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1.
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
2.
Early Paleozoic magmatism of the Tannuola terrane located in the northern Central Asian Orogenic Belt is important to understanding the transition from subduction to post-collision settings. In this study, we report in situ zircon U-Pb ages, whole rock geochemistry, and Sr-Nd isotopic data from the mafic and granitic rocks of the eastern Tannuola terrane to better characterize their petrogenesis and to investigate changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal three magmatic episodes for about 60 Ma from ∼510 to ∼450 Ma, that can be divided into the late Cambrian (∼510–490 Ma), the Early Ordovician (∼480–470 Ma) and the Middle-Late Ordovician (∼460–450 Ma) stages. The late Cambrian episode emplaced the mafic, intermediate and granitic rocks with volcanic arc affinity. The late Cambrian mafic rocks of the Tannuola terrane may originate from melting of mantle source that contain asthenosphere and subarc enriched mantle metasomatized by melts derived from sinking oceanic slab. Geochemical and isotopic compositions indicate the late Cambrian intermediate-granitic rocks are most consistent with an origin from a mixed source including fractionation of mantle-derived magmas and crustal-derived components. The Early Ordovician episode reveal bimodal intrusions containing mafic rocks and adakite-like granitic rocks implying the transition from a thinner to a thicker lower crust. The Early Ordovician mafic rocks are formed as a result of high degree melting of mantle source including dominantly depleted mantle and subordinate mantle metasomatized by fluid components while coeval granitic rocks were derived from partial melting of the high Sr/Y mafic rocks. The latest Middle-Late Ordovician magmatic episode emplaced high-K calc-alkaline ferroan granitic rocks that were formed through the partial melting the juvenile Neoproterozoic sources.These three episodes of magmatism identified in the eastern Tannuola terrane are interpreted as reflecting the transition from subduction to post-collision settings during the early Paleozoic. The emplacement of voluminous magmatic rocks was induced by several stages of asthenospheric upwelling in various geodynamic settings. The late Cambrian episode of magmatism was triggered by the slab break-off while subsequent Early Ordovician episode followed the switch to a collisional setting with thickening of the lower crust and the intrusion of mantle-induced bimodal magmatism. During the post-collisional stage, the large-scale lithospheric delamination provides the magma generation for the Middle-Late Ordovician granitic rocks. 相似文献
3.
Nd, Pb and Sr Isotopic Compositions of East African Carbonatites: Evidence for Mantle Mixing and Plume Inhomogeneity 总被引：14，自引：8，他引：14
New Pb isotopic data are presented for 10 young Mesozoic toCenozoic (0–116 Ma) carbonatites from a 1400 km long segmentof the East African Rift. Patterns observed in Pb vs Pb, Srvs Pb and Nd vs Pb isotope diagrams define unusual, nearly linear,trends that are interpreted as mixing between two componentsthat are broadly similar to the two mantle end-member components,HIMU and EM1, which were first recognized from ocean-islandbasalts. The two plutons with isotope signatures closest toHIMU and EM1 crop out within 140 km of each other. From thesedata, EM1 and HIMU are now known to occur in both continentaland oceanic settings that are associated with plumes or rifts.Moreover, these isotopic signatures tend to occur in regionswhere seismic tomography indicates prominent low-velocity zonesin the lower mantle. For these reasons, we favour a model forthe origin of the East African Rift carbonatites that involvesmelting and mixing of HIMU and EM1 components contained withinan isotopically heterogeneous mantle plume. We consider theHIMU and EM1 sources to be stored within the deep (lower 1000km) mantle, possibly the core–mantle boundary. The rolethat continental lithosphere plays in carbonatite generationis probably one of concentrating volatiles at the upper levelsof an ascending mantle plume. KEY WORDS: carbonatites; isotopes; rifts; plumes; FOZO 相似文献
4.
George D. Kamenov Michael R. Perfit Paul A. Mueller Ian R. Jonasson 《Contributions to Mineralogy and Petrology》2008,155(5):635-656
The Tabar–Lihir–Tanga–Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated
alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic
xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental
and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism
in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences
and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism
in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete
labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements),
most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated
that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases
exhibit REE patterns that range from flat to light REE depleted. Sr–Nd isotopic data for the ultramafic residues show a coupled
behavior of increasing 87Sr/86Sr with decreasing 143Nd/144Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd
with distinctly more radiogenic 87Sr/86Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the
involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific
Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively
low 206Pb/204Pb at elevated 207Pb/204Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle.
Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized,
it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to
the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating
that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction
of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated
the geochemical characteristics of the lavas in the area.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
5.
Younger and older zircons from rocks of the oceanic lithosphere in the Central Atlantic and their geotectonic implications 总被引：1，自引：0，他引：1
Local U-Pb dating of zircons separated from various rocks in the crest zone of the Mid-Atlantic Ridge (MAR) and Carter Seamount
(Sierra Leone Rise) is performed. Younger zircons formed in situ in combination with older xenogenic zircons are revealed
in enriched basalts, alkaline volcanic rocks, gabbroic rocks, and plagiogranites. Only older zircons are found in depleted
basalts and peridotites. Older zircons are ubiquitous in the young oceanic lithosphere of the Central Atlantic. The age of
the younger zircons from the crest zone of the MAR ranges from 0.38 to 11.26 Ma and progressively increases receding from
the axial zone of the ridge. This fact provides additional evidence for spreading of the oceanic floor. The rate of half-spreading
calculated from the age of the studied zircons is close to the rate of half-spreading estimated from magnetic anomalies. The
age of the younger zircons from Carter Seamount (58 Ma) corresponds to the age of the volcanic edifice. Older zircons make
up an age series from 53 to 3200 Ma. Clusters of zircons differing in age reveal quasiperiodicity of about 200 Ma, which approximately
corresponds to the global tectonic epochs in the geological evolution of the Earth. Several age groups of older zircons combine
grains close in morphology and geochemistry: (1) Neoproterozoic and Phanerozoic (53–700 Ma) prismatic grains with slightly
resorbed faces, well-preserved or translucent oscillatory zoning, and geochemical features inherent to magmatic zircons; (2)
prismatic grains dated at 1811 Ma with resorbed faces and edges, fragmentary or translucent zoning, and geochemical features
inherent to magmatic zircons; (3) ovoid and highly resorbed prismatic grains with chaotic internal structure and metamorphic
geochemical parameters; the peak of their ages is 1880 Ma. The performed study indicates that older xenogenic zircons from
young rocks in the crest zone of the MAR were captured by melt or incorporated into refractory restite probably in the sublithospheric
mantle at the level of magma generation in the asthenosphere. It is suggested that zircons could have crystallized from the
melts repeatedly migrating through the asthenosphere during geological history or were entrapped by the asthenosphere together
with blocks of disintegrated and delaminated continental lithosphere in the process of breakup of the continents older than
Gondwana. The variability in the age of older zircons even within individual samples may be regarded as evidence for active
stirring of matter as a result of periodically arising and destroyed within-asthenospheric convective flows varying in orientation
and scale. 相似文献
6.
班公湖-怒江缝合带西段阿翁错地区中酸性侵入岩的成因及其对区域构造演化的指示 总被引：2，自引：0，他引：2
班公湖-怒江缝合带西段出露大量中酸性侵入岩,为特提斯洋俯冲、拉萨地块与羌塘地块碰撞造山过程中岩浆响应的重要组成部分。本文对该缝合带西段阿翁错地区的闪长岩、花岗闪长岩和花岗岩进行了详细的岩石地球化学和锆石U-Pb年代学研究。锆石LA-ICP-MS U-Pb定年结果表明闪长岩、花岗闪长岩、花岗岩成岩年龄分别为119.3±1.8 Ma、114.7±1.4 Ma和103.2±1.3 Ma。岩石地球化学特征显示中酸性侵入岩属高钾钙碱性系列,具准铝质-弱过铝质I型花岗岩特征;其LREE分馏程度较高,而HREE近于平坦,存在Eu负异常;富集Rb、La等大离子亲石元素和Th、Zr、Hf等高场强元素,亏损Nb、Ta、P、Ti等高场强元素,具有岛弧岩浆岩的特征。研究结果表明在早白垩世晚期(103.0±1.3 Ma)班公湖-怒江特提斯洋壳仍在向北俯冲于南羌塘地块之下,随着俯冲深度增加,大洋板片发生大规模脱水,释放的流体交代地幔楔并引发其部分熔融,产生的幔源岩浆向上运移,与下地壳物质不同比例混合形成了闪长岩和花岗闪长岩;而花岗岩主要由古老下地壳物质部分熔融形成,并有少量地幔物质的参与。 相似文献
7.
全球幔源岩Pb-Sr-Nd同位素体系 总被引：5，自引：0，他引：5
根据各种同位素数据库得到的3万多个晚古生代以来的幔源岩(包括洋中脊玄武岩、洋岛玄武岩、岛弧火山岩、大陆与大洋溢流玄武岩以及大陆板内玄武岩)Pb-Sr-Nd同位素资料和图解分析,对各类火山岩的源区以及地幔的垂向与横向不均一性问题作了进一步讨论。笔者认为不存在具有公共性质的EM1、EM2和HIMU地幔端员,它们的源区可能来自上、下地幔过渡带,只在局部地区出现,独一无二。PREMA(FOZO)则是洋岛玄武岩和溢流玄武岩公共端员。DUAPAL异常现象不只是在洋中脊玄武岩中出现,在洋岛玄武岩、岛弧火山岩和大洋溢流玄武岩中也存在同步的地球化学分区现象。溢流玄武岩的同位素体系特征表明它们的源区涉及再循环地幔的壳幔混合、岩石圈减压熔融、上—下地幔过渡带和似原始-略亏损的下地幔。Pb同位素体系为鉴别俯冲带的存在提供了更严格的证据,这种鉴别表明,安第斯弧火山作用不是洋陆俯冲带产生的。 相似文献
8.
Geochemistry of Lavas from the Emperor Seamounts, and the Geochemical Evolution of Hawaiian Magmatism from 85 to 42 Ma 总被引：4，自引：1，他引：4
The Hawaiian–Emperor Seamount Chain (ESC), in the northernPacific Ocean, was produced during the passage of the PacificPlate over the Hawaiian hotspot. Major and trace element concentrationsand Sr–Nd–Pb isotopic compositions of shield andpost-shield lavas from nine of the Emperor Seamounts providea 43 Myr record of the chemistry of the oldest preserved Hawaiianmagmatism during the Late Mesozoic and Early Cenozoic (from85 to 42 Ma). These data demonstrate that there were large variationsin the composition of Hawaiian magmatism over this period. Tholeiiticbasalts from Meiji Seamount (85 Ma), at the northernmost endof the ESC, have low concentrations of incompatible trace elements,and unradiogenic Sr isotopic compositions, compared with youngerlavas from the volcanoes of the Hawaiian Chain (<43 Ma).Lavas from Detroit Seamount (81 Ma) have highly depleted incompatibletrace element and Sr–Nd isotopic compositions, which aresimilar to those of Pacific mid-ocean ridge basalts. Lavas fromthe younger Emperor Seamounts (62–42 Ma) have trace elementcompositions similar to those of lavas from the Hawaiian Islands,but initial 87Sr/86Sr ratios extend to lower values. From 81to 42 Ma there was a systematic increase in 87Sr/86Sr of boththoleiitic and alkalic lavas. The age of the oceanic lithosphereat the time of seamount formation decreases northwards alongthe Emperor Seamount Chain, and the oldest Emperor Seamountswere built upon young, thin lithosphere close to a former spreadingcentre. However, the inferred distance of the Hawaiian plumefrom a former spreading centre, and the isotopic compositionsof the oldest Emperor lavas appear to rule out plume–ridgeinteraction as an explanation for their depleted compositions.We suggest that the observed temporal chemical and isotopicvariations may instead be due to variations in the degree ofmelting of a heterogeneous mantle, resulting from differencesin the thickness of the oceanic lithosphere upon which the EmperorSeamounts were constructed. During the Cretaceous, when theHawaiian plume was situated beneath young, thin lithosphere,the degree of melting within the plume was greater, and incompatibletrace element depleted, refractory mantle components contributedmore to melting. KEY WORDS: Emperor Seamounts; Hawaiian plume; lava geochemistry; lithosphere thickness; mantle heterogeneity 相似文献
9.
D. A. Lykhin V. I. Kovalenko V. V. Yarmolyuk A. B. Kotov V. P. Kovach 《Geology of Ore Deposits》2010,52(4):289-301
An isotopic study of igneous and metamorphic rocks has been carried out at the Yermakovsky bertrandite-phenakite-fluorite
deposit. It has been established that the model age of the schists pertaining to the Zun-Morino Formation is 1360–1260 Ma.
In Nd and Sr isotopic composition, these schists deviate from the isotopic composition of the continental crust and are close
in this respect to the enriched mantle reservoir (EM-II). The model age of carbonate rocks of the Zun-Morino Formation is
1330–1020 Ma. The Middle Riphean model age of the Zun-Morino Formation is interpreted as the age of its protolith. According
to the Sr and Nd isotopic data, all preore igneous rocks (granitic dikes, gabbroic rocks, and gneissose granite of the Tsagan
Complex) were formed with the participation of continental crustal material. Synore basic dikes, alkali leucogranite stock,
and syenite intrusion are considered to be mixtures of mantle components (DM+HIMU) and various continental crustal components
(Tsagan gneissose granite, crystalline schists, the mean composition of granitoids of the Angara-Vitim batholith as an estimate
of average composition of the regional continental crust). Synore igneous rocks are genetically cognate and related to the
magmatic activity in the Western Transbaikal Rift Zone presumably formed in the Triassic under effect of a mantle plume. 相似文献
10.
The opening of the North Atlantic Ocean began in the Late Paleocene and was accompanied by the eruption of submarine and subaerial basalts, which built up submarine plateau and ridges, islands, and volcanoes. The volcanic rocks are dominated by low-K tholeiitic basalts, which associate with almost coeval alkaline rocks (subalkali and alkali basalts and their derivatives, basanites, nephelinites, and others). The oldest alkaline volcanics (58–56 Ma) were formed during the opening of the oceanic rift at its shoulders, in northeastern Greenland and the western Norwegian shelf. It was recently found that 55–53 Ma-old alkali-ultramafic rocks are much more widespread at the eastern coast of Greenland than it was previously thought. The younger occurrences of alkali volcanism with pulses at 30, 10, 5 Ma, and up to the present day were formed on the young oceanic plate and newly formed islands and seamounts. To compare the oceanic and continental volcanism of this region, oceanic volcanics dredged during Cruise 10 of the R/V Akademik Kurchatov were reanalyzed using modern analytical methods (XRF and ICP-MS). This study showed that the oceanic and continental alkaline rocks are significantly different in petrochemical and geochemical characteristics, which is caused by differences in magma generation depths and compositions of the mantle source material. The primary continental alkaline magmas were initially more enriched in incompatible trace elements than oceanic ones. During the shallow-level differentiation of oceanic magmas, trace elements and alkalis could be accumulated in residual melts, but these processes occurred on a minor scale and depended on tectonic conditions. 相似文献
11.
We present the first trace element and age data combined with new Sr, Nd, and Pb isotope ratios on lavas from San Felix Island in the Southeast Pacific. A 40 Ar/39 Ar plateau age of 421 ± 18 ka implies young intraplate volcanic activity in this region relative to the ∼22 Ma old volcanism on the neighbouring Easter seamount chain (ESC). The incompatible element compositions of the San Felix magmas are similar to those of EM1-type basalts from Gough, although the isotopic compositions differ. San Felix formed some 20 Ma after the ESC plume affected the plate in this region but no chemical signature of the ESC material is observed in the young volcanic rocks. The composition of the San Felix basalts indicates a mantle source containing old continental lithospheric material from either metasomatized mantle or recycled sediments, which ascends in a weak mantle plume. 相似文献
12.
This paper reports new petrological, geochemical and isotopic data for Carlsberg Ridge Basalts (CRB) of northwest Indian Ocean and evaluates their petrogenetic aspects in the context of the geochemical and tectonic evolution of the Indian Ocean mantle. The CRB samples exhibit tholeiitic to transitional composition of precursor melts derived by high degree, shallow level partial melting of a spinel peridotite mantle source. CRB reflects distinct E-MORB affinity with selective enrichment in incompatible trace elements. Higher values of Zr/Hf (33.8–47.3) and Zr/Sm (24.9–36.4) in conjunction with lower Nb/Ta (1.7–7.3) ratio corroborate their origin from an enriched mantle source. Negative Nb anomalies with lower Nb/Y (0.04–0.11) and Zr/Y (2.5–3.5) conform to a non-plume origin of these basalts. Higher Zr/Nb (25.5–71.5) and Th/Nb (0.6–0.42) compared to OIB substantiate contributions from recycled subduction-processed components in the source mantle. Lower Nb/U (6.2–37.9) values with higher Ba/Nb (6.1–21.9), Ba/Th (27.7–147.5), Zr/Nb (25.5–71.5) and Th/Nb (0.6–0.42) compared to OIB and N-MORB attest to role of a metasomatized oceanic lithosphere that recycled into the depleted upper mantle attributing to the source heterogeneity. Sr-Nd isotopic signatures (87Sr/86Sr: 0.702668 to 0.702841 and 143Nd/144Nd: 0.512972 to 0.513068) of CRB suggest a HIMU source component preserved in the northwest Indian Ocean Ridge mantle. The compositional diversity of the Indian Ocean mantle can be translated in terms of periodic refertilization of depleted N-MORB type mantle through delamination and recycling of oceanic (HIMU component) and continental lithosphere (EM I component) concurrent with Neoproterozoic-Palaeozoic amalgamation and Jurassic dispersal of Gondwana Supercontinent respectively. This study complies with the derivation of CRB from a geochemically heterogeneous Indian Ocean mantle that experienced a protracted residence beneath the Gondwana Supercontinent prior to the opening of Indian Ocean and trapped recycled metasomatized oceanic lithosphere genetically linked with multiple stages of paleo-ocean closure and continental convergence during Gondwana assembly. 相似文献
13.
Results of isotope Sr, Ns, and O analyses of volcanic rocks from the Uda sector of the West Transbaikal Rift Zone have allowed estimation of the character of interaction of their parental mantle melts with crustal rocks. The smallest magnitude of this interaction has been found in the compositions of Late Cretaceous (83–70 Ma) volcanics, the geochemical and isotope markers of which suggest their derivation from a moderately enriched mantle compositionally resembling OIB sources. The Early Cretaceous volcanics were derived from mantle sources that included a mantle enriched by subduction. While ascending through the crust, the parental melts of the Uda Complex (130–111 Ma) were contaminated by the lower crust matter. The Zazin Complex magmas (143–135 Ma) have features suggesting their interaction with upper crustal granitoids of the Angara–Vitim Batholith.
相似文献14.
A. K. Barker V. R. Troll R. M. Ellam T. H. Hansteen C. Harris C. J. Stillman A. Andersson 《Contributions to Mineralogy and Petrology》2012,163(6):949-965
The Cadamosto Seamount is an unusual volcanic centre from Cape Verde, characterised by dominantly evolved volcanics, in contrast to the typically mafic volcanic centres at Cape Verde that exhibit only minor volumes of evolved volcanics. The magmatic evolution of Cadamosto Seamount is investigated to quantify the role of magma-crust interaction and thus provide a perspective on evolved end-member volcanism of Cape Verde. The preservation of mantle source signatures by Nd–Pb isotopes despite extensive magmatic differentiation provides new insights into the spatial distribution of mantle heterogeneity in the Cape Verde archipelago. Magmatic differentiation from nephelinite to phonolite involves fractional crystallisation of clinopyroxene, titanite, apatite, biotite and feldspathoids, with extensive feldspathoid accumulation being recorded in some evolved samples. Clinopyroxene crystallisation pressures of 0.38–0.17 GPa for the nephelinites constrain this extensive fractional crystallisation to the oceanic lithosphere, where no crustal assimilants or rafts of subcontinental lithospheric mantle are available. In turn, magma-crust interaction has influenced the Sr, O and S isotopes of the groundmass and late crystallising feldspathoids, which formed at shallow crustal depths reflecting the availability of oceanic sediments and anhydrite precipitated in the ocean crust. The Nd–Pb isotopes have not been affected by these processes of magma-crust interaction and hence preserve the mantle source signature. The Cadamosto Seamount samples have high 206Pb/204Pb (>19.5), high εNd (+6 to +7) and negative Δ8/4Pb, showing affinity with the northern Cape Verde islands as opposed to the adjacent southern islands. Hence, the Cadamosto Seamount in the west is located spatially beyond the EM1-like component found further east. This heterogeneity is not encountered in the oceanic lithosphere beneath the Cadamosto Seamount despite greater extents of fractional crystallisation at oceanic lithospheric depths than the islands of Fogo and Santiago. Our data provide new evidence for the complex geometry of the chemically zoned Cape Verde mantle source. 相似文献
15.
V. P. Kovach V. V. Yarmolyuk V. I. Kovalenko A. M. Kozlovskyi A. B. Kotov L. B. Terent’eva 《Petrology》2011,19(4):399-425
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
16.
青藏高原分布有羌塘—囊谦—滇西和冈底斯两条新生代钾质-超钾质火山岩带。羌塘—囊谦—滇西超钾质岩浆活动的峰值时间为40～30Ma,主体岩石具有Ⅰ型超钾质岩的高MgO和低CaO、Al2O3含量特征;30～24Ma期间羌塘中、西部出现Ⅲ型钾质-超钾质岩浆活动,主体岩石以贫SiO2、高CaO、Al2O3和低MgO/CaO为特征。冈底斯新生代超钾质火山岩也显示I型超钾质岩的高MgO和低CaO、Al2O3含量特征,其形成时间为25～12Ma。综合超钾质岩石的实验资料,可知区内I型超钾质岩的源区以富硅、富钾流(熔)体交代形成的金云母方辉橄榄岩为主;Ⅲ型钾质-超钾质岩浆源区则以斜辉橄榄岩地幔为主。囊谦—滇西Ⅰ型超钾质岩带空间上严格受红河走滑构造带所控制,40～28Ma出现I型超钾质岩浆活动,16Ma转变为OIB型钾质火山岩。岩浆源区从岩石圈地幔向软流圈演变,暗示大型走滑断裂引起的岩石圈地幔减薄和软流圈上涌是导致交代岩石圈地幔金云母分解熔融产生区内I型超钾质岩浆的主控因素。羌塘中部35～34Ma有软流圈来源为主的钠质碱性玄武岩岩浆的喷发,30～24Ma转变为以岩石圈地幔为主要来源的Ⅲ型钾质-超钾质岩浆活动,岩浆源区从软流圈向岩石圈迁移,指示软流圈上涌伴随的富CO2流(熔)体活动是导致古交代岩石圈地幔升温熔融产生Ⅲ型钾质-超钾质岩浆的主控因素,软流圈上涌可能是俯冲板片断离或岩石圈地幔拆沉作用的结果。 相似文献
17.
《Gondwana Research》2014,26(4):1660-1679
New radiometric age and geochemical data of volcanic rocks from the guyot-type Marie Byrd Seamounts (MBS) and the De Gerlache Seamounts and Peter I Island (Amundsen Sea) are presented. 40Ar/39Ar ages of the shield phase of three MBS are Early Cenozoic (65 to 56 Ma) and indicate formation well after creation of the Pacific–Antarctic Ridge. A Pliocene age (3.0 Ma) documents a younger phase of volcanism at one MBS and a Pleistocene age (1.8 Ma) for the submarine base of Peter I Island. Together with published data, the new age data imply that Cenozoic intraplate magmatism occurred at distinct time intervals in spatially confined areas of the Amundsen Sea, excluding an origin through a fixed mantle plume. Peter I Island appears strongly influenced by an EMII type mantle component that may reflect shallow mantle recycling of a continental raft during the final breakup of Gondwana. By contrast the Sr–Nd–Pb–Hf isotopic compositions of the MBS display a strong affinity to a HIMU-type mantle source. On a regional scale the isotopic signatures overlap with those from volcanics related to the West Antarctic Rift System, and Cretaceous intraplate volcanics in and off New Zealand. We propose reactivation of the HIMU material, initially accreted to the base of continental lithosphere during the pre-rifting stage of Marie Byrd Land/Zealandia to explain intraplate volcanism in the Amundsen Sea in the absence of a long-lived hotspot. We propose continental insulation flow as the most plausible mechanism to transfer the sub-continental accreted plume material into the shallow oceanic mantle. Crustal extension at the southern boundary of the Bellingshausen Plate from about 74 to 62 Ma may have triggered adiabatic rise of the HIMU material from the base of Marie Byrd Land to form the MBS. The De Gerlache Seamounts are most likely related to a preserved zone of lithospheric weakness underneath the De Gerlache Gravity Anomaly. 相似文献
18.
YAN Ming LIU Shuwen LI Qiugen ZHANG Wanyi YANG Pengtao YAN Zhen WANG Ruiting WANG Wei GUO Rongrong 《《地质学报》英文版》2014,88(1):154-175
The Sanchahe quartz monzonite intrusion is situated in the middle segment of the North Qinling tectonic belt, Central China mainland, and consists chiefly of sanukitoid–like and granodioritic-monzogranitic rocks. The sanukitoid–like rocks are characterized by quartz monzonites, which display higher Mg#（55.0–59.0）, and enrichments in Na2 O＋K2 O（7.28–8.94 %）, Ni（21-2312 ppm）, Cr（56-4167 ppm）, Sr（553-923 ppm）, Ba（912-1355 ppm） and LREE（（La/Yb）N =9.47–15.3）, from negative to slightly positive Eu anomalies（δEu=＋0.61 to ＋1.10）, but also depletion in Nb, Ta and Ti. The granodioritic-monzogranitic rocks diaplay various Mg#of 6.00-53.0, high Na2 O＋K2 O（7.20– 8.30%）, Sr（455–1081 ppm） and（La/Yb）N（27.6–47.8）, with positive Eu anomalies（δEu=1.03–1.57） and depleted Nb, Ta and Ti. Laser ablation inductively coupled plasma mass spectrometry（LA-ICPMS） zircon U-Pb isotopic dating reveals that the sanukitoid-like rocks were emplaced at two episodes of magmatism at 457±3 Ma and 431±2 Ma, respectively. The monzogranites were emplaced at 445±7Ma. Sanukitoid–like rocks have their εHf（t） values ranging from ＋0.3 to ＋15.1 with Hf–depleted mantle model ages of 445 to 1056 Ma, and the monzogranite shows its εHf（t） values ranging from 21.6 to ＋10.8 with Hf–depleted mantle model ages of 635 to 3183 Ma. Petrological, geochemical and zircon Lu –Hf isotopic features indicate that the magmatic precursor of sanukitoid–like rocks was derived from partial melting of the depleted mantle wedge materials that were metasomatized by fluids and melts related to subduction of oceanic slab, subsequently the sanukitoid magma ascended to crust level. This emplaced mantle magma caused partial melting of crustally metamorphosed sedimentary rocks, and mixing with the crustal magma, and suffered fractional crystallization, which lead to formations of quartz monzonites. However, the magmatic precursor of the granodioritic-monzogranitic rocks were derived from partial melting of subducted oceanic slab basalts. Integrated previous investigation for the adackitic rocks in the south of the intrusion, the Sanchahe intrusion signed that the North Qinling tectonic zone was developed in an early Paleozoic transitionally tectonic background from an island arc to back–arc. 相似文献
19.
新疆准噶尔北缘北塔山组火山岩年龄及岩石成因 总被引：9，自引：7，他引：2
对准噶尔北缘北塔山组辉石玄武岩进行了LA-ICP-MS锆石U-Pb 年龄测定, 获得了玄武岩的喷发年龄380.5±2.2Ma,表明北塔山组火山岩形成于中泥盆世。该地层火山岩中辉石玄武岩和无斑玄武岩的SiO2含量为47.55%~52.97%、Al2O3的含量为8.44%~20.00%、TiO2为0.5%~1.2%,MgO含量为2.8%~15.35%、CaO为3.98%~14.83%、FeOT为9.46%~19.23%,具有亚碱性拉斑玄武岩的特征。其微量元素显示富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损Nb、Ta和Ti,Eu异常不明显。它们具有极低的初始87Sr/86Sr同位素比值(0.703835~0.704337)和高的εNd(381Ma)值(+6.84~+12.3,t=381Ma)的亏损地幔源区特征。结合区域地质背景,北塔山组火山岩形成于与俯冲作用相关的构造环境,是准噶尔古洋盆于泥盆世时发生的俯冲-消减所引发的岛弧岩浆作用的地质记录。岩浆源区为被流体或沉积物熔体交代改造的地幔楔和软流圈地幔,不同类型的岩石系不同成分的原始岩浆经不同演化过程的产物。 相似文献
20.
《International Geology Review》2012,54(11):1297-1312
ABSTRACTEarly Palaeozoic magmatic records and tectonic reconstructions along the northern margin of Gondwana are still pending problems. In this paper, Late Silurian Dawazi and Dazhonghe volcanics in SW Yunnan Province (China) were studied. The Dazhonghe volcanics (419 Ma) have variable chemical compositions with SiO2 ranging between 49.8 and 79.5 wt.%, whereas, the Dawazi volcanics (417–429 Ma) form a bimodal volcanic suite consisting mainly of silicic rocks and subordinate basaltic rocks with a SiO2 content gap of ca. 15 wt.%. The Dazhonghe volcanics display calc-alkaline elemental compositions with enrichment in light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) (e.g. Nb, Ta and Ti) and positive εNd(t) values (+ 4.0 to + 5.5). The Dawazi basaltic rocks are calc-alkaline, depleted in HFSEs, enriched in large ion lithophile elements (LILEs) (e.g. Cs, Rb, U and K), and have high εNd(t) values of ?1.7 to + 5.4. The Dawazi silicic rocks have high Na2O/K2O ratios and positive εNd(t) values of + 2.4 to + 5.0, which are equivalents of calc-alkaline I-type granites. The Dazhonghe volcanics are dominated by fractional crystallization (FC) from a calc-alkaline primary magma which originated from an enriched mantle source metasomatized by subduction-related, sediment-derived fluid. The Dawazi basaltic rocks were derived from partial melting of an enriched mantle source metasomatized by subducted oceanic sediment/slab-derived fluids; the Dawazi silicic rocks originated from partial melting of the juvenile mafic lower crust with extensive FC. Both the Dazhonghe and Dawazi volcanics were generated in a continental back-arc extension setting. Combined with previous geological observations, a Late Silurian Prototethyan arc and back-arc extension system is proposed along the northern margin of Gondwana in SW China. 相似文献