Isotope provinces, mechanisms of generation and sources of the continental crust in the Central Asian mobile belt_ geological and isotopic evidence

Institute of Ore Deposits Geology, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetnyi per., 35, Moscow 109017,Journal of Asian Earth Sciences 23 (2004) 605627/locate/jseaesIsotope provinces, mechanisms of generation and sourcesof the continental crust in the Central Asian mobile belt: geologicaland isotopic evidenceV.I. Kovalenkoa, V.V. Yarmolyuka, V.P. Kovachb,*, A.B. Kotovb, I.K. Kozakovb,E.B. Salnikovab, A.M. LarinbaRussian FederationbInstitute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Makarova emb., 2, St- Petersburg 199034, Russian FederationAbstractThe available geological, geochronological and isotopic data on the felsic magmatic and related rocks from South Siberia, Transbaikaliaand Mongolia are summarized to improve our understanding of the mechanisms and processes of the Phanerozoic crustal growth in theCentral Asian mobile belt (CAMB). The following isotope provinces have been recognised: Precambrian (TDM 3:3 2:9 and 2.50.9 Ga)at the microcontinental blocks, Caledonian (TDM 1.10.55 Ga), Hercynian (TDM 0.80.5 Ma) and Indosinian (TDM 0.3 Ga) thatcoincide with coeval tectonic zones and formed at 570475, 420320 and 310220 Ma. Continental crust of the microcontinents isunderlain by, or intermixed with, juvenile crust as evidenced by its isotopic heterogeneity. The continental crust of the Caledonian,Hercynian and Indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of oldcrustal material in the island arcs or active continental margin environments. The crustal growth in the CAMB had episodic character andimportant crust-forming events took place in the Phanerozoic. Formation of the CAMB was connected with break up of the Rodiniasupercontinent in consequence of creation of the South-Pacic hot superplume. Intraplate magmatism preceding and accompanyingpermanently other magmatic activity in the CAMB was caused by inuence of the long-term South-Pacic plume or the Asian plumedamping since the Devonian.q 2003 Published by Elsevier Ltd.Keywords: Central Asian mobile belt; Phanerozoic; Crustal growth; Isotope Provinces1. IntroductionThe formation and evolution of the terrestrialcontinental crust is undoubtedly one of the keyproblems in Earth science. It is generally suggestedthat the growth of the continental crust has beenessentially completed in the Early Precambrian (Windley,1995; Condie, 1998). However, recently it was demon-strated based on geochronological and isotopic data thatthe continental crust growth in many Phanerozoicorogenic belts was also very signicant (Samson et al.,1989, 1995; DePaolo et al., 1991; Kovalenko et al.,1996a,b; Jahn et al., 2000a,b; Chen and Jahn, 1998), butthe mechanisms of the production of new crust is still anissue of debate.* Corresponding author. Tel.: 7-812-328-4302; fax: 7-812-328-4801.E-mail address: (V.P. Kovach).1367-9120/03/$ - see front matter q 2003 Published by Elsevier Ltd.doi:10.1016/S1367-9120(03)00130-5One of the most expressive examples of such belts is theCentral Asian mobile belt (CAMB) (Zonenshain, 1972),also known as Altaid Tectonic Collage (Sengor et al., 1993)or Central Asian Orogenic Belt (Hu et al., 2000; Jahn et al.,2000a,b; Wu et al., 2000). This orogenic belt contains verylarge volumes of granitoids and related rocks that wereemplaced during the Paleozoic and Mesozoic. Until recentlythese granitoids, especially the granitoids in Tuva, SayanMts, Transbaikalia and Mongolia, have been poorly studiedand many aspects of their timing and origin, as well as theirsignicance as indicators of Phanerozoic crustal growth,were controversial. New geochemical, geochronologicaland isotopic data indicate that the CAMB is an importantsite of juvenile crustal growth during the Phanerozoic(Kovalenko et al., 1996a,b; Jahn et al., 2000a,b; Chen andJahn, 1998; Wu et al., 2000). In order to further improve ourunderstanding of the mechanisms and processes of suchcrustal generation we summarize in this paper the available606 V.I. Kovalenko et al. / Journal of Asian Earth Sciences 23 (2004) 605627geological, geochronological and isotopic data that wereobtained during the past decade on the felsic magmatic andrelated rocks from South Siberia, Transbaikalia, Mongoliaand adjacent terrains of the Siberian Craton. The mainpurposes of the paper are: (1) to document the possiblesources of the Phanerozoic granitoids from the CAMB, (2)to assess the role of basement rocks and intraplate igneousactivity in the genesis of Phanerozoic intrusive granites andPhanerozoic continental crust, and (3) to discuss themechanisms and processes that could lead to generation ofjuvenile continental crust in Phanerozoic mobile belts.2. General geologic setting and magmatic evolutionof the CAMBThe 1000 2000 km wide CAMB is situated betweentwo major Precambrian cratons of Central Asia(Siberian Craton in the north and Tarim and North ChinaCratons in the south) and extends across central Asia forabout 5000 km (Fig. 1). The tectonic architecture ofthe CAMB is dened by a combination of themicrocontinental composite blocks and sublinear mobilebelts of different ages (Neoproterozoic Cambrian EarlyOrdovician (Caledonian), Ordovician Early Carboniferous(Hercynian) and Carboniferous Permian (Indosinian)extending for hundreds and even thousands of kilometres(Fig. 1). It is necessary to note that Russian geologisttraditionally use the term Caledonian structures for CAMBsince rst half of the XIX century (Yanshin, 1966). As inother regions (Read and Watson, 1975), evolution of theCaledonian fold belts of the CAMB started at the end ofNeoproterozoic and nished to the end of Silurian EarlyDevonian time when deposition of molassa began (Mossa-kovsky, 1975). The Caledonia structures of the CAMB arecharacterised by several pulses of orogeny in various regions,f.e., Early Ordovician (Lake Zone), Late Ordovician(Sangilen block), Silurian (Mongolian and Gobi Altay).The microcontinental composite blocks containinghigh-grade rocks constitute a signicant proportion of thecontinental crust of the CAMB. The largest of them havebeen named the Dzabkhan, Khangai, Tuvino Mongolian,Barguzin and Altai (Fig. 1). The tectonic position of thesemicrocontinental blocks in the Neoproterozoic is the subjectof considerable discussion. Some authors (Mossakovskyet al., 1994; Didenko et al., 1994) proposed that they werederived from East Gondwana, whereas others (Belichenkoet al., 1994; Berzin et al., 1994) considered them fragmentsFig. 1. Simplied tectonic division of the Central Asia after (Yanshin, 1980) with additions of authors.V.I. Kovalenko et al. / Journal of Asian Earth Sciences 23 (2004) 605627 607rifted off the Siberian Craton. We suggest that micro-continents of the CAMB were derived from a singlesupercontinent Rodinia that included as East Gondwana asSiberia.The magmatic evolution of the CAMB was described indetail in many publications (Yanshin, 1980, 1989; Zanvile-vich et al., 1985; Gordienko, 1987; Kovalenko andYarmolyuk, 1990; Yarmolyuk and Kovalenko, 1991; Ney-mark et al., 1993, 1998; Berzin et al., 1994; Dobretsov andKidryashkin, 1994; Ruzhentsev and Mossakovsky, 1995;Pusharovsky, 1997; Kovalenko et al., 1999b; Dergunov et al.,2001; Khain, 2001) and was recently summarized byKovalenko et al. (1995, 1999b), Yarmolyuk et al. (2000)and Dergunov et al. (2001). According to Kovalenko et al.(1995, 199b) and (Yarmolyuk et al., 2000), orogenic andintraplate igneous activity in Central Asia continuedthroughout the entire Phanerozoic (from the Late Neoproter-ozoic to present time) without any signicant interruption.The correlation and sequence of tectonic and magmaticevents as well as the spatial temporal relationships of thePhanerozoic within-plate and orogenic magmatism in theCAMB are given in Table 1 and Fig. 2, that are compiled onthe base of available geological and geochronological data(Kovalenko and Yarmolyuk, 1990; Yarmolyuk and Kova-lenko, 1991; Kozakov, 1993; Khain et al., 1995a,b, 2002;Kotov et al., 1995a; Kogarko et al., 1995; Kovalenko et al.,1995, 1996a,b,c; Yarmolyuk et al., 1995, 1996, 1997a,b,1998, 1999a,b; Gusev and Peskov, 1996; Vladimirov et al.,1999; Kozakov et al., 1999a,b, 2001a,b, 2002a; Reznitskiiet al., 2000; Dergunov et al., 2001; Salnikova et al., 2001;Kuzmichev et al., 2001; and references in these publications).During the formation of the CAMB very large volumes oforogenic and intraplate granitic rocks of Paleozoic toMesozoic age were emplaced (Kovalenko et al., 1995,1999b). Paleozoic granitic intrusions are distributed mainlyin the northern part of the CAMB. They include (1) numerousplutons of the calc-alkaline series (granodiorite granite) thatwere intruded (500 440 Ma) during and after collision ofPrecambrian microcontinental blocks and ensimatic islandarcs of the Caledonian paleoocean; (2) Late Ordovician Devonian (440 360 Ma) granodiorite granite, granite,basalt andesite and andesite dacite rhyolite associationsof the Altai active continental margin; (3) Carboniferous andPermian granitoids of calc-alkaline series, represented by thevast Barguzin batholith (330 290 Ma) in northern Mongoliaand Transbaikalia and the Hangai batholith (270 250 Ma) inwest-central Mongolia (Table 1, Fig. 1). Mesozoic magmaticactivity in the CAMB was mainly connected with accretionaryprocesses up to collision of the North Asia (Siberian Craton,Caledonian and Hercynian mobile belts of the CAMB) andSino-Korean continents and with intensive intraplate activity.During this period within the CAMB the Early MesozoicKhentei (220 200 Ma) and Late Mesozoic Uda-Stanovoy(150 120 Ma) batholiths were emplaced (Table 1).The intraplate magmatic activity in the CAMB andadjacent terrains of the Siberian Craton continued sincethe Neoproterozoic up to Cenozoic (Kovalenko et al.,1999b; Yarmolyuk et al., 2000). The earliest impulses of theintraplate magmatism (ultrabasic-carbonatite type) occurredalong of southern margin of the Aldan shield (720 690 Ma)and along of the Eastern Sayan margin of the SiberianCraton (725 650 Ma) (see Table 1 and Fig. 2). At the 675660 Ma time interval the alkaline intraplate magmatism wasinitiated within Enisey Ridge and continued virtually to480 Ma (Kogarko et al., 1995; Yarmolyuk and Kovalenko,2001). From 490 Ma to Early-Middle Devonian the systemof grabens (the Minusinskaya, the Agulskaya, the NorthernMongolian etc.) were formed at the back zone of theSilurian Early Devonian Altai marginal belt (Vorontsovet al., 1997; Yarmolyuk and Kovalenko, 1991). The alkalineand subalkaline volcanics and plutonic rocks of intraplatemagmatic features characterize these grabens. Intraplatealkaline magmatism was also widespread within the LatePaleozoic Central Asian rift system. The formation of thissystem started at geodynamic setting when the continentalmargin override the spreading centre or mantle plume. Thenat the Mesozoic time this setting has changed by theMongolo Okhotsk type of tectonic scenario with simul-taneous continental collision and rifting processes. Thesetectonic environments prevailed during Early Mesozoictime and resulted in the generation of within-plate bimodaland alkaline basic magmatic associations in Western andEastern Transbaikalia, Khangai, Mongolian Altai, Sayanand Kuznetsky trough (Yarmolyuk et al., 2000, 2001).During the Late Mesozoic and Cenozoic the magmatism ofthe CAMB has an intraplate nature only (Yarmolyuk et al.,1995, 1996, 2000).Thus, intraplate magmatic activity occurred persistentlysince the Neoproterozoic until the Holocene with severalpeaks: at Early Middle Paleozoic, Late Paleozoic EarlyMesozoic and Late Early-Early Cenozoic. There iscorrelation of within-plate and active continental marginmagmatic activity, which xes the gradual closure of theCentral-Asia paleoocean (Table 1). For example, the EarlyMiddle Paleozoic period of the CAMB within-platemagmatism temporally relates with the formation of theCaledonides and Devonian active margin along of the Altaiborder of the Caledonian microcontinent. The LatePaleozoic intraplate activity was coeval with the closureof the Hercynian oceanic basin and active continentalmargin magmatism. The Early Mesozoic intraplate mag-matic event is well correlated with the closure of theIndosinian basins and respective island arc and collisionprocesses.3. Methods of the studyContinental crust is a result of relatively complexmagmatic processes of chemical differentiation frommantle. Due to large fractionation of Sm and Nd duringformation of granitoids and felsic volcanics from608 V.I. Kovalenko et al. / Journal of Asian Earth Sciences 23 (2004) 605627Table 1Sequence and correlation of tectonic and magmatic events in the Central-Asian Mobile BeltEpoch (Ma)Neoproterozoic(1000570)Oceanic segmentDunzhugur ophiolites (1020 Ma)Continental segmentConvergence settingFragments of continental-margin andriftogenious types magmatic belts in thePrecambrian TuvinoMongolian, Central-Mongolian and Barguzinskymicrocontinental blocks: bimodal volcanicsassociations, basic intrusions, granite andIntraplate settingGrabens along of southern and south-westernmargins of the Siberian craton: basaltic dykesswarms, ultrabasic rockcarbonatitecomplexes(720670, 600540 Ma).Riftogenesis and breackup of Rodinia,opening of the Paleo-Asian oceansubalkaline granite plutons (850700 Ma)?Neoproterozoic Cambrian (570510)Ordovician (500 450)Ensimatic island arcs (ophiolitebelts)? in the Altay and Sayanregions, northern Mongolia andTransbaikalia. Picritebasalt and?Altay, Sayan, northern Mongolia,Transbaikaliabasaltandesite associations,layered gabbro plutons (570,530 Ma)Late Ordovician Ensimatic island arcs (ophioliteDevonian (450360) belts) in the southern Mongolia(450400 Ma). Basalt and basalt-andesite associations, gabbro,gabbrodiorite and tonaliteplutonsLate DevonianEarlyCarboniferousCollision of Precambrian microcontinentalblocks and NeoproterozoicCambrian islandarcs of Caledonian paleoocean (500480 Ma), concolidation of Caledonianmobile belt (superterrain): collisional andpost-collisional granodioritegranitebatholites (500440 Ma)Collision of Caledonian superterrain and theSiberian craton, formation of the North-Asian paleocraton (450410 Ma)Mountain, Mongolian and Gobian AltayAltay active continental margin of the North-Asian paleocraton: granodioritegranite andgranite plutons, basaltandesite andandesitedaciteriolite associations (440360 Ma)Collision of the North-Asian paleocraton andisland arcs of Hercinian paleoocean,concolidation of Hercinian mobile belt(superterrain). Metamorphism, folding(350 Ma)Faults zones: layered gabbro (500480 Ma),alkaline and peralkaline syenites and granites(490, 460 Ma) plutonsAltay, Sayan and northern MongoliaFaults zones in the Altay active continentalmargin of the North-Asian paleocraton andintracontinental setting: alkaline gabbro(440360 Ma), alkaline and peralkalinesyenites and granites (450370) plutons,bimodal basalttrachiriolitekomenditeassociations (420410 Ma, 400380 Ma)Carboniferous Permian (340250)Ensimatic island arcs (ophiolitebelts) of the MongoloOkhotskMongolia, Transbaikalia(320 Ma) and Solonker (LateCarboniferousEarly Permian)basinsCarboniferous South-Mongolian activecontinental margin of the North-Asianpaleocraton: andesite, andesitedaciteriolite, riolitetrachiriolite association andgranodioritegranite and monzonitegranosyenitegranite plutonsLate CarboniferousEarly PermianMongoloTransbaikalian active continentalmargin of the North-Asian paleocraton:andesite, andesitedaciteriolite, riolitetrachiriolite associations; graniteleucogranite and monzonitegranosyeniteplutons (300270 Ma)Siberian mantle hot spot in the activecontinental margin: granodiorites andgranites of the Barguzine batholite (330290 Ma); alkaline and peralkaline granites,syenites, alkaline gabbro and carbonatites(310280 Ma) in the Sinnir and UdinoVitim rift zonesMongolian mantle hot spot in the in theactive continental margin: granodiorites,granites and leucogranites of the Khangaybatholite (270250 Ma); bimodal basalt-komendite association and alkaline andperalkaline granite plutons in the GobiTanShan (310380 Ma), Gobi-Altay(270260 Ma) and North-Mongolian (265250 Ma) rift zonesTriassic EarlyJurassic (240180)Collision of the North-Asian and Sino-Korean cratons at the western part ofMongoloOkhotsk trough: granodioritegranite, leucogranite and granosyeniteplutonsMongolian mantle hot spot in the collisionalzone of the North-Asian and Sino-Koreancratons: granodiorites and granites of theKhentey batholite (220200 Ma); alkaline,peralkaline and LiF granites plutons,basalts and bimodal basaltkomenditeassociations (230195 Ma)(continued on next page)Nd/ Nd 0.512638 and 147Sm/144Nd 0.1967V.I. Kovalenko et al. / Journal of Asian Earth Sciences 23 (2004) 605627 609Table 1 (continued)Epoch (Ma) Oceanic segment Continental segmentMiddle Jurassic,LateCretaceous (17070)Ceinozoic (600)Convergence settingCulmination of collision of the North-Asianand Sino-Korean cratons: granitoids of theUdaStanovoy batholite (150120 Ma)Intraplate settingWestern-Transbaikalian, Eastern-Mongolianand GobiAltay rift zones: bimodal basaltkomenditecarbonatite and alkaline basaltassciations (16070 Ma)Central-Asian mantle hot eld: alkalinebasalts associations of the Southern-Khangay, Southern-Transbaikalian,Western-Transbaikalian, Dariganga, etc. hotspotsthe mantle sources and relative constancy of the Sm/Ndratio in intracrustal processes (melting, metamorphism,erosion and sedimentation) Sm Nd isotope systematics offelsic igneous rocks are widely used to determine the age ofthe continental crust formation, and to evaluate theirpossible sources (DePaolo, 1988). Different aspects of Ndisotopic data interpretation are discussed in detail inn