Thermochronology of Early Paleozoic collisional and subduction-collisional structures of Central Asia

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ScienceDirect Russian Geology and Geophysics 57 (2016) 434–450 www.elsevier.com/locate/rgg

Thermochronology of Early Paleozoic collisional and subduction–collisional structures of Central Asia A.V. Travin a,b,* a

V.S. Sobolev Institute of Geology and Mineralogy, 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 Tomsk State University, pr. Lenina 36, Tomsk, 634050, Russia Received 10 June 2015; accepted 28 August 2015

Abstract The thermochronology of the Early Paleozoic collisional and subduction–collisional systems and blueschist complexes of the Central Asian Orogenic Belt has been reconstructed by the proposed method of “through” isotope dating. The evolution of these geologic structures is divided into short synchronous stages of active thermal events related to large-scale mantle–crustal magmatism, high-pressure/low-temperature and high-temperature/low-pressure metamorphism, and intense tectonic deformations. The plume activity of different intensities, both in intraoceanic and intracontinental environments, is presumed to be the deep mechanism of synchronization. © 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: collisional tectogenesis; metamorphic deformations; U–Pb and

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Ar–39Ar dating; thermochronology; Central Asia

Introduction Phanerozoic folded orogens are key to the geologic evolution of Central Asia. It is customary to distinguish suprasubductional and collisional orogens (Dobretsov et al., 2001; Lobkovskii et al., 2004; Shengör et al., 1993; Vladimirov et al., 2003, 2005). The estimation of duration of orogenic events, their periodicity, variations in their intensity, and style over time is highly important in the geodynamic modeling of orogeny. Depending on the type of collision (e.g., continent– continent or island arc–continent), the formation of collisional structures can last for 50 Myr or more (e.g., Himalayas) to 18 Myr (Dalradian orogeny, British Caledonides) (Dewey, 2005; Vladimirov et al., 2003; Yin, 2006). The Cambrian–Ordovician played important role in the evolution of the Paleoasian ocean (Dobretsov, 2011; Dobretsov and Buslov, 2007; Rudnev, 2013). At that stage, early collision terminated; most of the “primary” ocean closed; and “secondary” oceans and island arcs formed, including the Paleouralian and Ob’–Zaisan paleooceans. By analogy with that of the present-day Western Pacific, the evolution of the Central Asian Orogenic Belt (CAOB) is considered by some

* Corresponding author. E-mail address: [email protected] (A.V. Travin)

authors (Badarch et al., 2002; Buslov et al., 2001, 2004; Didenko et al., 1994; Fedorovskii et al., 1995; Khain et al., 2003; Kuzmichev et al., 2001; Laurent-Charvet et al., 2003; Mossakovskii et al., 1993; Wang and Liu, 1986; Yin and Nie, 1996; Zonenshain et al., 1990) in terms of the successive accretion of island arcs, oceanic islands, oceanic plateaus, accretionary prisms and/or microcontinents. Others (Kovalenko et al., 1999; Yarmolyuk et al., 2003, 2013) substantiate the hypothesis that the CAOB Caledonides belong to the accretionary superterrane which formed independently of the Siberian Platform as a result of collision (accretion) of a system of Vendian–Cambrian island arcs, backarc basins, and Precambrian terranes between them with a group of oceanic islands and lava plateaus marking a mantle hotfield. After that event, the superterrane joined the Siberian craton along a strike-slip boundary of the transform-fault type. The thermochronological approach can be used to obtain unique information on the metamorphic history of indicator rocks during ascent and cooling, the age of medium- and low-temperature tectonic events, and the time of formation of plutonic rocks and their ascent to the surface. Unfortunately, such studies within the CAOB are fragmentary rather than systematic. Pioneering works include studies of UHP metamorphic complexes—Kokchetav and Maksyutov (Dobretsov et al., 2006; Lepezin et al., 2006; Schertl and Sobolev, 2013),

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 + 6.03.006

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complexes of Cordilleran-type metamorphic cores in Transbaikalia (Sklyarov et al., 1997), and blastomylonites of the Irtysh shear zone (Eastern Kazakhstan) (Travin et al., 2001). Results of thermochronological reconstructions for the key Early Paleozoic collisional structures of Central Asia are presented with the use of the approach developed by the author during field studies and the interpretation of the data obtained in integrated expeditions (Baikal region, eastern Tuva, and Kazakhstan) in 1997–2014.

Methods As 40Ar/39Ar dating of a set of minerals covers a narrow range of closure temperatures (from 200 to 550 °C), the application of two dating methods (U–Pb dating of zircon and 40 Ar/39Ar dating of amphibole, mica, and feldspar) permits a more complete reconstruction of the thermal evolution of igneous and metamorphic rocks, from formation to ascent to the upper crust. An original system with a quartz reactor with an external fast-response furnace for 40Ar/39Ar studies by incremental heating was developed in the laboratory of isotope–analytical geochemistry of the Sobolev Institute of Geology and Mineralogy (Novosibirsk) (Fig. 1) (Travin et al., 2009). One of the main advantages of the device is the possibility of removal of processed samples from the reactor with the use of a magnet after incremental heating to 1300 ºC, because melt remains within a nickel cover. On the one hand, this permits minimizing the blank level (no more than 5 × 10–10 ncm3 40Ar/20 min at 1200 °C); on the other, this increases the reactor work resource by an order of magnitude. Temperature is controlled using a TXA thermoelectric converter immediately adjacent to the sample in the zone of the maximum heating (Fig. 1). The temperature of each step is controlled with an accuracy of ±5 °C, which is much higher than that in “double-vacuum” systems, which are used in most of the laboratories worldwide. When results of 40Ar/39Ar incremental heating are interpreted as age and Ca/K spectra, the generally accepted method is the plateau method, in which the average weighted age for several (at least three) successive temperature steps is considered reliable. The plateau steps have to meet the following requirements (Fleck et al., 1977): The age difference between any two of them cannot be significant; they are characterized by consistent Ca/K ratios; and at least 50% of 39Ar released corresponds to them. As the framing rocks emanate radiogenic 40Ar which accumulated in them at great depths in the crust when temperature increases during metamorphism or melt emplacement, the isotope composition of Ar trapped by the forming minerals can have a considerably higher value than that of atmospheric argon. To identify it and take its presence in the mineral into account, an isochron correlation diagram is most often used. Nevertheless, in the presence of all the above-mentioned intrinsic criteria for the reliability of 40Ar/39Ar age (plateau and isochron regression), the dating disagrees sometimes with geological data or dating by other methods.

Fig. 1. Schematic diagram for the apparatus for argon extraction, purification, and measurement with a Micromass 5400 mass spectrometer. 1, vacuum valve; 2, sample wrapped in Ni foil; 3, glass herringbone unit for preliminary degassing of samples at 150 ºC; 4, quartz reactor; 5, TXA thermoelectric converter; 6, furnace for the external heating of the sample; 7, nitrogen trap for the preliminary purification of argon; 8, SAES (Zr–V) getters for argon purification.

Multistage metamorphic, magmatic, and tectonic processes with long time intervals are typical of the collisional structures of Central Asia. Therefore, the isotopic ages of minerals and systems of different stabilities will be distributed over the time scale depending on thermal history and the intensity of superposed deformations and hydrothermal effects. More intense events (superposed heating, dramatic temperature decrease, rapid ascent to the surface, etc.) will yield a larger number of different ages, because complete age rejuvenation and the closure of the isotope system become more likely during such events. Coincidence between the ages of different minerals and isotope systems considerably increases the chance that they correspond to the age of a real geologic event and are reliable. This is what “couple criteria” are based on (Morozova and Rublev, 1987; Shanin, 1979). “Through” isotope dating has shown high efficiency in thermochronological reconstructions of collisional structures (Lepezin et al., 2006; Travin et al., 2003, 2006). This approach consists in the comparative study of a set of samples: (a) of the same geochemical composition and of primary (magmatic and metamorphic) origin, with different parameters of superposed alterations, or (b) of different compositions (including different mineral phases) but with the same thermal history. Generally, the final criteria for the validity of the obtained isotopic ages are (a) consistency of their relative succession with the series of closure temperatures of isotopic systems (Hodges, 2004) and (b) consistency with the succession of formation of the studied rocks from geological and petrographic data. Thermochronological reconstructions require thorough selection of parageneses corresponding to the indicator complexes of these systems (Table 1). The formation of collisional structures can be modeled adequately as a result of consistent interpretations of the thermochronological trends obtained for the objects of study and the trends of pressure and temperature evolution of metamorphic complexes, because the character of rock heating depends on geodynamic settings. The Ol’khon (western Baikal region) and Kokchetav (Northern Kazakhstan) regions were the main territories for developing the abovementioned approach in long-term studies.

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Table 1. Main subjects of study in thermochronological reconstructions of collisional systems of Central Asia Closure temperature (Tc), ºC

Interpretation

800–900

Age of formation

500–550

Age of formation, cooling to Tc

Micas ( Ar/ Ar)

300–350

Cooling to Tc

K-feldspar (40Ar/39Ar)

200–250

Cooling to Tc

Zircon, monazite (U–Pb)

800–900

Age of metamorphism

Amphibole ( Ar/ Ar)

500–550

Age of metamorphism, cooling to Tc

Micas (40Ar/39Ar)

300–350

Cooling to Tc

Subject of study

Paragenesis

Mineral (method)

Magmatic bodies (granitoids, basic rocks, and ultrabasic rocks), dikes

Primary-magmatic

Zircon, monazite (U–Pb) 40

Amphibole ( Ar/ Ar) 40

Metamorphic framing of magmatic bodies: (a) welded-on contact and (b) rocks of the enclosing frame

39

Least blastomylonitized metamorphic rocks

39

40

39

40

39

200–250

Cooling to Tc

40

39

500–550

Age of deformations

K-feldspar ( Ar/ Ar) Deformed zones

Syndeformation

Amphibole ( Ar/ Ar) 40

39

Micas ( Ar/ Ar)

300–350

Age of deformations, cooling to Tc

K-feldspar (40Ar/39Ar)

200–250

Age of deformations, cooling to Tc

Thermochronology of the Ol’khon region (western Baikal region) Multiple upliftings, compressions, and subsidences at different rates were substantiated for the Chernorud sheet (granulite-facies metamorphism) in this region, unlike the traditional interpretation of slow (tens of Myr) uplifting and cooling, as in granulite complexes worldwide (South India, Greenland, Africa, etc.). The Ol’khon region is a fragment of the Sayan–Baikal collisional belt (western Baikal region) (Donskaya et al., 2000; Gladkochub et al., 2010), formed by the Early Paleozoic events related to the closure of the Paleoasian ocean on the southern margin of the Siberian craton (in geographic coordinates). The geologic structure of the Ol’khon region is traditionally regarded as a stack of tectonic sheets differing in rock associations, metamorphic grade, and the specifics of associated magmatism (Fedorovsky et al., 2005). Three main zones are recognized: Chernorud, Anga-Sakhyurty, and Anga (Fig. 2), with the metamorphic grade increasing from epidote– amphibolite to granulite facies from southeast to northwest. The Ol’khon metamorphic rocks are separated from the Proterozoic rocks of the Siberian craton by the blastomylonites of the Primorsky fault and collisional suture (Fedorovskii, 2004; Fedorovsky et al., 2005; Vladimirov et al., 2011). Lens-shaped inclusions of granulite- and amphibolite-facies rocks, similar to the Chernorud rocks, are observed within the collisional suture in mylonites of different compositions (Sukhorukov, 2007). The rocks of both facies experienced retrograde metamorphism and then exhumation and shearing (Fig. 3). Each of the recognized tectonic zones in the Ol’khon region has a specific set of metamorphic and igneous complexes, but they are all characterized by the “total” evolution of the synmetamorphic granite–leucogranites “sealing” lithologic sheets of contrasting metamorphic grades (Makrygina and Petrova, 1996; Vladimirov et al., 2004).

By the proposed approach, integrated structure–petrological and isotope (U–Pb and 40Ar/39Ar) studies were made for: (a) the Ulan-Khargana and Chernorud pyroxenite–gabbro massifs and their metamorphic framing within the Chernorud zone (Khromykh, 2006; Mekhonoshin et al., 2001; Travin et al., 2009; Volkova et al., 2008); (b) restite ultrabasic bodies and their migmatite–gneiss framing on the Shida Peninsula, within the boundary between the structure–compositional complexes of the granulite and amphibolite facies (Mekhonoshin et al., 2013; Vladimirov et al., 2011); (c) the ultrabasic bodies of the Kharikta-Tog site and their framing within the Anga-Sakhyurty zone (Mekhonoshin et al., 2013; Volkova et al., 2008); (d) the Birkhin gabbro and Aya granite massifs and their framing within the Anga zone (Volkova et al., 2008; Yudin et al., 2005); (e) the synmetamorphic stress granites of the Shara-Nur complex sealing zones of granulite and amphibolite metamorphism (Makrygina and Petrova, 1996; Vladimirov et al., 2004); (f) narrow bands with the most intense mylonitization and blastomylonitization, namely, the collisional suture bordering on the Primorsky fault (it separates the Siberian Platform from the Ol’khon region) and the Orso complex (it separates the Anga-Sakhyurty zone with tholeiitic metavolcanics from the Anga zone with subalkalic metavolcanics) (Fedorovskii, 2004; Fedorovsky, 1997; Fedorovsky et al., 2010; Gladkochub et al., 2010; Mekhonoshin et al., 2013; Sukhorukov et al., 2005; Vladimirov et al., 2011; Volkova et al., 2008). In almost all the cases, 40Ar/39Ar ages estimated by the plateau method were regarded as the age of closure of the K–Ar isotope system in the studied minerals. In general, the interpretation of the obtained 40Ar/39Ar data on the coordinates time–temperature of closure of isotope systems (Fig. 4) confirms the internal structure of the Ol’khon terrane and reveals the dynamics of tectonic exposure of the strike-slip

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Fig. 2. Tectonic zonation, after V.S. Fedorovskii (2004), with a reference to isotope–geochronological data (U–Pb and Ar/ Ar) on the metamorphic and igneous complexes of the Ol’khon region. 1, Archean–Early Proterozoic structure–compositional complexes of the Siberian craton; 2, blastomylonites of the Primorsky fault and collisional suture; 3–7, Early Paleozoic structure–compositional complexes of the Ol’khon region: 3, metamorphic rocks of the Chernorud zone (granulite facies); 4, metamorphic rocks of the Shida zone (amphibolite facies); 5, metamorphic rocks of the Anga-Sakhyurty zone (amphibolite facies); 6, metamorphic rocks of the Orso complex (epidote–amphibolite facies); 7, metamorphic rocks of the Anga zone (epidote–amphibolite facies); 8–10, Early Paleozoic intrusive complexes: 8, massifs of gabbro, monzodiorites, and monzonites of the Birkhin complex (Anga zone); 9, bodies and veins of the granitoids of the Shara-Nur (Anga-Sakhyurty and Chernorud zones) and Aya (Anga zone) complexes; 10, Tazheran massif of alkali gabbro, nepheline syenites, and syenites; 11, main strike-slip sutures; 12, sampling sites for geochronological studies: a, U–Pb isotope dating of zircon; 40 39 40 39 b, Ar/ Ar dating (Ma); the geochronometer mineral (Amf, amphibole; Bi, biotite; Mu, muscovite) is specified for Ar/ Ar dating. The isotope data are after (Bibikova et al., 1990; Fedorovskii et al., 2005, 2010; Gladkochub et al., 2008, 2010; Letnikov et al., 1990; Sklyarov et al., 2001, 2009; Travin et al., 2009; Vladimirov et al., 2008, 2011; Volkova et al., 2008, 2010; Yudin et al., 2005).

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Fig. 3. Scheme showing the evolution of the metamorphic pressure and temperature of the Chernorud rocks. Red rectangle in the upper right corner shows the PT-range estimated with the use of mineral thermometers and barometers for the Chernorud two-pyroxene gneisses (Fedorovsky et al., 2005). Gray band indicates the trend corresponding to the traditionally presumed evolution of the pressures and temperatures of granulite complexes (slow rise and cooling). Colored rectangles and connecting arrows show the presumed evolution consisting of discrete stages.

sheets making up the regional collisional complex. According to the isotope data (Figs. 2, 4), an early collision stage (495 ± 5 Ma) can be distinguished for the Ol’khon region; it is related to the peak of collisional compression and the intrusion of the gabbro of the Birkhin complex (Anga zone) and the gabbro–pyroxenites, hypersthene plagiogranites, and quartz-bearing syenites of the Chernorud complex (Chernorud zone). The maximum temperatures of crustal metamorphism to the granulite facies at the stage of pressure decrease might have been due to additional heat supply during collapse and the exhumation of the metamorphic complex during the emplacement of the mantle magmas concentrated in the root of of the collisional system (Fedorovsky et al., 2010; Sklyarov et al., 2001). Based on the metamorphic conditions for two-pyroxene basic gneisses estimated with the use of a garnet–clinopyroxene thermometer and a garnet–orthopyroxene geobarometer (Fig. 3, 770–820 ºC, 7.7–8.6 kbar (Fedorovsky et al., 2005)), the Chernorud rocks were localized at a depth of ~27 km at that time (Fig. 4). For garnet–biotite–andalusite schist from a fragment of the contact aureole of the Birkhin gabbro massif, the following conditions are reconstructed: 490–650 ºC and 3.6–4.6 kbar (Fig. 3) (Volkova et al., 2008); this corresponds to the intrusion of the massif to a depth of 12–15 km (Fig. 4). Based on U–Pb zircon dating (Vladimirov et al., 2011), the age of the migmatites of garnet–biotite gneiss from the Shida blastomylonite complex was 30–35 Myr more than that of the early collision stage (Figs. 2, 4). A regional metamorphic event

(amphibolite facies) might have taken place at that time prior to granulite metamorphism (Mekhonoshin et al., 2013). The late collision stage (470 ± 5 Ma) was key to the regional evolution. The regional structure was formed by the collapse of the collisional system under a transition from the tectonic setting of compression to a long period of extension and the breakup of the folded structure. The change of the tectonic regime is evidenced by more intense sinistral tectonic movements, accompanied by the intrusion of the synkinematic stress granites of the Shara-Nur complex and mingling dikes (Fedorovsky et al., 2010; Sklyarov et al., 2001; Vladimirov et al., 2004). In the Anga zone, this stage corresponds to the subalkaline microgabbro, syenites, and nepheline syenites of the Tazheran complex, whose intrusion and evolution were accompanied by sinistral viscoplastic and brittle–plastic shearing (Fedorovsky et al., 2010; Sklyarov et al., 2009). Together with the rare-metal granites of the Aya complex, the subalkaline and alkaline rocks of the Tazheran massif are now considered indicators of plume activity in the Ol’khon region (Sklyarov et al., 2009; Vladimirov et al., 2011). The later (in our interpretation, intraplate) stages (435 ± 10 and 410 ± 10 Ma) were amagmatic, with widespread mylonitization and blastomylonitization of the rocks (sinistral deformations) at low pressures and temperatures of metamorphism (no higher than those for the epidote–amphibolite facies). The defined stages are characterized by short pulses of tectonic activity related to the evolution of a conjugate system of penetrating first-rank faults (Primorsky fault, collisional suture, and Orso complex, Figs. 2, 4). The activity of the main faults was synchronous with the closure of the K–Ar isotope system in the metamorphic and magmatic minerals of the early stages within three main zones of the Ol’khon region (Figs. 2, 4). In all probability, discrete pulses of slip of the sheets of the Ol’khon terrane along the edge of the Siberian craton were synchronous with their effective ascent to the upper crust, which led to the successive closure of isotope systems. Comparison between the 40Ar/39Ar ages of the southern and northern margins of the Birkhin gabbro massif suggests intense shearing at the last stage, ~410 (390–415) Ma. In the southern part of the massif, the K–Ar isotope system of biotite from a leucogranite vein of the Aya complex and from the preserved metamorphic rocks of the contact aureole closed at 430–435 Ma (Fig. 2). At the same time, biotite from a similar leucogranite vein in the northeastern endocontact zone yields a considerably younger age (415 ± 4 Ma). Also, biotite and amphibole from amphibolized monzogabbro in the northwestern exocontact zone yield 410–415 Ma. These samples were taken in the territory influenced by the zone of areal amphibolization which had formed together with the thick blastomylonite rim of the Birkhin massif during its “rolling” (rotation about its axis synchronous with intense shearing). Also, this zone influences the rocks of the Aya massif of rare-metal granites (Aya complex): The biotites from these rocks yield 40 Ar/39Ar ages of 391 ± 4 and 412 ± 4 Ma (Fig. 2). In general, successive closure of isotope systems, similar to that in many slow-cooling granulite complexes worldwide

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Fig. 4. Thermal evolution (age–temperature) of the lithologic sheets and blastomylonite complexes of the Ol’khon region. 1, U–Pb zircon dating; 2–5, 40Ar/39Ar dating: 2, of muscovite; 3, of amphibole; 4, of biotite; 5, of feldspar; 6, bulk Rb/Sr. Arrows show the thermochronological trends defined for individual lithologic sheets. The ages of tectonothermal activity are marked gray. Heavy lines indicate thermal trends for K-feldspar from the Chernorud zone, whereas dotted lines indicate confidence intervals. Separate scale on the right shows the evolution of depth for the Chernorud and Anga zones. The sources of the isotope data are cited in the caption to Fig. 2.

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A.V. Travin / Russian Geology and Geophysics 57 (2016) 434–450 Fig. 5. Comparison between the histories of active thermal events for the Caledonian folded structures of Central Asia (Travin et al., 2009). See legend in Fig. 4. For the Bayankhongor zone, K–Ar ages of biotite are given after (Travin et al., 2009).

(South India, Greenland, Africa, etc.), is reconstructed for the high-grade metamorphic rocks of the Chernorud zone, which correspond to the root of the collisional system. According to the data obtained, the Chernorud rocks moved from a depth of 27 km to <10 km for 100 Myr (Fig. 4). That process might have been continuous (by analogy with that for Precambrian shields), or it might have been due to some discrete tectonic events (by analogy with Franciscan-type thrusts (Agard et al., 2009; Dobretsov, 1995). The second hypothesis appears more logical with regard to all the data obtained for the main zones of the Ol’khon region. For example, the exhumation of the Chernorud rocks as a result of several tectonic events is evidenced by coincidence between the ages of closure of the isotope systems of minerals and stages of tectonic activity within large faults of the Ol’khon region. Apparently, the rocks of the tectonic sheets “froze” at an intermediate depth at low temperature during intervals tens of Myr long. The next period of tectonomagmatic activity was characterized by a short-time temperature increase accompanied by a pressure decrease—the exhumation of the Chernorud rocks to the next depth level (Fig. 3). At the early stages, mantle and mantle– crustal magmas were the main heating source (Fedorovsky et al., 2010; Khromykh, 2006; Mekhonoshin et al., 2013; Sklyarov et al., 2001; Vladimirov et al., 2004), whereas at the amagmatic late stages, the heating might have been caused by intense plastic and brittle–plastic deformations. This agrees with the hypothesis that the late stages were related to the slip of the Ol’khon terrane along the edge of the Siberian craton, accompanied by the exhumation of strike-slip sheets from great depths in the crust (Fedorovsky et al., 2010). The Early Paleozoic accretion–collisional events accompanying the closure of the Paleoasian ocean and the accretion of terranes of different geodynamic nature to the edges of the Siberian craton (Dobretsov and Buslov, 2007) resulted in the formation of several high-grade metamorphic complexes within the northern segment of the CAOB, along the southern flank of the Siberian craton (Donskaya et al., 2000; Gladkochub et al., 2010). Previously, we compared the stages of active thermal events for five Early Paleozoic collisional structures of the CAOB (Travin et al., 2009). Along with the Ol’khon region, those were the Sangilen Upland, Slyudyanka crystalline complex (southern Baikal region), Derba terrane (East Sayan), and Bayankhongor ophiolite zone (central Mongolia). For all these collisional structures, which correspond to the vast territory of the CAOB (more than 700 × 800 km), active thermal events of equal duration are reconstructed (Fig. 5); note that the ages obtained for the igneous and metamorphic rocks by different isotope methods are consistent with the discrete stages defined for the Ol’khon collisional system (Travin et al., 2009).

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Fig. 6. Tectonic sketch map of Northern Kazakhstan (Dobretsov et al., 2006). 1, Devonian–Late Paleozoic volcanosedimentary basins; 2, 3, fragments of the Kokchetav and Shatsky (northeast of Kokchetav) microcontinents: 2, slightly altered, with diaphthoresis in the greenstone facies; 3, with sediments metamorphosed to the amphibolite facies in the subduction zone; 4, 5, megamélange belt (terranes of the paleosubduction zone): 4, diamond-bearing gneisses and coesite eclogites (terranes: 1, Barchin; 2, Kumdy-Kol); 5, other terranes containing eclogites, garnet amphibolites, and garnet peridotites in granite-gneisses and micaceous schists (terranes: 3, Sulutobe; 4, Kulet; 5, Enbek-Berlyk; 6, its analogs north of Shchuchinsk); 6, Vendian (?) volcanosedimentary rocks in an accretionary prism; 7, Early Ordovician accretionary prism; 8, Vendian–Cambrian island-arc volcanosedimentary rocks (of the Ishim arc in the west and of the Selety arc in the east); 9, Late Arenigian–Early Caradocian syntectonic olistostrome; 10, Ordovician volcanosedimentary rocks of the Stepnyak trough; 11, Ordovician volcanics of the Stepnyak island arc; 12, Middle–Upper Ordovician shelf sediments; 13, Late Cambrian–Tremadocian ophiolites of the Zlatogorsk complex; 14, Middle Cambrian Krasnomaiskii alkaline ultramafic complex; 15, Silurian–Ordovician granites; 16, Devonian granites; 17, deformed Late Cambrian–Early Ordovician fault planes; 18, Late Arenigian–Early Caradocian front thrust of the Kokchetav massif over the Stepnyak trough; 19, Late Paleozoic strike-slip faults: a, actual; b, predicted; 20, U–Pb zircon dating; 21, Sm–Nd isochron age; 22, 40Ar/39Ar age (Phe, phengite; Tur, tourmaline; see Fig. 2 for the rest of the mineral abbreviations). The dating is after (Dobretsov et al., 2006; Schertl and Sobolev, 2013).

Thermochronology of the Kokchetav subduction–collisional zone (Northern Kazakhstan) The Early Paleozoic collisional zone of Central Asia (Berzin et al., 1994; Buslov et al., 2001; Dobretsov, 2003; Mossakovskii et al., 1993; Zonenshain et al., 1990), localized between Precambrian East European and Siberian continents, includes Northern Kazakhstan (Fig. 6). The UHP–HP rocks of the Kokchetav massif were formed at depths of ≤150–200 km in a subduction zone by the metamorphism of the rocks of the basement and cover of the Kokchetav microcontinent (Dobretsov et al., 1998; Schertl and Sobolev, 2013; Sobolev and Shatsky, 1990). The high velocity of their ascent, necessary for the preservation of high-pressure minerals and assemblages, is explained by different models, including those related to microcontinent–island arc collision and the rearrangement of the subduction zone (Dobretsov et al., 1995, 2005a,b) as well as the formation of large thrusts (Dobretsov et al., 1998) or the squeezing-out of a near-horizontal wedge (Maruyama and Parkinson, 2000; Okamoto et al., 2000). The Kokchetav subduction–collisional zone (KSCZ) (Figs. 6, 7) is a tectonic collage of repeatedly deformed fragments of the Precambrian

Kokchetav microcontinent and a Vendian–Cambrian megamélange belt (terranes of a paleosubduction zone); part of the sheets outside the Kokchetav zone belongs to an Early Ordovician accretionary wedge (Dobretsov et al., 1998, 1999, 2005a,b). Flakes and blocks of UHP and HP rocks are observed in two structural units: (1) the Cambrian megamélange belt including terranes of metamorphic rocks of the paleosubduction zone, which formed at depths of 60 to 150 (200) km, and (2) the Ordovician accretionary wedge (Figs. 6, 7) with eclogites which formed at depths of ~60 km, combined with tectonic blocks of the microcontinent rocks and ophiolite and island-arc terranes, which alternate with Early Ordovician turbidites containing olistostrome lenses (Dobretsov et al., 2005a,b). The megamélange belt shows different depth levels of the Vendian–Cambrian subduction zone. On the tectonic sketch map of Northern Kazakhstan, it is between a slightly altered fragment of the Kokchetav microcontinent and the Early Ordovician accretionary prism (Figs. 6, 7). The belt is a contrasting structure composed of flakes and blocks of UHP and HP rocks, which are separated by tectonic sheets of the rocks of the Kokchetav microcontinent metamorphosed from medium to highest (relict) and then low pressures. Five

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Fig. 7. Geological sketch map of the North Kokchetav tectonic zone, modified after (Zhimulev et al., 2011). 1–4, geologic complexes of the Kokchetav microcontinent: 1, gneisses of the basement of the Kokchetav microcontinent, Early–Middle Proterozoic; 2, mylonites and blastomylonites after the Proterozoic basement gneisses; 3, quartz–feldspar porphyry, Middle Proterozoic; 4, quartz–sericite schists, metasandstones, and dolomites of the Kokchetav microcontinent, Ilekty Group, Late Proterozoic; 5–9, rocks of the Kokchetav metamorphic belt: 5, gneisses with eclogite boudins; 6, garnet–kyanite schists with garnet amphibolite boudins; 7, quartz–garnet–muscovite (phengite) schists with eclogite boudins; 8, large eclogite bodies and accumulations of bodies; 9, amphibolites; 10, Early Ordovician andalusite–cordierite–biotite schists (Daulet Formation); 11–14, pre-Ordovician island-arc (?) structures: 11, greenstone basaltic porphyry; 12, boulder conglomerates with block inclusions; 13, andesite agglomerates; 14, mudstones, siltstones, and sandstones (flysch sediments); 15, Arenigian olistostrome; 16, 17, Lower–Middle Ordovician structures of the Stepnyak trough: 16, sandstones, silicified mudstones and siltstones, jasperoids, and breccia lenses; 17, basalts, red cherty siltstones, jasperoids; 18–20, Middle Ordovician sediments: 18, gray cherty shales and microquartzites; 19, volcaniclastic flysch; 20, andesite-basalts and their tuffs; 21, congomerates and sandstones, Middle–Upper Ordovician; 22, red-colored sandstones and conglomerates, Middle–Upper Devonian; 23, gray sandstones, siltstones, pinkish gray limestones, Tournaisian; 24, Proterozoic granites; 25–27, Shchuchinsk ophiolite belt (Precambrian): 25, basaltic porphyry; 26, gabbro; 27, serpentinites; 28, Paleozoic (O3, S, D1) granites; 29, dips and strikes of stratification (a) and foliation (b); 30, geologic boundaries between units of different ages (a) and bodies of different compositions within these units (b); 31, marker horizons traced in the territory; 32, thrusts (a) and other faults (b); 33, large thrusts delineating the North Kokchetav tectonic zone. Circles show sampling sites for isotope dating (Dobretsov et al., 2006; Schertl and Sobolev, 2013). See notation for the dating in Fig. 6. Inset shows a scheme with the main tectonic units. 1, Kokchetav microcontinent; 2, North Kokchetav subduction–collisional zone; 3, Stepnyak trough. The dating is after (Dobretsov et al., 2006; Schertl and Sobolev, 2013).

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terranes are recognized (Barchi-Kol, Kumdy-Kol, Sulutobe, Enbek-Berlyk, and Kulet), in which UHP–HP and HP rocks are observed (Figs. 6, 7). Terranes 1, 2, and, partly, 3 contain similar ranges of rocks. Apparently, lithologically different terranes (1 + 2), (3 + 4), and 5 characterize different levels of the paleosubduction zone, each with a specific range of rocks and evolution of PT-conditions (Fig. 8). In the KumdyKol and Barchi-Kol terranes, diamond-bearing rocks with a maximum pressure of 40–70 kbar and a maximum temperature of 1100–1200 °C, which have a complex multistage history of exhumation with pressure and temperature decrease, are exposed or stripped by boreholes (Dobretsov et al., 1995, 1996, 1999; Maruyama and Parkinson, 2000; Okamoto et al., 2000; Schertl and Sobolev, 2013; Sobolev and Shatsky, 1990) (Fig. 8a). The rocks in the Kulet and Sulu-Tyube terranes are diamond-free, but coesite occurs at some places (in the zone south of Lake Kulet). The eclogites of the Kulet terrane show prograde transformation from amphibolite to eclogite facies, maximum pressures and temperatures (Fig. 8b) of 34–36 kbar and 720–760 °C, respectively (Ota et al., 2000; Parkinson, 2000; Zhang et al., 2012); an exhumation stage is detected in garnet amphibolites at 7–13 kbar and 540–720 ºC (Ota et al., 2000). In the eclogites of the Sulu-Tyube terrane, P = 14–16 kbar and T = 700–860 °C; this corresponds to depths of 50–40 km (Dobretsov et al., 2006). The metamorphic schists of the Enbek-Berlyk Formation formed long before subduction or collision in the crust at 7 kbar and 650–700 ºC; this corresponds to depths of 20–25 km (Fig. 8b). Geochronological data on the high-pressure rocks of the Kumdy-Kol and Barchi-Kol terranes are the most abundant (Dobretsov et al., 2006; Schertl and Sobolev, 2013). Note the similarity between (a) SHRIMP U–Pb dating of zircon domains with inclusions indicating high-pressure parageneses (Claoue-Long et al., 1991; Hermann et al., 2001; Katayama et al., 2001); (b) Sm–Nd dating of high-pressure garnet and clinopyroxene, which, most likely, correspond to the closure of the isotope system (Jagoutz et al., 1990; Shatsky et al., 1993, 1999); (c) SHRIMP U–Pb dating of zircon domains with inclusions indicating granulite metamorphism (domain 3, Fig. 8a) (Hermann et al., 2001); (d) SHRIMP U–Pb dating of zircon from migmatites (Ragozin et al., 2009); (e) SHRIMP U–Pb dating of zircon rims corresponding to mineral inclusions indicating amphibolite metamorphism (domain 4, Fig. 8a) (Hermann et al., 2001), and three 40Ar/39Ar age estimates for high-pressure phengites and biotite (Hacker et al., 2003) (Figs. 6, 8a, 9). Therefore, shortly (~4 ± 4 Myr) after the UHP–HP metamorphism of the rocks of the KumdyKol terrane, their retrograde metamorphism and transport from the depths of formation (150–200 km) to depths of <10 km must have been terminated. Stages of fast exhumation (530– 535 and 524–530 Ma) are presumed. A later event with an age of 505–517 Ma is detected based on coincidence between the ages obtained using different isotope systems (Figs. 8a, 9). In all probability, the KumdyKol terrane experienced intense shearing at that time, with the formation of garnet–micaceous and micaceous schists, mylonitization, and partial melting and/or crystallization of granitic

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melts (Borisova et al., 1995; Dobretsov et al., 2001; Hacker et al., 2003; Katayama et al., 2001; Troesh and Jagoutz, 1993). The closure of the K–Ar isotope system in biotite from garnet–biotite gneiss at 484 Ma (Hacker et al., 2003) testifies to later events within the terrane. Also, this age estimate agrees with those obtained by the 40Ar/39Ar method for phengite and tourmaline from the Tour–Qtz–Kfs–Mu gneisses within the main adit level of the Kumdy-Kol deposit (Korsakov et al., 2009). The age of the last episode agrees with that of syndeformation muscovite from the numerous faults of the Kokchetav subduction–collisional zone, which are considered below. Two rims of zircons from diamond-bearing gneisses yield SHRIMP U–Pb ages of 456–460 Ma (Fig. 8a), which are attributed to late thermal events related to the intrusion of Ordovician–Silurian granitoids (Katayama et al., 2001). Thus, the high-pressure rocks were influenced by at least three active thermal events: 505–517, 481–497, and 456– 460 Ma. The fact that the isotope systems of the micas reflect their age suggests the low intensity or short duration of the events superposed on the high-pressure parageneses. The estimated dependence of the portion of radiogenic argon lost by the biotite lattice on the heating duration and temperature on the assumption of bulk diffusion is shown in Fig. 10 (Travin et al., 2001). The above-mentioned episodes of formation of garnet–micaceous and micaceous schists (505– 517 Ma), as well as Tour–Qtz–Kfs–Mu gneisses (481– 497 Ma), proceeded in a zone of transition from brittle to plastic deformations (depth 5–8 km) at 350–500 °C or more. The fact that high-pressure biotite cannot have lost more than 10% radiogenic argon suggests that the duration of the episodes of superposed brittle–plastic deformations was significantly less than the observed scatter of isotopic ages and did not exceed 1 Myr. Within the Kulet terrane, micaceous schists with a high-pressure paragenesis were dated by the 40Ar/39Ar method (Theunissen et al., 2000). They show the weakest penetrating deformations, which are typical of the matrix of the host rocks of eclogite lenses. Ages considerably older than the age of high-pressure metamorphism were obtained for phengite and biotite from several samples (Fig. 7). Considering the wide scatter of the ages and the lack of consistent ages obtained by other methods, we presume that they are related to excess radiogenic 40Ar* in the mica lattices. The ages of phengite from schist and biotite from gneiss (519.3 and 521.5 Ma, Figs. 7, 9) are consistent with the Sm–Nd age of amphibole– garnet–zoisite rock (Shatsky et al., 1993). Thus, the retrograde metamorphism and exhumation of the rocks of the Kulet terrane from the formation depth (~100 km) to depths of 5–8 km ended over 5 Myr after the termination of high-pressure metamorphism. The micas from garnet–kyanite–micaceous, pyrope–talc–kyanite–biotite, and garnet–muscovite– biotite–quartz schists (Hacker et al., 2003; Shatsky et al., 1993) are significantly younger (499–505 Ma) (Figs. 7, 9). As in the case of the Kumdy-Kol terrane, that was a short (no more than 1 Myr) episode of superposed brittle–plastic deformations.

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Fig. 8. Evolution of the pressures and temperatures of the metamorphic rocks of the North Kokchetav tectonic zone: a, for the Barchi-Kol (1) and Kumdy-Kol (2) terranes, based on the data summarized in (Dobretsov et al., 2006; Korsakov et al., 2009; Schertl and Sobolev, 2013); b, for the Kulet (3), Sulu-Tyube (4), and Enbek-Berlyk (5) terranes and accretionary prism (6), after (Buslov et al., 2010; Dobretsov et al., 2005a,b, 2006); 7, final stage of ascent; 8, superposed events. Arrows show the evolution of pressure and temperature over time. Coe, Coesite; Ab, albite; Qtz, quartz; Jd, jadeite.

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Fig. 10. Temperature dependence of the duration of isothermal heating of biotite for the cases of 10, 50, and 90% loss of radiogenic argon, after (Travin et al., 2001). Black band shows the hypothetical temperature range during schist formation and mylonitization.

Fig. 9. Thermal evolution (age–temperature) of the terranes of the Kokchetav subduction–collisional zone. The stages of thermally active events for the Chernorud granulite zone are marked gray. 1, U–Pb (zircon); 2, 40Ar/39Ar (amphibole); 3, 40Ar/39Ar (biotite); 4, 40Ar/39Ar (muscovite); 5, Sm–Nd (isochron); 6, 40Ar/39Ar (tourmaline); 7, bulk Rb/Sr. The dating is after (Dobretsov et al., 2006; Schertl and Sobolev, 2013).

Fault zones. The Kokchetav subduction–collisional zone is characterized by widespread Late Cambrian–Early Ordovician mylonitization and the formation of the garnet–micaceous and micaceous rocks marking the deep levels of fault zones after the rocks of the terranes making up the subduction–collisional

zone. According to geological data, the fault zones formed as a result of thrusting of the Kokchetav microcontinent, megamélange zone, and accretionary prism over the rocks of the Stepnyak trough (Dobretsov et al., 2005a,b, 2006). Within the Enbek-Berlyk terrane, 40Ar/39Ar dating was carried out for syndeformation muscovite (Dobretsov et al., 2006; Theunissen et al., 2000; Travin, 1999). Besides that, muscovite was studied from the fault zones of the accretionary prism at a distance of >120 km—from Kokchetav to Zhanatalap Village (Dobretsov et al., 2005a,b). The ages, which were obtained by the plateau method in all the cases, range from 478 to 492 Ma (Figs. 7, 9). The similarity between the temperatures of formation of the schists and closure of the K–Ar isotope system of muscovite (350–400 ºC) suggests that the obtained ages are consistent with the age of the schists and, correspondingly, to the Early Arenigian age of the accretionary prism and imbricated structure of the Enbek-Berlyk terrane. The rocks of the other KSCZ terranes are characterized by isotope–geochronological data to a considerably smaller extent. Note the SHRIMP U–Pb ages of the rims of zircon from the quartz–garnet–sillimanite–muscovite schist of the Daulet Formation (Katayama et al., 2001) (461–516 Ma). This is the range of the 40Ar/39Ar ages of muscovite from schists of different compositions from the Sulutobe terrane (Travin, 1999) and Daulet Formation (Buslov et al., 2010) (Figs. 6, 7, 9). These ages mainly agree with those of syntectonic muscovite from the quartz–micaceous schists, fault zones of the accretionary prism, and Enbek-Berlyk terrane and suggest that the present-day imbricated structure of the KSCZ formed as a result of several short-term pulses of brittle–plastic deformations at 470–490 Ma. Amphibole from the eclogite-bearing schist (Travin, 1999) of the Sulutobe terrane and biotite from the Daulet schists (Letnikov and Khalilov, 1994; Travin, 1999) yield 435 ± 5 and 396–402 Ma, respectively (Fig. 9). Such young ages might be due to heat effect on the KSCZ rocks during the formation of the granites of the Zerenda complex; this is confirmed by the Rb/Sr dating of granites (Shatagin, 1994, 1995).

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Thus, a succession of discrete short-term active thermal events of the same duration as that for the accretion–collisional structures considered above is defined for the Kokchetav subduction–collisional zone. Discussion An age estimate for the early stages of closure of some zones of the Paleoasian ocean can be obtained by isotope

dating of UHP and HP metamorphic complexes, eclogites, and blueschists. Like the KSCZ, most of the blueschist belts formed in an intraoceanic forearc setting with tectonic accretion, subduction, underplating, and obduction of mafic terranes. The preservation of HP/LT metamorphic associations implies that part of the subducted crust separated from the descending slab and returned quickly to the upper crust at the moment of collision. The exhumation might have been due to a return flow in the accretionary wedge (Dobretsov, 2000) during seamount–island arc collision and the release of large quantities of dehydrated water in a subducted serpentinite slab, which favors the exhumation of eclogites and blueschists. The history of active thermal events in the key Early Paleozoic collisional and subduction–collisional structures of the CAOB is compared in Fig. 11 to the dating of the blueschist complexes (Volkova and Sklyarov, 2007; Volkova et al., 2011). Note that each stage of formation of these systems corresponds to the age of one of the blueschist complexes. For example, one of the early ages of closure of the Paleoasian ocean corresponds to the dating of the UHP parageneses of the KSCZ (520–537 Ma). Such synchronization of events within the entire CAOB is possible only in the case of simultaneous activity with periods of igneous, metamorphic, and tectonic inactivity at the global level. According to N.L. Dobretsov (2003), it results from plume activity of different intensities. The maximum synchronization must occur during periods of superplume activity at intervals of 120 Myr; local synchronization of tectonic processes takes place between them with the activity of smaller local plumes at intervals of ~30 Myr: rearrangement of island arcs, local collision, and exhumation of eclogites and blueschists. The proposed model is confirmed by the pulsed curve of formation of ophiolite complexes (Helo et al., 2006; Vaughan and Scarrow, 2003) (Fig. 11). After 570 Ma, the Paleoasian ocean evolved in the regime of convergence; rock complexes related to mantle hotspots formed simultaneously with island arcs within the ocean (Safonova, 2008; Yarmolyuk et al., 2006, 2013). An intense pulse of intraoceanic mantle magmatism, also detected for the parageneses of the early stages of the collisional systems (Ol’khon region, western Sangilen, Fig. 11), resulted in the dramatic acceleration of subduction and the rearrangement of oceanic plates. Besides, the formation of the UHP complexes of the Kokchetav massif and their fast exhumation were related to that event. The newly formed structures (oceanic isles, island arcs, and backarc basins) of the Paleoasian ocean accreted to the Precambrian terranes within the ocean at 460–505 Ma and ended with the formation of the CAOB Caledonian superter-

Fig. 11. Comparison between the histories of active thermal events for terranes: a, Kokchetav metamorphic belt (Northern Kazakhstan); b, Chernorud zone (Ol’khon region); c, western Sangilen (southeastern Tuva); d, data from the 40 Ar/39Ar dating of the blueschist complexes of the CAOB. Complexes: U, Uimon; K, Kurtushiba; Ch, Chara; M, Maksyutov. See legend in Fig. 9. d, Solid line shows the intensity of formation of ophiolite complexes (Helo et al., 2006; Vaughan and Scarrow, 2003).

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rane (Dobretsov and Buslov, 2007; Yarmolyuk et al., 2013). The evolution from compression to extension proceeded under some intense pulses of intraplate plume activity. The Altai– Sayan LIP, including the area of numerous basic and granitoid batholiths, formed at that time (Vladimirov et al., 2013). The formation of the HT/LP metamorphic complexes considered above (Ol’khon region and western Sangilen) is closely related to ultrabasic–basic magmatism. These events were synchronous with short pulses of intense tectonic alterations of the Kokchetav metamorphic belt, localized far outside the area of plume activity (Fig. 11). The later igneous activity within the considered accretion– collisional structures became much weaker. The stages of 430–450 and 390–415 Ma were amagmatic in the Ol’khon region under conditions of extensive tectonic movements. Apparently, the final accretion of the Caledonian superterrane to the Siberian craton as a result of some short pulses of sinistral deformations occurred at that time. Intense intraplate magmatism took place at the continental edge, within the Altai–Sayan region, with the formation of a triple graben system. At the same time, mantle intraplate magmatism proceeded within the Paleoasian ocean (Yarmolyuk et al., 2006, 2007). These events correspond to the formation of syncollisional granitoids within the KSCZ (Shatagin, 1994, 1995).

Conclusions By the example of Central Asia, the “through” approach to thermochronological reconstructions of collisional and subduction–collisional structures has been proposed. It consists in the comparative study of series of samples by the U–Pb and 40 Ar/39Ar dating of sets of minerals; these samples (1) have the same geochemical composition and primary (igneous and metamorphic) age but different parameters of superposed alterations or (2) differ in composition (including different mineral phases) but experienced the same thermal history. The valid criteria for the reliability of a series of isotopic-age estimates are consistency between their relative succession and the series of closure temperatures of isotopic systems as well as consistency with the succession of formation of the studied rocks from geological and petrographic data and the intensity of superposed alterations. Stages of active thermal events related to large-scale mantle–crustal magmatism and metamorphism of the HP/LT and HT/LP types are defined for the Early–Middle Paleozoic collisional structures of the CAOB (from Northern Kazakhstan to western Baikal region) isolated from one another at intervals of 30–25 Myr: 520–530, 490–500, 460–470, 430–450, and 390–410 Ma. The observed synchronization between metamorphic, igneous, and tectonic events might be related to igneous, metamorphic, and tectonic activity at the global level during the manifestation of plumes of different intensities. The fast exhumation of the UHP rocks of the Kokchetav belt, Maksyutov complex, blueschist complexes of the Uimon and Kurtushiba zones and the Chara belt to the upper crust

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(no more than 5 Myr) has been defined for the metamorphic rocks in the paleosubduction zones of the CAOB. The radiogenic isotope systems of the UHP rocks of the KSCZ keep information on Early–Middle Paleozoic events. This permits using numerical modeling for the conclusion that the real duration of active thermal events (episodes of intense shearing and heating related to the intrusion of basic and/or granitoid massifs) is considerably less than the observed scatter of ages obtained by different isotope methods and it is usually no more than 1 Myr. The author thanks N.L. Dobretsov, Member of the Russian Academy of Sciences; M.M. Buslov, Doctor of Geology and Mineralogy; A.G. Vladimirov, Doctor of Geology and Mineralogy; V.S. Fedorovsky, Doctor of Geology and Mineralogy; A.S. Mekhonoshin, Doctor of Geology and Mineralogy; and N.I. Volkova, Doctor of Geology and Mineralogy, for useful discussion. The study was supported by the Russian Science Foundation (project no. 15-17-10010)—synthesis materials and preparation of the article, the Russian Foundation for Basic Research (projects no. 14-05-00712 and 14-05-00747)—preparation of initial data.

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