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441.
文章对钦杭结合带西南段大瑶山东南缘地区发育的早古生代花岗岩进行了LA-ICP-MS锆石U-Pb定年和岩石学、地球化学、TTG岩石组合的研究。该系列岩体主要沿大瑶山隆起东南缘呈规模大小不等的岩株(脉)成群、成带产出,大致呈北东向弧形带状展布。按其产出的时空分布特征,可划分为2条构造-岩浆岩带:1古龙—夏郢中-晚奥陶世辉长闪长岩+TTG组合构造-岩浆岩带(445~475 Ma);2罗平—古袍早志留世花岗闪长岩-花岗岩组合构造-岩浆岩带(432~436 Ma)。岩石SiO_2含量51.56%~73.12%,总体具低K_2O、高CaO、相对富钠(Na_2OK_2O)的特征;以偏铝质—弱过铝质(A/CNK值≈1)为主,岩石化学系列由低钾系列→钙碱性系列→高钾钙碱性系列演化;稀土总量低(∑REE67.82×10~(-6)~214.81×10~(-6)),为弱—中等铕亏损的轻稀土富集型的稀土配分曲线特征;富集大离子亲石元素Rb、Th、U和LREE,具明显Ta、Nb负异常,总体反映了活动陆缘弧花岗岩组合的地球化学特征。结合区域地质特征分析认为,该早古生代花岗岩属典型的活动大陆边缘弧环境岩浆岩组合,以壳幔同熔I型花岗岩为主,为大瑶山东南缘早古生代洋陆俯冲-碰撞的地质记录;由TTG组合→花岗闪长岩-花岗岩组合的组成极性显示:洋俯冲玄武岩板片由南东往北西方向俯冲,其与东邻的华夏云开陆缘(由北西往南东俯冲)构成双向俯冲-碰撞的地球动力学机制。 相似文献
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The character of mountain building processes in Palaeoproterozoic times is subject to much debate. Based on the discovery of high-pressure granulites in the Man Rise (Côte d'Ivoire), several authors have argued that Eburnean (Palaeoproterozoic) reworking of the Archean basement was achieved by modern-style thrust-dominated tectonics.
A mafic granulite of the Kouibli area (Archean part of the Man Rise, western Ivory Coast) displays a primary assemblage (M1) containing garnet, diopsidic clinopyroxene, red-brown pargasitic amphibole, plagioclase (andesine), rutile, ilmenite and quartz. This assemblage is associated with a subvertical regional foliation. Symplectites that developed at the expense of the M1 assemblage contain orthopyroxene, clinopyroxene, plagioclase (bytownite), green pargasitic amphibole, ilmenite and magnetite (M2). Multiequilibrium thermobarometric calculations and P–T pseudosections calculated with thermocalc suggest granulite facies conditions of ∼ 13 kbar, 850 °C and <7 kbar, 700–800 °C for M1 and M2, respectively. In agreement with the qualitative information obtained from reaction textures and chemical zoning of minerals, this suggests an evolution dominated by decompression accompanied by moderate cooling. A Sm–Nd garnet – whole-rock age of 2.03 Ga determined on this sample indicates that this evolution occurred during the Palaeoproterozoic. It is argued that from the geodynamic point of view the observed features are best explained by homogeneous thickening of the margin of the Archean craton, re-heated and softened due to the accretion of hot, juvenile Palaeoproterozoic crust, as well as coeval intrusion of juvenile magmas. Crustal shortening was mainly accommodated by transpressive shear zones and by lateral crustal spreading rather than large-scale thrust systems. 相似文献
A mafic granulite of the Kouibli area (Archean part of the Man Rise, western Ivory Coast) displays a primary assemblage (M1) containing garnet, diopsidic clinopyroxene, red-brown pargasitic amphibole, plagioclase (andesine), rutile, ilmenite and quartz. This assemblage is associated with a subvertical regional foliation. Symplectites that developed at the expense of the M1 assemblage contain orthopyroxene, clinopyroxene, plagioclase (bytownite), green pargasitic amphibole, ilmenite and magnetite (M2). Multiequilibrium thermobarometric calculations and P–T pseudosections calculated with thermocalc suggest granulite facies conditions of ∼ 13 kbar, 850 °C and <7 kbar, 700–800 °C for M1 and M2, respectively. In agreement with the qualitative information obtained from reaction textures and chemical zoning of minerals, this suggests an evolution dominated by decompression accompanied by moderate cooling. A Sm–Nd garnet – whole-rock age of 2.03 Ga determined on this sample indicates that this evolution occurred during the Palaeoproterozoic. It is argued that from the geodynamic point of view the observed features are best explained by homogeneous thickening of the margin of the Archean craton, re-heated and softened due to the accretion of hot, juvenile Palaeoproterozoic crust, as well as coeval intrusion of juvenile magmas. Crustal shortening was mainly accommodated by transpressive shear zones and by lateral crustal spreading rather than large-scale thrust systems. 相似文献
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河北省张宣地区的东坪金矿赋存于水泉沟碱性杂岩体与太古宙花岗岩一绿岩地体的接触带附近岩体一侧,是与碱性若有关的新型金矿床。通过对该区岩石的含矿性、稀土元素.Sr同位素等方面的研究,认为东坪金矿的形成与花岗岩一绿岩带的活化再造作用有关。其金质来源除幔源外,还有部分来自碱性岩形成时花岗岩一绿岩地体的重熔。含金质高的花岗岩一绿岩地体熔入岩浆后,随碱性岩的演化而富集,在岩体边部的构造有利部位形成超大型金矿。这类矿床与绿岩带的发展演化有关,属于与碱性岩或减质交代岩有关的绿岩带型金矿床。 相似文献
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Paleo-to Neoarchean granitoid gneisses (ca. 3.30 to 2.49 Ga) are well preserved in the Western Superior Craton. Protoliths of these gneisses are mainly I-type granitoids characterized by high Sr/Y and La/Yb ratios and low Mg#, consistent with Archean tonalite-trondhjemite-granodiorites. Zircons from granitoid gneisses commonly contain three growth phases: inherited cores (zircon I), magmatic rims (zircon II) and outer rims that have undergone Pb-loss (zircon III). The 3.12 Ga to 2.86 Ga zircon I represent early crustal material, that was captured in younger zircons; zircon II preserve crustal re-working and younger crustal additions that are constrained between 2.85 to 2.72 and 2.69 to 2.65 Ga.Zircon II contains both positive and negative εHf(t) values (−6.3 to +8.1), with both depleted-mantle and older crustal signatures. Half of the magmatic rims (II) are characterized by depleted mantle signatures with positive εHf(t) values representing juvenile crust-forming events, whereas the other half are characterized by recycled crustal signatures with negative εHf(t) values. εHf(t) results show that the North Caribou and the Island Lake terranes and the northern Uchi domain are isotopically more enriched than the southern Uchi, English River, Wabigoon and Winnipeg River terranes, suggesting the northern Uchi margin represents a major terrane boundary.Based on mass balance calculations, large volumes of juvenile material at circa 3.0 Ga mixed with smaller amounts of older crust. The vast majority of the granites were derived from a source with about 50% mantle material during the peak crust formation events after 2.8 Ga. The decline in the volume of felsic magmatism in the later Archean is coeval with a reduced supply of both heat and material from depleted mantle sources. Combined with previously published geochemical, geochronological and isotopic data, this suggests an evolution in felsic magma sources consistent with crustal thickening. 相似文献
449.
Arubam C.Khelen C.Manikyamba Li Tang M.Santosh K.S.V.Subramanyam Th Dhanakumar Singh 《地学前缘(英文版)》2020,11(1):229-242
Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cuddapah basin.Stromatolitic carbonates are well preserved in the Neoarchean greenstone belts of Dharwar Craton and Cuddapah Basin of Peninsular India displaying varied morphological and geochemical characteristics.In this study,we report results from U-Pb geochronology and trace element composition of the detrital zircons from stromatolitic carbonates present within the Dharwar Craton and Cuddapah basin to understand the provenance and time of accretion and deposition.The UPb ages of the detrital zircons from the Bhimasamudra and Marikanve stromatolites of the Chitradurga greenstone belt of Dharwar Craton display ages of 3426±26 Ma to 2650±38 Ma whereas the Sandur stromatolites gave an age of 3508±29 Ma to 2926±36 Ma suggesting Paleo-to Neoarchean provenance.The U-Pb detrital zircons of the Tadpatri stromatolites gave an age of 2761±31 Ma to1672±38 Ma suggesting Neoarchean to Mesoproterozoic provenance.The Rare Earth Element(REE)patterns of the studied detrital zircons from Archean Dharwar Craton and Proterozoic Cuddapah basin display depletion in light rare earth elements(LREE)and enrichment in heavy rare earth elements(HREE)with pronounced positive Ce and negative Eu anomalies,typical of magmatic zircons.The trace element composition and their relationship collectively indicate a mixed granitoid and mafic source for both the Dharwar and Cuddapah stromatolites.The 3508±29 Ma age of the detrital zircons support the existence of 3.5 Ga crust in the Western Dharwar Craton.The overall detrital zircon ages(3.5-2.7 Ga)obtained from the stromatolitic carbonates of Archean greenstone belts and Proterozoic Cuddapah basin(2.7-1.6 Ga)collectively reflect on^800-900 Ma duration for the Precambrian stromatolite deposition in the Dharwar Craton. 相似文献