首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   904篇
  免费   35篇
  国内免费   3篇
测绘学   31篇
大气科学   51篇
地球物理   221篇
地质学   346篇
海洋学   69篇
天文学   149篇
综合类   2篇
自然地理   73篇
  2021年   8篇
  2020年   14篇
  2019年   13篇
  2018年   17篇
  2017年   24篇
  2016年   22篇
  2015年   21篇
  2014年   22篇
  2013年   49篇
  2012年   24篇
  2011年   45篇
  2010年   38篇
  2009年   61篇
  2008年   37篇
  2007年   31篇
  2006年   39篇
  2005年   43篇
  2004年   36篇
  2003年   25篇
  2002年   22篇
  2001年   19篇
  2000年   19篇
  1999年   9篇
  1998年   13篇
  1997年   12篇
  1996年   7篇
  1995年   10篇
  1994年   11篇
  1993年   7篇
  1992年   8篇
  1991年   9篇
  1990年   8篇
  1989年   10篇
  1988年   9篇
  1987年   14篇
  1986年   8篇
  1985年   13篇
  1984年   20篇
  1983年   19篇
  1982年   9篇
  1981年   11篇
  1980年   7篇
  1979年   8篇
  1978年   5篇
  1977年   5篇
  1976年   5篇
  1975年   5篇
  1958年   6篇
  1950年   5篇
  1948年   6篇
排序方式: 共有942条查询结果,搜索用时 15 毫秒
871.
EDITORIAL   总被引:1,自引:0,他引:1  
  相似文献   
872.
David R. Nelson 《Lithos》1992,28(3-6):403-420
The potassic igneous rock suite (with molar K2O/Na2O > 1) can be divided into an “orogenic” subgroup that occur in subduction-related tectonic settings and an “anorogenic” sub-group that are confined to stable continental settings. Representatives of both sub-groups possess trace element and isotopic features consistent with the contamination of their magma sources by incompatible element rich and isotopically evolved “metasomatic” components. It is argued here that these metasomatic components are principally derived from subducted lithosphere, including subducted sediments. Most examples of orogenic potassic magmatism (e.g. Italian potassic rocks, Spanish lamproites, Sunda arc leucitites) have trace-element and Sr, Nd and Pb isotopic characteristics consistent with the contamination of their mantle sources by a component derived from marine sediments. Anorogenic sub-group potassic magmas have generally similar incompatible trace element and Sr and Nd isotopic characteristics to those of orogenic potassic magmas, but many examples have unusual Pb isotopic compositions with unradiogenic 206Pb/204Pb. Modern marine sediments characteristically have low U/Pb ratios and the unradiogenic 206Pb/204Pb of anorogenic potassic magmas may have evolved during long-term storage of subducted sediments (or components derived from them) within the subcontinental lithosphere. These unusual Pb isotopic compositions require substantial time periods (> 1 Ga) to have elapsed between the fractionation events lowering the U/Pb ratio (i.e. erosion and sedimentation at the Earth's surface) and subsequent potassic magmatism and it is therefore not surprising that most examples of anorogenic potassic magmatism are not associated with recent subduction processes. Although the eruption of potassic magmas is commonly related to rifting or hotspot activity, these processes do not necessarily play an important role in the genesis of the unusual sources from which potassic magmas are derived.  相似文献   
873.
The highly alkaline springs of the Maqarin area of northern Jordan are currently under examination as part of an international project testing the models used to analyse the safety of repositories for low and intermediate level radioactive waste (L/ILW). The Maqarin area contains a rock-groundwater system which is an ideal natural analogue of a concrete-filled L/ILW repository emplaced in a sedimentary host rock. The high pH (12.5) groundwaters at this site are the product of interaction with naturally occurring cement minerals and not of the alteration of ultramafic minerals. Stable isotope data for the hyperalkaline groundwaters lie below both the local meteoric water line and the eastern Mediterranean water line. At least part of the shift appears to be the result of groundwater interaction with the cement minerals. This paper introduces the site of the natural analogue study and, in presenting novel data on the chemistry and stable isotopic signature of the groundwater, provides background information which is used in a companion paper (Alexander et al., 1992) on testing the predictive capabilities of geochemical thermodynamic codes (and their associated databases) which will be utilised as part of the safety assessment of a L/ILW repository.  相似文献   
874.
The south-eastern Bohemian Massif consolidated during the Late Variscan orogeny by the oblique collision of two continental crustal blocks after closure of an oceanic realm. One microcontinent comprises portions which are now distributed among Moravian and Moldanubian units and which are characterized by Late Proterozoic tectonothermal events, especially by granitoid intrusions. The other microcontinent includes the Gföhl gneiss and granulites (Gföhl nappe) of probable Early Palaeozoic protolith ages. Both continental blocks are separated by an ophiolite-like assemblage, which is preserved in portions of the Raabs unit.Oblique crustal stacking is accompanied by north-eastward propagation of nappes in a dextral transpressive regime. Exhumation of previously thickened crust is achieved by equally oriented bulk extension but partitioned in distinct displacement paths. Coeval stacking and extension at different crustal levels is suggested.Correspondence to: H. Fritz  相似文献   
875.
876.
The Tinto and Odiel rivers drain 100 km from the Rio Tinto sulphide mining district, and join at a 20-km long estuary entering the Atlantic Ocean. A reconnaissance study of heavy metal anomalies in channel sand and overbank mud of the river and estuary by semi-quantitative emission dc-arc spectrographic analysis shows the following upstream to downstream ranges in ppm (μg g?1): As 3,000 to <200, Cd 30 to <0.1, Cu 1,500 to 10, Pb 2,000 to <10, Sb 3000 to <150, and Zn 3,000 to <200. Organic-rich (1.3–2.6% total organic carbon, TOC), sandysilty overbank clay has been analyzed to represent suspended load materials. The high content of heavy metals in the overbank clay throughout the river and estuary systems indicates the importance of suspended sediment transport for dispersing heavy metals from natural erosion and anthropogenic mining activities of the sulfide deposit. The organic-poor (0.21–0.37% TOC) river bed sand has been analyzed to represent bedload transport of naturally-occurring sulfide minerals. The sand has high concentrations of metals upstream but these decrease an order of magnitude in the lower estuary. Although heavy metal contamination of estuary mouth beach sand has been diluted to background levels estuary mud exhibits increased contamination apparently related to finer grain size, higher organic carbon content, precipitation of river-borne dissolved solids, and input of anthropogenic heavy metals from industrial sources. The contaminated estuary mud disperses to the inner shelf mud belt and offshore suspended sediment, which exhibit metal anomalies from natural erosion and mining of upstream Rio Tinto sulphide lode sources (Pb, Cu, Zn) and industrial activities within the estuary (Fe, Cr, Ti). Because heavy metal contamination of Tinto-Odiel river sediment reaches or exceeds the highest levels encountered in other river sediments of Spain and Europe, a detailed analysis of metals in water and suspended sediment throughout the system, and epidemiological analysis of heavy metal effects in humans is appropriate.  相似文献   
877.
Some allochthonous terranes form along active continental margins when slivers of forearc crust (or more extensive crust) are displaced along arc-parallel strike-slip faults. Such faults can be generated or reactivated in response to either oblique subduction or ridge collision (collision between an oceanic spreading ridge and the leading edge of the forearc). The mechanical and thermal effects of ridge collision are important factors in the origin crustal development of some forearc sliver terranes. Some of the effects of ridge collision are well illustrated in the South American forearc near the Chile triple junction (46° S) where the Chile Rise is colliding today. Impingement of the Chile Rise, in conjuction with oblique subduction, has caused an elongate forearc sliver terrane to move northward away from an extensional zone at the collision site. The terrane is bounded on the east by the arc-parallel Liquiñe-Ofqui fault system (LOF) which coincides roughly with the forearc-arc boundary, and on the south by the Golfo de Penas extensional basin. Fault fabrics, recent seismicity, and paleomagnetic results indicate a component of right-lateral strike-slip movement on the LOF. Neotectonic geomorphology and pre- and post-seismic vertical strain data from the 1960 Concepcíon earthquake indicate a west-down dip-slip component of movement. Three-dimensional finite element models of ridge collision in this region substantiate these shear strains and development of an arc-parallel fault at about 150–200 km from the trench.Development of the forearc crust during Miocene and younger collision also involved intrusion of silicic magmas and emplacement of the Pliocene(?) Taitao ophiolite within about 15 km of the trench. The ophiolite and the silicic magmas constitute anomalous additions to the forearc crust, and record tectonic events leading to the origin of the allochthonous terrane carrying them. Similar ophiolite/silicic plutonic associations may help unravel the origin of other allochthonous terranes.  相似文献   
878.
Exploration for Carlin-type gold orebodies in the western United States typically involves sampling and analysis of jasperoid, a distinctive alteration type formed by intense silicification of marine sediments. In this study, rock suites were collected from six orebodies and four similar but barren systems. Jasperoids at all ten systems contain episodically silicified breccias, quartz vein stockworks, elevated As, Sb, Hg, Ba and Tl, and, locally, anomalous Au and Ag. Jasperoids from the four barren systems are as anomalous as jasperoids from the six orebodies in all members of an epithermal geochemical suite, including gold and silver.A database suite containing 272 samples from six of the ten systems was analyzed for 45 elements. Q-mode factor analysis shows that geochemical variance in the epithermal geochemical suite is related to geochemical variance in Li, P, Mn, Ba, Mo, Cr, Co, V, Cd, Ni, U, Zn and Pb. Metalliferous marine black shales are enriched in these elements and are spatially related to Carlin-type deposits.A test suite containing 109 samples from the four remaining systems was analyzed to determine whether a discriminant function derived from the database suite could be applied to exploration. An 8-element function correctly assigned 40 of 42 jasperoid samples collected from the surface at the Carlin and Horse Canyon gold deposits. These results indicate that elements characteristic of metalliferous marine black shales can be used to identify Carlin-type systems with associated gold ore. Gold in Carlin-type orebodies may have been leached from a source-rock sequence which contains metalliferous marine black shales.  相似文献   
879.
The Taitao ophiolite of southern Chile lies 10 km from the buried extension of the Peru-Chile trench, and less than 50 km from the present position of the Nazca/South America/Antarctica triple junction. Plio-Pleistocene radiometric and paleontologic ages indicate its formation during ridge subduction, and an ultramafic rock, gabbro, sheeted dike, volcanic and sedimentary rock psuedostratigraphy suggests formation by typical accretionary processes for oceanic lithosphere. Yet major and trace element data show that mafic dikes and volcanic units are transitional from MORB to IAT, and there are abundant silicic volcanic units of calc-alkaline character that have high LIL element and light REE concentrations relative to oceanic plagiogranites. Sr and Nd isotopic data are consistent with that of modern oceanic suites, even though having a greater internal variability. Silicic volcanic units show the more enriched Sr and depleted Nd isotopic ratios relative to dike and gabbro samples. In addition to chemical distinctions, paleobathymetric data support a shallow water origin for some of the upper volcanic units and, assuming local compensation, suggest crustal thicknesses of continental proportion. In the vicinity of the Taitao ophiolite, and extending some 40 km landward of the plate margin, are a series of silicic stocks, sills, and plutons that were intruded into the forearc at the time of ridge collision and ophiolite generation. These calc-alkaline I-type granitoids are light REE enriched and have Sr and Nd isotopic compositions similar to those of the main volcanic chain 200 km landward. Chemically, some of the silicic intrusions are indistinguishable from volcanic units of the ophiolite. In general, major, trace, REE, and isotopic variations of both the ophiolite and the distributed intrusions are atypical of simple fractionation trends for basaltic liquids. Intermediate to silicic units lie along mixing hyperbolae between Taitao gabbro and either forearc sediment or metamorphic basement on a Nd---Sr correlation diagram, and these two crustal components support, respectively, either a 10–25% or 5–10% assimilation. Shutdown of magmatism, and therefore probably partial melting as well, appears to occur within 40 km of the trench, roughly spanning the depth interval for the disappearance of the plagioclase-lherzolite stability field as the zone of mantle upwelling is overridden by an increasing thickness of continental lithosphere. A deeper and more landward absence of partial melt related to the subducted ridge is supported by the correlation of the shutoff and re-initiation of arc volcanism over the northern and southern trailing edges of the postulated subcontinental asthenospheric window. Here, as well as elsewhere in the circum-Pacific, the general restriction of magmatism related to ridge subduction to near-trench settings supports a shallow (0 to 15 km) shutoff mechanism for adiabatic decompressive melting and a rather abrupt return to single-phase (solid) convective rise of mantle into an evolving asthenospheric window.  相似文献   
880.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号