排序方式： 共有15条查询结果，搜索用时 31 毫秒
1.
ALGIRDAS GAIGALAS LEONID SEREBRYANNY MAISA VALUEVA 《Boreas: An International Journal of Quaternary Research》1992,21(3):289-293
Lacustrine sediments, 13 m thick, have been discovered under a Late Valdaian (Late Weichselian) moraine complex in a palaeokarst depression at Biržai, northern Lithuania. Wood fragments from silty layers in the basal part of the section were 14 C dated at 34,440 ± 1500 (Vs-412) and 33,460 ± 1060 BP (LU-1633). Pollen analytical results suggest that taiga landscapes similar to the present-day environments in the Pechora Basin and eastern Urals occurred on the southern cryogenic grounds. Evidently, the ice sheet did not penetrate into the eastern Baltic region during the Middle Valdaian interval. 相似文献
2.
The Kanskaya formation in the Yenisey range, Eastern Siberia is a newly studied example of retrogression of granulite facies rocks. The formation consists of two stratigraphical units: the lower Kuzeevskaya group and the upper Atamanovskaya group. Rocks from both of these units show rare reaction textures such as replacement of cordierite by garnet, sillimanite and quartz, silimanite coronas around spinel and corundum, and garnet rims around plagioclase in metabasites, while plagioclase rims around garnet can be seen in associated metapelites. The paragenesis quartz + orthopyroxene + sillimanite is a feature of the Kuzeevskaya group. In many samples, chemical zoning of garnet and cordierite shows an increase in Mg from core to rim as well as the reverse.
Biotite-garnet-cordierite-sillimanite-quartz as well as spinel±biotite-garnet°Cordierite±sillimanite-quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net-transfer reactions (Perchuk & Lavrent'eva, 1983; Aranovich & Podlesskii, 1983; Gerya & Perchuk, 1989). Detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression (first stage) followed by the near isobaric cooling of the granulites. From geological studies, the 7 km total thickness of the sequence closely corresponds to the pressure difference (∼ 2.2kbar) measured by geobarometers in the samples taken from different levels in the sequence. Individual samples yield P-T paths ranging from 100°C/kbar to 140°C/kbar depending on their locations with respect to the large Tarakskiy granite pluton. In places the 100°C/kbar path changed to the 140°C/kbar due to the influence of the intrusion. In a P-T diagram these trajectories are subparallel lines, whose P-T maxima define the Archaean geotherm between 3.1 and 2.7 Ga, determined isotopically. A petrological model for P-T evolution of the Kanskaya formation is proposed. 相似文献
Biotite-garnet-cordierite-sillimanite-quartz as well as spinel±biotite-garnet°Cordierite±sillimanite-quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net-transfer reactions (Perchuk & Lavrent'eva, 1983; Aranovich & Podlesskii, 1983; Gerya & Perchuk, 1989). Detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression (first stage) followed by the near isobaric cooling of the granulites. From geological studies, the 7 km total thickness of the sequence closely corresponds to the pressure difference (∼ 2.2kbar) measured by geobarometers in the samples taken from different levels in the sequence. Individual samples yield P-T paths ranging from 100°C/kbar to 140°C/kbar depending on their locations with respect to the large Tarakskiy granite pluton. In places the 100°C/kbar path changed to the 140°C/kbar due to the influence of the intrusion. In a P-T diagram these trajectories are subparallel lines, whose P-T maxima define the Archaean geotherm between 3.1 and 2.7 Ga, determined isotopically. A petrological model for P-T evolution of the Kanskaya formation is proposed. 相似文献
3.
LEONID R. SEREBRYANNY REI V. ORLOV 《Boreas: An International Journal of Quaternary Research》1982,11(4):279-289
Composition and fabric of different moraines in the glaciated areas of Central Caucasus were studied to elucidate sedimentary environments and geomorphological processes. Methods used included particle-size distribution, pebble and mineral counts, shape analysis of pebbles, and till fabric analysis. Samples were taken from superglacial and basal debris, marginal moraine ridges and till horizons, and quantitative parameters were established for different glacial environments and till accumulation processes. Considerable differences in composition and fabric of superglacial and basal debris were caused by their specific transport conditions. Most marginal moraines consist of material similar to basal debris and only the topmost parts of some ridges have a noticeable incorporation of superglacial debris. These results point to considerable erosion of mountain valleys by temperate glaciers. Previous geomorphological investigations have emphasized the importance of superglacial debris in the formation of marginal moraines by alpine glaciers, but this viewpoint is not supported by the studies performed. 相似文献
4.
DANYUSHEVSKY LEONID V.; EGGINS STEPHEN M.; FALLOON TREVOR J.; CHRISTIE DAVID M. 《Journal of Petrology》2000,41(8):1329-1364
The H2O contents and trace-element abundances are presentedfor two well-studied suites of mid-ocean ridge basalt (MORB)glasses from the Northern East Pacific Rise (EPR, 9–11°N)and the South East Indian Ridge (SEIR, 127–129°E).Exactly the same region of the glass samples has been analysedfor these components using microbeam techniques. Our data allowexamination of the fine details of H2O geochemical behaviourduring MORB genesis. We demonstrate that relative H2O contents[i.e. H2O/(another incompatible element)] vary systematicallywith increasing (La/Sm)N in MORB glasses from both the EPR andSEIR. This indicates that H2O behaves like other incompatible(in peridotite mineralogies) elements during MORB petrogenesis,and is primarily controlled by solid–melt partitioning.However, the relative H2O contents of MORB glasses from theSEIR are higher than in glasses from the EPR at a given (La/Sm)N,demonstrating global variations in the H2O contents of MORB.Despite regional differences in relative H2O contents, the incompatiblebehaviour of H2O is similar in both studied regions. The relativeincompatibility of H2O varies systematically with increasing(La/Sm)N: in depleted MORB, H2O is similar to La whereas inEMORB, H2O is similar to Ce. Similar patterns of varying relativeincompatibility (to REE) are displayed by Zr, Hf, and P. Ourdata are best explained if H2O is stored in the mantle in thesame phase with LREE (clinopyroxene?) at sub-solidus. Regionalvariations in relative H2O contents in EMORB that have moreradiogenic Sr, Nd and Pb isotopes might be explained by differencesin the nature of enriched components recycled via subductionprocesses. However, when EMORB have the same radiogenic isotopecompositions as NMORB within a segment, relative H2O contentsin EMORB probably reflect local processes that lead to enrichmentin incompatible elements. Regional differences in relative H2Ocontents of NMORB may reflect either initial variations in theEarth’s mantle or inhomogeneities left after formationof the continental crust. KEY WORDS: glass; geochemistry; H2O; MORB; petrology 相似文献
5.
Peridotite Melting at 1 GPa: Reversal Experiments on Partial Melt Compositions Produced by Peridotite-Basalt Sandwich Experiments 总被引：3，自引：0，他引：3
FALLOON TREVOR J.; DANYUSHEVSKY LEONID V.; GREEN DAVID H. 《Journal of Petrology》2001,42(12):2363-2390
One of the goals of igneous petrology is to use the subtle andmore obvious differences in the geochemistry of primitive basaltsto place constraints on mantle composition, melting conditionsand dynamics of mantle upwelling and melt extraction. For thisgoal to be achieved, our first-order understanding of mantlemelting must be refined by high-quality, systematic data oncorrelated melt and residual phase compositions under knownpressures and temperatures. Discrepancies in earlier data onmelt compositions from a fertile mantle composition [MORB (mid-oceanridge basalt) Pyrolite mg-number 87] and refractory lherzolite(Tinaquillo Lherzolite mg-number 90) are resolved here. Errorsin earlier data resulted from drift of W/Re thermocouples at1 GPa and access of water, lowering liquidus temperatures by30–80°C. We demonstrate the suitability of the ‘sandwich’technique for determining the compositions of multiphase-saturatedliquids in lherzolite, provided fine-grained sintered oxidemixes are used as the peridotite starting materials, and thechanges in bulk composition are considered. Compositions ofliquids in equilibrium with lherzolitic to harzburgitic residueat 1 GPa, 1300–1450°C in the two lherzolite compositionsare reported. Melt compositions are olivine + hypersthene-normative(olivine tholeiites) with the more refractory composition producinga lower melt fraction (7–8% at 1300°C) compared withthe model MORB source (18–20% at 1300°C). KEY WORDS: mantle melting; sandwich experiments; reversal experiments; anhydrous peridotite melting; thermocouple oxidation; olivine geothermometry 相似文献
6.
7.
Melt Inclusions in Primitive Olivine Phenocrysts: the Role of Localized Reaction Processes in the Origin of Anomalous Compositions 总被引：9，自引：1，他引：8
DANYUSHEVSKY LEONID V.; LESLIE ROMAN A. J.; CRAWFORD ANTHONY J.; DURANCE PATRICIA 《Journal of Petrology》2004,45(12):2531-2553
Melt inclusions are small portions of liquid trapped by growingcrystals during magma evolution. Recent studies of melt inclusionshave revealed a large range of unusual major and trace elementcompositions in phenocrysts from primitive mantle-derived magmaticrocks [e.g. in high-Fo olivine (Fo > 85 mol %), spinel, high-Anplagioclase]. Inclusions in phenocrysts crystallized from moreevolved magmas (e.g. olivine Fo < 85 mol %), are usuallycompositionally similar to the host lavas. This paper reviewsthe chemistry of melt inclusions in high-Fo olivine phenocrystsfocusing on those with anomalous major and trace element contentsfrom mid-ocean ridge and subduction-related basalts. We suggestthat a significant portion of the anomalous inclusion compositionsreflects localized, grain-scale dissolution–reaction–mixing(DRM) processes within the magmatic plumbing system. The DRMprocesses occur at the margins of primitive magma bodies, wheremagma is in contact with cooler wall rocks and/or pre-existingsemi-solidified crystal mush zones (depending on the specificenvironment). Injection of hotter, more primitive magma causespartial dissolution (incongruent melting) of the mush-zone phases,which are not in equilibrium with the primitive melt, and mixingof the reaction products with the primitive magma. Localizedrapid crystallization of high-Fo olivines from the primitivemagma may lead to entrapment of numerous large melt inclusions,which record the DRM processes in progress. In some magmaticsuites melt inclusions in primitive phenocrysts may be naturallybiased towards the anomalous compositions. The occurrence ofmelt inclusions with unusual compositions does not necessarilyimply the existence of new geologically significant magma typesand/or melt-generation processes, and caution should be exercisedin their interpretation. KEY WORDS: melt inclusions; olivine; geochemistry; mush zones; MORB; subduction-related magmas 相似文献
8.
Late-and postglacial environments in the northern Barents Sea west of Franz Josef Land 总被引：1，自引：0，他引：1
A lithological and micropaleontological study of three sediment cores from the western Franz Josef Land area, two of them AMS l4 C dated, provides new data on the environmental evolution of the northern Barents Sea during and after the last deglaciation. Glacimarine conditions commenced in the deep Franz Victoria Trough by 13 kyr BP. and then presumably propagated into adjacent inter-island channels of Franz Josef Land. Pulses of Atlantic-derived water occurred during deglaciation and could have accelerated ice-sheet decay. Normal marine environments were established close to 10 kyr BP. Ameliorated conditions are recorded for the interval of approximately 9.5 to 5 kyr BP. After that, more severe environments existed probably associated with heavier sea-ice cover. 相似文献
9.
Petrology and Geochemistry of Boninites from the North Termination of the Tonga Trench: Constraints on the Generation Conditions of Primary High-Ca Boninite Magmas 总被引：14，自引：11，他引：14
We report here a detailed mineralogical, geochemical, and experimentalstudy of a high-Ca boninite suite from the northern terminationof the Tonga trench. Most samples are strongly olivine porphyriticand show a significant range of phenocryst compositions includinga very refractory olivine-spinel assemblage Fo94–CrN =87. They are also characterized by a wide range of incompatible-elementcontents, e.g., (La/Yb)N varies from 0.5 to 16, whereas compatiblemajor-element concentrations (Al2O3, FeO, CaO, SiO2, and MgO)remain essentially the same. Primary melt compositions for thesuite were established on the basis of an experimental studyof melt inclusions in phenocrysts and numerical modelling ofthe reverse of fractional crystallization. Tongan primary meltsare characterized by high MgO contents (22–24 wt.%) andoriginated in the mantle wedge at pressures of 20–25 kbarand temperatures of 1450–1550 C. H2O contents in primarymelts were estimated from direct measurements of melt inclusionsby ion probe, and range from 2.0 to 1.0 wt.%, and a strong correlationexists between H2O and other incompatible element contents.The primary melts crystallized in the presence of an H2O-richfluid in the temperature range 1390–1150 C and pressuresof 1.7–0.15 kbar. Continuous degassing of melts took placeduring crystallization. Trace-element concentrations in primarymelts were estimated using proton- and ion-probe analyses ofmelt inclusions in olivine, and whole-rock analyses. Our datasuggest that three independent components (D, E1, and E2) wereinvolved. Component D was a refractory mantle depleted in incompatibleelements, likely to be hot ‘dry’ Iherzolite producedby previous melting within a mantle plume. Component E1 wasan H2O-rich fluid containing LILE and Th, and had an H2O/K2Ovalue of 20; it was probably produced by dehydration of thesubducted slab. Component E2 is thought to have been an incompatible-element-enrichedsilicate melt of plume origin. Formation of high-Ca boninitesrequires interaction of hot ‘dry’ residual mantle,associated with plumes, with a subduction-related H2O-bearingcomponent. 相似文献
10.
DANYUSHEVSKY LEONID V.; SOKOLOV SERGUEI; FALLOON TREVOR J. 《Journal of Petrology》2002,43(9):1651-1671
A technique is described for determining the cooling historyof olivine phenocrysts. The technique is based on the analysisof the diffusive re-equilibration of melt inclusions trappedby olivine phenocrysts during crystallization. The mechanismof re-equilibration involves diffusion of Fe from and Mg intothe initial volume of the inclusion. The technique applies toa single crystal, and thus the cooling history of differentphenocrysts in a single erupted magma can be established. Weshow that melt inclusions in high-Fo olivine phenocrysts frommantle-derived magmas are typically partially re-equilibratedwith their hosts at temperatures below trapping. Our analysisdemonstrates that at a reasonable combination of factors suchas (1) cooling interval before eruption (<350°C), (2)eruption temperatures (>1000°C), and (3) inclusion size(<70 µm in radius), partial re-equilibration of upto 85% occurs within 3–5 months, corresponding to coolingrates faster than 1–2°/day. Short residence timesof high-Fo phenocrysts suggest that if eruption does not happenwithin a few months after a primitive magma begins cooling andcrystallization, olivines that crystallize from it are unlikelyto be erupted as phenocrysts. This can be explained by efficientseparation of olivine crystals from the melt, and their rapidincorporation into the cumulate layer of the chamber. Theseresults also suggest that in most cases erupted high-Fo olivinephenocrysts retain their original composition, and thus compositionsof melt inclusions in erupted high-Fo olivine phenocrysts donot suffer changes that cannot be reversed. Short residencetimes also imply that large unzoned cores of high-Fo phenocrystscannot reflect diffusive re-equilibration of originally zonedphenocrysts. The unzoned cores are a result of fast efficientaccumulation of olivines from the crystallizing magma, i.e.olivines are separated from the magma faster than melt changesits composition. Thus, the main source of high-Fo crystals inthe erupted magmas is the cumulate layers of the magmatic system.In other words, olivine-phyric rocks represent mixtures of anevolved transporting magma (which forms the groundmass of therock) with crystals that were formed during crystallizationof more primitive melt(s). Unlike high-Fo olivine phenocrysts,the evolved magma may reside in the magmatic system for a longtime. This reconciles long magma residence times estimated fromthe compositions of rocks with short residence times of high-Foolivine phenocrysts. KEY WORDS: melt inclusions; olivine; picrites; residence time; diffusion 相似文献