Phanerozoic continental growth in Central Asia

Journal of Asian Earth Sciences 23 (2004) 599–603 www.elsevier.com/locate/jseaes

Preface

Phanerozoic continental growth in Central Asiaq

Central Asia (sensu lato; apolitical) is variously known as the Central Asian Orogenic Belt (CAOB), the Central Asian Mobile Belt (CAMB) or the Altaid Tectonic Collage (Altaids). It is situated between the Siberian and SinoKorean-Tarim cratons, and encompasses an immense area from the Urals in the west, through Kazakhstan, NW China, Mongolia, NE China to the Okhotsk Sea in the Russian Far East. Suess (1908) first recognized several foldbelts arranged peripherally to the west and south of the Angaran nucleus ( ¼ Baikalian structures on the southern side of the Siberian Platform) with successively younger ages away from the craton. A common characteristic feature of the individual orogens in Central Asia is the complex but recurrent arrangement of dominantly accretionary-prism and magmatic-arc material, interspersed with massifs of older continental crust and slivers of oceanic crust (Sengo¨r et al., 1993). In contrast with the better known Caledonides, Hercynides and Himalayas, which are essentially linear belts formed by frontal collision between Precambrian cratons, Central Asia was viewed by Sengo¨r et al. (1993); Sengo¨r and Natal’in (1996) as resulted from growth of voluminous subduction –accretionary complexes and their duplication along large-scale strike-slip faults. Moreover, many isotope tracer studies in the last decade have shown that Central Asia was the world’s largest site of juvenile crustal formation in the Phanerozoic eon. Project IGCP-420 (Continental Growth in the Phanerozoic: Evidence from Central Asia) was created in 1997 and ran through a 5-year span (1998 – 2002) to tackle several problems concerning the tectonic and structural evolution of the orogenic belt(s) and the growth of the continent crust in the Phanerozoic. We have made a distinction between the two terms used in the growth of Asia: (1) amalgamation of dispersed continental fragments from the break-up of older continents (e.g. Gondwana). This process enlarged the size of Asia, but might not have added substantial amount of newly formed (juvenile) crust to the continent. (2) growth of the continental crust. This implies a net transfer of mantlederived material to the continental crust; GCP-420 emphasizes the latter. The tectonic model of Sengo¨r et al. (1993) was tested by a variety of field and laboratory approaches; q Selected papers presented at the IGCP-420 Second and Third Workshops held in Mongolia (1999) and Siberia (2001), respectively.

1367-9120/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1367-9120(03)00124-X

and most importantly, by geochemical and isotopic techniques. In the last 5 years, more than 150 papers have been published by the participating members from a dozen countries. However, while a few tectonic and structural studies have been published, the majority of the works involved age determination, petrology, geochemical and isotopic tracer studies applied to the generation of the juvenile continental crust in the CAOB. This is well reflected in the first special issue published in Tectonophysics (Jahn et al., 2000a), as well as in this volume. The major accomplishments of IGCP-420 may be summarized as follows: (1) Confirmation of significant mantle contribution in the generation of the continental crust in Central Asia. This was achieved by vigorous Sr –Nd isotope studies coupled with multi-chronometric dating of granitoids from many parts of the CAOB—Transbaikalia, Mongolia, Kazakhstan, southern Siberia (Sayan and Altai), northern Xinjiang, Inner Mongolia, and NE China (e.g. Kovalenko et al., 1996; Jahn et al., 2000b,c). (2) The CAOB appears to have formed by assembly of Precambrian continental slivers, some of which could be microcontinental fragments, and a lot more of Phanerozoic juvenile crust produced by both lateral accretion of arc complexes and vertical accretion of underplated material of mantle derivation. Arc accretion appears to have been the dominant process in the CAOB. On the other hand, the abundant Permo-Triassic granitoids in Transbaikalia and the Altai Mountains and the vast Siberian Trap basalts could have been generated by super-plume activity. In any case, complicated subduction– accretion and collision processes took place continuously throughout the Phanerozoic in Central Asia. (3) Recognition of voluminous juvenile granitoids emplaced in post-collisional and intra-continental settings in the Altai-Sayan region and in NE China. The Altai-Sayan granitoids are thought to be related to old shear zones and granitic magmas formed by melting of mantlelower crust mixture (Kruk et al., 1999). In NE China, the generation of abundant A-type granitoids are hypothesized to be related to slab break-off (for Permian granites), lithospheric delamination (for late Triassic to Jurassic granites) or continental rifting (for Cretaceous granites) (Wu et al., 2002). (4) Precise geochronology is critical for tectonic interpretation and for estimation of crustal growth rates. Recent dating using advanced and more rigorous

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techniques have generated many new results that completely changed the traditional views on the ages of many granitic massifs, and hence tectonic evolution, in the CAOB. The best examples are found in Xinjiang, Inner Mongolia, Mongolia and NE China. (5) The roˆle of Precambrian basement gneisses in the generation of granitic magmas has been evaluated using isotope tracer techniques. An excellent example is given by the granitoids emplaced in the Baydrag terrane in Mongolia (Kovalenko et al., 1996, this issue; Jahn et al., this issue), the Altai and Tianshan mountains in Xinjiang (Hu et al., 2000), as well as those emplaced in the Jiamusi Massif of NE China (Wu et al., 2000, 2002). In these cases, the Nd isotope data indicate that the presence of ‘old’ rocks is quite significant. (6) Metallogeny. Geochemical studies of granitoids and ore mineralization in Central Asia suggest juvenile sources for all ore elements. This is best demonstrated by the work of Heinhorst et al. (2000) on Kazakhstan. (7) Isotope provinces. Based on a very large Nd isotope database, Kovalenko and his colleagues (this issue) divided the CAOB into 4 isotope provinces: ‘Precambrian’ (model ages T DM ¼ 3.3 – 2.9 and 2.5 – 0.9 Ga), ‘Caledonian’ (1.1 –0.55 Ga), ‘Hercynian’ (0.8 – 0.5 Ga), and ‘Indosinian’ (0.3 Ga). The last three coincide with coeval tectonic zones and formed at 570 – 475, 420 – 320, and 310 –220 Ma. Note that the use of the terms ‘Caledonian’, ‘Hercynian’ and ‘Indosinian’ outside of their proper localities has been much criticized by American and West European geologists, but they are still frequently used by Russian and Chinese workers to imply only temporal sense or periods of events. In any case, the ‘Precambrian’ province in Central Asia represents the basement of continental blocks consisting of high-grade gneisses or arc backstops where these rocks are not exposed or identified. The Precambrian crust is often intruded and mixed with younger juvenile crust as evidenced from its isotopic heterogeneity. The continental crust of the younger provinces is isotopically homogeneous and was produced by melting of juvenile sources (e.g. oceanic crust), but with addition of old crustal material. Kovalenko et al. also showed that intraplate magmatism always marked the terminal phase of crust-forming event in any given province. While the Turkish group (Boris Natal’in, Celal Sengor) continues refining their tectonic model of the CAOB ( ¼ Altaı¨ds), another group working in Mongolia presented a different tectonic model based on the terrane concepts and newly acquired field and age data (Badarch et al., 2002). They divided the geology of Mongolia into blocks of island arcs, continental margin arcs, accretionary prisms, ophiolites, passive continental margins, old Precambrian cratons, and late overlap basins. Large-scale models are likely to involve many controversial points, especially in specific regions, for lack of details and oversimplification. However, they often give good insight for further studies. In the past 5 years, output of high-quality laboratory data (geochemistry, geochronology, petrology, geophysics) is quite significant.

These data must be fully integrated with other field, structural and possibly geophysical data to propose a more comprehensive and comprehensible tectonic model for the tectonic evolution and growth of the continental crust in Central Asia. The present volume represents the second special issue published as an outgrowth of the workshops held in Ulaan Baatar, Mongolia (1999) and Novosibirsk, South Siberia (2001). Some highlights of this collection are given as follows. Kovalenko et al. give a comprehensive synthesis of geological, age and isotopic data on felsic magmatic rocks from South Siberia, Transbaikalia and Mongolia. Based on these data, they discuss the mechanisms and processes of the Phanerozoic crustal growth in Central Asia. They divide the Central Asian Mobile Belt ( ¼ CAOB) into four isotope provinces: (1) Precambrian microcontinental blocks, (2) early paleozoic ‘Caledonian’ province, (3) late Paleozoic ‘Hercynian’ province, and (4) late paleozoic to Mesozoic ‘Indosinian’ province. These coincide with coeval tectonic zones and were formed at 2.5 – 0.9 Ga, 570– 475, 420– 320, and 310– 220 Ma, respectively. The authors emphasize the globally juvenile character of the Phanerozoic granitoids, the episodic nature of crustal growth, and the influence of Precambrian rocks in the generation of Phanerozoic granitoids emplaced within the microcontinental blocks. In the end, the authors conclude that the formation of the CAOB was connected with the break-up of Rodinia supercontinent as a consequence of the creation of the South-Pacific hot superplume. Mongolia is the heartland of the Central Asian Orogenic Belt and it has been subject to numerous investigations, particularly in metallogenesis, isotope geology and tectonic evolution, as shown by Kovalenko’s life-long research. Jahn et al. present newly acquired data of petrography, geochronology, geochemical and Sr –Nd isotopic analyses on the granitoids (ages from 500 to 120 Ma) collected from west-central Mongolia during the 1999 excursion of IGCP420 Workshop II. They discuss their source characteristics and provide implications for the Phanerozoic crustal growth in Central Asia. The Altai Mountains are the type locality where the concept of the Altaid Tectonic Collage (Suess, 1908; Sengo¨r et al., 1993), or the CAOB, was developed. The Altai Mountains straddle four countries—Russia, Kazakhstan, China and Mongolia, and are composed of continental slivers, which are interpreted by some workers as microcontinents ( ¼ terranes of Gondwana origin) surrounded by island arcs, accretionary prisms and ophiolite complexes. According to Buslov et al., the tectonic pattern of the Altai includes three groups of structural-tectonic units: (1) the Gondwana-derived Altai-Mongolian terrane, (2) terranes of different ages, composed of fragments of Paleozoic accretion– collision complexes, and (3) Strike-slip faults (e.g. the Irtysh strike-slip zone) and thrusts of different ages which separate the terranes. Based on a detailed field mapping,

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structural/kinematic and Ar – Ar age analyses, Buslov et al. reinterpret the tectonic development of this region. They see that the collage of terranes was achieved by strike-slip faulting resulted from two periods of collision—(a) a late Devonian to early Carboniferous oblique subduction and collision of the Gondwana-derived Altai-Mongolian terrane and the Siberian continent, and (b) a late Carboniferous to Permian closure of the Irtysh-Zaysan branch of the PaleoAsian Ocean and collision of the Kazakhstan and Siberian continents. The authors further presented a paleogeographic reconstruction of the Altai orogen. In the excursion of IGCP-420 workshop III (2001), Nick Dobretsov led us to examine an interesting and complicated tectonic unit in the Katun block of Gorny Altai. This unit contains a series of Vendian to Cambrian carbonates, terrigenous sediments and volcanic rocks (MORB, alkali basalts and island arc volcanic rocks). The sequence was interpreted by Dobretsov as shallow-water (slope) facies deposits in a seamount or ocean island tectonic setting (the Katun paleoceanic seamount), though some other Russian geologists hold different opinions. Recognition of fragments of ocean islands and plateaux in Paleozoic orogenic belts is very important for our understanding of the evolution of accretionary wedges and exhumation of high-pressure metamorphic rocks. In this issue, Dobretsov et al. presented a detailed account of three occurrences of paleo-ocean islands – two (Katun and Baratal) from Gorny Altai, and one from the Salair accretionary wedge. To the south of the Chinese Altai, the basement nature of the Junggar Terrane (or Basin) is highly controversial. Some authors contended that the basement represents a terrane or a Precambrian microcontinent, while others argued for a trapped Paleozoic oceanic crust and arc complexes, still others proposed that the basement mainly comprises mantle-derived basic/ultrabasic rocks that underplate the lower crust during a post-orogenic extensional process. Post-orogenic granitoids are often considered to be generated in the lower crust or at the crust-mantle interface, so they may show evidence of interaction with basement rocks during emplacement. The post-collisional magmatism of the Junggar Terrane is characterized by intrusion of large amounts of granitoids and minor basic/ultrabasic rocks. Chen and Jahn show that the granitoids comprise two magmatic suites: calc-alkaline and alkaline, which were emplaced contemporaneously at about 294 Ma. Geochemical and isotopic analyses revealed that all the granitoids are highly juvenile. The calc-alkaline rocks are most probably derived by dehydration –melting of a basic lower crust leaving behind a granulite residue. The process was likely triggered by underplating of mantle-derived basic magmas in an extensional regime. The alkaline granites are considered to have formed by differentiation of the calc-alkaline granitoids. This study argues for a juvenile continental crust for the basement of the Junggar terrane, which is dominated by early Paleozoic oceanic crust and arc complex that were deeply buried

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during the late Paleozoic subduction and accretion. It is definitely not a Precambrian microcontinent as proposed by some workers. Nappe structures and high-pressure metamorphic rocks are rare in accretionary orogens as represented by the Altaids (Sengo¨r et al., 1993). Recently, eclogites and blueschists of early Cambrian age (< 530 Ma) have been identified recently in Gorny Altai, and their metamorphism seems to be coeval with the UHP metamorphic event of the Koktchtav Massif in Kazakhstan. In Gorny Altai, except for the eclogites in the Chagan-Uzen ophiolites from the Kurai zone, the Terekta blueschist/greenschist complex is the only locality of high-pressure metamorphic rocks known in the Russian Altai. Here Volkova et al. (1) present the results of a petrological and geochemical study on these blueschists/ greenschists. They conclude that the metabasic rocks have the features typical of ocean island basalts and MORB. Together with layered Mn-rich chert, marble, metagreywacke, the Terekta rocks form part of a subduction– accretionary complex, and represent the beginning of the Altaid evolution. The southernmost border zone between China and Mongolia is characterized by an extensional structure where a large metamorphic core complex and voluminous Triassic and Cretaceous granitoids are exposed. Wang et al. present new results of zircon dating and geochemical analyses of some granitic rocks. The Triassic granitoids (228 ^ 7 Ma) show intraplate and post-collisional characteristics and their petrofabrics reveal a syn-emplacement extensional deformation. The Cretaceous granitoids (135 ^ 2 Ma), by contrast, are high-K calc-alkaline and emplaced in a domal structure. Nd isotope data suggest a significant role of juvenile mantle component in the generation of the early and late Mesozoic granitoids, as commonly observed in other parts of Central Asia. NE China is characterized by massive distribution of Phanerozoic granitoids, and it makes up the essential part of the ‘Manchuride Paleozoic arc’ that evolved along the northern side of the North China Block (Sengo¨r et al., 1993; Sengo¨r and Natal’in, 1996). Sr –Nd isotopic tracer studies in the past 5 years show that the continental crust in this region is quite juvenile, very similar to other parts of the Altaids. New studies have shown that magmatic activities extended to the Mesozoic. Wu and his colleagues have published many papers on geochemical, isotopic and age studies on the granitoids of NE China. Here, in a new study on the Dongqing Pluton, a peraluminous intrusion in the Zhangguangcai Range, Wu et al. (1) dated the pluton at < 160 Ma. Their Sr– Nd isotopic data suggest that the parental magma was derived by melting of a relatively juvenile crust, similar to most cases in Central Asia. In addition, some rock types show ‘tetrad’ rare earth distribution patterns and unusual trace element ratios, which strongly suggest that the parental magmas have undergone extensive differentiation and interaction between the late stage melt and hydrothermal fluids.

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To the west of the Altaids, the South Urals presents another high-pressure metamorphic suite, the Maksyutov complex. It was formed during a Devonian subduction and the following collision event between the East European Platform and an island arc of the Paleo-Ural ocean. The metamorphic event was dated at < 380 Ma by a variety of chronometers. A geochemical and petrological study on a banded eclogite boudin of 5 £ 30 m from the Lower Unit of the Maksyutov Complex led Volkova et al. (2) to trace its pre-metamorphic magmatic differentiation and estimate the metamorphic conditions. In the economic aspect, Central Asia is known to host numerous world-class gold, silver, copper – molybdenum, lead – zinc and nickel deposits of late Proterozoic to Mesozoic ages. A correct understanding of tectonic development in this area is obviously very important for the search of these metal deposits, which are a result of a prolonged and complex history of crustal growth and deformation in diverse tectonic settings. Yakubchuk provides a very good account of the definition and conceptual development of the ‘Altaids’, and the evolution of different schools of thought. He then relates metallogeny with plate tectonic evolution. He considers that the tectonic evolution and metallogeny have been strongly influenced by plume activities as proposed by Kovalenko et al. He further shows that major events of mineralization (Au, Cu – Mo porphyry and VMS deposits) seem to coincide with plate reorganization and oroclinal bending of magmatic arcs, and Pb – Zn and Cu sedimentary rock-hosted deposits occur in back-arc rifts developed on the amalgamated fragments of earlier arcs. In addition, the superplume activities produced the famous Ni –Cu – PGE deposits of Norilsk in the NW Siberian craton. In a study of a small-scale economic deposit, Wu et al. (2) present the result of zircon dating and geochemical analyses for two mafic-ultramafic complexes from NE China, one of them (Hongqiling) contains the second largest Cu – Ni deposit in China. Mafic-ultramafic complexes are an important component of the continental crust in Central Asia. They are widespread in the Altai and Tianshan Mountains, in Junggar, and also in NE China. However, their origins are often controversial. In this study, Wu et al. (2) conclude that the parental basic magmas were derived from a subcontinental lithospheric mantle in the late Triassic (216 ^ 5 Ma); the magmas underwent extensive differentiation, leading to formation of the Cu – Ni deposit. The complexes are post-orogenic intrusive bodies, but not ophiolite suites as suggested by some workers. Finally, Hong et al. presented a model speculating a causality between the massive production of juvenile crust in Central Asia and the assembly of the Laurasia supercontinent. They tried to show that the major episodes of crustal growth coincide with the supercontinental formation since at least the late Archean. They conclude that the supercontinental cycle and crustal growth are both governed by the changing patterns of mantle convection.

Acknowledgements The realization of this special issue was not possible without the in-depth review and constructive comments of the following reviewers (except the co-editors), even though some of their reviewed papers have not been accepted for publication: Yoji Arakawa, Michel Balle`vre, Bernard Bonin, Ramon Capdevila, Bernard Charoy, Jacques Charvet, Catherine Chauvel, Jiangfeng Chen, Kent Condie, Alain Corcherie, Fiona Darbyshire, E. Distanov, Jan Golonka, V.I. Kovalenko, Sung-Tack Kwon, Mary Leech, Bernd Lehmann, Jean-Paul Lie´geois, J.G. Liou, Boris Litvinovsky, Calvin Miller, Tsutomu Ota, Jonathan Patchett, Giampiero Poli, Louis Raimbault, Rolf Romer, Reimar Seltmann, Shen-su Sun, A.G. Vladimirov, and Alexander Yakubchuk.

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Bor-ming Jahn* Ge´osciences Rennes, Universite´ de Rennes 1, 35042 Rennes Cedex, France E-mail address: [email protected]

Boris Natal’in Istanbul Technical University, Maden Fakultesi, Jeoloji Bolumu, Ayazaga 80626 Istanbul, Turkey E-mail address: [email protected]

Brian Windley Department of Geology, University of Leicester, Leicester LE1 7RH, UK E-mail address: [email protected]

Nick Dobretsov Institute of Geology, UIGGM, Siberian Branch, RAS, Novosibirsk 630090, Russia E-mail address: [email protected]

* Corresponding author. Address: Department of Geosciences, National Taiwan University, 245 Choushan Toad, Taipei 106, Taiwan. Tel.: þ 886223630231/2378; fax: þ 886-23636095.