Geodynamics of Late Paleozoic Magmatism in the Tien ... - Springer Link

ISSN 00168521, Geotectonics, 2013, Vol. 47, No. 4, pp. 291–309. © Pleiades Publishing, Inc., 2013. Original Russian Text © Yu.S. Biske, D.L. Konopelko, R. Seltmann, 2013, published in Geotektonika, 2013, No. 4, pp. 61–81.

Geodynamics of Late Paleozoic Magmatism in the Tien Shan and Its Framework Yu. S. Biskea, D. L. Konopelkoa, and R. Seltmannb a

b

St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034 Russia CERCAMS, Natural History Museum, Cromwell Road, London, SW7 5BD United Kingdom email: [email protected] Received May 10, 2012

Abstract—The Devonian–Permian history of magmatic activity in the Tien Shan and its framework has been considered using new isotopic datings. It has been shown that the intensity of magmatism and composition of igneous rocks are controlled by interaction of the local thermal upper mantle state (plumes) and dynamics of the lithosphere on a broader regional scale (plate motion). The Kazakhstan paleocontinent, which partly included the presentday Tien Shan and Kyzylkum, was formed in the Late Ordovician–Early Silurian as a result of amalgamation of ancient continental masses and island arcs. In the Early Devonian, heating of the mantle resulted in the withinplate basaltic volcanism in the southern framework of the Kazakhstan paleo continent (Turkestan paleoocean) and development of suprasubduction magmatism over an extensive area at its margin. In the Middle–Late Devonian, the margins of the Turkestan paleoocean were passive; the area of withinplate oceanic magmatism shifted eastward, and the active margin was retained at the junction with the Balkhash–Junggar paleoocean. A new period of active magmatism was induced by an overall shortening of the region under the settings of plate convergence. The process started in the Early Carboniferous at the Jung gar–Balkhash margin of the Kazakhstan paleocontinent and the southern (Paleotethian) margin of the Kara kum–Tajik paleocontinent. In the Late Carboniferous, magmatism developed along the northern boundary of the Turkestan paleoocean, which was closing between them. The disappearance of deepwater oceanic basins by the end of the Carboniferous was accompanied by collisional granitic magmatism, which inherited the paleolocations of subduction zones. Postcollision magmatism fell in the Early Permian with a peak at 280 Ma ago. In contrast to Late Carbonif erous granitic rocks, the localization of Early Permian granitoids is more independent of collision sutures. The magmatism of this time comprises: (1) continuation of the suprasubduction process (Igranites, etc.) with transition to the bimodal type in the Tien Shan segment of the Kazakhstan paleocontinent that formed; (2) superposition of Agranites on the outer Hercynides and foredeep at the margin of the Tarim paleoconti nent (Kokshaal–Halyktau) and emplacement of various granitoids (I, S, and A types, up to alkali syenite) in the linear Kyzylkum–Alay Orogen; and (3) withinplate basalts and alkaline intrusions in the Tarim paleo continent. Synchronism of the maximum manifestation and atypical combination of igneous rock associa tions with spreading of magmatism over the foreland can be readily explained by the effect of the Tarim plume on the lithosphere. Having reached maximum intensity by the Early Permian, this plume could have imparted a more distinct thermal expression to collision. The localization of granitoids in the upper crust was controlled by postcollision regional strikeslip faults and antiforms at the last stage of Paleozoic convergence. DOI: 10.1134/S001685211304002X

INTRODUCTION The Tien Shan Orogen in its broad comprehension 1

from the Kyzylkum Hills in the west to the Gobi Altay in the east is the Late Paleozoic (Hercynian in the broad sense of this term) collisional structure up to 3000 km in extent. The Cenozoic withinplate activa tion has created a highly dissected mountain system. 1

The Tien Shan Mountains extend through China, Kazakhstan, Kyrgyzstan, and Uzbekistan. In each of these countries the cur rently used geographic names differ in spelling and cannot be rationally unified. In this translation, the proper names are spelled according to the local styles and thus are changeable, e.g., Tian Shan, Junggar, Yili in China and Tien Shan, Zhongar, Ili elsewhere. Translator’s comment.

The central position in the Tien Shan region is occu pied by the southern part of the Kazakhstan (Kyrgyz– Kazakh, Kazakhstan–Yili in other publications) pale ocontinent, the crust of which was formed in the Sil urian as a result of amalgamation of older, mainly Neoproterozoic continental masses and Early Paleo zoic island arcs. The Precambrian blocks comprise the microcontinents of the Middle Tien Shan, and the Moyinkum and YsykKöl massifs of the Northern Tien Shan (Fig. 1). The Central Tien Shan in publications by Chinese authors [44, 52, 78–80, 82] is a continua tion of one of these massifs (the YsykKöl Massif, in our opinion, though other interpretations are known). This narrow zone of exposed Precambrian crystalline

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Fig. 1. General scheme of tectonic regionalization of Tien Shan and location of dated granitoid plutons, modified after [74]. White background is postPaleozoic cover (main depressions are named); gray background is Paleozoic and Precambrian paleo continents paleocontinents shown by patterns. Kazakhstan paleocontinent: NETS, Northeastern Tien Shan; NTS, Northern Tien Shan; MTS, Middle Tien Shan; CTS, Central Tien Shan; KT, Karakum–Tajik and Tarim paleocontinents; STS, collision belts of the Southern Tien Shan. Sutures: B, Bayingou (Junggar); Southern Tien Shan: T, Turkestan; AI, Atbash–Inylchek; TF, Talas–Farghona StrikeSlip Fault. (1–3) Granitoid plutons dated by the authors: (1) Early Permian; (2) Late Carboniferous; (3) Early Devonian.

rocks in the eastern Tien Shan actually corresponds to its drainage divide and differs from the Northern Tien Shan in the occurrence of Silurian volcanic and sedi mentary complexes. The Northeastern (Northern in Chinese sources) Tien Shan was formed as a result of accretion of islandarc structures to the Kazakhstan paleocontinent in the Devonian and Carboniferous, whereas the Southern Tien Shan was formed due to collision of this paleocontinent with intermediate minor continental masses including the Karakum– Tajik (Baysun) paleocontinent in the extreme south west, which is also Proterozoic in age [9, 11, 13, 39]. The dynamics of the subsequent period is consid ered in this paper. The most acceptable reconstruction of this period assumes the existence of the Paleo tethian oceanic basin in the south (presentday coor dinates) and the Paleoasian oceanic basin (in a broad sense, including the Junggar–Balkhash basin) in the northeast. The Turkestan basin, which was situated between them, separated the Kazakhstan and Kara kum–Tajik paleocontinents [13, 50]. The period was completed by the Late Paleozoic collision and forma tion of contemporary northern Eurasia. Paleozoic magmatism of the Tien Shan and adja cent territories was discussed in [17–20, 31, 34, 80]. This discussion can be renewed on the basis of geo chronological data obtained over the last 10–15 years, largely using the U–Pb dating of individual zircon grains [52, 60–62, 74, 79]. The author’s results are shown in Fig. 1. The summary for the Chinese seg ment of the Tian Shan (east of 80° E and the southern slope east of Kashgar) is partly given in [52, 79]. Subdivision of the geodynamic settings of mag matic activity into divergent (rifting, spreading), con

vergent (subduction, collision), and withinplate types are now universally recognized. The latter type can be feasibly linked to a cause that does not proceed from the kinematics and energetics of lithospheric plate motion, but which is related to hot spots generated by local ascent of mantle plumes. Of course, hot spots can accompany and partly induce a breakup of continents and then spreading of the oceanic lithosphere, in par ticular, in marginal seas, and conversely, the conver gence of plates can develop with manifestations of hot spots related to autonomous deep sources. In this case, one type of magmatism is superposed on another. We will attempt to trace these combinations, having divided magmatic and geodynamic history into several stages. EARLY DEVONIAN AND EIFELIAN (416–391 Ma) As was shown in [50], the Kazakhstan paleoconti nent was surrounded in the Early Devonian by a ring of active margins with characteristic manifestations of suprasubduction magmatism. We do not consider here (Fig. 2) the initial configuration of these margins. Some of them most likely were rectilinear and then markedly modified by strikeslip faults against the background of oroclinal bending at the end of the Paleozoic [65]. The northern margin of the Kazakhstan paleoconti nent (presentday coordinates are used from here on) is more distinct and can be traced by the ranges of the Northern Tien Shan. The margin is a continuation of the Kazakhstan volcanic–plutonic belt in Central Kazakhstan to the southeast, separating its from the GEOTECTONICS

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Fig. 2. Early Devonian igneous complexes of Tien Shan and its framework. (1) Withinplate volcanic rocks: mafic and bimodal complexes; (2, 3) volcanic and plutonic com plexes at active margins of Kazakhstan paleocontinent and island arcs: (2) calc–alkaline and (3) alkaline; (4) grani toids; (5) late thrust faults and sutures that inherit mag matic fronts; (6) other faults (ticks denote thrust faults); (7) volcanic complexes of inner domains of Turkestan and Junggar–Balkhash ocean; (8) preDevonian paleoconti nents (letters in circles): TR, Tarim; KZ, Kazakhstan; KT, Karakum–Tajik; (9) postPaleozoic cover. Letter nota tions in figure: Ak, AkTüz; Al, Alay Range; Bk, Bayankol; JA, Junggar Alatau; Dn, Dananhu; Jj, JangyJer Range; Kg, Kyrgyz Range; Kk, Kekesu; Kr, Kurama Range; Ps, Pistalitau Mountains; Sk, Shokh; Tw, Tuwu; Tr, Terskey Range; He, Heyingshan; Ct, Chotqol; Tl, Talas Range. Insets to Figs 2–5 are geodynamic schemes for corre sponding periods. South or southwest on profiles is shown to the left. The continental and transitional lithosphere with suprasubduction volcanic and granitoid rocks is light; the oceanic lithosphere and withinplate mafic igneous rocks are dark; zones of anomalously hot mantle is hatched.

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The more complete Devonian volcanic section has been retained in the Kyrgyz part of the Northern Tien Shan as a continuation of the back zone of the Kaza khstan volcanic belt. The felsic volcanics that occur at the base of the section give way upsection to the vari eties more contrasting in silica content, as a rule, alka line (trachyte porphyry, trachyandesite, leucitophyre,

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It is suggested that this margin had already existed since the Silurian, as indicated, for example, by calc alkaline granitoids dated at 425–426 Ma close to the Kucha River at the northern margin of Tarim [84]. It is more probable, however, that these rocks, like similar volcanic rocks of the Southern Tien Shan [6, 7], cor respond to the preDevonian period of the Turkestan ocean that arose in the Ordovician [24]. Reconstruc tions that take into account the northward subduction of the Turkestan ocean beneath Kazakhstan before the Devonian have also been proposed for the Chinese segment [49, 69].

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inner zone of the Junggar–Balkhash paleoocean [3, 5].The width of the zone of intrusive and volcanic activity in the Early–Middle Devonian from the southern Balkhash region and the southern Junggar Alatau in the north to the Kyrgyz, Talas, and Terskey ranges in the south is about 300 km and thus is com mensurable with the size of this belt in Central Kazakhstan, as well as with the width of younger struc tures of the same kind, for example, the Cretaceous Okhotsk–Chukotka volcanic belt or the recent Andian belt. The geochemical zoning of volcanism shows that paleoseismofocal zone sloped to the south from the side of the Junggar–Balkhash paleoocean [17]. The dynamics of subduction in the eastern, Chi nese segment of the Tian Shan has also been recon structed as a continuation of the active margin of the Junggar–Balkhash (Northern Tien Shan) paleoocean [78, 79].

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etc.) and less frequent calcalkaline rocks. They are combined with coarse clastic continental sedimentary rocks with plant remains of the Early–Middle Devonian. The thickness of these sequences reaches 2000 m [34]. The U–Pb timing of accessory zircons has sup ported Early Devonian age of some rhyolites [3]. The widespread Early–Middle Devonian leucogranite, granite porphyry, syenite, and shonkinite are dated more reliably. U–Pb (SHRIMP) estimates at 414 ± 7 Ma are available for the AkTüz granite [74]; grani toids from the Talas and Kyrgyz ranges are dated at 420–391 Ma (U–Pb and Rb–Sr methods) [3, 22]. The eastern continuation of the Devonian volcanic– plutonic belt is marked by Early Devonian granite at the Bayankol River (402–413 Ma [3]) and in the Chi nese Northern Tian Shan. A Late Silurian–Early Devonian age (433–398 Ma) was established here for highK calcalkaline granite and monzodiorite and for the volcanic rocks exposed along the Kekesu River and to the west of it, which were previously regarded as Early Carboniferous [51]. These estimates have been confirmed by the peak ages of clastic zircon grains from alluvium in the basin of the Tekes River (398 Ma near the Kekesu River) [73]. Early Devonian granites that cut through the Neoproterozoic basement in the eastern part of the Chinese Northern Tian Shan are known further at a longitude of 86° E (Mount Baluntai and Mishigou dis trict), where the Itype plutons have U–Pb zircon ages of 411 and 402 Ma [49]. All of them make up a conti nental arc, i.e., an active margin, and occasionally contain older xenogenic zircons [51, 79]. To the east, a continuation of the margin is documented to the south of the Turpan–Hami Depression in the Tuwu district, where the Devonian calcalkaline volcanic rocks with positive εNd(t) = 5.6–8.8 crop out [55]. It cannot be ruled out that beyond this depression, another island arc proper existed within the Junggar– Balkhash ocean. The so far poorly studied Early– Middle Devonian (?) volcanic rocks of the Dananhu Group have been retained in the Chinese Northern Tian Shan to the west of Urumqi [66]. The southern margin of the Kazakhstan paleoconti nent was also active in the Early Devonian, at least in its western sector, as indicated by volcanic and intru sive rocks locally exposed in the Kurama and Chotqol ranges [19, 20, 34]. Subduction developed here from the side of Turkestan (Southern Tien Shan in the cited sources) basin. The volcanic rocks of its margin are known in the Kurama Range and in the Pistalitau Ridge at the southern margin of Kyzylkum. These are continuously fractionated calcalkaline and subalka line series tnat formed under subaerial conditions and are characterized by the prevalence of felsic and inter mediate rocks over basalts and by abundant volcani clastic rocks. The thickness of the section together with coarse terrigenous rocks at the base reaches 2000–3000 m.

To the north, inward the continent, the K/Na ratio in volcanics increases up to the level of shoshonite– latite series [20]. The same may be said about minor leucogranite–monzonite intrusions of the Karagata Complex, which cut through volcanic rocks. The intrusive rocks are dated at the Early Devonian, e.g., 415–416 Ma for the minor intrusion at the Karakiya River [74] and 414.3 ± 6 Ma for granodiorite at the southwestern end of the Kurama Range [28]. The width of the volcanic margin is about 200 km. The accretionary complex composed of the Lower Paleozoic and Silurian oceanic and islandarc rocks that partly metamorphosed up to green and blue schists [6, 7, 11, 36] also participates in the structure of the Early Devonian active margin in the Kurama Range and the adjacent areas of the southern and east ern Farghona. The rhyolites and andesites on the left bank of the Shokh River in the Turkestan Range [6] probably indicate that one more, southern island arc did exist in the Turkestan paleoocean and continued to evolve up to the Early Devonian. In the northern Farghona and to the east of it, suprasubduction complexes are unknown, so that the existence of the active margin of the Kazakhstan pale ocontinent remains here hypothetical. It can be sug gested, however, that its eastern part is strongly nar rowed due to the collisional leftlateral strikeslip off set and thrusting in the Late Paleozoic [6, 39] and Cenozoic [26] against the background of continental subduction along the Atbashi–Inylchek segment of the Southern Tien Shan ophiolitic suture. The frag ments of the southern active margin of the Kazakhstan paleocontinent displaced along the longitudinal left lateral strikeslip faults could have been partly retained in the Chinese Tian Shan. Pillowbasalt flows, tuffs, and hyaloclastites reach ing more than 3000 m in thickness in combination with picrites and layered mafic–ultramafic intrusions [1, 6, 25] are typical of the inner part of the Turkestan paleoocean. The age of this sequence determined from fossils is mainly Lower Devonian and locally Upper Silurian and Eifelian. The basalts occur along the entire Southern Tien Shan from the southern Aral region in the west to the Heyingshan in the eastern Chinese segment but are the most abundant in the southern and eastern Farghona. Being displaced dur ing the Late Paleozoic collision, they occur now as upper tectonic nappes. In the eastern Alay, basalts and trachybasalts occur more locally. They are everywhere associated with bathyal cherty sediments and are often overlapped by limestone with shallowwater benthic fauna. Dikes and sills of highTi dolerite, including dikeindike packets, are widespread. The Early Devonian basalts are close in composition to oceanic tholeiites but commonly differ from standard MORB by a high alka linity, prevalence of K over Na, and levated TiO2 and P2O5 contents. GEOTECTONICS

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In the southern Farghona, Devonian basalts have probably built up the Ordovician ophiolitic sections [25]; however, reliably continuous Ordovician–Devo nian sections are unknown. Overlapping of the Sil urian bathyal sedimentary rocks (graptolite slates) by basalts in the Alay Range, overlapping of the Lower Devonian limestone and chert by basalts in the Jangy Jer Range [6], and the occurrence of limestone reef lenses among basalts are more consistent with within plate (OIB) or marginalsea types of volcanism. In some places of the eastern Farghona and in the JangyJer Range, basalts intercalate with rhyolite and dacite lavas and tuffs, the chemical composition of which corresponds to their crustal sources localized in microcontinents [10]. The easternmost fragments of the Early and Middle Devonian basalts and gabbro– peridotite cumulates (392 ± 5 Ma, U–Pb method) were established in the Heyingshan–Kulehu district on the southern slopes of the Harkeshan Range. It is suggested that ophiolites and basic rocks contaminated with conti nental crustal material occur here [79]. GIVETIAN AND LATE DEVONIAN (392–359 Ma) In the southeastern Kazakhstan paleocontinent (northern and northeastern parts of the Tien Shan), the Late Devonian granitoid magmatism is related to the active margin of the Junggar–Balkhash paleo ocean. Subduction of its lithosphere beneath the Kazakhstan paleocontinent is the best substantiated for the Central Kazakhstan sector (to the north of the region under consideration), where volcanic and accretionary complexes corresponding in age occur [17, 18]. In the Northern Tien Shan, i.e., behind the subduction zone, large bodies of alkali leucogranite and granosyenite were emplaced in the Late Devo nian; alkali basaltic and gabbroic rocks, trachytes, and leucite porphyries are known as well [18]. In the Kyr gyz Range and along the Nikolaev Line, extension has been noted. Judging by plant remains, the large basins were filled with thick (up to 3000 m) Middle and Late Devonian variegated sandstone and conglomerate [34]. It cannot be ruled out that these basins are related to bimodal alkaline magmatism (Aral and Taldysui formations). Granites occur in the Shu–Ili Mountains of the Southern Kazakhstan [17] and are traced to the Chi nese Northern Tian Shan. According to [79], gabbro, diorite and granite with a U–Pb (SHRIMP) zircon age of 383 ± 9 and 357 ± 6 Ma are located closer to the frontal part of the Kazakhstan volcanic–plutonic belt in the southern Junggar Alatau and in the Borokhoro Range of the northeastern Tian Shan. To the east, the granites dated at 368–361 Ma are known to the north of the Kumux Settlement [79]. The emplacement of granitic intrusions has completed the geological his tory of the marginal Kazakhstan magmatic belt and is related to the kinematic rearrangement at the margin GEOTECTONICS

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of the Junggar–Balkhash ocean, which led to the pro gradation of the volcanic front and the origination of a new Balkash–Ili volcanic–plutonic belt. In the Chi nese Tian Shan, the position of islandarc magmatism in the Devonian and Early Carboniferous remained almost unchanged, so that active development of the Kazakhstan–Yili margin is suggested as continuous in the Devonian–Early Carboniferous [49, 69]. In the eastern part of the region, the Late Devonian magmatism of marginal continental or collision type also spread over the Southern Tien Shan up to the margin of Tarim. Granite in the Bayinbulak district dated at 378 Ma (U–Pb method) [80], and granodior ite and monzonite (382 ± 6 Ma) and gabbro–granite series (363 ± 2 Ma) in the Serikia–Kulehu district and to the east of it, were formed in the second half of the Devonian, as well as thermal (locally granulitefacies) metamorphism similar in age (Ar/Ar method, SHRIMP) [52, 79]. Late Devonian volcanic rocks of the islandarc type occur in the Middle Paleozoic sed imentary cover to the north of the Kuruktagh Moun tains. The geodynamic setting of these occurrences remains debatable. It is suggested that they were local ized behind the same seismofocal zone plunging southward from the side of the Junggar–Balkhash ocean. Such a reconstruction requires preceding clo sure of the ocean located between Tarim and Kazakh stan, which comes in conflict with stratigraphic data on the Southern Tien Shan. According to a more probable explanation, the suprasubduction magma tism in the second half of the Devonian is related to the terrane (now allochthonous) that accreted at the end of Devonian to the Kazakhstan margin from the south. We suppose that the formation of such an accreted ter rane (Erbinshan–KumuxTala, after [8]) took place at the Kazakhstan rather than the Tarim margin and did not imply that these continents were involved in the collision. On the Tarim paleocontinent proper, the Late Devonian transgression was accompanied by deposi tion of red beds followed by carbonate sedimentary rocks [45]. It is also known that, at least, to the west of KhanTengri, suprasubduction magmatism did not develop at that time [6, 10]. Stabilization is clearly noted at the northern Kazakhstan margin of the Turkestan paleoocean (the presentday Middle Tien Shan and the Kyzylkum plains), where quartz sand stone are overlapped by carbonate rocks (commonly Famennian) that mark a mature stage of passive mar gin evolution. In the inner regions of the Turkestan paleoocean, Givetian and Upper Devonian limestone and dolo mite make up carbonate platforms that conformably or transgressively overlie older rocks, including Early Devonian basalts. In other zones, the basalts are over lapped by bathyal cherty–carbonate sediments. In the south of this oceanic domain, i.e., in the Kyzylkum–Alay system of carbonate platforms, and to the north of the Tarim paleocontinent, a new region

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of withinplate magmatism was formed, which was displaced to the east relative to the Early Devonian one. As a rule, the volcanic rocks overlie shelf lime stones and give way to them upsection. The western most occurrence has been noted at the headwater of the Shokh River in the Alay Range, where highly explosive volcanic and subvolcanic rocks occur in a wide range of silica contents [29]. The Givetian and probably Frasnian volcanic rocks of Mount Baubas hata in the northeastern Farghona and eastward, in the JangyJer and Ulan ranges of the Southern Tien Shan in Kyrgyzstan, have a more homogeneous sodium subalkaline composition [10]. Similar but often more alkaline basalts of the Borkoldoy Range also erupted in the Famennian. The age of volcanic rocks was determined by organic remains in the under and overlying marine sedimentary rocks and coeval lime stone interlayers and volcanic–lithoclastic aprons on slopes [6]. Basalt on the southern slope of the Kok shaal Range has been dated at 392 ± 15 Ma (Sm–Nd isochron) [79]. EARLY CARBONIFEROUS (359–318 Ma) The suprasubduction magmatism at the northern margin of the Kazakhstan paleocontinent resumed in the Early Carboniferous with accumulation of thick piles of andesites (including highMg varieties), rhyo dacite lavas and tuffs, and less frequent basalts [17, 18, 50, 65]. The suprabsubduction magmatism in the northeast of the Tien Shan covers the southeastern limb of the Balkhash–Ili volcanic–plutonic belt and the TransIli Alatau Mountains, the contemporary depression of the Ili (Yili) River and its mountainous framework further to the north and the east, as well as southeast and south of the Junggar Alatau. The volca nic rocks occurring in the Uzun and Borokhoro Mountains are dated at 363–313 Ma (U–Pb method, SHRIMP); the oldest rocks correspond to the late Famennian [51]. In Southern Kazakhstan and the Chinese Northern Tian Shan, the intermediate and felsic volcanics are Tournaisian in age. The main phase of volcanic activ ity fell on the late Visean–Serpukhovian and is close in this respect to the western (Balkhash) sector [14, 38, 75]. The positive εNd = 0.32–4.90 (occasionally up to +10.0) and ISr = 0.7015–0.7068 are constant attributes of the volcanic rocks. The increasing К2О/Na2O ratio in some volcanic series [77, 78] does not contradict the model with southern polarity of subduction. The granitic rocks are Early Carboniferous in age [78]. The large granitic plutons of the marginal conti nental type dated as Early Carboniferous (~350 Ma) occur in the Borokhoro Range and to the south in the Narat Range up to the Southern Tian Shan Suture. In chemical composition, the rocks correspond to Itype granites related to the mantle source with insignificant participation of crustal contamination [51, 78]. The igneous rocks of marginal continental type are com

bined with shallowwater marine and continental sed imentary rocks, including coalbearing sequences. Farther to the east, the Early Carboniferous supra subduction magmatism developed in the Bogdashan Range (the Bogda–Choltagh arc) [78]. The basement of this arc is not reliably known, and it is assumed to be continental. This is partly supported by diverse igne ous rocks (basalt, andesite, dacite, largely pyroclastic) and shallowwater limestone with brachiopods [66]. Most likely, the rocks correspond to the upper part of the Lower Carboniferous sequence, which is enor mous in thickness (> 8000 m?). No reliable datings are available for gabbrodiorite and granitic intrusions. Subduction is suggested in the southern direction from the side of the Junggar ocean [13, 43]. Similar magma tism is also known to the south of the Turpan–Hami Depression in the Kangurtag or to the Choltagh Mountains, where Lower Carboniferous calcalkaline volcaniclastic rocks (354 ± 1 Ma) and related gabbro– plagiogranite intrusions (331 ± 2 Ma, SHRIMP) make up the Dananhu–Tuzkan arc [38, 53]. The arc rests on the basement of the Devonian islandarc vol canics and is accreted to the Ili (Yili), or Kazakh con tinent. As is seen from Fig. 4, the Early Carboniferous Balkhash–Ili volcanic–plutonic belt in the Southern Kazakhstan and the Junggar–Turpan sector reaches 250 km in width and thus is appreciably wider than in the northern Balkhash region. A secondary increase in width in the Permian and Mesozoic is likely related to rifting [78]. Such an expansion can hardly be signifi cant, because no manifestations of oceanic crust of that age have been noted. It is more likely that we are dealing here with remnants of two volcanic zones: the island arc proper in the north (the Bogdashan and Choltagh mountains) and the active margin of the Kazakhstan paleocontinent in the south, to the south west of the ophiolitic Bayingou Suture in the Borokhoro Range. The suture consists of ophiolitic melange with gabbro and plagiogranite blocks (325 and 344 Ma, respectively, U–Pb method, SHRIMP); Lower Carboniferous turbidites; and older bathyal cherts with Famennian–Visean microfossils [78, 79, 83, 84]. The recent strikeslip–thrust struc ture of the suture generally inherits the paleolocation of the subduction suture zone formed after the oceanic lithosphere subduction to the south in the Early Car boniferous with its subsequent accretion to the conti nent. Judging by the age of the stitching granitic intru sion (316 Ma), the Bayingou basin was closed with the formation of the suture at the end of the Early Carbon iferous [69]. It cannot be ruled out that remnants of this basin occur in a more complete form in the base ment of the Turpan–Hami Basin displaced along the rightlateral Junggar StrikeSlip Fault. The magmatic events resumed at the end of the Early Carboniferous in the Kurama Range at the southern margin of the Kazakhstan paleocontinent; this margin remained passive for a long time. The events GEOTECTONICS

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Fig. 3. Givetian–Late Devonian igneous complexes of Tien Shan and its framework. (1) Complexes of Late Devonian riftrelated basins in the Kazakhstan paleocon tinent; see Fig. 2 for other symbols. Letters in figure: Bb, Mount Baubashta; Bl, Bayinbulak; Br, Borkoldoy Range; Bk, Borokhoro Range; Km, Kumux; Kg, Kyrgyz Range; Ku, Kurugtagh Mountains; Se, Serikia; Sk, Shokh; Ul, Ulan Range; SI, Shu–Ili Mountains.

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The onset of magmatic activity is established more distinctly at the southwestern boundary of the Tien Shan, which is related to the active margin of the Paleotethys [50], and also traced in the northern Pam irs and the western Kunlun [13, 22]. In the Gissar Range and its southwestern spurs, the Visean (proba bly, also late Tournaisian) felsic volcanics overlap the Precambrian basement with deep scouring and basal conglomerate. As judged from marine fossils in sedi mentary interlayers, the Sioma Formation of rhyolitic and dacitic tuffs and lavas (up to 2200 m) in the axial zone of the Gissar Range [31] is mainly Visean in age. The bimodal association of subalkali basalts of the Karatag and Vakhshivar formations, dated as Ser pukhovian and Bashkirian from fossils in interbeds of bathyal sediments combined with rhyolite having K– Ar age of 316–320 Ma [20], occurs in the southern Gissar Range. Minor bodies of gabbro and serpenti nized peridotite of the Kundadjuaz Complex are localized in fault zones. Their relationship to basalts is not evident; no complete ophiolitic sections have been documented [19, 31]. These occurrences mark back arc rifting and subsequent partial breakup of the Kar akum–Gissar continent into the southern (Baysun) and the northern (Gissar) parts. The shortening started as early as Visean in the back zone of volcanic margin between the Gissar Range and the Zarafshon River, where the thrust faults are overlapped by the upper Visean conglomerate (data of V.I. Lavrusevich, cited in [9]). The thrusting and related thickening of the crust were accompanied by metamorphism, in particular, by the appearance of gneissic granites and migmatites super posed on the Lower Paleozoic protolith in the Zarafs hon Range. The age of this process is Visean (339– 327 Ma, SHRIMP) [28]. Later on, the marginal basin was closed (Figs. 1, 4). The calcalkaline volcanic rocks of the activemargin type were formed in the southern Gissar Range already in the Middle Carbon iferous and then overlain by marine molasse.

72°

comprised emplacement of peridotites, gabbro, and anorthosite (Shavaz Complex to the southeast of Toshkent, 345–343 Ma in Rb–Sr age) and monzogab bro (327 ± 3 Ma, U–Pb method [20]). At least the lat ter date is confirmed by the lower Serpukhovian age of comagmatic trachybasalt and trachyte of the Uin For mation. In general, magmatism of the Kurama Range is related to the recurrent subduction. The igneous rocks are characterized by elevated alkalinity of the initial phase and by the occurrence of ultramafic rocks.

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Fig. 4. Early Carboniferous igneous complexes of Tien Shan and its framework. (1) Alkaline mafic and ultramafic rocks; (2) ophiolitic complexes in sutures of Early Carbon iferous marginal basins; (3) boundaries of continental massifs inactive in Early Carboniferous (subsequent offsets are shown by arrows); (4) accretionary complexes. See Fig. 2 for other symbols. Letters in figure: Bd, Mount Bogdas han; Bn, Bayingou; Bk, Borokhoro Range; Bs, Baysun Mountains; Gs, Gissar Range; Jn, Junggar StrikeSlip Fault; Ti, TransIli Range; Zr, Zarafshon Range; Kt, Kan gurtagh Mountains; Nr, Narat Mountains; Tu, Turpan– Hami Basin; Uz, Uzun Mountains.

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The active development of the Bogdashan island arc continued in the Late Carboniferous at the north eastern margin of the Kazakhstan paleocontinent (Fig. 5) [43, 66]. The thick pile of calcalkaline volca nic rocks occurs in the Bogdashan Range together with marine sedimentary rocks. Judging by the fossil complex with Profusulinella, these rocks belong to the Bashkirian–Moscovian stages. After closure of the Bayingou marginal basin (Figs. 1, 4) no later than 316 Ma ago, the magmatic activity at the place of the former Junggar–Balkhash paleoocean should be regarded as a collisionrelated event, though it is described as a postcollision one [78]. The island arc– continental margin collision was apparently soft. In any case, the tectonic nappes of the Southern Tien Shan type have not been revealed here. The intensity of volcanic activity at the end of Late Carboniferous probably decreased, though reliable paleontological data for this period became scant because of passage to a continental setting. In the Chi nese Northern Tian Shan, i.e., within the former accretionary system at the margin of Kazakhstan pale ocontinent, widespread granites were emplaced from the Serpukhovian (327 Ma) to the Early Permian, inclusive [79]. The main body of collisionrelated granites were emplaced 320–310 Ma ago. In the Narat Range, the U–Pb age of the youngest volcanic rocks is 313 Ma and granodiorite is dated at 308 Ma [78]. In the entire region, the predominant calcalkaline gran itoids are characterized by ISr = 0.703–0.705 and the latest model Nd age is 600–460 Ma. These parameters are regarded as evidence for slab breakoff and partici pation of juvenile mantle material in magmatism. Magmatic activity waned after 310 Ma ago, and then the postcollision stage proper followed [68]. The same gap is probable in the more western and inner parts of the paleocontinent that underwent collision, i.e., in the Northern and most of the Middle (Kyrgyz) Tien Shan, where Late Carboniferous granitic rocks are much less abundant than suggested earlier [34]. The southern margin of the Kazakhstan paleoconti nent 318–320 Ma ago was a place of almost coeval events indicating rearrangement of the geodynamics. We are dealing with (1) the first southverging tectonic GEOTECTONICS

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Fig. 5. Late Paleozoic igneous complexes of Tien Shan and its framework. (1) Itype granitoids; (2) collision suture; (3) ultrahighpressure metamorphic rocks. See Fig. 2 for other symbols. Letters in figure: Aa, Almalyk; At, Atbashi Range; Bd, Mount Bogdashan; Kk, Kekesu–Yili; Nr, Narat Range; Nn, Naryn Depression; Nn, Northern Nurota Mountains.

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nappes; (2) appearance of the early Bashkirian or occasionally Serpukhovian flysch and olistostromes [9]; (3) metamorphism and exhumation of high and ultrahighgrade metamorphic rocks along the tec tonic suture from Kyzylkum to Xinjiang; and (4) vig orous, though not continuous magmatism in its north ern limb. The metamorphic complexes, including both high temperature facies, fragments of highpressure meta morphic rocks, and oceanic basalts, have been retained in the southern limb of the Southern Tien Shan collision suture at the northern slope of the Atbashi Range and to the east at the headwaters of the Kekesu River. In the Atbashi Range, the complex comprises melange with gneisses and eclogites, the peak of metamorphism (18–24 kbar) of which is dated by Sm–Nd isochron at 319 ± 4 Ma and subsequent exhumation at 316 ± 3 Ma (40Ar/39Ar age of phengite). At the Akiyaz River (headwaters of the Kekesu River), eclogites and glaucophane schists occur in a wide (up to 20 km) zone, where the outer rims of zircon grains from eclogites are dated at 319 Ma, on average [54]. In the western sector of the region, the Southern Tien Shan (Turkestan) Suture is accompanied by glau cophane schists, retrograde greenschist, and occa sional amphibolite metamorphism [4, 19]. Exhuma tion and erosion of greenschists began in the Ser pukhovian [7]. In the northern segment of the same suture zone, i.e., at the southern margin of the Kazakhstan paleo continent, magmatic activation is wellstudied in the Chotqol–Kurama region, where it is accompanied by important mineralization. The initial (Uin) magmatic phase with simulta neous transition from marine to continental paleoen vironment is characterized by trachybasalt and tra chyte eruptions. The interbeds with marine fauna are dated at the Serpukhovian or early Bashkirian. Upsec tion, separated by a hiatus, basalt, andesite, and dacite with elevated alkalinity follow; the Rb–Sr age of 317 ± 6 Ma also corresponds Bashkirian [19]. The granite porphyry minor intrusions in the Almalyk (Almaliq) ore field correspond to the same time (315 ± 1 Ma, U–Pb method, SHRIMP), as well as rearrangement of the isotopic system in the older (Karakiya) granite in the Kurama Range [74]. Judging by geological rela tionships and the Rb–Sr isochron (316 Ma), gabbro, peridotite, anorthosite, syenite, and monzonite of the Karamazar Complex are coeval [19]. The second and major magmatic phase is represented by the thick Aqcha–Nadaq Complex of trachyandesites and dac

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ites with less abundant subalkali basalts and rhyolites dated at 298–300 Ma (Rb–Sr method) [19], 305 ± 3 and 301 ± 4 Ma (U–Pb method, SHRIMP) [74]. In the Southern Tien Shan, where active collision took place at that time, Late Carboniferous magma tism most likely did not develop. The previous age esti mates based on insufficiently reliable geological con straints and K–Ar datings [31, 34] have been revised toward rejuvenation [74]. In contrast, some volcanic sequences of the Kyzylkum Hills and Northern Nurota Mountains formerly related to the Middle Carboniferous [19] turned out to be allochthonous in their structural position and they are dated as Silurian or Devonian in age based on new findings of fossils [7]. In the southern Gissar district of the southwestern Tien Shan, the lower part of the Upper Carboniferous with Bashkirian fauna in marine interbeds is com posed of andesite and rhyolite [31], which are gener ally close to the marginal continental type and locally overlap ophiolites with bathyal sediments, thus denot ing the end of backarc spreading. Minor gabbro–granite intrusive bodies, whose geological age is consistent with K–Ar datings at 320– 310 Ma, are related to the same stage. The subsequent preMoscovian deformations mark the transition to the collision stage, when large granitoid plutons of the Gissar Range were formed. The oldest of them are composed of diorite and granodiorite; they were eroded at the end of the Moscovian or somewhat later with the appearance of pebbles in the marine molasse. In general, the Gissar granites are close to the I type; their late (Early Permian ?) phases partly belong to the S type. Unfortunately, reliable isotopic datings are not yet available. EARLY PERMIAN (299–270 Ma) We refer completion of collision to the moment when the last deepwater basins disappeared in the Southern Tien Shan. A case in point is the clayey– cherty bathyal sedimentary rocks, which are no younger than Late Carboniferous here, and the turbid ites in the foredeep, which are as young as Asselian in the eastern Farghona [7, 11, 39]. Meanwhile, marine sedimentary rocks, including carbonates, are known up to Artinskian in the Tien Shan and up to Kungurian in the Tarim paleocontinent [44]. Thus, the Early Per mian stage of the Paleozoic history of the Tien Shan may be regarded as postcollision. As the datings for the western Tien Shan show (Figs. 1, 6), magmatism sharply intensified at the beginning of the Early Permian, locally predated by a break about 10 Ma long (Fig. 7). The area of magmatic activity expanded and acquired new outlines. The fol lowing regions are distinguished by the composition of igneous rocks. The former Kazakhstan paleocontinent and its active margins have been transformed into the median mass of the Central Asian Hercynides. This tectonic

unit is characterized by bimodal subalkaline magma tism that followed suprasubduction magmatism. In the northeast of the Tien Shan, i.e., at the former active margin of the Junggar–Balkhash ocean, the Early Permian granitoids are mainly exposed in the framework of the Yili Depression, where they cut through the collision structure immediately after the Late Carboniferous intrusions [69]. The temporal compositional trend is expressed in the gradual transi tion from calcalkaline to highK intrusive rocks. Both bimodal basalt–rhyolite volcanic series and calcalka line series with andesite occur here as well. The latter are also noted up to the Kyrgyz Range (Ashukoltor Formation [34]). In southern Kazakhstan and north ern Xinjiang, the magmatic activity partly continued in the MidPermian. Granitic rocks of the Borokhoro Range (on the right bank of the Yili River) have a U–Pb age of 285–266 Ma. The age of volcanic rocks in the Aulale and Narat ranges is 296–260 Ma [17, 78]. Basalts and mafic dikes in the Junggar and Turpan– Hami depressions are dated at 278–264 Ma [85]. The Permian alkali granite and quartz syenite of two plu tons at the southern margin of the Central Tian Shan in China were formed 296–276 Ma ago [58, 69]. Hightemperature granulite metamorphism dated at 299 ± 5 Ma developed in the same zone [67]. The relationship of Early Permian magmatism to rifting and strikeslip faulting is commonly empha sized [40, 71]. Indeed, judging by Ar/Ar datings of newly formed biotite in the shear zones, offsets along the Junggar (Northern Tian Shan) and other strike slip faults took place 285–245 Ma ago [64, 78]. It is noteworthy that mafic and ultramafic intru sions localized along strikeslip faults in the eastern Chinese Tian Shan and known from the related Cu– Ni mineralization (Huangshan and other deposits located to the east of the territory shown in Fig. 5) are also dated at 280 Ma. It has been noted that the initial lowTi and highMg magma parental for these intrusions melted at a high temperature. Granitic plutons were emplaced synchronously with basaltic eruptions [72]. The granitic pluton intruded along the collision boundary between the Tarim and Kazakhstan paleo continents (Atbashi–Inylchek Suture), which was also then modified by strikeslip faults. The elongated and tectonized Terekty pluton (294–291 Ma) with calc alkaline specialization is typical. Granites contain xenoliths of Precambrian rocks [62]. A similar age has been established for SongKöl–Ulan calcalkaline granitoids exposed as blocks elongated in the latitudi nal directions with offsets along the leftlateral strike slip faults [2]. In addition, the basalt–andesite–dacite rocks associated with Asselian sedimentary rocks con taining marine fauna occur in the lower part of the sec tion in the Atbashi–Inylchek StrikeSlip Fault Zone (BaybicheToo and JananToo). HighK granites of I type are localized along the late rightlateral Talas– Farghona StrikeSlip Fault, including the Aubearing GEOTECTONICS

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Fig. 6. Early Permian igneous complexes of Tien Shan and its framework. (1) Felsic and bimodal volcanics; (2) basalts, including beneath cover of younger sedimen tary rocks, after [85]; (3–5) granitoids: (3) Stype, (4) Atype, including alkaline intrusions; (5) unspecified, mainly leucogranites; (6) alkaline gabbroic rocks; (7) Permian strikeslip faults. See Figs. 2 and 4 for other symbols. Let ters in figure; At, Atbashi Range; Jd, JamanDavan Range; Yi, Yili Depression; Kl, Kalpin Mountains; Ks, Kokshaal Range; Bc, Bachu Uplift; magmatic aureoles and clusters: CK, Chotqol–Kurama; KN, Kyzylkum–Nurota; ZA, Zerafshon–Alay; SU, SonKöl–Ulan; granitioc plutons: Tr, Terekty; Mk, Makmal. Numerals in circles: 1, Kazakh stan paleocontinent and its margins; 2, South Tien Shan; 3, Tarim paleocontinent.

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Makmal pluton. They are also somewhat younger (286–279 Ma [74]. The inner regions of the former Kazakhstan paleo continent, which mainly correspond to the Northern Tien Shan, was an area of leucogranite–syenite (up to nepheline syenite) magmatism in the Early Permian. Nepheline syenite of the Akkulen pluton in the western YsykKöl region is dated at 292 Ma (U–Pb method, SHRIMP) [74]. Atype granites 285–270 Ma in age occur at the headwaters of the Yili River [85]. In the Chotqol–Kurama region, isotopic datings, though not representative so far, confirm a significant gap in time between the Late Carboniferous and Per mian stages of magmatic activity, and this is consistent with the deep erosion during that gap [19]. The Early Permian stage of magmatism begins here with the widespread Oyasay rhyolite–trachyrhyolite lavas and tuffs related to rifts and calderas combined with hypabyssal felsic intrusions, including leucogranite. The age has been determined by Asselian fauna in marine members at the base of the section. Only con tinental sedimentary rocks are known upsection. Shurabsay trachybasalt, trachyte, and trachyandesite with an Rb–Sr age of 285–282 Ma [20] are somewhat older, as well as alkaline gabbro, monzonite, syenite, and granosyenite. The felsic volcanic rocks of the Qiz ilnura Formation are the youngest. They were previ ously dated at the Early Triassic; however, according to subsequent dating, they are no younger than Early Permian [16]. The Southern Tien Shan is distinguished almost solely by intrusive granitoid magmatism, which differs in composition in the western and eastern segments of this linear orogen. In the western Kyzylkum–Alay segment, the fold– nappe structure of the Hercynides involves diverse Early Permian granitoids and alkaline intrusions occa sionally accompanied by felsic volcanics. To date, a large, but insufficiently representative in area, body of information on the age of granitic rocks has been obtained using U–Pb (SHRIMP), Rb–Sr, and Ar/Ar methods [23, 30, 59, 60, 62, 63, 74]. The available esti mates indicate a relatively narrow time interval of fel sic magmatism (295–280, less frequently 275 Ma).

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Fig. 7. Bar chart of U–Pb ages of granitoid plutons in Western and Central Tien Shan, modified after [73]. Main occurrences of Devonian–Permian basaltic magmatism (black arrows) are shown without estimation of intensity.

Most of the Early Permian intrusions in the Kyzyl kum–Alay region are normal granites and grano diorites of the I type; locally occurring synchronous Sgranites, Agranites, and nepheline syenites are also noted. Such a character is inherent to the Kyzylkum– Nurota granitic aureole in the extreme west of the Southern Tien Shan, which stands out in its high grade ore mineralization. The Zarafshon–Alay gra nitic cluster is situated to the east and oriented obliquely to the strike of the Hercynides. The granitic and alkaline magmas were especially diverse here, and occasionally they mixed with one another [30]. The datings for this district need confirmation. The high temperature cordierite–sillimanite metamorphism related to Sgranites is typical of the Nurota Moun tains and the high mountains of the Turkestan–Alay. As follows from Pb isotope ratios [46] and geological data (occurrence of Cambrian shelf sedimentary rocks), the Kyzylkum–Nurota granites could have been related to the mobilization of material from the Precambrian basement reworked and involved in thrusting. The accreted preDevonian [7] oceanic and

islandarc volcanic and sedimentary rocks, i.e., a partly juvenile material, has suggested as another source of Igranites in the Turkestan–Alay region [76]. The Permian Igranitoids in the Kyzylkum–Turke stan Range are not related to the Late Carboniferous subduction zone and are commonly localized in the lower tectonic sheets of the collision complex. In the eastern Tarim sector of the Southern Tien Shan Orogen (Kokshaal Range–Khalyktau), the Early Permian magmatism is characterized by the prevalence of Agranites, which make up a linear belt subdivided into several clusters. They are largely located in the fold–nappe complexes of the outer zone of the Southern Tien Shan Hercynides but also occur to the south in basement inliers and in the Tarim Fore deep [52, 68]. Atype granites occupy wide areas in the Kokshaal aureole. Their composition varies from rapakivilike biotite–anphibole granite to topaz leucogranite [57, 60, 62, 74, 77]. Mafic rocks are asso ciated with granites as inclusions and separate minor intrusions and dikes. Dikes are also known in the west GEOTECTONICS

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ern wall of the Talas–Farghona StrikeSlip Fault but remain undated. Alkaline gabbro and syenite occur in the Atbashi Range. As follows from U–Pb dating, all aforementioned intrusions were emplaced in the Per mian 295–275 Ma ago [57, 60–62, 74]. The early fel sic volcanics associated with fossiliferous sedimentary rocks are dated close to the Carboniferous–Permian boundary [9]. The aureoles of granitic plutons are accompanied by lowgrade thermal metamorphism, mostly in the eastern Kokshaal Range, in the moun tainous cluster of the KhanTengry– Jengish (Pobeda) peaks, and further to the east. The Tarim crystalline basement affected by hot mantle material probably was a source of granites. This is supported by the model Nd age (1430–1050 Ma) and low negative εNd values (–1.6 to –6.9) [60]. Thus, the localization of granites is consistent with northward underthrusting of the Tarim continent beneath the Southern Tien Shan nappes up to the Atbashi–Inylchek Suture, as was noted by E.V.Khris tov, cited after [10], and confirmed by new seismic sounding results [26]. In that case, the formation of granitic magma seemingly could have been related to the collisional underthrusting and stacking of tectonic nappes with significant thickening of the continental Tien Shan crust [76]. Attention should be drawn (Fig. 8), however, to the very short time span between the propa gation of nappes with the participation of Asselian rocks, i.e., no earlier than 300 Ma, and the emplace ment of granitic magma therein (297 ± 4 Ma for the Jangart pluton) [73]. This suggests that the crust of the Tarim paleocontinent began to heat before thrusting. Thus, the arrangement of granitic rocks and the most probable geodynamics of magmatic activity in the Tarim sector were approximately the same as in the Qizilkum–Alay region; however, the composition of granites is more uniform due to a common continental magma source. In the Tarim paleocontinent in front of the Kokshaal nappes, the Early Permian magmatism involves bimo dal withinplate association with widespread basalts. The plateau basalts of the Tarim paleocontinent, which have been known for a long time from the Kal pin Mountains [37, 42, 58], have been recently drilled over an area no less than 200000 km2, where their thickness reaches a few hundred meters. The subalkali basalts contain 2–5 wt % TiO2 and are depleted in MgO [72]. Typical εNd(t) values vary from –9.2 to –1.7, probably reflecting a different degree of contamina tion of asthenospheric mantle material with crustal rocks of the Tarim basement. The age of basalts and related alkaline gabbro, which was estimated using modern methods, is close to 280 Ma [52, 72]. Gabbro, ultramafic rocks, and sporadic quartz syenite of the A type crop out at the Bachu Uplift in the field of basalts. Their age is 285–275 Ma, and comagmatic volcanic rocks are dated at 289–267 Ma [43, 71, 85]. In the Karakum–Tajik paleocontinent, i.e., in the extreme southwest of the Tien Shan, the published K–Ar GEOTECTONICS

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age determinations and paleontological data [19, 27, 34] broadly confirm the abundance of Early Permian alkali granites and less frequent felsic volcanic rocks. Their combination with alkali basalts also indicates bimodal postcollision magmatism; however, the lack of reliable datings does not allow more definite time constraints. DISCUSSION AND CONLUSIONS The Early Devonian magmatic processes in the Tien Shan part of the Kazakhstan paleocontinent are described in terms of the classic model of active mar gins. The boundary with the Junggar–Balkhash ocean expressed in the Kazakhstan volcanic–plutonic belt has been retained better than the southern boundary in the Kurama Range. In general, it is possible to draw an analogy with Cenozoic evolution of the Philippine Archipelago. The northern Kazakhstan belt is, at least, rather wider. By analogy with similar younger struc tures, the large width may be a result of lowangle sub duction zone, which, in turn, reflects a high tempera ture, insignificant thickness, and buoyancy of the sub ducting plate. The general predominance of convergence in the Early Devonian Paleoasian ocean and corresponding shortening [50] was apparently caused by the conver gence of lithospheric plates on a regional scale. It is important, however, that this process was accompa nied mainly by withinplate basaltic magmatism over the entire length of its Turkestan branch. Independent heating of the sublithospheric mantle over a vast terri tory due to internal causes (ascending heat and mass transfer in form of plumes) should have led to intensi fication and expansion of magmatism at the continen tal margins. In the second half of the Devonian (Fig. 3), the convergence at the northern margin of the Kazakhstan paleocontinent continued with a shift of the volcanic belt and intensification of granitic magmatism due to accretion of the island arc to the paleocontinent. To the east of 82° E, the suprasubduction magma genera tion has also penetrated the Southern Tien Shan. It cannot be ruled out that subduction developed here from the side of the Turkestan ocean, unless we are dealing here with fragments of the same northern mar gin of the Kazakhstan paleocontinent, which were transported to the south by susbsequent tectonic nappes. The development of another part of the region was passive with indications of rifting in the Kazakh paleocontinent. A hotspot shift is marked by basaltic eruptions in the Turkestan basin. First they were local ized in the eastern Farghona (the Ulan Range), where their presentday aureole is about 500 km long, and then migrated to the east. In the Late Devonian, igne ous complexes pertaining to hotspottype and coeval rifting are also widespread in Northern Kazakhstan [14] and in other continents of northern Eurasia.

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Fig. 8. Tectonic scheme of the area adjacent to the Jangart pluton in eastern Kokshaal Range. (1) Granites of A and S types (J, Jangart pluton); (2) collisionrelated tectonic nappes and later strikeslip faults; (3) Lower Permian tur bidites and olistostromes in the roof of the Jangart pluton; (4) Paleozoic and Precambrian rocks of the Middle Tien Shan (MTS) and Tarim (Tr); (5) Paleozoic rocks in nappes of the South Tien Shan (Devonian withinplate basalts are shown); (6) strike and dip symbol.

In the Early Carboniferous (Fig. 4), the formation of calcalkaline series of marginal continental type, which resumed from Tournaisian in the northeast of the presentday Tien Shan and from Visean in the Gis sar and the northern Pamirs, was without any doubt related to the renewal of subduction at the boundaries of the Kazakhstan paleocontinent. The general con vergence of the Paleoasian and Paleotethian oceanic plates was a background that resembles the recent set ting of the Indopacific. Ophiolites that formed at the end of the Early Carboniferous are known at the boundaries of the Karakum–Tajik continents (south ern Gissar, northern Pamirs) and along the northeast ern margin of the Kazakhstan paleocontinent (Bay ingou). Opening of the marginal basins designated by these ophiolites could not have been significant under general shortening settings irrespective of magma type, so that deepwater basins disappeared right away. Only mafic–ultramafic rocks with Visean isotopic age may likely be classified as manifestations of within plate magmatism in the Kurama Range. Since the Bashkirian, plate convergence (Fig. 5) has led to collision of continents with formation of nappes, olistostromes, and exhumation of high and ultrahighpressure metamorphic rocks along the southern margins of the Kazakhstan paleocontinent. These features are supplemented by granitoid magma tism with predominance of calcalkaline volcanic rocks and Itype granites. At the northern Junggar– Balkhash continental margin and the southern Kara kum–Tajik margin, this magmatism inherited the activemarginal processes of the preceding stage. As

concerns the southern boundary of the ancient Kazakhstan, it does not reveal such inheritance. Mag matism of the Beltau–Quarama belt developed only at the collision stage in the back zone of thrusting and was related to the thickening of the Earth’s crust. Local intensification of magmatism in the west of the Tien Shan could also have been induced by the same cause as the appearance of mafic and ultramafic melts in the Visean (345 Ma ago). This phenomenon has been described as the Chotkol–Quarama plume [20, pp. 156, 157]. However, the localization of this plume in the same district from Silurian to Mesozoic does not seem realistic. The nature of the Early Permian postcollision mag matic events and related ore resources, including gold, is perhaps the most interesting in the Tien Shan. The orthogonal rigid collision of continents, which is especially evident in the central and eastern parts of the Tien Shan, should induce fold–nappe thickening of the continental crust up to values close to the presentday 50–60 km and subsequent heating of its lower part above the solidus temperature. The dissipa tive heating related to movement of lithospheric plates and the hampered release of the radioactive heat could have been additional factors. The model of simple col lision thickening of the crust is applicable to the colli sion orogens with the predominance of palingenetic granites close to the S type, which are products of melting of the ancient granitegneiss basement or sed imentary rocks deposited owing to its denudation. Such an origin should be reflected in the older model Nd ages of granites, negative εNd(t) values, and high initial 87Sr/86Sr ratios. In addition, the scenario of the partial subduction of a delaminated crustal sheet is discussed. Advanced subduction of such a sheet is hampered by its own ongoing collision and buoyancy. In this scenario, the melts creating orogenic granitoids are products of interaction of both the subducting continental crust and the mantle of the hanging plate with the hot asthenospheric material penetrating the fault zone [15, 35, 47]. As applied to the eastern Tien Shan, delamination of the collision structure roots in the Early Permian has been advocated by many authors, e.g., [52, 69]. The assumption of complete slab brea koff allows us to deal with the diversity of granitoid magmas and to explain the appearance of highalka line melts with attributes of partial mantle origin, as well as a new temperature pulse for the generation of Itype magma in the lower crust. The plunge of the slab should be vertical or retains a horizontal compo nent of the former subduction (Fig. 9), giving birth to diverse magmas in the suprasubduction zone, includ ing due to metasomatic alteration of the mantle wedge in the case of Itype granitoid generation. Nevertheless, the localization of Early Permian granitoids in the Tien Shan (Fig. 6) cannot be explained completely by these two assumptions. The intrusive bodies and volcanic fields are not strictly GEOTECTONICS

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Fig. 9. Model of postcollision Early Permian magmatism in the Tien Shan. (1) Basalts; (2) felsic volcanics; (3) A and Itype granitoids; (4) suture with subsequent strike slip offset; (5) tectonic delamination of crust by collision related thrust faults.

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The authors, proceeding from the plume hypothe sis, suggest that the Tarim basalts are a manifestation of an ascending mantle flow spreading beneath the lithosphere, whereas highMg gabbro and ultramafic rocks in the eastern Tian Shan mark the head of the plume [72]. Meanwhile, the Early Permian ultramafic rocks are also known in other districts of the Tian Shan. For example, the Permian basalts cover the basement of the Junggar and the Turpan–Hami depressions in the northeastern framework of the Tian Shan. Their age here is even younger [85], though col lision was completed simultaneously or somewhat ear lier than in the Southern Tian Shan. Mafic rocks (300–274 Ma) occur in the Beishan rift basins [72] and are known to the east in Mongolia [71]. Thus, here we are more likely dealing with almost coeval different plumes or with a Permian large igneous province. The idea of the Tarim plume as a particular manifestation of this giant event in northern Asia has been discussed rather thoroughly [41, 71, 72]. At present, we have no crucial petrologic or geochemical evidence that would show that the territory of the Tien Shan belongs to a

Tarim

Therefore, bimodal magmatism at the foreland cannot be a direct implication of collision and must have a separate cause of its own. A slab breakoff can be such a cause. According to some calculations, this is possible, owing to density instability, when the lithos phere doubles its thickness up to 300–400 km [15]. It also should be kept in mind that the unstable density and related delamination of the mantle lithosphere are more feasible if the temperature of the asthenosphere rises in advance due to the autonomous ascending flow, i.e., in mantle plume scenario.

NTLE ERIC MA H P S O H LIT

confined to any suprasubduction zone(s). They are distributed nonuniformly and, in addition, are com bined in time and space with direct derivatives of man tle melts, i.e., with basalts variable in alkalinity. The appearance of alkali plateau basalts at the foreland of subducting Tarim paleocontinent simultaneously with an outburst of granitic magmatism ~280 Ma ago is especially characteristic. It is as though the last colli sion nappes of the Kokshaal Range have been almost immediately cut through by granites. Such an event is not typical of orogeny. This event occurs with a long lag (for example, Permian basalts of northern Ger many that formed after the Variscan collision in the Early Carboniferous [40] or Triassic basalts on the western margin of the Urals; elsewhere basalts do not occur at all.

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single area of the lower mantle superplume that con trols upper mantle ascending flows [21]. We are not sure that its formation took place only in the Early Permian. The data presented above allow us to suggest that the ascending mantle flows reached the lithos phere at the end of Devonian and at the beginning of Late Carboniferous. In the latter case (Chotqol– Kurama aureole), we can also see a precursor of the more extensive Early Permian event. From the practical viewpoint, it is more important to discuss the ways along which the deepseated matter and energy of the plume are transferred to the surface. In addition to the abovementioned conjectural frac ture zones in the subducting slab, a case in point is extension zones in the crust related to the deep and long strikeslip faults [40, 47, 59, 70, 81, 82]. It is assumed that Early Permian rifting (Yili Basin, etc.) is entirely a result of postcollision transtension and not related to a plume [78]. Some cases of granite emplacement into the faultline zone have been dis cussed above. It may be added that the linear, nearly meridional arrangement of granitic plutons at the northern margin of the Tarim Platform suggests a deep pullapart system in the nearlatitudinal direction. Granites mainly occur in the areas of highest colli sionrelated stacking of upper crustal masses and are often situated in the cores of antiforms intruding into the lower tectonic nappes. In general, however, the geological map does not show any significant links of granitic plutons to collision structures. Thus, the convergence of lithospheric plates in the Late Paleozoic, which was completed by collision of continental masses in the Late Carboniferous and cre ated the recent Tien Shan, can satisfactorily explain the known magmatic processes. Withinplate magma tism, however, remains beyond the scope of this model. This magmatic process was intense in the Early Devonian and less intense in the Late Devonian– Early Carboniferous, but is mainly expressed in the Early Permian postcollision setting. Sporadic distribu tion of granitoid intrusions unrelated to collision sutures and particularly intense basaltic magmatism covering even the continental foreland compel us to suggest an additional internal source of heating of the sublithospheric mantle (plume) independent of colli sion, which reached the bottom of the lithosphere 290–275 Ma ago. ACKNOWLEDGMENTS This study was supported by research grants of St. Petersburg State University (nos. 3.0.93.2010 and 3.37.91.2011) and the IGCP592 project funded by IUGSUNESCO.

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