Composition,sources, and mechanisms of formation of the continental crust of the Lake zone of the Central Asian Caledonides. II. Geochemical and Nd isotope data

Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological, geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents, which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile continental crust and allow us to distinguish a corresponding stage of juvenile crust formation.     

Crustal growth stages in the Songino block of the Early Caledonian superterrane in Central Asia: I. Geological and geochronological data

The Early Caledonian folded area in Central Asia (Early Caledonian superterrane) hosts micro-continent fragments with an Early and Late Precambrian crystalline basement, the largest of them being the Dzabkhan and Tuva-Mongolian fragments. Their junction zone hosts exposures of crystalline rocks that were previously thought to be part of the Early Precambrian Dzabkhan microcontinent. The Bayannur zone in the southern part of the Songino block hosts the Baynnur gneiss-migmatite and Kholbonur metavolcanic-terrigenous metamorphic complexes. The former is believed to be the Early Proterozoic crystalline basement, and the latter is thought to unconformably overly the Late Riphean cover complex of the Songino block. Various rocks of the tectono-stratigraphic complexes in the Bayannur zone were studied geologically and geochronologically (by the U-Pb technique of zircon). Regional metamorphism and folding in the Bayannur Complex were dated at 802 ± 6 Ma. The Nd model ages lie within the range of 1.5–2.0 Ga and thus preclude the correlation of these rocks with those in the Archean and Early Proterozoic basement of the Dzabkhan microcontinent. The upper age limit for folding and metamorphism in the Bayannur zone is marked by postkinematic granites dated at 790 ± 3 Ma, and the lower limit of the volcano-sedimentary complex is determined by the Nd model age of the sandstone (1.3 Ga). The upper age limit of the volcano-plutonic rocks in this zone is set by the gabbroids and anorthosites: 783 ± 2 and 784 ± 3 Ma, respectively. The complex of island-arc granitoids in the Bayannur zone is dated at 859 ± 3 Ma. The age constraints make it possible to correlate crystalline rocks in the Bayannur Complex of the Sangino block and the Dzhargalant Complex in the Tarbagatai block. Currently available data testify that the Precambrian Khangai group of blocks in the Early Caledonian Central Asian superterrane includes continental crustal blocks related to the processes of Early Precambrian, Late Riphean, and Vendian tectonism.     

Collisional granitoids of the Dzhida zone of the Central Asian Fold belt,Southwestern Transbaikalia: Age and conditions of the formation

This paper considers the geological structure, composition, and age of the Darkhintui, Barun-Gol, and Khuldat granitoid plutons of the Dzhida zone of Caledonides of the Central Asian Fold belts. These plutons were formed in the Late Cambrian-Early Ordovician in the range between 490 ± 2 and 477 ± 6 Ma, after tectonic juxtaposition of the oceanic and island-arc complexes of the Dzhida Zone and volcanogenic-carbonate-terrigenous rocks of the Khamardaban zone, i.e., at the collisional stage of the region evolution. Geological, geochronological, geochemical, and Nd isotope data indicate that the collisional granitoids of the Dzhida zone were derived by melting of continental crust thickened through accretion. The sources for parental melts of the granitoids were presumably Vendian-Early Cambrian juvenile igneous rocks of ophiolite and island-arc complexes, as well as the crustal material of the Lower Paleozoic flyschoid sediments of the back-arc basin of the Dzhida zone and metaterrigenous rocks of the Khamardaban zone.     

Isotopic structure and evolution of the continental crust in the East Transbaikalian segment of the Central Asian Foldbelt

New data on the geology and tectonics of the main structural elements of the East Transbaikalian segment of the Central Asian Foldbelt are discussed. Correlation charts of the main stratified and igneous complexes are compiled. The rocks of the Baikal-Patom and Baikal-Muya belts, as well as the Barguzin-Vitim Superterrane, are characterized by new Nd isotopic data, which have allowed us to establish the sources of these rocks, to separate isotopic provinces, and to distinguish two stages of crust-forming processes: the Early Baikalian (1.0–0.8 Ga) and the Late Baikalian (0.70–0.62 Ga). The Early Baikalian crust was formed in relatively narrow and spatially isolated troughs of the Baikal-Muya Belt and probably in the Amalat Terrane, whereas the Late Baikalian continental crust was formed and reworked in the Karalon-Mamakan, Yana, and Katera-Uakit zones of the Baikal-Muya Belt. The Baikal-Patom Belt and most of the Anamakit-Muya Zone in the Baikal-Muya Belt are characterized by remobilization of the Early Precambrian continental crust and by a subordinate role of Late Riphean juvenile sources. Reworking of the mixed Late Riphean and Early Precambrian crustal sources is typical of the Barguzin-Vitim Superterrane. The origination and evolution of the continental crust in the studied region are considered in light of new data; alternative versions of paleogedynamic reconstructions are discussed.     

Sources of granitoids in the Tuva-Mongolian microcontinent and surrounding structures: Evidence from Pb, Nd, and O isotopic systematics

Newly obtained data on Pb isotopic ratios in feldspars from Early Proterozoic granitoids of the Sangilen block of the Tuva-Mongolian microcontinent and Caledonian structures surrounding it, considered together with earlier data on the O and Nd isotopic systems, indicate that ancient (approximately 2 Ga) crustal material mixed in the sources of granites of the Tuva-Mongolian microcontinent with younger juvenile material. Positive ɛ_Nd values of granitoids from the Bashkimugor and Chgargalant massifs are accounted for by processes of crustal contamination during the interaction of the melt with crustal material. Similar Nd isotopic characteristics of granitoids in the Khoromnug pluton were caused by the melting of the Late Riphean crust. In granitoids of the massifs in the Kaakhem and Eastern Tannuola zones of the surrounding Caledonian structures, the involvement of juvenile material from oceanic crust increases away from the Sangilen block. Granites in the junction zone between the Tuva-Mongolian microcontinent and surrounding structures display evidence of the presence (up to 10–20%) of an ancient crustal component, and the melting history of granitoids in the Eastern Tannuola zone is dominated by an Early Paleozoic juvenile component in combination with material similar to the Vendian ophiolites of the Agardag-Erzin zone. An increase in the δ¹⁸O value, the ²⁰⁶Pb/²⁰⁴Pb ratio, and the T_Nd(DM) values within a single complex (from older to younger granitoid phases) is explained by the systematic involvement of crustal material in the melting processes.     

Age and sources of granitoids in the junction zone of the Caledonides and Hercynides in southwestern Mongolia: Geodynamic implications

The paper reports newly obtained geological, geochronological (U-Pb zircon method), Nd isotopic, and geochemical data on Middle and Late Paleozoic granitoids and metamorphic rocks from the southern slope of the Mongolian Altai and Gobi Altai and on granitoids from the Trans-Altai Gobi. Tectonically, the former rocks are hosted in the margin of a Caledonian paleocontinent, and the latter are localized among island-arc and oceanic complexes related to the development of the Hercynian Southern Mongolian Ocean. According to their geological setting, the intrusive complexes are subdivided into two major groups: (i) related to processes of regional metamorphism and (ii) separated from these processes. Geochemical data suggest that the source of most of the granitoids and metamorphic rocks contained island-arc rocks and their erosion products. Nd isotopic evidence indicates that practically all of the allochthonous granitoids, regardless of their composition, age, and structural setting, have positive ?_Nd(T) values [i.e., belong to the ?(+) type] and could not be formed by the melting of metaterrigenous rocks widespread at the modern erosion level. These granitoids in both the Caledonian and the Hercynian structures have practically identical Late Riphean Nd model ages [T_Nd(DM) = 0.97–0.60 Ga], which become slightly younger in the granitoids of the Hercynides. The exception are ultrametamorphic subautochthonous ?(?) granites of the first group localized in the peripheral part of migmatite fields. The sources of these granitoids could be the host metaterrigenous rocks. The results obtained in the course of this research suggest, with regard for preexisting data on granitoids in the isotopic provinces in Central Asia, that the sources of the Paleozoic granitoids were the rocks of the “juvenile” Caledonian and Hercynian island-arc crust and of the older crust of cratonic blocks with a Early Precambrian and Late Riphean basement, respectively. The Late Riphean crustal material in Caledonian and Hercynian structures related to the development of the corresponding oceanic basins most probably consisted of clastic sediments or relatively small fragments of the Late Riphean crust. The occurrence of this crustal material in the sources of the granitoids can be explained by the involvement of sediments in subduction zones and the participation of these sediments and fragments of Late Riphean complexes in the accretionary-collision processes during the closure of the paleoceanic basins. Simultaneously, the subduction zones received juvenile material that could be later involved in the melting processes together with older rocks.     

Late Riphean episode in the formation of crystalline rock complexes in the Dzabkhan microcontinent: Geological,geochronologic, and Nd isotopic-geochemical data

The Early Caledonian Central Asian Orogenic Belt hosts fragments of continental blocks with Early and Late Precambrian crystalline basement. One of the structures with an Early Precambrian basement was thought to be the Dzabkhan microcontinent, which was viewed as an Early Precambrian “cratonal terrane”. The first geochronologic data suggest that the basement of the Dzabkhan microcontinent includes a zone of crystalline rocks related to Late Riphean tectonism. Geological, geochronological (U-Pb zircon dates), and Nd isotopic-geochemical data were later obtained on the northwestern part of the Dzabkhan microcontinent. The territory hosts the most diverse metamorphic complexes thought to be typical of the Early Precambrian basement. The complexes were determined to comprise the Dzabkhan-Mandal and Urgamal zones of high-grade metamorphic rocks. Gabbrodiorites related to the early metamorphic episode and dated at 860 ± 3 Ma were found in the Dzabkhan-Mandal zone, and the gneiss-granites marking the termination of this episode were dated at 856 ± 2 Ma. The granitoids of the Dzabkhan batholith, whose emplacement was coeval with the termination of the late high-grade metamorphic episode in rocks of both zones, have an age of 786 ± 6 Ma. Similar age values were determined for the granitoids cutting across the Late Precambrian rocks of the Songino and Tarbagatai blocks, which mark the stage when the mature Late Riphean continental crust was formed. The Late Riphean magmatic and metamorphic rocks of the Dzabkhan microcontinent were found out to have Nd model ages mostly within the range of 1.1–1.4 Ga at ?_Nd(T) from +1.9 to +5.5. The Nd model age of the metaterrigenous rocks is 2.2?1.3 Ga at ?_Nd(T) from ?7.2 to +3.1. The results of our studies provide evidence of convergence processes, which resulted in the Late Riphean (880?780 Ma) continental crust in Central Asia. Simultaneously with these processes, divergence processes that were responsible for the breakup of Rodinia occurred in the structures of the ancient cratons. It is reasonable to suggest that divergence processes within ancient continental blocks and Rodinia shelf were counterbalanced by the development of the Late Riphean continental crust in the convergence zones of its surrounding within established interval.     

Sedimentary complexes of the cover of the Dzabkhan continental block: Different sedimentary basins and source areas

The geochemical and Sm–Nd isotope characteristics of Late Precambrian and Early Cambrian sandstones previously related to the sedimentary cover of the Dzabkhan continental block are reported. It is established that the Riphean and Vendian sedimentary rocks of the Ul’zitgol’skaya and Tsaganolomskaya Formations were accumulated within the Dzabkhan continental block as a result of recycling of the terrigenous deposits formed at the expense of destruction of basement rocks and younger granite. The formation of terrigenous rocks of the Bayangol’skaya Formation after a gap in sedimentation occurred in the sedimentary basin, where only the Late Riphean formations of the juvenile crust, probably of the Dzabkhan–Mandal block were the sources, without the contribution of the ancient crustal material. The Tsaganolomskaya and Bayangol’skaya Formations were formed in different sedimentary basins and cannot be related to the same complex.     

Continental crust of Gorny Altai: stages of formation and evolution; indicative role of granitoids

The composition and mechanisms of formation of continental crust in Gorny Altai and the role of granitoid magmatism in its evolution are considered. Geochemical and isotope data for major types of rocks of primary crust and for Early–Middle Paleozoic granitoids of the region are presented. The role of granitoids as indicators of the different stages of the continental-crust evolution is discussed. A review of the main models of continental crust formation is maid, and their applicability to the Gorny Altai segment of the Central Asian Fold Belt is shown. Based on the complex of geological, geochemical, isotope, and geochronological data, it has been established that the formation of continental crust in the Early and Late Caledonian terranes of Gorny Altai proceeded nearly synchronously (in the Middle–Late Devonian).In the Early Caledonian terranes, this process was the consequence of the multistage fractionation of primary juvenile crust of basic composition, and in the Late Caledonian ones it was the result of one-cycle intracrustal melting of hybrid andesitic crust rich in recycled material.     

Early Precambrian granitoids of the Batomga inlier of the southeastern Siberian Platform basement: Age and geodynamic formation settings

New data on the age, composition, sources, and formation conditions of the Early Precambrian granitoids of the Batomga inlier of the southeastern Siberian Platform basement are discussed. Geochronological SRHIMP II U–Pb study of the zircons reveals that the calc-alkaline granitoids of the Khoyunda Complex are 2056–2057 Ma in age and their formation was related to the Early Proterozoic stage in the development of the Batomga granite–greenstone domain. It is established that the primary melts for these rocks formed in subduction settings through melting of the depleted mantle source with some contribution of ancient crustal material. In terms of temperature, partial melting followed by crystallization of the granitoids under peak metamorphic conditions corresponds to the transition between amphibolite and granulite facies at elevated pressure; high temperature and high-grade metamorphism are subduction-related phenomena reflected in the back-arc settings of the active continental margin. The protoliths of calc-alkaline metavolcanics of the Batomga Group are found to be chronologically and compositionally analogous to the subduction granitoids of the Khoyunda and Dzhagdakan complexes; i.e., these granitoids are coeval with the Batomga island arc. The lower age limit of the Batomga Group is estimated at 2.2 Ga and its upper age limit is defined by the age of the intruded Khoyunda granitoids. The formation of the rocks of the Batomga Group and associated granitoids of the Khoyunda and Dzhagdakan complexes reflects the formation of the continental crust at the Early Paleoproterozoic stage of the evolution of the Batomga lithosphere block (2.2–2.0 Ga ago).     

Nd and Pb Isotopic Composition of Granitoids in the Khangai Batholith as an Indicator of Crust-Forming Processes in the Terranes of the Central Asian Orogenic Belt

The Khangai batholith is one of the largest groups of granitoid plutons produced in Central Asia in the Late Permian–Early Triassic, at 270–240 Ma. The batholith occurs in the Khangai collage of Precambrian terranes, which include Early Precambrian crustal blocks (Dzabkhan and Tarbagatai) and Early to Late Neoproterozoic structures of the Songino block in their surroundings. The axial zone of this collage is overprinted by a basin filled with Devonian volcanic–siliceous rocks and Early to Middle Carboniferous terrigenous rocks. The isotopic parameters (Nd and Pb) of granitoids in the Khangai batholith indicate that the melts were derived from compositionally contrasting crustal sources and a single mantle one. The massifs hosted in the Precambrian blocks were produced with the involvement of lower crustal material, with various ages of the origin of the crust and its differentiation into upper and lower ones. The crust of the Tarbagatai and Dzabkhan blocks was produced in the Early Archean and was differentiated at the Archean–Proterozoic boundary. The crust of the Songino block was formed in the Paleoproterozoic and differentiated in the Early Neoproterozoic. According to the Pb and Nd isotopic parameters of granitoids in the Khangai Basin, the regional continental crust was close to the juvenile one, i.e., the continental crust of the Khangai Basin had still not been differentiated by the time when the Khangai batholith was produced. A single mantle source was involved in the origin of the melts of granitoids of the Khangai batholith in various tectonic blocks. The evolution of the Pb isotopic composition of this sources is consistent with the Stacey–Kramers model at µ = 9.5. This source can be identified with the enriched mantle, which has a higher U/Pb ratio than the depleted mantle and lower ε_Nd(T) of 0 to +2.     

Pb isotope composition of granitoids from the Hercynides of the Central Asian orogenic Belt: Evidence for growth of the juvenile crust

Based on study of the Middle Paleozoic (Hercynian) structures of the Central Asian Fold Belt located in Southwest Mongolia, the problem of the sources of juvenile crust growth registered in the Pb isotope composition of postaccretional granitoids is considered. Our study shows that the Pb isotope system of Late Paleozoic granitoids from the Hercynides of the Central Asian Fold Belt provides evidence for the juvenile nature of the continental crust in this region. The evolution of the Pb isotope composition in the Hercynides of Mongolia corresponds to the model parameter μ = 9.25, according to the Stacey–Kramers twostage model. The juvenile source of the Hercynian crust should be characterized by a lower μ value. In addition, according to the Stacey–Kramers two-stage model, the Th/U value in this crust is 3.58.     

Reassessment of continental growth during the accretionary history of the Central Asian Orogenic Belt

We argue that the production of mantle-derived or juvenile continental crust during the accretionary history of the Central Asian Orogenic Belt (CAOB) has been grossly overestimated. This is because previous assessments only considered the Palaeozoic evolution of the belt, whereas its accretionary history already began in the latest Mesoproterozoic. Furthermore, much of the juvenile growth in Central Asia occurred in late Permian and Mesozoic times, after completion of CAOB evolution, and perhaps related to major plume activity. We demonstrate from zircon ages and Nd–Hf isotopic systematics from selected terranes within the CAOB that many Neoproterozoic to Palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous Precambrian crust or through mixing of old continental crust with juvenile or short-lived material, most likely in continental arc settings. At the same time, juvenile growth in the CAOB occurred during the latest Neoproterozoic to Palaeozoic in oceanic island arc settings and during accretion of oceanic, island arc, and Precambrian terranes. However, taking together, our data do not support unusually high crust-production rates during evolution of the CAOB. Significant variations in zircon εHf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources. We reiterate that the chemical characteristics of crustally-derived granitoids are inherited from their source(s) and cannot be used to reconstruct tectonic settings, and thus many tectonic models solely based on chemical data may need re-evaluation. Crustal evolution in the CAOB involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history, and we see many similarities with the evolution of the SW Pacific and the Tasmanides of eastern Australia.     

The formation processes and isotopic structure of continental crust of the Chingiz Range Caledonides (Eastern Kazakhstan)

According to this paper, the juvenile crust of the Chingiz Range Caledonides (Eastern Kazakhstan) was formed due to suprasubduction magmatism within the Early Paleozoic island arcs developed on the oceanic crust during the Cambrian–Early Ordovician and on the transitional crust during the Middle–Late Ordovician, as well as to the attachment to the arcs of accretionary complexes composed of various oceanic structures. Nd isotopic compositions of the rocks in all island-arc complexes are very similar and primitive (ε_Nd(t) from +4.0 to +7.0) and point to a short crustal prehistory. Further increase in the mass and thickness of the crust of the Chingiz Range Caledonides was mainly due to reworking of island-arc complexes in the basement of the Middle and Late Paleozoic volcanoplutonic belts expressed by the emplacement of abundant granitoids. All Middle and Late Paleozoic granitoids have high positive values of ε_Nd(t) (at least +4), which are slightly different from Nd isotopic compositions of the rocks in the Lower Paleozoic island-arc complexes. Granitoids are characterized by uniform Nd isotopic compositions (<2–3 ε units for granites with a similar age), and thus we can consider the Chingiz Range as the region of the Caledonian isotope province with an isotopically uniform structure of the continental crust.     

Late Riphean age of conglomerates from the Kholbonur complex of Songino block, Central Asian Caledonides

The Early Caledonian folded area of Central Asia comprises a variety of continental crust fragments with Early to Late Precambrian crystalline basement. Crystalline rocks, which form part of the Songino block, outcrop at the junction between the Dzabkhan and Tuva-Mongolian terranes. The Bayannur zone in the southern part of the Songino block contains the Bayannur migmatite-gneiss and Kholbonur terrigenous-metavolcanic metamorphic complexes. Previous studies provide the 802 ± 6 Ma age for the regional metamorphism and folding within the Bayannur complex. On the basis of the minimum Nd model age of 1.5 Ga, gneisses from this complex cannot be regarded as Early Precambrian. Two main rock associations were distinguished in the Kholbonur complex. Mafic metavolcanics compose the dominant lithology of the first rock association, whereas the second association comprises terrigenous-volcanic and predominantly terrigenous suites. The rocks of the predominantly terrigenous suite, including mudstones, sandstones, and conglomerates, are interpreted to derive from the Late Riphean accretionary prism. The lithology and composition of metaterrigenous rocks suggest that they were possibly derived from erosion of a volcanic arc. The upper age limit of this suite is constrained by postkinematic granites (790 ± 3 Ma; U-Pb zircon), the lower age is given by plagiogranite (874 ± 3 Ma; U-Pb zircon) from comglomerate pebbles. Therefore, the timing of deposition of this terrigenous suite can be bracketed by the 874–790 Ma time interval. These ages and compositional features of the Kholbonur complex terrigenous rocks suggest that the convergence took place at around 870–880 Ma and thus it can be correlated with the divergent processes between the blocks of continental crust composing the supercontinent Rodinia.     

Carboniferous Highly Fractionated I‐type Granites from the Kalamaili Fault Zone,Eastern Xinjiang,NW China: Petrogenesis and Tectonic Implications

Carboniferous magmatism is one of the most important tectonothermal events in the Central Asian Orogenic Belt(CAOB). However, the final closure time of the Kalamaili Ocean between East Junggar and Harlik Mountain is still debated. Early Carboniferous(332 Ma) and late Carboniferous(307–298 Ma) granitic magmatism from Kalamaili fault zone have been recognized by LA-ICP-MS zircon U-Pb dating. They are both metaluminous highly fractionated I-type and belong to the high-K calc-alkaline. The granitoids for early Carboniferous have zircon ε_(Hf)(t) values of-5.1 to +8.5 with Hf model ages(T_(DM2)) of 1.78–0.83 Ga, suggesting a mixed magma source of juvenile material with old continental crust. Furthermore, those for late Carboniferous have much younger heterogeneous zircon ε_(Hf)(t) values(+5.1 to +13.6) with Hf model ages(T_(DM2)=1.03–0.45 Ga) that are also indicative of juvenile components with a small involvement of old continental crust. Based on whole-rock geochemical and zircon isotopic features, these high-K granitoids were derived from melting of heterogeneous crustal sources or through mixing of old continental crust with juvenile components and minor AFC(assimilation and fractional crystallization). The juvenile components probably originated from underplated basaltic magmas in response to asthenospheric upwelling. These Carboniferous highly fractionated granites in the Kalamaili fault zone were probably emplaced in a post-collisional extensional setting and suggested vertical continental crustal growth in the southern CAOB, which is the same or like most granitoids in CAOB. This study provides new evidence for determining the post-accretionary evolution of the southern CAOB. In combination with data from other granitoids in these two terranes, the Early Carboniferous Heiguniangshan pluton represents the initial record of post-collisional environment, suggesting that the final collision between the East Junggar and Harlik Mountain might have occurred before 332 Ma.     

Tectonic evolution of Early Paleozoic island-arc systems and continental crust formation in the Caledonides of Kazakhstan and the North Tien Shan

The extended Saryarka and Shyngyz-North Tien Shan volcanic belts that underwent secondary deformation are traced in the Caledonides of Kazakhstan and the North Tien Shan. These belts are composed of igneous rocks pertaining to Early Paleozoic island-arc systems of various types and the conjugated basins with oceanic crust. The Saryarka volcanic belt has a complex fold-nappe structure formed in the middle Arenigian-middle Llanvirnian as a result of the tectonic juxtaposition of Early-Middle Cambrian and Late Cambrian-Early Ordovician complexes of ensimatic island arcs and basins with oceanic crust. The Shyngyz-North Tien Shan volcanic belt is characterized by a rather simple fold structure and consists of Middle-Late Ordovician volcanic and plutonic associations of ensialic island arcs developing on heterogeneous basement, which is composed of complexes belonging to the Saryarka belt and Precambrian sialic massifs. The structure and isotopic composition of the Paleozoic igneous complexes provide evidence for the heterogeneous structure of the continental crust in various segments of the Kazakh Caledonides. The upper crust of the Shyngyz segment consists of Early Paleozoic island-arc complexes and basins with oceanic crust related to the Saryarka and Shyngyz-North Tien Shan volcanic belts in combination with Middle and Late Paleozoic continental igneous rocks. The deep crustal units of this segment are dominated by mafic rocks of Early Paleozoic suprasubduction complexes. The upper continental crust of the Stepnyak segment is composed of Middle-Late Ordovician island-arc complexes of the Shyngyz-North Tien Shan volcanic belt and Early Ordovician rift-related volcanics. The middle crustal units are composed of Riphean, Paleoproterozoic, and probably Archean sialic rocks, whereas the lower crustal units are composed of Neoproterozoic mafic rocks.     

兴蒙造山带正ε(Nd,t)值花岗岩的成因和大陆地壳生长

大陆地壳的生长速率和地壳生长的位置均是地球科学中的最基本的问题。现有的许多大陆地壳生长模式认为 ,90 %的大陆地壳生长于 18亿年以前 ,显生宙以来的地壳生长不到整个地壳的 10 % ,主要位于活动大陆边缘。近年来在兴蒙造山带发现大量具有新生地壳来源性质的花岗岩产生于 50 0～ 10 0Ma ,对上述传统看法提出了挑战。现有的Nd同位素资料表明 ,兴蒙造山带的显生宙花岗岩 ,不论形成于什么时代和什么构造背景 ,也不论属于何种成因类型 ,几乎都具有正ε(Nd ,t)值和年轻的Nd模式年龄tDM 。从西往东 ,随着时代逐渐变新ε(Nd ,t)值有逐渐降低的趋势。花岗岩的tDM同由蛇绿岩和岛弧杂岩记录的古亚洲洋扩张的时间基本一致。只有一些在新元古代微陆块上的花岗岩才显示负ε(Nd ,t)值和较老的tDM,反映了其源岩包括前寒武纪地壳同地幔来源物质的不同程度混合。兴蒙造山带的花岗岩具有地幔来源的ε(Nd ,t)值 ,说明这些花岗岩中有一部分 (例如加里东期和海西早期 )可能同板块俯冲作用有关 ,花岗岩的来源是被交代的地幔楔。而大面积的晚古生代—中生代花岗岩则可能是由 80 0～6 0 0Ma前俯冲的洋壳形成的新生大陆地壳在拉伸体制下部分熔融而成。如果情况是这样 ,显生宙就曾发生过大规模的地壳生长。板内岩浆活动 ,特别是     

Juvenile versus recycled crust in the Central Asian Orogenic Belt: Implications from ocean plate stratigraphy,blueschist belts and intra-oceanic arcs

New or “juvenile” crust forms and grows mainly through mafic to andesitic magmatism at Pacific-type or accretionary type convergent margins as well as via tectonic accretion of oceanic and island-arc terranes and translation of continental terranes. During the last decades the juvenile or recycled nature of crust has been commonly evaluated using whole-rock isotope and Hf-in-zircon isotope methods. However, evidence for the accretionary or Pacific-type nature of an orogenic belt comes from geological data, for example, from the presence of accretionary complexes (AC), intra-oceanic arcs (IOA), oceanic plate stratigraphy units (OPS), and MORB-OIB derived blueschist belts (BSB). The Central Asian Orogenic Belt (CAOB) represents the world's largest province of Phanerozoic juvenile crustal growth during ca. 800 m.y. between the East European, Siberian, North China and Tarim cratons. From geological point of view, the CAOB is a typical Pacific-type belt as it hosts numerous occurrences of accretionary complexes, intra-oceanic arcs, OPS units, and MORB-OIB derived blueschist belts. In spite of its accretionary nature, supported by positive whole rock Nd isotope characteristics in CAOB granitoids, the Hf-in-zircon isotope data reveal a big portion of recycled crust. Such a controversy can be explained by presence of accreted microcontinents, isotopically mixed igneous reservoirs and by the tectonic erosion of juvenile crust. The most probable localities of tectonic erosion in the CAOB are the middle and southern Tienshan and southern Transbaikalia because these regions comprise a predominantly recycled crust (based on isotope data), but the geological data show the presence of intra-oceanic arcs, blueschist belts and accreted OPS with oceanic island basalts (OIB) and tectonically juxtaposed coeval arc granitoids and accretionary units. This warrants combination of detailed geological studies with isotopic results, as on their own they may not reflect such processes as tectonic erosion of juvenile crust and/or arc subduction.     

Dzhida island-arc system in the Paleoasian Ocean: structure and main stages of Vendian-Paleozoic geodynamic evolution

We propose a model of the geodynamic evolution of the Dzhida island-arc system of the Paleoasian Ocean margin which records transformation of an oceanic basin into an accretion-collision orogenic belt. The system includes several Vendian-Paleozoic complexes that represent a mature oceanic island arc with an accretionary prism, oceanic islands, marginal and remnant seas, and Early Ordovician collisional granitoids. We have revealed a number of subunits (sedimentary sequences and igneous complexes) in the complexes and reconstructed their geodynamic settings. The tectonic evolution of the Dzhida island-arc system comprises five stages: (1) ocean opening (Late Riphean); (2) subduction and initiation of an island arc (Vendian-Early Cambrian); (3) subduction and development of a mature island arc (Middle-Late Cambrian); (4) accretion and formation of local collision zones and remnant basins (Early Ordovician-Devonian); and (5) postcollisional strike-slip faulting (Carboniferous-Permian).