The passive margin of northern Gondwana during Early Paleozoic: Evidence from the central Tibet Plateau

The central–south domain of the Tibet Plateau represents an important part of the northern segment of Gondwana during the early Paleozoic. Here we present zircon U–Pb, Lu–Hf isotope, and whole–rock geochemical data from a suite of early Paleozoic magmatic rocks from the central Tibet Plateau, with a view to gain insights into the nature and geotectonic evolution of the northern margin of Gondwana. Zircon grains in four granitic rocks yielded ages of 532−496 Ma with negative ε_Hf(t) values (−13.7 to −0.6). Zircon grains in meta–basalt and mafic gneiss yielded ages of 512 ± 5 Ma and 496 ± 6 Ma, respectively. Geochemically, the granitic rocks belong to high–K calc–alkaline and shoshonitic S–type granite suite, with the protolith derived from the partial melting of ancient crustal components. The mafic gneiss and meta–basalt geochemically resemble OIB (Oceanic Island Basalt) and E–MORB (Enriched Mid–Ocean Ridge Basalt), respectively. They were derived from low degree (∼5–10%) partial melting of an enriched mantle (garnet and spinel lherzolite) that was contaminated by upper crustal components. The parental magmas experienced orthopyroxene–dominated fractional crystallization. Sedimentological features of the Cambrian–Ordovician formations indicate that the depositional cycle transformed from marine regression to transgression leading to the formation of parallel/angular unconformities between the Cambrian and Ordovician strata. The hiatus associated with these unconformities are coupled with the peak of the early Paleozoic magmatism in Tibet Plateau, indicating a tectonic control. We conclude that the Cambrian–Ordovician magmatic suite and sedimentary rocks formed in an extensional setting, and we correlate this with the post–peak stage of the Pan–African orogeny. The post–collision setting associated with delamination, orogenic collapse or lithospheric extension along the northern margin of Gondwana, can account for the Cambrian–Ordovician magmatism and sedimentation, rather than oceanic subduction along the external margin. We thus infer a passive margin setting for the northern Gondwana during the Early Paleozoic.     

Evolution of the Proto-Tethys in the Baoshan block along the East Gondwana margin: constraints from early Palaeozoic magmatism

Zircon U–Pb ages and geochemical and isotopic data for Late Ordovician granites in the Baoshan Block reveal the early Palaeozoic tectonic evolution of the margin of East Gondwana. The granites are high-K, calc-alkaline, metaluminous to strongly peraluminous rocks with A/CNK values of 0.93–1.18, are enriched in SiO₂, K₂O, and Rb, and depleted in Nb, P, Ti, Eu, and heavy rare earth elements, which indicates the crystallization fractionation of the granitic magma. Zircon U–Pb dating indicates that they formed at ca. 445 Ma. High initial ⁸⁷Sr/⁸⁶Sr ratios of 0.719761–0.726754, negative ?_Nd(t) values of –6.6 to –8.3, and two-stage model ages of 1.52–1.64 Ga suggest a crustal origin, with the magmas derived from the partial melting of ancient metagreywacke at high temperature. A synthesis of data for the early Palaeozoic igneous rocks in the Baoshan Block and adjacent Tengchong Block indicates two stages of flare-up of granitic and mafic magmatism caused by different tectonic settings along the East Gondwana margin. Late Cambrian to Early Ordovician granitic rocks (ca. 490 Ma) were produced when underplated mafic magmas induced crustal melting along the margin of East Gondwana related to the break-off of subducted Proto-Tethyan oceanic slab. In addition, the cession of the mafic magmatism between late Cambrian-Early Ordovician and Late Ordovician could have been caused by the collision of the Baoshan Block and outward micro-continent along the margin of East Gondwana and crust and lithosphere thickening. The Late Ordovician granites in the Baoshan Block were produced in an extensional setting resulting from the delamination of an already thickened crust and lithospheric mantle followed by the injection of synchronous mafic magma.     

东天山巴里坤地区早石炭世高镁闪长岩的确定及地质意义

东天山巴里坤地区两个闪长岩体分别获得327 Ma 和333 Ma 的LA-ICP-MS 锆石U-Pb年龄,代表早石炭世一期岩浆活动。这些岩体具中等略高的SiO₂（51.33%～62.48%）、高MgO（2.04%～11.16%）、高TiO₂（0.67%～1.29%）、富钠（Na₂O/K₂O = 1.39～2.95）,相对富集LILE和LREE、贫HFSE,亏损Nb、富集Pb,具中等略高的轻重稀土分馏,无或弱负铕异常的右倾谱型,与赞岐质高镁闪长岩相同。岩体基本没有受到陆壳物质混染,其锆石ε_Hf（t）均为正值,且变化范围较大,指示来自受富集组分改造的亏损地幔。结合岩石中出现较多角闪石等含水矿物以及Pb富集,证明它们来自俯冲带之上有流体加入的地幔楔,高的TiO₂和中等略高的轻重稀土分馏进一步表明,是俯冲消减板片部分熔融的长英质熔体与地幔橄榄岩相互作用后,交代改造地幔熔融岩浆侵位的产物。结合区域同期弧火山岩和花岗岩类的形成,以及于晚石炭世-二叠纪广泛出现碰撞/后碰撞高钾花岗岩、A 型花岗岩以及镁铁质-超镁铁质岩体和基性岩脉的研究认为,博格达-哈尔里克构造带在早石炭世处于与大洋俯冲消减有关的岛弧环境,此后于晚石炭世晚期-早二叠世开始转入后碰撞伸展的构造环境。     

Geochemistry and Sm–Nd isotopic systematics of Ediacaran–Ordovician,sedimentary and bimodal igneous rocks in the western Acatlán Complex,southern Mexico: Evidence for rifting on the southern margin of the Rheic Ocean

Ordovician igneous rocks in the western Acatlán Complex (Olinalá area) of southern Mexico include a bimodal igneous suite that intrudes quartzites and gneisses of the Zacango Unit, and all these rocks were polydeformed and metamorphosed in the amphibolite facies during the Devono-Carboniferous. The Ordovician igneous rocks consist of the penecontemporaneous amphibolites, megacrystic granitoids and leucogranite, the latter dated at ca. 464 Ma. Geochemical and Sm–Nd data indicate that the amphibolites have a differentiated tholeiitic signature, and that its mafic protoliths formed in an extensional setting transitional between within-plate and ocean floor. The amphibolites are variably contaminated by a Mesoproterozoic crustal source, inferred to be the Oaxacan basement exposed in the adjacent terrane. The most primitive samples have εNdt (t = 465 Ma) values significantly below that of the contemporary depleted mantle and were probably derived from the sub-continental lithospheric mantle. The megacrystic granites were most probably derived by partial melting of an arc crustal source (similar to the Oaxacan Complex) and triggered by the ascent of mafic magma from the lithospheric mantle. Sm–Nd isotopic signatures suggest that metasedimentary rocks from Zacango Unit were derived from adjacent Oaxacan Complex. Trace elements relationships (e.g. La/Th vs. Hf) and REE patterns suggest provenance in felsic-intermediate igneous rocks with a calc-alkaline signature. The Ordovician bimodal magmatism is inferred to have resulted from rifting on the southern flank of the Rheic Ocean and is an expression of a major rifting event that occurred along much of the northern Gondwanan margin in the Ordovician.     

Crustal segments in the North Patagonian Massif,Patagonia: An integrated perspective based on Sm–Nd isotope systematics

New insights on the Paleozoic evolution of the continental crust in the North Patagonian Massif are presented based on the analysis of Sm–Nd systematics. New evidence is presented to constrain tectonic models for the origin of Patagonia and its relations with the South American crustal blocks. Geologic, isotopic and tectonic characterization of the North Patagonian Massif and comparison of the Nd parameters lead us to conclude that: (1) The North Patagonian Massif is a crustal block with bulk crustal average ages between 2.1 and 1.6 Ga T_DM (Nd) and (2) At least three metamorphic episodes could be identified in the Paleozoic rocks of the North Patagonian Massif. In the northeastern corner, Famatinian metamorphism is widely identified. However field and petrographic evidence indicate a Middle to Late Cambrian metamorphism pre-dating the emplacement of the ca. 475 Ma granitoids. In the southwestern area, are apparent 425–420 Ma (?) and 380–360 Ma metamorphic peaks. The latter episode might have resulted from the collision of the Antonia terrane; and (3) Early Paleozoic magmatism in the northeastern area is coeval with the Famatinian arc. Nd isotopic compositions reveal that Ordovician magmatism was associated with attenuated crust. On the southwestern border, the first magmatic recycling record is Devonian. Nd data shows a step by step melting of different levels of the continental crust in the Late Palaeozoic. Between 330 and 295 Ma magmatism was likely the product of a crustal source with an average 1.5 Ga T_DM (Nd). Widespread magmatism represented by the 295–260 Ma granitoids involved a lower crustal mafic source, and continued with massive shallower-acid plutono volcanic complexes which might have recycled an upper crustal segment of the Proterozoic continental basement, resulting in a more felsic crust until the Triassic. (4) Sm–Nd parameters and detrital zircon age patterns of Early Paleozoic (meta)-sedimentary rocks from the North Patagonian Massif and those from the neighboring blocks, suggest crustal continuity between Eastern Sierras Pampeanas, southern Arequipa-Antofalla and the northeastern sector of the North Patagonian Massif by the Early Paleozoic. This evidence suggests that, at least, this corner of the North Patagonian Massif is not allochthonous to Gondwana. A Late Paleozoic frontal collision with the southwestern margin of Gondwana can be reconcilied in a para-autochthonous model including a rifting event from a similar or neighbouring position to its post-collision location. Possible Proterozoic or Early Paleozoic connections of the NPM with the Kalahari craton or the western Antartic blocks should be investigated.     

西藏班戈寒武纪辉长闪长岩体的发现及其构造意义

西藏班戈地区构造单元属北拉萨地体,到目前为止,该地体尚未有寒武纪岩浆活动的报道.本文就班戈地区首次发现的辉长闪长岩体,在野外地质调查、元素地球化学及同位素年代学研究的基础上,得出以下结论:该岩体属高钾钙碱性系列,具有富集大离子亲石元素（如Rb、Ba）和轻稀土元素（La、Ce）,亏损Nb、Ta、Zr、Hf和Ti等高场强元素,呈现出安第斯型岛弧岩浆特征,显示该岩体是俯冲沉积物部分熔融形成的熔体交代上覆地幔楔的产物.锆石U-Pb年代学指示其侵入年龄为512 ±3 Ma,形成于寒武纪,为北拉萨地体存在早古生代岩浆活动提供了可靠的年代学证据.综合证实其属于冈瓦纳大陆北缘的一部分,该成果为进一步探讨青藏高原的构造演化提供了新的约束.      

东昆仑祁漫塔格早中生代大陆地壳增生过程中的岩浆活动与成矿作用

东昆仑祁漫塔格地区位于青藏高原北缘,为典型的大陆边缘增生造山带,经历了漫长的古生代—早中生代增生造山过程,其中以早中生代岩浆活动与成矿作用最为发育。文章系统总结了区内早中生代侵入岩分布及成因,对与其相关矿床地质、成矿流体特征及成矿物质来源进行分析,进一步探讨了祁漫塔格地区早中生代大陆地壳增生过程中的壳幔混合岩浆活动与成矿作用的关联。研究结果认为,中二叠世—早三叠世以俯冲阶段的侧向增生为主,中-晚三叠世以碰撞-后碰撞阶段的垂向增生为主,与成矿有关的岩浆岩主要为中-晚三叠世石英闪长岩、花岗闪长岩、二长花岗岩、正长花岗岩、花岗斑岩等,以I型、A型花岗岩为主,且多见暗色包体,Sr-Nd-Hf同位素组成表明其源于古陆壳物质的重熔,有地幔物质的参与,由地幔底侵古老陆壳,幔源基性岩浆与壳源花岗质岩浆发生不同程度混合作用而形成。与该时期岩浆活动关系密切的主要为斑岩型铜钼矿床、矽卡岩型铁多金属矿床、层控矽卡岩型铅锌矿床、与碱性花岗岩有关稀有金属矿化等。成矿时代集中于248~210 Ma,成矿流体主要来源于岩浆热液,成矿物质具有壳幔混合来源,区内中-晚三叠世大陆垂向增生过程中的壳-幔岩浆混合作用为区域大规模金属成矿提供大量热能、成矿流体及成矿物质。     

Evolution of Cambrian and Early Ordovician arcs in the Kyrgyz North Tianshan: Insights from U-Pb zircon ages and geochemical data

Geochronological, geochemical, and structural studies of magmatic and metamorphic complexes within the Kyrgyz North Tianshan (NTS) revealed an extensive area of early Palaeozoic magmatism with an age range of 540–475 Ma. During the first episode at 540–510 Ma, magmatism likely occurred in an intraplate setting within the NTS microcontinent and in an oceanic arc setting within the Kyrgyz-Terskey zone in the south. During the second episode at 500–475 Ma, the entire NTS represented an arc system. These two phases of magmatism were separated by an episode of accretionary tectonics of uncertain nature, which led to obduction of ophiolites from the Kyrgyz-Terskey zone onto the microcontinent. The occurrence of zircon xenocrysts and predominantly negative whole-rock ɛ_Nd(t) values and ɛ_Hf(t) values of magmatic zircons suggest a continental setting and melting of Precambrian continental sources with minor contributions of Palaeozoic juvenile melts in the generation of the magmatic rocks. The late Cambrian to Early Ordovician 500–475 Ma arc evolved mainly on Mesoproterozoic continental crust in the north and partly on oceanic crust in the south. Arc magmatism was accompanied by spreading in a back-arc basin in the south, where supra-subduction ophiolitic gabbros yielded ages of 496 to 479 Ma. The relative position of the arc and active back-arc basin implies that the subduction zone was located north of the arc, dipping to the south. Variably intense metamorphism and deformation in the NTS reflect an Early Ordovician orogenic event at 480–475 Ma, resulting from closure of the Djalair-Naiman ophiolite trough and collision of the Djel'tau microcontinent with the northern margin of NTS. Comparison of geological patterns and episodes of arc magmatism in the NTS and Chinese Central Tianshan indicate that these crustal units constituted a single early Palaeozoic arc and were separated from the Tarim Craton by an oceanic basin since the Neoproterozoic.     

Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension     

胶北早前寒武纪变质基底多期岩浆-变质热事件:来自TTG片麻岩和花岗质片麻岩中锆石U-Pb定年的证据

本文采用LA-ICP-MS技术,对胶北地体TTG片麻岩和花岗质片麻岩中锆石进行系统原位U-Pb定年和稀土、微量元素的分析,发现研究区早前寒武变质结晶基底存在多期岩浆-变质热事件。4件TTG片麻岩和2件花岗质片麻岩锆石样品记录了2909±13Ma、2738±23Ma、2544±15~2564±12Ma和2095±12Ma 4组岩浆事件年龄,以及2504±16~2513±32Ma和1863±41Ma 2组变质事件年龄。结合以往TTG片麻岩和花岗质片麻岩的地球化学及Nd同位素研究发现,约2738Ma的TTG岩浆事件可能代表胶北地体地壳最主要的生长事件,而2544~2564Ma的岩浆事件则可能代表古老地壳重熔的最强烈岩浆事件,约2095Ma岩浆事件则反映了胶-辽-吉构造带内部在该时期与地壳拉张作用有关的岩浆活动。2504~2513Ma是研究区以及华北克拉通早前寒武基底最主要的一期变质热事件,可能与地幔柱(热点)岩浆的底侵作用有关,而TTG片麻岩记录的约1863Ma的变质年龄与研究区基性和泥质高压麻粒岩相岩石记录的麻粒岩相变质时代一致,暗示TTG片麻岩可能也经历了古元古代高压麻粒岩相变质作用,上述研究进一步表明胶北地体在古元古代的确存在一期陆-陆碰撞的重要造山事件。该项研究成果对于进一步深入探讨胶北乃至华北克拉通早前寒武纪变质基底的形成演化、岩浆-变质热事件序列及其构造背景具有重要的科学意义。     

Timing, styles, and kinematics of Cambro–Ordovician extension in the Teplá–Barrandian Unit, Bohemian Massif, and its bearing on the opening of the Rheic Ocean

This paper describes late Cambrian dikes and Early Ordovician volcano-sedimentary successions of the Prague Basin, Bohemian Massif, to discuss the timing and kinematics of breakup of the northern margin of Gondwana. Andesitic dikes indicate minor E–W crustal extension in the late Cambrian, whereas the Tremadocian to Dapingian lithofacies distribution and linear array of depocenters suggest opening of this Rheic Ocean rift-related basin during NW–SE pure shear-dominated extension. This kinematic change was associated with the onset of basic submarine volcanism, presumably resulting from decompression mantle melting as the amount of extension increased. We conclude from these inferences and from a comparison with other Avalonian–Cadomian terranes that the rifting along the northern Gondwana margin was a two-stage process involving one major pulse of terrane detachment in the early Cambrian and one in the Early Ordovician. While the geodynamic cause for the former phase remains unclear, but still may include effects of Cadomian subduction (roll-back, slab break-off), isostatic rebound, or mantle plume, the incipient stage of the latter phase may have been triggered by the onset of subduction of the Iapetus Ocean at around 510 Ma, followed by advanced extension broadly coeval (Tremadocian to Darriwilian) in large portions of the Avalonian–Cadomian belt. Unequal amounts of extension resulted in the separation and drift of some terranes, while other portions of the belt remained adjacent to Gondwana.     

U–Pb geochronology and Lu–Hf isotopes of zircons from newly identified Permian–Early Triassic plutons in western Liaoning province along the northern margin of the North China Craton: constraints on petrogenesis and tectonic setting

Mafic to felsic gneisses along the northern margin of the North China Craton (NMNCC), in western Liaoning province, China, were previously assumed to be part of Archean metamorphic basement but are here identified as younger (Permian–Early Triassic) intrusions. LA–ICP–MS zircon U–Pb isotopic dating reveals that the magmatic precursors of the mafic gneisses were emplaced from 295 ± 3 to 259 ± 2 Ma and that the magmatic precursors of the dioritic and monzogranitic gneisses were emplaced at 267 ± 1 and 251 ± 2 Ma, respectively, thus recording a continuum of Permian to Early Triassic magmatism. The mafic and dioritic rocks exhibit zircon ε_Hf(t) values from ?20.7 to ?3.3, suggesting they were mainly derived from a metasomatized lithospheric mantle source, possibly involving some crustal contamination. The monzogranitic rocks display their zircon ε_Hf(t) values of +0.9 to +4.7, indicating the acidic magma was derived from partial melting of juvenile crustal materials from the depleted mantle source. Crustal model ages (T _DM ^C ) obtained from zircon Hf isotopes of these monzogranitic rocks range from 976 to 1,215 Ma, with an average of 1,074 ± 32 Ma, possibly implying an episode of Grenvillian crustal growth in western Liaoning province. These new lines of evidence show that the NMNCC witnessed abundant magmatic activity and interaction of the crust and mantle during the Permian and Early Triassic and that the mafic magmatism was earlier than the monzogranitic activity. These findings indicate that the monzogranitic activity was the result of underplating of mafic magma with an enriched mantle source. In the context of regional Late Paleozoic to Early Mesozoic magmatic activity, the Permian magmatism occurred in an Andean-style continental margin setting when the Paleo-Asian oceanic plate was subducted beneath the NMNCC, and in this context, the Late Permian to Early Triassic magmatism may have been linked to post-collisional extension and asthenospheric upwelling, suggesting that the western Liaoning province in the NMNCC may be an eastward extension of the Late Paleozoic to Early Mesozoic active continental margin.     

拉萨地体南部早侏罗世岩浆岩的成因和构造意义

本文从拉萨地体南部原来被认为是前寒武纪变质基底的冈底斯岩群中厘定出了一套早侏罗世的岩浆岩.锆石U-Pb年代学研究表明,这些岩浆岩侵位于202～ 180Ma.岩石类型包括辉长闪长岩、二长岩和花岗闪长岩,是一套中酸性、偏铝质钙碱性、Ⅰ型花岗岩类.微量元素表现出消减带富集大离子亲石元素、亏损高场强元素的特征,并具有岛弧花岗岩的亲缘性.锆石Hf同位素研究表明,加查地区中酸性岩石来自新生地壳物质的熔融,偏基性岩石来自于亏损地幔.而桑日地区的酸性岩石来自于古老地壳物质的重熔.本文认为包括研究区在内的南拉萨地体中的晚三叠世-早侏罗世岩浆岩为俯冲到南拉萨地体之下的松多洋壳断离或回卷,软流圈地幔上涌,地幔楔熔融并加热上覆地壳的产物.     

Post-collisional granitoids from the Dabie orogen in China: Zircon U–Pb age, element and O isotope evidence for recycling of subducted continental crust

While recycling of subducted oceanic crust is widely proposed to be associated with oceanic island, island arc, and subduction-related adakite magmatism, it is less clear whether recycling of subducted continental crust takes place in continental collision belts. A combined study of zircon U–Pb dating, major and minor element geochemistry, and O isotopes in Early Cretaceous post-collisional granitoids from the Dabie orogen in China demonstrates that they may have been generated by partial melting of subducted continental crust. The post-collisional granitoids from the Dabie orogen comprise hornblende-bearing intermediate rocks and hornblende-free granitic rocks. These granitoids are characterized by fractionated REE patterns with low HREE contents and negative HFSE anomalies (Nb, Ta and Ti). Although zircon U–Pb dating gives consistent ages of 120 to 130 Ma for magma crystallization, occurrence of inherited cores is identified by CL imaging and SHRIMP U–Pb dating; some zircon grains yield ages of 739 to 749 Ma and 214 to 249 Ma, in agreement with Neoproterozoic protolith ages of UHP metaigneous rocks and a Triassic tectono-metamorphic event in the Dabie–Sulu orogenic belt, respectively. The granitoids have relatively homogeneous zircon δ¹⁸O values from 4.14‰ to 6.11‰ with an average of 5.10‰ ± 0.42‰ (n = 28) similar to normal mantle zircon. Systematically low zircon δ¹⁸O values for most of the coeval mafic–ultramafic rocks and intruded country rocks preclude an AFC process of mafic magma or mixing between mafic and felsic magma as potential mechanisms for the petrogenesis of the granitoids. Along with zircon U–Pb ages and element results, it is inferred that the granitic rocks were probably derived from partial melting of intermediate lower crust and the intermediate rocks were generated by amphibole-dehydration melting of mafic rocks in the thickened lower crust, coupled with fractional crystallization during magma emplacement. The post-collisional granitoids in the Dabie orogen are interpreted to originate from recycling of the subducted Yangtze continental crust that was thickened by the Triassic continent–continent collision. Partial melting of orogenic lithospheric keel is suggested to have generated the bimodal igneous rocks with the similar crustal heritage. Crustal thinning by post-collisional detachment postdated the onset of bimodal magmatism that was initiated by a thermal pulse related to mantle superwelling in Early Cretaceous.     

A review of the geodynamic setting of large-scale Late Mesozoic gold mineralization in the North China Craton: an association with lithospheric thinning

Abundant gold deposits are distributed along the margins of the North China Craton (NCC). Occurring throughout the Precambrian basement and located in or proximal to Mesozoic granitoids, these deposits show a consistent spatial–temporal association with Late Jurassic–Early Cretaceous magmatism and are characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock. Their ages are mainly Early Cretaceous (130–110 Ma) and constrain a very short period of metallogenesis. Sr–Nd–Pb isotopic tracers of ores, minerals and associated rocks indicate that gold and associated metals mainly were derived from multi-sources, i.e., the wall rocks (Precambrian basement and Mesozoic granites) and associated mafic rocks.Previous studies, including high surface heat flow, uplift and later basin development, slow seismic wave speeds in the upper mantle, and a change in the character of mantle xenoliths sampled by Paleozoic to Cenozoic magmas, have been used to suggest that ancient, cratonic mantle lithosphere was removed from the base of the NCC some time after the Ordovician, and replaced by younger, less refractory lithospheric mantle. The geochemistry and isotopic compositions of the mafic rocks associated with gold mineralization (130–110 Ma) indicate that they were derived from an ancient enriched lithospheric mantle source; whereas, the mafic dikes and volcanic rocks younger than 110 Ma were derived from a relatively depleted mantle source, i.e., asthenospheric mantle. According to their age and sources, relation to magmatism and geodynamic framework, the gold deposits were formed during lithospheric thinning. The removal of lithospheric mantle and the upwelling of new asthenospheric mantle induced partial melting and dehydration of the lithospheric mantle and lower crust due to an increase of temperature. The fluids derived from the lower crust were mixed with magmatic and meteoric waters, and resulted in the deposition of gold and associated metals.     

Zedong terrane revisited: An intra-oceanic arc within Neo-Tethys or a part of the Asian active continental margin?

Precise timing of the India-Asia collision is important to constrain the evolution history of both the Himalayan orogen and the Tibetan Plateau. It has been proposed that the Indian plate first collided with an intra-oceanic arc at ∼55 Ma, and then the composite terrane collided with the Asian continent at ∼35 Ma. The Zedong terrane has been suggested to represent the vestige of such an intra-oceanic arc developed within the Neo-Tethys Ocean, as some volcanic rocks with high K₂O have been classified as shoshonites. In this study, we present detailed geochemical and geochronological data of various types of magmatic rocks (including volcanic, cumulate and granitic rocks) widely exposed in the Zedong terrane to constrain the formation age and tectonic setting of the Zedong terrane. We found that the Zedong volcanic rocks belong to calc-alkaline series rather than shoshonites and high K₂O contents in some volcanic rocks resulted from alteration. The basalts are highly enriched in LREE and LILE, but strongly depleted in HFSE, indicating they were derived from a metasomatized mantle. Presence of hornblende phenocryst in both gabbros and hornblendites indicates that the cumulates were produced from hydrous basalts through crystallization. The granitic rocks have adakite-like compositional characteristics, i.e., high Sr/Y ratios but low Y contents, which were formed by melting of a thickened lower crust. Zircons from six samples, including a volcanic rock (an andesite), three cumulates (a hornblendites, a hornblende-bearing gabbro and a gabbro) and two granitic (a tonalite and a granodiorite) rocks, have been dated to yield identical ages of ∼155–160 Ma. This suggests that the volcanic eruption and plutonic emplacement were coevally developed in the Zedong terrane. Zircons from both the andesite and the cumulates have similar positive ε_Hf(t) values (∼+11.6 to +16.7), indicating they were stemmed from similarly depleted mantle sources. Meanwhile, zircons from the granitic rocks also have positive ε_Hf(t) values of ∼+12.6 to +15.2, implying their derivation from a juvenile lower crust. Therefore, we proposed that the basalts in the Zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab-released fluids/melts. A part of hydrous basalts were underplated in the thickened lower crust beneath the Zedong terrane, which gave rise to the cumulate and granitic rocks. By comparison, magmatic rocks in the Zedong terrane show compositional similarities with the Jurassic rocks exposed in the Gangdese arc. This suggests that the Zedong terrane represents a slice of the active continental margin developed on the southern margin of the Lhasa terrane as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic, rather than the vestige of an intra-oceanic arc.     

Shrimp U–Pb zircon geochronology,geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province,SW China

The Cida A-type granitic stock (∼ 4 km²) and Ailanghe I-type granite batholith (∼ 100 km²) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO^⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites.The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios (ε_Nd(t) = − 0.25 to + 1.35 and (⁸⁷Sr/⁸⁶Sr)_i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε_Nd(t) ratios (− 6.34 to − 6.26) and high (⁸⁷Sr/⁸⁶Sr)_i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks.In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.     

Early Palaeozoic rift-related magmatism in Variscan Europe: fragmentation of the Armorican Terrane Assemblage

Early Palaeozoic bimodal rift-related magmatism is widespread throughout much of the Variscides of Europe. It is traceable from the Polish Sudetes to NW Iberia. Granitic plutonism generally predates Cambro–Ordovician bimodal magmatism. In the N Bohemian Massif this early Palaeozoic granitic plutonism was generated by partial melting of Cadomian basement, whereas contemporaneous alkali granites with a mantle component are typical of the NW Iberian Massif. Silurian-Devonian mafic magmatism in the N Bohemian Massif, Massif Central and NW Iberian Massif is partly preserved as obducted ophiolites. Compositional diversity displayed by Cambro-Ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume. This indicates that combined tensional forces and mantle plume convection assisted the early Palaeozoic dispersal of terranes from the N Gondwana margin. Continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust.     

敦化-密山断裂带的平移距离:来自松嫩-张广才岭-佳木斯-兴凯地块古生代-中生代岩浆作用的制约

敦化-密山断裂带是郯庐断裂北段的重要分支之一,其大规模左行走滑发生的时限以及平移距离一直存在较大争议。本文系统地总结了松嫩-张广才岭地块东缘、佳木斯地块以及兴凯地块之上古生代-中生代火成岩的锆石U-Pb年代学资料,结合其空间分布特征,对敦化-密山断裂带的平移时限及距离提供了制约。研究表明,松嫩-张广才岭地块东缘与兴凯地块在古生代-中生代期间具有类似的岩浆活动历史,两个地块之上该时期的岩浆作用可以划分为8个主要期次:中-晚寒武世(ca.500~516Ma)、早奥陶世(ca.480~486Ma)、晚奥陶世(ca.450~456Ma)、中志留世(ca.426~430Ma)、早二叠世(ca.285~292Ma)、晚二叠世(ca.255~260Ma)、晚三叠世(ca.202~210Ma)和早侏罗世(ca.185~186Ma)。相比之下,佳木斯地块中的古生代-中生代早期岩浆事件则集中在晚寒武世(~492Ma)、晚泥盆世(~388Ma)、早二叠世(~288Ma)、晚二叠世(~259Ma)和早侏罗世(~176Ma),而晚奥陶世-志留纪和晚三叠世的岩浆活动在佳木斯地块未见报道。早白垩世晚期(ca.105~110Ma)和晚白垩世(ca.90~94Ma)的岩浆活动在三个地块均存在。上述结果表明兴凯地块东缘与松嫩-张广才岭地块东缘在早古生代经历了共同的地质演化历史,而中生代早期,兴凯地块西缘与松嫩-张广才岭地块东缘经历了同样的岩浆作用历史。上述结果暗示,敦化-密山断裂可能经历了至少两次平移,分别发生在中-晚二叠世-早三叠世和中-晚侏罗世-早白垩世,推测其总的平移距离约400km。结合研究区中生代期间的构造演化历史,敦化-密山断裂中生代的左行平移应与中-晚侏罗世-早白垩世期间古太平洋板块(Izanagi板块)的斜向俯冲相联系。     

Pre-collisional (> 0.5 Ga) complexes of the Olkhon terrane (southern Siberia) as an echo of events in the Central Asian Orogenic Belt

The Olkhon terrane is a part of the Early Palaeozoic accretionary-collisional system in the northern Central Asian Orogenic Belt (CAOB). The terrane was produced by an Ordovician collision as a collage of numerous chaotically mixed tectonic units composed of rock complexes of different ages originated in different tectonic settings. The pre-collisional history of the terrane is deciphered using new data on zircon ages and chemistry of rocks from several complexes. The oldest Olkhon rocks are the 1.87–1.83 Ga granulite and gneissic granites of the Kaltygey complex, which is an exotic Palaeoproterozoic tectonic slice. The next age group consists of the Ust-Zunduk orthogneisses (807 ± 9 Ma) and the Orso amphibolites and gneisses (792 ± 10 and 844 ± 6 Ma). Samples of both complexes have negative ε_Nd(t) values. The Ust-Zunduk and Orso complexes can have formed in active margins of continents or in crustal blocks other than southern Siberia. The Ediacaran subduction-related rocks of the Olkhon complex may have formed in an island arc setting within the Palаeo-Asian Ocean (PAO). The protolith of schists after volcanic rocks has an age of 637 ± 4 Ma and shows positive ɛ_Nd(t) values. The Ediacaran/Cambrian Tonta mafic granulites (ca.545 Ma), with OIB affinity and slightly positive ɛ_Nd(t), were derived from an enriched mantle source and may represent a fragment of an oceanic island. The Cambrian Shebarta gneisses after continental-arc greywackes with negative ɛ_Nd(t) values were deposited in a back-arc basin of a microcontinent within the PAO, between 530 and 500 Ма. The Cambrian Birkhin metamorphics after PAO mature island-arc rocks have U-Pb ages of ca. 500–490 Ma and positive ɛ_Nd(t) values. All pre-collisional complexes in the Olkhon terrane have their analogues among the rocks formed during main events in the northern CAOB history. Thus the reconstructed milestones in the Olkhon terrane history appear to be an echo of events in the CAOB northern segment.