Structure and evolution of the continental crust in the Baikal Fold Region

A summary of original Nd isotopic data on granitoids, silicic volcanics, and metasediments of the Baikal Fold Region is presented. The available Nd isotopic data, in combination with new geological and geochronological evidence, allowed recognition of the Early Baikalian (1000 ± 100 to 720 ± 20 Ma) and Late Baikalian (700 ± 10 to 590 ± 5 Ma) tectonic cycles in the geological evolution. The tectonic stacking, deformation, metamorphism, and granite formation are related to orogenic events that occurred 0.80–0.78 Ga and 0.61–0.59 Ga ago. The crust-forming events dated at 1.0–0.8 Ga and 0.70–0.62 Ga pertain to each cycle. The Early Baikalian crust formation developed largely in the relatively narrow and spatially separated Kichera and Param-Shamansky zones of troughs in the Baikal-Muya Belt. The formation and reworking of the Late Baikalian continental crust played the leading role in the Karalon-Mamakan, Yana, and Kater-Uakit zones and in the Svetlinsky Subzone of the Anamakit-Muya Zone in the Baikal-Muya Belt. In general, three large historical periods are recognized in the evolution of the Baikal Fold Region. The Early Baikalian period was characterized by prevalence of reworking of the older continental crust. The Late Baikalian-Early Caledonian period is distinguished by more extensive formation and transformation of the juvenile crust. The third, Late Paleozoic period was marked by reworking of the continental crust with juxtaposition of all older crustal protoliths. Two models of paleogeodynamic evolution of the Baikalian fold complexes are considered: (1) the autochthonous model that corresponds to the formation of suboceanic crust in rift-related basins of the Red Sea type and its subsequent reworking in the course of collision-related squeezing of paleorifts and intertrough basins and (2) the allochthonous model that implies the formation of fragments of the Baikal-Muya Belt at the shelf of the Rodinia supercontinent, their subsequent participation in the evolution of the Paleoasian ocean, and their eventual juxtaposition during Late Baikalian and Early Caledonian events in the structure of the Caledonian Siberian Superterrane of the Central Asian Foldbelt.     

The eastern boundary of the Baikal collisional belt: Geological,geochronological, and Nd isotopic evidence

New geological. geochronological, and Nd isotopic data are reported for the rocks occurring at the interfluve of the Barguzin, Nomama, and Katera rivers, where the main structural elements of the Early Paleozoic collisional system have been established. The strike-slip and thrust Tompuda-Nomama and Barguzin boundary sutures separate the Svetlaya and the Katera zones of the Baikal-Muya Belt from the Barguzin terrigenous-carbonate terrane. The age estimates of syntectonic (prebatholithic) gneissic granite and gabbrodiorite intrusive bodies (469 ± 4 and 468 ± 8 Ma, respectively) coincide with the age of collisional events in the Ol’khon, Southwest Baikal, and Sayan regions (480–470 Ma). A linear zone with zonal metamorphism and granite-gneiss domes dated at 470 Ma is revealed in the allochthonous fold-nappe packet of the Upper Riphean Barguzin Formation. This zone of Caledonian remobilization marks the collisional front between the Riphean structural units of the Barguzin Terrane consolidated 0.60–0.55 Ga ago and the Baikal-Muya Belt. New data allow us to recognize this zone as the northeastern flank of the Baikal Collisional Belt. The Nd isotopic data for the reference igneous complexes of the collisional zone indicate that the Late Riphean juvenile crust was involved in the Ordovician remobilization in the zone of conjugation of the consolidated Baikalian structural elements at the northeastern flank of the Baikal Belt and likely was a basement of the entire Barguzin Terrane or, at least, its frontal portion. The lateral displacements of the terranes to the northeast during the Early Ordovician collision were constrained by the rigid structural framework of the Baikalides in the Muya segment of the Baikal-Muya Belt, where the Riphean blocks were involved in strike-slip faulting and the Vendian-Cambrian superimposed basin underwent deformation. Finally, it may be concluded that the Early Ordovician was an epoch of collision, complex in kinematics, between heterogeneous blocks of the continental crust: the Baikalides of the Baikal-Muya Belt and polycyclic Barguzin-Vitim Superterrane.     

Age of granitoids from the conjunction zone of the Baikal-Muya foldbelt and the Kalar metamorphic terrane

One of the key segments in the conjunction zone between the Baikal folded area of Baikalides, the Early Precambrian Aldan-Stanovoi shield, and the Barguzin-Vitim superterrane involving fragments of the Early Precambrian, Baikalian, and Paleozoic folded complexes is discussed. Within this segment, complicated tectonic contacts between the Late Riphean complexes of the Param-Shaman paleotrough zone in the Baikal-Muya foldbelt of Baikalides and Lower Precambrian complexes of the Kalar metamorphic terrane are mapped. The results of the U-Pb zircon isotopic dating (TIMS and SHRIMP-II) of gneisses-syenites from the Burgai Complex and gneissoid granites of the Drevnestanovoi Complex of the Early Precambrian age, as well as results of the Nd-isotope study of reference magmatic and stratified complexes of the region are presented. The ages of the oldest gneiss-syenites from the Burgai Complex and overlying plagiomigmatites in the conjunction zone have been established to differ by less than 1 Ma, making up 601 ± 5 Ma. Drevnestanovoi gneissoid granites in the conjunction zone are of the Late Paleozoic age (325–270 Ma). According to Nd isotopic data, the age of the source, from which Vendian gneisses-syenites and granites were melted, was established to be not older than the Riphean, and the material of the old continental crust to be the protolith of the upper Paleozoic granites. It has been inferred that the collision junction of Baikalian and Early Precambrian structures of the Baikal folded area and the Aldan-Stanovoi Shield into a single block took place 600 Ma ago.     

Crustal growth stages in the Songino block of the early caledonian superterrane in Central Asia: II. Geochemical and Nd-isotope data

Geochemical and Nd isotope data are reported for Late Riphean metamorphic complexes and granitoids of the Bayannur zone of the Songino block in the Early Caledonian superterrane of Central Asia. Geological, geochronolgical, geochemical, and isotope data were integrated to discuss rock sources and main mechanisms responsible for the formation and evolution of the Late Riphean continental crust. It was established that lithotectonic complexes of the Bayannur zone were formed on a convergent plate during Late Proterozoic tectonogenesis (around 1.3–0.78 Ga). This period primarily produced a juvenile crust represented by paleooceanic (N- and E-MORB types) and island arc basalts. An interval of 800–880 Ma was marked by the formation of rocks of the Bayannur complex and metaterrigenous sequence (accretionary wedge) of the Kholbonur complex, and the emplacement of quartz diorites and granodiorites of the Gashunnur pluton due to erosion and melting of both Late Riphean juvenile sources and ancient possibly Early Precambrian crustal material in a setting of ensialic island arc. At the final stage of the Late Riphean evolution of the Bayannur zone, postkinematic granitoids of the Bayannur pluton, and gabbrodiorites and anorthosites of the Ontsula pluton were derived from mantle juvenile and crustal sources in a within-plate setting. In terms of isotope characteristics, the crystalline complexes of the Bayannur zone are comparable with the Japan-type modern island arc systems. A synthesis of geological, geochronological, and isotope-geochemical data indicates a much wider distribution of the Late Riphean juvenile crust-forming processes than considered previously and remobilization of continental crust in the eastern segment of the Central Asian Fold Belt. The Vendian-Paleozoic stage in the evolution of this segment was characterized by an intense growth of juvenile crust, while magmatism during Late Riphean stage was determined by mixing of Late Riphean juvenile and ancient Early Precambrian sources.     

The Early Baikalian crystalline complex in the basement of the Dzabkhan microcontinent of the Early Caledonian orogenic area, Central Asia

Fragments of continental blocks or microcontinents are represented in the Early Caledonian orogenic area of Central Asia (or Early Caledonian superterrane); the largest of these are the Dzabkhan and Tuva-Mongolian microcontinents, with Early and Late Precambrian crystalline basements, respectively. In the linkage zone of these microcontinents, crystalline rocks of the Tarbagatai and Songino blocks that are considered as units of the Early Precambrian ensialic basement of the superterrane are also known. They are composed of strongly metamorphosed rocks formed during the Early Baikalian orogeny about 790 to 820 Ma. U-Pb zircon dating and Nd isotope studies revealed, within the northwestern Dzabkhan microcontinent, the Dzabkhan-Mandal zone of crystalline rocks associated with the Riphean crust-forming process. The age of the gneiss substrate of this zone is estimated as 1.3 to 0.86 Ga. An early episode of metamorphism is dated at about 856 ± 2 Ma. The data available so far indicate a heterogeneous structure of the Dzabkhan microcontinent basement represented by Early Precambrian and Early and Late Baikalian crystalline formations.     

Late Riphean episode in the formation of crystalline rock complexes in the Dzabkhan microcontinent: Geological,geochronologic, and Nd isotopic-geochemical data

The Early Caledonian Central Asian Orogenic Belt hosts fragments of continental blocks with Early and Late Precambrian crystalline basement. One of the structures with an Early Precambrian basement was thought to be the Dzabkhan microcontinent, which was viewed as an Early Precambrian “cratonal terrane”. The first geochronologic data suggest that the basement of the Dzabkhan microcontinent includes a zone of crystalline rocks related to Late Riphean tectonism. Geological, geochronological (U-Pb zircon dates), and Nd isotopic-geochemical data were later obtained on the northwestern part of the Dzabkhan microcontinent. The territory hosts the most diverse metamorphic complexes thought to be typical of the Early Precambrian basement. The complexes were determined to comprise the Dzabkhan-Mandal and Urgamal zones of high-grade metamorphic rocks. Gabbrodiorites related to the early metamorphic episode and dated at 860 ± 3 Ma were found in the Dzabkhan-Mandal zone, and the gneiss-granites marking the termination of this episode were dated at 856 ± 2 Ma. The granitoids of the Dzabkhan batholith, whose emplacement was coeval with the termination of the late high-grade metamorphic episode in rocks of both zones, have an age of 786 ± 6 Ma. Similar age values were determined for the granitoids cutting across the Late Precambrian rocks of the Songino and Tarbagatai blocks, which mark the stage when the mature Late Riphean continental crust was formed. The Late Riphean magmatic and metamorphic rocks of the Dzabkhan microcontinent were found out to have Nd model ages mostly within the range of 1.1–1.4 Ga at ?_Nd(T) from +1.9 to +5.5. The Nd model age of the metaterrigenous rocks is 2.2?1.3 Ga at ?_Nd(T) from ?7.2 to +3.1. The results of our studies provide evidence of convergence processes, which resulted in the Late Riphean (880?780 Ma) continental crust in Central Asia. Simultaneously with these processes, divergence processes that were responsible for the breakup of Rodinia occurred in the structures of the ancient cratons. It is reasonable to suggest that divergence processes within ancient continental blocks and Rodinia shelf were counterbalanced by the development of the Late Riphean continental crust in the convergence zones of its surrounding within established interval.     

Age and sources of granitoids in the junction zone of the Caledonides and Hercynides in southwestern Mongolia: Geodynamic implications

The paper reports newly obtained geological, geochronological (U-Pb zircon method), Nd isotopic, and geochemical data on Middle and Late Paleozoic granitoids and metamorphic rocks from the southern slope of the Mongolian Altai and Gobi Altai and on granitoids from the Trans-Altai Gobi. Tectonically, the former rocks are hosted in the margin of a Caledonian paleocontinent, and the latter are localized among island-arc and oceanic complexes related to the development of the Hercynian Southern Mongolian Ocean. According to their geological setting, the intrusive complexes are subdivided into two major groups: (i) related to processes of regional metamorphism and (ii) separated from these processes. Geochemical data suggest that the source of most of the granitoids and metamorphic rocks contained island-arc rocks and their erosion products. Nd isotopic evidence indicates that practically all of the allochthonous granitoids, regardless of their composition, age, and structural setting, have positive ?_Nd(T) values [i.e., belong to the ?(+) type] and could not be formed by the melting of metaterrigenous rocks widespread at the modern erosion level. These granitoids in both the Caledonian and the Hercynian structures have practically identical Late Riphean Nd model ages [T_Nd(DM) = 0.97–0.60 Ga], which become slightly younger in the granitoids of the Hercynides. The exception are ultrametamorphic subautochthonous ?(?) granites of the first group localized in the peripheral part of migmatite fields. The sources of these granitoids could be the host metaterrigenous rocks. The results obtained in the course of this research suggest, with regard for preexisting data on granitoids in the isotopic provinces in Central Asia, that the sources of the Paleozoic granitoids were the rocks of the “juvenile” Caledonian and Hercynian island-arc crust and of the older crust of cratonic blocks with a Early Precambrian and Late Riphean basement, respectively. The Late Riphean crustal material in Caledonian and Hercynian structures related to the development of the corresponding oceanic basins most probably consisted of clastic sediments or relatively small fragments of the Late Riphean crust. The occurrence of this crustal material in the sources of the granitoids can be explained by the involvement of sediments in subduction zones and the participation of these sediments and fragments of Late Riphean complexes in the accretionary-collision processes during the closure of the paleoceanic basins. Simultaneously, the subduction zones received juvenile material that could be later involved in the melting processes together with older rocks.     

Age and source of terrigenous rocks of the turan group of the bureya terrane of the eastern part of the central Asian foldbelt: Results of geochemical (Sm-Nd) and geochronological (U-Pb LA-ICP-MS) studies

According to Sm-Nd isotopic-geochemical studies, the t _Nd(DM) of the terrigenous rocks of the Turan Group of the Bureya terrane is 1.4–1.5 Ga and their sources are Precambrian rocks and (or) younger effusive rocks, the formation of which is related to the reworking of the Late Precambrian continental crust. The U-Pb LA-ICP-MS geochronological studies indicate dominant Vendian-Cambrian (588–483 Ma) and Late Riphean (865–737 Ma) detrital zircons. Our data point to their accumulation at the beginning of the Paleozoic rather than in the Precambrian as is accepted in modern stratigraphic schemes.     

Sedimentary complexes of the cover of the Dzabkhan continental block: Different sedimentary basins and source areas

The geochemical and Sm–Nd isotope characteristics of Late Precambrian and Early Cambrian sandstones previously related to the sedimentary cover of the Dzabkhan continental block are reported. It is established that the Riphean and Vendian sedimentary rocks of the Ul’zitgol’skaya and Tsaganolomskaya Formations were accumulated within the Dzabkhan continental block as a result of recycling of the terrigenous deposits formed at the expense of destruction of basement rocks and younger granite. The formation of terrigenous rocks of the Bayangol’skaya Formation after a gap in sedimentation occurred in the sedimentary basin, where only the Late Riphean formations of the juvenile crust, probably of the Dzabkhan–Mandal block were the sources, without the contribution of the ancient crustal material. The Tsaganolomskaya and Bayangol’skaya Formations were formed in different sedimentary basins and cannot be related to the same complex.     

Tectonic evolution of Early Paleozoic island-arc systems and continental crust formation in the Caledonides of Kazakhstan and the North Tien Shan

The extended Saryarka and Shyngyz-North Tien Shan volcanic belts that underwent secondary deformation are traced in the Caledonides of Kazakhstan and the North Tien Shan. These belts are composed of igneous rocks pertaining to Early Paleozoic island-arc systems of various types and the conjugated basins with oceanic crust. The Saryarka volcanic belt has a complex fold-nappe structure formed in the middle Arenigian-middle Llanvirnian as a result of the tectonic juxtaposition of Early-Middle Cambrian and Late Cambrian-Early Ordovician complexes of ensimatic island arcs and basins with oceanic crust. The Shyngyz-North Tien Shan volcanic belt is characterized by a rather simple fold structure and consists of Middle-Late Ordovician volcanic and plutonic associations of ensialic island arcs developing on heterogeneous basement, which is composed of complexes belonging to the Saryarka belt and Precambrian sialic massifs. The structure and isotopic composition of the Paleozoic igneous complexes provide evidence for the heterogeneous structure of the continental crust in various segments of the Kazakh Caledonides. The upper crust of the Shyngyz segment consists of Early Paleozoic island-arc complexes and basins with oceanic crust related to the Saryarka and Shyngyz-North Tien Shan volcanic belts in combination with Middle and Late Paleozoic continental igneous rocks. The deep crustal units of this segment are dominated by mafic rocks of Early Paleozoic suprasubduction complexes. The upper continental crust of the Stepnyak segment is composed of Middle-Late Ordovician island-arc complexes of the Shyngyz-North Tien Shan volcanic belt and Early Ordovician rift-related volcanics. The middle crustal units are composed of Riphean, Paleoproterozoic, and probably Archean sialic rocks, whereas the lower crustal units are composed of Neoproterozoic mafic rocks.     

Precambrian terranes in the southwestern framing of the Siberian craton: isotopic provinces,stages of crustal evolution and accretion-collision events

We studied geology and main rock assemblages of the Precambrian Kan, Arzybei, and Derba terranes of the Central Asian Fold Belt which border the Siberian craton in the southwest. The Precambrian terranes include three isotopic provinces (Paleoproterozoic, Mesoproterozoic, and Neoproterozoic) distinguished from the Sm-Nd isotope compositions of granitoids, felsic metavolcanics, and metasediments. The terranes formed in three stages of crustal evolution: 2.3–2.5, 0.9–1.1, and 0.8–0.9 Ga. Proterozoic juvenile crust was produced by subduction-related magmatism; it was originally of transitional composition and transformed into continental crust by potassic plutonism as late as the Late Vendian-Cambrian. Terrigenous sediments in the Arzybei and Derba terranes vary in T(DM) Nd model ages from 1.0 to 2.0 Ga. The Nd ages of the underlying metavolcanics and lowest T(DM) of metasediments indicate that terrigenous sedimentation started in the Neoproterozoic. It was maintained by erosion of Mesoproterozoic-Neoproterozoic crust and, to a lesser extent, of Early Precambrian rocks on the craton margin or in Paleoproterozoic terranes. Ar-Ar dating of amphiboles and biotites from metamorphic rocks and U-Pb dating of zircons from granitoids yielded 600–555 and 500–440 Ma, respectively, corresponding to the Vendian and Early Paleozoic stages of nearly synchronous metamorphism and plutonism. Accretion and collision events caused amalgamation of the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic terranes in the Vendian and their collision with the Siberian craton. The lateral growth of the paleocontinent completed in the Late Ordovician.     

Late Riphean age of conglomerates from the Kholbonur complex of Songino block, Central Asian Caledonides

The Early Caledonian folded area of Central Asia comprises a variety of continental crust fragments with Early to Late Precambrian crystalline basement. Crystalline rocks, which form part of the Songino block, outcrop at the junction between the Dzabkhan and Tuva-Mongolian terranes. The Bayannur zone in the southern part of the Songino block contains the Bayannur migmatite-gneiss and Kholbonur terrigenous-metavolcanic metamorphic complexes. Previous studies provide the 802 ± 6 Ma age for the regional metamorphism and folding within the Bayannur complex. On the basis of the minimum Nd model age of 1.5 Ga, gneisses from this complex cannot be regarded as Early Precambrian. Two main rock associations were distinguished in the Kholbonur complex. Mafic metavolcanics compose the dominant lithology of the first rock association, whereas the second association comprises terrigenous-volcanic and predominantly terrigenous suites. The rocks of the predominantly terrigenous suite, including mudstones, sandstones, and conglomerates, are interpreted to derive from the Late Riphean accretionary prism. The lithology and composition of metaterrigenous rocks suggest that they were possibly derived from erosion of a volcanic arc. The upper age limit of this suite is constrained by postkinematic granites (790 ± 3 Ma; U-Pb zircon), the lower age is given by plagiogranite (874 ± 3 Ma; U-Pb zircon) from comglomerate pebbles. Therefore, the timing of deposition of this terrigenous suite can be bracketed by the 874–790 Ma time interval. These ages and compositional features of the Kholbonur complex terrigenous rocks suggest that the convergence took place at around 870–880 Ma and thus it can be correlated with the divergent processes between the blocks of continental crust composing the supercontinent Rodinia.     

U–Pb zircon geochronology of Late Devonian to Early Carboniferous extension‐related silicic volcanism in the northern New England Fold Belt

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age (ca 360–350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U–Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km² and ≥500 km inboard of plate margin) and protracted, occurring over a period of ～15 million years. Zircon inheritance is commonplace in the Late Devonian — Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385–370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation. This contrasts with a lack of significant zircon inheritance in younger Permo‐Carboniferous igneous rocks intruded through, and emplaced on top of, the Devonian‐Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr–Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent.     

华北克拉通南缘太华杂岩组成及演化

太华杂岩位于华北克拉通南部,其组成复杂,记录了几乎所有早前寒武纪各阶段重要的地质事件;此外,由于其所处特殊地理位置,研究太华杂岩对于华北克拉通早前寒武纪地壳形成和演化、构造单元划分和基底拼合等都具有举足轻重的科学价值。本文综合已有的岩石学、变质作用、地球化学以及同位素年代学等诸多研究工作,得到以下阶段性结论和认识:1)将鲁山地区太华划分为以深成侵入岩为主的片麻岩系和以变质沉积-火山岩为主的表壳岩系;前者形成于中太古代晚期-新太古代早期,后者形成于古元古代晚期。而小秦岭地区太华杂岩中变质深成侵入岩形成时间跨度较大,为中太古代晚期-古元古代早期;而其上覆的火山-沉积岩可与鲁山太华杂岩的表壳岩类比,形成时间亦为古元古代晚期。2)中太古代-新太古代(2.91～2.50Ga)为华北克拉通南部大陆最主要的地壳形成时期。提出太华杂岩在太古宙经历了两期明显的地壳生长时间,一期发生在2.85～2.70Ga,以鲁山太华片麻岩系中的深成侵入岩和斜长角闪岩为代表;另一期发生在～2.50Ga,以小秦岭华山和崤山地区太华杂岩中各类花岗质岩石为代表。3)太华杂岩在所谓的全球陆壳生长"沉寂期(2.45～2.20Ga)"岩浆活动异常发育,推测这一时期的岩石形成于古元古代俯冲-汇聚环境,可能是与华北克拉通南部太古宙陆块和其他陆块汇聚-碰撞相关。4)太华杂岩在古元古代晚期普遍遭受了强烈的变质和变形,其变质程度主体为高角闪岩相,局部可达麻粒岩相,且记录了包含近等温降压退变质片段的顺时针变质作用P-T轨迹,经历了一个漫长的变质演化过程(1.97～1.80Ga),变质作用的时限跨度可达150Myr。5)提出华北克拉通南部曾经为一个统一基底,称之为"南部太古宙地块",此地块形成时间为新太古代末期(～2.5Ga)。该古老陆块经历了如下5个构造-演化阶段:(1)冥古宙-始太古代初始陆核形成;(2)中太古代-新太古代陆壳快速生长;(3)古元古代早期(～2.3Ga)岩浆活动异常活跃;(4)古元古代(2.30～1.97Ga)陆内拉伸-破裂;和(5)古元古代末期(1.97～1.80Ga)陆块最终拼合。     

Nd and Pb Isotopic Composition of Granitoids in the Khangai Batholith as an Indicator of Crust-Forming Processes in the Terranes of the Central Asian Orogenic Belt

The Khangai batholith is one of the largest groups of granitoid plutons produced in Central Asia in the Late Permian–Early Triassic, at 270–240 Ma. The batholith occurs in the Khangai collage of Precambrian terranes, which include Early Precambrian crustal blocks (Dzabkhan and Tarbagatai) and Early to Late Neoproterozoic structures of the Songino block in their surroundings. The axial zone of this collage is overprinted by a basin filled with Devonian volcanic–siliceous rocks and Early to Middle Carboniferous terrigenous rocks. The isotopic parameters (Nd and Pb) of granitoids in the Khangai batholith indicate that the melts were derived from compositionally contrasting crustal sources and a single mantle one. The massifs hosted in the Precambrian blocks were produced with the involvement of lower crustal material, with various ages of the origin of the crust and its differentiation into upper and lower ones. The crust of the Tarbagatai and Dzabkhan blocks was produced in the Early Archean and was differentiated at the Archean–Proterozoic boundary. The crust of the Songino block was formed in the Paleoproterozoic and differentiated in the Early Neoproterozoic. According to the Pb and Nd isotopic parameters of granitoids in the Khangai Basin, the regional continental crust was close to the juvenile one, i.e., the continental crust of the Khangai Basin had still not been differentiated by the time when the Khangai batholith was produced. A single mantle source was involved in the origin of the melts of granitoids of the Khangai batholith in various tectonic blocks. The evolution of the Pb isotopic composition of this sources is consistent with the Stacey–Kramers model at µ = 9.5. This source can be identified with the enriched mantle, which has a higher U/Pb ratio than the depleted mantle and lower ε_Nd(T) of 0 to +2.     

Crustal growth stages in the Songino block of the Early Caledonian superterrane in Central Asia: I. Geological and geochronological data

The Early Caledonian folded area in Central Asia (Early Caledonian superterrane) hosts micro-continent fragments with an Early and Late Precambrian crystalline basement, the largest of them being the Dzabkhan and Tuva-Mongolian fragments. Their junction zone hosts exposures of crystalline rocks that were previously thought to be part of the Early Precambrian Dzabkhan microcontinent. The Bayannur zone in the southern part of the Songino block hosts the Baynnur gneiss-migmatite and Kholbonur metavolcanic-terrigenous metamorphic complexes. The former is believed to be the Early Proterozoic crystalline basement, and the latter is thought to unconformably overly the Late Riphean cover complex of the Songino block. Various rocks of the tectono-stratigraphic complexes in the Bayannur zone were studied geologically and geochronologically (by the U-Pb technique of zircon). Regional metamorphism and folding in the Bayannur Complex were dated at 802 ± 6 Ma. The Nd model ages lie within the range of 1.5–2.0 Ga and thus preclude the correlation of these rocks with those in the Archean and Early Proterozoic basement of the Dzabkhan microcontinent. The upper age limit for folding and metamorphism in the Bayannur zone is marked by postkinematic granites dated at 790 ± 3 Ma, and the lower limit of the volcano-sedimentary complex is determined by the Nd model age of the sandstone (1.3 Ga). The upper age limit of the volcano-plutonic rocks in this zone is set by the gabbroids and anorthosites: 783 ± 2 and 784 ± 3 Ma, respectively. The complex of island-arc granitoids in the Bayannur zone is dated at 859 ± 3 Ma. The age constraints make it possible to correlate crystalline rocks in the Bayannur Complex of the Sangino block and the Dzhargalant Complex in the Tarbagatai block. Currently available data testify that the Precambrian Khangai group of blocks in the Early Caledonian Central Asian superterrane includes continental crustal blocks related to the processes of Early Precambrian, Late Riphean, and Vendian tectonism.     

Within-plate (intracontinental) and postorogenic magmatism of the East European Craton as reflection of the evolution of continental lithosphere

A comparative analysis of within-plate (intracontinental) and orogenic magmatic series formed during various evolution stages of the East European Craton (EEC) was performed using geological-petrological, geochemical, and isotopic data. The example of Baltic shield indicates that the compositions and tectonic settings of mantle melts in the Early Precambrian (Archean and Early Paleoproterozoic) significantly differed from those in the Phanerozoic. The Early Precambrian magmas were dominated by high-Mg low-Ti melts of the komatiite-basaltic and boninite-like series; this tectonomagmatic activity was determined by the ascent of mantle superplumes of the first generation, which originated in the depleted mantle. In the interval of 2.3–2.0 Ga, high-Mg mantle melts gradually gave place to the Fe-Ti picrites and basalts that are typical of within-plate Phanerozoic magmatism; at ～2 Ga, plume tectonics of the Early Precambrian gave way to plate tectonics. This is considered to be linked to the activity of mantle superplumes of the second generation (thermochemical), which originated from the liquid metallic core/mantle interface. Owing to the presence of fluid components, these superplumes reached much higher levels, where spreading of their head portions led to the active interaction with overlaying thinned rigid lithosphere. Sm-Nd isotopic studies showed that orogenic Neoarchean and Middle Paleoproterozoic magmatism of the Baltic shield was connected to the melting of the lithospheric mantle and crust; the melting of crustal sources gave rise to felsic members of the considered complexes. The systematic geochemical variations observed in these rocks with time presumably reflect a general trend toward an increase of the thickness of the continental crust serving as the basement for orogens. Beginning at ～2 Ga, the Meso, Neoproterozoic, and Phanerozoic including, no systematic variations were observed in the isotopic-geochemical characteristics of within-plate magmatism. All considered age sections demonstrate that isotopic-geochemical characteristics of parental mantle melts were strongly modified by crustal contamination. Mesoproterozoic magmatism of EEC was unique in the development of giant anorthosite-rapakivi granite complexes. Kimberlites and lamproites were repeatedly formed within EEC in the time interval from 1.8 to 0.36 Ga; their maximal development was noted in the Late Devonian. It was shown that only kimberlites derived from weakly enriched mantle are diamondiferous in the Arkhangelsk province; in the classic diamond provinces (Africa and Yakutia), diamondiferous kimberlites were derived from both depleted and enriched mantle.     

白云鄂博地区早前寒武纪基底岩浆作用和变质作用:锆石SHRIMP U-Pb定年和LA-MC-ICPMS Hf同位素分析

白云鄂博位于华北克拉通北缘,是阴山地块的重要组成部分.本文对该区变质基底的5个花岗质岩石样品进行了锆石SHRIMP U-Pb定年,获得了2.63Ga和1.89～2.20Ga的岩浆锆石年龄以及2.47～2.51Ga和1.86～ 1.94Ga的变质锆石年龄.新太古代早期片麻状英云闪长岩的岩浆锆石εHf(t)值和tDM2(C...     

The Baikal-Vitim Fold System: Structure and geodynamic evolution

New data on the stratigraphy, structure, isotopic age, geochemistry, and geodynamic characteristics of the lithotectonic complexes of the Baikal-Vitim Fold System are reported. In particular, it is shown that Middle and Upper Paleozoic rocks are widespread along with Precambrian and Lower Paleozoic sequences. The Baikal-Vitim Fold System is characterized by cyclic evolution and comprises four structural stages: Baikalian (Riphean-Vendian), Caledonian (Cambrian-Early Silurian), Variscan (Late Silurian-Early Carboniferous), and Hercynian (Middle Carboniferous-Permian). A specific set of lithotectonic complexes formed in certain geodynamic settings corresponds to each stage. According to the proposed model, the Variscan and Hercynian complexes developed under conditions of progressively changing geodynamic settings of passive (Late Silurian-Middle Devonian), Andean-type active (Middle Devonian-Early Carboniferous), and Californian-type (Middle Carboniferous-Permian) continental margins. The Middle and Late Paleozoic evolution of the Baikal-Vitim Fold System is correlated with that of the Mongolia-Okhotsk Belt (Aga paleooceanic basin).     

Formation stages of the Early Precambrian crust in the Sharyzhalgai basement uplift,southwestern Siberian craton: Synthesis of Sm-Nd and U-Pb data

Newly obtained U-Pb and Sm-Nd isotopic data on Early Precambrian metamorphic and granitoid complexes in the southwestern margin of the Siberian craton (Sharyzhalgai basement uplift) are synthe-sized in order to elucidate the crustal evolution starting at the Paleoarchean (∼3.6 Ga) to Late Paleoproterozoic (∼1.85 Ga), evaluate the lateral extent of the Paleo-Mesoarchean crust, and identify major stages in its growth and recycling. Two crustal growth stages were determined in the Onot and Bulun granite-greenstone terranes: at 3.6–3.3 and 2.8–2.9 Ga. The earliest recycling processes (at ∼3.4 and 3.2 Ga) involved partial melting, metamorphism, and migmatization and produced a stable continental crust. Crustal growth in the Mesoarchean (∼2.8–2.9 Ga) due to basaltoid magmatism was associated with the recycling of the Paleoarchean crust, which served as a source of felsic melts and of detrital material for terrigenous sediments. The Archean crust of the Irkut granulite-gneiss terrane was formed by two pulses of intermediate-felsic and basic volcanism at ∼3.6-3.4 and ∼2.7 Ga. In the terminal Archean (at ∼2.55 Ga), the preexisting crust was involved in metamorphic and magmatic processes. Traces of recycling of the Paleoproterozoic crust are identified in the isotopic parameters of the intermediate-felsic granulites. Two discrete stages in the influx of juvenile material are identified in the Paleoarchean: at ∼2.0 and 1.88–1.85 Ga, with the latter stage associated with the large-scale recycling of the Archean crust during the origin of granitoids.