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   1708篇
  免费   16篇
  国内免费   127篇
测绘学   62篇
大气科学   94篇
地球物理   415篇
地质学   994篇
海洋学   125篇
天文学   37篇
综合类   4篇
自然地理   120篇
  2024年   15篇
  2023年   38篇
  2022年   46篇
  2021年   67篇
  2020年   161篇
  2019年   89篇
  2018年   119篇
  2017年   177篇
  2016年   114篇
  2015年   133篇
  2014年   230篇
  2013年   354篇
  2012年   214篇
  2010年   5篇
  2009年   1篇
  2007年   2篇
  2006年   4篇
  2005年   11篇
  2004年   12篇
  2003年   8篇
  2002年   21篇
  2001年   4篇
  2000年   7篇
  1999年   6篇
  1998年   5篇
  1997年   5篇
  1996年   1篇
  1995年   1篇
  1990年   1篇
排序方式: 共有1851条查询结果,搜索用时 31 毫秒
991.
This article reports our new interpretations of the depositional environment and provenance of the Dawashan Formation in the Longmuco–Shuanghu–Lancangjiang suture zone (LSLSZ), in the Southern Qiangtang terrane of northern Tibet, in order to gain a better understanding of the Ordovician tectonic evolution of the northern margin of Gondwana. The Dawashan Formation is dominated by greywacke and shale, with interlayered bimodal volcanic rocks that were deposited in a bathyal to abyssal marine basin. The detrital zircons in the greywacke of the Dawashan Formation have peak ages of 550, 988, 1640, and 2500 Ma, indicating a northern Gondwana margin provenance. The bimodal metavolcanic rocks from the Dawashan Formation are dominated by metarhyolite with subordinate metabasalt. The results of zircon LA-ICP-MS U–Pb dating indicate that the metarhyolite formed between 470 and 455 Ma. The metavolcanic samples are bimodal (SiO2 = 45.27–55.05 and 66.09–74.59 wt.%). In comparison, the metabasalt has a wide range of MgO concentrations and Mg# values, contains variable Cr and low Ni concentrations, is depleted in Rb, Ba, and Sr, and is enriched in TiO2, Th, U, Nb, and Ta. Geochemical diagrams show that the metabasalt erupted in an intra-plate environment. The metarhyolites have high SiO2, Th, and U concentrations, low concentrations of MgO, P2O5, Nb, Sr, and Ti, and negative Eu anomalies. The metarhyolites yield negative zircon εHf(t) values (–2.08 to – 4.50) and TCDM model ages of 1436–1567 Ma. The metarhyolites formed from magma derived from the partial melting of old continental crust. These data indicate that the Dawashan Formation records Middle–Upper Ordovician bathyal to abyssal turbidite deposition in a deep-water rift basin at the northern margin of Gondwana.  相似文献   
992.
The Haobugao Zn–Fe deposit is a typical skarn deposit located in the southern part of the Great Xing’an Range that hosts polymetallic mineralization over a large region. The main ore minerals at the deposit include sphalerite, magnetite, galena, chalcopyrite and pyrite, and the main gangue minerals include andradite, grossular garnet, hedenbergite, diopside, ilvaite, calcite and quartz. There are broadly two mineralizing periods represented by the relatively older skarn and younger quartz–sulfide veins. In detail, there are five metallogenic stages consisting of an early skarn, late skarn, oxide, early quartz–sulfide, and late quartz–sulfide–calcite stages. Electron microprobe analyses show that the garnet at the deposit varies in composition from And97.95Gro0.41Pyr1.64 to And30.69Gro66.69Pyr2.63, and pyroxene is compositionally in the diopside–hedenbergite range (i.e. Di90.63Hd8.00Jo1.37–Hd88.98Di4.53Jo6.49). Petrographic observations and electron microprobe analyses indicate that the sphalerite has three generations ([Zn0.93Fe0.08]S–[Zn0.75Fe0.24]S). The Zn associated with the first generation sphalerite replaced Cu and Fe of early xenomorphic granular chalcopyrite (i.e. [Cu1.01Fe1.03]S2–[Cu0.99Fe0.99]S2), and part of the first generation sphalerite is coeval with late chalcopyrite (i.e. [Cu0.96Fe0.99Zn0.03]S2–[Cu1.00Fe1.03Zn0.01]S2). Magnetite has a noticeable negative Ce anomaly (δCe = ∼0.17 to 0.54), which might be a result of the oxidized ore-fluid. Thirty δ34SV-PDB analyses of sulfides from the ore range from −2.3 to −0.1‰ in value, which are indicative of a magmatic source. The δ13C‰ and δ18O‰ values for calcite from the ore formed at quartz–sulfide–calcite stage vary from −9.9 to −5.5‰ and from −4.2 to 1.1‰, respectively, contrasting with δ13C‰ (2.9–4.8‰) and δ18O‰ (9.8–13.9‰) values for calcite from marble. It is suggested that the ore-forming fluid associated with late stage of mineralization was predominantly magmatic in origin with some input of local meteoric water.Molybdenite from the Haobugao deposit defines an isochron age of 142 ± 1 Ma, which is interpreted as the mineralization age being synchronous, within error, with the zircon U–Pb ages of 140 ± 1, 141 ± 2, and 141 ± 1 Ma for granite at the deposit. These data and characteristics of lithology and mineralization further show that the Zn–Fe mineralization is temporally and spatially related to the emplacement of granite in an extensional tectonic setting during the Mesozoic.  相似文献   
993.
994.
The Xishan deposit, located in the western Guangdong Province in South China, is a quartz-vein type W-Sn deposit with an average Sn grade of 0.1–0.4 wt%. The deposit is temporally and spatially associated with Xishan alkali feldspar granite. The W–Sn mineralization is present mainly as veins that are hosted by the granite. In this paper we present new zircon U–Pb age, whole-rock geochemical data, Sr–Nd–Pb–Hf isotopic data and Re–Os age in order to constrain the nature and timing of magmatism and mineralization in the Xishan mining district with implications on geodynamic settings. LA–ICP–MS zircon U–Pb analyses yielded an age of 79.14 ± 0.31 Ma for the alkali feldspar granite, consistent with the molybdenite Re–Os age of 79.41 ± 1.11 Ma. The alkali feldspar granite shows high contents of SiO2 (71.52–76.25 wt%), high total alkalis (Na2O + K2O = 9.35–13.51 wt%), high field strength elements (e.g. Zr = 95.4–116 ppm, Y = 97.1–138 ppm, Nb = 36.1–55.5 ppm, Ga = 97.1–138 ppm), and rare earth elements (total REE = 171.8–194.0 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 3.23–3.82) suggesting that it has the geochemical characteristics of A-type granite and shows an A2 subtype affinity. Sr–Nd isotopes of the alkali feldspar granite show that (87Sr/86Sr)i values range from 0.7111 to 0.7183, and the εNd(t) values and Nd model ages (T2DM) vary from −6.8 to −6.5 and 1414 to 1433 Ma, respectively. The Pb isotopic compositions are variable, with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values ranging from 18.783 to 18.947, 15.709 to 15.722 and 38.969 to 39.244, respectively, indicating that the alkali feldspar granite was derived from a mantle-crust mixed source. In situ Hf isotopic analyses reveal that the alkali feldspar granite has εHf(t) values ranging from −9.69 to −0.04 and two-stage Hf model ages from 1145 Ma to 1755 Ma, indicating that the alkali feldspar granite was formed by the partial melting of Mesoproterozoic crusts of the Cathaysia Block with additions of mantle-derived materials. These results, together with previously presented regional geological relationships, suggest that the formation of the Xishan granite and associated W–Sn mineralization is related to lithospheric extension and asthenospheric upwelling that are attributed to a directional change of Pacific plate motion.  相似文献   
995.
The Donggebi Mo deposit located in NW China is a newly discovered, large, stockwork-type Mo deposit with ore reserves of 441 Mt @ 0.115% Mo. Ore bodies occur along faults and fractures at the external contact zone of a concealed porphyritic granite and volcaniclastic rocks of Gandun Formation, spatially associated with a fine-grained granite. Mo-bearing veins are mainly assemblages of volatile-rich K-feldspar-quartz-oxide, K-feldspar-quartz, polymetallic sulfides and calcite-quartz. Zircon LA-ICP-MS U–Pb dating yielded concordant ages of 234.6 ± 2.7 Ma and 231.8 ± 2.4 Ma for the porphyritic granite and the fine-grained granite, respectively; molybdenite Re–Os dating gave an isochron age of 234.0 ± 2.0 Ma. These ages further confirm an important and extensive magmatic-metallogenic event in Eastern Tianshan during the Triassic Indosinian orogeny. Whole-rock major and trace element analyses indicate that the granitic rocks associated with Mo mineralization are high in Si, K, Rb, Th, Nb, Ta, Ga and LREE, but low in P, Ti, Sr and Ba, belonging to high-K calc-alkaline granites with A-type features. Magma was likely derived from the re-melting of thickened lower crust in a post-collision compression environment in the Late Permian, experienced strong crystal fractionation and formed the large Donggebi Mo deposit under an intra-plate extension setting in the Early to Middle Triassic.  相似文献   
996.
The Tieshan Fe–Cu deposit is located in the Edong district, which represents the westernmost and largest region within the Middle–Lower Yangtze River Metallogenic Belt (YRMB), Eastern China. Skarn Fe–Cu mineralization is spatially associated with the Tieshan pluton, which intruded carbonates of the Lower Triassic Daye Formation. Ore bodies are predominantly located along the contact between the diorite or quartz diorite and marbles/dolomitic marbles. This study investigates the mineral chemistry of magnetite in different skarn ore bodies. The contrasting composition of magnetite obtained are used to suggest different mechanisms of formation for magnetite in the western and eastern part of the Tieshan Fe–Cu deposit. A total of 178 grains of magnetite from four magnetite ore samples are analyzed by LA–ICP–MS, indicating a wide range of trace element contents, such as V (13.61–542.36 ppm), Cr (0.003–383.96 ppm), Co (11.12–187.55 ppm) and Ni (0.19–147.41 ppm), etc. The Ti/V ratio of magnetite from the Xiangbishan (western part of the Tieshan deposit) and Jianshan ore body (eastern part of the Tieshan deposit) ranges from 1.32 to 5.24, and 1.31 to 10.34, respectively, indicating a relatively reduced depositional environment in the Xiangbishan ore body. Incorporation of Ti and Al in magnetite are temperature dependent, which hence propose that the temperature of hydrothermal fluid from the Jianshan ore body (Al = 3747–9648 ppm, with 6381 ppm as an average; Ti = 381.7–952.0 ppm, with 628.2 ppm as an average) was higher than the Xiangbishan ore body (Al = 2011–11122 ppm, with 5997 ppm as an average, Ti = 302.5–734.8, with 530.8 ppm as an average), indicating a down–temperature precipitation trend from the Jianshan ore body to the Xiangbishan ore body. In addition, in the Ca + Al + Mn versus Ti + V diagram, magnetite is plotted in the skarn field, consideration with the ternary diagram of TiO2–Al2O3–MgO, proposing that the magnetite ores are formed by replacement, instead of directly crystallized from iron oxide melts, which provide a better understanding regarding the composition of ore fluids and processes responsible for Fe mineralization in the Tieshan Fe–Cu deposit.  相似文献   
997.
Fish remains from over 100 localities in the Upper Silurian to Lower Devonian (traditional Lower Old Red Sandstone: LORS: Přidolí–Pragian) of Wales and the Welsh Borderland Anglo-Welsh Basin, southwest Britain have been investigated. Work on microfossils of fish (‘microvertebrates’, generally <5–8 mm) is reviewed, covering agnathan thelodonts, heterostracans, cephalaspids, anaspids, and gnathostomes including acanthodians, placoderms, and chondrichthyans, including the first from Pembrokeshire. Scales of the following taxa are newly identified: acanthodians Euthacanthus sp., Nostolepis musca, Parexus recurvus and Cheiracanthoides sp. cf. C. rarus; early “sharks” including Altholepis sp.; and a (?)radotinid placoderm. Species ranges in space, time and environment reveal interesting patterns, the most significant being a wide geographic distribution, which does not support a wholly freshwater provenance for the Anglo-Welsh Basin; endemic taxa are few. Using the International mid-Palaeozoic Microvertebrate zonal scheme, the presence of a Thelodus parvidensParalogania ludlowiensis-osteostracan Assemblage within the Ludlow Bonebed at the base of the former Downton Group (now in part in the new lithostratigraphically defined Daugleddau Group) supports a basal Přidolí age for the member. A mid-Přidolí dearth with few taxa, mainly acanthodians and cephalaspids is followed by an upper Přidolí TrimerolepisParalogania kummerowiLoganellia cuneata-poracanthodid-Toombsaspis pococki Assemblage. The Silurian–Devonian boundary is equated with the appearance of Turinia pagei and associated taxa including Phialaspis symondsi at a level about 30 m below the local Chapel Point Limestone. This biozone can be correlated across the Old Red Sandstone continent. All vertebrate species including newly recognised Lochkovian chondrichthyans indicate marine environments were present in the LORS.  相似文献   
998.
New zircon LA-ICP-MS U–Pb age, zircon Hf isotope, and whole-rock major and trace elemental data of the Late Cretaceous Ageledaban complex in the Karakorum Terrane (KKT), northwest Tibet, provide new constraints on the tectonic processes of the collision and thickening of the terrane between the Lhasa and Qiangtang terranes. The granitoids from the Ageledaban complex have a variable SiO2 content, from 62.83 to 73.35 wt.% and A/CNK<1.1 (except for YM61-2). They have rare earth element and trace element patterns that are enriched in light rare earth elements, Rb, Pb, Th, and U, and are depleted in Ba, P, Sr, Ti, and Nb, indicative of weakly peraluminous-metaluminous I-type affinity. Zircon U–Pb dating reveals that the Ageledaban complex was emplaced at ca. 80 Ma. Zircons from the monzogranite and monzonite samples with concordant 206Pb/238U ages about 80 Ma have a zircon εHf(t) of ?6.6 to ?1.1, corresponding to the Mesoproterozoic Hf crustal model ages (TDMC = 1.2–1.6 Ga); the remaining inherited zircons from the monzonite with concordant 206Pb/238U ages of about 108.1 Ma have εHf(t) values that range from ?8.3 to ?5.0, corresponding to the Mesoproterozoic Hf crustal model ages with an average of 1.6 Ga. These signatures indicate that the Ageledaban granitoids may have been derived from the partial melting of a mixed mantle-crust source. Together with the age and geochemical data in the literature, we propose that the collisional event in the KKT in northwestern Tibet would postdate the northern Lhasa–southern Qiangtang collision, which occurred first in the Amdo in the east and later in the Shiquanhe in central Tibet. Our results support the previous view that the collision of the Bangong–Nujiang suture zone (BNSZ) may be diachronous.  相似文献   
999.
Through the ?zmir–Ankara–Erzincan and the Vardar oceans suture zones, convergence between the Eurasian and African plates played a key role in controlling Palaeogene magmatism in northwestern Anatolia, northern Aegean, and eastern Balkans. LA-ICP-MS dating of U and Pb isotopes on zircon separates from the tuffs of the Harmankaya Volcanic Rocks, which are inter-fingered with the lower-middle Eocene deposits of the Gaziköy Formation to the north of the Ganos Fault and the Karaa?aç Formation in the Gelibolu Peninsula, yielded a late Ypresian (51 Ma) age. The chemical characteristics suggest that the lavas and tuffs of the Harmankaya Volcanic Rocks are products of syn- or post-collision magmas. These volcanic rocks show also close affinities to the subduction-related magmas. In addition to the already known andesitic volcanic rocks, our field observations in Gökçeada Island indicate also the existence of granitic and rhyolitic rocks (Marmaros Magmatic Assemblage). Our U–Pb zircon age data has shown that the newly discovered Marmaros granitic plutons intruded during late Oligocene (26 Ma) into the deposits of the Karaa?aç Formation in Gökçeada Island. LA-ICP-MS dating of U and Pb isotopes on zircon separates from the Marmaros rhyolitic rocks yielded a late Oligocene (26 Ma) crystallization age. Geochemical characteristics indicate that the more-evolved Oligocene granitic and rhyolitic rock of the Marmaros Magmatic Assemblage possibly assimilated a greater amount of crustal material than the lower Eocene Harmankaya Volcanic Rocks. Geochemical features and age relationships suggest increasing amounts of crustal contamination and a decreasing subduction signature during the evolution of magmas in NW Turkey from the early Eocene to the Oligocene. The magmatic activity developed following the northward subduction of the ?zmir–Ankara–Erzincan oceanic lithosphere and the earliest Palaeocene final continental collision between the Sakarya and Anatolide–Tauride zones.  相似文献   
1000.
The latest Carboniferous to lower Permian volcanism of the southern Variscides in Sardinia developed in a regional continental transpressive and subsequent transtensile tectonic regime.Volcanism produced a wide range of intermediate-silicic magmas including medium-to high-K calc-alkaline andesites,dacites,and rhyolites.A thick late Palaeozoic succession is well exposed in the four most representative Sardinian continental basins(Nurra,Perdasdefogu,Escalaplano,and Seui-Seulo),and contains substantial stratigraphic,geochemical,and geochronological evidence of the area's complex geological evolution from the latest Carboniferous to the beginning of the Triassic.Based on major and trace element data and LA-ICP-MS U-Pb zircon dating,it is possible to reconstruct the timing of postVariscan volcanism.This volcanism records active tectonism between the latest Carboniferous and Permian,and post-dates the unroofing and erosion of nappes in this segment of the southern Variscides.In particular,igneous zircon grains from calc-alkaline silicic volcanic rocks yielded ages between299±1 and 288±3 Ma,thereby constraining the development of continental strike-slip faulting from south(Escalaplano Basin)to north(Nurra Basin).Notably,andesites emplaced in medium-grade metamorphic basement(Mt.Cobingius,Ogliastra)show a cluster of older ages at 332±12 Ma.Despite the large uncertainty,this age constrains the onset of igneous activity in the mid-crust.These new radiometric ages constitute:(1)a consistent dataset for different volcanic events;(2)a precise chronostratigraphic constraint which fits well with the biostratigraphic data and(3)insights into the plate reorganization between Laurussia and Gondwana during the late Palaeozoic evolution of the Variscan chain.  相似文献   
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