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1.
Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological, geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents, which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile continental crust and allow us to distinguish a corresponding stage of juvenile crust formation.  相似文献   

2.
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   

3.
With U-Pb zircon dating, the ages of the Ul'degit (228 ± 1 Ma) and Chek-Chikan (203 ± 1 Ma) mafic massifs were determined. These massifs were earlier considered to form at the Early Precambrian stage of the geologic evolution of the Dzhugdzhur–Stanovoi superterrane. In geochemical features the igneous rocks of the massifs show relation with a within-plate source, on the one hand, and are similar to igneous rocks of subduction zones, on the other. They might have formed after subduction, which caused the intrusion of gabbroids of the Lucha massif (248 ± 1 Ma) and diorites of the Tok-Algoma complex (238 ± 2 Ma), followed by the fracturing of the subducted plate.  相似文献   

4.
Complex mineralogical, geochemical, and geochronological studies of the gabbroids from the Dzhigdinskii Massif located in the western part of the Dzhugdzhur–Stanovoy Superterrane are performed. It is established that the age of the rocks from the Dzhigdinskii Massif is Middle Triassic (244 ± 5 Ma), rather than Early Archean, as was previously assumed. The age of the Dzhigdinskii Massif is close to the age of the formation of the other Triassic gabbroid massifs, such as the Amnunaktinskii (~240 Ma), Lukindinskii (~250 Ma), and Luchinskii (~248 Ma) in the southeastern environ of the North Asian Craton. One of the stages in the formation of the Selenga–Vitim volcanoplutonic belt falls in this period as well. This indicates that the Selenga–Vitim volcanoplutonic belt, along with the granitoids and volcanic rocks, is composed of ultrabasic–basic and basic massifs and that this belt is superposed on the structures of the Selenga–Stanovoy Superterrane, as well as on the western part of the Dzhugdzhur–Stanovoy Superterrane. The gabbro, gabbro–diorite, and series of gabbro and gabbro–diorite with high sodic alkalinity from the Dzhigdinskii Massif show obvious geochemical features of duality, including combination of intraplate and super-subduction origin. In this relation, we can assume that the origin of the gabbroids of the Dzhigdinskii Massif is related to the detachment of the oceanic lithosphere and its subduction into the mantle with the formation of an “asthenospheric window.”  相似文献   

5.
Data on the composition, age, and source of material of Aptian rocks composing a bimodal volcanic complex and related granitoids in the northern margin of the Amur microcontinent indicate that the granodiorites of the Talalinskii Massif and subalkaline granites of the Dzhiktandiunskii Massif crystallized at 117 ± 2 and 119 ± 2 Ma, respectively (40Ar/39Ar method), and their crystallization ages coincide with the age of volcanic rocks of the Gal’kinskii bimodal complex. These data make it possible to combine the rocks within a single volcano-plutonic association. Geochemical and isotopic-geochemical features of trachybasaltic andesites of the Gal’kinskii bimodal complex suggest that the parental melts were derived from such sources as PREMA (or DM) and an enriched source of the EMII type at a subordinate contribution of a crustal source. The parental melts of rhyolites of the Gal’kinskii Complex and granitoids of the Talalinskii and Dzhiktandinskii massifs were derived from crustal material with minor amounts of juvenile material. The bimodal volcanic association and related granitoids dated at 119–115 Ma were most likely formed in geodynamic environments implying the ascent of the asthenospheric mantle.  相似文献   

6.
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   

7.
The paper reports data on the Nd isotopic composition and the evaluated composition of the sources of magmatism that produced massifs of alkali and basic rocks of the Khaldzan-Buregtei group. The massifs were emplaced in the terminal Devonian at 392–395 Ma in the Ozernaya zone of western Mongolia. The host rocks of the massifs are ophiolites of the early Caledonian Ozernaya zone, which were dated at 545–522 Ma. The massifs were emplaced in the following succession (listed in order from older to younger): (1) nordmarkites and dolerites syngenetic with them; (2) alkali granites and syngenetic dolerites; (3) dike ekerites; (4) dike pantellerites; (5) rare-metal granitoids; (6) alkali and intermediate basites and quartz syenites; and (7) miarolitic rare-metal alkali granites. Our data on the Nd isotopic composition [?Nd(T)] and conventionally used (canonical) ratios of incompatible elements (Nb/U, Zr/Nb, and La/Yb) in rocks from the alkaline massifs and their host ophiolites indicate that all of these rocks were derived mostly from mantle and mantle-crustal enriched sources like OIB, E-MORB, and IAB with a subordinate contribution of N-MORB (DM) and upper continental crustal material. The variations in the ?Nd(T) values in rocks of these massifs suggest multiple mixing of the sources or magmas derived from them when the massifs composing the Khaldzan-Buregtei group were produced. The OIB and E-MORB sources were mixed when the rocks with mantle signatures were formed. The occurrence of nordmarkites, alkali granites, and other rocks whose isotopic and geochemical signatures are intermediate between the values for mantle and crustal sources testifies to the mixing of mantle and crustal magmas. The crustal source itself, which consisted of rocks of the ophiolite complex, was obviously isotopically and geochemically heterogeneous, as also were the magmas derived from it. The model proposed for the genesis of alkali rocks of the Khaldzan-Buregtei massifs implies that the magmas were derived at two major depth levels: (1) mantle, at which the plume source mixed with an E-MORB source, and (2) crustal, at which the ophiolites were melted, and this gave rise to the parental magmas of the nordmarkites and alkali granites. The basites were derived immediately from the mantle. The mantle syenites, pantellerites, and rare-metal granitoids were produced either by the deep crystallization differentiation of basite magma or by the partial melting of the parental basites and the subsequent crystallization differentiation of the generated magmas. Differentiation likely took place in an intermediate chamber at depth levels close to the crustal (ophiolite) level of magma generation. Only such conditions could ensure the intense mixing of mantle and crustal magmas. The principal factor initiating magma generation in the region was the mantle plume that controlled within-plate magmatism in the Altai-Sayan area and the basite magmas related to this plume, which gave rise to small dikes and magmatic bodies in the group of intrusive massifs.  相似文献   

8.
The Paleoarchean (ca. 3.5–3.3 Ga) Onverwacht Suite (OS) of the Barberton Greenstone Belt consists of a 15‐km thick imbricate tectonic stack of seven complexes consisting predominantly of volcanic rocks and intrusions. Tectonostratigraphically from base to top they are the Sandspruit, Theespruit, Komati, Hooggenoeg, Noisy, Kromberg and Mendon Complexes. The Hooggenoeg and Noisy Complexes in the middle of the OS are separated by a significant unconformity resulting from the uplift of the submarine lavas and deep erosion, demonstrating the onset of tectonic accretion prior to 3455 Ma. The basic lavas of the tectonostratigraphic lower (Theespruit, Sandspruit and Komati) and upper (Mendon) complexes are composed of komatiite, komatiitic basalt and high-MgO basalt, whereas those in the middle part (Hooggenoeg and Kromberg) are predominantly high- to low-MgO tholeiitic basalts. Felsic volcanic rocks and intrusions are important in two of the complexes (Theespruit and Noisy). The ultramafic to basaltic lavas show REE patterns that are almost flat and resemble those of modern MORB, whereas those of the felsic rocks are flat from Lu to Gd and moderately to strongly enriched in LREE, similar to modern arcs. Average εNd (T) values are close to depleted mantle growth curves. In MORB-normalised multi-element diagrams, the komatiitic to basaltic rocks exhibit flat patterns from Lu through La and consistent relative enrichment in the elements Pb, U, Th, Ba and Cs. Apart from the Komati Complex, the majority of the lavas show significant negative Nb and Ta anomalies. Enrichment in non-conservative incompatible elements (Cs, Ba, Th, LREE) relative to conservative elements (Ta, Nb, Zr, Hf, Ti, Y, HREE) shows that the komatiitic to basaltic magmas were generated from metasomatised mantle above subducting altered oceanic crust. The geochemistry of the felsic rocks indicates an origin by melting of subducted amphibolite and eclogite. The tectonostratigraphy and the geochemical characteristics of the lavas and intrusions are consistent with successive obduction and accretion of segments of oceanic crust formed in back-arc basins and volcanic arcs.  相似文献   

9.
The U-Pb zircon dates obtained for the Sutara (480 ± 4 Ma), Kabalinskii (471 ± 10 Ma), and Durilovskii (461 ± 5 Ma) massifs reliably confirm an Early Proterozoic orogenic event, which took place after granulite metamorphism at approximately 500 Ma (Wilde et al., 2003) in the Lesser Khingan (Jiamusi) terrane. The rocks emplaced most shortly after the main metamorphic event are the granites of the Sutara Massif and leucogranites of the Kabalinskii Massif, whose geochemistry is close to that of collision granites. The quartz diorites and subalkaline granites of the Durilovskii Massif, whose geochemistry suggests their origin in a postcollision environment with the participation of an enriched mantle source, were emplaced longer after metamorphic event and after the aforementioned massifs.  相似文献   

10.
宋述光  杨立明 《地球科学》2019,44(12):4167-4172
岛弧的形成和演化对于理解板块构造和大陆生长有重要意义.祁连山-西秦岭一带发育两条不同类型的弧岩浆岩带,其北侧为北祁连增生杂岩带,由蛇绿岩、高压变质岩和大陆型弧岩浆岩带组成,形成时代为520~440 Ma.岩浆岩以中酸性火山岩-侵入岩为主,部分地区发育典型双峰式火山岩.南侧为祁秦增生杂岩带,由寒武纪蛇绿岩（525~490 Ma）和奥陶纪IBM型洋内弧岩浆岩（470~440 Ma）组成,蛇绿岩以拉脊山-永靖洋底高原型蛇绿岩为代表,蛇绿岩的上部熔岩部分由夏威夷型苦橄岩、板内碱性玄武岩和板内拉斑玄武岩组成,为大洋板块内部地幔柱活动产物.洋内弧岩浆岩以高镁玄武岩、玄武安山岩、高铝安山岩、玻安岩为主,局部发育赞岐岩.祁秦增生杂岩带的蛇绿岩和弧火山岩组合很好地说明洋底高原与海沟碰撞和俯冲带阻塞是造成俯冲带起始和新的洋内弧形成和发展主要因素.   相似文献   

11.
The paper reports newly obtained K-Ar isotopic-geochronological data on the age of three lava flows (Khrami, Mashavera, and Kura), which begin at the Dzhavakheti volcanic highland in southern Georgia. All of the dated rocks, including those from the Kura Flow, which was previously considered as the Pleistocene, are demonstrated to have a Pliocene age. The lavas of the longest Khrami Flow were erupted at 3.25–3.10 Ma, and those of the Kura and Mashavera Flows at 2.20–2.05 Ma, a fact testifying to two pulses of volcanic activity at the Dzhavakheti Highland. The petrogeochemical and isotopic characteristics of the rocks (87Sr/86Sr = 0.7039–0.7042; ∈Nd = 3.4–5.1) indicate that they are subalkaline within-plate basalts formed by the fractional crystallization of a basic mantle melt with the usually discontinuous selective or rarely continuous contamination with material that was not in geochemical equilibrium with the melt. The volcanics of the Khrami Flow are characterized by the less radiogenic Sr isotopic composition and the highest ∈Nd values, while the younger rocks of the Mashavera and Kura Flows have similar and more “crustal” isotopic signatures. The 87Sr/86Sr ratios of the Dzhavakheti subalkaline basalts are close to the initial Sr isotopic ratios of the Quaternary and Middle Pliocene dacite lavas from the same territory. Considered together with petrogeochemical and geological data, this suggests that all young rocks in Southern Georgia were produced in similar tectonic and geodynamic environments.  相似文献   

12.
A GIS layout of the map of recent volcanism in North Eurasia is used to estimate the geodynamic setting of this volcanism. The fields of recent volcanic activity surround the Russian and Siberian platforms—the largest ancient tectonic blocks of Eurasia—from the arctic part of North Eurasia to the Russian Northeast and Far East and then via Central Asia to the Caucasus and West Europe. Asymmetry in the spatial distribution of recent volcanics of North Eurasia is emphasized by compositional variations and corresponding geodynamic settings. Recent volcanic rocks in the arctic part of North Eurasia comprise the within-plate alkaline and subalkaline basic rocks on the islands of the Arctic Ocean and tholeiitic basalts of the mid-ocean Gakkel Ridge. The southern, eastern, and western volcanic fields are characterized by a combination of within-plate alkaline and subalkaline basic rocks, including carbonatites in Afghanistan, and island-arc or collision basalt-andesite-rhyolite associations. The spatial distribution of recent volcanism is controlled by the thermal state of the mantle beneath North Eurasia. The enormous mass of the oceanic lithosphere was subducted during the formation of the Pangea supercontinent primarily beneath Eurasia (cold superplume) and cooled its mantle, having retained the North Pangea supercontinent almost unchanged for 200 Ma. Volcanic activity was related to the development of various shallow-seated geodynamic settings and deep-seated within-plate processes. Within-plate volcanism in eastern and southern North Eurasia is controlled, as a rule, by upper mantle plumes, which appeared in zones of convergence of lithospheric plates in connection with ascending hot flows compensating submergence of cold lithospheric slabs. After the breakdown of Pangea, which affected the northern hemisphere of the Earth insignificantly, marine basins with oceanic crust started to form in the Cretaceous and Cenozoic in response to the subsequent breakdown of the supercontinent in the northern hemisphere. In our opinion, the young Arctic Ocean that arose before the growth of the Gakkel Ridge and, probably, the oceanic portion of the Amerasia Basin should be regarded as a typical intracontinental basin within the supercontinent [48]. Most likely, this basin was formed under the effect of mantle plumes in the course of their propagation (expansion, after Yu.M. Pushcharovsky) to the north of the Central Atlantic, including an inferred plume of the North Pole (HALIP).  相似文献   

13.
High-Mg basalt-andesite suites are extremely rare in the modern Earth but genetically important for indicating essential crust-mantle interactions, ascertaining critical geodynamic settings, and understanding the formation of porphyry copper deposits. Secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating and geochemistry document an early Palaeozoic high-Mg basalt-andesite suite from the Duobaoshan porphyry Cu deposit, eastern Central Asian Orogenic Belt (CAOB). Dating results reveal that the Duobaoshan high-Mg basalt and andesite erupted ca. 506 Ma and ca. 485 Ma, respectively. These high-Mg samples are typical subduction-related volcanic rocks with SiO2 contents of 47.73 to 55.16 wt%, high MgO (6.08 to 10.82 wt%), and high Mg# (58 to 67) and feature enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs). The samples have juvenile whole-rock initial 87Sr/86Sr ratios of 0.70272 to 0.70451, ɛNd(t) of 4.8 to 8.7 (average 7.23), and zircon ɛHf(t) of 7.3 to 15.9. Additionally, they have high Sr (average 619.36 ppm) and low Y (average 11.92 ppm) and Yb (average 1.21 ppm) contents that show affinity with adakitic high-Mg basalts and andesites worldwide. These high-Mg, depleted mantle-like isotopic and adakitic geochemical features imply a depleted mantle source variably assimilated by slab-derived melts under a sustained subduction tectonic setting. We also propose that the Duobaoshan high-Mg basalt-andesite suite, as the parental source magma, fertilized the overlying Duobaoshan porphyry Cu deposit by providing water, copper, and sulphur and high oxygen fugacity. A comprehensive comparison of the post-ore volcanic rocks shows that they might have originated from the slab-derived fluid metasomatized depleted mantle wedge, which had different properties from the mantle that produced the pre- and syn-ore volcanic intrusive rocks. The post-ore volcanic rocks underwent little crustal evolution en route to the surface during a reworked subduction event, which indicates a relatively immature island arc environment.  相似文献   

14.
Mantle fragments of ultramafic composition are widespread in the Scandinavian Caledonides (SC). Lenses and boudins of Alpine-type peridotites in the Scandinavian Caledonides represent parts of dismembered ophiolite sequences and fragments of sub-continental upper mantle. Metaperidotites of nappes in internal positions are generally isofacial with the metamorphic envelope, usually Caledonian metasediments but in places also Precambrian metagranitoids forming the basement cores of the nappes. Caledonian metamorphism strongly modified the texture and mineralogy of the peridotites and resulted in a systematic metamorphic pattern which is consistent with the pattern observed in the envelope.

Metaperidotites of the external massifs display at least a two-stage metamorphic history: an early Caledonian high-pressure high-temperature phase related to early crustal stacking and a late Caledonian regional metamorphic overprint which produced a regular Barrovian-type metamorphic pattern of in-situ metamorphism.

Metaperidotites from nappes in intermediate positions (Iapetus Ocean ophiolites and ultramafic rocks from island arc environments) show strongly diverging histories. Metaperidotites from internal ophiolites (oceanic ophiolites, Köli) lack any evidence of subduction metamorphism, are serpentinized to various degrees, show abundant primary mantle relic mineralogies and the Caledonian metamorphic overprint is low. Metaperidotites from external (island arc) ophiolites and other associations (Seve) often show relic high-pressure metamorphism related to the Finnmarkian phase of the Caledonian orogeny. The Seve metaperidotites are occasionally associated with eclogites and show a weak overprint of late Caledonian regional metamorphism. Alpine-type peridotites are absent in the foreland of the Baltic Shield and in the innermost nappes (Lofoten).

The metamorphic characteristics and evolution recorded by the metaperidotites in the Scandinavian Caledonides allow a general reconstruction of the dynamics of collision belt formation.  相似文献   


15.
The Veselki peridotite-websterite-gabbronorite massif was dated by U-Pb zircon method at 154 ± 1 Ma. This is the first Late Jurassic date obtained for ultramafic-mafic massifs in the eastern part of the Selenga-Stanovoy superterrane bounding the southeastern margin of the Siberian craton. The mineralogical specifics of the massif is expressed in the presence of three-pyroxene assemblage [bronzite-pigeonite-augite (diopside)] and exsolution lamellae of Cr-spinel and Cr-magnetite in Fe-Cr picotite and suggests unstable crystallization at shallow depths. Geochemical similarity between the Upper and Lower series attests to their genetic relation through intrachamber differentiation. The massif was generated from a highly evolved melt, as is seen from the LREE enrichment (La/Yb)N = 3.89–30. Plagioclase varieties display a weak positive Eu anomaly (Eu/Eu* = 1.1–1.25), whereas other rocks have an insignificant negative Eu anomaly (Eu/Eu* = 0.85–0.97). Model calculations show that parental melt was close to subalkaline picrite, which evolved along two fractionation trends into dunites and subalkaline gabbroids and monzodiorites.  相似文献   

16.
We present new data on the age and geochemistry of the Veselyi and Petropavlovsk ultramafic-mafic massifs of the Selenga-Stanovoy (West Stanovoy) superterrane on the southeastern framing of the North Asian craton. The massifs are composed of rocks of peridotitewebsterite-gabbro and peridotite-gabbro-monzodiorite associations, respectively. The latter combine normal, subalkalic, and alkaline rocks and thus are of diverse composition: from ultrabasites and pyroxenites through gabbroids to monzodiorites. The U-Pb zircon age of these massifs is 154 ± 1 and 159 ± 1 Ma, respectively, which permits them to be referred to as the youngest rocks of ultramafic-mafic complexes on the southern framing of the North Asian craton. The rocks of the studied massifs are enriched in LILE (K, Rb, Sr, Ba, LREE) and are depleted in HFSE (Zr, Nb, Hf, Ta). These rocks formed, most likely, in the rear of subduction zone or in the setting of the subducting-slab detachment.  相似文献   

17.
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 (TNd(DM)-2st = 1490–1650 Ma and εNd(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   

18.
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   

19.
产于扬子板块东南缘皖南祁门北部地区的铺岭组和扬子板块东北缘皖东张八岭地区的西冷岩组分别发育一套火山岩。前者为陆相火山岩,岩性以玄武安山岩为主;后者为海相火山岩,岩性以细碧-角斑岩系为主。两者之上均角度不整合覆盖着一套砂泥质沉积岩。铺岭组玄武安山岩LA-ICP-MS锆石U-Pb年龄为(765±15) Ma,西冷岩组石英角斑岩LA-ICP-MS锆石U-Pb年龄为(726.8±1.4) Ma,均属南华纪(新元古代中期)。二者主量元素含量差别较大(铺岭组为玄武-安山岩;西冷岩组为石英角斑岩),但它们均属于钙碱性系列,并具有相似的微量元素特征:它们均富集轻稀土元素和Ba、Th、U等大离子亲石元素,高场强元素Nb、Ta、Ti强烈亏损,δEu、Zr、Hf异常不明显。这些特征显示它们与岛弧火山岩类似,可能是受俯冲板片流体影响的亏损地幔楔和新生地壳熔体的混合形成的。这些火山岩系形成于华夏向扬子俯冲形成的沟弧盆体系消失之后。结合前人研究成果,我们认为:扬子板块晋宁期深部的俯冲作用可能一直持续到约700 Ma,新元古代中期可能存在多岛弧拼贴过程,到了南华纪晚期才正式进入板内伸展阶段。  相似文献   

20.
New data are reported on the content of radioactive elements in the Precambrian Na-K granitoids from the southwestern margin of the Siberian Craton, Aldan and Ukrainian shields, and Kursk-Voronezh Massif. Analytical data on other regions were generalized for comparison. Two global epochs of Na-K granitoid magmatism bearing elevated contents of radioactive elements (U, Th, K) were distinguished in the Early Precambrian (in Ga): Neoarchean (2.8-2.6) and Late Paleoproterozoic (1.9-1.75). Mesoarchean (3.1-2.8 Ga) epoch of Na-K granite formation has been additionally distinguished at the Australian, South African, and Canadian shields. These epochs of granitization provided high maturity of the crust: geochemical differentiation of the oldest continental blocks and their geochemical and metallogenic specialization for trace elements and RAE. In the southern margin of the Siberian Craton, the most intense granite formation occurred in the Late Paleoproterozoic. The extended South Siberian belt of collisional and within-plate Na-K granitoids is characterized by intense influx of RAE and other trace elements in the upper crustal shell. The southwestern margin of the craton (Yenisei Range) was spanned by repeated Late Neoproterozoic Na-K granite formation, with wide development of collisional and within-plate Na-K granites having elevated Th content and [Th]/[U] ratio. The higher RAE concentrations are typical of within-plate Paleo and Neoproterozoic granitoids. The highest uranium content was found in the postcollisional and within-plate Na-K granites and subalkaline leucogranites. Uranium ore concentrations were formed at the riftogenic stages of evolution of these crustal blocks, when within-plate subalkaline acid magmatism and accompanying hydrothermal metamorphism overprinted granitized crystalline massifs, including high-U sedimentary and volcanic complexes. Areas with the most favorable geological-geochemical environments for the formation of uranium mineralization were distinguished in the southern margin of the Siberian Craton and its nearest folded framing.  相似文献   

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