Neoproterozoic accretionary and collisional events on the western margin of the Siberian craton: new geological and geochronological evidence from the Yenisey Ridge     

Valery A. Vernikovsky  A. E. Vernikovskaya  A. B. Kotov  E. B. Sal'nikova  V. P. Kovach 《Tectonophysics》2003,375(1-4):147

The geological, structural and tectonic evolutions of the Yenisey Ridge fold-and-thrust belt are discussed in the context of the western margin of the Siberian craton during the Neoproterozoic. Previous work in the Yenisey Ridge had led to the interpretation that the fold belt is composed of high-grade metamorphic and igneous rocks comprising an Archean and Paleoproterozoic basement with an unconformably overlying Mesoproterozoic–Neoproterozoic cover, which was mainly metamorphosed under greenschist-facies conditions. Based on the existing data and new geological and zircon U–Pb data, we recognize several terranes of different age and composition that were assembled during Neoproterozoic collisional–accretional processes on the western margin of the Siberian craton. We suggest that there were three main Neoproterozoic tectonic events involved in the formation of the Yenisey Ridge fold-and-thrust belt at 880–860 Ma, 760–720 Ma and 700–630 Ma. On the basis of new geochronological and petrological data, we propose that the Yeruda and Teya granites (880–860 Ma) were formed as a result of the first event, which could have occurred in the Central Angara terrane before it collided with Siberia. We also propose that the Cherimba, Ayakhta, Garevka and Glushikha granites (760–720 Ma) were formed as a result of this collision. The third event (700–630 Ma) is fixed by the age of island-arc and ophiolite complexes and their obduction onto the Siberian craton margin. We conclude by discussing correlation of these complexes with those in other belts on the margin of the Siberian craton.  相似文献   

Reinterpretation of the RRISP-77 Iceland shear-wave profiles   总被引：1，自引：0，他引：1  

William Menke  Bryndís Brandsdóttir  Páll Einarsson  Ingi Th. Bjarnason 《Geophysical Journal International》1996,126(1):166-172

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Metamorphic evolution of oceanic gabbros: recrystallization from subsolidus to hydrothermal conditions in the MARK area (ODP Leg 153)     

L. Gaggero  L. Cortesogno 《Lithos》1997,40(2-4):105-131

The 117.38 m of gabbroic core drilled during the Ocean Drilling Program (ODP) Leg 153 at Sites 921 to 924 in the Mid-Atlantic Ridge (MAR) between 23 °N and the Kane Fracture Zone, exhibits a remarkable primary compositional heterogeneity, such as magmatic layering, intrusive contacts and late magmatic veining, which express a succession of magmatic events. Textural indicators suggest that the cooling of the crystal mush occurred in a dynamic environment, with infiltration of progressively evolved liquids. Magmatic features include random shape fabric and magmatic lamination; the subsequent deformational overprint occurred in subsolidus conditions. The ductile deformation, generally concentrated in discrete domains of the gabbro, is associated with continuous re-equilibration of the metamorphic assemblages of (1) olivine + clinopyroxene + orthopyroxene + plagioclase + ilmenite + Ti-magnetite, (2) olivine + clinopyroxene + plagioclase + ilmenite + Ti-magnetite + red hornblende. At lower temperatures brittle deformation prevails and subsequent fractures control the development of metamorphic assemblages: (3) clinopyroxene + plagioclase + red brown hornblende + Ti-magnetite + magnetite (?) + ilmenite, (4) plagioclase + brown hornblende + Ti-magnetite + magnetite + hematite + titanite ± Ti-oxide, (5) plagioclase + green hornblende + magnetite + titanite, (6) plagioclase + actinolite + chlorite + titanite + magnetite, (7) albite + actinolite + chlorite + prehnite ± epidote ± titanite and (8) albite + prehnite + chlorite ± smectite. Assemblages 1 to 8 express increasing water/rock ratios and decreasing degrees of recrystallization.

During the ductile phase, red hornblende is stable and its abundance increases with deformation intensity, possibly as an effect of the introduction of hydrous fluids. During the brittle phase, water diffusion controls the development of the fracture-filling mineral assemblages and re-equilibration of the adjacent rock; temperatures decrease further, as demonstrated by mineral zoning and incompletely re-equilibrated assemblages. The lowest temperatures correspond to the development of hydrothermal assemblages.

Compared with oceanic gabbros from fast-spreading transform environments, high-temperature ductile phases (granulite and amphibolite) are well developed, whereas brittle phases are widespread, as microcracks, prevalent on fracturing associated with discrete veins.  相似文献   

Crystal morphologies in pillow basalts: implications for mid-ocean ridge processes     

Franois Faure  Pierre Schiano 《Earth and Planetary Science Letters》2004,220(3-4):331-344

We present the results of a detailed petrological study of a sparsely phyric basalt (MAPCO CH98-DR11) dredged along the Mid-Atlantic Ridge (30°41′N). The sample contains microphenocrysts of olivine that display four different rapid-growth morphologies. Comparison of these morphologies with those obtained in dynamic crystallization experiments allows us to constrain the thermal history of the sample. The dendritic morphology (swallowtail, chain and lattice olivine) is directly related to the final quenching during magma–seawater interaction. In contrast, the three other morphologies, namely the complex polyhedral crystal, the closed hopper and the complex swallowtail morphology result from several cycles of cooling–heating (corresponding to a maximum degree of undercooling of 20–25°C) during crystal growth. These thermal variations occurred before eruption and are interpreted to be the result of turbulent convection in a small magmatic body beneath the ridge. The results suggest that the Mid-Atlantic Ridge is underlain by a mush zone that releases batches of liquid during tectonic segregation. Aphyric basalts are emitted during eruptions controlled by the tectonic activity, whereas phyric basalts correspond to small fractions of magma from the mush zone mobilized by reinjections of primitive magmas.  相似文献   

Seismic reflection imaging of an oceanic detachment fault: Atlantis megamullion (Mid-Atlantic Ridge, 30°10′N)     

J.P. Canales  B.E. Tucholke  J.A. Collins 《Earth and Planetary Science Letters》2004,222(2):543-560

We present multichannel seismic reflection data collected over the Atlantis megamullion, at the eastern ridge-transform intersection of Atlantis fracture zone on the northern Mid-Atlantic Ridge, and over its conjugate crust. These data image for the first time the internal structure of a young, well-developed megamullion dome formed by tectonic extension across a long-lived oceanic detachment fault. The exposed, corrugated detachment-fault surface exhibits a sharp, coherent reflection that contrasts with less organized reflectivity of surrounding basaltic seafloor. At the termination of the megamullion the fault is imaged ∼13 km along-strike beneath a volcanic hanging-wall block at a sub-seafloor depth of 0.2-0.5 s two-way travel time, reaching north as far as 30°19′N. The eastward dipping of the fault beneath the hanging-wall block is estimated to be ∼6-14°. The corrugated fault surface is underlain by a continuous, strong, and relatively smooth reflection (D) at 0.2-0.25 s sub-bottom below the central axis of the dome. This reflection deepens up to 0.6 s sub-bottom beneath the western slope and it appears to intersect the seafloor on the eastern slope. We suggest that Atlantis massif formed by sequential slip on two different detachment faults that merged at depth, with breakaways as little as ∼2 km apart. The initial detachment is represented by reflection D, and the second corresponds to the presently exposed fault surface. In this interpretation, much of the sliver between the faults is interpreted to be strongly serpentinized peridotite with reduced seismic velocity; it lies in contact with less altered, higher-velocity mantle below the first detachment, resulting in the strong, smooth character of reflection D. Mantle rocks exposed in the megamullion indicate that the feature formed during a period of extreme tectonic extension and probably limited magmatism. In conjugate crust corresponding to termination of the megamullion, observed sub-bottom reflections are interpreted as base of seismic layer 2A. This layer is as thick as or thicker (∼570-900 m) than layer 2A in normal Atlantic crust, and it suggests that relatively normal crustal accretion occurred by the time the megamullion stopped forming.  相似文献   

Cenozoic bottom current sedimentation in the Cape basin, South Atlantic   总被引：1，自引：0，他引：1  

Etienne Wildeboer Schut  Gabriele Uenzelmann-Neben 《Geophysical Journal International》2005,161(2):325-333

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东南印度洋中脊澳大利亚-南极洲不整合带地区岩浆供应与构造作用沿洋脊的变化     

刘守金  林间  周志远  张帆 《海洋学报(英文版)》2020,39(1):118-129

We analyzed seafloor morphology and geophysical anomalies of the Southeast Indian Ridge(SEIR) to reveal the remarkable changes in magma supply along this intermediate fast-spreading ridge. We found systematic differences of the Australian-Antarctic Discordance(AAD) from adjacent ridge segments with the residual mantle Bouguer gravity anomaly(RMBA) being more positive, seafloor being deeper, morphology being more chaotic, M factors being smaller at the AAD. These systematic anomalies, as well as the observed Na_(8.0) being greater and Fe_(8.0) being smaller at AAD, suggest relatively starved magma supply and relatively thin crust within the AAD.Comparing to the adjacent ridges segments, the calculated average map-view M factors are relatively small for the AAD, where several Oceanic Core Complexes(OCCs) develop. Close to 30 OCCs were found to be distributed asymmetrically along the SEIR with 60% of OCCs at the northern flank. The OCCs are concentrated mainly in Segments B3 and B4 within the AAD at ～124°–126°E, as well as at the eastern end of Zone C at ～115°E. The relatively small map-view M factors within the AAD indicate stronger tectonism than the adjacent SEIR segments.The interaction between the westward migrating Pacific mantle and the relatively cold mantle beneath the AAD may have caused a reduction in magma supply, leading to the development of abundant OCCs.  相似文献   

Groundwater mixing between different aquifer types in a complex structural setting discerned by elemental and stable isotope geochemistry          下载免费PDF全文

Dorina Murgulet  Marlon Cook  Valeriu Murgulet 《水文研究》2016,30(3):410-423

Karst aquifers are well known for their intricate stratigraphy and geologic structures, which make groundwater characterization challenging because flowpaths and recharge sources are complex and difficult to evaluate. Geochemical data, collected from ten closely spaced production wells constructed in two karst aquifers (Bangor Limestone (Mb) and Tuscumbia Limestone/Fort Payne Chert (Mftp)) in Trussville, north‐central Alabama, illustrate two distinctive groundwater end‐members: (1) higher major ion, dissolved inorganic carbon, conductivity, alkalinity concentrations, heavier δ¹³C ratios (max: −10.2 ± 0.2‰ Vienna Pee Dee Belemnite (PDB)) and lower residence times (mean: 19.5 ± 2 years, n = 2) of groundwater in the Mb aquifer and (2) lower constituent concentrations, lighter δ¹³C ratios (min: −13.4 ± 0.2‰ PDB) and longer residence times of groundwater (mean: 23.6 ± 2 years, n = 4) in the Mftp aquifer. Summer and fall data and the binary mixing model show aquifer inter‐flow mixing along solution fractures and confirms the distinctive groundwater geochemistry of the two aquifers. Lowering of static water levels over the summer (drawdown from 2 to 5.2 m) leads to more reducing groundwater conditions (lower Eh values) and slightly enriched δ¹⁸O and δD ratios during the fall [δ¹⁸O: −4.8 ± 0.1 to −5.4 ± 0.1‰ Vienna Standard Mean Oceanic Water (VSMOW), n = 9; δD: −25.4 ± 1 to −27.4 ± 1‰ VSMOW, n = 9] when compared with summer season samples (δ¹⁸O: −5.1 ± 0.1 to −5.7 ± 0.1‰ VSMOW, n = 11; δD: −25.0 ± 1 to −30.6 ± 1‰ VSMOW, n = 11). GIS analyses confirm the localized origin of recharge to the investigated aquifers. The combination of GIS, field parameters and geochemistry analyses can be successfully used to identify recharge sources, evaluate groundwater flow and transport pathways and to improve understanding of how groundwater withdrawals impact the sustainability and susceptibility to contamination of karst aquifers. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

西北印度洋中脊玄武岩源区地幔特征   总被引：1，自引：0，他引：1  

淳明浩  于增慧  李怀明  翟世奎 《海洋科学》2016,40(8):108-118

利用全球岩石地球化学数据库(Pet DB)中有关卡尔斯伯格洋脊(CR)、北中印度洋脊(NCIR)及南中印度洋脊(SCIR)玄武岩的微量元素及同位素组成数据,分析了玄武岩的元素地球化学特征及其沿脊轴的变化,旨在探讨玄武岩源区地幔的(不)均一性及岩浆作用过程的差异。初步研究结果表明:CR、NCIR及SCIR玄武岩组成相近,仅在个别脊段表现有微量稀土元素和同位素组成上的差异,玄武岩整体与N-MORB组成特征相近,与先前通常认为的典型印度洋中脊玄武岩不同。玄武质岩浆主要源自尖晶石二辉橄榄岩地幔的熔融,岩浆源区主要由两个地幔端元构成,即以亏损型地幔(DMM)为主(69%),其次为富集型地幔(EMⅡ,27%)。富集组分可能源自古老陆壳物质的混染。自CR经NCIR到SCIR整个印度洋中脊西北分支玄武岩的Sr、Nd及Pb同位素组成表现出均一性,表明岩浆源区地幔组成相近。在SCIR 19°S附近脊段岩浆源区地幔存在有不均一性,有EMⅡ型地幔端元混入的迹象。在CR 3.5°N附近脊段,玄武岩明显富集K、Ba、La及U等微量元素,但由于缺少同位素数据,源区地幔特征有待进一步研究。在上述研究成果的基础上,提出了该区大比例尺的调查填图及密集采样和精细室内分析是CR深入研究的基础,同时加强Sr、Nd、Pb及Re、Os、Be等同位素分析测试,可提供揭示CR地幔不均一性的可靠依据,而厘清印度洋型地幔对CR的影响程度则有助于深入认识地幔不均一性的成因及地幔动力学过程。  相似文献   

Nodular monazite from placers in the Kular Ridge (Arctic Siberia,Russia): Composition and age     

E.V. Lazareva  S.M. Zhmodik  A.V. Prokopiev  N.S. Karmanov  A.I. Sergeenko 《Russian Geology and Geophysics》2018,59(10):1330-1347

Nodular monazite occurs in metamorphic rocks worldwide and has zonal REE patterns. This paper focuses on the composition of nodular monazite hosted by Permian black shales of the Kular Ridge in the Kular-Nera terrane. This monazite variety (called kularite in the Russian literature) reaches commercial amounts in placers of the area. The contents of Ce, Nd, and La in the analyzed monazite nodules show correlations at Ce/Nd = 14.39La + 0.0919 (in apfu) and Ce/Nd = 0.2318La + 0.1135 (in wt.%) and vary regularly from core to rim. All monazite compositions fall on this trend, but specific grains may plot in its different parts. Thermodynamic calculations indicate that monazite forms via an intermediate precursor (LnPO₄·2H₂O). The Ce:La:Nd changes in different grains record Eh-pH variations during nucleation and a gradual temperature increase during subsequent growth. The Ce:La:Nd ratio changes partly in grain rims as a result of oxidative dissolution. Judging by the tectonic setting, REE came to the Kular-Nera rocks from the weathered Tomtor Nb-REE deposit, being transported by the Paleo-Khatanga River with monazite nanoparticles bound to the surface of clay minerals.  相似文献