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.     

Intra-oceanic arcs of the Paleo-Asian Ocean

The paper reviews and integrates geological, geochronological, geochemical and isotope data from 21 intra-oceanic arcs (IOA) of the Paleo-Asian Ocean (PAO), which have been identified in the Central Asian Orogenic belt, the world largest accretionary orogeny. The data We discuss structural position of intra-oceanic arc volcanic rocks in association with back-arc terranes and accretionary complexes, major periods of intra-oceanic arc magmatism and related juvenile crustal growth, lithologies of island-arc terranes, geochemical features and typical ranges of Nd isotope values of volcanic rocks. Four groups of IOAs have been recognized: Neoproterozoic – early Cambrian, early Paleozoic, Middle Paleozoic and late Paleozoic. The Neoproterozoic – early Cambrian or Siberian Group includes eleven intra-oceanic arcs of eastern and western Tuva-Sayan (southern Siberia, Russia), northern and southwestern Mongolia and Russian Altai. The Early Paleozoic or Kazakhstan Group includes Selety-Urumbai, Bozshakol-Chingiz and Baydaulet-Aqastau arc terranes of the Kazakh Orocline. The Middle Paleozoic or Southern Group includes six arc terranes in the Tienshan orogen, Chinese Altai, East-Kazakhstan-West Junggar and southern Mongoia. Only one Late Paleozoic intra-oceanic arc has been reliably identified in the CAOB: Bogda in the Chinese Tienshan, probably due to PAO shrinking and termination. The lithologies of the modern and fossil arcs are similar, although the fossil arcs contain more calc-alkaline varieties suggesting either their more evolved character or different conditions of magma generation. Of special importance is identification of back-arc basins in old accretionary orogens, because boninites may be absent in both modern and fossil IOAs. The three typical scenarios of back-arc formation - active margin rifting, intra-oceanic arc rifting and fore-arc rifting were reconstructed in fossil intra-oceanic arcs. Some arcs might be tectonically eroded and/or directly subducted into the deep mantle. Therefore, the structural and compositional records of fossil intra-oceanic arcs in intracontinental orogens allow us to make only minimal estimations of their geometric length, life span, and crust thickness.     

http://www.sciencedirect.com/science/article/pii/S1674987113001606

The paper reviews previous and recently obtained geological, stratigraphic and geochronological data on the Russian-Kazakh Altai orogen, which is located in the western Central Asian Orogenic Belt （CAOB）, between the Kazakhstan and Siberian continental blocks. The Russian-Kazakh Altai is a typical Pacific-type orogen, which represents a collage of oceanic, accretionary, fore-arc, island-arc and continental margin terranes of different ages separated by strike-slip faults and thrusts. Evidence for this comes from key indicative rock associations, such as boninite- and turbidite （graywacke）-bearing volcanogenic-sedimentary units, accreted pelagic chert, oceanic islands and plateaus, MORB-OIB-protolith blueschists. The three major tectonic domains of the Russian-Kazakh Altai are： （1） Altai-Mongolian terrane （AMT）; （2） subduction-accretionary （Rudny Altai, Gorny Altai） and collisional （Kalba-Narym） terranes; （3） Kurai, Charysh-Terekta, North-East, Irtysh and Char suture-shear zones （SSZ）. The evolution of this orogen proceeded in five major stages： （i） late Neoproterozoic-early Paleozoic subduction-accretion in the Paleo-Asian Ocean; （ii） Ordovician-Silurian passive margin; （iii） Devonian-Carboniferous active margin and collision of AMT with the Siberian conti- nent; （iv） late Paleozoic closure of the PAO and coeval collisional magmatism; （v） Mesozoic post-collisional deformation and anarogenic magmatism, which created the modern structural collage of the Russian- Kazakh Altai orogen. The major still unsolved problem of Altai geology is origin of the Altai-Mongolian terrane （continental versus active margin）, age of Altai basement, proportion of juvenile and recycled crust and origin of the middle Paleozoic units of the Gorny Altai and Rudny Altai terranes.     

No excessive crustal growth in the Central Asian Orogenic Belt: Further evidence from field relationships and isotopic data

We provide new field observations and isotopic data for key areas of the Central Asian Orogenic Belt (CAOB), reiterating our previous assessment that no excessive crustal growth occurred during its ca. 800 Ma long orogenic evolution. Many Precambrian blocks (microcontinents) identified in the belt are exotic and are most likely derived from the northern margin of Gondwana, including the Tarim craton. Ocean opening in the Palaeo-Asian Ocean, arc formation and accretionary processes began in the latest Mesoproterozoic along the southern margin of the Siberian craton and continued into the Neoproterozoic, giving rise to tectono-metamorphic terranes distinct from the exotic microcontinents in that they include tectonically mixed ancient crust as well as juvenile, mantle-derived igneous rocks. Several previous assessments of crustal growth based on the distribution of oceanic and island arc complexes and on Nd isotopic data for post-accretion igneous rocks are questionable, and we show that such data, in combination with the occurrence of old zircon xenocrysts, frequently signify tectonic mixing of juvenile and ancient crustal components.The only truly juvenile terranes, including oceanic crust and intra-oceanic arcs, seem to occur in northeastern Kazakhstan, in the Altai-Sayan region of Siberia and in the Lake and Trans-Altai zones of Mongolia. The largest area of pre-CAOB continental crust forms a broad belt from northwestern Kazakhstan via the Kyrgyz North and Middle Tianshan to the Yili Block and Chinese Central Tianshan in NW China. Most arcs in the CAOB formed on older continental crust, or with substantial addition of old crustal material via sediment recycling, similar to the situation in the present Southwest Pacific in southern Indonesia, and we suspect that the volume of old material in the lower crust of the CAOB is considerable but largely unaccounted for because of lack of geophysical data. Comparing the lithospheric mantle domains as revealed by Os model ages, with ancient crust at least Mesoproterozoic in age and predating formation of the CAOB significantly reduces the volume of new juvenile crust generated during the orogeny. We conclude that the volume of truly juvenile crustal material in the CAOB is about 20%, similar to that in other accretionary orogens through Earth history, and considering the ca. 800 Ma history of the belt this is not anomalous.     

The building blocks of continental crust: Evidence for a major change in the tectonic setting of continental growth at the end of the Archean

Oceanic arcs are commonly cited as primary building blocks of continents, yet modern oceanic arcs are mostly subducted. Also, lithosphere buoyancy considerations show that oceanic arcs (even those with a felsic component) should readily subduct. With the exception of the Arabian–Nubian orogen, terranes in post-Archean accretionary orogens comprise < 10% of accreted oceanic arcs, whereas continental arcs compose 40–80% of these orogens. Nd and Hf isotopic data suggest that accretionary orogens include 40–65% juvenile crustal components, with most of these (> 50%) produced in continental arcs.Felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs. They have lower Th/Yb, Nb/Yb, Sr/Y and La/Yb ratios, reflecting shallow mantle sources in which garnet did not exist in the restite during melting. The bottom line of these geochemical differences is that post-Archean continental crust does not begin life in oceanic arcs. On the other hand, the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs.During the Archean, however, oceanic arcs may have been thicker due to higher degrees of melting in the mantle, and oceanic lithosphere would be more buoyant. These arcs may have accreted to each other and to oceanic plateaus, a process that eventually led to the production of Archean continental crust. After the Archean, oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges, hence, oceanic lithosphere is more subductable. Widespread propagation of plate tectonics in the late Archean may have led not only to rapid production of continental crust, but to a change in the primary site of production of continental crust, from accreted oceanic arcs and oceanic plateaus in the Archean to primarily continental arcs thereafter.     

Recycling oceanic crust for continental crustal growth: Sr–Nd–Hf isotope evidence from granitoids in the western Junggar region,NW China

The juvenile component of accretionary orogenic belts has been declining since the Archean. As a result, there is often controversy regarding the contribution of oceanic basalts to Phanerozoic crustal growth, as in the case of the Central Asian Orogenic Belt (CAOB). Here we report on three groups of Late Carboniferous (316–305 Ma) granitoids in the western Junggar region of northern Xinjiang, NW China, which is part of the southwestern CAOB. They consist of adakites and I and A-type granites, and as a whole have the most depleted isotopic compositions (ε_Nd(t) = + 6–+9, (⁸⁷Sr/⁸⁶Sr)_i = 0.7030–0.7045, and ε_Hf(t) = + 12–+16) among the granitoids of the CAOB. These features are nearly identical to those of pre-Permian ophiolites in northern Xinjiang, and are clearly different from those of Carboniferous basalts in the western Junggar region. These relationships indicate that the granitoids were mainly derived from recycled oceanic crust by melting of subducted oceanic crust (e.g., adakites), and of the middle–lower crust of intra-oceanic arc that mainly consisted of oceanic crust (e.g., I and A-type granites). Based on evidence from the CAOB, we suggest that recycling of oceanic crust has made a significant contribution to continental crustal growth and evolution during the Phanerozoic.     

Fore-arc evolution and continental growth: a general model

The extended evolution of fore-arc regions which leads to their eventual incorporation into stable kratonic continental crust is elucidated by a general model based upon observations from the modern circum-Pacific and the Palaeozoic Tasman Geosyncline.Fore-arc regions widen during subduction in the manner described by Karig & Sharman (1975). Their history, after subduction has ceased, depends upon the thickness of the accretionary prism formed during subduction. Where the prism is thick (ca. 20 km) kratonization is a single-step process. The fore-arc region remains above sea-level; post-arc silicic volcanics accumulate due to granitoid plutonism, the magmas being derived by melting of the subduction complex and from the oceanic lithosphere trapped beneath it. The volcanic arc subsides, becoming the site of a fore-deep.Intermediate-thickness accretionary prisms (ca. 16 km) are kratonized in a two-step process. They remain at shelf depths, while their associated volcanic arcs sink to comparable depths. Both acquire a post-arc shallow marine sequence of typical platform-cover facies. They are then deformed and intruded by granitoids when the crust attains critical thickness (ca. 20 km).Thin accretionary prisms (≤ 12 km) require a three-step process for kratonization. They and their associated arcs sink to bathyal depths. They are overwhelmed by prograding post-arc flysch deposits of continental origin. Deformation of the post-arc flysch and plutonism occur when critical crustal thickness (ca. 20 km) is attained. A transitional tectonic regime ensues, with molasse-like transitional basins preferentially sited over the extinct volcanic arcs and the thinner parts of buried accretionary prisms.The model satisfactorily explains the Late Proterozoic-Palaeozoic evolution of southeast Australia, where a 1000 km wide tract of continental crust was accreted to the Australian Kraton in 250–300 Ma, beginning as a S.W. Pacific-type oceanic terrain. It has been found useful for interpreting geosynclinal terrains in other continents.According to the model, the dynamic processes that contribute to kratonization are systematically causally connected. Kratonization is a unified, internally deterministic and self-sustaining phenomenon. The model has implications for the origin, ‘stratigraphy’ and composition of upper and lower continental crust; the origins and tectonic settings of ophiolites, granitoids, paired metamorphic belts and transitional basins; and for the nature and causes of orogenesis.     

Asia: a frontier for a future supercontinent Amasia

Asia is the world’s largest but youngest continent, in which Pacific-type (P-type) and collision-type (C-type) orogenic belts coexist with numerous amalgamated continental blocks. P-type orogens represent major sites of continental growth through tonalite-trondhjemite-granodiorite type (TTG-type) juvenile granitoid magmatism and accretion of oceanic crust and intra-oceanic arcs. The Asian continent includes several P-type orogenic belts, of which the largest are the Central Asian and Western Pacific. The Central Asian Orogenic Belt is dominated by P-type fossil orogens arranged with a regular northward subduction polarity. The Western Pacific is characterized by ongoing P-type orogeny related to the westward subduction of the Pacific plate. Asia has a multi-cratonic structure and its post-Palaeozoic history has witnessed amalgamation of the Laurasia composite continent and Pangaea supercontinent. Nowadays, Asia is surrounded by double-sided subduction zones, which generate new TTG-type crust and supply oceanic crust and microcontinents to its active margins. The TTG-crust can be tectonically eroded and subducted down to the mantle transition zone to form a ‘second’ continent, which may generate mantle upwelling, plumes, and extensive intra-plate volcanism. Moreover, recent plate movements around Asia are dominated by northward directions, which resulted in the India–Eurasia and Arabia–Eurasia collisions beginning at 50–45 and 23–20 Ma, respectively, and will result in Africa–Eurasia collision in the near future. Therefore, Asia is the best candidate to serve as the nucleus for a future supercontinent ‘Amasia’, likely to form 200–250 Ma in the future. In this paper we unravel a puzzle of continental growth in Asia through P-type orogeny by discussing its tectonic history and geological structure, subduction polarity in P-type orogens, tectonic erosion of TTG-type crust and arc subduction at convergent margins, generation of mantle plumes, and prospects of Asia growth and overgrowth.     

西藏南羌塘晚三叠世陆缘俯冲增生造山带的褶皱-冲断与增生杂岩双层结构厘定

增生型造山带形成于活动大陆边缘,以宽阔且延伸稳定的增生杂岩为代表,在大洋板块向大陆板块发生缓慢而复杂的俯冲、碰撞过程中,大洋板块、火山岛弧、海山、大陆碎块等沿逐渐后退的海沟拼贴,仰冲板块前端发生刮削作用、底垫作用和构造剥蚀等作用,使得洋壳物质在海沟内壁增生,具体表现为增生杂岩的形成、垂向和侧向的生长,最终实现陆壳的横向生长。陆陆碰撞期间,加入俯冲通道的被动陆缘也将遭受类似的构造作用,从而形成规模较大的陆缘增生杂岩。因此,造山带增生杂岩的物质组成与结构、形成机制和演化过程对解剖洋陆转换过程中的复杂地球动力学过程具有极为关键的作用。西藏南羌塘增生杂岩是近年来通过走廊性地质填图以及多学科交叉工作得到的研究认识。然而,该增生杂岩的物质组成和结构等关键内容还未得到系统的研究,严重阻碍了对其形成机制和演化过程的理解。因此,本文以时空演化为主线,解剖杂岩物质组成和结构,结合俯冲期和同碰撞期大地构造单元,洞察南羌塘增生杂岩的形成演化过程。本次研究显示:(1)南羌塘增生杂岩具有俯冲杂岩在下、褶皱-冲断带在上的双层结构,二者间为大规模的拆离断层系统;(2)俯冲杂岩内不只含有洋板块地层单元,还含有大量的南羌塘被动陆缘物质;(3)褶皱-冲断带虽主要由被动陆缘物质变形改造而来,也含有属于洋板块地层系统的海山和洋内岛弧等物质。结合同俯冲期弧前盆地和楔顶盆地、同碰撞期晚三叠世岩浆的时空分布,高压变质岩的形成与折返时限,南羌塘增生杂岩内的双层结构应主要是陆陆碰撞过程中被动陆缘俯冲的结果,少量形成于大洋俯冲期间的俯冲反向过程中。本文提出的陆缘俯冲导致南羌塘增生杂岩双层结构的研究认识,对理解南羌塘地壳结构、中生代盆地基底形成演化具有较为重要的意义。     

http://www.sciencedirect.com/science/article/pii/S1674987111001125

<正>Greenstone belts of the eastern Dharwar Craton,India are reinterpreted as composite tectonostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new high-precision elemental data.The former are dominated by a komatiile plus Mg-tholeiitic basalt volcanic association,with deep water siliciclastic and banded iron formation(BIF) sedimentary rocks.Plumes melted at90 km under thin rifted continental lithosphere to preserve inlraoceanic and continental margin aspects.Associated alkaline basalts record subduction-recycling of Mesoarchean oceanic crust,incubated in the asthenosphere.and erupted coevally with Mg basalts from a heterogeneous mantle plume.Together.komaliites-Mg basalts-alkaline basalts plot along the Phanerozoic mantle array in Th/Yb versus Nb/Yb coordinate space,representing zoned plumes,establishing that these reservoirs were present in the Neoarchean mantle. Convergent margin magmatic associations are dominated by tholeiitic to calc-alkaline basalts eompositionally similar to recent intraoceanic arcs.As well,boninitic flows sourced in extremely depleted mantle are present,and the association of arc basalts with Mg-andesites-Nb enriched basalts-adakites documented from Cenozoic arcs characterized by subduction of young(20 Ma),hot,oceanic lithosphere. Consequently.Cenozoic style "hot" subduction was operating in the Neoarchean.These diverse volcanic associations were assembled to give composite terranes in a subduction-accretion orogen at～2.1 Ga,coevally with a global accretionary orogen at ~2.7 Ga,and associated orogenic gold mineralization. Archean lithospheric mantle,distinctive in being thick,refractory,and buoyant,formed complementary to the accreted plume and convergent margin terranes.as migrating arcs captured thick plumeplateaus. and the refractory,low density.residue of plume melting coupled with accreted imbricated plume-arc crust.     

Cryptic tectonic domains of the Klamath Mountains, California and Oregon

Numerous fragments of oceanic crust and island arcs make up the Klamath Mountains province. These fragments were joined together (amalgamated) in an oceanic setting during Paleozoic and Mesozoic collisional events and were accreted to North America as a composite unit during latest Jurassic or earliest Cretaceous time. The roughly arcuate and concentric distribution of the terranes of the Klamath Mountains does not now seem to be a result of simple oroclinal bending as earlier believed. Although commonly described as a west-facing arcuate structure, the province is cut diagonally by a vaguely defined NW-trending zone of discontinuity, or hinge line, that divides the province into NE and SW tectonic domains. The zone of discontinuity is marked by a number of lithic and structural anomalies, and particularly by the distribution of a remarkable series of belts of plutonic rocks. The terranes, regional structures, and plutonic belts of the NE domain trend NE and are generally wider and more coherent than the narrow NW-trending terranes and plutonic belts of the SW domain. Most plutonic belts of the NE domain do not have equivalents in the SW domain. Paleomagnetic evidence suggests that all the plutonic belts, except possibly the youngest (the earliest Cretaceous Shasta Bally belt), were emplaced before the Klamath Mountains terranes finally accreted to North America.     

大陆的起源

太阳系固体星球都有类似的核-幔-壳结构,但唯独人类居住的地球具有长英质组成的大陆壳.太古宙大陆克拉通主要由英云闪长岩(Tonalite)-奥长花岗岩(Trondhjemite)-花岗闪长岩(Granodiorite)为主的TTG深成侵入体变质而成的正片麻岩和由基性-超基性酸性火山岩及少量沉积岩变质的表壳岩(绿岩)组成....     

Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean

In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time.     

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.     

Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt (CAOB) in NE China is a key area for investigating continental growth. However, the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood. NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts. The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities, respectively. In contrast, the Xing ’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes. These blocks and terranes were separated by the Xinlin-Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans from north to south, and these oceans closed during the Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), early Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (260 –240 Ma), respectively, forming the Xinlin-Xiguitu, Mudanjiang-Yilan, Hegenshan-Heihe, Solonker-Linxi, and Changchun-Yanji suture zones. Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean (PAO), namely, the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans. The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west. The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south. The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner. A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO, which led to “soft collision” of tectonic units on each side, forming huge accretionary orogenic belts in central Asia.©2022 China Geology Editorial Office.     

Accreted Terranes in the Paleo‐Asian Ocean

Terrane accretion is considered to be one of the main contributors to the growth of continental crust (Stern and Scholl, 2010). Allochthonous terranes can be categorized into three general groups, including island arcs, seamounts (oceanic plateaus, submarine ridges), and continental fragments. The Central Asian Orogenic Belt (CAOB) is one of the largest and long‐lived accretionary collages on the globe, and is composed of numerous island arcs, and accretionary wedges, seamounts, and microcontinents associated with the growth and consumption of the Paleo‐Asian Ocean (Fig.1; ?engör et al., 1993; Windley et al., 2007; Xiao et al., 2015; Yang et al., 2015).     

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.     

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.     

The western Central Asian Orogenic Belt: A window to accretionary orogenesis and continental growth

The architecture of accretionary orogens is a key to understand continental growth. Here we present an overview of the orogenic components and their amalgamation in the western Central Asian Orogenic Belt (CAOB). The CAOB records the convergence and interactions among various types of orogenic components including the Japan-type, Mariana-type, and Alaska–Aleutian-type arc systems, as well as the active marginal sequences of the Siberia Craton, which incorporated wide accretionary complexes and accreted arcs and terranes. During construction of the CAOB, the Kazakhstan arc chain was characterized by multiple subduction, whereas the northern fringe of the Tarim Craton remained mostly as a passive margin. The multiple convergence and accretions among these various orogenic components generated huge orogenic collages in the late Paleozoic and even in the early Triassic, involving parallel amalgamation, circum-microcontinent amalgamation and oroclinal bending. The preservation of trapped basins played a significant role in orogenesis with some parts of the oceanic plate being subducted and others behaving as rigid units. The orogenesis in the CAOB was long-lived, lasting for more than 800 m.y., involving multiple-subduction and long, continuous accretion, and featuring the complexity of accretionary orogenesis and continent growth.     

How many sutures in the southern Central Asian Orogenic Belt：Insights from East Xinjiang-West Gansu（NW China）？

How ophiolitic mèlanges can be defined as sutures is controversial with regard to accretionary orogenesis and continental growth.The Chinese Altay,East junggar,Tianshan,and Beishan belts of the southern Central Asian Orogenic Belt（CAOB） in Northwest China,offer a special natural laboratory to resolve this puzzle.In the Chinese Altay,the Erqis unit consists of ophiolitic melanges and coherent assemblages,forming a Paleozoic accretionary complex.At least two ophiolitic melanges（Armantai,and Kelameili） in East Junggar,characterized by imbricated ophiolitic melanges,Nb-enriched basalts,adakitic rocks and volcanic rocks,belong to a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites,superimposed by Permian arc volcanism.In the Tianshan,ophiolitic melanges like Kanggurtag,North Tianshan,and South Tianshan occur as part of some Paleozoic accretionary complexes related to amalgamation of arc terranes.In the Beishan there are also several ophiolitic melanges,including the Hongshishan,Xingxingxia-Shibangjing,Hongliuhe-Xichangjing,and Liuyuan ophiolitic units.Most ophiolitic melanges in the study area are characterized by ultramafic,mafic and other components,which are juxtaposed,or even emplaced as lenses and knockers in a matrix of some coherent units.The tectonic settings of various components are different,and some adjacent units in the same melange show contrasting different tectonic settings.The formation ages of these various components are in a wide spectrum,varying from Neoproterozoic to Permian.Therefore we cannot assume that these ophiolitic melanges always form in linear sutures as a result of the closure of specific oceans.Often the ophiolitic components formed either as the substrate of intra-oceanic arcs,or were accreted as lenses or knockers in subduction-accretion complexes.Using published age and paleogeographic constraints,we propose the presence of （1） a major early Paleozoic tectonic boundary that separates the Chinese Altay-East Junggar multiple subduction system