Hercynian post-collisional magmatism in the context of Paleozoic magmatic evolution of the Tien Shan...

ISSN 0016�8521, 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, Vol. 47, No. 4, pp. 61–81.

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INTRODUCTION

The Tien Shan Orogen in its broad comprehension

from the Qizilqum Hills1 in the west to the Gobi Altay

in the east is the Late Paleozoic (Hercynian in thebroad sense of this term) collisional structure up to3000 km in extent, which is accessible for investigationowing to the Cenozoic within�plate activation, which

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 berationally unified. In this translation, the proper names arespelled 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.

has created a highly dissected mountain system. Thecentral position in the Tien Shan region is occupied bythe southern part of the Kazakhstan (Kyrgyz–Kazakh,Kazakhstan–Yili in other publications) paleoconti�nent, the crust of which was formed in the Silurian asa result of amalgamation of older, mainly Neoprotero�zoic continental masses and Early Paleozoic islandarcs. The Precambrian blocks comprise the microcon�tinents of the Middle Tien Shan, and the Moyynqumand Ysyk�Köl massifs of the Northern Tien Shan (Fig. 1).The Central Tien Shan in publications by Chineseauthors [44, 52, 78–80, 82] is a continuation of one ofthese massifs (the Ysyk�Köl Massif, in our opinion,though other interpretations are known). This narrow

Geodynamics of Late Paleozoic Magmatism in the Tien Shan and Its Framework

Yu. S. Biskea, D. L. Konopelkoa, and R. Seltmannb

a St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034 Russiab CERCAMS, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

e�mail: [email protected] May 10, 2012

Abstract—The Devonian–Permian history of magmatic activity in the Tien Shan and its framework has beenconsidered using new isotopic datings. It has been shown that the intensity of magmatism and compositionof igneous rocks are controlled by interaction of the local thermal upper mantle state (plumes) and dynamicsof the lithosphere on a broader regional scale (plate motion). The Kazakhstan paleocontinent, which partlyincluded the present�day Tien Shan and Qizilqum, was formed in the Late Ordovician–Early Silurian as aresult of amalgamation of ancient continental masses and island arcs. The Devonian began here with heatingof the mantle that resulted in the within�plate basaltic volcanism in the southern framework of the Kazakh�stan paleocontinent (Turkestan paleoocean) and development of suprasubduction magmatism over an exten�sive area at its margin. In the Middle–Late Devonian, the margins of the Turkestan paleoocean were passive;the area of within�plate oceanic magmatism shifted eastward, and the active margin was retained at the junc�tion with the Balqash–Junggar paleoocean. A new period of active magmatism was induced by an overallshortening of the region under conditions of plate convergence. The process started in the Early Carbonifer�ous at the Junggar–Balqash margin of the Kazakhstan paleocontinent and the southern (Paleotethian) mar�gin of the Qarakum–Tajik paleocontinent. In the Late Carboniferous, magmatism developed along thenorthern boundary of the Turkestan paleoocean, which was closing between them. The disappearance ofdeepwater oceanic basins by the end of the Carboniferous was accompanied by collisional granitic magma�tism, which inherited the 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 (I�granites, etc.)with transition to the bimodal type in the Tien Shan segment of the Kazakhstan paleocontinent that formed;(2) superposition of A�granites on the outer Hercynides and foredeep at the margin of the Tarim paleoconti�nent (Kakshaal–Halyktau) and emplacement of various granitoids (I, S, and A types, up to alkali syenite) inthe linear Qizilqum–Alay Orogen; and (3) within�plate basalts and alkaline intrusions in the Tarim paleocon�tinent. Synchronism of the maximum manifestation and atypical combination of igneous rock associationswith spreading of magmatism over the foreland can be readily explained by the effect of the Tarim plume onthe lithosphere. Having reached maximum intensity by the Early Permian, this plume could have imparted amore distinct thermal expression to collision. The localization of granitoids in the upper crust was controlledby postcollision regional strike�slip faults and antiforms at the last stage of Paleozoic convergence.

DOI: 10.1134/S001685211304002X

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zone of exposed Precambrian crystalline rocks in theeastern Tien Shan actually corresponds to its drainagedivide and differs from the Northern Tien Shan in theoccurrence of Silurian volcanic and sedimentary com�plexes. The Northeastern (Northern in Chinesesources) Tien Shan was formed as a result of accretionof island�arc structures to the Kazakhstan paleoconti�nent in the Devonian and Carboniferous, whereas theSouthern Tien Shan was formed due to collision of thispaleocontinent with intermediate minor continentalmasses including the Qaraqum–Tajik (Baysun) paleo�continent in the extreme southwest, which is also Pro�terozoic in age [9, 11, 13, 39].

The dynamics of the subsequent period is consid�ered in this paper. The most acceptable reconstructionof this period assumes the existence of the Pale�otethian oceanic basin in the south (present�day coor�dinates) and the Paleoasian oceanic basin (in a broadsense, including the Junggar–Balqash basin) in thenortheast. The Turkestan basin, which was situatedbetween them, separated the Kazakhstan andQaraqum–Tajik paleocontinents [13, 50]. The periodwas completed by the Late Paleozoic collision and for�mation 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 zircongrains [52, 60–62, 74, 79]. The author’s results areshown in Fig. 1. The summary for the Chinese seg�ment of the Tian Shan (east of 80° E and the southernslope 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 within�plate typesare now universally recognized. The latter type can befeasibly linked to a cause that does not proceed fromthe kinematics and energetics of lithospheric platemotion, but which is related to hot spots generated bylocal ascent of mantle plumes. Of course, hot spots canaccompany and partly induce a breakup of continentsand then spreading of the oceanic lithosphere, in par�ticular, in marginal seas, and conversely, the conver�gence of plates can develop with manifestations of hotspots related to autonomous deep sources. In this case,one type of magmatism is superposed on another. Wewill attempt to trace these combinations, havingdivided magmatic and geodynamic history into severalstages.

EARLY DEVONIAN AND EIFELIAN(416–391 Ma)

As was shown in [50], the Kazakhstan paleoconti�nent was surrounded in the Early Devonian by a ringof active margins with characteristic manifestations ofsuprasubduction magmatism. We do not consider here(Fig. 2) the initial configuration of these margins.Some of them most likely were rectilinear and thenmarkedly modified by strike�slip faults against thebackground of oroclinal bending at the end of thePaleozoic [65].

The northern margin of the Kazakhstan paleoconti�nent (present�day coordinates are used from here on)is more distinct and can be traced by the ranges of theNorthern Tien Shan. The margin is a continuation of

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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 post�Paleozoic cover (main depressions are named); gray background is Paleozoic and Precambrian paleo�continents paleocontinents shown by patterns. Kazakhstan paleocontinent: NETS, Northeastern Tien Shan; NTS, NorthernTien Shan; MTS, Middle Tien Shan; CTS, Central Tien Shan; QT, Qaraqum–Tajik and Tarim paleocontinents; STS, collisionbelts of the Southern Tien Shan. Sutures: B, Bayingou (Junggar); Southern Tien Shan: T, Turkestan; AI, Atbash–Inylchek; TF,Talas–Farghona Strike�Slip Fault. (1–3) Granitoid plutons dated by the authors: (1) Early Permian; (2) Late Carboniferous; (3)Early Devonian.

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GEODYNAMICS OF LATE PALEOZOIC MAGMATISM IN THE TIEN SHAN 293

the Kazakhstan volcanic–plutonic belt in CentralKazakhstan to the southeast, separating its from theinner zone of the Junggar–Balqash paleoocean [3,5].The width of the zone of intrusive and volcanicactivity in the Early–Middle Devonian from thesouthern Balqash region and the southern ZhongarAlatau in the north to the Kyrgyz, Talas, and Terskeyranges in the south is about 300 km and thus is com�mensurable with the size of this belt in Central Kaza�khstan, as well as with the width of younger structuresof the same kind, for example, the CretaceousOkhotsk–Chukotka volcanic belt or the recentAndian belt. The geochemical zoning of volcanismshows that paleoseismofocal zone sloped to the southfrom the side of the Junggar–Balqash 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 theJunggar–Balqash (Northern Tien Shan) paleoocean[78, 79].

It is suggested that this margin had already existedsince the Silurian, as indicated, for example, by calc�alkaline granitoids dated at 425–426 Ma close to theKucha River at the northern margin of Tarim [84]. It ismore probable, however, that these rocks, like similarvolcanic rocks of the Southern Tien Shan [6, 7], cor�respond to the pre�Devonian period of the Turkestanocean that arose in the Ordovician [24]. Reconstruc�tions that take into account the northward subductionof the Turkestan ocean beneath Kazakhstan before theDevonian have also been proposed for the Chinesesegment [49, 69].

The more complete Devonian volcanic section hasbeen retained in the Kyrgyz part of the Northern TienShan as a continuation of the back zone of the Kaza�khstan volcanic belt. The felsic volcanics that occur atthe base of the section give way upsection to the vari�

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Fig. 2. Early Devonian igneous complexes of Tien Shanand its framework. (1) Within�plate volcanic rocks: maficand bimodal complexes; (2, 3) volcanic and plutonic com�plexes at active margins of Kazakhstan paleocontinent andisland 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 andJunggar–Balqash ocean; (8) pre�Devonian paleoconti�nents (letters in circles): TR, Tarim; KZ, Kazakhstan; QT,Qaraqum–Tajik; (9) post�Paleozoic cover. Letter nota�tions in figure: Ak, Ak�Tüz; Al, Alay Range; Bk, Bayankol;JA, Junggar Alatau; Dn, Dananhu; Jj, Jangy�Jer Range;Kg, Kyrgyz Range; Kk, Kekesu; Qr, Qurama Range; Ps,Pistalitau Mountains; Sk, Shokh; Tw, Tuwu; Tr, TerskeyRange; 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 shownto the left. The continental and transitional lithospherewith suprasubduction volcanic and granitoid rocks is light;the oceanic lithosphere and within�plate mafic igneousrocks are dark; zones of anomalously hot mantle ishatched.

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eties more contrasting in silica content, as a rule, alka�line (trachyte porphyry, trachyandesite, leucitophyre,etc.) and less frequent calc�alkaline rocks. They arecombined with coarse clastic continental sedimentaryrocks with plant remains of the Early–Middle Devo�nian. 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]. Thewidespread Early–Middle Devonian leucogranite,granite porphyry, syenite, and shonkinite are datedmore reliably. U–Pb (SHRIMP) estimates at 414 ± 7Ma are available for the Ak�Tüz granite [74]; grani�toids from the Talas and Kyrgyz ranges are dated at420–391 Ma (U–Pb and Rb–Sr methods) [3, 22].The eastern continuation of the Devonian volcanic–plutonic belt is marked by Early Devonian granite atthe Bayankol River (402–413 Ma [3]) and in the Chi�nese Northern Tian Shan. A Late Silurian–EarlyDevonian age (433–398 Ma) was established here forhigh�K calc�alkaline granite and monzodiorite and forthe volcanic rocks exposed along the Kekesu River andto the west of it, which were previously regarded asEarly Carboniferous [51]. These estimates have beenconfirmed by the peak ages of clastic zircon grainsfrom alluvium in the basin of the Tekes River (398 Manear the Kekesu River) [73].

Early Devonian granites that cut through theNeoproterozoic basement in the eastern part of theChinese Northern Tian Shan are known further at alongitude of 86° E (Mount Baluntai and Mishigou dis�trict), where the I�type plutons have U–Pb zircon agesof 411 and 402 Ma [49]. All of them make up a conti�nental arc, i.e., an active margin, and occasionallycontain older xenogenic zircons [51, 79]. To the east,a continuation of the margin is documented to thesouth of the Turpan–Hami Depression in the Tuwudistrict, where the Devonian calc�alkaline volcanicrocks with positive εNd(t) = 5.6–8.8 crop out [55]. Itcannot be ruled out that beyond this depression,another island arc proper existed within the Junggar–Balqash ocean. The so far poorly studied Early–Mid�dle Devonian (?) volcanic rocks of the DananhuGroup have been retained in the Chinese NorthernTian Shan to the west of Urumqi [66].

The southern margin of the Kazakhstan paleoconti�nent was also active in the Early Devonian, at least inits western sector, as indicated by volcanic and intru�sive rocks locally exposed in the Qurama and Chotqolranges [19, 20, 34]. Subduction developed here fromthe side of Turkestan (Southern Tien Shan in the citedsources) basin. The volcanic rocks of its margin areknown in the Qurama Range and in the PistalitauRidge at the southern margin of Qizilqum. These arecontinuously fractionated calc�alkaline and subalka�line series tnat formed under subaerial conditions andare 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 reaches2000–3000 m.

To the north, inward the continent, the K/Na ratioin volcanics increases up to the level of shoshonite–latite series [20]. The same may be said about minorleucogranite–monzonite intrusions of the KaragataComplex, which cut through volcanic rocks. Theintrusive rocks are dated at the Early Devonian, e.g.,415–416 Ma for the minor intrusion at the KarakiyaRiver [74] and 414.3 ± 6 Ma for granodiorite at thesouthwestern end of the Qurama Range [28]. Thewidth of the volcanic margin is about 200 km.

The accretionary complex composed of the LowerPaleozoic and Silurian oceanic and island�arc rocksthat partly metamorphosed up to green and blueschists [6, 7, 11, 36] also participates in the structure ofthe Early Devonian active margin in the QuramaRange and the adjacent areas of the southern and east�ern Farghona. The rhyolites and andesites on the leftbank of the Shokh River in the Turkestan Range [6]probably indicate that one more, southern island arcdid exist in the Turkestan paleoocean and continued toevolve up to the Early Devonian.

In the northern Farghona and to the east of it,suprasubduction complexes are unknown, so that theexistence 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 left�lateral strike�slip off�set and thrusting in the Late Paleozoic [6, 39] andCenozoic [26] against the background of continentalsubduction along the Atbashi–Inylchek segment ofthe Southern Tien Shan ophiolitic suture. The frag�ments of the southern active margin of the Kazakhstanpaleocontinent displaced along the longitudinal left�lateral strike�slip faults could have been partly retainedin the Chinese Tian Shan.

Pillow�basalt flows, tuffs, and hyaloclastites reach�ing more than 3000 m in thickness in combinationwith picrites and layered mafic–ultramafic intrusions[1, 6, 25] are typical of the inner part of the Turkestanpaleoocean. The age of this sequence determined fromfossils is mainly Lower Devonian and locally UpperSilurian and Eifelian. The basalts occur along theentire Southern Tien Shan from the southern Aralregion in the west to the Heyingshan in the easternChinese segment but are the most abundant in thesouthern and eastern Farghona. Being displaced dur�ing the Late Paleozoic collision, they occur now asupper tectonic nappes.

In the eastern Alay, basalts and trachybasalts occurmore locally. They are everywhere associated withbathyal cherty sediments and are often overlapped bylimestone with shallow�water benthic fauna. Dikesand sills of high�Ti dolerite, including dike�in�dikepackets, are widespread. The Early Devonian basaltsare close in composition to oceanic tholeiites butcommonly differ from standard MORB by a high alka�

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linity, prevalence of K over Na, and levated TiO2 andP2O5 contents.

In the southern Farghona, Devonian basalts haveprobably 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) bybasalts in the Alay Range, overlapping of the LowerDevonian limestone and chert by basalts in the Jangy�Jer Range [6], and the occurrence of limestone reeflenses among basalts are more consistent with within�plate (OIB) or marginal�sea types of volcanism.

In some places of the eastern Farghona and in theJangy�Jer Range, basalts intercalate with rhyolite anddacite lavas and tuffs, the chemical composition ofwhich corresponds to their crustal sources localized inmicrocontinents [10]. The easternmost fragments ofthe Early and Middle Devonian basalts and gabbro–peridotite cumulates (392 ± 5 Ma, U–Pb method) wereestablished in the Heyingshan–Kulehu district on thesouthern slopes of the Harkeshan Range. It is suggestedthat 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 tothe active margin of the Junggar–Balqash paleoocean.Subduction of its lithosphere beneath the Kazakhstanpaleocontinent is the best substantiated for the CentralKazakhstan sector (to the north of the region underconsideration), where volcanic and accretionary com�plexes corresponding in age occur [17, 18]. In theNorthern Tien Shan, i.e., behind the subduction zone,large bodies of alkali leucogranite and granosyenitewere emplaced in the Late Devonian; alkali basalticand gabbroic rocks, trachytes, and leucite porphyriesare known as well [18]. In the Kyrgyz Range and alongthe Nikolaev Line, extension has been noted. Judgingby plant remains, the large basins were filled with thick(up to 3000 m) Middle and Late Devonian variegatedsandstone and conglomerate [34]. It cannot be ruledout that these basins are related to bimodal alkalinemagmatism (Aral and Taldysui formations).

Granites occur in the Shu–Ili Mountains of theSouthern Kazakhstan [17] and are traced to the Chi�nese Northern Tian Shan. According to [79], gabbro,diorite and granite with a U–Pb (SHRIMP) zirconage of 383 ± 9 and 357 ± 6 Ma are located closer to thefrontal part of the Kazakhstan volcanic–plutonic beltin the southern Junggar Alatau and in the BorokhoroRange of the northeastern Tian Shan. To the east, thegranites dated at 368–361 Ma are known to the northof the Kumux Settlement [79]. The emplacement ofgranitic intrusions has completed the geological his�tory of the marginal Kazakhstan magmatic belt and is

related to the kinematic rearrangement at the marginof the Junggar–Balqash ocean, which led to the pro�gradation of the volcanic front and the origination of anew Balkash–Ili volcanic–plutonic belt. In the Chi�nese Tian Shan, the position of island�arc magmatismin the Devonian and Early Carboniferous remainedalmost unchanged, so that active development of theKazakhstan–Yili margin is suggested as continuous inthe Devonian–Early Carboniferous [49, 69].

In the eastern part of the region, the Late Devonianmagmatism of marginal continental or collision typealso spread over the Southern Tien Shan up to themargin of Tarim. Granite in the Bayinbulak districtdated at 378 Ma (U–Pb method) [80], and granodior�ite and monzonite (382 ± 6 Ma) and gabbro–graniteseries (363 ± 2 Ma) in the Serikia–Kulehu district andto the east of it, were formed in the second half of theDevonian, as well as thermal (locally granulite�facies)metamorphism similar in age (Ar/Ar method,SHRIMP) [51, 79]. Late Devonian volcanic rocks ofthe island�arc type occur in the Middle Paleozoic sed�imentary cover to the north of the Kuruktagh Moun�tains. The geodynamic setting of these occurrencesremains debatable. It is suggested that they were local�ized behind the same seismofocal zone plungingsouthward from the side of the Junggar–Balqashocean. Such a reconstruction requires preceding clo�sure of the ocean located between Tarim and Kazakh�stan, which comes in conflict with stratigraphic dataon the Southern Tien Shan. According to a moreprobable explanation, the suprasubduction magma�tism in the second half of the Devonian is related to theterrane (now allochthonous) that accreted at the endof Devonian to the Kazakhstan margin from the south.We suppose that the formation of such an accreted ter�rane (Erbinshan–Kumux�Tala, after [8]) took place atthe Kazakhstan rather than the Tarim margin and didnot imply that these continents were involved in thecollision.

On the Tarim paleocontinent proper, the LateDevonian transgression was accompanied by deposi�tion of red beds followed by carbonate sedimentaryrocks [45]. It is also known that, at least, to the west ofKhan�Tengri, suprasubduction magmatism did notdevelop at that time. Stabilization is clearly noted atthe northern Kazakhstan margin of the Turkestanpaleoocean (the present�day Middle Tien Shan andthe Qizilqum plains), where quartz sandstone areoverlapped by carbonate rocks (commonly Famen�nian) that mark a mature stage of passive margin evo�lution.

In the inner regions of the Turkestan paleoocean,Givetian and Upper Devonian limestone and dolo�mite make up carbonate platforms that conformablyor transgressively overlie older rocks, including EarlyDevonian basalts. In other zones, the basalts are over�lapped by bathyal cherty–carbonate sediments.

In the south of this oceanic domain, i.e., in theQizilqum–Alay system of carbonate platforms, and to

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the north of the Tarim paleocontinent, a new region ofwithin�plate magmatism was formed, which was dis�placed to the east relative to the Early Devonian one.As a rule, the volcanic rocks overlie shelf limestonesand give way to them upsection. The westernmostoccurrence has been noted at the headwater of theShokh River in the Alay Range, where highly explosivevolcanic and subvolcanic rocks occur in a wide rangeof silica contents [29]. The Givetian and probablyFrasnian volcanic rocks of Mount Baubashata in thenortheastern Farghona and eastward, in the Jangy�Jerand Ulan ranges of the Southern Tien Shan in Kyr�gyzstan, have a more homogeneous sodium�subalka�line composition [10]. Similar but often more alkalinebasalts of the Borkoldoy Range also erupted in theFamennian. The age of volcanic rocks was determinedby organic remains in the under� and overlying marinesedimentary rocks and coeval limestone interlayersand volcanic–lithoclastic aprons on slopes [6]. Basalton the southern slope of the Kakshaal Range has beendated at 392 ± 15 Ma (Sm–Nd isochron) [79].

EARLY CARBONIFEROUS (359–318 Ma)

The suprasuduction magmatism at the northernmargin of the Kazakhstan paleocontinent resumed inthe Early Carboniferous with accumulation of thickpiles of andesites (including high�Mg varieties), rhyo�dacite lavas and tuffs, and less frequent basalts [17, 18,50, 65]. The suprasuduction magmatism in the north�east of the Tien Shan covers the southeastern limb ofthe Balqash–Ili volcanic–plutonic belt and the Trans�Ili Alatau Mountains, the contemporary depression ofthe Ili (Yili) River and its mountainous framework fur�ther to the north and the east, as well as southeast andsouth of the Junggar Alatau. The volcanic rocks occur�ring in the Uzun and Borokhoro Mountains are datedat 363–313 Ma (U–Pb method, SHRIMP); the oldestrocks correspond to the late Famennian [51].

In Southern Kazakhstan and the Chinese NorthernTian Shan, the intermediate and felsic volcanics areTournaisian in age. The main phase of volcanic activ�ity fell on the late Visean–Serpukhovian and is close inthis respect to the western (Balqash) sector [14, 38,75]. The positive εNd = 0.32–4.90 (occasionally up to+10.0) and ISr = 0.7015–0.7068 are constant attributesof the volcanic rocks. The increasing К2О/Na2O ratioin some volcanic series [77, 78] does not contradict themodel 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 at Early Carboniferous (~350 Ma)occur in the Borokhoro Range and to the south in theNarat Range up to the Southern Tian Shan Suture. Inchemical composition, the rocks correspond to I�typegranites related to the mantle source with insignificantparticipation of crustal contamination [51, 78]. Theigneous rocks of marginal continental type are com�

1 bined with shallow�water marine and continental sed�imentary rocks, including coal�bearing sequences.

Farther to the east, the Early Carboniferous supra�subduction magmatism developed in the BogdashanRange (the Bogda–Choltagh arc) [78]. The basementof this arc is not reliably known, and it is assumed to becontinental. This is partly supported by diverse igne�ous rocks (basalt, andesite, dacite, largely pyroclastic)and shallow�water limestone with brachiopods. Mostlikely, the rocks correspond to the upper part of theLower Carboniferous sequence, which is enormous inthickness (> 8000 m ?). No reliable datings are avail�able for gabbrodiorite and granitic intrusions. Subduc�tion is suggested in the southern direction from theside of the Junggar ocean [13, 43]. Similar magmatismis also known to the south of the Turpan–HamiDepression in the Kangurtag or to the CholtaghMountains, where Lower Carboniferous calc�alkalinevolcaniclastic rocks (354 ± 1 Ma) and related gabbro–plagiogranite intrusions (331 ± 2 Ma, SHRIMP)make up the Dananhu–Tuzkan arc [38, 53]. The arcrests on the basement of the Devonian island�arc vol�canics and is accreted to the Ili (Yili), or Kazakh con�tinent.

As is seen from Fig. 4, the Early CarboniferousBalqash–Ili volcanic–plutonic belt in the SouthernKazakhstan and the Junggar–Turpan sector reaches250 km in width and thus is appreciably wider than inthe northern Balqash region. A secondary increase inwidth in the Permian and Mesozoic is likely related torifting [78]. Such an expansion can hardly be signifi�cant, because no manifestations of oceanic crust ofthat age have been noted. It is more likely that we aredealing here with remnants of two volcanic zones: theisland arc proper in the north (the Bogdashan andCholtagh mountains) and the active margin of theKazakhstan paleocontinent in the south, to the south�west of the ophiolitic Bayingou Suture in theBorokhoro Range. The suture consists of ophioliticmelange with gabbro and plagiogranite blocks (325and 344 Ma, respectively, U–Pb method, SHRIMP);Lower Carboniferous turbidites; and older bathyalcherts with Famennian–Visean microfossils [78, 79,83, 84]. The recent strike�slip–thrust structure of thesuture generally inherits the southern direction of theoceanic lithosphere subduction in the Early Carbonif�erous with its accretion to the continent. Judging bythe age of the stitching granitic intrusion (316 Ma), theBayingou basin was closed with the formation of thesuture at the end of the Early Carboniferous [69]. Itcannot be ruled out that remnants of this basin occurin a more complete form in the basement of the Tur�pan–Hami Basin displaced along the right�lateralJunggar Strike�Slip Fault.

The magmatic events resumed at the end of theEarly Carboniferous in the Qurama Range at thesouthern margin of the Kazakhstan paleocontinent; thismargin remained passive for a long time. The eventscomprised emplacement of peridotites, gabbro, and



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Fig. 3. Givetian–Late Devonian igneous complexes ofTien Shan and its framework. (1) Complexes of LateDevonian rift�related 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.

anorthosite (Shavaz Complex to the southeast ofToshkent, 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 ofcomagmatic trachybasalt and trachyte of the Uin For�mation. In general, magmatism of the Qurama Rangeis related to the recurrent subduction. The igneousrocks are characterized by elevated alkalinity of theinitial phase and by the occurrence of ultramaficrocks.

The onset of magmatic activity is established moredistinctly at the southwestern boundary of the TienShan, which is related to the active margin of the Pale�otethys [50], and also traced in the northern Pamirsand the western Kunlun [13, 22]. In the Gissar Rangeand its southwestern spurs, the Visean (probably, alsolate Tournaisian) felsic volcanics overlap the Precam�brian basement with deep scouring and basal con�glomerate. As judged from marine fossils in sedimen�tary interlayers, the Sioma Formation of rhyolitic anddacitic tuffs and lavas (up to 2200 m) in the axial zoneof the Gissar Range [31] is mainly Visean in age. Thebimodal association of subalkali basalts of the Karatagand Vakhshivar formations, dated as Serpukhovianand Bashkirian from fossils in interbeds of bathyal sed�iments combined with rhyolite having K–Ar age of316–320 Ma [20], occurs in the southern GissarRange. Minor bodies of gabbro and serpentinizedperidotite of the Kundadjuaz Complex are localized infault zones. Their relationship to basalts is not evident;no complete ophiolitic sections have been docu�mented [19, 31]. These occurrences mark backarc rift�ing and subsequent partial breakup of the Qaraqum–Gissar continent into the southern (Baysun) and thenorthern (Gissar) parts. The compressive deforma�tions started as early as Visean in the back zone of volca�nic margin between the Gissar Range and the ZarafshonRiver, where the thrust faults are overlapped by the upperVisean conglomerate (data of V.I. Lavrusevich, cited in[9]). The thrusting and related thickening of the crustwere accompanied by metamorphism, in particular, bythe appearance of gneissic granites and migmatitessuperposed on the Lower Paleozoic protolith in theZarafshon Range. The age of this process is Visean(339–327 Ma, SHRIMP) [28]. Later on, the marginalbasin was closed (Figs. 1, 4). The calc�alkaline volca�nic rocks of the active�margin type were formed in thesouthern Gissar Range already in the Middle Carbon�iferous and then overlain by marine molasse.

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Fig. 4. Early Carboniferous igneous complexes of TienShan and its framework. (1) Alkaline mafic and ultramaficrocks; (2) ophiolitic complexes in sutures of Early Carbon�iferous marginal basins; (3) boundaries of continentalmassifs inactive in Early Carboniferous (subsequent offsetsare 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, BaysunMountains; Gs, Gissar Range; Jn, Junggar Strike�SlipFault; Ti, Trans�Ili Range; Zr, Zarafshon Range; Kt, Kan�gurtagh Mountains; Nr, Narat Mountains; Tu, Turpan–Hami Basin; Uz, Uzun Mountains.

3

LATE CARBONIFEROUS(BASHKIRIAN–GZELIAN) (318–300 Ma)

The active development of the Bogdashan islandarc continued in the Late Carboniferous at the north�eastern margin of the Kazakhstan paleocontinent(Fig. 5) [43, 66]. The thick pile of calc�alkaline volca�nic rocks occurs in the Bogdashan Range togetherwith marine sedimentary rocks. Judging by the fossilcomplex with Profusulinella, these rocks belong to theBashkirian–Moscovian stages. After closure of theBayingou marginal basin (Figs. 1, 4) no later than316 Ma ago, the magmatic activity at the place of theformer Junggar–Balqash paleoocean should beregarded as a collision�related event, though it isdescribed as a postcollision one [78]. The island arc–continental margin collision was apparently soft. Inany case, the tectonic nappes of the Southern TienShan type have not been revealed here.

The intensity of volcanic activity at the end of LateCarboniferous probably decreased, though reliablepaleontological data for this period became scantbecause of passage to a continental setting. In the Chi�nese Northern Tian Shan, i.e., within the formeraccretionary system at the margin of Kazakhstan pale�ocontinent, widespread granites were emplaced fromthe Serpukhovian (327 Ma) to the Early Permian,inclusive [79]. The main body of collision�relatedgranites were emplaced 320–310 Ma ago. In the NaratRange, the U–Pb age of the youngest volcanic rocks is313 Ma and granodiorite is dated at 308 Ma [78]. Inthe entire region, the predominant calc�alkaline gran�itoids are characterized by ISr = 0.703–0.705 and thelatest model Nd age is 600–460 Ma. These parametersare regarded as evidence for slab breakoff and partici�pation of juvenile mantle material in magmatism.

Magmatic activity waned after 310 Ma ago, andthen the postcollision stage proper followed [68]. Thesame gap is probable in the more western and innerparts of the paleocontinent that underwent collision,i.e., in the Northern and most of the Middle (Kyrgyz)Tien Shan, where Late Carboniferous granitic rocksare much less abundant than suggested earlier [34].

The southern margin of the Kazakhstan paleoconti�nent 318–320 Ma ago was a place of almost coevalevents indicating rearrangement of the geodynamics.We are dealing with (1) the first south�verging tectonic



nappes; (2) appearance of the early Bashkirian oroccasionally Serpukhovian flysch and olistostromes[9]; (3) metamorphism and exhumation of high� andultra�high�grade metamorphic rocks along the tec�tonic suture from Qizilqum to Xinjiang; and (4) vigor�ous, though not continuous magmatism in its north�ern limb.

The metamorphic complexes, including both high�temperature facies, fragments of high�pressure meta�morphic rocks, and oceanic basalts, have beenretained in the southern limb of the Southern TienShan collision suture at the northern slope of theAtbashi Range and to the east at the headwaters of theKekesu River. In the Atbashi Range, the complexcomprises melange with gneisses and eclogites, thepeak of metamorphism (18–24 kbar) of which is datedby Sm–Nd isochron at 319 ± 4 Ma and subsequentexhumation at 316 ± 3 Ma (Ar/Ar age of phengite). Atthe Akiyaz River (headwaters of the Kekesu River),eclogites and glaucophane schists occur in a wide (upto 20 km) zone, where the outer rims of zircon grainsfrom eclogites are dated at 319 Ma, on average [54]. Inthe western sector of the region, the Southern TienShan (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 limb of the same suture zone, i.e.,at the southern margin of the Kazakhstan paleoconti�nent, magmatic activation is well�studied in theChotqol–Qurama region, where it is accompanied byimportant mineralization.

The initial (Uin) phase with simultaneous transi�tion from marine to continental conditions is charac�terized by trachybasalt and trachyte eruptions. Theinterbeds with marine fauna are dated at the Ser�pukhovian or early Bashkirian. Upsection, separatedby a hiatus, basalt, andesite, and dacite with elevatedalkalinity follow; the Rb–Sr age of 317 ± 6 Ma alsocorresponds Bashkirian [19]. The granite porphyryminor intrusions in the Almalyk (Almaliq) ore fieldcorrespond to the same time (315 ± 1 Ma, U–Pbmethod, SHRIMP), as well as rearrangement of theisotopic system in the older (Karakiya) granite in theQurama Range [74]. Judging by geological relation�ships and the Rb–Sr isochron (316 Ma), gabbro, peri�dotite, anorthosite, syenite, and monzonite of theQaramazar Complex are coeval [19]. The second andmajor magmatic phase is represented by the thickAqcha–Nadaq Complex of trachyandesites and dac�

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Fig. 5. Late Paleozoic igneous complexes of Tien Shan andits framework. (1) I�type granitoids; (2) collision suture;(3) ultrahigh�pressure metamorphic rocks. See Fig. 2 forother symbols. Letters in figure: Aa, Almalyk; At, AtbashiRange; Bd, Mount Bogdashan; Kk, Kekesu–Yili; Nr,Narat Range; Nr, Naryn Depression; Nn, NorthernNurota Mountains.

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ites with less abundant subalkali basalts and rhyolitesdated at 298–300 Ma (Rb–Sr method) [19], 305 ± 3and 301 ± 4 Ma (U–Pb method, SHRIMP) [74].

In the Southern Tien Shan, where active collisiontook 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 revisedtoward rejuvenation [74]. In contrast, some volcanicsequences of the Qizilqum Hills and Northern NurotaMountains formerly related to the Middle Carbonifer�ous [19] turned out to be Silurian or Devonian in agebased on new findings of fossils and allochthonous intheir structure [7].

In the southern Gissar district of the southwesternTien Shan, the lower part of the Upper Carboniferouswith Bashkirian fauna in marine interbeds is com�posed of andesite and rhyolite [31], which are gener�ally close to the marginal continental type and locallyoverlap ophiolites with bathyal sediments, thus denot�ing the end of backarc spreading.

Minor gabbro–granite intrusive bodies, whosegeological age is consistent with K–Ar datings at 320–310 Ma, are related to the same stage. The subsequentpre�Moscovian deformations mark the transition tothe collision stage, when large granitoid plutons of theGissar Range were formed. The oldest of them arecomposed of diorite and granodiorite; they wereeroded at the end of the Moscovian or somewhat laterwith 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 theS type. Unfortunately, reliable isotopic datings are notyet available.

EARLY PERMIAN (297–270 Ma)

We refer completion of collision to the momentwhen the last deepwater basins disappeared in theSouthern Tien Shan. A case in point is the clayey–cherty bathyal sedimentary rocks, which are noyounger than Late Carboniferous here, and the turbid�ites in the foredeep, which are as young as Asselian inthe eastern Farghona [7, 11, 39]. Meanwhile, marinesedimentary rocks, including carbonates, are knownup to Artinskian in the Tien Shan and up to Kungurianin the Tarim paleocontinent [44]. Thus, the Early Per�mian stage of the Paleozoic history of the Tien Shanmay be regarded as postcollision.

As the datings for the western Tien Shan show(Figs. 1, 6), magmatism sharply intensified at thebeginning of the Early Permian, locally predated by abreak about 10 Ma long (Fig. 7). The area of magmaticactivity expanded and acquired new outlines. The fol�lowing regions are distinguished by the composition ofigneous rocks.

The former Kazakhstan paleocontinent and itsactive margins have been transformed into the medianmass of the Central Asian Hercynides. This tectonic

1

unit is characterized by bimodal subalkaline magma�tism that followed suprasubduction magmatism.

In the northeast of the Tien Shan, i.e., at the formeractive margin of the Junggar–Balqash ocean, theEarly Permian granitoids are mainly exposed in theframework of the Yili Depression, where they cutthrough the collision structure immediately after theLate Carboniferous intrusions [69]. The temporalcompositional trend is expressed in the gradual transi�tion from calc�alkaline to high�K intrusive rocks. Bothbimodal basalt–rhyolite volcanic series and calc�alka�line series with andesite occur here as well. The latterare also noted up to the Kyrgyz Range (AshukoltorFormation [34]). In southern Kazakhstan and north�ern Xinjiang, the magmatic activity partly continuedin the Mid�Permian. Granitic rocks of the BorokhoroRange (on the right bank of the Yili River) have a U–Pb age of 285–266 Ma. The age of volcanic rocks inthe 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]. ThePermian alkali granite and quartz syenite of two plu�tons at the southern margin of the Central Tian Shanin China were formed 296–276 Ma ago [58, 69].High�temperature granulite metamorphism dated at299 ± 5 Ma developed in the same zone [67].

The relationship of Early Permian magmatism torifting and strike�slip faulting is commonly empha�sized [40, 71]. Indeed, judging by Ar/Ar datings ofnewly formed biotite in the shear zones, offsets alongthe 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 strike�slip faults in the easternChinese Tian Shan and known from the related Cu–Ni mineralization (Huangshan and other depositslocated to the east of the territory shown in Fig. 5) arealso dated at 280 Ma. It has been noted that the initiallow�Ti and high�Mg magma parental for these intrusionsmelted at a high temperature. Granitic plutons wereemplaced synchronously with basaltic eruptions [72].

The granitic pluton intruded along the collisionboundary between the Tarim and Kazakhstan paleo�continents (Atbashi–Inylchek Suture), which was alsothen modified by strike�slip faults. The elongated andtectonized Terekty pluton (294–291 Ma) with calc�alkaline specialization is typical. Granites containxenoliths of Precambrian rocks [62]. A similar age hasbeen established for Song�Köl–Ulan calc�alkalinegranitoids exposed as blocks elongated in the latitudi�nal directions with offsets along the left�lateral strike�slip faults [2]. In addition, the basalt–andesite–daciterocks associated with Asselian sedimentary rocks con�taining marine fauna occur in the lower part of the sec�tion in the Atbashi–Inylchek Strike�Slip Fault Zone(Baybiche�Too and Janan�Too). High�K granites of Itype are localized along the late right�lateral Talas–Farghona Strike�Slip Fault, including the Au�bearing

3

1



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 NorthernTien Shan, was an area of leucogranite–syenite (up tonepheline syenite) magmatism in the Early Permian.Nepheline syenite of the Akkulen pluton in the westernYsyk�Köl region is dated at 292 Ma (U–Pb method,SHRIMP) [74]. A�type granites 285–270 Ma in ageoccur at the headwaters of the Yili River [85].

In the Chotqol–Qurama region, isotopic datings,though not representative so far, confirm a significantgap in time between the Late Carboniferous and Per�mian stages of magmatic activity, and this is consistentwith the deep erosion during that gap [19]. The EarlyPermian stage of magmatism begins here with thewidespread Oyasay rhyolite–trachyrhyolite lavas andtuffs related to rifts and calderas combined withhypabyssal felsic intrusions, including leucogranite.The age has been determined by Asselian fauna inmarine members at the base of the section. Only con�tinental sedimentary rocks are known upsection.Shurabsay trachybasalt, trachyte, and trachyandesitewith an Rb–Sr age of 285–282 Ma [20] are somewhatolder, 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 tosubsequent dating, they are no younger than EarlyPermian [16].

The Southern Tien Shan is distinguished almostsolely by intrusive granitoid magmatism, which differsin composition in the western and eastern segments ofthis linear orogen.

In the western Qizilqum–Alay segment, the fold–nappe structure of the Hercynides involves diverseEarly Permian granitoids and alkaline intrusions occa�sionally accompanied by felsic volcanics. To date, alarge, but insufficiently representative in area, body ofinformation on the age of granitic rocks has beenobtained using U–Pb (SHRIMP), Rb–Sr, and Ar/Armethods [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. 6. Early Permian igneous complexes of Tien Shan andits framework. (1) Felsic and bimodal volcanics; (2)basalts, including beneath cover of younger sedimentaryrocks, after [85]; (3–5) granitoids: (3) S�type, (4) A�type,including alkaline intrusions; (5) unspecified, mainlyleucogranites; (6) alkaline gabbroic rocks; (7) Permianstrike�slip faults. See Fig. 2 for other symbols. Letters infigure; At, Atbashi Range; Jd, Jaman�Davan Range; Yi,Yili Depression; Kl, Kalpin Mountains; Ks, KakshaalRange; Bc, Bachu Uplift; magmatic aureoles and clusters:CQ, Chotqol–Quarama; QN, Qizilqum–Nurota; ZA,Zerafshon–Alay; SU, Son�Köl–Ulan; granitioc plutons:Tr, Terekty; Mk, Makmal. Numerals in circles: 1, Kazakh�stan paleocontinent and its margins; 2, South Tien Shan;3, Tarim paleocontinent.1

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Most of the Early Permian intrusions in the Qiz�ilqum–Alay region are normal granites and granodior�ites of the I type; locally occurring synchronous S�granites, A�granites, and nepheline syenites are alsonoted. Such a character is inherent to the Qizilqum–Nurota granitic aureole in the extreme west of theSouthern Tien Shan, which stands out in its high�grade ore mineralization. The Zarafshon–Alay gra�nitic cluster is situated to the east and orientedobliquely to the strike of the Hercynides. The graniticand alkaline magmas were especially diverse here, andoccasionally they mixed with one another [30]. Thedatings for this district need confirmation. The high�temperature cordierite–sillimanite metamorphismrelated to S�granites is typical of the Nurota Moun�tains and the high mountains of the Turkestan–Alay.As follows from Pb isotope ratios [46] and geologicaldata (occurrence of Cambrian shelf sedimentaryrocks), the Qizilqum–Nurota granites could havebeen related to the mobilization of material from thePrecambrian basement reworked and involved inthrusting. The accreted pre�Devonian [7] oceanic and

island�arc volcanic and sedimentary rocks, i.e., apartly juvenile material, has suggested as anothersource of I�granites in the Turkestan–Alay region [76].

The Permian I�granitoids in the Qizilqum–Turke�stan Range are not related to the Late Carboniferoussubduction zone and are commonly localized in thelower tectonic sheets of the collision complex.

In the eastern Tarim sector of the Southern TienShan Orogen (Kakshaal Range–Khalyktau), theEarly Permian magmatism is characterized by theprevalence of A�granites, which make up a linear beltsubdivided into several clusters. They are largelylocated in the fold–nappe complexes of the outer zoneof the Southern Tien Shan Hercynides but also occurto the south in basement inliers and in the Tarim Fore�deep [52, 68]. A�type granites occupy wide areas in theKakshaal aureole. Their composition varies fromrapakivi�like biotite–anphibole granite to topazleucogranite [57, 60, 62, 74, 77]. Mafic rocks are asso�ciated with granites as inclusions and separate minorintrusions and dikes. Dikes are also known in the west�

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N = 106

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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 ofDevonian–Permian basaltic magmatism (black arrows) are shown without estimation of intensity.



ern wall of the Talas–Farghona Strike�Slip Fault butremain undated. Alkaline gabbro and syenite occur inthe Atbashi Range. As follows from U–Pb dating, allaforementioned intrusions were emplaced in the Per�mian 295–275 Ma ago [57, 60–62, 74]. The early fel�sic volcanics associated with fossiliferous sedimentaryrocks are dated close to the Carboniferous–Permianboundary [9]. The aureoles of granitic plutons areaccompanied by low�grade thermal metamorphism,mostly in the eastern Kakshaal Range, in the moun�tainous cluster of the Khan�Tengry– Jengish (Pobeda)peaks, and further to the east.

The Tarim crystalline basement affected by hotmantle material probably was a source of granites. Thisis 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 withnorthward underthrusting of the Tarim continentbeneath the Southern Tien Shan nappes up to theAtbashi–Inylchek Suture, as was noted by E.V.Khris�tov, cited after [10], and confirmed by new seismicsounding results [26]. In that case, the formation ofgranitic magma seemingly could have been related tothe collisional underthrusting and stacking of tectonicnappes with significant thickening of the continentalTien Shan crust [76]. Attention should be drawn, how�ever, to the very short time span between the propaga�tion of nappes with the participation of Asselian rocks,i.e., no earlier than 300 Ma, and the emplacement ofgranitic magma therein (297 ± 4 Ma for the Jangartpluton) [73]. This suggests that the crust of the Tarimpaleocontinent began to heat before thrusting.

Thus, the arrangement of granitic rocks and themost probable geodynamics of magmatic activity inthe Tarim sector were approximately the same as in theQizilkum–Alay region; however, the composition ofgranites is more uniform due to a common continentalmagma source.

In the Tarim paleocontinent in front of the Kakshaalnappes, the Early Permian magmatism involves bimo�dal within�plate association with widespread basalts.

The plateau basalts of the Tarim paleocontinent,which have been known for a long time from the Kal�pin Mountains, have been recently drilled over an areano less than 200 000 km2, where their thicknessreaches a few hundred meters. The subalkali basaltscontain 2–5 wt % TiO2 and are depleted in MgO [72].Typical εNd(t) values vary from –9.2 to –1.7, proba�bly reflecting a different degree of contamination ofasthenospheric mantle material with crustal rocks ofthe Tarim basement. The age of basalts and relatedalkaline gabbro, which was estimated using modernmethods, is close to 280 Ma [52, 72]. Gabbro, ultra�mafic rocks, and sporadic quartz syenite of the A typecrop out at the Bachu Uplift in the field of basalts.Their age is 285–275 Ma, and comagmatic volcanicrocks are dated at 289–267 Ma [43, 71, 85]

In the Qaraqum–Tajik paleocontinent, i.e., in theextreme southwest of the Tien Shan, the published K–

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Ar age determinations and paleontological data [19,27, 34] broadly confirm the abundance of Early Per�mian alkali granites and less frequent felsic volcanicrocks. Their combination with alkali basalts also indi�cates bimodal postcollision magmatism; however, thelack of reliable datings does not allow more definitetime constraints.

DISCUSSION AND CONLUSIONS

The Early Devonian magmatic processes in theTien Shan part of the Kazakhstan paleocontinent aredescribed in terms of the classic model of active mar�gins. The boundary with the Junggar–Balqash oceanexpressed in the Kazakhstan volcanic–plutonic belthas been retained better than the southern boundary inthe Qurama Range. In general, it is possible to draw ananalogy with Cenozoic evolution of the PhilippineArchipelago. The northern Kazakhstan belt is, at least,rather wider. By analogy with similar younger struc�tures, the large width may be a result of low�angle 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 theEarly Devonian Paleoasian ocean and its relativeshortening [50] was apparently caused by the conver�gence of lithospheric plates on a regional scale. It isimportant, however, that this process was accompa�nied mainly by within�plate basaltic magmatism overthe entire length of its Turkestan branch. Independentheating of the sublithospheric mantle over a vast terri�tory due to internal causes (ascending heat and masstransfer 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), theconvergence at the northern margin of the Kazakhstanpaleocontinent continued with a shift of the volcanicbelt and intensification of granitic magmatism due toaccretion of the island arc to the paleocontinent. Tothe east of 82° E, the suprasubduction magma genera�tion has also penetrated the Southern Tien Shan. Itcannot be ruled out that subduction developed herefrom the side of the Turkestan ocean, unless we aredealing here with fragments of the same northern mar�gin of the Kazakhstan paleocontinent, which weretransported to the south by susbsequent tectonicnappes. The development of another part of the regionwas passive with indications of rifting in the Kazakhpaleocontinent. A hotspot shift is marked by basalticeruptions in the Turkestan basin. First they were local�ized in the eastern Farghona (the Ulan Range), wheretheir present�day aureole is about 500 km long, andthen migrated to the east. In the Late Devonian, igne�ous complexes pertaining to hotspot�type and coevalrifting are also widespread in Northern Kazakhstan[14] and in other continents of northern Eurasia.

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In the Early Carboniferous (Fig. 4), the formationof calc�alkaline series of marginal continental type,which resumed from Tournaisian in the northeast ofthe present�day Tien Shan and from Visean in the Gis�sar and the northern Pamirs, was without any doubtrelated to the renewal of subduction at the boundariesof the Kazakhstan paleocontinent. The general con�vergence of the Paleoasian and Paleotethian oceanicplates was a background that resembles the recent set�ting of the Indopacific. Ophiolites that formed at theend of the Early Carboniferous are known at theboundaries of the Qaraqum–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 bythese ophiolites could not have been significant undercompressive conditions irrespective of magma type, sothat deepwater basins disappeared right away. Onlymafic–ultramafic rocks with Visean isotopic age maylikely be classified as manifestations of within�platemagmatism in the Qurama Range.

Since the Bashkirian, plate convergence (Fig. 5)has led to collision of continents with formation ofnappes, olistostromes, and exhumation of high� andultrahigh�pressure metamorphic rocks along thesouthern margins of the Kazakhstan paleocontinent.These features are supplemented by granitoid magma�tism with predominance of calc�alkaline volcanicrocks and I�type granites. At the northern Junggar–Balqash continental margin and the southernQaraqum–Tajik margin, this magmatism inherited theactive�marginal processes of the preceding stage. As

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concerns the southern boundary of the ancient Kaza�khstan, it does not reveal such inheritance. Magma�tism of the Beltau–Quarama belt developed only atthe collision stage in the back zone of thrusting andwas related to the thickening of the Earth’s crust.Local intensification of magmatism in the west of theTien Shan could also have been induced by the samecause as the appearance of mafic and ultramafic meltsin the Visean (345 Ma ago). This phenomenon hasbeen described as the Chotkol–Quarama plume [20,pp. 156, 157]. Truth be told, the localization of thisplume in the same district from Silurian to Mesozoicdoes 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, whichis especially evident in the central and eastern parts ofthe Tien Shan, should induce fold–nappe thickeningof the continental crust up to values close to thepresent�day 50–60 km and subsequent heating of itslower part above the solidus temperature. The dissipa�tive heating related to movement of lithospheric platesand the hampered release of the radioactive heat couldhave been additional factors. The model of simple col�lision thickening of the crust is applicable to the colli�sion orogens with the predominance of palingeneticgranites close to the S type, which are products ofmelting of the ancient granite�gneiss basement or sed�imentary rocks deposited owing to its denudation.Such an origin should be reflected in the older modelNd ages of granites, negative εNd(t) values, and highinitial 87Sr/86Sr ratios.

In addition, the possibility of the subducting sheetof the lithospheric mantle being torn from the crust isdiscussed. Further subduction of such a sheet is ham�pered by its own ongoing collision and buoyancy. Inthis scenario, the melts creating orogenic granitoidsare products of interaction of both the subductingcontinental crust and the mantle of the hanging platewith the hot asthenospheric material penetrating thefault zone [15, 35, 47]. As applied to the eastern TienShan, delamination of the collision structure roots inthe Early Permian has been advocated by manyauthors, e.g., [52, 69]. The assumption of completeslab breakoff allows us to deal with the diversity ofgranitoid magmas and to explain the appearance ofhigh�alkaline melts with attributes of partial mantleorigin, as well as a new temperature pulse for the gen�eration of I�type magma in the lower crust. The plungeof the slab should be vertical or retains a horizontalcomponent of the former subduction (Fig. 9), givingbirth to diverse magmas in the suprasubduction zone,including due to metasomatic alteration of the mantlewedge in the case of I�type granitoid generation.

Nevertheless, the localization of Early Permiangranitoids in the Tien Shan (Fig. 6) cannot beexplained completely by these two assumptions. Theintrusive bodies and volcanic fields are not strictly

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Fig. 8. Tectonic scheme of the area adjacent to the Jangartpluton in eastern Kakshaal Range. (1) Granites of A and Stypes (J, Jangart pluton); (2) collision�related tectonicnappes and later strike�slip faults; (3) Lower Permian tur�bidites and olistostromes in the roof of the Jangart pluton;(4) Paleozoic and Precambrian rocks of the Middle TienShan (MTS) and Tarim (Tr); (5) Paleozoic rocks in nappesof the South Tien Shan (Devonian within�plate basalts areshown); (6) strike and dip symbol.

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confined to any suprasubduction zone(s). They aredistributed 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. Theappearance of alkali plateau basalts at the foreland ofsubducting Tarim paleocontinent simultaneously withan outburst of granitic magmatism ~280 Ma ago isespecially characteristic. It is as though the last colli�sion nappes of the Kakshaal Range waited here for aheated bed to be almost immediately cut through bygranites. Such an event is not typical of orogeny. Thisevent occurs with a long lag (for example, Permianbasalts of northern Germany that formed after theVariscan collision in the Early Carboniferous [40] orTriassic basalts on the western margin of the Urals;elsewhere basalts do not occur at all.

Therefore, bimodal magmatism at the forelandcannot be a direct implication of collision and musthave a separate cause of its own. A slab breakoff can besuch a cause. According to some calculations, this ispossible, owing to density instability, when the lithos�phere doubles its thickness up to 300–400 km [15]. Italso should be kept in mind that the unstable densityand related delamination of the mantle lithosphere aremore feasible if the temperature of the asthenosphererises in advance due to the autonomous ascendingflow, i.e., in any mantle plume variant.

The authors, proceeding from the plume hypothe�sis, suggest that the Tarim basalts are a manifestationof an ascending mantle flow spreading beneath thelithosphere, whereas high�Mg gabbro and ultramaficrocks in the eastern Tian Shan mark the head of theplume. Meanwhile, the Early Permian ultramaficrocks are also known in other districts of the TianShan. For example, the Permian basalts cover thebasement of the Junggar and the Turpan–Hamidepressions in the northeastern framework of the TianShan. Their age here is even younger [85], though col�lision has finished simultaneously or somewhat earlierthan in the Southern Tian Shan. Mafic rocks (300–274 Ma) occur in the Beishan rift basins [72] and areknown to the east in Mongolia [71]. Thus, here we aremore likely dealing with almost coeval differentplumes or with a Permian large igneous province. Theidea of the Tarim plume as a particular manifestationof this giant event in northern Asia has been discussedrather thoroughly [41, 71, 72]. At present, we have nocrucial petrologic or geochemical evidence that wouldshow that the territory of the Tien Shan belongs to a

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

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single area of the lower mantle superplume that con�trols upper mantle ascending flows [21]. We are notsure that its formation took place only in the EarlyPermian. The data presented above allow us to suggestthat the ascending mantle flows reached the lithos�phere at the end of Devonian and at the beginning ofLate Carboniferous. In the latter case (Chotqol–Qurama aureole), we can also see a precursor of themore extensive Early Permian event.

From the practical viewpoint, it is more importantto discuss the ways along which the deep�seated matterand energy of the plume are transferred to the surface.In addition to the above�mentioned conjectural frac�ture zones in the subducting slab, a case in point isextension zones in the crust related to the deep andlong strike�slip faults [40, 47, 59, 70, 81, 82]. It isassumed that Early Permian rifting (Yili Basin, etc.) isentirely a result of postcollision transtension and notrelated to a plume [78]. Some cases of graniteemplacement into the fault�line zone have been dis�cussed above. It may be added that the linear, nearlymeridional arrangement of granitic plutons at thenorthern margin of the Tarim Platform suggests a deeppull�apart system in the near�latitudinal direction.Granites mainly occur in the areas of highest colli�sion�related stacking of upper crustal masses and areoften situated in the cores of antiforms intruding intothe lower tectonic nappes. In general, however, thegeological map does not show any significant links ofgranitic plutons to collision structures.

Thus, the convergence of lithospheric plates in theLate Paleozoic, which was completed by collision ofcontinental masses in the Late Carboniferous and cre�ated the recent Tien Shan, can satisfactorily explainthe known magmatic processes. Within�plate magma�tism, however, remains beyond the scope of thismodel. This magmatic process was intense in the EarlyDevonian and less intense in the Late Devonian–Early Carboniferous, but is mainly expressed in theEarly Permian postcollision setting. Sporadic distribu�tion of granitoid intrusions unrelated to collisionsutures and particularly intense basaltic magmatismcovering even the continental foreland compel us tosuggest an additional internal source of heating of thesublithospheric mantle (plume) independent of colli�sion, which reached the bottom of the lithosphere290–275 Ma ago.

ACKNOWLEDGMENTS

This study was supported by research grants of St.Petersburg State University (nos. 3.0.93.2010 and3.37.91.2011) and the IGCP�592 project.

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Reviewers: K.E. Degtyarev,M.G. Leonov, and V.V. Yarmolyuk

Translated by V. Popov

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SPELL: 1. Tarim, 2. Mao, 3. ultramafic, 4. Asamoah, 5. Han

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