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11.
The Petrogenetic Significance of Interstratified High- and Low-Ti Basalts in Central Nicaragua 总被引:3,自引:0,他引:3
WALKER JAMES A.; CARR MICHAEL J.; FEIGENSON MARK D.; KALAMARIDES RUTH I. 《Journal of Petrology》1990,31(5):1141-1164
Basalts erupted from recent volcanoes in central Nicaragua canbe divided into distinct high-and low-Ti suites. Low-Ti basaltshave higher concentrations of LILE and LREE than high-Ti basalts.In addition, low-Ti basalts have obviously higher Ba/La, La/Sm,and 87Sr/86Sr, and lower Ti/Zr, than high-Ti basalts. In contrast,there are no mineralogical or petrographic differences betweenthe two suites. The differences between the high-and low-Ti basalts of centralNicaragua are inherited from their source regions. The primarymagmas of both are generated in the mantle wedge. However, low-Tiprimary magmas come from parts of the wedge which bear a strongsubduction zone signature, including that of subducted pelagicsediment. On the other hand, the primary magmas of the high-Tibasalts are generated in parts of the wedge relatively freeof subduction zone influence. Subducted pelagic sediment can therefore be a key source componentat active continental margins as well as at island arcs. Pelagicsediment could also be responsible for subtle high-field-strengthelement fractionations within subduction zone magmas. The mantlewedge beneath Nicaragua, which is variably modified by the subductingplate, is relatively enriched suboceanic mantle. 相似文献
12.
VINCENT R. RINTERKNECHT IRINA E. PAVLOVSKAYA PETER U. CLARK GRANT M. RAISBECK FRANCOISE YIOU EDWARD J. BROOK 《Boreas: An International Journal of Quaternary Research》2007,36(3):307-313
We measured 10Be concentrations in boulders collected from the Orsha and Braslav moraines, associated with the Last Glacial Maximum extent and a recessional stage of the Scandinavian Ice Sheet (SIS), respectively, providing a direct dating of the southeastern sector of the ice-sheet margin in Belarus. By combining these data with selected existing radiocarbon ages, we developed a chronology for the last deglaciation of Belarus. The northeastern part of the country remained ice free until at least 19.2±0.2 cal. kyr BP, whereas the northwestern part of the country was ice free until 22.3±1.5 cal. kyr BP. A lobate ice margin subsequently advanced to its maximum extent and deposited the Orsha Moraine. The ice margin retreated from this moraine at 17.7±2.0 10Be kyr to a position in the northern part of the country, where it deposited the Braslav Moraine. Subsequent ice-margin retreat from that moraine at 13.1±0.5 10Be kyr represented the final deglaciation of Belarus. Direct dating of these moraines better constrains the relation of ice-margin positions in Belarus to those in adjacent countries as well as the SIS response to climate change. 相似文献
13.
Mineral Chemistry of Peridotites from Paleozoic, Mesozoic and Cenozoic Lithosphere: Constraints on Mantle Evolution beneath Eastern China 总被引:24,自引:0,他引:24
ZHENG JIANPING; GRIFFIN WILLIAM L.; O'REILLY SUZANNE Y.; YANG JINSUI; LI TIANFU; ZHANG MING; ZHANG RUTH Y.; LIOU JOHN G. 《Journal of Petrology》2006,47(11):2233-2256
Major- and trace-element data on the constituent minerals ofgarnet peridotite xenoliths hosted in early Paleozoic (457500Ma) kimberlites and Neogene (1618 Ma) volcanic rockswithin the North China Craton are compared with those from thepre-pilot hole of the Chinese Continental Scientific DrillingProject (CCSD-PP1) in the tectonically exhumed Triassic (220Ma) Sulu ultrahigh-pressure (UHP) terrane along its southernmargin. PT estimates for the Paleozoic and Neogene peridotitexenoliths reflect different model geotherms corresponding tosurface heat flows of 40 mW/m2 (Paleozoic) and 80 mW/m2 (Neogene).Garnet peridotite xenoliths or xenocrysts from the Paleozoickimberlites are strongly depleted, similar to peridotites fromother areas of cratonic mantle, with magnesium olivine (meanFo92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet(and spinel) peridotite xenoliths hosted in Neogene basaltsare derived from fertile mantle; they have high Al2O3 and TiO2contents, low-Mg-number olivine (mean Fo89.5), low-Cr garnetand diopside with flat rare earth element (REE) patterns. Thedifferences between the Paleozoic and Neogene xenoliths suggestthat a buoyant refractory lithospheric keel present beneaththe eastern North China Craton in Paleozoic times was at leastpartly replaced by younger, hotter and more fertile lithosphericmantle during MesozoicCenozoic times. Garnet peridotitesfrom the Sulu UHP terrane have less magnesian olivine (Fo91.5),and lower-Cr garnet than the Paleozoic xenoliths. The diopsideshave low heavy REE (HREE) contents and sinusoidal to light REE(LREE)-enriched REE patterns. These features, and their highMg/Si and low CaO and Al2O3 contents, indicate that the CCSD-PP1peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experiencedcomplex metasomatism by asthenosphere-derived melts or fluidsin Mesoproterozoic, and subsolidus re-equilibration involvingfluids/melts derived from the subducted Yangtze continentalcrust during UHP metamorphism in the early Mesozoic. Tectonicextension of the subcontinental lithospheric mantle of the NorthChina Craton and exhumation of the Sulu UHP rocks in the earlyMesozoic induced upwelling of the asthenosphere. Peridotitessampled by the Neogene basalts represent newly formed lithospherederived by cooling of the upwelling asthenospheric mantle inJurassicCretaceous and Paleogene time. KEY WORDS: garnet peridotite xenoliths; North China Craton; lithospheric thinning; Sulu UHP terrane; UHP lithosphere evolution; mantle replacement 相似文献
14.
ATLE NESJE SVEIN OLAF DAHL HENRIETTE LINGE COLIN K. BALLANTYNE DANNY MCCARROLL EDWARD J. BROOK GRANT M. RAISBECK FRANCOISE YIOU 《Boreas: An International Journal of Quaternary Research》2007,36(3):227-239
Blockfields, weathering boundaries and marginal moraines have been mapped along a longitudinal transect from northern Andøya to Skånland in northern Norway. The degree of rock-surface weathering above and below glacial trimlines, clay-mineral assemblages and surface exposure dating based on in situ cosmogenic 10 Be have been used to reconstruct the vertical dimensions and timing of the Last Glacial Maximum (LGM) of the Scandinavian Ice Sheet in this region. The cosmogenic exposure dates suggest that the lower blockfield boundary/trimline along the Andøya-Skånland transect represents the upper limit of the Late Weichselian ice sheet, with an average surface gradient of c . 9.5 m/km. The surface exposure dates from Andøya pre-date the LGM, suggesting that the LGM ice sheet did not reach mountain plateaux at northwest Andøya. The results thus support evidence from lake sediment records that the northern tip of Andøya was not covered by the Scandinavian Ice Sheet during the LGM. 相似文献
15.
A Petrogenetic Model for the Origin of the Calc-alkaline suite of Grenada, Lesser Antilles 总被引:2,自引:0,他引:2
Basanitoid, alkal basalt, andesite and dacite compositions fromGrenada, Lesser Antilles, show a differentation trend from nepheline-normative,through hypersthene-normative to quartz-normative. This unusualsequence, in which the liquids migrate through the low pressurethermal divide of olivine-clinopyroxene-plagioclase from criticallyundersaturated compositions, is interpreted from experimentaland chemical data as the result of crystallization of amphiboleunder a hydrous pressure of several kilobars. 相似文献
16.
The Rb/Sr ratios of magmas increase significantly during fractionalcrystallization. Hence, the rate of change of the 87Sr/86Srratio in successive residual liquids increases as crystallizationproceeds. In slowly solidifying igneous complexes the totalchange in 87Sr/86Sr ratio exceeds analytical uncertainty andis manifested in several ways. Whole rock 87Rb/86Sr versus 87Sr/86Sr plots for an entire complexwill produce errorchrons, which may be improved by consideringsmall subsets of the complex. These subsets will show a generalincrease in initial 87Sr/86Sr with increasing average Rb/Srratio. Thus, quenched liquids, mineral ages and fractionatedcumulates from the same complex can show different ages andinitial 87Sr/86Sr ratios. The Murrumbidgee granitoid complex shows parallel isochronsfor geochemically controlled subsets which give an age co-incidentwith the metamorphic aureole while an isochron for the entirecomplex gives an unrealistically old age. Quenched liquids inthe Rosses granitic complex are apparently younger than theirassociated crystal accumulates using conventional isochron interpretations,but this can be reconciled within the magmatic ageing model.Increasing initial 87Sr/86Sr ratios with height within the BushveldComplex need not be due to several independent magmas but resultfrom slow cooling of two magmas having the same initial ratio. 相似文献
17.
Origin of the Pegmatitic Pyroxenite in the Merensky Unit, Bushveld Complex, South Africa 总被引:2,自引:1,他引:2
The genesis of the pegmatitic pyroxenite that often forms thebase of the Merensky Unit in the Bushveld Complex is re-examined.Large (>1 cm) orthopyroxene grains contain tricuspidate inclusionsof plagioclase, and chains and rings of chromite grains, whichare interpreted to have grown by reaction between small, primaryorthopyroxene grains and superheated liquid. This superheatedliquid may have been an added magma or be due to a pressurereduction as a result of lateral expansion of the chamber. Therewould then have been a period of non-accumulation of grains,permitting prolonged interaction with the crystal mush at thecrystalliquid interface. Crystal ageing and grain enlargementof original orthopyroxene grains would ensue. Only after thepegmatitic pyroxenite had developed did another layer of chromiteand pyroxenite, with normal grain size, accumulate above it.Immiscible sulphide liquids formed with the second pyroxenite,but percolated down as a result of their density contrast, evenas far as the footwall anorthosite in some cases. Whole-rockabundances of incompatible trace elements in the pegmatiticpyroxenite are comparable with or lower than those of the overlyingpyroxenite, and so there is no evidence for addition and/ortrapping of large proportions of interstitial liquid, or ofan incompatible-element enriched liquid or fluid in the productionof the pegmatitic rock. Because of the coarse-grained natureof the rock, modal analysis, especially for minor minerals,is unreliable. Annealing has destroyed primary textures, suchthat petrographic studies should not be used in isolation todistinguish cumulus and intercumulus components. Geochemicaldata suggest that the Merensky pyroxenite (both pegmatitic andnon-pegmatitic) typically consists of about 7080% cumulusorthopyroxene and 1020% cumulus plagioclase, with a further10% of intercumulus minerals, and could be considered to bea heteradcumulate. KEY WORDS: Bushveld Complex; Merensky Reef; pegmatitic textures; cumulate processes; heteradcumulates; recrystallization; incompatible trace elements 相似文献
18.
VINCENT R. RINTERKNECHT LESZEK MARKS JAN A. PIOTROWSKI GRANT M. RAISBECK FRANÇOISE YIOU EDWARD J. BROOK PETER U. CLARK 《Boreas: An International Journal of Quaternary Research》2005,34(2):186-191
Rinterknecht, V. R., Marks, L., Piotrowski, J. A., Raisbeck, G. M., Yiou, F., Brook, E. J. & Clark, P. U. 2005 (May): Cosmogenic 10 Be ages on the Pomeranian Moraine, Poland. Boreas , Vol. 34, pp. 186–191. Oslo. ISSN 0300–9483.
We measured the10 Be concentrations in boulders collected from the Pomeranian Moraine in Poland, providing the first direct dating of the southern margin of the Scandinavian Ice Sheet (SIS) in the Polish Lowland. The mean age of 8 10 Be ages of the Pomeranian Moraine in northwestern Poland is 14.30.8 10 Be ka, while in northeastern Poland the mean age of 19 10 Be ages of the moraine is 15.00.5 10 Be ka. Given the excellent agreement between the two age groups, we calculate a mean age of 14.80.4 10 Be ka for final deposition of the Pomeranian Moraine of northern Poland. The age of the Pomeranian Moraine suggests that the southern margin of the SIS was near its maximum extent in Poland at a younger time than previously inferred, and that retreat from the moraine at 14.80.4 10 Be ka probably occurred in response to the onset of the Bølling interstade. 相似文献
We measured the
19.
20.
GRANT HUNTER 《New Zealand geographer》2000,56(2):47-51
There are opportunities for more effectively integrating resource‐based recreation and tourism with goals for primary production and nature conservation across a range of landscapes and land tenure types. ‘Naturalness’ sought for recreation and tourism, like biodiversity, is not confined to the conservation estate, and the ‘recreation estate’ extends into private land. Although it may be argued that ongoing privatisation of land ownership reduces public opportunities for recreation, diversity in ownership, at least at the regional level, enhances the overall spectrum of recreational opportunities. Diversification into recreation and tourism on private land may also help protect biodiversity through such processes as ‘sympathetic management’, as well as by making production land‐use systems more economically robust. 相似文献