Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the Altai–Sayan folded area and in the Lake Zone of western Mongolia (geochronological, geochemical, and isotope data)

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ScienceDirect Russian Geology and Geophysics 54 (2013) 1272–1287 www.elsevier.com/locate/rgg

Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the Altai–Sayan folded area and in the Lake Zone of western Mongolia (geochronological, geochemical, and isotope data) S.N. Rudnev a,*, V.P. Kovach b, V.A. Ponomarchuk a a

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Koptyuga 3, Novosibirsk, 630090, Russia b Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034, Russia Received 14 March 2013; received in revised form 13 May 2013; accepted 23 May 2013

Abstract We generalize results of geological, geochronological, geochemical, and isotope-geochemical studies of the Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the Altai–Sayan folded area and in the Lake Zone of western Mongolia. Based on these data, we analyzed the scales of development of plagiogranitoid magmatism, studied the petrologic composition and isotope characteristics of granitoids, and established the main sources of plagiogranitoid-generating melts and the leading mechanisms of formation of Early Caledonian juvenile crust. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Vendian–Early Cambrian island-arc plagiogranitoid magmatism; geochronology; geochemistry and isotope geochemistry; Central Asian Fold Belt; Altai–Sayan folded area; Lake Zone of western Mongolia

Introduction The Central Asian Fold Belt (CAFB) is a large accretionary orogen (Berzin et al., 1994; Kröner et al., 2007; Mossakovskii et al., 1993; Sengör et al., 1993; Windley et al., 2007) with a long geologic history (from Late Riphean through Mesozoic) and several stages of formation of juvenile continental crust (Jahn et al., 2000; Kovalenko et al., 1996, 2003, 2004c). The eastern part of the CAFB is composed of Late Riphean, Vendian–Early Paleozoic (Caledonian), and Paleozoic (Hercynian) fold belts and composite Precambrian microcontinents sutured by intrusive complexes of different ages, compositions, and geodynamic settings (Berzin et al., 1994; Didenko et al., 1994; Kovalenko et al., 2004c; Mossakovskii et al., 1993; Zonenshain et al., 1990). The largest fragment of the eastern CAFB with juvenile crust of Vendian–Early Paleozoic age is the Altai–Sayan folded area (ASFA) and the Lake Zone of western Mongolia (Kovach et al., 2011; Kovalenko et al., 2003; Kruk et al., 2010, 2011; Yarmolyuk et al., 2002, 2003, 2011). The

* Corresponding author. E-mail address: [email protected] (S.N. Rudnev)

beginning of formation of island arcs in this region (~570 Ma) is estimated from the age of small plagiogranite bodies localized in ophiolite complexes of the Han-Taishiri and Daribi Ridges in the Lake Zone and in the Agardag zone of southeastern Tuva (Gibsher et al., 2001; Kozakov et al., 2002; Pfänder et al., 2002). In the period 570–514 Ma, the global evolution of juvenile and mature island arcs (Berzin et al., 1994; Kruk et al., 2010; Yarmolyuk et al., 2011) and intimately associated felsic and basic intrusions (Babin et al., 2013; Izokh et al., 1998; Kruk et al., 2010, 2011; Mongush et al., 2011; Rudnev et al., 2006a, 2008a,b, 2009, 2012, 2013; Shokal’skii et al., 2000; Yarmolyuk et al., 2006, 2011) took place, which now form extended volcanoplutonic belts in different segments of the CAFB (Fig. 1). The Vendian–Early Cambrian stage of the CAFB evolution was studied by the example of island-arc terranes in the western and central parts of the ASFA (Kuznetsk Alatau, Gornaya Shoriya, Gorny Altai, West Sayan, and eastern Tuva) and the Lake Zone (Fig. 1). Despite the detailed geological observations and results of geochronological, geochemical, and isotope–geochemical studies of volcanic, terrigenous, and plutonic complexes, the problem of the regularities of crustal growth at this stage and its scales and specifics in different structures of the CAFB Caledonides is still unsolved. In some

1068-7971/$ - see front matter D 201 3, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2013.09.010 +

Fig. 1. Schematic occurrence of granitoid massifs in the Vendian–Early Cambrian volcanic belts of the Altai–Sayan folded area and Lake Zone, after Babin et al. (2003), Rudnev (2013), Shokal’skii et al. (2000), and Yarmolyuk et al. (2011). 1, volcanoplutonic belts with oceanic or sea-marginal associations (I, Altai–Salair; II, Altai–Kuznetsk; III, Tuva–West Sayan); 2, volcanoplutonic belts with island-arc associations (IV, Salair; V, Alatau; VI, Altai–North Sayan; VII, Tannu-Ola; VIII, Lake); 3–4, Vendian–Cambrian paleobasins (3, turbidite; 4, terrigenous-carbonate); 5, Vendian–Cambrian plagiogranitoid massifs; 6, disjunctions; 7–9, boundaries of: 7, paleobasins of Paleozoic troughs; 8, outcrops of structure-lithologic complexes; 9, Meso-Cenozoic deposits; 10, plagiogranitoid massifs of geochemical types: a, tholeiitic low-alumina; b, calc-alkalic low-alumina; c, calc-alkalic high-alumina. Numerals in squares mark age (Ma, U–Pb and Ar–Ar dating) and ε Nd, parenthesized are the numbers and names of massifs (plutons): granitoid massifs and plutons: 1, Tyla massif; 2, Kshta massif; 3, Taraskyr massif; 4, Yenisei pluton; 5, Kaakhem batholith; 6, Biikhem batholith; 7, East Tannu-Ola batholith; 8, Sharatologoi pluton; 9, Kharanur pluton; 10, Tri Kholma massif; 11, Bumbat-Khairkhan pluton; 12, West Bayan-Khairkhan; 13, Tugrik massif; 14, Meshtueryk; gabbroid massifs: 15, Ust’-Kozhukhovka; 16, Bol’shoi Atalyk and Malyi Atalyk; 17, Lysaya Gora; 18, Brungan; 19, Kolbagdag; 20, Irbitei; 21, Kharachulu; 22, Sair-Khairkhan; 23, Dzavhan; 24, Bayan-Tsagaan; 25, Tugrik.

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regions of early Caledonides, e.g., the Lake Zone and Gorny Altai, these problems were considered in detail on a huge material, including data on the entire complex of volcanic, sedimentary, and plutonic associations (Kovach et al., 2011; Kruk et al., 2010, 2011; Yarmolyuk et al., 2011), whereas other regions of the ASFA call for additional studies, in particular, their volcanic and sedimentary complexes. Plutonic associations (granitoids and gabbroids) in this part of the CAFB have been studied more comprehensively, both the scales of their development and the temporal sequence of their formation and their petrologic composition, isotope-geochemical characteristics, and sources (Gordienko et al., 2006; Kovach et al., 2011; Kruk et al., 2010, 2011; Mongush et al., 2011; Rudnev et al., 2004, 2006a, 2007a,b, 2008a, 2009, 2013; Yarmolyuk et al., 2002, 2003, 2011). In this work we generalized available data on geological, geochronological, geochemical, and isotope-geochemical studies of Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the ASFA and Lake Zone, analyzed the scales of development of plagiogranitoid magmatism, considered the petrologic composition and isotope characteristics of granitoids, and established the main sources of the parental melts of plagiogranitoids and the leading mechanisms of formation of the Early Caledonian juvenile crust.

Geologic position and types of Vendian–Early Cambrian island-arc plagiogranitoids of the Altai–Sayan folded area and Lake Zone The position of Vendian–Early Cambrian volcanic belts in the eastern CAFB, with areas of granitoid associations, is schematically shown in Fig. 1. Island-arc plagiogranitoid associations and related gabbroids are distributed extremely unevenly throughout the belts and are of different abundance. These associations are most widespread in the Lake Zone in western Mongolia and in West Sayan, where they are part of large plutons; they are scarcer in Early Caledonian structures in eastern Tuva and are extremely rare in Kuznetsk Alatau and Gornaya Shoriya. Analysis of the available geochronological data (Table 1) shows that in the Early Caledonian structures of Central Asia, island-arc plagiogranitoids formed in the period 563 ± 2–514 ± 8 Ma and reached a maximum within 535 ± 6–518 ± 2 Ma (Figs. 1 and 2). It was established that the scales of occurrence, time interval of formation, petrologic composition, melting conditions, and sources of plagiogranitoids in particular island-arc belts of the ASFA and Lake Zone are different. Geological, mineralogical, and petrographic studies of plagiogranitoids from the ASFA and Lake Zone revealed several rock associations within plutons or massifs: diorite– tonalite–plagiogranite, tonalite–plagiogranite, and plagiogranite. The first two associations are most abundant. Their tonalites and plagiogranites usually strongly dominate over quartz diorites by quantity, though in some massifs these rocks are present in commensurate amounts. Plagiogranite associations, on the contrary, are extremely rare, though in places

they form large massifs. These associations include plagiogranites and leucoplagiogranites in equal quantities, with dikes and veins of pegmatites of similar composition intruded at the final stage of their formation. Geological observations showed that plagiogranitoid associations in almost all magmatic areas (Fig. 1) formed after gabbroid complexes of different petrologic compositions (Fig. 2) (Izokh et al., 1998, 2003; Khain et al., 1995). Petrological systematics of granitoids was made using a set of data on the contents of major and trace elements. Today, the most common classification of granitoids is based on their separation into rocks of tholeiitic (M-type) and calc-alkalic (I-type) series (Chappell and White, 1974; White, 1979). By Al2O3, Yb, Y, and Sr contents, plagiogranitoids (quartz diorites, tonalites, and trondhjemites) are separated into highand low-alumina types of tholeiitic and calc-alkalic series (Arth, 1979; Drummond and Defant, 1990; Martin et al., 2005), which reflects their different PT-conditions of formation and tectonic positions. This separation is based on experimental works on N-MORB melting at different pressures and temperatures and on the similar compositions of the produced plagiogranite melts and natural plagiogranites. It was established that high-alumina plagiogranites have Al2O3 > 15 wt.%, Yb < 1.2 ppm, and low contents of heavy REE (HREE). Such experiments performed at 8–32 kbar and 900–1000 ºC (Rapp and Watson, 1995) and model calculations (Turkina et al., 2000) showed that high-alumina plagiogranites are produced either through the melting of oceanic slab subsided in subduction zone or as a result of melting of metabasites in the basement of thickened crust at P ≥ 15 kbar, in equilibrium with garnet-containing amphibolite, granulite, and eclogite restites. The model of formation of high-alumina plagiogranitoids in subduction settings is based on their compositional similarity to adakites, low-K volcanics of normal–acid composition with high LaN/YbN (≥10) and Sr/Y (≥50) (Castillo, 2006; Martin et al., 2005). Low-alumina plagiogranites have Al2O3 < 15 wt.%, Yb > 1.2 ppm, LaN/YbN ≤ 10, Sr/Y ≤ 50, and higher HREE contents than high-alumina plagiogranitoids. As was shown by experimental data (Beard and Lofgren, 1991; Rapp and Watson, 1995] and model calculations (Turkina, 2000), low-alumina plagiogranitoids are produced through the partial melting of metabasalts in the lower zone or basement of island arcs at 3–8 kbar under the effect of heat generated during the intrusion of gabbroids, in equilibrium with Pl ± Cpx ± Opx and Hb + Pl ± Cpx ± Opx restites. We used this approach to separate the island-arc plagiogranitoid associations of the ASFA and Lake Zone into the above-mentioned types.

Plagiogranitoid associations of low-alumina type Analysis of data on the chemical composition and geochemical characteristics of low-alumina island-arc plagiogranitoid associations of the ASSA and Lake Zone permitted us to recognize rocks of tholeiitic and calc-alkalic series.

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S.N. Rudnev et al. / Russian Geology and Geophysics 54 (2013) 1272–1287 Table 1. Results of U–Pb and Ar–Ar isotope dating of Vendian–Early Cambrian island-arc plagiogranitoid associations of the ASFA and Lake Zone Pluton, massif

Association, geochemical type

Age, Ma

Method

Reference

Diorite–tonalite–plagiogranite (M-type,
532 ± 2

TIMS

Rudnev et al., 2008a

Kuznetsk Alatau Tyla massif Gornaya Shoriya and West Sayan Kshta massif

Diorite–tonalite–plagiogranite (I-type, >Al)

545 ± 4

SHRIMP-II

Rudnev et al., 2013

Taraskyr massif

Plagiogranite (I-type,
545 ± 4

SHRIMP-II

Rudnev et al., 2013

Yenisei pluton

Tonalite-plagiogranite (I-type,
524 ± 2

TIMS

Rudnev et al., 2005

Diorite-tonalite-plagiogranite (M-type,
563 ± 2 536 ± 4

SHRIMP-II Ar-Ar

Rudnev et al., 2006 Rudnev et al., 2006

Tonalite-plagiogranite (M-type,
518 ± 2 522 ± 4 518 ± 2

SHRIMP-II SHRIMP-II TIMS

Rudnev et al., 2008b Rudnev et al., 2008b Mongush et al., 2011

Kharanur pluton

Diorite-tonalite-plagiogranite (I-type, Al)

529 ± 6 531 ± 10

SHRIMP-II SHRIMP-II

Rudnev et al., 2009 Rudnev et al., 2009

Bumbat-Khairkhan pluton

Diorite-tonalite-plagiogranite (I-type, Al) Diorite-tonalite-plagiogranite (I-type,
551 ± 13 535 ± 6 524 ± 10

SHRIMP-II SHRIMP-II SHRIMP-II

Rudnev et al., 2012 Rudnev et al., 2012 Rudnev et al., 2012

Sharatologoi pluton

Tonalite-plagiogranite (I-type,
519 ± 8

SHRIMP-II

Rudnev et al., 2009

West Bayan-Khairkhan massif

Tonalite-plagiogranite (I-type, >Al) Tonalite-plagiogranite (I-type, Al)

514 ± 8 514 ± 8 514 ± 8

TIMS

Yarmolyuk et al., 2011

Tugrik pluton (this work)

Diorite-tonalite-plagiogranite (I-type, >Al)

530 ± 7

SHRIMP-II

This work

Eastern Tuva Kaakhem batholith Kopti massif Buren’ massif East Tannu-Ola batholith

Lake Zone

Note. Al—high-alumina plagiogranitoid associations, after Arth (1979) and Martin et al. (2005); I-type— calc-alkalic and M-type—tholeiitic, after Chappell and White (1974) and White (1979). U–Pb isotope studies of zircons by TIMS were carried out on a Finnigan MAT-261 multichannel mass spectrometer at the Institute of Precambrian Geology and Geochronology, St. Petersburg, and on an SHRIMP-II ion microprobe at the Center of Isotope Studies of the Karpinsky Geological Institute, St. Petersburg. Ar–Ar isotope amphibole dating was performed on a Noble Gas 5400 mass spectrometer at the Institute of Geology and Mineralogy, Novosibirsk. For location of massifs, see Fig. 1.

Low-alumina plagiogranitoid associations of tholeiitic series are not only extremely rare but are also unevenly distributed in island-arc belts. They are present in the Tyla massif (532 ± 2 Ma) of the Alatau island-arc belt in the north of Kuznetsk Alatau (Fig. 1, Table 1) and in the Meshtueryk massif (~525 Ma) of the Altai–North Sayan island-arc belt in the south of Gorny Altai (Kruk et al., 2010; Rudnev et al., 2008a). In eastern Tuva, such associations are widespread in the Ondum and Tannu-Ola zones of the Tannu-Ola island arc, where they occur as small intrusions among Vendian–Early Cambrian island-arc volcanics in the Kaakhem batholith (Baisyut, 563 ± 2 Ma; Buren’, 536 ± 4 Ma) and East TannuOla batholith (Khol’-Ozu Village region, 518 ± 2 Ma) (Rudnev et al., 2006a, 2008b). In the Lake Zone, Vendian–Early Cambrian plagiogranitoids of tholeiitic series were found in the suprasubductional ophiolites of the Daribi and HanTaishiri Ridges (Gibsher et al., 2001; Kozakov et al., 2002). The plagiogranitoids of tholeiitic series are characterized by low contents of alkalies (especially K2O), Al2O3, TiO2, Rb, Ba, Zr, Hf, and REE. They show mainly a fractionated

REE pattern with a predominance of HREE over LREE and negative Eu anomalies and, more seldom, a gentle REE pattern with close contents of HREE and LREE and, sometimes, positive Eu anomalies and the absence of positive Sr anomalies (Fig. 3). Other specific features are negative anomalies of Nb, Ta, and Ti, which suggests the subduction type of magma generation source. The same is confirmed by the localization of these plagiogranitoids in the field of island-arc basalts on the Y/Nb–Yb/Ta (Eby, 1990) diagram and their spatial correlation with Vendian–Early Cambrian island-arc volcanic complexes. In contents of Al2O3 (<15 wt.%) and Yb (>1.2 ppm), LaN/YbN (0.4–2.0), and Sr/Y (3–27) the tholeiitic plagiogranitoids of the ASFA and Lake Zone correspond to the low-alumina plagiogranitoids produced through the partial melting of metabasites at 3–8 kbar, in equilibrium with Pl + Cpx + Opx and Hb + Pl ± Cpx ± Opx restites (Fig. 4a). The studied plagiogranitoid associations of tholeiitic series are characterized by positive εNd(T) values of +4.7 to +6.5 and low initial ratios (87Sr/86Sr)0 = 0.7034–0.7046 (Table 2). The ASFA plagiogranites show some regional difference in

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Fig. 2. The age boundaries of Vendian–Early Paleozoic intrusive magmatism in the ASFA and western Mongolia, after Rudnev (2013). a, Periodicity of formation of Early Paleozoic granitoid batholiths in the ASFA and western Mongolia. Constructed using results of U–Pb (zircon) and Ar–Ar (amphibole) isotope dating of granitoid and gabbroid associations. N, Number of analytical data; numerals above arrows show the confidence significance level (%) of the minima calculated in accordance with the unimodality criterion (Belousov, 1967); b, periodicity of formation of Vendian–Early Paleozoic basic (ultramafic-mafic, alkaline, and alkali-gabbroid) magmatism in the ASFA and western Mongolia.

isotope characteristics. The plagiogranitoids of the Kaakhem and East Tannu-Ola batholiths in eastern Tuva, independently of the time of their formation (563 ± 2 to 518 ± 2 Ma), show stable high εNd(T) values, +6.3 to +6.5, whereas the plagiogranitoids of similar composition and age (532 ± 2 Ma) of the Tyla massif in the Alatau island-arc belt (Fig. 1, Table 2) are characterized by lower εNd(T) values, +4.7, though the 87 86 Sr/ Sr ratios of these rocks are close (Rudnev et al., 2007a,b, 2008a). In general, the geochemical and Sr–Nd isotope data on the ASFA island-arc plagiogranitoid associations of tholeiitic series point to the leading role of depleted juvenile sources and the participation of ancient crustal material in the generation of parental melts. Vendian–Early Cambrian low-alumina plagiogranitoids of calc-alkalic series are the major products of island-arc granitoid magmatism in the ASFA and Lake Zone. The largest massifs of such plagiogranitoids are located in the Altai–North Sayan and Tannu-Ola belts and in the Lake Zone, where they, together with gabbroids, form intrusive areas among the host Vendian–Early Cambrian island-arc complexes (Fig. 1). These plagiogranitoids formed in the period from 551 ± 13 to 518 ± 2 Ma (Table 1). They make up large meso- and hypabyssal massifs and plutons in the Altai–North Sayan belt

(Yenisei and Tabat, 524 ± 2 Ma; Taraskyr, 545 ± 4 Ma), the Tannu-Ola belt in the Khamsara batholith (Ak-Sug Village region, 532 ± 3 and 524 ± 4 Ma) and East Tannu-Ola batholith (Irbitei River region, 522 ± 4 and 518 ± 2 Ma), and the Lake Zone (Sharatologoi pluton, 519 ± 8 Ma; Kharanur pluton, 529 ± 6 Ma; Bumbat-Khairkhan pluton, 551 ± 13 and 524 ± 10 Ma) (Mongush et al., 2011; Rudnev et al., 2005, 2008b, 2009, 2012, 2013). The low-alumina plagiogranitoids of calc-alkalic series are characterized by a domination of LREE over HREE (Fig. 5), the presence of a negative Eu anomaly, and low LaN/YbN (1.5–8.8) and Sr/Y (3–47) ratios. Like the plagiogranitoids of tholeiitic series, they show negative anomalies of Nb, Ta, and Ti and no positive anomalies of Sr. On the Al2O3–Yb and Eu–Yb diagrams (Fig. 4b), the composition points of the low-alumina plagiogranitoids of calc-alkalic series fall into the field of plagiogranitoids that were produced (according to the experimental data and model calculations) through the partial melting of metabasites at 3–8 kbar, in equilibrium with the Pl ± Cpx ± Opx and Hb + Pl ± Cpx ± Opx restites. The low-alumina plagiogranitoids of calc-alkalic series of the ASFA and Lake Zone are characterized by high positive εNd(T) values, +7.6 to +4.9, Late Riphean Nd model age (TNd(DM) = 0.65–0.85 Ga), and low initial (87Sr/86Sr)0 ratios,

S.N. Rudnev et al. / Russian Geology and Geophysics 54 (2013) 1272–1287

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Fig. 3. Trace-element and REE patterns of Vendian–Early Cambrian island-arc tholeiitic plagiogranitoids of low-alumina type (Rudnev et al., 2006a, 2008a,b). a, Tyla massif; b, Kaakhem batholith; c, East Tannu-Ola batholith.

0.7034–0.7053 (Table 2). In general, they are similar in isotope characteristics to the island-arc plagiogranitoids of tholeiitic series. At the same time, they show some regional differences. For example, such plagiogranitoids in the Altai– North Sayan island-arc belt (Taraskyr and Yenisei plutons) are characterized by εNd(T) values of +6.7 to +4.9, TNd(DM) = 0.77–0.85 Ga, and (87Sr/86Sr)0 = 0.7042–0.7053. Plagiogranitoids of this age in the Tannu-Ola belt (Kaakhem and East

Tannu-Ola batholiths) and Lake Zone (Sharatologoi, Kharanur, and Bumbat-Khairkhan plutons) show higher εNd(T) values, +7.9 to +6.5, and their Nd model age (TNd(DM) = 0.65 0.73 Ga) and primary Sr isotope ratios ((87Sr/86Sr)0 = 0.7034– 0.7039) vary over narrower ranges. This suggests a more depleted composition of their source or a smaller portion of ancient crustal material, or its more juvenile character as compared with the plagiogranitoids of the Altai–North Sayan belt.

Fig. 4. Al2O3–Yb and Eu–Yb diagrams for Vendian–Early Cambrian island-arc tholeiitic plagiogranitoids of the ASFA and Lake Zone. a–b, Plagiogranitoids of low-alumina type: a, tholeiitic series: 1, Tyla massif; 2, Kaakhem batholith; 3, East Tannu-Ola batholith; b, calc-alkalic series: 4, Tri Kholma massif; 5, Taraskyr massif; 6, Yenisei pluton; 7, East Tannu-Ola batholith; 8, Bumbat-Khairkhan pluton; 9, Sharatologoi pluton; c, plagiogranitoids of high-alumina type of calc-alkalic series: 10, Kshta massif; 11, Kharanur pluton; 12, Bumbat-Khairkhan pluton; 13, West Bayan-Khairkhan massif; 14, Tugrik massif. Triangles mark element contents in melts produced during dehydration (solid lines) and hydration (dashed lines) melting of TH1, TH2, and MORB (Arth, 1979; Beard and Lofgren, 1991; Rapp et al., 1991; Turkina, 2000) in equilibrium with five types of restites: I, Pl + Cpx + Opx; II, Hb + Pl ± Cpx ± Opx; III–IV, Hb + Cpx + Pl + Gar; V, Cpx + Gar ± Hb; Pl, plagioclase; Cpx, clinopyroxene; Opx, orthopyroxene; Hb, amphibole; Gar, garnet.

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532

1.83

ppm

Age, Ma Sm

5.71

Nd

0.1934

Sm/144Nd

147

545

524

Taraskyr massif (2, 4)

Yenisei pluton (2, 4)

551 535 524

519

514 514 514 514

530

Bumbat-Khairkhan pluton (1, 3)

Sharatologoi pluton (1, 3)

West Bayan-Khairkhan massif (1)

Tugrik pluton (2, 4)

2.76

2.20 2.22 2.05 6.78

2.76

2.89 0.43 1.99

13.93

10.34 10.14 9.51 23.61

13.02

11.06 2.00 9.68

6.23 3.40

8.21 14.27 22.1

8.60

5.36

6.04

35.3

4.13

0.1198

0.1288 0.1324 0.1300 0.1736

0.1283

0.1578 0.1307 0.1243

0.1614 0.1243

0.2557 0.1517 0.1703

0.2097

0.2345

0.1469

0.1355

0.1046

7.9 7.4 7.6 8.8 6.9 6.6 8.9 9.0 8.6 7.9 7.5

0.512887 ± 3 0.512859 ± 6 0.512745 ± 4 0.512742 ± 2 0.512865 ± 6 0.512884 ± 12 0.512854 ± 5 0.512966 ± 5 0.512753 ± 8

6.3 6.9 6.9

0.513163 ± 8 0.512839 ± 9 0.512900 ± 7

0.512921 ± 5 0.512768 ± 4

6.4

0.513012 ± 13

4.9

0.512720 ± 8

6.5

6.7

0.512760 ± 10

0.513107 ± 11

7.5

4.7

εNd(T)

0.512692 ± 10

0.512869 ± 15

Nd/144Nd, ±2σ

143

648

514 501 542

730

538 693

654

984

764

643

15.10

25.49 19.10

17.52 8.30

0.59 1.95

11.2 14.1

0.31 0.19 0.21

1.12

0.373

7.93

3.20

TNd(DM), Rb Ma ppm

874

525 525

78 558

507 663

153 248

153 143 108

257

69.3

410

238

Sr

Rb/86Sr

0.04984

0.1405 0.1053

0.1822 0.0431

0.0034 0.0085

0.21840 0.16429

0.00579 0.00393 0.00555

0.01241

0.01558

0.05596

0.03833

87

0.7043

(87Sr/86Sr)0

0.7046 0.7040 0.7040

0.7042

0.7053

0.7037 0.7039

0.70412 ± 1

0.7037

0.704863 ± 6 0.7038 0.704643 ± 4 0.7039

0.704784 ± 5 0.7034 0.703836 ± 5 0.7035

0.70376 ± 3 0.70397 ± 7

0.70494 ± 11 0.7034 0.70490 ± 13 0.7037

0.70469 ± 6 0.70411 ± 8 0.70596 ± 9

0.70410 ± 3

0.70545 ± 9

0.70462 ± 14 0.7042

0.70461 ± 9

Sr/86Sr, ±2σ

87

This work

Kovach et al., 2011

Rudnev et al., 2009

This work

Rudnev et al., 2009

Mongush et al., 2011

Rudnev, 2013

Rudnev et al., 2006a, 2007b

Rudnev et al., 2005

Rudnev et al., 2013

Rudnev et al., 2013

Rudnev et al., 2007a

Reference

Note. Sm–Nd isotope studies were carried out on a TRITON TI (1) multichannel mass spectrometer at the Institute of Precambrian Geology and Geochronology, St. Petersburg, and on a Finnigan MAT-262 (2) seven-channel mass spectrometer at the Geological Institute of the Kola Scientific Center, Apatity, following the techniques of Kovach et al. (2011) and Zhuravlev et al. (1987). Rb-Sr isotope studies were performed on a TRITON TI (3) multichannel mass spectrometer at the Institute of Precambrian Geology and Geochronology, St. Petersburg, and on an MI-1201 T (4) mass spectrometer at the Institute of Geology and Mineralogy, Novosibirsk, following the techniques of Kovalenko et al. (2004b) and Sotnikov et al. (1995).

529 531

Kharanur pluton (1, 3)

Lake Zone 1.66 0.70

3.48 3.61 6.23

East Tannu-Ola batholith (1, 4) 518 522 518

Buren’ massif (2, 4)

2.98

2.08

1.46

7.72

0.71

563 563 536

Kaakhem batholith Kopti massif (2, 4)

Eastern Tuva (Tannu-Ola belt)

545

Kshta massif (2, 4)

West Sayan, Gornaya Shoriya, and Gorny Altai (Altai–North Sayan belt)

Tyla massif (2, 4)

Kuznetsk Alatau (Alatau belt)

Pluton, massif

Table 2. Results of Sm–Nd and Rb–Sr isotope studies of Vendian–Early Cambrian plagiogranitoid associations of the ASFA and Lake Zone

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Plagiogranitoid associations of high-alumina type The Vendian–Early Cambrian high-alumina plagiogranitoids of the ASFA and Lake Zone belong to calc-alkalic series. They are much scarcer than the low-alumina plagiogranitoids of calc-alkalic series. High-alumina plagiogranitoids compose both small intrusions (few km2 in area) and large massifs (>100 km2). High-alumina plagiogranitoid associations are most widespread in the Lake Zone (Fig. 1), where they are present in the Bumbat-Khaerkhan (535 ± 6 Ma, Table 1), Kharanur (531 ± 10 Ma), and Tugrik (530 ± 7 Ma) plutons and West Bayan-Khairkhan massif (514 ± 8 Ma) (Kovach et al., 2011; Rudnev et al., 2009, 2012; Yarmolyuk et al., 2011). In the Altai–North Sayan island-arc belt they are extremely rare and compose small intrusions, with the Kshta massif (545 ± 4 Ma) being the best studied (Rudnev et al., 2013). The high-alumina plagiogranitoids, in contrast to the lowalumina ones of calc-alkalic series, have higher contents of alkalies (K2O), Al2O3, Sr, and LREE, LaN/YbN = 4–35 (mainly 9–35, seldom, 4–8), and Sr/Y = 60–287 and lower contents of TiO2, MgO, P2O5, Rb, Zr, Hf, Th, U, Y, and HREE (Fig. 6). In these geochemical characteristics the studied plagiogranitoid associations of the ASFA and Lake Zone are similar to world adakites (Figs. 6 and 7), which are volcanic analogs of high-alumina tonalite–trondhjemite–granodiorite (TTG) complexes (Arth, 1979; Castillo, 2006; Drummond and Defant, 1990; Drummond et al., 1996; Luchitskaya, 2001; Martin, 1994; Martin et al., 2005; Pavlis et al., 1988; Turkina, 2002). These plagiogranites show both positive and negative anomalies of Eu and negative anomalies of Nb, Ta, and Ti. As seen from the Al2O3–Yb and Eu–Yb diagrams (Fig. 4c), the composition points of high-alumina plagiogranitoids of the ASFA and Lake Zone lie in the field of rocks formed through the partial melting of metabasites compositionally corresponding to N-MORB during their subsidence into subduction zone to depths with P ≥ 15 kbar, in equilibrium with the Hb + Cpx + Pl + Gar restite. Therefore, they differ strongly in the formation conditions of initial melts from the low-alumina (tholeiitic and calc-alkalic) plagiogranitoids. Compared with the low-alumina tholeiitic and calc-alkalic plagiogranitoid associations, the high-alumina plagiogranitoids of the ASFA and Lake Zone are characterized by higher εNd(T) values (+7.4 to +9.0), younger Nd model ages (TNd(DM) = 0.50–0.66 Ga), and (87Sr/86Sr)0 = 0.7035–0.7042 (Table 2) (Kovach et al., 2011; Rudnev et al., 2009, 2012, 2013). The obtained isotope data point to the essentially metabasic composition of the sources of plagiogranitoids and the subordinate role of ancient crustal material or its young (probably, Late Riphean) age.

Discussion Analysis of geological and geochronological data on the sequence of formation of island-arc intrusive associations, the scales of their occurrence in the ASFA and Lake Zone, and their geochemical and Sr–Nd isotope characteristics leads us

to the conclusion about the mechanisms and sources of formation of juvenile crust in the eastern part of the CAFB at the Vendian–Early Cambrian stage of its evolution. The available geochronological data on the Vendian–Early Cambrian associations of the ASFA and Lake Zone show that island-arc granitoid magmatism took place there in the period from 563 ± 2 to 514 ± 8 Ma (Table 1). From the early stages of formation of plagiogranitoid intrusions, 563 ± 2–545 ± 4 Ma, to 535 ± 6–518 ± 2 Ma, the scales of plagiogranitoid magmatism increased, as evidenced from the formation of large plutons. At the final stage (~514 ± 8 Ma), the magmatism became less active and is expressed now as occasional small intrusions. This large-scale plagiogranitoid magmatism in the period 535 ± 6–518 ± 2 Ma is, most likely, due to the maximum heating of crust by ascending mantle melts. A similar situation is observed with the change of volcanic activity at the island-arc stage. Geological observations show that in the first half of the Early Cambrian, the volcanic activity in the ASFA and Lake Zone reached a maximum (Kruk et al., 2010; Rudnev et al., 2013; Yarmolyuk et al., 2011). It was drastically reduced in the second half of the Early Cambrian (sedimentary-terrigenous deposits are predominant, and basal conglomerates are widespread) and ended in the Middle Cambrian. In the Lake Zone, there is no gap between island-arc and accretion processes (Yarmolyuk et al., 2011). This indicates the somewhat different evolution and duration of subduction processes in different segments of the CAFB or might be related to different degrees of their geochronologic activity. Note that the errors of determination of the time of plagiogranite crystallization do not permit the separation of the island-arc and accretion stages of the Lake Zone evolution (Yarmolyuk et al., 2011). According to the chemical composition and geochemical features of the Vendian–Early Cambrian island-arc plagiogranitoid associations of the ASFA and Lake Zone, plagiogranitoids of tholeiitic and calc-alkalic series of low- and highalumina types are recognized, which indicates that their parental melts were generated at different depths (3–8 and ≥15 kbar) and from different sources. The generalized results of Sr–Nd isotope studies of islandarc plagiogranitoid associations in the ASFA and Lake Zone permit their separation into three groups differing in isotope characteristics (Fig. 8). Group I includes high-alumina plagiogranitoids of the Kharanur, Bumbat–Khairkhan, and Tugrik plutons and the West Bayan–Khairkhan massif in the Lake Zone and those of the Kshta massif in the Altai–North Sayan island-arc belt in Gornaya Shoriya. These plagiogranitoids are characterized by high positive values of εNd(T), +9.0 to +7.4, close to those of depleted mantle, and TNd(DM) = 0.50–0.66 Ga and low (87Sr/86Sr)0 = 0.7035–0.7042 (Fig. 8, Table 2). In chemical composition and geochemical characteristics they are similar to adakites, which indicates that their parental melts were generated through the partial melting of subducted oceanic slab (N-MORB, probably, with a minor admixture of crustal material) at >15 kbar, in equilibrium with the garnet-containing restite (Kovach et al., 2011; Rudnev et al., 2009, 2013).

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Fig. 5 (to be continued).

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Fig. 5. Trace-element and REE patterns of Vendian–Early Cambrian island-arc calc-alkalic plagiogranitoids of low-alumina type (Rudnev et al., 2005, 2008b, 2009, 2012, 2013). a, Tri Kholma massif; b, Taraskyr massif; c, Yenisei pluton; d, East Tannu-Ola batholith; e, Bumbat-Khairkhan pluton; f, Sharatologoi pluton.

Group II includes low-alumina plagiogranitoids of tholeiitic and calc-alkalic series in the large plutons and small massifs of the Tannu-Ola island arc in eastern Tuva (Kaakhem and East Tannu-Ola batholiths, Table 1), the Altai–North Sayan island-arc belt in West Sayan (Taraskyr massif), and the Lake Zone (Sharatologoi, Kharanur, and Bumbat–Khairkhan plutons). They differ from group I plagiogranitoids in lower εNd(T), varying from +7.9 to +6.3, older Nd model ages, TNd(DM) = 0.65–0.85 Ga, and (87Sr/86Sr)0 = 0.7034–0.7053 (Fig. 8, Table 2) (Kovach et al., 2011; Mongush et al., 2011; Rudnev et al., 2006a, 2009, 2013). The obtained isotope data also indicate their formation from juvenile parental melts, when ancient crustal material got into the magma generation zone, and/or from a metabasic source of different composition. The presence of a crustal component in the magma generation zone is evidenced from the variation in the Sr–Nd isotope parameters of plagiogranitoids and their deviation from the mantle sequence line to the field of crustal contamination

(Fig. 8b). Taking into account the geochemical (Nb, Ta, and Ti minima) and isotope characteristics of these plagiogranitoids, we assume that their parental melts were generated through the partial melting of metabasites localized in the lower parts of island-arc systems at 3–8 kbar, in equilibrium with the plagioclase- and amphibole-containing restites. Group 3, like group 2, includes low-alumina plagiogranitoid associations of tholeiitic and calc-alkalic series. They form plagiogranite massifs in the Altai–North Sayan (Yenisei pluton) and Alatau (Tyla massif) island-arc belts. These associations, in contrast to those of group 2, are characterized by still lower εNd(T) values, varying from +4.7 to +4.9, and more ancient Nd model ages, TNd(DM) = 0.85–0.86 Ga, but close (87Sr/86Sr)0 ratios, 0.7042–0.7043 (Rudnev, 2013; Rudnev et al., 2005, 2007a, 2008a). The isotope data, together with the geochemical characteristics of these plagiogranitoids, indicate that the rocks formed through the melting of juvenile Caledonian crust when ancient crustal material got into the

Fig. 6. Chondrite-normalized (Taylor and McLennan, 1985) and primitive-mantle-normalized (Sun and McDonough, 1989) trace-element and REE patterns of Vendian–Early Cambrian island-arc calc-alkalic plagiogranitoids of high-alumina type from the ASFA and Lake Zone (Kovach et al., 2011; Rudnev et al., 2009, 2012, 2013; Yarmolyuk et al., 2011). a, Kshta massif, 545 ± 4 Ma; b, Kharanur pluton, 531 ± 10 Ma; c, Bumbat-Khairkhan pluton, 535 ± 6 Ma; d, Tugrik pluton, 530 ± 6 Ma; e, West Bayan-Khairkhan massif, 514 ± 8 Ma. Gray composition field corresponds to adakites, after Martin et al. (2005).

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magma generation zone. By the example of the diorite–plagiogranite association of the Tyla massif in the Alatau island-arc belt, we suggest several reasons for this low εNd(T) value. On the one hand, it might be due to the involvement of sedimentary material with a longer crustal history in the melting, which was supplied from ancient buried continental blocks. The presence of such blocks is assumed based on the presence of Late Riphean plagiogranitoids with Early Proterozoic Nd model ages in the Talanovka–Bogorodsk block (Gremyachinsk massif, ~876 Ma, TNd(DM) = 2.2 Ga (Rudnev et al., 2006b)). The latter is expressed as volcanogenic and sedimentary deposits of the Altai–Kuznetsk volcanic belt of oceanic type, joining the Alatau island-arc belt (Fig. 1). On the other hand, the low positive εNd(T) values of the Tyla massif plagiogranites might be due to the involvement of compositionally heterogeneous mantle sources (both depleted and enriched) in the melting. This is indirectly evidenced from the localization of the island-arc complexes of the Alatau belt in the areas of the Kuznetsk Alatau belt volcanics with geochemical characteristics of OIB (Rudnev et al., 2008a). Nevertheless, these hypotheses call for more detailed isotope-geochemical studies of the volcanogenic and terrigenous island-arc com-

plexes hosting the Tyla massif plagiogranitoids and of the volcanogenic deposits of the Kuznetsk Alatau belt. In general, the recognized three groups of plagiogranitoids and variations in their isotope parameters and geochemical characteristics clearly show their formation from sources of different types (N-MORB, island-arc, and, probably, OIB) and/or the increased contribution of crustal material.

Conclusions Analysis of the results of geochronological studies of plagiogranitoids formed at the Vendian–Early Cambrian stage of the CAFB evolution has shown their age of 563 ± 2 to 514 ± 8 Ma. The peak of plagiogranitoid magmatism was in the period 535 ± 6–518 ± 2 Ma, when large plagiogranitoid plutons formed in the Early Caledonian structures of the ASFA and Lake Zone. By chemical composition, the plagiogranitoid associations of the island-arc stage belong to tholeiitic and calc-alkalic series. By contents of Al2O3, trace elements, and REE, they are separated into low- and high-alumina types. The presence of these elements reflects not only the different depths of

Fig. 7. SiO2–MgO, CaO + Na2O–Sr, Sr/Y–Y, and Cr/Ni–TiO2 diagrams for Vendian–Early Cambrian island-arc calc-alkalic plagiogranitoids of high-alumina type from the ASFA and Lake Zone. 1, Kshta massif; 2, Kharanur pluton; 3, Bumbat-Khairkhan pluton; 4, West Bayan-Khairkhan massif; 5, Tugrik massif. Gray composition fields correspond to low-Si adakites (LSA), dashed line marks the field of high-Si adakites (HSA) from different world regions, after Castillo (2006) and Martin et al. (2005).

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Fig. 8. εNd(T)–age (a) and εNd(T)–(87Sr/86Sr)0 (b) diagrams for Vendian–Early Cambrian island-arc plagiogranitoids from the ASFA and Lake Zone. 1, high-alumina plagiogranitoids of calc-alkalic series; 2–3, low-alumina plagiogranitoids of: 2, calc-alkalic series; 3, tholeiitic series. Gray field marks the evolution of Nd isotope composition of island-arc volcanics of the Lake Zone, and speckled field, of metamorphic rocks of the Dzavhan microcontinent (5), after Kovach et al. (2011). Arrow shows the evolution trend of the Nd isotope composition of middle continental crust.

formation of initial melts (3–8 kbar and ≥15 kbar) but also their sources. High-alumina plagiogranitoid associations of calc-alkalic series formed at the island-arc stage are of limited occurrence; they are present in small massifs or, more seldom, large plutons. The rocks are characterized by low contents of REE (mainly HREE), high LaN/YbN (mainly 9–35; seldom, 4–8) and Sr/Y (60–287) ratios, and distinct positive Sr anomalies, which is typical of high-alumina TTG complexes. The parental magmas were probably generated through the partial melting of metabasites similar in composition to N-MORB at P ≥ 15 kbar, in equilibrium with the garnet-containing restite, during the subsidence into the subduction zone of oceanic slab. This mechanism is confirmed by the similarity of the highalumina plagiogranitoids to world high-Si adakites and their high positive values of εNd(T) close to those of depleted mantle. The low-alumina plagiogranitoid associations of tholeiitic and calc-alkalic series, in contrast to the high-alumina ones, have higher REE contents but lower LaN/YbN (0.4–9) and Sr/Y (3–47) ratios and show no positive anomaly of Sr. The geochemical features of the low-alumina plagiogranitoids indicate that these rocks formed, most likely, through the melting of metabasites localized in the lower zone or basement of island-arc systems at 3–8 kbar, in equilibrium with plagioclase- and amphibole-containing restites, respectively. This points to the shallower depths of formation of parental melts in contrast to the high-alumina plagiogranitoid associations. The Sr–Nd isotope studies of the Vendian–Early Cambrian plagiogranitoid associations formed at the island-arc stage of evolution of the ASFA and Lake Zone showed their high εNd(T) values, +9.0 to +4.7, TNd(DM) = 0.50–0.85 Ga, and

(87Sr/86Sr)0 = 0.7034–0.7053. The high-alumina plagiogranitoids are characterized by the highest εNd(T) values, +9.0 to +7.4, indicating a basic (N-MORB-type) source with isotope characteristics similar to those of depleted mantle. The low-alumina plagiogranitoids, which formed through the melting of metabasites localized in the lower zone of the basement of island-arc systems, are characterized by reduced εNd(T) values, +7.6 to +4.7, which points to the presence of crustal component in the parental melts. The Sr–Nd isotope data, together with the geochemical characteristics of the studied plagiogranitoid associations, indicate a predominance of juvenile mafic sources during the formation of parental melts with contribution of ancient crustal material in various amounts as well as the different compositions of magma-generating sources (N-MORB, island-arc) and generation conditions of plagiogranitoid melts. The geological, geochemical, and Sr–Nd isotope data testify to the widespread processes of Vendian–Early Cambrian juvenile crust formation in the eastern CAFB. We thank A.E. Izokh and O.M. Turkina for critical remarks and valuable advice on the manuscript. This work was supported by grants 13-05-00381 and 11-05-92003 from the Russian Foundation for Basic Research and Project 10.2 of the Department of Geosciences.

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