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ScienceDirect Russian Geology and Geophysics 57 (2016) 69–81 www.elsevier.com/locate/rgg
The evolution of the subduction zone magmatism on the Neoproterozoic and Early Paleozoic active margins of the Paleoasian Ocean I.V. Gordienko a,*, D.V. Metelkin b,c a
Geological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakhyanovoi 6a, Ulan-Ude, 670047, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia c A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Received 10 August 2014; accepted 28 August 2015
Abstract The geodynamic reconstruction using new data on the composition, age, and paleomagnetism of Neoproterozoic and Vendian–Early Paleozoic island arc complexes has provided new insights into the evolution of the subduction zone magmatism over extensive areas of the Central Asian Orogenic Belt, including eastern Altai–Sayan, Transbaikalia, and Northern Mongolia. Comparison of the igneous complexes of modern and ancient ensimatic and ensialic island arcs in the subduction zone forms a basis for possible geodynamic scenarios of the subduction zone magmatism in Neoproterozoic and Vendian–Early Paleozoic island arcs in the zone of interaction between the Siberian paleocontinent and the Paleoasian Ocean, which take into account the composition of crustal and mantle (including mantle plume) components. © 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: island arcs; subduction zone magmatism; Neoproterozoic; Early Paleozoic; petrological and geochemical composition; isotope age; paleomagnetism; Siberian continent; Paleoasian Ocean; geodynamic reconstructions
Introduction Tracking the magmatic history of active continental margins and elucidating its relation to processes operating within the crust and mantle is crucial for fundamental petrological and geodynamic studies. According to the most recent tectonic models, all the major orogenic belts owe their origin to paleooceans and result from evolution of their active margins and transformation of oceanic lithosphere into continental lithosphere. This evolution is governed by internal endogenic processes and mantle geodynamics, which are manifested in structural, petrological, and geochemical variations of subduction-related magmatic products. Deciphering magma evolution at modern and ancient active plate margins has been widely discussed in the literature (Dobretsov, 2003, 2010a, 2010b, 2011; Dobretsov et al., 2005, 2012, 2013, 2015). Interpreting even modern island arcs is complicated due to structural complexity and a wide variety of magma sources and magmatic manifestations, whereas studies of ancient island arcs are even less satisfactory because they usually deal
with few remnant arcs. It is obvious that reconstruction of paleoarcs is possible by comparing their geochemical and petrological signatures with those of modern subduction-zone complexes. This information can provide insights into geodynamic conditions under which they formed and reveal certain distinctive features of subduction magmatism in the studied island arc system. The reconstruction of the structural architecture of active margins and paleoenvironmental settings of the remnant arcs are based on geochronological and paleomagnetic data. In this study, we use actualistic geodynamic models and new geochemical, geochronological and paleomagnetic data on Neoproterozoic and Vendian–Early Paleozoic island-arc complexes of eastern Altai–Sayan, Transbaikalia, and Mongolia to shed light on many controversial issues of the evolution of magmatism in subduction zones and possible mechanisms associated with transformation of the Siberian continental margin of the Paleoasian Ocean during the early accretionary phase of the Central Asian Orogenic Belt (CAOB).
* Corresponding author. E-mail address:
[email protected] (I.V. Gordienko) 1068-7971/$ - see front matter D 201 6, 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.201 + 6.01.005
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The composition of igneous rocks in modern subduction zones The pioneering study of Bogatikov and Tsvetkov (1988) described the compositional variations in magmatism associated with different types of island arc structures as a successive change from tholeiitic to calc-alkaline magma series (Fig. 1). Although these models were first proposed some 25 years ago, they still form the basis for reconstruction of magmatic arc evolution of the Paleoasian, Uralian, Mongol-Okhotsk, and other paleooceans, which comprised fragments of both juvenile and mature island arcs. In the primitive ensimatic volcanic arc settings, adiabatic transformation of amphibolized mafic and ultramafic rocks of the subducting oceanic slab and generation of quartz eclogites at depths of 60–70 km releases up to 5% water, which rises
into the overlying mantle resulting in partial melting of the subducting plate and generation of primary tholeiitic melts with strong REE depletion (more depleted than N-MORB-type mantle). Further melting in the subducting oceanic slab produces more depleted melts of the marianite-boninite series. The temperature of boninitic melts may reach 1500 °C at high water contents, which requires specific melting conditions, which can be established in a hot mantle wedge of ensimatic island arcs (Crawford, 1989). The model of Bogatikov and Tsvetkov (1988) suggests that the calc-alkaline volcanic suites in a mature ensialic arc setting are derived by dehydration of serpentine at depths of about 100 km, resulting in partial melting of quartz eclogites in the upper part of the subducting slab and in generation of felsic (dacitic and rhyodacitic) melts. These rising melts promote melting in the lherzolite layer of the mantle wedge and
Fig. 1. Schematic representation of the magmatic evolution of an island arc, after Bogatikov and Tsvetkov (1988). a, Juvenile ensimatic arc; b, mature ensialic arc. 1, oceanic crust; 2, harzburgite (depleted) mantle passing at depth to lherzolite (fertile) mantle; 3, dehydration of subducting oceanic crust; 4, tholeiitic magmas and rocks; 5, tholeiitic magma chambers; 6, accretionary prism sedimentary units; 7, quartz eclogite and partial melting of quartz eclogite; 8, calc-alkaline magmas and rocks; 9, calc-alkaline magma chambers; 10, recycled and metasomatically altered oceanic crust in the island-arc basement; 11, granite-metamorphic crust beneath the arc; 12, the core of the arc composed of tholeiitic and calc-alkaline rocks; 13, marianite-boninite magmas; 14, submarine volcanoes with subalkaline (K-Na) lavas; 15, generation of K-subalkaline magmas; 16, shoshonitic rocks; 17, calc-alkaline plutonic rocks of the diorite-granite series; 18, direction of terrigenous sediment transport and subduction beneath the island arc.
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Fig. 2. General features of subduction zones and subduction-related magmatism, after Dobretsov (2010a). a, Sectorial structure of the vertical section of a subduction zone (1–5), three types of volcanism (I, tholeiite-boninite, boninite; II, andesite, andesite-dacite-rhyolite, basaltic andesite; III, trachyandesite-trachybasalt, shoshonite, and tholeiite in backarc basins), and the temperature distribution with depth during the initial (800a, 1000a, 1400a) and final (800b, 1000b, 1400b) stages subduction stages; b, the distribution volcanoes of the Kuril-Kamchatka arc, according to groups (1–10), straight lines and numbers (100 to 500 km) show the depth isopleths of the seismic focal plane.
generate andesite and basalt-andesite magmas parental to the mature arc’s volcanic front. At the same time, emplacement and crystallization of calc-alkaline plutonic rocks occur in the peripheral magmatic chambers (Bogatikov and Tsvetkov, 1988). Rocks of the adakite and shoshonite-latite suite are relatively rare in ensialic island arcs. Typical adakites are intermediate to felsic porphyritic volcanic rocks, ranging in composition from Na-rich and high-Mg andesite to dacite and rhyolite, with high Cr, Ni, and Sr contents and strongly fractionated REE patterns (LaN/YbN > 10) (Bogatikov et al., 2010; Martin et al., 2005). Recent experimental works (Dobretsov, 2010a; Dobretsov et al., 2001, 2012, 2015) allowed revision of the previous petrological and geochemical models of the subduction zone magmatism. For example, the subduction zone was subdivided into five depth sectors (Fig. 2a) in the upper part of the subducted oceanic slab, suprasubduction mantle and lithosphere (1–5, Fig. 2a) and three volcanic zones typifying mostly boninite-tholeiite (I, Fig. 2a), calc-alkaline, predominantly andesite (II, Fig. 2a), and alkali basalt of the shoshonite-latite series (III, Fig. 2a). The boundaries of sectors 1–5 and volcanic zones I, II, and III are controlled by the temperature distribution in the subducted slab and the subduction angle. Large amounts of melts and volatiles expelled from the dehydrating slab at a certain depth trigger partial melting in the suprasubduction mantle wedge, variations in melt composition, emplacement of magma into the upper lithosphere, and, consequently, ongoing island-arc volcanism. Fluid migration and magma transport pathways at different depths
above the subducting slab control the structure of nascent island arcs. It was shown that the volcanoes of the Kuril-Kamchatka arc line up closely on the 100 and 200 km contour lines of the Benioff zone at a spacing of ~100 km between large volcanic centers (Fig. 2b). The eruptive products are dominated by andesites (about 65% of the total erupted volume) and roughly equal proportions of basalts and felsic rocks (Bogatikov et al., 2010). Andesite melts in subduction zones are strongly enriched in La (9–10 times), Cs, Rb, Ba, U, Th, K, Sr, Zr, and Hf (2–4 times), moderately enriched or not enriched in Nb, Nd, Ti, Sm, and Eu, and markedly depleted in heavier lanthanides (from Dy to Lu), compared to the original basaltic magma. The behavior of many other trace elements (U, Nb, Ti, Sr, and Eu) primarily depends on the oxidizing conditions, e.g., the H2O and CO2 content of the fluid, as well as the degree of hydrothermal alteration of lavas (Martin et al., 2005). Backarc spreading processes can also play a role in the evolution of subduction-zone magmas (Martynov et al., 2015). Therefore, modern subduction zones are represented by calc-alkaline volcanic suites ranging from basaltic to rhyolitic composition, with a dominance of andesites, and display strong variations in silica, alkali, and iron contents. These are mostly low-Ti, low to moderately alkaline and high-alumina rocks. The discovery of such magmatic rock complexes within fold-and-nappe structures is of great importance for reconstruction of the evolution of geodynamic and structural settings at active margins.
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Certain features of Neoproterozoic and Early Paleozoic subduction magmatism on the Siberian continental margin Neoproterozoic and Early Paleozoic subduction processes and magmatism along the active margins of the Siberian paleocontinent are intimately related to the evolution of the Paleoasian Ocean (PAO) (Dobretsov, 2003). Most reconstructions show that the opening of the PAO began with Rodinia breakup, but the timing, mechanisms and early Neoproterozoic tectonic history of this event still remain a matter of hot debate (Dobretsov, 2003, 2010b; Gordienko, 2006; Kheraskova et al., 2010; Li et al., 2008; Metelkin et al., 2007). The Rodinia supercontinent is postulated to have formed by continental collision as a result of the Grenville orogeny at ~1100– 950 Ma. Amongst the existing paleogeographic models, the southern margin of the Siberian craton is reconstructed against northern Laurentia, which in turn formed the core of the supercontinent. In such a configuration, Siberia formed a kind of gigantic northeastern peninsula of Rodinia (ancient coordinates) (Li et al., 2008; Metelkin et al., 2007; Pisarevsky et al., 2008). As a result, continental shelf settings existed around most of the Siberian craton in the Early Neoproterozoic except its southern margin (Pisarevsky and Natapov, 2003; Pisarevsky et al., 2008). The Neoproterozoic–Early Paleozoic breakup of Rodinia and the opening of the PAO were controlled by rising of mantle plumes (Kuzmin and Yarmolyuk, 2014; Maruyama et al., 2014). The subalkaline basaltic dike/sill complexes with an age of 1000–950 Ma intruded the sedimentary cover of the Uchur–Maya region and correspond to the initial phase of rifting and subsequent opening of an oceanic basin in southern Siberia, which is referred to as the PAO (Khudoley et al., 2001; Pavlov et al., 2002; Rainbird et al., 1998). Therefore, the PAO must have been located in the extreme southeast (present coordinates) of the Siberian craton during the earliest Neoproterozoic. At the same time, isotope dating indicated that rift-related magmatic suites along the southern margin of Siberia tend to exhibit progressively younger ages from east to west (Pisarevsky et al., 2008; Yarmolyuk and Kovalenko, 2001; Yarmolyuk et al., 2005). The youngest rocks (750 Ma and younger) represent the products of intraplate alkaline magmatism in the Sayan and Baikal regions (Gladkochub et al., 2007, 2010; Yarmolyuk et al., 2005). Available geochronological and paleomagnetic data allow us to assume that the disintegration processes along the southern margin of the Siberian craton took place over a period of 200 Myr period concurrently with displacement and rotation of Siberia (Metelkin et al., 2012). However, a narrow basin of the Red Sea type extending from the Uchur–Maya to Baikal margins of Siberia is assumed to have existed in the earliest Neoproterozoic (Metelkin et al., 2012). The maximum opening of the PAO at 850–800 Ma led to the formation of subduction zones and related ensimatic island arcs. However, it was suggested that the oldest (Early Neoproterozoic) island-arc associations of the CAOB accretionary-collisional structures were formed outside the PAO within certain cratonic blocks (microcontinents) belonging to other basins surrounding the Siberian
craton (Kheraskova et al., 2010; Kuzmichev and Larionov, 2013; Yarmolyuk et al., 2006), and therefore their assignment to the active margin of the PAO is tentative. The later stage of the PAO evolution is marked by two-phase island-arc volcanism, which finished with accretion and orogeny on the southwestern margin of Siberia during Late Neoproterozoic (Early Ediacaran) and Late Cambrian– Early Ordovician times (Dobretsov et al., 2003, 2005, 2013; Gordienko, 2006; Kheraskova et al., 2010). The island-arc complexes of these two phases contain magmatic associations of ensialic and ensimatic island arcs, backarc and forearc basins. The magmatic complexes of the accretionary wedges contain material that was formed in different geodynamic settings, including relics of oceanic crust and volcanic islands (Dobretsov et al., 2004; Gordienko, 2006). Some of the Neoproterozoic and Vendian–Early Paleozoic island arcs of East Sayan, Transbaikalia, and Mongolia, which are of primary importance for the reconstruction of the Siberian active margin of the PAO, are discussed in more details below. The Neoproterozoic active margin. Previous studies revealed a system of Neoproterozoic island arcs along the southern folded periphery of the Siberian craton, where paleomagnetic data are not available: Dunzhugur (1050– 850 Ma), Arzybei (1100–800 Ma), Shishkhidgol (820– 775 Ma), Sarkhoi (805–770 Ma), Yenisei (697–637 Ma), Shumikha–Kirel (687 Ma) in East Sayan (Fedotova and Khain, 2002; Izokh et al., 2012; Kuzmichev, 2004; Kuzmichev and Larionov, 2011, 2013; Turkina et al., 2007; Vernikovsky et al., 1999, 2001), and Nyurundyukan (1050–1035 Ma), Kelyana (950–790 Ma), Meteshikha (840–810 Ma), Kataev (890– 830 Ma) in Transbaikalia (Gordienko et al., 2009; Orsoev et al., 2015; Rytsk al., 2001, 2007). The only exception is the Yenisei arc, which was located near the present-day Sayan– Yenisei margin of the craton at 640 Ma, immediately before the onset of accretion (Metelkin et al., 2004). The problem of identification and assignment of other arcs to the PAO active margin was discussed above. Nevertheless, the similarities in the petrological features, geochemical signatures, and ages of suprasubduction zone magmatism provide a basis for considering all of these island arcs as being parts of a single Neoproterozoic active margin, which may have experienced several phases of restructuring as an indirect response to the evolution of the PAO. During the time interval (850–800 Ma) selected for the reconstruction, suprasubduction zone magmatism was manifested in most of the island arcs. According to the existing paleotectonic reconstructions, the PAO represented at the time a narrow spreading a basin between the southern (present coordinates) margin of Siberia and northern (present coordinates) margin of Laurentia (Kheraskova et al., 2010; Metelkin et al., 2012), where all the above island arcs and related basins are tentatively reconstructed (Fig. 3). Some models assume that the blocks composing Arctida before the breakup of Rodinia were located along the continental margin of Laurentia or formed part of it (Li et al., 2008; Metelkin et al., 2015; Vernikovsky et al., 2013). This fact, along with the presence of similar magmatic complexes of
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Fig. 3. Paleogeodynamic reconstruction of the Neoproterozoic active margin of the Siberian continent at 850–800 Ma. a, Global reconstruction (modified after Metelkin et al., 2012); b, regional reconstruction (modified after Gordienko, 2006). 1, uplifts and terranes within the Siberian craton (GR, Gargan; KB, Kan–Biryusa; M, Muya; PR, Proterosayan; TM, Tuva–Mongolia; CA, Central Angara; Sh, Sharyzhalgai); 2, platform complexes of microcontinents; 3, marginal continental basins (BP, Baikal–Patom; EA, East Angara); 4, Neoproterozoic accretionary orogen; 5, island arcs, orientation of a subduction zone (triangles) and positions of the accretionary wedge (dashed line); 6, inferred spreading (and backarc) zones; 7, major strike-slip faults; 8, paleolatitudes at 750–700 Ma (shown by dashed line). Note: Island arcs, spreading zones, and microcontinents, except the Siberian craton, are not to scale.
suprasubduction origin within the Central Taimyr accretionary belt, the oldest of which were emplaced from 960 to 600 Ma along the northwestern periphery of Siberia (Vernikovsky and Vernikovskaya, 2001; Vernikovsky et al., 2011), allow us to conclude that the active margin may have extended well into Taimyr. This assumption is supported by new paleomagnetic and geochronological data, which indicate the presence of a vast oceanic basin between the Central Angara terrane and Siberia (Vernikovsky et al., 2016). Thus, it can be supposed that during the early evolution of the PAO the entire Siberian margin was characterized by discrete pulses of subduction zone magmatism, which led to formation of a system of island arcs with various types of magmatism (Fig. 3). One of the most prominent arcs is the Dunzhugur island arc, the magmatic associations of which were found in the southeast of East Sayan and the upper reaches of the Oka River. The Dunzhugur suprasubduction complexes have geochemical signatures of an ensimatic arc setting (Sklyarov et al., 1988) with tholeiitic and calc-alkaline differentiation trends. The volcanic suites comprise very low-Ti and low-Fe andesites and basaltic andesites with high MgO and Cr2O3 contents, which can be classified as typical boninites and marianites. The REE patterns are characterized by the negative Ta, Nb, and Ti anomalies. Forearc complexes are represented by felsic volcaniclastic rocks. The Dunzhugur arc appears to
be the oldest island arc on the Neoproterozoic active margin. The age of magmatism was constrained at 1010 ± 10 Ma (U–Pb) or 1020 ± 0.7 Ma (Pb-Pb) (Khain et al., 2002). Detrital zircons from the suprasubduction volcanosedimentary complex have ages ranging from 1048 ± 12 Ma to 844 ± 8 Ma (Kuzmichev and Larionov, 2013). This arc may have been active outside the PAO limits before the breakup of Rodinia (Kuzmichev, 2004), suggesting its similarity with the oldest island arcs of Taimyr (Vernikovsky et al., 2011). Even older (or close) ages of magmatism were also identified for the Arzybei island arc in the northeastern part of East Sayan. This arc is correlated with the 1100 Ma (U–Pb) Argydzhek pyroxenite-anorthosite-gabbro massif in the upper reaches of the Mana River. The ages of plagiogranites and trondhjemites confined to this arc yielded U–Pb ages of 1017 ± 47 and 800 ± 6 Ma, respectively (Izokh et al., 2012; Turkina et al., 2007). All these rocks have suprasubduction zone geochemical signatures, including low concentrations of incompatible elements (Ti, K, and Zr). Their REE patterns exhibit a slight LREE enrichment, a weak positive Eu minimum, also manifested by ultramafic differentiates, and strong Ta, Nb, Zr, and Hf minima. The isotope characteristics εNd (+4–6) and 86Sr/87Sr (
