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991.
992.
993.
U. Harms D. P. F. Darbyshire T. Denkler M. Hengst H. Schandelmeier 《International Journal of Earth Sciences》1994,83(3):591-603
In the Delgo basement area of northern Sudan, low to medium grade metamorphosed volcanic, sedimentary and plutonic rocks are surrounded by high grade gneisses. A NNE-SSW trending suture zone can be defined by the lithological, chemical and structural characteristics of several distinct units. The early Proterozoic gneiss terrain is overlain by metasedimentary units, the metamorphism of which has been dated by the Sm-Nd whole rock-mineral technique (702 ± 27 Ma in the west, 592 ± 16 Ma in the east). In the central part, the Abu Sari volcanic rocks show geochemical signatures of formation at an arc, with a protracted tholeiitic, calc-alkaline and shoshonitic evolution. The overlying El Hamri ophiolite contains chemical features of a back-arc tectonic environments. The ophiolite was dated by the Sm-Nd whole rock method on metagabbros at 752 ± 48 Ma. The further extension of this oceanic basin into the Jebel Rahib in the south-west was dated at 707 ± 54 Ma (Sm-Nd whole rock and minerals).Widespread suite of syn-tectonic granitoid intrusives displays subduction-related characteristics. They where emplaced between 650 to 760 Ma (Pb-zircon evaporation method). Their Nd and Sr isotopic compositions indicate a changing pattern of island arc to active continental margin character along an east-west transect and suggest a west to north-west dipping subduction zone. All units were juxtaposed at the minimum age of 600 Ma and rearranged during an extensional event, which was dated by the Rb-Sr thin slab technique (546 ± 19 Ma) on a migmatite. The Delgo suture provides evidence of a complex terrane pattern in north-east Africa and crustal growth during the Pan-African event by the addition of oceanic material to pre-existing continental crust. 相似文献
994.
Origin of the fumarolic fluids of Vulcano Island,Italy and implications for volcanic surveillance 总被引:2,自引:2,他引:0
Variations in D and 18O values with H2O contents and outlet temperatures indicate that the fumaroles of La Fossa crater have discharged mixtures of magmatic water and marine hydrothermal water, since 1979. The contribution of meteoric water was low in the period 1979–1982 and very low afterwards. The 18O values of the marine-hydrothermal component of +5 to +7.2 are due to isotopic exchange with the 18O-rich silicates of the rocks under high-temperature and low-permeability conditions. The 18O value of the magmatic end-member is generally +3.5 to +4.3, although values as high as +5.5 to +6.5 were reached in the summer of 1988, when magma degassing appears to have extended into the core of the magma body. The D values of the end-member were close to -20, typical of andesitic waters. Both the isotopic values and chemical data strongly support a dry model, consisting of a central magmatic gas column and a surrounding hydrothermal envelope, in which marine hydrothermal brines move along limited fracture zones to undergo total evaporation on approaching the conduits of magmatic fluids. The vents at the eastern and western boundaries of the fumarolic field are fed by fluids whose pressure is governed by the coexistence of vapor, liquid and halite, giving rise to a high risk of phreato magmatic explosions, should magma penetrate into these wet environments. Most La Fossa eruptions were triggered by an initial hydrothermal blast and continued with a series of phreatomagmatic explosions. The fluids discharged by the Forgia Vecchia fumaroles are mixed with meteoric water, which is largely evaporated, although subordinate loss of condensed steam may be responsible for scrubbing most of the acidic gas species. The temperatures and pressures, and the risk of a sudden pressure increase, are low. A boiling hydrothermal aquifer at 230° C is present underneath the Baia di Levante beach. This area has a minor risk of hydrothermal explosions. 相似文献
995.
Deception Island is the most active volcano of the South Shetland Islands-Antarctic Peninsula group, experiencing eruptions in 1967, 1969 and 1970. Local attenuation derived from coda analysis and source parameters derived from Brune's model, for well located seismic events, have been studied in order to complement the available geophysical information. Results show abnormally lowQ
0 values and an abnormally high frequency dependence, as well as large dispersion. These factors are strongly dependent on the path travelled by the seismic wave. Retrieved values of the source parameters (stress drop, seismic moment and source radius), are again abnormally low compared to world-wide average values, for example, those obtained for the Oroville, California aftershock series between June and September, 1976. These results are consistent with some aspects of the geology of Deception Island, such as the very high degree of fracturing and faulting, and the existence of a strong hydrothermal alteration affecting most of the subaerial volcanic rocks. Moreover, the pattern defined for the lateral variations ofQ
0 shows minimum values in the inner bay of the island, close to the most recent eruption vents. A large reduction in spectral amplitudes over a particular frequency range occurs in several observations, corresponding to the path crossing the zone of highest attenuation. This observation suggests the existence of a hot magmatic intrusion produced during the most recent eruption, and coincides with the superficial low density mass distribution obtained from the gravimetric model and the long wave magnetic field component obtained from magnetic surveys. The width of this intrusion is estimated to be about 200 m, in agreement with the previous results obtained analyzing residuals of the location of seismic events. 相似文献
996.
本文详细介绍了1992年2月18日南日岛地震前,福建地下水动态观测网中4口深井所记录到的水位前兆异常。其异常时间以短临为主,基本同步。异常形式主要表现为水位升降,水位固体潮形态畸变和相位错动以及典型的“临震回跳”等。本文还对提高井网监控能力以及沿海地区井受“复合潮”影响等诸问题进行了讨论。 相似文献
997.
新的区域地质调查在海南岛东北部木栏头地区识别出一套从前未知的中级变质杂岩。木栏头变质杂岩主要沿林新—木栏头—虎威岭—赤坡—七星岭—新埠海—铺前海边沿岸呈基岩或不同尺度的无根岩块断续出露,其主体是钙硅酸盐岩和正、副片麻岩,含有少量斜长角闪岩、石英岩和大理岩,并按分布区域可进一步区分出林新片麻岩- 斜长角闪岩组合、木栏头变质火山岩- 钙硅酸盐岩组合、虎威岭- 七星岭片麻岩- 钙硅酸盐岩- 大理岩组合和新埠海- 铺前片麻岩组合等四套岩石组合。对30件变质基性岩、变质中酸性岩、变质碎屑沉积岩、钙硅酸盐岩以及花岗和伟晶岩脉等不同类型岩石的锆石U- Pb定年结果表明,木栏头变质杂岩的原岩主体是一套二叠纪火山- 沉积岩系,其内含有少量二叠纪花岗质侵入岩以及前寒武纪结晶基底的残留。前寒武纪结晶基底主要包括古元古代晚期(1670 Ma)碎屑沉积岩和中元古代早期(1460~1410 Ma)花岗质片麻岩,晚二叠世碱性花岗岩中还存在大量单一的中元古代晚期(1180 Ma)继承锆石。变质沉积岩中的早期碎屑锆石年龄峰值为2550~2490 Ma、1850~1780 Ma、1600~1560 Ma、1450 Ma和1100 Ma,表明其物源主要来自于海南岛中部的抱板群、石碌群和石灰顶组。二叠纪花岗岩的侵入时代主要为280 Ma和260 Ma,与陆缘弧前盆地环境下形成的火山- 沉积岩系的时代基本一致。这些沉积岩中的碎屑锆石除具有395~345 Ma和280~256 Ma两个年龄峰值外,部分样品还含有960~930 Ma和450~410 Ma两个重要年龄峰值,与前人在海南岛晚古生代地层中获得的年代学结果相似。木栏头变质杂岩经历了晚二叠世—中三叠世(254~235 Ma)高角闪岩相区域变质和深熔作用以及花岗和伟晶岩脉的大规模侵入,独居石U- Pb定年表明中侏罗世(159 Ma)花岗岩脉也侵入其中。结合近年发表的研究资料,我们认为海南岛应属于印支陆块的一部分,由中元古代结晶基底和早古生代盖层构成的琼南地体以及该地体演化而来的琼北构造混杂岩带两个次级构造单元组成,邦溪- 晨星构造带或昌江- 琼海断裂不能被视为华南和印支陆块间的构造边界,真正的古特提斯缝合带(即金沙江- 哀牢山- 马江缝合带的东延)应位于木栏头北部,大致相当于现今琼州海峡断裂的位置。华南和印支陆块间古特提斯洋盆的关闭始于石炭纪(340~300 Ma)洋壳的南向俯冲,形成北部的潮滩鼻榴辉岩和南部的邦溪- 晨星弧后盆地,二叠纪时期(280~255 Ma)洋盆持续俯冲形成海南岛主体大陆岛弧以及木栏头弧前盆地,而后洋盆最终关闭并进入到陆- 陆碰撞和碰撞后伸展阶段,从而形成木栏头变质杂岩以及海南岛内部其他三叠纪变质岩和同期花岗质岩石。 相似文献
998.
海南岛万宁地区寒武纪地层及其沉积特征 总被引:1,自引:0,他引:1
海南岛万宁地区的寒武系为海南岛除三亚地区外迄今所见惟一有化石依据的寒武纪地层。该区寒武纪地层暂命名为美子林组,自下而上可分为3个岩性段:一段为滨—浅海相为主体的陆源碎屑沉积,厚度大于673m ;二段为滨海—浅海相含灰质的细碎屑岩沉积,局部见灰岩条带,厚156m 以上;三段以浅海—近滨相碳酸盐岩沉积为主体,主要为灰岩、泥灰岩、砂质—粉砂质灰岩、灰质粉砂岩,厚度大于196m ,寒武纪瓶虫和海绵骨针化石即产于该段灰岩中 相似文献
999.
1000.
Characteristics and dynamics origin of the Lower Cretaceous sill in northern Beilaishi section of Lingshan Island,Shandong Province 下载免费PDF全文
Zhong Jian-Hua Sun Ning-Liang Hao Bing Ni Liang-Tian Song Guan-Xian Xue Chun-Qi Mao Cui Shao Zhu-Fu Ge Yu-Zhu Chen Bin Liu Sheng-Xin Peng Chao-Feng Gu Dong-Hui Wang Yong-Qiang Li Cong 《古地理学报》1999,22(4):635-646
A lot of sand dikes are found in the Lower Cretaceous Laiyang Formation in northern Beilaishi section located in the Lingshan Island of Shandong Province. There are two types including bedding sand dikes and unbedding sand dikes based on their relationship with the bedding plane. The bedding sand dikes, which are often associated with soft sedimentary deformation, are mainly developed in the black mudstone of delta front, and their top and bottom interfaces are generally uneven. Based on the characteristics of sand dikes, the physical model and formula of bedding sand dikes under non-seismic condition are established. The relationship between the depth of liquefied sand layer, the emplacement depth and thickness of overlying sediment layer is deduced. It is considered that the depth of liquefied sand layer which forms large sand veins is very deep. This paper is of great significance in studying the origin of sand veins, lake related deposition and basin tectonic movement. 相似文献