Regional metamorphic belts of the Japanese Islands

Abstract An overview of the regional metamorphic belts of Japan is given in the context of the tectonic evolution of the Japanese Islands. The Japanese Islands were situated on an active margin of the Eurasian continent or its constituent landmass before their assembly during the Phanerozoic. The Japanese Islands are composed mainly of metamorphosed and unmetamorphosed accretionary complexes, granitoids and their effusive equivalents that were formed by the Cordilleran-type orogeny. The metamorphic belts are regarded essentially as a deep-seated portion of an accretionary complex. In spite of continuous subduction of oceanic plates beneath the continents, these orogenic rocks were formed quite episodically, as evidenced by discontinuous matrix ages of the accretionary complexes and a striking concentration of isotopic ages of the granitoids. A systematic along-arc age shift of Cretaceous large-scaled granitic magmatism and regional metamorphism suggests a tectonic control such as ridge subduction, which triggered the episodic orogeny. A tectonic model based on the paired metamorphic belts, combined with the non-steady tectonic control, works well to explain this magmatism and metamorphism in a single arc-trench system as a continental margin process. However, the juxtapositional process of the paired metamorphic belts is still a problem. Two possible cases, namely transcurrent displacement and back-arc overthrusting are discussed.     

Dating the lower crust by ion microprobe

Ion microprobe UThPb ages of zircons from granulite facies lower crustal xenoliths from north Queensland, Australia, correlate well with the ages of major orogenic episodes manifest at the earth's surface. About half of the xenoliths contain Proterozoic zircons which are similar in age to the episodes of high-grade metamorphism of the older surface rocks. All the xenoliths contain late Paleozoic zircons which show a real 100 Ma range in²⁰⁶Pb²³⁸/U ages (from 320 to 220 Ma), which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust. The Paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash-flows and intrusion of high-level granites in this region (320-270 Ma). Mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths, and coupled with the age information, can be used to infer the geological processes operating in the lower crust. The zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two-stage history for these rocks, involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt. Some of the zircons from the felsic xenoliths contain CO₂-rich fluid inclusions, indicating that those zircons grew during high-grade metamorphism. The isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic, mantle-derived, lithologies. The major orogenic event responsible for the formation of that crust occurred in the late Paleozoic, when Proterozoic supracrustal rocks were emplaced into the lower crust, possibly along thin-skinned thrust slices. This was accompanied by intrusion of high-temperature, mantle-derived melts which caused partial melting of pre-existing crust. The most likely setting for such tectonism is a continental margin subduction zone.     

The supercontinent cycle and gondwanaland assembly: Component cratons and the timing of suturing events

The Neoproterozoic Gondwana supercontinent cycle includes the break-up of a Mesoproterozoic supercontinent here termed Pangea Y, followed by the fusion of several cratons and the large East Gondwana continent to form Gondwanaland. The accretion can be analysed in terms of plate tectonics. Rifting of Pangea Y was active in the 1100-650 Ma interval. Collision and deformation events occurred in the 820-540 Ma interval. The earliest collision event at 820 Ma between the Sao Francisco-Congo craton and East gondwana formed the Zambezi belt. Major shear zones in transpressional mobile belts developed from 820 to 550 Ma. Post orogenic magmatism and extension events affected the Gondwana supercontinent in the 660-500 Ma interval.     

A numerical model of exhumation of the orogenic lower crust in the Bohemian Massif during the Variscan orogeny

We present a numerical model of the main phase (370?C335 Ma) of the Variscan orogeny in the central part of the Bohemian Massif. The crustal deformation in our model is driven by radiogenic heating in the felsic lower crust, the lateral contraction of the Moldanubian domain due to convergence with the Saxothuringian plate (in the early stage of orogeny), and the indentation of the Brunovistulian basement into the weakened orogenic root (in the late stage). Our model explains the main geological events inferred from the geological record in the Moldanubian domain: formation of the orogenic plateau and onset of sedimentation at about 345 Ma, rapid exhumation of the orogenic lower crust at about 340 Ma and subsurface flow of crustal material (?? 335 Ma and later). The results of our modeling suggest that delamination of the lithosphere, often invoked to explain the high temperature metamorphism in the orogenic lower crust of the Bohemian Massif, is not the only physical mechanism which can transfer a sufficient amount of heat to the crust to trigger its overturn.     

扬子板块东北缘中元古代的大地构造划分

扬子板块东北缘存在四条主要的中元古代变质带,自南向北依次为江南变质带、沿江变质带、云台一张八岭变质带和连云港一泗阳变质带。它们分别为中元古代的古弧后盆地、火山岛弧、裂谷及弧前盆地,扬子板块东北缘中元古代为活动大陆边缘构造体系。苏（北）胶（南）变质造山带应解体,其中一部分属扬子大陆边缘体系。     

Disintegration and age of basement metamorphic rocks in Qiangtang,Tibet, China

Comprehensive studies on lithologic association, provenance of metacongelometre, characteristics of metamorphism and deformation, and²⁰⁷Pb/²⁰⁶Pb-dating of single-zircon for metamorphic rocks distributed in Chabu-Chasang areas in Qiangtang block indicate that most of them belong to Middle Proterozoic metamorphic basement except silicilith member ascribed to Triassic. Disintegrated basement strata are called Gemuri group and Guoganjianianri group; they are different in histories of metamorphism and deformation. The single-zircon²⁰⁷Pb/²⁰⁶Pb-ages provide excellent evidence for the existence of an Archean continent nucleus around study areas. Some thermal event ages such as 929–1016 and 509–548 Ma are recorded in Gemuri group.     

The Lhasa Terrane: Record of a microcontinent and its histories of drift and growth

The Lhasa Terrane in southern Tibet has long been accepted as the last geological block accreted to Eurasia before its collision with the northward drifting Indian continent in the Cenozoic, but its lithospheric architecture, drift and growth histories and the nature of its northern suture with Eurasia via the Qiangtang Terrane remain enigmatic. Using zircon in situ U–Pb and Lu–Hf isotopic and bulk-rock geochemical data of Mesozoic–Early Tertiary magmatic rocks sampled along four north–south traverses across the Lhasa Terrane, we show that the Lhasa Terrane has ancient basement rocks of Proterozoic and Archean ages (up to 2870 Ma) in its centre with younger and juvenile crust (Phanerozoic) accreted towards its both northern and southern edges. This finding proves that the central Lhasa subterrane was once a microcontinent. This continent has survived from its long journey across the Paleo-Tethyan Ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the Qiangtang Terrane to the north and with the Indian continent to the south. Zircon Hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks (e.g., ~ 50–90%, 0–70%, and 30–100% to the Mesozoic magmatism in the southern, central, and northern Lhasa subterranes, respectively). We suggest that much of the Mesozoic magmatism in the Lhasa Terrane may be associated with the southward Bangong–Nujiang Tethyan seafloor subduction beneath the Lhasa Terrane, which likely began in the Middle Permian (or earlier) and ceased in the late Early Cretaceous, and that the significant changes of zircon ε_Hf(t) at ~ 113 and ~ 52 Ma record tectonomagmatic activities as a result of slab break-off and related mantle melting events following the Qiangtang–Lhasa amalgamation and India–Lhasa amalgamation, respectively. These results manifest the efficacy of zircons as a chronometer (U–Pb dating) and a geochemical tracer (Hf isotopes) in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics.     

Disintegration and age of basement metamorphic rocks in Qiangtang,Tibet, China

Comprehensive studies on lithologic association, provenance of metacongelometre, characteristics of metamorphism and deformation, and²⁰⁷Pb/²⁰⁶Pb-dating of single-zircon for metamorphic rocks distributed in Chabu-Chasang areas in Qiangtang block indicate that most of them belong to Middle Proterozoic metamorphic basement except silicilith member ascribed to Triassic. Disintegrated basement strata are called Gemuri group and Guoganjianianri group; they are different in histories of metamorphism and deformation. The single-zircon²⁰⁷Pb/²⁰⁶Pb-ages provide excellent evidence for the existence of an Archean continent nucleus around study areas. Some thermal event ages such as 929–1016 and 509–548 Ma are recorded in Gemuri group.

     

Blueschist-facies metamorphism during Paleozoic orogeny in southwestern Japan: Phengite K–Ar ages of blueschist-facies tectonic blocks in a serpentinite melange beneath early Paleozoic Oeyama ophiolite

Blueschist-bearing Osayama serpentinite melange develops beneath a peridotite body of the Oeyama ophiolite which occupies the highest position structurally in the central Chugoku Mountains. The blueschist-facies tectonic blocks within the serpentinite melange are divided into the lawsonite–pumpellyite grade, lower epidote grade and higher epidote grade by the mineral assemblages of basic schists. The higher epidote-grade block is a garnet–glaucophane schist including eclogite-facies relic minerals and retrogressive lawsonite–pumpellyite-grade minerals. Gabbroic blocks derived from the Oeyama ophiolite are also enclosed as tectonic blocks in the serpentinite matrix and have experienced a blueschist metamorphism together with the other blueschist blocks. The mineralogic and paragenetic features of the Osayama blueschists are compatible with a hypothesis that they were derived from a coherent blueschist-facies metamorphic sequence, formed in a subduction zone with a low geothermal gradient (~ 10°C/km). Phengite K–Ar ages of 16 pelitic and one basic schists yield 289–327 Ma and concentrate around 320 Ma regardless of protolith and metamorphic grade, suggesting quick exhumation of the schists at ca 320 Ma. These petrologic and geochronologic features suggest that the Osayama blueschists comprise a low-grade portion of the Carboniferous Renge metamorphic belt. The Osayama blueschists indicate that the 'cold' subduction type (Franciscan type) metamorphism to reach eclogite-facies and subsequent quick exhumation took place in the northwestern Pacific margin in Carboniferous time, like some other circum-Pacific orogenic belts (western USA and eastern Australia), where such subduction metamorphism already started as early as the Ordovician.     

Initiation and evolution of intra-oceanic subduction in the Uralides: Geochemical and isotopic constraints from Devonian oceanic rocks of the Southern Urals, Russia

Abstract The Southern Uralides are a collisional orogen generated in the Late Devonian–Early Carboniferous by the collision of the Magnitogorsk island arc (MA) generated in the Early to Middle Devonian by intra‐oceanic convergence opposite to the continental margin, and the continental margin of the East European craton. A suture zone of the arc to the continental margin, the Main Uralian Fault (MUF), is marked by ophiolites and exhumed high‐pressure–low‐temperature metamorphic rocks of continental origin. The pre‐orogenic events of the Southern Urals and their geodynamic setting are traced by means of fluid‐immobile incompatible trace elements (rare earth elements and high field strength elements) and Sr–Nd–Pb isotope geochemistry of the MA suites, in particular the protoarc suite with boninites and probably ankaramites, and the mature arc comprised of island arc tholeiitic (IAT) suites, transitional IAT to calc‐alkaline (CA), and CA suites. The MA volcanics result in genetically distinct magmatic source components. In particular, depleted normal‐mid‐oceanic ridge basalt‐type mantle sources with various enrichments in a slab‐derived aqueous fluid component are evident. The enriched component is not involved in significant amounts, as testified by the rather radiogenic Nd isotopes and unradiogenic Pb isotopes. Further information on the pre‐orogenic events is provided by the Mindyak Massif metagabbros derived from diverse gabbroic protoliths that were affected by oceanic rodingitization, and subsequently by a high‐temperature (HT) metamorphism related to the development of a metamorphic sole. The HT metamorphism has the same age as the protoarc volcanism, and constrains the initiation of subduction at approximately 410 Ma. Consequently, the maximum timespan between initial intra‐oceanic convergence and final collision is approximately 31 my, a duration consistent with that of present‐day ongoing collisions in the western Pacific. The characteristics of early volcanism and the traces of a metamorphic sole provide useful criteria to attribute most MUF ophiolites to the Tethyan type with a complex pre‐orogenic evolution.     

The chemical Th-U-total Pb isochron ages of Jiaodong and Jiaonan metamorphic rocks in the Shandong Peninsula, eastern China

Abstract The chemical Th-U-total Pb isochron method (CHIME) was applied to determine the age of monazite and thorite in five gneisses and zircon in an ultra high-pressure (UHP) phengite schist from the Su-Lu region, eastern China. The CHIME ages and isotopic ages reported in the literature show that gneisses in the Su-Lu region are divided into middle Proterozoic (1500–1720 Ma) and Mesozoic (100–250 Ma) groups. The Proterozoic group includes paragneiss and orthogneiss of the amphibolite-granulite facies, and their protolith age is late Archean-early Proterozoic. The Mesozoic group is mainly composed of orthogneiss of the greenschist-epidote amphibolite facies, and the protolith age is Middle Paleozoic-Early Proterozoic. The Proterozoic and Mesozoic gneisses occupy northern and southern areas of the Su-Lu region, respectively, which are divided by a major Wulian-Qingdao-Yantai fault. Ultra high-pressure metamorphic rocks occur as blocks in the Mesozoic gneisses, and form a UHP complex.
The UHP phengite schist in the Mesozoic orthogneiss contains detrital zircons with late Proterozoic CHIME age ( ca 860 Ma). Age of the UHP metamorphism is late Proterozoic or younger, and protolith age of the UHP metamorphic rocks is probably different from that of the surrounding Mesozoic gneisses.     

Global scale patterns of continental fragmentation: Wilson's cycles as a constraint for long-term sea-level changes

We propose to characterize land–ocean distributions over Late Proterozoic to Phanerozoic times from measurement of perimeters and areas of continental fragments, based on paleomagnetic reconstructions. These measurements serve to calculate geophysically constrained breakup and scatter indexes of continental land masses from 0 to 1100 Ma. We then provide quantitative investigation and modelling of relationships between scatter of continental landmasses and mean age of the oceanic lithosphere during Mesozoic times, which appears to range from 56 to 62 Ma over the last 170 My. We then inverse the scatter of continental landmasses in terms of global oceanic crust mean age over the last 600 My, i.e. back in times where no measurement of seafloor accretion history is possible because of subduction. We finally show that the inferred evolution of oceanic lithosphere mean age over the Phanerozoic remarkably correlates in time with long-term sea-level changes since the Cambrian.     

The Palaeo-Tethyan suture: A line of demarcation between two fundamentally different architectural styles in the structure of Asia

Abstract The Palaeo-Tethyan suture separates regions characterized by two fundamentally different tectonic styles in the structure of the Tethysides. North of the suture in Iran, Turkmenistan, Afghanistan, Tadjikistan, Kirgizstan, Uzbekistan, Kazakhstan and large parts of the Russian Federation and China, orogenic development is characterized by very large subduction-accretion complexes developed since the late Proterozoic. Magmatic arc axes migrated radially outwards from the 'Old Vertex of Eurasia' and consolidated the accretionary prisms into a 'basement complex' dominated by a pelitic composition. In such orogens, called the 'Turkic-type' after the dominant ethnic population of Central Asia, ophiolites are unreliable indicators of sutures, because they are present throughout the 'basement' as in-faulted shreds and rarely as nappes. By contrast, south of the Palaeo-Tethyan suture, orogeny was commonly characterized by a Sumatra- or Andean-type continental margin arc (e.g. the Transhimalaya arc) that in places became an island arc by back-arc basin rifting (e.g. the Black Sea behind the Rhodope-Pontide fragment) and later collided with an Atlantic- (as in the Himalaya) or California-type (as in the Alps) continental margin to create Alpine- or Himalayan-type orogenic belts. Turkic-type orogenic belts result from the exaggeration of the Himalayan-type as a result of the subduction of very large oceanic areas that contain great amounts of sediment. They contribute to the enlargement and also possibly the growth of the continental crust which has a composition more silicic than basalt. The Palaeo-Tethyan suture is thus a line across which the rate of continental enlargement by subduction-accretion changed dramatically.     

Geophysical characteristics of the Namaqua-Natal Belt and its boundaries, South Africa

A study of the available gravity, magnetic and geoelectrical data for the Proterozoic Namaqua-Natal Belt of South Africa shows that the boundaries of this tectonic province have distinct geophysical signatures. The southern boundary is marked by a large static field magnetic anomaly and a transition from electrically resistive to conductive crust. The northern margin of the belt is associated with a distinctive gravity signature. These geophysical signatures proved ideal to extrapolate and interpolate the boundaries of the Namaqua-Natal Belt, especially through areas where the transition is covered by Phanerozoic sediments. Numeraial modelling of the gravity data shows that a simple model of uplift in the Namaqua-Natal Belt associated with differential movement along a faulted transition to the older provinces can explain the gravity signature. This model is consistent with the development of the Namaqua-Natal Belt in its final stages as part of an Andean mountain belt with northward subduction and the associated trench south of the southern boundary of the belt. The Andean model of crustal development for this Proterozoic province is consistent with features such as the large quantity of calcalkaline magmatism, the low-pressure, high-temperature metamorphism, the metamorphic zonation and differential uplift along the northern margin, the major mantle derived contribution to the crust between 1300 and 1200 Ma ago and the deformational history.     

The early Paleozoic carbon cycle

A review of O, C, Sr and S isotope trends for the entire Phanerozoic shows that the present-day values of isotope signals are similar to those at the Proterozoic termination. The sharp rise in ⁸⁷Sr/⁸⁶Sr since 65 Ma has been attributed to an uplift and subsequent metamorphism and erosion associated with the Himalayas and Tibet. This orogenic evolution has been postulated to have influenced the global organic and inorganic carbon cycles and climate as well. A similar large-scale orogeny, the Pan-African event, also dominated the Neoproterozoic (Vendian) times, and the similarity of modern and Neoproterozoic isotope values for seawater may therefore have had a comparable tectonic cause. In this contribution, we present the results of a numerical model of the coupled C–alkalinity–S–Sr cycles suggesting that the early Paleozoic (from early Cambrian to late Devonian) evolution of Sr, O, C and S seawater isotope signals could have been the consequence of progressive oxidation of a large reduced carbon reservoir exhumed during the Pan-African orogeny. The δ¹⁸O measured in brachiopod shells is used as a forcing of the model, postulating that any change in the oxygen isotopic composition of seawater is the result of a disequilibrium in the organic carbon subcycle through the coupling of the oxygen isotopic and carbon cycles. The calculated δ¹³C, ⁸⁷Sr/⁸⁶Sr and δ³⁴S are in good agreement with the data, as is the reasonable calculated history for atmospheric pCO₂ and its relation to global climate.     

Geology of the Grove Mountains in East Antarctica

Grove Mountains consists mainly of a series of high-grade (upper amphibolite to granulite facies) metamorphic rocks, including felsic granulite, granitic gneiss, mafic granulite lenses and charnockite, intruded by late tectonic gneissic granite and post-tectonic granodioritic veins. Geochemical analysis demonstrates that the charnockite, granitic gneiss and granite belonged to aluminous A type plutonic rocks, whereas the felsic and mafic granulite were from supracrustal materials as island-arc, oceanic island and middle oceanic ridge basalt. A few high-strained shear zones disperse in regional stable sub-horizontal foliated metamorphic rocks. Three generations of ductile deformation were identified, in which D₁ is related to the event before Pan-African age, D₂ corresponds to the regional granulite peak metamorphism, whereas D₃ reflects ductile extension in late Pan-African orogenic period. The metamorphic reactions from granitic gneiss indicate a single granulite facies event, but 3 steps from mafic granulite, with P-T condition of M1 800°C, 9.3×10⁵ Pa; M₂ 800–810°C, 6.4 × 10⁵ Pa; and M₃ 650°C have been recognized. The U-Pb age data from representative granitic gneiss indicate (529±14) Ma of peak metamorphism, (534±5) Ma of granite emplacement, and (501±7) Ma of post-tectonic granodioritic veins. All these evidences suggest that a huge Pan-African aged mobile belt exists in the East Antarctic Shield extending from Prydz Bay via Grove Mountains to the southern Prince Charles Mountains. This orogenic belt could be the final suture during the Gondwana Land assemblage.     

华南岩石层与大陆动力学

华南大陆记录和保存了自太古代至今大陆生长层完整的历史过程.以杨子克拉通为核心，地壳不断向东南生长，古扬子块前寒武系以灰色片麻岩、古元古代科马提岩绿岩、新元古代蛇绿岩、绿岩为特征，为相对稳定高速高阻冷的残存地幔“残烃柱”；而沿海一带火成岩以中、新生代壳－幔混合源火山－侵入杂岩、碱性花岗岩和正长岩带以及不同类型的玄武岩类为特征，为相对活动低速高导热的超地幔柱．巨型裂解构造是物质热传输的主要形式，地幔柱迁移是华南大陆构造演化的原动力．     

Seismic tomography beneath the orogenic belts and adjacent basins of northwestern China

Three-dimensional velocity images of the crust and upper mantle beneath orogenic belts and adjacent basins of the northwestern continent of China are reconstructed by seismic tomography, based on arrival data of P wave recorded in seismic networks in Xinjiang, Qinghai, Gansu of China and Kyrgyzstan. The velocity images of upper crust demonstrate the tectonic framework on the ground surface. High velocities are observed beneath orogenic belts, and low velocities are observed in the basins and depressions that are obviously related to unconsolidated sediments. The velocity image in mid-crust maintains the above features, and in addition low velocities appear in some earthquake regions and a low velocity boundary separates the western Tianshan Mts. from eastern Tianshan Mts. The orogenic belts and the northern Tibetan plateau have a Moho depth over 50 km, whereas the depths of the Moho in basins and depressions are smaller than 50 km. The velocity images of upper mantle clearly reveal the colliding relationship and location of deep boundaries of the continental blocks in northwestern China, indicating a weakness of the upper mantle structure of orogenic belts. The top depth of upper mantle asthenosphere varies from place to place. It seems shallower under the northern Tibetan plateau, Altay and Qilian Mts., and deeper under the Tarim and Tianshan regions. Hot mantle probably rose to the bottom of some orogenic belts along tectonic boundaries when continental blocks collided to each other. Therefore their dynamic features are closely correlated to the formation and evolution of orogenic belts in northwestern China.     

Zircon U–Pb ages and tectonic implications of 'Early Paleozoic' granitoids at Yanbian, Jilin Province, northeast China

Abstract   The Yanbian area is located in the eastern part of the Central Asian Orogenic Belt (CAOB) of China and is characterized by widespread Phanerozoic granitic intrusions. It was previously thought that the Yanbian granitoids were mainly emplaced in the Early Paleozoic (so-called 'Caledonian' granitoids), extending east–west along the northern margin of the North China craton. However, few of them have been precisely dated; therefore, five typical 'Caledonian' granitic intrusions (the Huangniling, Dakai, Mengshan, Gaoling and Bailiping batholiths) were selected for U–Pb zircon isotopic study. New-age data show that emplacement of these granitoids extended from the Late Paleozoic to Late Mesozoic (285–116 Ma). This indicates that no 'Caledonian' granitic belt exists along the northern margin of the North China craton. The granitoids can be subdivided into four episodes based on our new data: Early Permian (285 ± 9 Ma), Early Triassic (249–245 Ma), Jurassic (192–168 Ma) and Cretaceous (119–116 Ma). The 285 ± 9 Ma tonalite was most likely related to subduction of the Paleo-Asian Oceanic Plate beneath the North China craton, followed by Triassic (249–245 Ma) syn-collisional monzogranites, representing the collision of the CAOB orogenic collage with the North China craton and final closure of the Paleo-Asian Ocean. The Jurassic granitoids resulted from subduction of the Paleo-Pacific plate and subsequent collision of the Jiamusi–Khanka Massif with the existing continent, assembled in the Triassic. The Early Cretaceous granitoids formed in an extensional setting along the eastern Asian continental margin.