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   84篇
  免费   12篇
  国内免费   35篇
测绘学   1篇
大气科学   2篇
地球物理   3篇
地质学   98篇
海洋学   12篇
天文学   1篇
综合类   8篇
自然地理   6篇
  2024年   1篇
  2023年   4篇
  2022年   6篇
  2021年   3篇
  2020年   10篇
  2019年   5篇
  2018年   7篇
  2017年   4篇
  2016年   3篇
  2015年   4篇
  2014年   12篇
  2013年   3篇
  2012年   7篇
  2011年   4篇
  2010年   1篇
  2009年   6篇
  2008年   3篇
  2007年   5篇
  2006年   5篇
  2005年   3篇
  2004年   2篇
  2003年   4篇
  2002年   5篇
  2001年   8篇
  2000年   3篇
  1999年   5篇
  1998年   1篇
  1997年   1篇
  1996年   3篇
  1993年   2篇
  1988年   1篇
排序方式: 共有131条查询结果,搜索用时 15 毫秒
41.
The Tabletop Domain of the Rudall Province has been long thought an exotic entity to the West Australian Craton. Recent re-evaluation of this interpretation suggests otherwise, but is founded on limited data. This study presents the first comprehensive, integrated U–Pb geochronology and Hf-isotope analysis of igneous and metasedimentary rocks from the Tabletop Domain of the eastern Rudall Province. Field observations, geochronology and isotope results confirm an endemic relationship between the Tabletop Domain and the West Australian Craton (WAC), and show that the Tabletop Domain underwent a similar Archean–Paleoproterozoic history to the western Rudall Province. The central Tabletop Domain comprises Archean–Paleoproterozoic gneissic rocks with three main age components. Paleo–Neoarchean (ca 3400–2800 Ma) detritus is observed in metasedimentary rocks and was likely sourced from the East Pilbara Craton. Protoliths to mafic gneiss and metasedimentary rocks are interpreted to have been emplaced and deposited during the early Paleoproterozoic (ca 2400–2300 Ma), and exhibit age and isotopic affinities to the Capricorn Orogen basement (Glenburgh Terrane). Mid–late Paleoproterozoic mafic and felsic magmatism (ca 1880–1750 Ma) is assigned to the Kalkan Supersuite, which is exposed in the western Rudall Province. The Kalkan Supersuite provided the main source of detritus for mid–late Paleoproterozoic metasedimentary rocks in the Tabletop Domain. Similarities in the age and Hf-isotope compositions of detrital zircon from these metasedimentary rocks and Capricorn Orogeny basin sediments suggests that a regionally extensive, linked basin system may have spanned the northern WAC at this time. The Tabletop Domain records evidence for two metamorphic events. Mid–late Paleoproterozoic deformation (ca 1770–1750 Ma) was high-grade, regional and involved the development of gneissic fabrics. In contrast, early Mesoproterozoic (ca 1580 Ma) high-grade deformation was localised and associated with more widespread, late-stage, greenschist facies alteration. These new findings highlight that the Tabletop Domain experienced a much higher grade of deformation than previously assumed, with a Paleoproterozoic metamorphic history similar to that of the western Rudall Province.  相似文献   
42.
Paleoproterozoic to Mesoproterozoic cratonic basins in the São Francisco Craton are classified into three megasequences: Statherian, Calymmian–early Ectasian, and Stenian–early Tonian sequences. The location, depositional age, and provenance of these sequences are reflected in their detrital zircon U–Pb age distributions. A minimum of five detrital zircon age distributions are distinguishable: i) the Archean and Rhyacian distribution, which is restricted to the Calymmian interval at Chapada Diamantina; ii) the Rhyacian distribution, which occurs in the Statherian, Calymmian, and Stenian intervals and generally precedes the Proterozoic distribution; iii) the Proterozoic distribution, which always succeeds the Rhyacian distribution and appears in the Brasília Belt and Southern Espinhaço; iv) the mixed Archean and Proterozoic distribution, restricted to the Stenian interval at the Southern Brasília Belt; and v) the Statherian distribution, which is present in the Statherian interval at Espinhaço Basin and is recycled into the Calymmian and Stenian intervals at Chapada Diamantina. Based on the Hf isotope compositions of the detrital zircons, most of the sequences are predominantly sourced from Archean juvenile terrains and from Archean and Paleoproterozoic evolved terrains. The exception is the Paranoá Group, western São Francisco Craton, whose positive ɛHf(t) values suggest juvenile Paleoproterozoic sources such as the Goiás Massif. The Archean zircons from Chapada Diamantina have Hf depleted mantle model ages ranging from Paleoarchean to Neoarchean and are mainly derived from the Gavião Block within northeastern São Francisco Craton. The model unify a dataset spanning hundreds of millions of years of the depositional record over a large area of central and eastern Brazil. The records will assist researchers in unraveling the sedimentary and tectonic history of the Archean and Paleoproterozoic continents from new data collected regionally and in other continents.  相似文献   
43.
To constrain the provenance of the Ordos Basin and the evolution history of the Qinling Orogen Belt from the Triassic to the Jurassic, 10 samples from the Dongsheng area and 28 samples from the Yan’an area were analyzed for U–Pb ages and Lu–Hf and Sm–Nd isotopic compositions. The results indicate that Middle Jurassic sediments in the Dongsheng area were derived from the Khondalite Belt, Langshan Mountain and the Yinshan Terrane. Mesozoic sediments in the Yan’an area consist of two parts. One part is derived from the North China Craton (NCC), which has U–Pb age groups of ∼1.8 Ga and ∼2.5 Ga, and Hf model ages of ∼2.8 Ga. The other part is derived from the Qilian–Qinling Orogenic Belt, which has U–Pb age groups of 600–1500 Ma and 100–500 Ma, and Nd and Hf isotopic model ages of less than 2.2 Ga. Combining the U–Pb ages with the Hf and Nd isotopic model ages, Mesozoic detrital zircons with U–Pb age groups of ∼1.8 Ga and ∼2.5 Ga in the Yan’an area are found to also be derived from the Khondalite Belt, Langshan Mountain and the Yinshan Terrane, not from the Trans-China Orogen Belt. From the late–Late Triassic sediments of the Yan’an area, the low average values of the Hf (2.03 Ga) and Nd (2.03 Ga) model ages and the characteristic age population of 600–1500 Ma reveal that the main collision or continental subduction between the NCC and the South China Craton (SCC) occurred in the late–Late Triassic. After the main collision or continental subduction, the proportion of sediments from the Qinling–Qilian Orogenic Belt began to decrease (recorded in the early Jurassic samples), which may be in response to the gradual slowing of the uplift speed of the Qinling Orogenic Belt. In the early-middle Jurassic, the sediments have a main U–Pb age population of 100–500 Ma, low detrital zircon Hf model ages (average value is 1.17 Ga) and low whole rock Nd model ages (average value is 1.13 Ga), which suggests that the Qilian–Qinling Orogenic Belt may have a fast uplift history in the early-middle Jurassic.  相似文献   
44.
本文分析测试了骑田岭花岗岩芙蓉锡矿的各类花岗岩的锆石U-Pb同位素年龄和Lu-Hf同位素组成。本区有两期花岗岩,早期角闪石黑云母花岗岩锆石U-Pb谐和年龄为160.02.7 Ma,晚期黑云母花岗岩的锆石U-Pb谐和年龄为156.5±1.8 Ma。在晚期矿化的黑云母花岗岩中出现了蜕晶化锆石,蜕晶化锆石强烈富集LREE和U、Th,是典型热液锆石的特征。四个花岗岩锆石的Lu-Hf同位素组成说明骑田岭花岗岩的成岩物质主要来源于下地壳,它们的原始岩浆源于中元古代(1.3~1.5Ga)下地壳物质的部分熔融,并有部分地幔物质的参与。还分析测定了骑田岭芙蓉锡矿黄铁矿、石榴子石流体包裹体的He-Ar同位素组成,其3 He/4 He比值较低(0.059~0.432),介于地壳和地幔流体3 He/4 He比值之间,表明芙蓉锡矿的成矿流体主要来源于地壳,同时有部分地幔流体的参与成矿。  相似文献   
45.
The North China Craton (NCC) witnessed a prolonged subduction–accretion history from the early to late Palaeoproterozoic, culminating with final collision at ca. 1.85 Ga and assembling the continental blocks into the cratonic framework. Subsequently, widespread post-collisional magmatism occurred, particularly along the Trans-North China Orogen (TNCO) that sutures the Eastern and Western blocks of the NCC. Here we present petrological, geochemical, and zircon U–Pb geochronological and Lu–Hf data from a pyroxenite (websterite)–gabbro–diorite suite at Xinghe in Inner Mongolia along the northern segment of the TNCO. The internal structures and high Th/U values of the zircons from the gabbro–diorite suite suggest magmatic crystallization. LA-ICP-MS U–Pb age data on three gabbros and one diorite from the suite yield emplacement ages of 1786.1 ± 4.8, 1783 ± 15 ,1754 ± 16 and 1767 ± 13 Ma, respectively. The εHf(t) shows mostly positive values (up to 5.8), with the lowest value at –4.2, suggesting that the magma was derived from dominantly juvenile sources. The generally low SiO2 and high MgO values, and other trace element features of the Xinghe suite are consistent with fractionation from a mantle-derived magma with a broadly E-MORB affinity, with no significant crustal contamination. Recent studies clearly establish that the major magmatic pulse associated with rifting of the NCC within the Columbia supercontinent occurred in the late Mesoproterozoic at ca. 1.3–1.2 Ga associated with mantle plume activity. This, together with the lack of robust geochemical imprints of rift-related magmatism in the Xinghe suite, prompts us to suggest a tectonic model that envisages magma genesis associated with post-collisional extension during slab break-off, following the westward subduction of the Eastern Block and its collision with the Western Block. The resulting asthenospheric upwelling and heat input might have triggered the magma generation from a heterogeneous, subduction-modified sub-lithospheric mantle source for the Xinghe rocks, as well as for similar late Palaeoproterozoic suites in the TNCO.  相似文献   
46.
In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.  相似文献   
47.
李美娇  何凡能  刘浩龙 《地理研究》2016,35(12):2322-2332
基于历史文献研究的方法,利用北宋中期垦田数据及宋代人口粮食需求量、粮食亩产量等史料,考察了北宋中期南北方的垦田隐匿特点、北宋户均垦田数的合理范围及西南五路的户均基本垦田需求,并据此对北宋中期路域耕地面积进行了再估算。主要结论:北宋垦田隐匿比例不具有北高南低的区域分异特征;北宋户均垦田数的合理范围为20~100亩;西南五路的户均基本垦田需求数约为20~30亩;北宋中期境内耕地面积约为731.9×106今亩,土地垦殖率为16.9%,其中,北方耕地面积约占29.7%,南方约占70.3%;黄淮海平原、长江中下游平原、两湖平原和成都平原等是北宋耕地的主要分布区,而西南地区垦殖率较低。合理订正北宋中期路域耕地面积,对重建中国过去千年LUCC数据集具有重要意义。  相似文献   
48.
在查阅、综合前人资料的基础上,通过野外观察、采样,结合偏光显微镜和热分析的实验数据,分析了几种常见鲁砚石的矿物成分及形成的地质环境。研究结果表明:沉积岩类的鲁砚石矿物成分几乎以泥晶方解石为主,另含少量陆源矿物。变质岩类鲁砚石矿物成分以绢云母、绿泥石为主要矿物,另含石英,电气石等矿物。鲁砚石主要产于鲁西地块,形成期以寒武纪居多,形成环境多滨、浅海相,河湖相。  相似文献   
49.
通过设定不同的氮(N)、磷(P)浓度一次性培养东海原甲藻,研究了该藻的生长状况及不同生长阶段单位细胞DNA,RNA及蛋白质含量的变化,结果表明,不同氮磷浓度对东海原甲藻的生长产生显著影响,最终生物量与初始的氮磷浓度呈正相关,不同培养组的最大生长率存在显著差异(P<0.05);DNA含量与培养液中氮浓度相关性极显著(P<0.01),但与磷浓度相关性不显著(P>0.05);RNA含量与氮磷浓度均呈极显著相关(P<0.01),而且在不同的生长阶段其含量也会发生变化,随着培养时间的延长,各培养组的RNA含量显著下降(P<0.05);RNA/DNA比值与氮磷浓度均显著相关(P<0.05),而且与生长率呈线性相关关系。培养液中氮浓度显著影响到东海原甲藻单位细胞总蛋白含量,磷缺乏也会使总蛋白含量显著降低。  相似文献   
50.
The Qinling Orogenic Belt, linking the Kunlun and Qilian Mountains to the west and continuing farther east to the Dabie Mountain, was assembled by the convergence and collision between the Greater South China and the North China blocks. The precise timing of the subduction and collision processes between these continental blocks and tectonic regime switchover is very equivocal. Zircon in-situ LA-ICP-MS U–Pb dating in this contribution indicates that the biotite monzogranite and monzogranite phases of the Dangchuan complex were crystallized at ca. 239.8 ± 2.3 Ma and 227.8 ± 1.2 Ma, respectively. The ca. 240 Ma biotite monzogranite displays εHf(t) values ranging from −2.4 to +2.9, and corresponding TDM2 of 1.72–1.94 Ga and TDM1 of 0.77–0.88 Ga. The ca. 228 Ma monzogranite exhibits εHf(t) values ranging from −4.3 to +1.9, and corresponding TDM2 of 1.73–2.08 Ga and TDM1 of 0.81–0.88 Ga. Lutetium–Hf isotopic composition indicates that the biotite monzogranite and monzogranite probably have the same parental magmas which were originated from hybrid sources of both reworking of Paleoproterozoic ancient crust and partial melting of the Neoproterozoic juvenile crust. The more negative εHf(t) values of the monzogranite suggest more contribution of the ancient crust during the source contamination, or more possible crustal assimilation during their crystallization at ca. 228 Ma than precursor biotite monzogranite. Integrated with previous research and our detailed petrography, we propose that the Dangchuan complex underwent an episodic growth documenting the tectonic regime switchover from early Paleozoic to Triassic. The ca. 439 Ma inherited zircon recorded the persistent subduction of the oceanic crust, the ca. 240 Ma biotite monzogranite emplaced during the northward subduction of the Mianlue oceanic crust beneath the South Qinling block, and the ca. 228 Ma monzogranite emplaced during the syn-collisional process in a compressional setting.  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