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991.
Asymmetric aggregates of fine-grained leucoxene and quartz are reported from siliceous L-S tectonites deformed by progressive simple shear. The leucoxene fish, morphologically similar to mica fish, consistently yield shear senses in agreement with other kinematic criteria. We interpret the leucoxene as a pre- or early syntectonic alteration product of detrital heavy-minerals in a siliciclastic protolith. The leucoxene fish behave as passive markers, and they record kinematics by modification of pre-existing aggregates rather than syndeformational mineral growth. For siliceous metasedimentary rocks otherwise lacking in micro-kinematic indicators, leucoxene fish may provide an alternative to quartz c-axis analysis. 相似文献
992.
993.
994.
The basic-ultrabasic Treknattan intrusion is an important example of a late intrusion in a solidified, evolved, layered complex and sheds light on possible mechanisms by which such associations may develop. The Treknattan intrusion, emplaced into the basic Fongen-Hyllingen intrusion shortly after the latter had solidified, consists mainly of massive or weakly layered peridotite (olivine ± Cr-spinel cumulate) and troctolite (plagioclase + olivine ± Cr-spinel cumulate). The mineral compositional range partially overlaps the most primitive end of the much larger variation-interval in the Fongen-Hyllingen intrusion. The margin of the Treknattan intrusion is sometimes outlined by massive feldspathic websterite which appears to have formed by reaction between magma and melts of gabbroic country rock. The parental magma appears to have been a relatively water-rich picritic basalt with a possible genetic relationship to the magma parental to the enveloping Fongen-Hyllingen intrusion, both displaying tholeiitic relationship between olivine and Ca-poor pyroxene, and having crystallized from relatively water-rich magmas with an early crystallization order of olivine ± Cr-spinel-plagioclase-Ca-rich pyroxene. The recognition of the Treknattan intrusion as a separate body suggests that the bulk composition of the Fongen-Hyllingen intrusion is dioritic rather than gabbroic as previously thought. 相似文献
995.
996.
Wolfgang H. Berger Memorie K. Yasuda Torsten Bickert Gerold Wefer 《International Journal of Earth Sciences》1996,85(3):466-495
We provide a reconstruction of atmospheric CO2 from deep-sea sediments, for the past 625000 years (Milankovitch chron). Our database consists of a Milankovitch template
of sea-level variation in combination with a unique data set for the deep-sea record for Ontong Java plateau in the western
equatorial Pacific. We redate the Vostok ice-core data of Barnola et al. (1987). To make the reconstructions we employ multiple
regression between deep-sea data, on one hand, and ice-core CO2 data in Antarctica, on the other. The patterns of correlation suggest that the main factors controlling atmospheric CO2 can be described as a combination of sea-level state and sea-level change. For best results squared values of state and change
are used. The square-of-sea-level rule agrees with the concept that shelf processes are important modulators of atmospheric
CO2 (e.g., budgets of shelf organic carbon and shelf carbonate, nitrate reduction). The square-of-change rule implies that, on
short timescales, any major disturbance of the system results in a temporary rise in atmospheric CO2. 相似文献
997.
The focus of this research was to determine if abandoned mines constitute a major environmental hazard in the Black Hills. Many abandoned gold mines in the Black Hills contribute acid and heavy metals to streams. In some areas of sulfide mineralization local impacts are severe, but in most areas the impacts are small because most ore deposits consist of small quartz veins with few sulfides. Pegmatite mines appear to have negligible effects on water due to the insoluble nature of pegmatite minerals. Uranium mines in the southern Black Hills contribute some radioactivity to surface water, but the impact is limited because of the dry climate and lack of runoff in that area. 相似文献
998.
The age and thermal history of Cerro Rico de Potosi, Bolivia 总被引:1,自引:0,他引:1
C. G. Cunningham R. E. Zartman E. H. McKee R. O. Rye C. W. Naeser O. Sanjinés V G. E. Ericksen F. Tavera V 《Mineralium Deposita》1996,31(5):374-385
Cerro Rico de Potosi, Bolivia, is the world’s largest silver deposit and has been mined since the sixteenth century for silver,
and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily
of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and
explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite,
and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena,
lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage
in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger
magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided
its initial emplacement. The dome was extruded at 13.8±0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and
alteration occurred within about 0.3 my of dome emplacement, as indicated by a 40Ar/39Ar date of 13.76±0.10 Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage,
high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5±1.1 Ma
(1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around
11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and widespread
volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing
events; the exceptionally flat thermal release pattern of 39Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough,
and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores
indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show
the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal
system.
Received: 14 October 1995/Accepted: 29 January 1996 相似文献
999.
Ordering and exsolution processes in Or-rich alkali feldspar megacrysts from the Eldzhurtinskiy granite (Caucasus) 总被引:3,自引:0,他引:3
G. Witt-Eickschen C. Evangelakakis H. A. Seck H. Kroll A. G. Gurbanov 《Contributions to Mineralogy and Petrology》1996,124(1):71-81
The extremely young (2.5 Ma) I-type Eldzhurtinskiy granite complex (Central Caucasus) is uniform with respect to modal composition,
major and trace element chemistries of bulk rocks and mineral phases. In contrast, it reveals two types of alkali feldspar
megacrysts differing in tetrahedral Al-content (2t1) and exsolution microtextures:
1. Alkali feldspar megacrysts (Or70An2Ab28) from the top of the body consist of ideally coherent intergrowths of fine-scale regular Or- and Ab-rich lamellae. The exsolved
K-feldspar host is monoclinic (2t1=0.7), the exsolved Na-rich phase consists of Albite- and/or Pericline-twinned albite.
2. Megacrysts from greater depths have the same bulk composition, but the exsolved Ab-rich phase occurs in the form of optically
visible, broad lamellae and patches of low albite. In addition, the K-rich host yields a higher degree of (Al, Si) ordering
(2t1=0.8). The evolution of the distinct types of megacrysts reflects differences in the cooling history within the upper and
lower part of the granite body. The occurrence of the coherent lamellae in the megacrysts from the top of the body is attributed
to exsolution under dry conditions during fast cooling, whereas coarsening of lamellae and formation of albite patches in
the megacrysts from the lower part are caused by fluid-feldspar interaction. The transition zone in the body between the two
types of megacrysts is sharp (in a depth interval of 100–200 m) and not related to shear zones.
Received: 12 June 1995 / Accepted: 29 January 1996 相似文献
1000.
M. H. Ort Beatriz L. Coira Mario M. Mazzoni 《Contributions to Mineralogy and Petrology》1996,123(3):308-322
Cerro Panizos, a large caldera in the central Andes Mountains, produced two large dacitic ignimbrites at 7.9 Ma and 6.7 Ma
and many andesitic and dacitic lava flows and domes. The older rhyodacitic Cienago Ignimbrite represents the most silicic
magma erupted by the system. The younger, much larger volume dacitic Cerro Panizos Ignimbrite is very crystal-rich, containing
up to 50% biotite, plagioclase, and quartz crystals in the pumice. It is weakly zoned, with most of the zoning apparent between
two main cooling units. Major and most trace elements show little variation through the Cerro Panizos Ignimbrite, but the
small range of composition is consistent with typical fractionation trends. Sr, Nd, and Pb isotopic ratios are very “crustal”,
with initial 87Sr/86Sr values of 0.711 to 0.715, ɛNd values of –7.5 to –10.2, and nearly invariant Pb isotopic ratios (206Pb/204Pb=18.85, 207Pb/204Pb=15.67, and 208Pb/204Pb=38.80). The limited zonation observed in the Cerro Panizos Ignimbrite is explained by impeded crystal settling due to high
crystal content. The magma body was a crystal-liquid mush before ascent to the pre-eruption crustal levels. Crystals formed,
but did not separate easily from the magma. Limited fractionation of plagioclase and biotite may have occurred, but the composition
was largely controlled by lower crustal MASH processes. AFC modeling shows that the Cerro Panizos magmas resulted from a mixture
of roughly equal proportions of late Miocene mantle-derived basalts and melts from ∼1.0 Ga (Grenville age) lower crust. This
occurred in a MASH zone in the lower crust, and set the crustal isotopic ratios observed in the Cerro Panizos magmas. The
great thickening of the crust beneath the central Andes Mountains sent upper and middle crustal rock types to lower crustal
(and deeper) depths, and this explains the “upper crustal” isotopic signatures of the Cerro Panizos rocks. Minor upper crustal
assimilation of early Miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system. The
nearly invariant high Pb isotopic values and high Pb concentrations indicate that Pb came almost entirely from the crustal
source, and was little altered by any subsequent upper crustal assimilation. This Pb signature is isotopically similar to
that of the southern Bolivian Tin Belt, suggesting a widely distributed Pb source. The great difference between compositions
of Miocene and Quaternary central Andean volcanic rocks is explained by crustal thickening in early Miocene time leading to
abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions. The lower crust dried
out considerably by Quaternary time, so less crustal component is present.
Received: 22 December 1994 / Accepted: 13 September 1995 相似文献