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Russian Geology and Geophysics 53 (2012) 1176–1196 www.elsevier.com/locate/rgg
Neoproterozoic alkaline magmatism and associated igneous rocks in the western framing of the Siberian craton: petrography, geochemistry, and geochronology I.V. Romanova a, A.E. Vernikovskaya a,*, V.A. Vernikovsky a,b, N.Yu. Matushkin a,b, A.N. Larionov c a
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia c A.P. Karpinsky Russian Geological Research Institute, Srednii pr. 74, St. Petersburg, 199106, Russia Received 22 March 2012; accepted 21 June 2012
Abstract The formation and evolution conditions for alkaline magmatism and associated igneous rocks in the western framing of the Siberian craton are shown by the example of alkaline and subalkaline intrusive bodies of the Yenisei Ridge. Here we present petrographic, mineralogical, geochemical, and geochronological data for the rocks of the Srednetatarka and Yagodka plutons located within the Tatarka–Ishimba suture zone. Ferroan and metaluminous varieties enriched with rare elements (Nb, Ta, Zr, Hf, and REE) are making up most of the studied rocks. They formed at the stages of fractional crystallization of alkaline magma in a setting of active continental margin in the west of the Siberian craton in the Late Neoproterozoic (710–690 Ma). As differentiates of mantle magmas, these rocks associate with Nb-enriched rocks—A-type leucogranites and carbonatites. Sm/Nd and Rb/Sr isotopic data imply a predominance of the mantle component in the magmatic sources of the mafic and intermediate rocks as well as contamination processes of various volumes of continental crustal material by this magma. © 2012, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: alkaline magmatism; mineralogy; petrography; geochemistry; geochronology; Neoproterozoic; active continental margin; southwestern framing of the Siberian craton
Introduction Alkaline igneous rocks in the Neoproterozoic accretionarycollisional structure of the Yenisei Ridge (southwestern framing of the Siberian craton) form small plutons, located in the Tatarka–Ishimba N–NW trending suture zone, which demarks the accreted terranes from the passive continental margin (Vernikovsky et al., 2003, 2007) (Fig. 1). This suture zone is one of the major and long-lived structural elements of the region (Vernikovsky et al., 2011). It hosts collisional granites with U/Pb ages 760–750 Ma (Vernikovskaya et al., 2002; Vernikovsky et al., 2003) and younger Late Neoproterozoic alkaline rocks, which are the subject of our study. The Late Neoproterozoic complex of alkaline and associating rocks marks the end of the Precambrian magmatic evolution in the
* Corresponding author. E-mail address:
[email protected] (A.E. Vernikovskaya)
region. It formed synchronously and in a subparallel line with island arc igneous rocks of the Yenisei ophiolites and island arcs belt, which accreted to the Siberian margin 700–630 Ma (Vernikovsky et al., 1999, 2001, 2008). The island arc formations and ophiolites in the west of the orogen mark the Yenisei suture zone that also is host to the latest subalkaline and alkaline anorogenic igneous formations—Devonian A-type granitoids as well as Triassic alkaline syenites, nepheline syenites and associating carbonatites (Vernikovskaya et al., 2010). The northern part of the Tatarka–Ishimba suture zone contains subalkaline and alkaline volcanic and subvolcanic rocks of the Zakhrebetnyi complex, forming the structure of the Verkhnevorogovka graben-syncline. They are represented by subalkaline basalts, trachyandesibasalts, trachyandesites, trachydolerites, teschenites, alkaline trachytes, alkaline syenites, nepheline syenites and other rocks (Diner, 2000). The 40 Ar/39Ar age of biotite from a trachydolerite of this complex is 696 Ma (Postnikov et al., 2005). This tectonothermal event
1068-7971/$ - see front matter D 201 2, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2012.09.005 +
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Fig. 1. Tectonic scheme of the Yenisei Ridge and the geological position of the Tatarka complex plutons, from (Vernikovsky et al., 2008) with additions. 1, Tatarka active continental margin complex (alkaline nepheline and quartz syenites, ijolites, trachybasalts, trachyandesibasalts, trachyandesites, trachydolerites, teschenites, alkaline trachytes, carbonatites, A-type granites), 711–630 Ma; 2, post-collisional Glushikha leucogranitic complex, 750–720 Ma; 3, syncollisional Ayakhta granitoid complex, 760–750 Ma; 4, terrane boundaries; 5, faults (a), thrusts (b); 6, Tatarka–Ishimba suture zone. Roman numerals in circles are terranes: I, Isakovka, ophiolite and island arc complex with plagiogranites (697 Ma); II, Central Angara, flyshoid and carbonate deposits, metamorphosed in greenschist-amphibolite facies conditions (MP–NP), Rybnaya–Panimba ophiolite belt (MP), Teya collisional granitoids (880–865 Ma); III, East Angara, terrigenous-carbonate deposits of the Siberian craton passive continental margin (MP-NP); IV, Predivinsk, ophiolite and island arc complexes with plagiogranites (628 Ma) and rhyolites (637 Ma); V, Angara–Kan, mostly granuliteamphibolite complexes (PP3). Numbers in rectangles are plutons: 1, Zakhrebetnyi; 2, Tatarka; 3, Srednetatarka; 4, Yagodka; 5, Chistopol’e. Letters in rectangles are faults: A, Angara; An, Ankinov, I, Ishimba; T, Tatarka; Y, Yenisei.
took place at the same time as the formation of the Kutukass complex A-type leucogranites in the marginal part of the Verkhnevorogovka graben-syncline, considering their zircon U/Pb age 690 Ma (Nozhkin et al., 2008). In the central part of the Tatarka–Ishimba suture zone the Tatarka syenites, granites, and A-type leucogranites with zircon age 629 Ma (Vernikovsky and Vernikovskaya, 2006) have been established, as well as intruding steeply dipping carbonatite bodies of the Penchenga complex (Zabrodin and Malyshev, 1975). The latter associate with Nb-enriched fenites. 40Ar/39Ar age
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estimates for the carbonatites have a wide range (Vrublevskii et al., 2011): 725 Ma for magnesioarfvedsonite and 637 Ma for phlogopite. In the same part of the tectonic zone the Srednetatarka nepheline syenites are located. K/Ar, Rb/Sr and Sm/Nd age estimates for various minerals (nepheline, lepidomelane, albite, aegirine, fluorite, biotite and muscovite) and whole rock samples from the alkaline rocks of this pluton are also not decisive and fall into a wide time range from 675 to 610 Ma (Sazonov et al., 2007; Sveshnikova et al., 1976). In the southern part of the Tatarka–Ishimba suture zone the Yagodka pluton alkaline syenites associating with granites (Krendelev, 1971; Kuznetsov, 1941, 1988) and Chistopol’e A-type leucogranites with U/Pb zircon age 683 Ma (Vernikovskaya et al., 2007) have been identified. Similar plutons have been described in suture zones of the southern framing of the Siberian craton at the margins of large orogenic belts—East Sayan, Cis-Baikal, Transbaikalian (Yarmolyuk and Kovalenko, 1991; Yarmolyuk et al., 2006). In this paper we show the conditions of formation and evolution of the alkaline and associating magmatism in the western framing of the Siberian craton on the example of alkaline and subalkaline intrusions of the Transangarian and South-Yenisei fragments of the Yenisei Ridge. Petrographic, mineralogical, geochemical and geochronological data are presented for the rocks of the Srednetatarka and Yagodka plutons, located in the Tatarka–Ishimba suture zone. These investigations are based on the study of a samples collection we accumulated in field trips from 2005 to 2008. The results we obtained are important in understanding the nature and age of alkaline rocks and magmatic bodies associating with them, as well as their place and role in the formation of active continental margin orogens.
Analytic methods The determinations of major elements contents in the rocks were done by X-ray fluorescence method with a 1–5% error in Vinogradov Institute of Geochemistry SB RAS (Irkutsk). Rare-earth and other trace elements determinations were obtained by ICP-MS with a 5–10% error. For the Yagodka pluton this was done using a quadrupole Agilent7500ce mass spectrometer and with a high resolution magnetic sector ELEMENT2 mass spectrometer in IG SB RAS (Irkutsk) by the procedure given in (Smirnova et al., 2010). For the Srednetatarka pluton element concentrations were determined applying an ELEMENT mass spectrometer in IGM SB RAS (Novosibirsk) by the procedure published in (Nikolaeva et al., 2008). Minerals analyses were performed on a Comebax-Micro X-ray microanalyzer in IGM SB RAS (Novosibirsk). Isotopic U–Pb analyses of sphene from the Srednetatarka foyaite (sample 05-01-9-6) were performed using a multicollector Finnigan MAT-261 mass spectrometer in IPGG RAS (St. Petersburg). The zircons dissolution and U and Pb extraction were performed by a modified procedure of Krogh (1973). The accuracy of U/Pb ratio determinations is 0.5%. The blank was below 0.1 ng for Pb and 0.005 ng for U. The
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aeroabrasion of zircons was done after Krogh (1982). The preliminary (HF + HNO3) acid etching of zircons was performed with varying exposition at 220 °C (Mattinson, 1994). The experimental data were processed with the PbDAT and ISOPLOT programs (Ludwig, 1991a,b; 1999). The decay constants of uranium of Steiger and Jäger (1976) were used in age calculations. Corrections for common lead were introduced according to model values (Stacey and Kramers, 1975). Additionally, U, Th and Pb isotope analyses of zircons from the same Srednetatarka foyaite sample 05-01-9-6, as well as the Yagodka quartz syenite sample V-07-6 and granite sample V-07-5-2 were performed using the SIMS SHRIMP-II at CIR VSEGEI (St. Petersburg) according to standard procedure (Larionov et al., 2004; Williams, 1998) using a secondary electron multiplier in peak-jumping mode through mass range from 196(Zr2O) to 254(UO) (4 mass specra). The primary O−2 ion current for the elliptic analytical point of ~25 × 20 µm was –4.0 / –5.0 nA. The mass resolution M/∆M ≥ 4300 (at 254 amu) excludes isobaric overlapping in the analyzed mass range. Only areas without visible fractures and inclusions in euhedral grains were selected for analysis. The zircons were cast in an epoxy matrix along with standard 91500 (Wiedenbeck, 1995) and Temora (Black et al., 2003) zircons, polished approximately to half thickness and vacuum-coated with a ~100 Å layer of 99.999% gold. The zircons internal structure was studied using optical microscope imaging and cathodoluminescence (CL). The analytical results were processed with the SQUID v1.12 and ISOPLOT/Ex. 3.22 programs (Ludwig, 2005a,b) using the decay constants recommended by Steiger and Jäger (1976). Corrections for common lead were made according to the Stacey and Kramers (1975) model using measured 204Pb/206Pb ratio. The isotopic Sm, Nd, Rb, and Sr analysis was performed using a 7-collector Triton T1 mass spectrometer in VSEGEI (St. Petersburg) according to techniques described in (Pervov et al., 2005; Skublov et al., 2010).
ing to geophysical data they are marked by a single gravitational anomaly elongated towards the northwest. These intrusions were emplaced in Neoproterozoic weakly metamorphosed deposits, folded in a N–S trending brachysynclinal structure and consisting mostly of limestones with interbeds of quartz–chlorite–sericite, sometimes carbon-bearing schists, and argillaceous schists northeasterly of the pluton. The foyaite contact zones have a steep dip (65°–90°) and are complicated by multiple offshoots (apophyses) 2–50 m thick and up to 100 m long that are confined to jointing areas. In these contact zone areas as well as in the more elongated ENE and WNW trending fracturing zones within the pluton multiple foyaite–pegmatite dikes and veins with rare element mineralization are located. Contact metamorphic formations (the aureole’s width is up to 200–500 m) are represented by marbleized limestones and closer to the contact also include argillaceous schists: quartz– mica, andalusite and pyroxene hornfels with rutile, tourmaline and garnet. In the ijolites, which form a NW trending elongated body in the apical part of the pluton, at the contact with foyaites multiple limestone and schists xenoliths have been identified reaching hundreds of meters in size. Xenolith schists retain schistosity elements, whose orientation corresponds to the bedding of host rocks. The ijolites close to the contacts and in jointing zones are intensively microclinised and albitized and at times display a taxitic structure, while the foyaites are enriched by mafic minerals. The presence of ijolite xenoliths in foyaites and, on the other hand, the presence of injections of the foyaites in ijolites shows that ijolites crystallized somewhat earlier than foyaites. The geologic position of the Srednetatarka pluton and its satellite as well as that of the Penchenga carbonatites (Zabrodin and Malyshev, 1975) located in the same tectonic zone 80 km to the northeast, clearly demonstrates that their emplacement took place after the accretion of terranes and establishment of the Tatarka–Ishimba suture zone. The orientation of structural elements and of the igneous bodies themselves shows their genetic connection with tectonic processes within this zone.
Geologic setting Srednetatarka pluton Geologists have studied the Srednetatarka nepheline syenite pluton (previously named the Transangarian pluton) since the 50s of the previous century, and its detailed geological study in the 1960–1970s was motivated by particular interest in the discovered rare element mineralization of alkaline rocks (Savanovich and Sergeeva, 1970; Sveshnikova, 1965; Sveshnikova et al., 1976; etc.). The Srednetatarka alkaline pluton (Fig. 2, a) located in the middle reach of Tatarka River is a small stock (the outcropping area is ~15 km2) of heterogeneous composition—its central part is composed of feldspar ijolites with tributary urtites, while the marginal zone consists of alkaline syenites and dominating nepheline syenites (foyaites). A similar but smaller satellite-stock has been identified to the southeast of the pluton. Both stocks are located in an intercrossing zone between sublatitudinal and northwest trending faults. Accord-
Yagodka pluton The geologic description of the Yagodka pluton alkaline rocks is based on the works of Yu.A. Kuznetsov (1941) and F.P. Krendelev (1971), and also on materials of various geological maps (Glazyrin and Vrublevich, 1967; Kachevsky et al., 1998; Savanovich and Sergeeva, 1970). The Yagodka pluton, located in the Yagodkina and Malaya Yagodkina river basin, consists of a group of elongated syenite stocks, including alkaline variations, intruding biotite and biotite– muscovite, often gneissic granites and leucogranites (Fig. 2, b). Yu.A. Kuznetsov (1941) who was the first to study the alkaline syenites of this pluton noted that these rocks are distinguished by exceptional freshness and lack of cataclasis. The alkaline rock intrusions have a sublongitudinal strike and are oval in shape (length 1–2.5 km, width ~0.5 km). In association with the syenites small trachybasalt occurrences in the form of rope lava flows have been identified. The granites
Fig. 2. Geological schemes of the Srednetatarka (a) and Yagodka (b) alkaline plutons, after (Glazyrin and Vrublevich, 1967; Kachevsky et al., 1998; Savanovich and Sergeeva, 1970; Sveshnikova et al., 1976). 1, sedimentary cover: clays, sands, sand loams, gravels (Pg3–N1); 2, Shirokinsk series, Sukhokhrebtinsk (Kirgiteisk) Formation: argillaceous schists, metasandstones, trachybasalts (NP1–2); 3–5, Tokminsk (Gorevka) Formation limestones (NP): 3, Lower Subformation, 4, Middle Subformation, 5, Upper Subformation; 6, Kuzeevsky complex of the Angara–Kan terrane: plagiogneisses, gneisses (PP3); 7–10, Tatarka magmatic complex: 7, granites, gneiss-granites, A-type leucogranites, 711–683 Ma; 8–9, Srednetatarka pluton alkaline and nepheline syenites (8) and ijolites (9), 711 Ma; 10, Yagodka pluton alkali feldspar syenites, alkaline quartz syenites, trachybasalts, 691 Ma; 11, contact metamorphism aureoles; 12, carbonate xenoliths (a) and terrigeneous-carbonate metaschists and limestones xenoliths (b); 13, unconformable boundaries; 14, verified fault (a), inferred fault (b), upthrow (c); 15, bedding; 16, sampling sites for U/Pb geochronological studies.
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and syenites are located in a mostly metapelite—metasandstones formation, with the level of metamorphic alteration corresponding to the greenschist facies of regional metamorphism. The acid intrusive rocks belong to the calc-alkaline and alkali-calcic magmatic series. Among them are also slightly peraluminous, iron-enriched rocks, belonging to A-type leucogranites. Southwesterly of these magmatic bodies the Late Neoproterozoic Chistopol’e pluton is located. It is composed of A-type leucogranites and its host rocks are Paleoproterozoic gneisses and plagiogneisses of the Kuzeevsky complex. The granitoids form a wide contact aureole of hornfelsed rocks. The igneous bodies of the Yagodka and Chistopol’e plutons associate with large NNW striking faults, which are part of the southern fragment of the Tatarka–Ishimba tectonic zone, located in the NNW part of the South-Yenisei Ridge (Kachevsky et al., 1998, and other geological mapping data). The proximity, orientation of the acid intrusions and their association with the same structures suggest their close emplacement ages. Petrography and mineralogy Srednetatarka pluton Most of the studied alkaline rocks samples were taken from outcrops on the left bank of Tatarka River (right tributary of
Angara River) and eluvial blocks on the north and northwestern rims of the Srednetatarka pluton. Among them are nepheline syenites (foyaites), alkaline syenites, and feldspar ijolites. Chemical compositions of the minerals in the studied igneous rocks are given in Tables 1–4. Foyaites are represented by medium- and coarse-grained as well as pegmatoid rocks with poikilitic structure, consisting mostly of potassic feldspar (microcline) (40–65 vol.%), nepheline (30–40 vol.%) and aegirine (5–15 vol.%). These rocks also contain sphene, fluorite, individual grains of arfvedsonite, biotite, astrophyllite, as well as eudialite, pyrochlore, and analcime mostly occurring in pegmatites. Potassic feldspar and nepheline grains length varies from 0.5–2 to 15 cm in pegmatites and aegirine grains reach 0.1–0.3 to 8 cm in pegmatites, taking into account data from E.V. Sveshnikova et al. (1976). Arfvedsonite grains length varies from 0.1–4 to 2 cm in pegmatites, whereas for biotite it does not exceed first millimeters. Astrophyllite forms individual blades
