3D model of deep structure of the Early Precambrian crust in the East European Craton and paleogeodynamic implications

The integral 3D model of the deep structure of the Early Precambrian crust in the East European Craton is based on interpretation of the 1-EU, 4B, and TATSEIS seismic CDP profiles in Russia and the adjacent territory of Finland (FIRE project). The geological interpretation of seismic images of the crust is carried out in combination with consideration of geological and geophysical data on the structure of the Fennoscandian Shield and the basement of the East European platform. The model displays tectonically delaminated crust with a predominance of low-angle boundaries between the main tectonic units and the complex structure of the crust-mantle interface, allowing correlation of the deep structure of the Archean Kola, Karelian, and Kursk granite-greenstone terrane with the Volgo-Uralia granulite-gneiss terrane, as well as the Paleoproterozoic intracontinental collision orogens (the Lapland-Mid-Russia-South Baltia orogen and the East Voronezh and Ryazan-Saratov orogens) with the Svecofennian accretionary orogen. The lower crustal “layer” at the base of the Paleoproterozoic orogens and Archean cratons was formed in the Early Paleoproterozoic as a result of underplating and intraplating by mantle-plume mafic magmas and granulite-facies metamorphism. The increase in the thickness of this “layer” was related to hummocking of the lower crustal sheets along with reverse and thrust faulting in the upper crust. The middle crust was distinguished by lower rigidity and affected by ductile deformation. The crust of the Svecofennian Orogen is composed of tectonic sheets plunging to the northeast and consisting of island-arc, backarc, and other types of rocks. These sheets are traced in seismic sections to the crust-mantle interface.     

The Bothnian coupled oroclines of the Svecofennian Orogen: a Palaeoproterozoic terrane wreck

The accretion of magmatic arcs gives rise to elongate, linear orogens and is a key process in forming new continental crust. Many Precambrian continents are, however, presently equidimensional or have large areas without any clear linearity, such as the central part of the Palaeoproterozoic Svecofennian Orogen (1.92–1.77 Ga). One way of forming an equidimensional continental domain is by buckling of a linear orogen about vertical axes of rotation into one or more coupled oroclines. Here, we reinterpret existing data and demonstrate the occurrence of coupled Bothnian oroclines in the Svecofennian Orogen. Palinspastic restoration of the southern and northern Bothnian oroclines brings a 1000‐km‐long segment of the Svecofennian Orogen into an originally linear, NW‐striking geometry that restores the lithological belts, metamorphic zones and structural vergences to a common direction, and which indicates that the orogen consists of a SW‐facing arc, which has been shortened along NE‐verging folds and thrust faults.     

西天山的增生造山过程

西天山位于中亚造山带的西南缘,经历了复杂的增生造山过程。它也是标志塔里木地块北部被动陆缘与西伯利亚地块南侧宽阔活动陆缘最后拼合的构造带。根据近年来的研究进展,将西天山划分为北天山弧增生体、伊犁地块北缘活动陆缘、伊犁地块、伊犁地块南缘活动陆缘、中天山复合弧地体、西天山（高压）增生楔和塔里木北部被动大陆边缘。同时综述了西天山蛇绿岩、高压变质岩、花岗岩类的年代学新资料,讨论了其增生造山的过程。西天山增生造山与早古生代帖尔斯克依古洋、早古生代晚期—晚古生代南天山洋和晚古生代北天山洋3个代表洋盆的演化相关,增生造山结束的时间可能是早石炭世末。二叠纪时期,西天山至整个中亚地区进入后碰撞演化阶段。现有资料证实西天山为晚古生代增生造山带,并非三叠纪碰撞造山带。     

造山带中增生楔识别与地质意义

增生楔主要由海沟复理石、远洋—半远洋沉积和洋岛/海山等大洋板块地层岩石及蛇绿岩共同构成,是汇聚板块边缘古俯冲带构造—沉积作用的综合产物,以发育叠瓦状逆冲断层、双冲断层和紧闭—倒转褶皱,以及片理、碎裂构造、小型褶皱、膝折等不同尺度的构造为特征。增生楔组成包括连续单元和混杂岩两部分,严格受滑脱面位置控制。增生楔是增生型造山带最基本大地构造相,它与弧前盆地、岛弧/大陆边缘弧的时空配置关系可直接指示大洋板块俯冲极性、揭示洋盆演化与造山作用方式。准确识别增生楔,详细解剖其结构特征与组成特征,综合判别并恢复大洋板块地层,可为古板块边界与古大洋盆地消亡位置确定、造山带结构精细划分及造山作用过程重建提供基本依据。大比例尺地质填图是研究增生楔结构和组成、以及大洋板块地层重建的有效手段。以南祁连拉脊山石灰窑增生楔为例,地质填图结果表明它是由中寒武世—早奥陶世洋壳物质构成,自北而南依次可分为海沟—大洋盆地—海山OPS和六道沟SSZ型蛇绿岩,被南倾逆冲断层分割成多个岩片。海沟—大洋盆地OPS岩片基本保留完整地层序列,海山OPS岩片包括连续单元和混杂岩两部分。顶帽山增生楔主要由海山OPS构成,内部结构层序相对完好,是南祁连构造带中保留最为完整、规模最大的海山OPS岩石组合序列。上述不同类型OPS片段和六道沟SSZ型蛇绿岩组成的增生楔与其南侧岛弧带同时代。增生楔—岩浆弧的时空配置暗示寒武纪—早奥陶世时期原特提斯洋自北向南俯冲极性。这些认识为南祁连早古生代火山—岩浆—沉积构造演化以及是否存在古洋盆与古洋盆演化重建提供基本佐证。     

Crustal reworking and orogenic styles inferred from zircon Hf isotopes:Proterozoic examples from the North Atlantic region

Zircon Hf evolutionary patterns are powerful tools to investiage magma petrogenesis and crustal evolution. The ~(176)Hf/~(177)Hf isotopic signature of a rock is particularly informative and can be used to derive an estimation of the time when mantle extraction and diagnose closed system reworking where successive samples through time define an Hf evolution array dependant on the source Lu/Hf ratio. However, many magmatic events require new mantle addition as the thermal impetus for melting pre-existing crust. In this situation, rather than simply reflecting reworking, the isotopic signature indicates mixing with contributions from both reworked crust and new radiogenic input. Different geodynamic settings have different propensities for either reworking or addition of new mantle-derived magma. Hence, Hf-time trends carry within them a record, albeit cryptic, of the evolving geodynamic environment as different tectonic configurations recycle and add new crust at different rates, magnitudes, and from different sources. As an example of the difference in apparent Hf evolution slopes, we present Hf-time compilations from three geographically distinct Meso-to Neoproterozoic orogenic belts in the North Atlantic Region whose geodynamic configurations remain a subject of debate. We use the εHf/Ma trajectory to assist in understanding their evolution. The εHf/Ma trajectory of the Sveconorwegian Orogen corresponds to a ~(176)Lu/~(177) Hf ratio of 0.012, which implies a process driven primarily by reworking of preexisting crust that is balanced with input from the depleted mantle resulting in a relatively shallowεHf/Ma slope. The Valhalla Orogen reveals a similar comparatively shallow εHf/Ma path. In stark contrast to these patterns is the steep εHf/Ma trajectory of the Grenville Orogen that requires a mixing process involving a greater contribution of old crust of at least ～ 1.8 Ga age. The degree of reworking required to produce the εHf/Ma trend of the Grenville Orogen is consistent with a continent-continent collisional orogeny whereas both Sveconorwegian and Valhalla orogens appear more consistent with accretionary margins.     

增生型造山带结构解析与时空制约

增生型造山带记录复杂的板块汇聚过程,增生造山作用演化历史漫长,发育多期次造山作用.就增生型造山带的特征与复杂性进行简要总结与评述,并讨论增生造山作用时空分析思路.增生造山作用是多种性质(汇聚、转换和离散)的板块边缘,沿一个核心大陆边缘最终发生复杂相互作用动力学过程的总和.弧前发育增生杂岩和各种混杂岩或者构造岩片,上叠有以弧前盆地为代表的各类沉积盆地,共同制约增生过程的时空演化特征.增生型造山带多发育多岛海复杂古地理格局,增生造山作用具有多组分、多岛海、多盆地类型、多种性质的岩浆活动、宽阔的增生杂岩、多俯冲极性、多地体拼贴、长期演化与面状增生等特性.以古地磁、古地理、古生物与古气候等资料为基本依据,划分一级大地构造单元界线.以构造地质解析和关键地区详细的地质填图,结合物质成分和年代学分析,进行二级大地构造单元及其相互关系的详细解剖.卷入增生造山事件中最年轻的地质体或者组分,提供了该期增生事件时限的下限;卷入增生造山事件中最年轻的角度不整合,以及最年轻的高压-低温变质事件,可能提供了最晚增生事件时限的下限;而未卷入增生造山事件中最老的区域性角度不整合,则可能提供了最晚增生事件时限的上限.     

Tectonic evolution of the Lachlan Orogen,southeast Australia: Historical review,data synthesis and modern perspectives

The Lachlan Orogen,like many other orogenic belts,has undergone paradigm shifts from geosynclinal to plate-tectonic theory of evolution over the past 40 years. Initial plate-tectonic interpretations were based on lithologic associations and recognition of key plate-tectonic elements such as andesites and palaeo-subduction complexes. Understanding and knowledge of modern plate settings led to the application of actualistic models and the development of palaeogeographical reconstructions, commonly using a non-palinspastic base. Igneous petrology and geochemistry led to characterisation of granite types into ‘I’ and ‘S’, the delineation of granite basement terranes, and to non-mobilistic tectonic scenarios involving plumes as a heat source to drive crustal melting and lithospheric deformation. More recently, measurements of isotopic tracers (Nd, Sr, Pb) and U–Pb SHRIMP age determinations on inherited zircons from granitoids and detrital zircons from sedimentary successions led to the development of multiple component mixing models to explain granite geochemistry. These have focused tectonic arguments for magma genesis again more on plate interactions. The recognition of fault zones in the turbidites, their polydeformed character and their thin-skinned nature, as well as belts of distinct tectonic vergence has led to a major reassessment of tectonic development. Other geochemical studies on Cambrian metavolcanic belts showed that the basement was partly backarc basin- and forearc basin-type oceanic crust. The application of ⁴⁰Ar–³⁹Ar geochronology and thermochronology on slates,schist and granitoids has better constrained the timing of deformation and plutonism,and illite crystallinity and b_o mica spacing studies on slates have better defined the background metamorphic conditions in the low-grade parts. The Lachlan deformation pattern involves three thrust systems that constitute the western Lachlan Orogen, central Lachlan Orogen and eastern Lachlan Orogen. The faults in the western Lachlan Orogen show a generalised east-younging (450–395 Ma), which probably relates to imbrication and rock uplift of the sediment wedge, because detailed analyses show that the décollement system is as old in the east as it is in the west. Overall, deformation in the eastern Lachlan Orogen is younger (400–380 Ma), apart from the Narooma Accretionary Complex (ca 445 Ma). Preservation of extensional basins and evidence for basin inversion are largely restricted to the central and eastern parts of the Lachlan Orogen. The presence of dismembered ophiolite slivers along some major fault zones, as well as the recognition of relict blueschist metamorphism and serpentinite-matrix mélanges requires an oceanic setting involving oceanic underthrusting (subduction) for the western Lachlan Orogen and central Lachlan Orogen for parts of their history. Inhibited by deep weathering and a general lack of exposure, the recent application of geophysical techniques including gravity, aeromagnetic imaging and deep crustal seismic reflection profiling has led to greater recognition of structural elements through the subcrop, a better delineation of their lateral continuity, and a better understanding of the crustal-scale architecture of the orogen. The Lachlan Orogen clearly represents a class of orogen, distinct from the Alps, Canadian Rockies and Appalachians, and is an excellent example of a Palaeozoic accretionary orogen.     

Paleoproterozoic sediment recycling during multiphase orogenic evolution in Fennoscandia, the Tampere and Pirkanmaa belts, Finland

Geochronology of detrital zircons and their overgrowths combined with whole-rock geochemical and Sm–Nd isotopic data can be used to distinguish different stages of sediment recycling and metamorphism during multiphase orogenic evolution. This approach is applied to the Paleoproterozoic sedimentary rocks of the Tampere and Pirkanmaa belts (southern Finland) in the center of the composite Svecofennian orogen.The lower part of the Tampere belt succession and bulk of the Pirkanmaa belt are characterized by turbidites whereas the upper part of the Tampere belt succession is dominated by 1.90–1.89 Ga mature arc-type volcanic rocks. Detrital zircon geochronology indicates that the Tampere and Pirkanmaa belts have a coeval 1.92–1.89 Ga depositional and tectonic history. Ages of pre-depositional zircon overgrowths vary from 1.91 Ga to 2.0 Ga with clusters at 1.92 Ga and 1.98 Ga. Within the Pirkanmaa belt, post-depositional zircon overgrowths indicate metamorphic culmination at c. 1885 Ma in the Vammala Ni-zone and at c. 1875 Ma in the northern part.The lower conglomerates and graywackes in the Tampere belt and their equivalents in the Pirkanmaa belt are rich both in Neoarchean and Paleoproterozoic grains, the latter ranging in age from 1.9 to 2.1 Ga. Compared to these, a sample from the Vammala Ni-zone has an exotic provenance with at least c. 1.90 Ga, 2.04–2.15 Ga, 2.38–2.48 Ga and 2.57–2.63 Ga components.A sedimentary recycling and tectonic model for the central Fennoscandia is proposed, in which the Paleoproterozoic Keitele + Bergslagen continent was formed during an unnamed orogeny at 1.98–1.97 Ga. The Archean Norrbotten microcontinent was attached to the continent at 1.97–1.93 Ga. Upper Kaleva turbidites, derived from the Lapland-Kola orogen in the north, were deposited before 1.92 Ga on a passive margin of the Archean Karelia craton. The Karelia craton collided with the Keitele + Bergslagen + Norrbotten continent at c. 1.92 Ga forming the Lapland-Savo orogen. Subsequent evolution led to rifting and break-up of the latter continent into two microcontinents in the hinterland. At 1.92–1.91 Ga the rift was developed into a subsiding passive margin of the Keitele microcontinent with voluminous turbidite deposition, now seen as graywackes in the Tampere, Pirkanmaa and Pohjanmaa (western Finland) belts. The turbidite material was derived from the rising Lapland-Savo orogen and included recycled Upper Kaleva, recycled (sandstones) and first-cycle 2.03–1.97 Ga detritus from Keitele, 1.93–1.92 Ga Savo arc material, as well as detritus from the Archean craton and its cover deposits. The collision between Karelia and Keitele caused a subduction reversal and the onset of Tampere arc volcanism at 1.90 Ga. Arc-derived materials started to deposit and were mixed with older sedimentary rocks, and trench-parallel distal turbidites from exotic source were being deposited in the accretionary wedge.     

Subduction,ophiolite genesis and collision history of Tethys adjacent to the Eurasian continental margin: new evidence from the Eastern Pontides,Turkey

This paper presents several types of new information including U–Pb radiometric dating of ophiolitic rocks and an intrusive granite, micropalaeontological dating of siliceous and calcareous sedimentary rocks, together with sedimentological, petrographic and structural data. The new information is synthesised with existing results from the study area and adjacent regions (Central Pontides and Lesser Caucasus) to produce a new tectonic model for the Mesozoic–Cenozoic tectonic development of this key Tethyan suture zone.

The Tethyan suture zone in NE Turkey (Ankara–Erzincan–Kars suture zone) exemplifies stages in the subduction, suturing and post-collisional deformation of a Mesozoic ocean basin that existed between the Eurasian (Pontide) and Gondwanan (Tauride) continents. Ophiolitic rocks, both as intact and as dismembered sequences, together with an intrusive granite (tonalite), formed during the Early Jurassic in a supra-subduction zone (SSZ) setting within the ?zmir–Ankara–Erzincan ocean. Basalts also occur as blocks and dismembered thrust sheets within Cretaceous accretionary melange. During the Early Jurassic, these basalts erupted in both a SSZ-type setting and in an intra-plate (seamount-type) setting. The volcanic-sedimentary melange accreted in an open-ocean setting in response to Cretaceous northward subduction beneath a backstop made up of Early Jurassic forearc ophiolitic crust. The Early Jurassic SSZ basalts in the melange were later detached from the overriding Early Jurassic ophiolitic crust.

Sedimentary melange (debris-flow deposits) locally includes ophiolitic extrusive rocks of boninitic composition that were metamorphosed under high-pressure low-temperature conditions. Slices of mainly Cretaceous clastic sedimentary rocks within the suture zone are interpreted as a deformed forearc basin that bordered the Eurasian active margin. The basin received a copious supply of sediments derived from Late Cretaceous arc volcanism together with input of ophiolitic detritus from accreted oceanic crust.

Accretionary melange was emplaced southwards onto the leading edge of the Tauride continent (Munzur Massif) during latest Cretaceous time. Accretionary melange was also emplaced northwards over the collapsed southern edge of the Eurasian continental margin (continental backstop) during the latest Cretaceous. Sedimentation persisted into the Early Eocene in more northerly areas of the Eurasian margin.

Collision of the Tauride and Eurasian continents took place progressively during latest Late Palaeocene–Early Eocene. The Jurassic SSZ ophiolites and the Cretaceous accretionary melange finally docked with the Eurasian margin. Coarse clastic sediments were shed from the uplifted Eurasian margin and infilled a narrow peripheral basin. Gravity flows accumulated in thrust-top piggyback basins above accretionary melange and dismembered ophiolites and also in a post-collisional peripheral basin above Eurasian crust. Thickening of the accretionary wedge triggered large-scale out-of-sequence thrusting and re-thrusting of continental margin and ophiolitic units. Collision culminated in detachment and northward thrusting on a regional scale.

Collisional deformation of the suture zone ended prior to the Mid-Eocene (~45?Ma) when the Eurasian margin was transgressed by non-marine and/or shallow-marine sediments. The foreland became volcanically active and subsided strongly during Mid-Eocene, possibly related to post-collisional slab rollback and/or delamination. The present structure and morphology of the suture zone was strongly influenced by several phases of mostly S-directed suture zone tightening (Late Eocene; pre-Pliocene), possible slab break-off and right-lateral strike-slip along the North Anatolian Transform Fault.

In the wider regional context, a double subduction zone model is preferred, in which northward subduction was active during the Jurassic and Cretaceous, both within the Tethyan ocean and bordering the Eurasian continental margin.     

Tectonic evolution,superimposed orogeny,and composite metallogenic system in China

Continental China is a mosaic of numerous tectonic blocks, which amalgamated from Neoarchean to Cenozoic broadly coeval with the cycles of global supercontinents such as Kenorland, Columbia, Rodinia, Gondwana, and Pangaea. By reviewing the long-lasting geological evolution in the different tectonic blocks, it reveals that more than two episodes of tectonic events, including accretionary and collisional orogeny, and dismantling, as well as mantle plume, occurred successively or simultaneously within a single tectonic belt. This is called superimposed orogeny in this study. Examples of the dominant types of superimposed orogeny in China include: (1) Cenozoic continental collision superimposed on Paleo- to Mesozoic accretionary orogeny in the Tibet and Sanjiang orogenic belts; (2) Reactivation of Paleozoic accretionary orogen in later Mesozoic oceanic subduction in the eastern part of Qinling–Qilian–Kunlun and Central Asian orogenic belts; (3) Mesozoic oceanic subduction under the paleo-suture in the South China Block; (4) Mesozoic demantling along the Paleo- and Neoproterozoic, and Paleozoic sutures in the eastern part of North China Craton; and (5) mantle plume rising through metasomatized lithospheric mantle or stagnant oceanic slab in the Emeishan large igneous province. A comprehensive review of the spatial-temporal distribution of ore deposits and their salient features shows that the superimposed orogeny has exerted significant control on metallogeny in China. The giant porphyry and skarnore deposits, as well as orogenic gold deposits were preferentially formed along previous tectonic suture, craton margin, and arc during later orogenesis due to the remobilization of previously enriched metals. Superimposed orogeny has reworked the lithospheric structure with concomitant granitoid-associated metallogeny. The mixing of magmas from juvenile lower crust, ancient lower crust, and middle crust, which tends to induce the different mineralization of Cu–Au, Mo, and Pb–Zn–W–Sn deposits respectively, was considered to generate a wide variety of combinations of metal species. The superimposed orogeny caused the overlapping of diverse genetic types of deposit formed in different tectonic periods in the same tectono-metallogenic belt. The stratiform ore deposit, including BIF, VMS, SEDEX, or sedimentary sulfide layers, formed from Neoarchean to Paleozoic, were modified by later mineralization, resulting in the enrichment of the various metal species and enhancement of ore resources. This study brings up the concept of composite metallogenic system to summarize the regional metallogeny driven by superimposed orogeny. The composite metallogenic system was dominantly characterized by the multi-episodic and diverse mineralization concomitant with one or more features, including mineralization evolved from the previous metal enrichment, later overlapping or modification on previous ore belt, and diversifying of metal species derived from reworked lithosphere.     

Paleoproterozoic supercontinent: Origin and evolution of accretionary and collisional orogens exemplified in Northern cratons

The evolution of the North American, East European, and Siberian cratons is considered. The Paleoproterozoic juvenile associations concentrate largely within mobile belts of two types: (1) volcanic-sedimentary and volcanic-plutonic belts composed of low-grade metamorphic rocks of greenschist to low-temperature amphibolite facies and (2) granulite-gneiss belts with a predominance of high-grade metamorphic rocks of high-temperature amphibolite to ultrahigh-temperature granulite facies. The first kind of mobile belt includes paleosutures made up of not only oceanic and island-arc rock associations formed in the process of evolution of relatively short-lived oceans of the Red Sea type but also peripheral accretionary orogens consisting of oceanic, island-arc, and backarc terranes accreted to continental margins. The formation of the second kind of mobile belt was related to the activity of plumes expressed in vigorous heating of the continental crust; intraplate magmatism; formation of rift depressions filled with sediments, juvenile lavas, and deposits of pyroclastic flows; and metamorphism of lower and middle crustal complexes under conditions of granulite and high-temperature amphibolite facies that, in addition, spreads over the fill of rift depressions. The evolution of mobile belts pertaining to both types ended with thrusting in a collisional setting. Five periods are recognized in Paleoproterozoic history: (1) origin and development of a superplume in the mantle that underlay the Neoarchean supercontinent; this process resulted in separation and displacement of the Fennoscandian fragment of the supercontinent (2.51–2.44 Ga); (2) a period of relatively quiet intraplate evolution complicated by locally developed plume-and plate-tectonic processes (2.44–2.0 (2.11) Ga); (3) the origin of a new superplume in the subcontinental mantle (2.0–1.95 Ga); (4) the complex combination of intense global plume-and plate-tectonic processes that led to the partial breakup of the supercontinent, its subsequent renascence and the accompanying formation of collisional orogens in the inner domains of the renewed Paleoproterozoic supercontinent, and the emergence of accretionary orogens along some of its margins (1.95–1.75 (1.71) Ga); and (5) postorogenic and anorogenic magmatism and metamorphism (<1.75 Ga).     

Marginal accretion processes of Jiamusi Block in NE China: Evidences from detrital zircon U-Pb age and deformation of the Wandashan Terrane

The eastern segment of Central Asian Orogenic Belt underwent not only a long evolution history related to the Paleo-Asian Ocean during Paleozoic but also the tectonic overprinting by the westward subduction of Paleo-Pacific Ocean crust during Mesozoic. When the subduction of Paleo-Pacific Ocean crust started has been long debated issue for understanding the tectonic evolution of the eastern Asian continental margin. The eastern margin of the Jimusi Block (Wandashan Terrane) preserved complete records for the accretionary process of the westward subduction of Paleo-Pacific Ocean crust. Comprising the Yuejinshan Complex and Raohe Accretionary Complex (RAC), the Wandashan Terrane is located in the eastern margin of Jiamusi Block, NE China, and is considered to be an accretionary wedge of the westward subducting oceanic crust. To reconstruct the marginal accretion processes of the Jiamusi Block, the structural deformation of the Wandashan Terrane was investigated in the field and the geochronology of the Dalingqiao and Yongfuqiao formations were studied, which were formed syn-and-post RAC accretion respectively. The Yuejinshan and Raohe complexes were discontinuously accreted to the eastern margin of the Jiamusi Block. Contrary to the previous consideration of the Late Triassic to Early Jurassic, this study suggests that the Yuejianshan Complex in southwest Wandashan Terrane probably accreted from Late Carboniferous to Middle Permian, which was driven by unknown oceanic crust subduction existing to the east (present position) of the Jiamusi Block at that time. The siltstones of the Dalingqiao Fm. yield the youngest zircon U-Pb age of 142 ± 2 Ma, indicating the emplacement of the RAC not earlier than the Late Jurassic. Thus, the RAC might start to accrete from the Jurassic and emplace during 142–131 Ma, resulted from the Paleo-Pacific subduction which started from the Late Triassic to Early Jurassic.     

内蒙古中—东部地区基底岩石、花岗岩和玄武岩的Nd同位素组成及其对该区地壳演化的指示

对出露于内蒙古地区的华北地台北缘中段及兴蒙造山带内共21件不同岩性的样品进行Nd同位素研究。这些样品的Nd模式年龄值表明:兴蒙造山带与华北地台具完全不同的特征,兴蒙造山带以年轻的亏损地幔模式年龄为特征(tDM=0.4~1.1 Ga),普遍低于华北地台西段的tDM值(1.8~3.4 Ga)。锡林浩特地块作为独立块体具与兴蒙造山带不同的特征,锡林浩特地块的亏损地幔模式年龄介于兴蒙造山带年龄与华北地台年龄之间。Nd模式年龄计算结果表明内蒙古地区华北地台北缘的地壳增生事件主要集中于中元古代之前,而兴蒙造山带地壳增生事件自新元古代开始。通过对内蒙古地区华北地台北缘εNd(t)值随时间的变化分析可知,在中元古代及海西期均存在古老地壳的再循环及新地壳的增生事件。内蒙古兴蒙造山带地壳增生速率表明该区地壳主要增生事件发生于1 000~700 M a,其后形成的岩浆岩所反映的增生过程表明有古老地壳组分的参与。     

Structural-kinematic evolution of the central Belomorian-Lapland Belt in the Paleoproterozoic

A model of the evolution of the central Belomorian-Lapland Granulite-Gneiss Belt is proposed on the basis of analysis of the Paleoproterozoic structural-kinematic assemblages. It is shown that this tectonic zone is a long-lived mobile structural unit that evolved through several stages of tectonic transformation and metamorphism of rocks, including (1) the Reboly stage, which comprises subduction (2.88–2.82 Ga) and collision (2.74–2.53 Ga) substages; (2) the Selet stage of rifting and extension of the continental crust according to the model of simple shear (2.45–2.35 Ga); and (3) the Svecofennian stage, characterized by collision and general transpression (1.94–1.75 Ga). The results of structural-kinematic study indicate that tectonic flow in the Svecofennian time was nonuniform and related to the formation of the Kolvitsa-Umba near-horizontal protrusion. The propagation of this protrusion was caused by transpressional extrusion of plastic lower-crustal masses to the surface as a gently ascending tectonic flow directed to the northwest (in present-day coordinates). Thereby, a thrust-normal-fault kinematic effect was expressed in pushing-out of deep-seated complexes contemporaneously with tectonic erosion of the upper portions of the sequence owing to the development of lowangle normal faults.     

Late Mesozoic Huangbeiling S-type granite in the East Qinling Orogen,China: Geochronology,petrogenesis and implications for tectonic evolution

S-type granites are typical features of collisional orogenic belts and could provide insights into the tectonic process associated with the final phase of orogeny. The East Qinling Orogen, one of significant segments in the Central China Orogen, witnessed complex tectonic evolution during the Late Mesozoic. The rare S-type granites in this orogen can be used as important proxies to understand the Late Mesozoic tectonic processes. Although a few previous studies suggested that the Huangbeiling pluton in the East Qinling Orogen might be S-type granite, detailed studies are lacking. Thus, we report the results from a systematic petrological, whole-rock geochemical and zircon U-Pb-Lu-Hf isotopic studies on the Huangbeiling pluton, with a view to constrain the timing of magmatism, petrogenetic evolution and genetic type, and to evaluate the implications for Late Mesozoic tectonic evolution of the East Qinling Orogen. Zircon U-Pb analysis yield ²⁰⁶Pb/²³⁸U spot ages in the range of 156.7–132.2 Ma, with weighted ²⁰⁶Pb/²³⁸U mean ages varying from 146.8 to 141.9 Ma, suggesting the Huangbeiling pluton formed during the Late Jurassic to Early Cretaceous. Zircon Lu-Hf isotopic data show negative εHf(t) values of ?21.5 to ?14.9 and two-stage Hf model ages of 2546–2131 Ma, which are correlated with the Neoarchean to Paleoproterozoic (3.0–2.1 Ga) meta-sedimentary rocks from nearby Taihua Group, indicating that the magma was sourced from reworked ancient crustal components involving meta-sedimentary rocks. Whole-rock geochemical data display enrichment of LREEs, Pb, Hf and Y as well as depletion in HREEs, Ba and HFSEs (e.g., Ta, P, Ti), with weakly negative Eu anomalies. The Huangbeiling granitoids are identified as S-type granites, which generated through partial melting of lower-middle crust and upper crustal fractional crystallization in syn-collisional settings. In conjunction with published information related to the tectonic evolution of the East Qinling Orogen, we propose that the Late Mesozoic Huangbeiling S-type granites might response to the complex tectonic evolution related to extensional tectonics induced by multi-directional (intra-continental) subductions from the Yangtze and North China Cratons as well as the Paleo-Pacific Plate during the Late Mesozoic.     

Anatexis constraints on the post-collisional evolution of the Sulu Orogen,China: evidence from pegmatite veins in migmatite

ABSTRACT

The widespread migmatites in the northwestern part of the Sulu Orogen, China, indicate regional anatexis that is of great significance when discussing the tectonic evolution of this continental orogenic belt. Cathodoluminescence (CL) images, U–Pb ages, and in situ trace element compositions of zircons from four pegmatite veins within these migmatites provide clear evidence for the nature of the post-collisional evolution of the Sulu Orogen. The inherited zircon cores reveal that the protoliths of the migmatites were middle Neoproterozoic magmatic rocks (810–620 Ma) of the South China Block. The protoliths underwent two partial melting events. The mantle domains of the inherited zircons record a Late Triassic (222.0–204.0 Ma) partial melting event that occurred during the exhumation and retrograde metamorphism, after ultrahigh-pressure (UHP) metamorphism. Subsequent newly grown zircons record a Middle–Late Jurassic to Early Cretaceous (164.1–125.5 Ma) anatexis event, indicating that the late Mesozoic anatexis started before ca. 164.1 Ma, reached a peak at ca. 152.1 Ma, and ceased at ca. 125.5 Ma. Combined with previous results of studies on the Sulu orogen, the late Mesozoic anatexis suggested that the thickened crust of the Sulu Orogen had started to become unstable before 164.1 Ma. The duration of ~164.1–137 Ma corresponds to a period of transition in the tectonic regime of the Sulu Orogen, enabling the early high-temperature ductile deformation. After ca. 137 Ma, the tectonic regime was fully transformed into extension and the Sulu Orogen underwent rapid thinning and collapse, thus leading to the late medium–low temperature ductile deformation (137–121 Ma) and laying the foundations for the large-scale magmatic emplacement during the late Early Cretaceous (127–115 Ma). These two partial melting events together promoted the rapid exhumation of the Sulu UHP rocks.     

A Neoarchean–Proterozoic Supercontinent (~2.8–0.9 Ga): An Alternative to the Model of Supercontinent Cycles

The model of supercontinent cycles is revisited on the basis of reevaluation of existing ideas on the geodynamics and tectonics of granulite gneiss belts and areals. Granulite-gneiss belts and areals of a regional scale correspond to mantle–plume (superplume) activity and form the major components of intracontinental orogens. The evolution of geodynamic settings of the Earth’s crust origin can be imagined as a “spiral sequence”: (1) interaction of mantle plumes and “embryonic” microplate tectonics during the Paleo- Mesoarchean (~3.80–2.75 Ga); (2) plume-tectonics and local plume-driven plate-tectonics within supercontinent during Neoarchean and Proterozoic (~2.75–0.85 Ga); (3) plate tectonics in the Phanerozoic along with a reduced role of mantle plumes starting from ~0.85 Ga.     

High-T, low-P metamorphism in the Palaeoproterozoic Halls Creek Orogen, northern Australia: the middle crustal response to a mantle-related transient thermal pulse

High‐T, low‐P metamorphic rocks of the Palaeoproterozoic central Halls Creek Orogen in northern Australia are characterised by low radiogenic heat production, high upper crustal thermal gradients (locally exceeding 40 °C km^?1) sustained for over 30 Myr, and a large number of layered mafic‐ultramafic intrusions with mantle‐related geochemical signatures. In order to account for this combination of geological and thermal characteristics, we model the middle crustal response to a transient mantle‐related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere. This mechanism has the potential to raise mid‐crustal temperatures by 150–400 °C within 10–20 Myr following initiation of the mantle temperature anomaly, via conductive dissipation through the crust. The magnitude and timing of maximum temperatures attained depend strongly on the proximity, duration and lateral extent of the thermal anomaly in the mantle lithosphere, and decrease sharply in response to anomalies that are seated deeper than 50–60 km, maintained for <5 Myr in duration and/or have half‐widths <100 km. Maximum temperatures are also intimately linked to the thermal properties of the model crust, primarily due to their influence on the steady‐state (background) thermal gradient. The amplitudes of temperature increases in the crust are principally a function of depth, and are broadly independent of crustal thermal parameters. Mid‐crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust, and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly. Although pluton‐related thermal signatures significantly dissipate within <10 Myr (even for very large, high‐temperature intrusive bodies), the interaction of pluton‐ and mantle‐related thermal effects has the potential to maintain host rock temperatures in excess of 400–450 °C for up to 30 Myr in some parts of the mid‐crust. The numerical models presented here support the notion that transient mantle‐related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high‐T, low‐P metamorphic belts, especially in those characterised by low surface heat flow, very high peak metamorphic geothermal gradients and abundant mafic intrusions.     

Proterozoic evolution and tectonic setting of the northwest Paterson Orogen,Western Australia

The Proterozoic igneous, deformation and metamorphic histories of the Palaeoproterozoic Rudall Complex in the northwestern Paterson Orogen can be linked to those of the Arunta Inlier in central Australia, and in part with the Capricorn Orogen in central Western Australia. The similarities in deformation and metamorphic histories for these widely separated regions indicate a Palaeoproterozoic continent–continent collisional event between the Palaeoproterozoic West Australian and North Australian cratons between c. 1830 and 1765 Ma. In the Paterson Orogen this Palaeoproterozoic collisional event resulted in the Yapungku Orogeny, which included thrust stacking of clastic sedimentary and volcanic rocks, deposition of the protoliths for the c. 1790 Ma siliciclastic paragneiss succession contemporaneous with granitic intrusion, and metamorphism up to granulite facies. During this 65-million-year period, the Arunta Inlier and Capricorn Orogen were deformed, metamorphosed at medium to high grades and intruded by granitoids during the Strangways Orogeny in the Arunta Inlier and the Capricorn Orogeny in the Capricorn Orogen.The Neoproterozoic Tarcunyah, Throssell and Lamil groups are clastic sedimentary sequences that were deposited after 1070 Ma in the northwestern Paterson Orogen, and deformed by the Miles Orogeny before 678 Ma. The Miles Orogeny produced a northwesterly trending fold and fault system of tight to isoclinal upright and overturned folds and thrust faults. The orogeny may have been coincident with the c. 750–720 Ma Areyonga tectonic movement affecting the Arunta Inlier and the lower Neoproterozoic part of the Amadeus Basin in central Australia. At c. 550 Ma the Paterson Orogeny, which is most likely equivalent to the Petermann Orogeny in the Musgrave Complex of central Australia, deformed the northwestern Paterson Orogen and was preceded by local intrusion of granites.The similarities of styles and timing of deformation in the northwestern Paterson Orogen, Arunta Inlier and Capricorn Orogen indicate that these three regions were probably linked during most of the Proterozoic.     

Crustal complexity in the Lachlan Orogen revealed from teleseismic receiver functions

There is an ongoing debate about the tectonic evolution of southeast Australia, particularly about the causes and nature of its accretion to a much older Precambrian core to the west. Seismic imaging of the crust can provide useful clues to address this issue. Seismic tomography imaging is a powerful tool often employed to map elastic properties of the Earth's lithosphere, but in most cases does not constrain well the depth of discontinuities such as the Mohorovi?i? (Moho). In this study, an alternative imaging technique known as receiver function (RF) has been employed for seismic stations near Canberra in the Lachlan Orogen to investigate: (i) the shear-wave-velocity profile in the crust and uppermost mantle, (ii) variations in the Moho depth beneath the Lachlan Orogen, and (iii) the nature of the transition between the crust and mantle. A number of styles of RF analyses were conducted: H-K stacking to obtain the best compressional–shear velocity (V _P /V _S) ratio and crustal thickness; nonlinear inversion for the shear-wave-velocity structure and inversion of the observed variations in RFs with back-azimuth to investigate potential dipping of the crustal layers and anisotropy. The thick crust (up to 48 km) and the mostly intermediate nature of the crust?mantle transition in the Lachlan Orogen could be due to the presence of underplating at the base of the crust, and possibly to the existing thick piles of Ordovician mafic rocks present in the mid and lower crust. Results from numerical modelling of RFs at three seismic stations (CAN, CNB and YNG) suggest that the observed variations with back-azimuth could be related to a complex structure beneath these stations with the likelihood of both a dipping Moho and crustal anisotropy. Our analysis reveals crustal thickening to the west beneath CAN station which could be due to slab convergence. The crustal thickening may also be related to the broad Macquarie volcanic arc, which is rooted to the Moho. The crustal anisotropy may arise from a strong N–S structural trend in the eastern Lachlan Orogen and to the preferred crystallographic orientation of seismically anisotropic minerals in the lower and middle crust related to the paleo-Pacific plate convergence.