The formation processes and isotopic structure of continental crust of the Chingiz Range Caledonides (Eastern Kazakhstan)     

K. E. Degtyarev  K. N. Shatagin  V. P. Kovach  A. A. Tretyakov 《Geotectonics》2015,49(6):485-514

According to this paper, the juvenile crust of the Chingiz Range Caledonides (Eastern Kazakhstan) was formed due to suprasubduction magmatism within the Early Paleozoic island arcs developed on the oceanic crust during the Cambrian–Early Ordovician and on the transitional crust during the Middle–Late Ordovician, as well as to the attachment to the arcs of accretionary complexes composed of various oceanic structures. Nd isotopic compositions of the rocks in all island-arc complexes are very similar and primitive (ε_Nd(t) from +4.0 to +7.0) and point to a short crustal prehistory. Further increase in the mass and thickness of the crust of the Chingiz Range Caledonides was mainly due to reworking of island-arc complexes in the basement of the Middle and Late Paleozoic volcanoplutonic belts expressed by the emplacement of abundant granitoids. All Middle and Late Paleozoic granitoids have high positive values of ε_Nd(t) (at least +4), which are slightly different from Nd isotopic compositions of the rocks in the Lower Paleozoic island-arc complexes. Granitoids are characterized by uniform Nd isotopic compositions (<2–3 ε units for granites with a similar age), and thus we can consider the Chingiz Range as the region of the Caledonian isotope province with an isotopically uniform structure of the continental crust.  相似文献   

Geodynamic settings and formation conditions of crystalline complexes in the south Altai and south Gobi metamorphic belts     

I. K. Kozakov  A. N. Didenko  P. Ya. Azimov  T. I. Kirnozova  E. B. Sal’nikova  I. V. Anisimova  Ch. Erdenejargal 《Geotectonics》2011,45(3):174-194

The Hercynian mobile belts in Central Asia comprise the Hercynian proper and the Late Hercynian (Indosinian) belts separated by the South Gobi microcontinent, the origin of which is related to the evolution of the South Mongolian and Inner Mongolian basins with the oceanic crust. Crystalline complexes within these belts occur as tectonic sheets of a variety of sizes. At the early stages, the metamorphic grade of these complexes reached conditions of high-temperature subfacies of amphibolite and locally developed granulite facies. In tectonic terms, the Hercynian belt of metamorphic rocks is situated at the margin of the North Asian Caledonian continent and extends from the southeast to the northwest along the southern slope of the Gobi, Mongolian, and Chinese Altai to East Kazakhstan, where metamorphic rocks are localized in the Irtysh Shear Zone. All these rocks are combined into the South Altai metamorphic belt of more than 1500 km in extent. Another belt of isolated outcrops of crystalline rocks conventionally combined into the Indosinian South Gobi metamorphic belt is traced along the junction of the Hercynides with the South Gobi microcontinent. The high-grade metamorphic rocks within both belts are not fragments of an ensialic Caledonian or older basement. These rocks were formed 390–360 and 230–220 Ma ago as a result of the closure of the Tethian South Mongolian and Inner Mongolian oceanic basins (Paleotethys I and Paleotethys II). The spatial position of the South Altai and South Gobi metamorphic belts is caused by the asymmetric structure of the Tethian basins, where active continental margins are expressed most distinctly along their northern parts, while passive margins extend along the southern parts (in present-day coordinates).  相似文献   

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

I. K. Kozakov  V. P. Kovach  E. V. Bibikova  T. I. Kirnozova  D. A. Lykhin  Yu. V. Plotkina  E. V. Tolmacheva  M. M. Fugzan  Ch. Erdenezhargal 《Petrology》2014,22(5):480-506

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.  相似文献   

Late Vendian postcollisional leucogranites of Yenisei Ridge     

A. D. Nozhkin  I. I. Likhanov  V. V. Reverdatto  T. B. Bayanova  S. V. Zinoviev  P. S. Kozlov  N. V. Popov  N. V. Dmitrieva 《Doklady Earth Sciences》2017,474(2):674-679

The Late Vendian (540–550 Ma) U–Pb zircon age of postcollisional granitoids in the Osinovka Massif was obtained for the first time. The Osinovka Massif is located in rocks of the island-arc complex of the Isakovka Terrane, in the northwestern part of the Sayany–Yenisei accretion belt. These events stand for the final stage of the Neoproterozoic history of the Yenisei Ridge, related to the completing accretion of the oceanic crust fragments and the beginning of the Caledonian orogenesis. The petrogeochemical composition and the Sm–Nd isotopic characteristics support the fact that the granitoid melt originated from a highly differentiated continental crust of the southwestern margin of the Siberian Craton. Hence, the granite-bearing Late Riphean island-arc complexes were thrust over the craton margin at a distance considerably exceeding the dimensions of the Osinovka Massif.  相似文献   

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

V. P. Kovach  I. K. Kozakov  E. B. Salnikova  V. V. Yarmolyuk  A. M. Kozlovsky  L. B. Terent’eva 《Petrology》2013,21(5):409-426

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.  相似文献   

Collisional granitoids of the Dzhida zone of the Central Asian Fold belt,Southwestern Transbaikalia: Age and conditions of the formation     

I. V. Gordienko  V. P. Kovach  A. L. Elbaev  A. B. Kotov  E. B. Sal’nikova  L. Z. Reznitskii  S. Z. Yakovleva  I. V. Anisimova 《Petrology》2012,20(1):40-58

This paper considers the geological structure, composition, and age of the Darkhintui, Barun-Gol, and Khuldat granitoid plutons of the Dzhida zone of Caledonides of the Central Asian Fold belts. These plutons were formed in the Late Cambrian-Early Ordovician in the range between 490 ± 2 and 477 ± 6 Ma, after tectonic juxtaposition of the oceanic and island-arc complexes of the Dzhida Zone and volcanogenic-carbonate-terrigenous rocks of the Khamardaban zone, i.e., at the collisional stage of the region evolution. Geological, geochronological, geochemical, and Nd isotope data indicate that the collisional granitoids of the Dzhida zone were derived by melting of continental crust thickened through accretion. The sources for parental melts of the granitoids were presumably Vendian-Early Cambrian juvenile igneous rocks of ophiolite and island-arc complexes, as well as the crustal material of the Lower Paleozoic flyschoid sediments of the back-arc basin of the Dzhida zone and metaterrigenous rocks of the Khamardaban zone.  相似文献   

Sources of granitoids in the Tuva-Mongolian microcontinent and surrounding structures: Evidence from Pb, Nd, and O isotopic systematics     

G. V. Ovchinnikova  D. P. Krylov  I. K. Kozakov  V. P. Kovach  N. A. Sergeeva 《Petrology》2009,17(6):570-578

Newly obtained data on Pb isotopic ratios in feldspars from Early Proterozoic granitoids of the Sangilen block of the Tuva-Mongolian microcontinent and Caledonian structures surrounding it, considered together with earlier data on the O and Nd isotopic systems, indicate that ancient (approximately 2 Ga) crustal material mixed in the sources of granites of the Tuva-Mongolian microcontinent with younger juvenile material. Positive ɛ_Nd values of granitoids from the Bashkimugor and Chgargalant massifs are accounted for by processes of crustal contamination during the interaction of the melt with crustal material. Similar Nd isotopic characteristics of granitoids in the Khoromnug pluton were caused by the melting of the Late Riphean crust. In granitoids of the massifs in the Kaakhem and Eastern Tannuola zones of the surrounding Caledonian structures, the involvement of juvenile material from oceanic crust increases away from the Sangilen block. Granites in the junction zone between the Tuva-Mongolian microcontinent and surrounding structures display evidence of the presence (up to 10–20%) of an ancient crustal component, and the melting history of granitoids in the Eastern Tannuola zone is dominated by an Early Paleozoic juvenile component in combination with material similar to the Vendian ophiolites of the Agardag-Erzin zone. An increase in the δ¹⁸O value, the ²⁰⁶Pb/²⁰⁴Pb ratio, and the T_Nd(DM) values within a single complex (from older to younger granitoid phases) is explained by the systematic involvement of crustal material in the melting processes.  相似文献   

Continental crust in Gorny Altai: nature and composition of protoliths     

N.N. Kruk  A.G. Vladimirov  G.A. Babin  S.P. Shokalsky  N.V. Sennikov  S.N. Rudnev  N.I. Volkova  V.P. Kovach  P.A. Serov 《Russian Geology and Geophysics》2010,51(5):431-446

The history of the Vendian–Early Paleozoic formation of protoliths of continental crust in the Gorny Altai segment of the Central Asian fold belt is considered, and their composition, isotopic characteristics, and formation mechanisms are estimated. We have established two stages of crust-forming processes in Gorny Altai: Early and Late Caledonian, with the different structures of formed geoblocks and nature and compositions of crustal protoliths. At the Early Caledonian stage, fragments of oceanic lithosphere of basic composition (MORB, OIT, OIB) (T_Nd(DM-2st) = 0.65–1.1 Ga) formed, as well as island arcs with andesite-basaltic and andesitic protoliths with low contents of incompatible elements (T_Nd(DM-2st) = 0.7–0.9 Ga). At the Late Caledonian stage, the redistribution of the substance of these blocks and the external supply of material led to the formation of heterogeneous crust of turbidite basins with an oceanic basement and andesite-dacitic upper-crustal protoliths (T_Nd(DM-2st) varies from 0.8–0.9 Ga in the framing of the volcanic arc of Altaids to 1.4–1.6 Ga at the boundary of the Altai–Mongolian microcontinent).  相似文献   

First data on late vendian granitoid magmatism of the Northwestern Sayan–Yenisei accretionary belt     

A. D. Nozhkin  I. I. Likhanov  T. B. Bayanova  P. A. Serov 《Geochemistry International》2017,55(9):792-801

Late Vendian (540–550 Ma) U–Pb age was established for zircon from postcollisional granites of the Osinovsky Massif located among island-arc complexes of the Isakovka terrane in the northwestern Sayan–Yenisei accretionary belt. The granites were formed 150 Ma after the formation of the host island-arc complexes and 50–60 Ma after the beginning of their accretion to the Siberian Craton. These events mark the final stage of the Neoproterozoic history of the Yenisei Ridge related to the end of accretion of oceanic fragments and the beginning of the Caledonian Orogeny. The granites are subalkaline leucoractic Na–K rocks enriched in Rb, U, and Th. The petrogeochemical and Sm–Nd isotope data (T_Nd(DM)-2st = 1490–1650 Ma and ε_Nd(T) from–2.5 to–4.4) indicate that their source was highly differentiated continental crust of the SW margin of the Siberian Craton. Therefore, the host Late Riphean island-arc complexes were thrust over the craton margin for distance significantly exceeding the size of the Osinovsky Massif.  相似文献   

Crust-forming processes in the Hercynides of the Central Asian Foldbelt     

V. V. Yarmolyuk  V. I. Kovalenko  A. M. Kozlovsky  V. P. Kovach  E. B. Sal’nikova  D. V. Kovalenko  A. B. Kotov  E. A. Kudryashova  V. I. Lebedev  G. Eenzhin 《Petrology》2008,16(7):679-709

The paper reports data on the evolutionary history of magmatism, its conditions, and sources in the process of the development of the Southern Mongolian Hercynides during the pre-accretion, continental-margin, and rifting stages within the time span from the Silurian to Early Permian. The Hercynian continental crust in the southern Mongolian segment of the Central Asian Foldbelt (CAFB) was determined to have grown in the environment of ensimatic island arcs, backarc basins, spreading centers, and oceanic islands or plateaus, with material coming from the depleted and, perhaps, also enriched mantle sources in the open ocean that surrounded the Siberian paleocontinent on the side of the Caledonian margin. This made it possible to recognize the Early-Middle Paleozoic epoch of juvenile crustal growth in CAFB and the corresponding isotopic crustal province with a total area of more than 200 thousand km². The principal differences between the composition and structure of the blocks surrounding the Hercynian regions (Caledonides in the Gobi Altai and Grenwillides in the South Gobi microcontinent) testify that the southern margin of the Caledonian Siberian continent and the Grenvillides of the South Gobi microcontinent had different geological histories and were spatially separated. The structural complex of the Paleoasian ocean, including the terranes of the South Gobi microcontinent, were transformed into a continental block in the latest Devonian-earliest Carboniferous, in relation with accretion processes, folding, metamorphism, and tectonic delamination along the boundaries of structurally heterogeneous domains. The subsequent recycling of the crust by magmatic processes was related to the development of an active continental margin (ACM). The development of an ACM in the Hercynides resulted from and was a continuation of the motions of the continental and oceanic lithospheric plates, i.e., processes that brought about the Hercynian accretion. The evolution history of the ACM was subdivided into two stages: early (a continental-margin stage proper) and late (rifting stage). The rocks of the early stage were produced at 350–330 Ma and compose a differentiated basalt-andesite-rhyodacite complex and related massifs of the granodiorite-plagiogranite and banatite (diorite-monzonite-granodiorite) associations. During the rifting stage at 320–290 Ma, a bimodal basalt-comendite-trachyrhyolite association was formed, along with accompanying alkali granite massifs. In the southern Mongolian segment of the Hercynides, the rocks of the rifting stage compose two subparallel rift zones: Gobi-Tien Shan, which extends along the boundaries of the South Gobi microcontinent, and the Main Mongolian lineament, which marks the boundaries between the Hercynides and Caledonides in the CAFB. The rift structures are made up of alkali granitoids and normal-alkalinity granitoids, which are atypical of rift zones. Their genesis is thought to have been related to crustal anatexis, a process that was triggered by rift-related magmas at an unusual combination of rifting and ACM tectonic setting. The basic rocks of the rift associations have geochemical signatures atypical of continental rifting. They show Ta and Nb minima and K and Pb maxima, as is typical of rocks generated at convergent plate boundaries. Nevertheless, the broad variations in the concentrations and ratios of some major and incompatible trace elements and in the Sr, Nd, and O isotopic composition of the rift basaltoids allowed us to distinguish their high-and low-Ti varieties, which were produced with the participation of three mantle sources: depleted mantle similar to the source of basalts in midoceanic ridges, enriched mantle like the source of basalts in oceanic islands, and the mantle material of the metasomatized mantle wedge. The origin of andesites in the rift zones is explained by the contamination of mantle basaltoid melts with sialic (predominantly sedimentary) material of the continental crust or the assimilation of anatectic partial granite melts.  相似文献   

Mechanisms of continental crust formation in the Central Asian Foldbelt   总被引：1，自引：0，他引：1  

V. V. Yarmolyuk  V. P. Kovach  I. K. Kozakov  A. M. Kozlovsky  A. B. Kotov  E. Yu. Rytsk 《Geotectonics》2012,46(4):251-272

Geological and isotopic study of rocks occurring in the Early and Late Baikalian, Caledonian, Hercynian, and Indosinian fold regions of Central Asia is carried out. The juvenile crust formation occurred in these fold regions have determined the systematic differences in isotopic compositions of the crust. In the course of the subsequent (postaccretion) evolution, the crust of these domains underwent multiple reworking. These processes were accompanied by variations in the Nd isotopic compositions of the crust, which, in turn, are recorded in the isotopic compositions of granites and felsic volcanics as products of crust melting. Three types of crust differing in Nd isotopic composition and structure and, as a consequence, in formation mechanisms, are distinguished. The isotopically homogeneous crust is a source of igneous rocks with constant model Nd isotopic age (T_Nd(DM2st)) irrespective of the age of the crustal igneous rocks. These are the isotopic provinces, the crust of which remained isolated from addition of alien materials during postaccretion evolution. The axial zone of the Hercynides in the Central Asian Foldbelt is an example. The isotopically heterogeneous layered crust consists of fragments differing in isotopic composition. The products of its melting are characterized by widely scattered ɛ_Nd(T) and (T_Nd(DM2st). The appearance of alien sources of melt is considered in terms of underplating. This mechanism develops either due to subduction of the juvenile oceanic lithosphere beneath the mature continental lithosphere at convergent boundaries or as a result of plume-lithosphere interaction. The first mechanism operated during the formation of granitoids pertaining to the Tuva-Mongolia microcontinent. The second mechanism was responsible for the formation of batholiths in the zonal Hangay, Barguzin, and Mongolia-Transbaikalia magmatic fields. The isotopically heterogeneous mixed crust is composed of fragments differing in isotopic composition, which are tectonically mixed, resulting in the formation of an isotopically uniform reservoir in the domain of magma generation. As a result, the products of melting acquire isotopic parameters substantially distinct from the juvenile rocks of the corresponding structural zone. The formation of such a crust is related to the tectonic delamination, which provides for juxtaposition and a high degree of tectonic mingling of heterogeneous fragments at deep levels. The Caledonides of the Central Asian Foldbelt are characterized by such a mechanism of crust formation.  相似文献   

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

I. K. Kozakov  E. B. Sal’nikova  V. V. Yarmolyuk  V. P. Kovach  A. M. Kozlovskii  I. V. Anisimova  Yu. V. Plotkina  A. M. Fedoseenko  S. Z. Yakovleva  Ch. Erdenezhargal 《Petrology》2013,21(3):203-220

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.  相似文献   

Late Paleozoic anorogenic magmatism in Southern Mongolia: Evolutionary stages and structural control     

V. V. Yarmolyuk  A. M. Kozlovsky  A. V. Travin 《Doklady Earth Sciences》2017,475(1):753-757

Geochronological studies of anorogenic magmatic complexes in the South Mongolian Hercinide have been carried out. Series of the massifs composed of alkaline and subalkaline granitoids with some monzonite formed during the interval of 318–316 Ma have been found. Taking into account previously obtained data, two groups of Late Paleozoic A-type granitoids different in age have been identified in the region: Late Carboniferous and Early Permian. These two occupy different structural positions. The location the early magmatic massifs is controlled by the extended Trans–Altai fault zone of northwest strike: the massifs cluster in areas of the fault intersection with boundaries of structural blocks formed by the pull-apart mechanism. The later Early Permian igneous complexes are associated with the Gobi–Tienshan rift zone of sublatitudinal strike.  相似文献   

Continental crust of Gorny Altai: stages of formation and evolution; indicative role of granitoids     

《Russian Geology and Geophysics》2015,56(8):1097-1113

The composition and mechanisms of formation of continental crust in Gorny Altai and the role of granitoid magmatism in its evolution are considered. Geochemical and isotope data for major types of rocks of primary crust and for Early–Middle Paleozoic granitoids of the region are presented. The role of granitoids as indicators of the different stages of the continental-crust evolution is discussed. A review of the main models of continental crust formation is maid, and their applicability to the Gorny Altai segment of the Central Asian Fold Belt is shown. Based on the complex of geological, geochemical, isotope, and geochronological data, it has been established that the formation of continental crust in the Early and Late Caledonian terranes of Gorny Altai proceeded nearly synchronously (in the Middle–Late Devonian).In the Early Caledonian terranes, this process was the consequence of the multistage fractionation of primary juvenile crust of basic composition, and in the Late Caledonian ones it was the result of one-cycle intracrustal melting of hybrid andesitic crust rich in recycled material.  相似文献   

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

K. E. Degtyarev 《Geotectonics》2011,45(1):23-50

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.  相似文献   

Two types of magma sources of rare-metal alkali granites   总被引：2，自引：0，他引：2  

V. I. Kovalenko  V. V. Yarmolyuk  A. M. Kozlovsky  V. P. Kovach  E. B. Sal’nikova  A. B. Kotov  N. V. Vladykin 《Geology of Ore Deposits》2007,49(6):442-466

The geological setting, age, and magma sources of rare-metal ores at the Khalzanburgetdei deposit in western Mongolia and the Khan Bogd occurrence in southern Mongolia are considered. The Khalzanburgetdei deposit, 392 Ma in age, is situated in the Lake Zone of the Early Caledonides of the Central Asian Foldbelt at a branch of a triple junction of grabens. The Khan Bogd occurrence, about 290 Ma in age, is located in the South Mongolian Hercynian Zone and related to a continental rift conjugated in space and time with Hercynian island-arc systems. To estimate features of rare-metal magma sources, the Nd and O isotopic compositions were used, as well as ratios of incompatible elements (Nb-U, Nb-Zr, La-Yb, Th-Ta, and Ce-Pb). Since the granitic magma was commonly saturated with these elements, their ratios may be used for estimation of magma source compositions, trends of magma fractionation, and accumulation of rare-metal minerals. A mixture of an OIB source and crustal material (Caledonian ophiolitic and island-arc complexes) served as a source of rare-metal granitic rocks of the Khalzanburgetdei deposit, while the Khan Bogd occurrence was related to a source of subduction-related basic rocks, probably, with participation of the depleted and enriched mantle and continental crust.  相似文献   

Isotopic structure and evolution of the continental crust in the East Transbaikalian segment of the Central Asian Foldbelt     

E. Yu. Rytsk  V. P. Kovach  V. V. Yarmolyuk  V. I. Kovalenko  E. S. Bogomolov  A. B. Kotov 《Geotectonics》2011,45(5):349-377

New data on the geology and tectonics of the main structural elements of the East Transbaikalian segment of the Central Asian Foldbelt are discussed. Correlation charts of the main stratified and igneous complexes are compiled. The rocks of the Baikal-Patom and Baikal-Muya belts, as well as the Barguzin-Vitim Superterrane, are characterized by new Nd isotopic data, which have allowed us to establish the sources of these rocks, to separate isotopic provinces, and to distinguish two stages of crust-forming processes: the Early Baikalian (1.0–0.8 Ga) and the Late Baikalian (0.70–0.62 Ga). The Early Baikalian crust was formed in relatively narrow and spatially isolated troughs of the Baikal-Muya Belt and probably in the Amalat Terrane, whereas the Late Baikalian continental crust was formed and reworked in the Karalon-Mamakan, Yana, and Katera-Uakit zones of the Baikal-Muya Belt. The Baikal-Patom Belt and most of the Anamakit-Muya Zone in the Baikal-Muya Belt are characterized by remobilization of the Early Precambrian continental crust and by a subordinate role of Late Riphean juvenile sources. Reworking of the mixed Late Riphean and Early Precambrian crustal sources is typical of the Barguzin-Vitim Superterrane. The origination and evolution of the continental crust in the studied region are considered in light of new data; alternative versions of paleogedynamic reconstructions are discussed.  相似文献   

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

I. K. Kozakov  E. B. Salnikova  V. P. Kovach  I. V. Anisimova  A. M. Fedoseenko  S. Z. Yakovleva 《Stratigraphy and Geological Correlation》2013,21(5):482-495

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.  相似文献   

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     

V. M. Savatenkov  V. V. Yarmolyuk  A. M. Kozlovsky  Z. B. Smirnova  O. E. Sviridova 《Petrology》2018,26(4):351-367

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.  相似文献   

Mantle and Crustal Sources in the Genesis of Late-Hercynian Granitoids (NW Portugal): Geochemical and Sr-Nd Isotopic Constraints     

《Gondwana Research》2002,5(2):287-305

Large volumes of granitoids were emplaced in the Hercynian Central Iberian Zone during the last ductile deformation phase (D3, 300-320 Ma). The biotite-rich granitoids are the most abundant: (1) syn-D3 granodiorites-monzogranites (313-319 Ma) with calc-alkaline and aluminopotassic affinities; (2) late-D3 granodiorites-monzogranites (306-311 Ma), related to subalkaline and aluminopotassic series. These granitoids are associated with coeval gabbro-norite to granodiorite bodies and/or mafic microgranular enclaves. Both granitoids and basic-intermediate rocks show petrological, geochemical and isotopic evidence of interaction between felsic and mafic magmas.The mantle-derived melts, represented by shoshonitic gabbro-norites, were probably derived from an enriched and isotopically homogeneous source (Sr_i = 0.7049 to 0.7053, e_Nd = -2.1 to -2.5). In some syn- and late-D3 plutons there are evidences of essentially crustal granites, represented by moderately peraluminous monzogranites of aluminopotassic affinity. They have similar Nd model ages (1.4 Ga) but different isotopic compositions (Sr_i = 0.7089 to 0.7106, e_Nd = -5.6 to -6.8), revealing a heterogeneous crust. Potential protoliths are metasedimentary (immature sediments) and/or felsic meta-igneous lower crust materials. Large amounts of hybrid magmas were generated by the interaction of these coeval mantle- and crust-derived liquids, giving rise to slightly peraluminous monzogranites/granodiorites of calc-alkaline and subalkaline affinities, which display more depleted isotopic compositions than the crustal end-members (Sr_i = 0.7064 to 0.7085, e_Nd = -4.4 to -6.2). Petrogenetic processes involving mingling and/or mixing and fractional crystallization (at variable degrees) in multiple reservoirs are suggested.A major crustal growth event occurred in late-Hercynian times (∼305-320 Ma) related to the input of juvenile mantle magmas and leading to the genesis of composite calc-alkaline and subalkaline plutons, largely represented in the Central Iberian Zone.  相似文献