冈底斯地区林子宗群火山岩岩石地球化学特征及地球动力学意义   总被引：11，自引：2，他引：11       下载免费PDF全文

贾建称温长顺  王根厚高春光 杨国东 《中国地质》2005,32(3):396-404

错卧莫-惩香错火山-沉积盆地位于西藏冈底斯陆块中部南缘。盆地内火山-沉积地层称古新世-渐新世林子宗群，岩石地球化学数据分析表明：这套火山岩系属钙碱性系列，轻稀土富集，不同程度表现出负Eu、负Ce、负Nb异常。微量元素K、Rb、Th、Ce富集，Sr、Ba，尤其是Nb、P、Ti亏损。初步认为这套火山-沉积岩系形成于俯冲造弧的构造背景中，是来自于俯冲带的地幔源区基性分异岩浆与陆壳重熔的酸性岩浆在不同的时间段内，按不同的比例混合形成的。同时，俯冲板块上深海沉积物与基性岩浆混合作用也是不容忽视的因素。在综合分析基础上，笔者认为印度板块与欧亚板块的碰撞启动时间在45Ma以后。  相似文献   

白云鄂博碳酸岩型REE-Nb-Fe矿床——一个罕见的中元古代破火山机构成岩成矿实例   总被引：16，自引：4，他引：16  

郝梓国  王希斌  李震  肖国望  张台荣 《地质学报》2002,76(4):525-540

白云鄂博碳酸岩型REE-Nb-Fe矿床是世界上最大的稀土矿床。稀土矿石产于整个白云石碳酸岩体和部分脉状碳酸岩中。对比世界上20余个火成碳酸岩地区的特征后发现,白云鄂博地区完全具备国外火成碳酸岩区的地质特征。在岩石、矿石组合上,本区也发育一套碳酸岩+超基性岩+碱性基性岩(含基性熔岩)+碱性岩+稀土矿石+铁矿石组合;在矿物组合上,以白云石为主,方解石次之,伴生一套碱性闪石、长石、霓石、磷灰石、萤石、磁铁矿、稀土矿物组合;在全岩化学成分、微量元素、稀土元素和Sr、Nd、Pb、C、O同位素上,这些岩石具有一定的亲缘关系,有着共同的来源;在岩体的形态与岩石组构上,它们以岩席、岩筒和脉岩的形式出现,并发育有强烈的熔离作用与流动构造;在区域构造上,发育隐伏穹窿构造、岩筒构造和巨型断裂汇聚构造。综合分析上述特征表明:白云鄂博地区具有中元古代破火山机构的痕迹,赋矿白云石碳酸岩体则是顺层侵入的火成碳酸岩体,东矿、主矿可能是一个火山颈构造控矿,而赋矿白云石碳酸岩体西南侧的苏木图矿床则是隐伏岩筒构造控矿。  相似文献   

准噶尔盆地基底火山岩中的辉石及其对盆地基底性质的示踪   总被引：4，自引：0，他引：4  

曾广策  王方正等 《地球科学》2002,27(1):13-18

根据钻井和航磁资料，准噶尔盆地基底可划分为西、北、南三区。3个地区火山岩中辉石的化学成分、种属名称各不相同。辉石化学成分反映出来的寄主岩的碱度、碱度演化趋势以及寄主岩形成的构造环境、构造环境演化史均各有差异，佐证了准噶尔盆地基底是由哈萨克斯坦板块东南缘、西伯利亚板块西南缘和塔里木板块北缘增生大陆拼合而成。其中西、北两区拼合较早，早石炭世末的早海西运动时抬升成陆；南区成陆较晚，晚古炭世末的晚海西运动使南区与西、北两区联合大陆对接，形成完整的准噶尔盆地海西褶皱基底。  相似文献   

青藏高原北羌塘半岛湖新生代粗面玄武岩橄榄石电子探针和激光探针分析   总被引：4，自引：0，他引：4  

赖绍聪  伊海生  刘池阳  O'Reilly S Y 《矿物学报》2002,22(2):107-112

利用电子探针和激光探针剥蚀系统（LA-ICP-MS），对北羌塘新第三纪粗面玄武岩中的橄榄石主元素和微量、稀土元素进行了系统分析。结果表明，本区橄榄石Fo平均为88，属贵橄榄石种属。相对富集Ni、Co和重稀土，而强烈亏损轻稀土及Rb、Sr、Ba、Zr等大离子亲石元素。其稀土元素配分型配分型式与粗面玄武岩全岩稀土配分型式呈互补状态。  相似文献   

Large-scale mass wasting in ancient volcanic materials     

Vicki Moon  Christine J Simpson 《Engineering Geology》2002,64(1):41-64

Giant landslides are significant hazards associated with many active volcanic edifices. We describe a similar feature on ancient (>4 Ma) volcanic deposits subject to active tectonism. The landslide is approximately 3 km long by 1 km wide, with an estimated depth of 400 m. Side margins are straight and parallel, mimicking regional structure; narrow valleys incised down these margins provide low-strength side-release surfaces. Between these is a giant slump consisting of at least four, largely intact, discrete blocks that have moved down-dip a distance of >500 m. A series of flows with areal extents ranging from 0.01 to 0.5 km² extends from the front of the failure. The materials represent an eroded sequence of andesite flows on the flanks of a stratovolcano. These have undergone two phases of hydrothermal alteration, and are deeply weathered to low-density (1040±80 kg m⁻³) silt (59%) and clay (35%) materials with strength properties typical of weathered silts (c=26±3 kN m⁻²; φ=42±8°). The size and location of this landslide preclude detailed geotechnical investigation of the failure. The worth of numerical stability analysis as an alternative technique in assessing the nature of the failure and hence the risk it poses to nearby communities is investigated. Sensitivity analysis identified likely conditions under which initial failure may have occurred: analyses for sensitivity to strength and earthquake acceleration needed conversion to critical combinations (F=1.0) of water table and strength/acceleration to remove the overriding influence of water table fluctuations. Failure was likely initiated either by a high water table level (83-84%), or some combination of intensity VII-IX earthquake waves together with water table heights of 40-80%. A general hazard assessment indicates that the risk associated with creep and catastrophic failure of the main mass is small, whereas the risk from flow failures near the toe of the landslide may be high. Important parameters (hydrological regime, flow failure morphology, age of initiation, and rates of movement) requiring closer investigation are identified. Development of a model is crucial to assessing the hazard associated with a feature such as that described here. With limited resources, a detailed stability analysis is a powerful tool as an initial stage in hazard analysis.  相似文献   

内蒙古宝音图岩群变质基性火山岩锆石U-Pb年龄及意义     

张玉清  苏宏伟 《华北地质》2002,(Z1)

内蒙古达茂旗宝音图岩群是在达茂旗、查干呼绍地区 1:5万区调和白云鄂博幅 1:2 5万区调过程中从过去的巴特敖包群和白云鄂博群未分岩组解体出来的中级变质岩系。通过对宝音图岩群中阳起钠长片岩、二云片岩 (原岩均为基性火山岩 )进行单颗粒锆石U -Pb法年龄测定 ,获得了两条不一致线上交点年龄 (2 486± 42 )Ma、(2 496± 2 6 )Ma ,代表了宝音图岩群基性火山岩及宝音图岩群的形成时代 ;在不一致线之外不同成因的锆石的2 0 7Pb/ 2 0 6Pb表面年龄 (2 2 2 7± 15 )Ma ,寓示着 2 2 0 0Ma左右发生了一次区域性变质热事件 ;下交点年龄 (319±486 )Ma和 (40 6± 46 )Ma ,代表了 2 5 0～ 480Ma时期该区大面积岩浆侵位及宝音图岩群后期变形变质改造的时期 ,两个年龄样的获得为研究古元古代古陆的裂解及白云鄂博海槽的形成提供了直接依据。  相似文献   

2-D crustal structure of Changbaishan-Tianchi volcanic region determined by seismic traveltime inversion     

王夫运  张先康  杨卓欣 《地震学报(英文版)》2002,15(2):152-161

IntroductionTrial-and-error forward modeling of wide-angle seismic reflection/refraction traveltimes for 2-D velocity structure is extremely time-consuming, even for experienced data interpreters. For wide-angle seismic reflection/refraction experiments that consist of numerous shots along a single line, it is quite difficult through repeated trial-and-error forward modeling to construct a 2-D model that fits the data within acceptable limits (Cerveny, et al, 1977; ZHANG, et al, 200 . In ad…  相似文献   

A methodology for the evaluation of long-term volcanic risk from pyroclastic flows in Campi Flegrei (Italy)     

I. Alberico  L. Lirer  P. Petrosino  R. Scandone 《Journal of Volcanology and Geothermal Research》2002,116(1-2)

The volcanological history of Campi Flegrei suggests that the most frequent eruptions are characterized by the emplacement of pyroclastic flow and surge deposits erupted from different vents scattered over a 150-km² caldera. The evaluation of volcanic risk in volcanic fields is complex because of the lack of a central vent. To approach this problem, we subdivided the entire area of Campi Flegrei into a regular grid and evaluated the relative spatial probability of opening of vents based on geological, geophysical and geochemical data. We evaluated the volcanic risk caused by pyroclastic flows based on the formula proposed by UNESCO (1972), R=H×V×Va, where H is the hazard, V is the vulnerability and Va is the value of the elements at risk. The product H×V was obtained by performing simulations of type eruptions centered in each cell of the grid. The simulation is based on the energy cone scheme proposed by Sheridan and Malin [J. Volcanol. Geotherm. Res. 17 (1983) 187–202], hypothesizing a column collapse height of 100 m for eruptions of VEI=3 and 300 m for eruptions of VEI=4 with a slope angle of 6°. Each simulation has been given the relative probability value associated with the corresponding cell. We made use of the GIS software ArcView 3.2 to evaluate the intersection between the energy cone and the topography. The superposition of the areas invaded by pyroclastic flows (124 simulations for VEI=3 and 37 for VEI=4) was used to obtain the relative hazard map of the area. The relative volcanic risk map is obtained by superimposing the urbanization maps.  相似文献   

Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties   总被引：1，自引：0，他引：1  

D.V. ChandrasekharD.C. Mishra  G.V.S. Poornachandra RaoJ. Mallikharjuna Rao 《Earth and Planetary Science Letters》2002,201(2):277-292

The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10⁻⁵m/s²) and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10⁻² SI units with an average Koenigsberger ratio (Q_n) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m³, respectively and susceptibility of 3.14×10⁻² SI units with a Q_n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.  相似文献   

Deformation of a uniform half-space due to a long inclined tensile fault     

Sarva Jit Singh  Anil Kumar  Sunita Rani  Mahabir Singh 《Geophysical Journal International》2002,148(3):687-691

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