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1.
Crystalline continental rocks and associated crust‐contaminated basaltic rocks were unexpectedly dredged on the crest and at seamounts of the Rio Grande Rise, South Atlantic. Zircon U–Pb ages of one gabbro (ca. 2,200 Ma) and four granitoids (between ca. 1,430–480 Ma) indicate that the breakup of SW Gondwana left behind continental fragments of dominantly African age. These rocks may have been incorporated into the oceanic lithosphere by complex processes including rifting and interaction of the Tristan‐Gough mantle plume with hyperextended continental margins. Until ca. 80–70 Ma, the Rio Grande Rise and an old portion of the Walvis Ridge formed a conjugate pair of aseismic ridges, and the Tristan‐Gough plume was positioned at the Mid‐Atlantic Ridge. The finding of continental rock fragments in one of these conjugate pairs opens new perspectives on the mechanisms of continental break‐up, the nature of this conjugate pair, and the geodynamic evolution of rifted Gondwana margins in the South Atlantic.  相似文献   

2.
Comparative tectonic analysis of passive margins of the Atlantic Ocean has been performed. Tectonotypes of both volcanic and nonvolcanic margins are described, and their comparison with other passive Atlantic margins is given. The structural features of margins, peculiarities of magmatism, its sources and reasons for geochemical enrichment of melts are discussed. The important role of melting of the continental lithosphere in the development of magmatism is demonstrated. Enriched EM I and EM II sources are determined for the lower parts of the volcanic section, and a depleted or poorly enriched source is determined for the upper parts of the volcanic section based on isotope data. The conclusions of the paper relate to tectonic settings of the initial occurrence of magmatism and rifting and breakup during the period of opening of the Mesozoic Ocean. It was found out that breakup and magmatism at proximal margins led only to insignificant structural transformations and reduction of the thickness of the ancient continental crust, while very important magmatic events happened later in the distal zone. New growth of magmatic crust at the stage of continental breakup is determined as a typical feature of distal zones of the margins under study. The relationship of development of margins with the impact of deep plumes as the source of magmatic material or a heat source only is discussed. Progradation of the zone of extension and breakup into the areas of cold lithosphere of the Atlantic and the formation of a single tectonomagmatic system of the ocean are under consideration.  相似文献   

3.
A tectonotype of volcanic passive margins exemplified in the conjugate Norwegian and East Greenland margins is considered, with discussion of the Paleogene igneous complexes and the regional rift structure before continental breakup. Fragments of asymmetrical rift have been retained on both sides of the ocean. Large Cretaceous pre-rift sedimentation basins marking the initial stage of the ocean opening are included into the passive margin as well. The continental breakup was accompanied by intense basaltic magmatism over a short time span. This magmatic episode was distinguished by (1) the formation of widespread plateau-basalt complexes on continents and in near-shore areas of the ocean; (2) the development of thick lava series that are recorded in seaward dipping reflector wedges; (3) thick high-velocity lower crust, resulting from magmatic underplating; (4) asymmetrical accretion of the crust and structure formation. The discussion is based on published seismic data and reference sections selected for each margin with consideration of the composition and thickness of the igneous rocks, their lateral variations, source composition, and eruption and crust formation conditions. The characteristic feature of both sections is the two-member structure of volcanic complexes with substantial geochemical differences between the rocks from the lower and upper parts of the section, which correspond to the pre-breakup and breakup phases. At the initial phase, small magma volumes were melted out from the lithosphere. The geochemical signatures of the upper parts of the sections testify to the melting of the asthenospheric mantle. Their spatiotemporal variations reflect the ascent and melting of the deep plume, which was active during and after continental breakup. In the Greenland area, near the central part of the plume, a N-MORB-type mantle magma source gave way to a depleted Iceland-type mantle, while apart from the central part of the plume, its effect is expressed only in the enormous volume of mantle-derived melt without migration of its source. A variety of evidence is provided for the plume’s activity: the great thickness of the volcanic complexes and the relatively stable composition of the melt; the elevated temperature in the mantle; the specific geochemistry of the breakup-related lavas and their lateral zoning; conclusions on the necessity of dynamic support of volcanic eruptions; and recent results of seismographic tomography. The continental breakup inherited a system of older sedimentary basins in the zone of prolonged extension of the lithosphere in the North Atlantic. The continuous dynamic support of extension was most likely provided by long-term ascent of the Iceland plume. The comparison of the considered tectonotype with other volcanic and non-volcanic margins opens the way to further elucidation of the geodynamic processes responsible for the ocean opening.  相似文献   

4.
Oblique-shear margins are divergent continental terrains whose breakup and early drift evolution are characterized by significant obliquity in the plate divergence vector relative to the strike of the margin. We focus on the Rio Muni margin, equatorial West Africa, where the ca. 70-km-wide Ascension Fracture Zone (AFZ) exhibits oblique–slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole lithosphere failure and continental breakup (ca. 117 Ma). Oblique extension is recorded also by strike–slip and oblique–slip fault geometry within the AFZ, and buckling of Aptian synrift rocks in response to block rotation and local transpression. Rio Muni shares basic characteristics of both rifted and transform margins, the end members of a spectrum of continental margin kinematics. At transform margins, continental breakup and the onset of oceanic spreading (drifting) are separate episodes recorded by discrete breakup and drift unconformities. Oceanic opening will proceed immediately following breakup on a rifted margin, whereas transform and oblique-shear margins may experience several tens of millennia between breakup and drift. Noncoeval breakup and drift have important consequences for the fit of the equatorial South American and African margins because, in reconstructing the configuration of conjugate continental margins at the time of their breakup, it cannot be assumed that highly segmented margins like the South Atlantic will match each other at their ocean–continent boundaries (OCBs). Well known ‘misfits’ in reconstructions of South Atlantic continental margins may be accounted for by differential timing of breakup and drifting between oblique-shear margins and their adjacent rifted segments.  相似文献   

5.
The spatial distribution of recent (under 2 Ma) volcanism has been studied in relation to mantle hotspots and the evolution of the present-day supercontinent which we named Northern Pangea. Recent volcanism is observed in Eurasia, North and South America, Africa, Greenland, the Arctic, and the Atlantic, Indian, and Pacific Oceans. Several types of volcanism are distinguished: mid-ocean ridge (MOR) volcanism; subduction volcanism of island arcs and active continental margins (IA + ACM); continental collision (CC) volcanism; intraplate (IP) volcanism related to mantle hotspots, continental rifts, and transcontinental belts. Continental volcanism is obviously related to the evolution of Northern Pangea, which comprises Eurasia, North and South America, India, Australia, and Africa. The supercontinent is large, with predominant continental crust. The geodynamic setting and recent volcanism of Northern Pangea are determined by two opposite processes. On one hand, subduction from the Pacific Ocean, India, the Arabian Peninsula, and Africa consolidates the supercontinent. On the other hand, the spreading of oceanic plates from the Atlantic splits Northern Pangea, changes its shape as compared with Wegener’s Pangea, and causes the Atlantic geodynamics to spread to the Arctic. The long-lasting steady subduction beneath Eurasia and North America favored intense IA + ACM volcanism. Also, it caused cold lithosphere to accumulate in the deep mantle in northern Northern Pangea and replace the hot deep mantle, which was pressed to the supercontinental margins. Later on, this mantle rose as plumes (IP mafic magma sources), which were the ascending currents of global mantle convection and minor convection systems at convergent plate boundaries. Wegener’s Pangea broke up because of the African superplume, which occupied consecutively the Central Atlantic, the South Atlantic, and the Indian Ocean and expanded toward the Arctic. Intraplate plume magmatism in Eurasia and North America was accompanied by surface collisional or subduction magmatism. In the Atlantic, Arctic, Indian, and Pacific Oceans, deep-level plume magmatism (high-alkali mafic rocks) was accompanied by surface spreading magmatism (tholeiitic basalts).  相似文献   

6.
The paper considers the conditions and mechanisms of the formation of linear volcanic structures in the Brazil Basin, South Atlantic. Among these objects, those related to the ascent of deep mantle plumes predominate. It is shown that the ascent of melts from plume sources leads to the formation of (a) hot spot tracks in the form of linear volcanic ridges and (b) active hot lines in the form of submarine mountain chains with trends differing from those of hot spot tracks and with a more variable character of the age distribution of volcanic rocks. Fault tectonics affects the character of plume activity. In addition, plume material from a hot spot area is dragged by a moving plate as a flow or a sublithospheric lens, which leads to the long-term existence of particular independent segments of linear structures and sometimes to late volcanism reactivation within their limits. Decompression melting of the asthenospheric mantle in zones where thin lithosphere undergoes tension causes the formation of passive hot lines. The main mantle source for the considered volcanic rocks was a mixture of DMM and HIMU mantle components, with the latter abruptly dominating. In marginal oceanic regions, the EM1 component is also present (the EM2 component is found more rarely) within fragments of tectonically delaminated continental mantle that was trapped by the oceanic mantle during the breakup of Gondwana.  相似文献   

7.
Magmatism synchronous to the formation of passive margins of the North Atlantic is discussed. The main features and causes of the geochemical enrichment of the primary magmas at the margins have been established. This paper is based on the published data on the Norwegian-Greenland tectonotype of volcanic margins and the West Iberia-Newfoundland tectonotype of nonvolcanic margins. In the first tectonotype the hot rifting and active magmatism gave rise to the formation of a thick crust at the margin and the adjacent oceanic zone. The second tectonotype is characterized by cold amagmatic rifting and slow initial spreading, which led to the widespread occurrence of ancient continental complexes and serpentinized mantle rocks at the margin, as well as the thin and disturbed oceanic crust nearby. In order to characterize the magmatism and initial oceanic opening, the geological and geochemical data pertaining to the reference sections chosen for each margin were compared in detail. In particular, the geochemical and isotopic data on the flood basalts and suites of parallel dikes related to the pre- and synbreakup magmatic phases were involved for the Norwegian-Greenland region. The predominance of tholeiites enriched in lithophile elements and radiogenic isotopes, as well as a significant contribution of continental material to them, are typical of the volcanic margins. No less than two enriched magma sources for the lower part of the volcanic complex are suggested, whereas a depleted or slightly enriched source is established for the upper part. A more enriched source as compared with the volcanic margins of the Norwegian-Greenland region is suggested for the low-volume magmatic manifestations at the nonvolcanic Iberian margin. The tectonic settings of margins development and their relationships with the effect of deep plumes and the propagation of the extension zone toward the cold Atlantic lithosphere are discussed.  相似文献   

8.
The study provides new understanding of magmatism at extinct and modern spreading zones around the western margin of East Antarctica from Bransfield Strait to the Bouvet Triple Junction (BTJ) in the Atlantic Ocean and reveals causes of geochemical heterogeneity of mantle magmatism during the early opening of the Southern Ocean. The results indicate the involvement of an enriched source component in the generation of parental melts, which was formed in several tectonic stages. The enriched (metasomatized) mantle generated at rift zones has geochemical characteristics typical of the western Gondwana lithosphere (with isotopic compositions similar to those inferred for the enriched HIMU and EM-2 sources). This mantle source may have been produced by the thermal erosion of the continental mantle during the early stages of the Karoo–Maud–Ferrar superplume activity. This enriched mantle generated in the apical parts of the plume (sub-oceanic) began to melt during tectonic displacement and fragmentation of Gondwana. The Bouvet Triple Junction, located along modern spreading zones between the Antarctic and South American plate, is characterized by a greater depth of melting and a higher degree of enrichment of primary tholeiitic magmas. The highest enrichment of magmas in this region is controlled by a contribution from a pyroxenite-rich component, which was also identified in the extinct spreading center in Powell Basin.  相似文献   

9.
The main features of the volcanic and nonvolcanic passive margins of the North and Central Atlantic are considered. The margins are compared using rather well-studied reference tectonotypes as examples. The conjugate margins of the Norwegian-Greenland region and the margins of West Iberia and Newfoundland are chosen as tectonotypes of volcanic and nonvolcanic margins, respectively. The structural and magmatic features of the margins and their preceding history are discussed. A complex of interrelated attributes is shown for each tectonotype. The Norwegian-Greenland region close to the Iceland plume is distinguished by narrow zones of stretched continental crust, rapid localization of stretching with breakup of the continent, a high rate of subsequent spreading, and intense magmatism with the formation of a thick new crust at the margin and the adjacent oceanic zone. The Iberia-Newfoundland region, remote from the plumes, is characterized by wide zones of stretched continental crust, long-term and diachronous prebreakup extension propagating northward, extremely restricted mantle melting during rifting and initial spreading, and frequent occurrence of ancient crustal complexes and serpentinized mantle rocks at the margin. Crustal faults and a thin tectonized oceanic crust appear along the margin under conditions of slow spreading. A model of hot and fast spreading with a high degree of melting in the mantle is applicable to the Norwegian-Greenland region, whereas a model of cold and slow amagmatic rifting with a long pre-breakup stretching and thinning of the lithosphere is appropriate to the Iberia-Newfoundland margins. The differences in the development of the margins is determined by the interaction of many factors: deep temperature, rheology of the underlying lithosphere, heterogeneities in the previously formed crust, and the duration and rate of stretching. All of these factors can be related to the effect of deep plumes and propagation of the extension zone toward the segments of the cold Atlantic lithosphere. Both types of margins also reveal similar features, in particular asymmetry. It is suggested that the rotation forces superimposed on the general tectonomagmatic pattern controlled by plumes could have been the cause of structural asymmetry.  相似文献   

10.
During the late Mesozoic, an unusually broad range of alkalic magma compositions was erupted along the southern border of the São Francisco craton of Brazil. This magmatic activity includes carbonatite, kimberlite, lamprophyre, lamproite, syenite and the largest known example of extrusive kamafugite, the Mata da Corda formation. To determine the nature of the sources of this magmatism, and their geochemical history, an Os isotope study along with major and trace element and Sr, Nd and Pb isotope analyses of kimberlitic, lamproitic and kamafugitic rocks from the Alto Paranaíba province of Brazil was undertaken. This complements recent geochemical and isotopic studies of these magmas. The Os isotope data for Alto Paranaíba samples point to a peridotitic lithospheric mantle source for the kimberlites and lamproites that was variably depleted in Re, presumably by melt removal at some time between the late Archean and mid-Proterozoic. These lithospheric peridotites experienced LIL-element enrichment by fluid/melt metasomatism at roughly 1 Ga, most likely during mobile belt formation along the western border of the São Francisco craton. Kamafugitic samples have very radiogenic Os, suggestive of mafic (e.g. pyroxenite, websterite, eclogite) source materials that again appear to have been stabilized in the lithospheric mantle of Brazil in the mid to late Proterozoic. The Os isotope evidence for lithospheric sources for the Alto Paranaíba activity, coupled with Sr, Nd and Pb isotopic characteristics that overlap those of the Walvis Ridge hot-spot trace indicate that the EM1 component in South Atlantic ocean island basalts most likely represents the influence of delaminated Brazilian lithospheric mantle mixed into mantle circulation beneath the South Atlantic and is not related to the plume(s) commonly associated with this ocean island magmatism.  相似文献   

11.
The paper reports results of the analysis of the spatial distribution of modern (younger than 2 Ma) volcanism in the Earth’s northern hemisphere and relations between this volcanism and the evolution of the North Pangaea modern supercontinent and with the spatial distribution of hotspots of the Earth’s mantle. Products of modern volcanism occur in the Earth’s northern hemisphere in Eurasia, North America, Greenland, in the Atlantic Ocean, Arctic, Africa, and the Pacific Ocean. As anywhere worldwide, volcanism in the northern hemisphere of the Earth occurs as (a) volcanism of mid-oceanic ridges (MOR), (b) subduction-related volcanism in island arcs and active continental margins (IA and ACM), (c) volcanism in continental collision (CC) zones, and (d) within-plate (WP) volcanism, which is related to mantle hotspots, continental rifts, and intercontinental belts. These types of volcanic areas are fairly often neighboring, and then mixed volcanic areas occur with the persistent participation of WP volcanism. Correspondingly, modern volcanism in the Earth’s northern hemisphere is of both oceanic and continental nature. The latter is obviously related to the evolution of the North Pangaea modern supercontinent, because it results from the Meso-Cenozoic evolution of Wegener’s Late Paleozoic Pangaea. North Pangaea in the Cenozoic comprises Eurasia, North and South America, India, and Africa and has, similar to other supercontinents, large sizes and a predominantly continental crust. The geodynamic setting and modern volcanism of North Pangaea are controlled by two differently acting processes: the subduction of lithospheric slabs from the Pacific Ocean, India, and the Arabia, a process leading to the consolidation of North Pangaea, and the spreading of oceanic plates on the side of the Atlantic Ocean, a process that “wedges” the supercontinent, modifies its morphology (compared to that of Wegener’s Pangaea), and results in the intervention of the Atlantic geodynamic regime into the Arctic. The long-lasting (for >200 Ma) preservation of tectonic stability and the supercontinental status of North Pangaea are controlled by subduction processes along its boundaries according to the predominant global compression environment. The long-lasting and stable subduction of lithospheric slabs beneath Eurasia and North America not only facilitated active IA + ACM volcanism but also resulted in the accumulation of cold lithospheric material in the deep mantle of the region. The latter replaced the hot mantle and forced this material toward the margins of the supercontinent; this material then ascended in the form of mantle plumes (which served as sources of WP basite magmas), which are diverging branches of global mantle convection, and ascending flows of subordinate convective systems at the convergent boundaries of plates. Subduction processes (compressional environments) likely suppressed the activity of mantle plumes, which acted in the northern polar region of the Earth (including the Siberian trap magmatism) starting at the latest Triassic until nowadays and periodically ascended to the Earth’s surface and gave rise to WP volcanism. Starting at the breakup time of Wegener’s Pangaea, which began with the opening of the central Atlantic and systematically propagated toward the Arctic, marine basins were formed in the place of the Arctic Ocean. However, the development of the oceanic crust (Eurasian basin) took place in the latter as late as the Cenozoic. Before the appearance of the Gakkel Ridge and, perhaps, also the oceanic portion of the Amerasian basin, this young ocean is thought to have been a typical basin developing in the central part of supercontinents. Wegener’s Pangaea broke up under the effect of mantle plumes that developed during their systematic propagation to the north and south of the Central Atlantic toward the North Pole. These mantle plumes were formed in relation with the development of global and local mantle convection systems, when hot deep mantle material was forced upward by cold subducted slabs, which descended down to the core-mantle boundary. The plume (WP) magmatism of Eurasia and North America was associated with surface collision- or subduction-related magmatism and, in the Atlantic and Arctic, also with surface spreading-related magmatism (tholeiite basalts).  相似文献   

12.
Assessment of the isotope systematics and the magmatotectonic history of mainly Cenozoic igneous rocks from Italy shows them to be inconsistent with subduction-related magmatism. We attempt to fit these data into an alternative model involving long-term, recurrent plume activity that extended over a period of about 100 Ma, that involved mantle expansion and subsequent mixing between isotopically-distinct, mantle components. Sr, Nd and Pb isotopic compositions of Cenozoic Italian igneous rocks, rather than being random, reflect binary mixing involving a common end-member similar to FOZO. Most isotopic data from along the entire length of Italy, from the Aeolian Islands to the Alpine belt, define a Main Italian Radiogenic Trend (MIRT), characterized by mixing between FOZO and a highly radiogenic Sr, mantle end-member (ITEM, ITalian Enriched Mantle). Data from the Adria foreland, Sicily and the south-western Tyrrhenian Sea and Sardinia deviate from MIRT suggesting mixing with other components, perhaps HIMU and EM1. Both the absence of pure DMM, and the presence of isotopic end-members not recognized in present-day consuming-plate margins are incompatible with subduction-related models. Two models are discussed, one in which ITEM is attributed to melting of pre-Alpine sediments/upper continental crust entrained in a FOZO-like mantle and the other to widespread metasomatic activity involving deep-seated plume activity. In the latter, the widespread nature of FOZO is attributed to a late Triassic–early Jurassic plume that preceded the opening of the Alpine Tethys and led to modification of the lithosphere and/or asthenosphere. Late Jurassic–early Cretaceous plume activity produced mantle expansion and the opening of the Alpine Tethys. A new phase of plume activity started during the Oligocene with the opening of the western and central Mediterranean Basins. Stretching and large-scale extension of the Mediterranean lithosphere was caused by the progressive eastward growth and volume increase of a plume head trapped within the Transition Zone. Plume-generated fluids/melts enriched in K–Ca–CO2–H2O, produced mantle sources capable of generating widespread alkaline, mafic, and carbonatitic magmatism. Lithospheric unloading controlled the Tyrrhenian and peri-Tyrrhenian magmatic activity.  相似文献   

13.
This paper reports original data on the composition of volcanic rocks in the western part of the Andrew Bain Fault of the South-West Indian Ridge obtained in the 23rd voyage of R/V Akademik Nikolai Strakhov. In accordance with high La/Th and low Nb/U ratios, the basalt compositions of stations S2317, S2318, and S2330 could result from melting of the DM-type source with HIMU traces. Meanwhile, the enriched samples of station S2326 correspond to a mantle source with a considerable contribution of recycled sediments (EM). Sample S2326/35, which is composed of a melt almost completely depleted in EM material, corresponds to the volcanic rocks of the Marion and Prince Edward islands. The obtained and available data on the SWIR segment from Bouvet Island to Andrew Bain Fault are indicative of small mantle heterogeneities in this region. Two possible variants of their origin are considered: either preservation of the enriched material fragments in the depleted mantle during the split of Gondwana or “contamination” of the mantle with plume material with the formation of vein irregularities before opening of the ocean in this region. In the latter case, the plume material could cover a huge area not constrained by the young plume magmatism regions on Bouvet, Marion, and Prince Edward islands.  相似文献   

14.
Data on the composition of rocks in linear tectono-magmatic rises in the Brazil Basin indicate that volcanic rocks in the Vitoria—Trindade seamount chain were derived from a mantle reservoir unevenly enriched in phosphorus under the effect of melts close to subalkaline picrobasalt. These melts contained much of the EM I mantle component because the plume material was contaminated with continental lithospheric component. A long-lived isotopic homogeneity of the source is typical of the whole structure, including the Trindade and Martin Vaz Islands and the Abrolhos Plateau. The analogous isotopic ratios of rocks at the Fernando de Noronha Islands are reportedly explained by a similar mechanism of melt derivation and the similar evolution of the mantle plume material, which was originally situated beneath the South American continent. Compared to the melts of volcanic rocks of all other seamounts discussed herein, the parental melts of volcanics at the Victoria—Trindade Seamounts were derived at lower degrees of melting of enriched source material at a greater depth. The overwhelming majority of volcanic rocks at the northern chain of the Bahia Seamounts were produced by melts generated with the involvement of material of the HIMU type. At the same time, one of our rock samples was derived from a source of composition close to DM with a certain admixture of enriched material like EM I. The mantle source of rocks of the Pernambuco Seamounts consisted of a mixture of DM and HIMU material with a certain admixture of EM I (or, perhaps, EM II). The 10°–11° S Seamounts were formed near the MAR axial zone at the decompressional melting of chemically homogeneous mantle source that consisted of DM material with an admixture of EM I (or, perhaps, EM II) component.  相似文献   

15.
梁光河  杨巍然 《地学前缘》2022,29(1):328-341
南大西洋裂解造成的非洲和南美洲的大陆分离到了广泛认可,该区域也与大陆漂移学说的诞生密切相关。但大陆漂移的驱动力从其提出至今一直存在争议,定量化分析大西洋裂解过程中板块运动的驱动力显得尤为重要。我们研究了南大西洋两侧被动大陆边缘盆地区域的两条深反射地震勘探剖面,在构造地质解译基础上,详细估算了非洲大陆的莫霍面倾角,得到了沿莫霍面地壳重力滑移剪切力的大小,用于解释大西洋裂解过程中非洲大陆运动的动力机制。结果说明,非洲大陆板块在地幔上涌形成的倾斜界面上能够产生强大的重力滑移力,且南部驱动力大于中部。大陆板块依靠连续的地幔热上涌和重力滑移力会持续漂移。该模型能够合理解释大西洋上诸多线状分布的大陆残片的成因机制,也能合理解释南大西洋南部宽度大于中部的内在原因,最后对南大西洋的打开过程进行了精细的构造演化史恢复。该研究为板块运动提供了一个新的动力模式,为认识板块运动驱动力提供了更为精确的约束信息。  相似文献   

16.
The minor intrusions of the Edel'veis alkaline–carbonatite complex are bounded by the spurs of the North Chuya Ridge in southeastern Gorny Altai. According to Ar–Ar isotope data, the complex formed in the Middle Cambrian (～507 Ma). All of its components (alkali clinopyroxenite–melanogabbro–alkali syenite + Ca-carbonatite) occur in only one pluton. Silicate igneous rocks are equivalent in silica content and alkalinity to potassic alkaline and subalkalic mafic rocks. Apatite-phlogopitic Ca-carbonatites are enriched in P2O5 (up to 3.6 wt.%), Sr (～2500–5500 ppm), and REE (up to ～2000 ppm) and are, presumably, of liquation genesis. A PREMA-type plume component was a predominant magma source for the complex (?Nd(T) = +6.56 to +6.85). According to isotope data (87Sr/86Sr(T) ～ 0.7032–0.7039; δ18O ～ 7.5–14.9‰; δ13C ～ –2.7 to –8.4‰), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the rocks suggests that the complex developed on a continental margin and its development was accompanied by late-collisional rifting and the mixing of moderately depleted (PREMA) and enriched suprasubductional lithospheric mantle (EM I or EM II) with continental crust. It is presumed that the alkaline and carbonatite complexes in the western Central Asian Fold Belt are of primary plume origin and form a LIP within this belt together with other associations produced by Early Paleozoic (510–470 Ma) magmatism.  相似文献   

17.
地幔柱构造是基于全地慢对流模式、主要依据热点火山活动提出的新的全球构造理论。它的主要表现形式和产物是地幔柱头上部地壳抬升、岩浆活动形成大火成岩省、大型放射状岩墙群，并导致大陆裂解、板块运动和大规模成矿，是生物灭绝、磁极倒转的诱因。中国大陆的地质演化历史中保存了多期地幔柱活动印记，它们主要是华南新元古代Rodinia地幔柱、古生代古特提斯和峨眉山地幔柱和中一新生代中国东部地慢柱构造事件。上述地幔柱活动产生了地壳抬升、强烈岩浆活动、大陆伸展与裂解、岩石圈剧烈减薄和大规模成矿等重要地质事件。  相似文献   

18.
The Bocaina Plateau, which is situated on the eastern flank of the continental rift of southeastern Brazil, is the highest part of the Serra do Mar. Topographic relief in this area is suggested to be closely related to its complex tectono-magmatic evolution since the breakup of Western Gondwana and opening of the South Atlantic Ocean. Apatite fission track ages and track length distributions from 27 basement outcrops were determined to assess these hypotheses and reconstruct the denudation history of the Bocaina Plateau. The ages range between 303 ± 32 and 46 ± 5 Ma, and are significantly younger than the stratigraphic ages. Mean track lengths vary from 13.44 ± 1.51 to 11.1 ± 1.48 μm, with standard deviations between 1.16 and 1.83 μm. Contrasting ages within a single plateau and similar ages at different altitudes indicate a complex regional tectonothermal evolution. The thermal histories inferred from these data imply three periods of accelerated cooling related to the Early Cretaceous continental breakup, Early Cretaceous alkaline magmatism, and the Paleogene evolution of the continental rift of southeastern Brazil. The oldest fission track ages (> 200 Ma) were obtained in the Serra do Mar region, suggesting that these areas were a long-lived source of sediments for the Paraná, Bauru, and Santos basins.  相似文献   

19.
火山型被动陆缘是大陆解体过程中形成的一类陆缘类型,其演化过程与活动陆缘一样复杂多变。随着近年来对大陆解体过程与被动陆缘演化的深入研究,对其沉积过程、岩浆活动以及变质作用研究都有了很大的进展。陆壳减薄解体的过程有许多不同的模式,不对称的简单剪切模式可能是火山型被动陆缘的成因,其机制是软流圈隆起的最大位置从剖面上看与地壳减薄最大位置不在一条垂线上,造成软流圈上升的岩浆在解体的大陆一侧形成火山型被动陆缘。被动陆缘的沉积建造由两套沉积物组成,一套是大陆解体的裂谷阶段所形成的陆相沉积物和双模式火山岩组合,另一套是稳定陆缘的复理石组合;岩浆作用中基性岩类反应了物质直接源于上地幔的主要特点,并有部分受到地壳混染的特征;变质作用中高温低压环境主要发生在裂谷作用阶段,其特点反映了大陆解体过程中随着时间的增温和减压过程,而拆离伸展阶段则被脆性变形所代替。  相似文献   

20.
The tectonotype of nonvolcanic passive margins is discussed on the basis of data on the conjugate margins of West Iberia and Newfoundland. Magmatic, structural, and historical aspects are considered. The Late Mesozoic structural elements related to rifting and transition to spreading are considered, as well as the Early Mesozoic sedimentary basins that begin the history of oceanic opening. The problem is set to determine the tectonic conditions of the early opening of the ocean in the framework of the chosen tectonoptype. These conditions are compared with the setting at the volcanic margins. The formation of the conjugate Iberia-Newfoundland margins is reconstructed as an asymmetric rift system developing in an almost amagmatic regime. All three segments of the margins on both sides of the ocean reveal similar features of transverse zoning with zones of the tectonized continental, transitional, and oceanic crust oriented nearly parallel to the margin. Special attention is called to the old age of the continental crust and subcontinental mantle and the absence of newly formed crystalline crust; the stadial tectonic and rheological evolution of the crust and lithospheric mantle; the specific features of the transitional zone; the serpentinization and exhumation of mantle peridotites and their role in the development of detachment at the crust-mantle interface, related listric faults and the Peridotite Ridge, attenuation of the medium, further localization of continental breakup, and the eventual development of asymmetric conjugate margins. Two papers characterizing the tectonotypes of volcanic and nonvolcanic passive margins ([2] and this paper) determine the line of further comparative analysis necessary for insights into the geodynamics of ocean opening.  相似文献   

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