CONCLUSIONS (1) The study of reference sections of the Losevo suture zone located within the Voronezh crystalline massif carried out using geodynamic analysis revealed contradictions in the modern stratigraphic scheme of this suture zone. This allows us to make appropriate adjustments: younger formations of the Baygora vol canic complex, previously attributed to the Voronezh Formation, are distinguished as a separate formation (Terentiev and Chuvashina, 2003). The Somovo (Voronezh) Formation is represented by metaterrige nous rocks. On the basis of the presence of the attrib utive metavolcanogenic formation, the Losevo For mation is subdivided into two sequences: Podgornoe and Strelitsa. A new hypabyssal Novovoronezh met agabbrodiabase complex is distinguished. As a result of our study, the problem of the stratigraphic position of gneisses and amphibolites, which were previously attributed to the Losevo Series, was raised and the pos sibility of distinguishing a plutonic dioritegranitoid complex in the Losevo Series was discussed. (2) The main features of volcanogenic formations of the Strelitsa sequence are the following: formational similarity with typical contrast basaltrhyolite associa tions; facies variation in the presence of pyroclastic, effusive and associated volcanogenicsedimentary rocks, and also subvolcanic rocks, which are not genetically connected with surrounding terrigenous rocks; an absence of relict amygdaloidal structures in rocks of volcanic and subvolcanic facies; an occur rence of protolith structures (aphyric and fine porphy Vol. 22
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ritic) in effusive species and coarse porphyritic and gabbroophitic textures in subvolcanic species; simi larity between metabasalts represented by sodium highTi moderately Mg tholeiites and modern mar ginal sea basaltoids; the belonging of acid metavolca nites to lowalkali highalumina rhyodacites of sodium, potassiumsodium, and calcalkaline series; the distribution of REE and other rare and trace ele ments in metabasites, similar to that of marginal sea basaltoids (NMORB and TMORB basalt types). (3) The main characteristics of volcanites of the Podgornoe sequence are the following: similarity to differentiated formations of basaltandesiterhyolite composition; the presence of two petrochemical rock series: tholeiitic metaferrobasaltandesite basalt in association with metaplagiorhyolites and differenti ated calcalkaline metaplagiobasaltandesiteplagio dacite; facies variation of volcanites: volcanogenic sedimentary, pyroclastic, effusive, extrusive, often jux taposed in one section; numerous relict amygdales: quartz and chloritequartz in tholeiitic metabasal toids, epidote and epidotechlorite sometimes with carbonates in the calcalkaline metabasaltsandesites; the presence of relict structures: porphyritic, interser tal, ophitic, hyalopilitic, etc.; the belonging of meta ferrobasaltandesite basalts to lowalumina basites of normal alkalinity of the sodium and highTi series; the belonging of metaplagiobasaltandesite basalts, meta andesite–andesidacites, and metaplagiodacitepla giorhyolites to moderate, high, and very highalu mina formations of sodium and potassiumsodium series; extensive development products of explosive volcanism, tuff turbidite flows, and volcanogenicsed imentary rocks; the distribution of REE and other rare and scattered elements in metavolcanites, similar to that in tholeiitic and calcalkaline basalts, andesites, and dacites of the island arcs. (4) The age range (2120 ± 11–2158 ± 43 Ma) of formation of premetamorphic gabbroids of the Rozh destvenskoe complex, which is complementary to metabasites of the contrast formation, is considered to be the upper age boundary of the Strelitsa sequence. The age of cores of zircon grains (2172 ± 17 Ma) from rocks of the Usman complex is probably its lower boundary. The age of Usman granitoides (2047 ± 11– 2066 ± 28 Ma) has been clarified. It turned out to be younger than the reference age of 2096.8 ± 3.3 Ma (Bibikova et al., 2009) which was obtained without taking into account an occurrence of ancient zircons cores from surrounding rocks. Thus, crystallization of Usman granitoides was associated with the collision which began at around 2050 Ma with intrusion of the Bobrovsky granite complex east of the Losevo Mamon Fault. (5) The pattern of paleogeodynamic evolution of the Losevo suture zone in the first half of the Paleopro terozoic was established. It includes the following stages: (1) tholeiitic volcanism in spreading zones and accumulation of terrigenous strata in the marginal sea
basins; (2) eruptions of Nbdepleted tholeiites and plagiorhyolites in the subduction setting, which is confirmed by geochemical data; (3) gabbro intrusions of the Rozhdestvenskoe complex; (4) formation of an island arc synchronously with stage 2 and tholeiitic and calcalkaline volcanism, which led to formation of Podgornoe strata; (5) intrusions of gabbrodiabases, subsynchronous to volcanism, of the Novovoronezh complex and dioritegranitoides and crystallization of granitoides of the Usman complex; (6) a break in sed imentation and formation of molasses of the Voronezh (Somovo) Formation. ACKNOWLEDGMENTS I am grateful to V.Yu. Skryabin for providing core samples of granitoides of the Usman complex and use ful discussion of all materials used for preparation of this article. I would like to especially thank analysts G.S. Zolotareva (Faculty of Geology, Voronezh State University) for selection of zircon monofractions and A.N. Larionov (VSEGEI) for the isotopegeochrono logical study of zircons and assistance in learning the Isoplot Ex ver. 3.6 program. Reviewers V.A. Glebovitskii, E.V. Bibikova REFERENCES Bagdasarova, V.V., Comparative analysis of structural and formational position, material composition, and ore bear ance of protogeosyncline basite intrusives of the southeast of the Voronezh Crystalline Massif (VCM), in Petrologiya i metallogeniya magmaticheskikh i metamorficheskikh kompleksov KMA i smezhnykh raionov (Petrology and Met allogeny of Magmatic and Metamorphic Complexes of the KMA and Adjacent Areas), Voronezh: Izd. VGU, 1983, pp. 61–69. Balashov, Yu.A., Geokhimiya redkozemel’nykh elementov (Geochemistry of Rare Earth Elements), Moscow: Nauka, 1976 [in Russian]. Bhatia, M.K., Plate tectonics and geochemical composi tions of sandstones, J. Geol., 1983, vol. 91, no. 6, pp. 611– 627. Bibikova, E.V., Bogdanova, S.V., Postnikov, A.V., et al., Sar matia–VolgoUralia junction zone: isotopicgeochrono logic characteristic of supracrustal rocks and granitoids, Stratigr. Geol. Correl., 2009, vol. 17, no. 6, pp. 561–573. Black, L.P., Kamo, S.L., Allen, C.M., et al., Temoral: a new zircon standard for U–Pb geochronology, Chem. Geol., 2003, vol. 200, pp. 155–170. Bocharov, V.L. and Chernyshov, N.M., Endogenic regimes of the Early Precambrian of the Voronezh Crystalline Mas sif, in Endogennye rezhimy formirovaniya zemnoi kory i rudoobrazovaniya v rannem dokembrii (Endogenous Regimes of the Earth’s Crust Formation and Ore Genesis in the Early Precambrian), Leningrad: Nauka, 1985, pp. 192– 205. Bocharov, V.L., Geology, geochemistry, and metallogeny of ultramafitemafite formations of the Voronezh Crystalline
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PALEOPROTEROZOIC SEQUENCES AND MAGMATIC COMPLEXES Massif (VCM), Extended Abstract of Doctoral (Geol.Mineral.) Dissertation, Voronezh, 1988. Bondarenko, S.V., Zolotareva, G.S., and Nenakhov, V.M., The nature of andesite porphyrites of the Baigora volcanic plutonic structure of the Voronezh Crystalline Massif, in Vulkanizm i geodinamika. T. 1 (Volcanism and Geodynamics), PetropavlovskKamchatsky: IViS DVO RAN, 2009, vol. 1, pp. 283–286. Borukaev, Ch.B., Struktura dokembriya i tektonika plit (Pre cambrian Structure and Plate Tectonics), Novosibirsk: Nauka, 1985 [in Russian]. Bouma, A.H., Sedimentology of Some Flysch Deposits: a Graphic Approach to Facies Interpretation, Amsterdam: Elsevier, 1962. Bush, V.A., Ermakov, Yu.N., and Uimanova, L.N., Geody namic model for formation of Late ArcheanPaleoprotero zoic structures of the Voronezh massif, Geotektonika, 2000, no. 4, pp. 14–24. Carey, S.N. and Sigurdson, H., A model of volcanogenic sed imentation in marginal basins, in Marginal Basin Geology: Vol canic and Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basins, Kokelaar, B.P. and Howells, M.F., Eds., 1984, Geol. Soc. London, Spec. Publ., vol. 16, pp. 37–58. Chernyshov, N.M. and Nenakhov, V.M., The major types of geodynamic and mineragenic series in the general model of the Precambrian lithosphere (as exemplified by the VCM), Vestn. Voronezh. Univ. Ser. Geol., 2010, no. 2, pp. 47–59. Chernyshov, N.M. and Egipko, O.I., Specific features of some Precambrian magmatogenic and ultrametagenic for mations of the Voronezh Crystalline Massif (VCM), in Problemy dokembriiskogo magmatizma (Problems of Precam brian Magmatism), Leningrad: Nauka, 1974, pp. 53–59. Chernyshov, N.M., Nenakhov, V.M., Lebedev, I.P., and Strik, Yu.N., The model of geodynamic evolution of the Voronezh Crystalline Massif in the Early Precambrian, Geotektonika, 1997, no. 3, pp. 21–30. Chernyshov, N.M., Bayanova, T.B., Levkovich, N.V., and Ryborak, M.V., Age of rocks of the early phase of the Ol’khovatka gabbronorite–quartz monzonite–granite ring complex of the Voronezh Crystalline Massif, Dokl. Akad. Nauk, 1998, vol. 359, no. 5, pp. 680–682. Evolution of the Igneous Rocks, Yoder, H.S., Ed., New Jer sey: Princeton Univ., 1978. Fisher, R.V. and Schminckle, H.U., Pyroclastic Rocks, Heidelberg: Springer, 1984. Fisher, R.V., Submarine volcanoclastic rocks, in Marginal Basin Geology: Volcanic and Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basins, Kokelaar, B.P. and Howells, M.F., Eds., 1984, Geol. Soc. London, Spec. Publ., vol. 16, pp. 5–27. Frolov, V.T., Litologiya (Lithology), Moscow: Izd. Mosk. Univ., 1993, vol. 2. Govindaraju, K., Complication of working values and sam ple description for 170 international reference samples of mainly silicate rocks and minerals, Geostandards Newsletter, 1984, vol. VIII, Spec. Iss. Hastie, A.R., Kerr, A.C., Pearce, J.A., and Mitchell, S.F., Classification of altered volcanic island arc rocks using immobile trace elements: development of the Th–Co dis crimination diagram, J. Petrol., 2007, vol. 48, no. 12, pp. 2341–2357. STRATIGRAPHY AND GEOLOGICAL CORRELATION
145
Kotorgin, N.F., Arkheiskie zelenokamennye poyasa Tsen tral’nogo regiona Rossii (Archean Greenstone Belts of the Central Russia), Moscow: Izd. “Sovrem. tetradi,” 2001 [in Russian]. Kremenetskii, A.A., Alekseeva, A.K., and Skryabin, V.Yu., Structure, evolution, and minerageny of ancient suture structures of the crust as evidenced from results of investiga tions along geotraverses and scientific drilling data interpre tation, in Fundamental’nye problemy geotektoniki (Funda mental Problems of Geotectonics), Moscow: GEOS, 2007, pp. 355–360. Ludwig, K.R., Isoplot/Ex ver. 3.6, Berkeley Geochronol. Center. Spec. Publ., 2008, no. 4. Makdonal’d, G., Volcanoes, New Jersey: Prentice Hall, Inc., 1972. Maleev, E.F., Kriterii diagnostiki fatsii i geneticheskikh tipov vulkanitov (Diagnostic Criteria for Facial and Genetic Types of Volcanites), Moscow: Nauka, 1975 [in Russian]. Martin, H., Smithies, R.H., Rapp, R., et al., An overview of adakite, tonalitetrondhjemitegranodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution, Lithos, 2005, vol. 79, pp. 1–24. McBirney, A.R., Factors governing the nature of submarine volcanism, Bull. Volcanol., 1963, vol. 26, pp. 455–469. Mints, M.V., Filippova, I.B., Suleimanov, A.K., et al., 3D model of the deep crustal structure of the Ryazan’Sara tov and EastVoronezh collision orogens (Eastern Voronezh crystalline massif, EastEuropean Platform): the 1EV reflection seismic profile, 2450–3500 km, in Modeli zemnoi kory i verkhnei mantii po rezul’tatam glubinnogo seismoprofi lirovaniya (Models of the Earth’s Crust and Upper Mantle by Deep Seismic Profiling), St. Petersburg: VSEGEI, 2007, pp. 120–125. Moore, J.G., Water content of basalt erupted on the ocean floor, Contrib. Mineral. Petrol., 1970, vol. 28, pp. 272–279. Nenakhov, V.M., Strik, Yu.N., Tregub, A.I., et al., Miner agenicheskie issledovaniya territorii s dvukh’’yarusnym stroe niem na primere Voronezhskogo kristallicheskogo massiva (Mineragenic investigations of Areas with TwoLevel Struc ture, Exemplified by the Voronezh Crystalline Massif), Moscow: Geokart, GEOS, 2007 [in Russian]. Nenakhov, V.M. and Bondarenko, S.V., Paleoproterozoic Tectonic Evolution of the Losevo suture zone in the Voronezh Crystalline Massif, Geotectonics, 2011, vol. 45, no. 4, pp. 302–316. Sedimentary Environments and Facies Reading, H.G., Ed., Blackwell Sci., Oxford, 1978. Osnovy metallogenicheskogo analiza pri geologicheskom kartirovanii. Metallogeniya geodinamicheskikh obstanovok (Foundations of Metallogenic Analysis at Geological Map ping. Metallogeny of geodynamic Settings) Mezhelovskii, N.V., Ed., Moscow: Roskomnedra, Geokart, MANPO, 1995 [in Russian]. Osobennosti izucheniya i geologicheskogo kartirovaniya kol lizionnykh granitoidov (The Features of the Study and Geo logical Mapping of Collision Granitoides), Moscow: Roskomnedra, Geokart, 1992 [in Russian]. Rittman, A., Volcanoes and Their Activity, New York: WileyIntersci. Publ., 1962. Saunders, A.D. and Tarney, J., Geochemical characteristics of basaltic volcanism within backarc basins, in Geology of Vol. 22
No. 2
2014
146
TERENTIEV
Marginal Basins, Kokelaar, B.P. and Howells, M.F., Eds., Geol. Soc. London, 1984, pp. 59–76. Shchipanskii, A.A., Samsonov, A.V., Petrova, A.Yu., and Larionova, Yu.O., Geodynamics of the Eastern margin of Sarmatia in the Paleoproterozoic, Geotektonika, 2007, no. 1, pp. 43–70. Shvanov, V.N., Petrografiya osadochnykh porod (komponentnyi sostav, sistematika i opisanie mineral’nykh vidov) (Petrogra phy of Sedimentary Rocks (Component Composition, Sys tematics, and Description of Mineral Species), Leningrad: Nedra, 1987 [in Russian]. Sun, S.S. and McDonough, W.F., Chemical and isotopic systematic of oceanic basalts: implications for mantle com position and processes, J. Geol. Soc. London, 1989, vol. 42, pp. 313–345. Tarkov, A.P., Glubinnoe stroenie Voronezhskogo massiva po geofizicheskim dannym (Deep Crustal Structure of the Vor onezh Massif from Geophysical Data), Moscow: Nedra, 1974 [in Russian]. Terentiev, R.A., Metavolcanites of the Losevo Series and their formational affiliation, Vestn. Voronezh. Univ., Ser. Geol., 2002, no. 1, pp. 150–160.
Terentiev, R.A. and Chuvashina, G.A., On the relation between stratified formations of the Losevo suture zone (Voronezh Crystalline Massif), Vestn. Voronezh. Univ., Ser. Geol., 2003, no. 2, pp. 91–104. Terentiev, R.A., Paleogeodynamic sedimentation settings in the Precambrian Losevo Basin, Central Russia, Tr. Molodykh Uchenykh VGU, 2004, no. 1, pp. 177–185. Terentiev, R.A., Paleoproterozoic paleobasin of the Losevo suture (Voronezh Crystalline Massif), Vestn. Voronezh. Univ., Ser. Geol., 2005, no. 1, pp. 81–94. Yerino, L.N. and Maynard, J.B., Petrolography of modern sands from the Peru–Chile trench and adjacent areas, Sedi mentol., 1984, vol. 31, pp. 83–89. Zaitsev, Yu.S., Ageikin, A.S., Golyshkina, R.I., and Shiba nov, V.M., New data on the Precambrian geology of the SE Voronezh crystalline massif, in Petrografiya dokembriya Russkoi platformy (Precambrian Petrography of the Russian Platform), Kiev: Naukova dumka, 1970, pp. 59–74.
Translated by D. Voroschuk
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