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81.
H. Sommer A. Kröner C. Hauzenberger S. Muhongo M. T. D. Wingate 《Journal of Metamorphic Geology》2003,21(9):915-934
New data on the metamorphic petrology and zircon geochronology of high‐grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan‐African event. The metamorphic rocks are characterized by a clockwise P–T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite‐bearing metapelites. The meta‐granitoid rocks are of calc‐alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet–clinopyroxene–plagioclase–quartz–biotite–amphibole ± K‐feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet‐biotite‐plagioclase ± K‐feldspar ± kyanite/sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12–13 kbar and 750–800 °C. Pressures of 5–8 kbar and temperatures of 550–700 °C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwise P–T path is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970–2500 Ma) and Palaeoproterozoic (c. 2124–1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages of c. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan‐African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan‐African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west. 相似文献
82.
Zuoheng Zhang Jingwen Mao Jianmin Yang Zhiliang Wang Zhaochong Zhang 《Resource Geology》2003,53(2):101-114
Abstract. The Ta'ergou tungsten deposit in Gansu province, northwestern China, is located in the western part of the North Qilian Caledonian orogen, and consists of scheelite skarn bodies and wolframite quartz veins. The tungsten‐bearing skarn developed by the replacement of carbonate layers intercalated in the Precambrian schist and amphibolite whereas wolframite‐quartz ore veins developed along a group of fractures that cut through horizontal skarns. The Ta'ergou tungsten deposit is genetically related to the Caledonian Yeniutan granodiorite intrusion and occurs ca. 500 m wide in the exo‐contact zone 300 ~ 500 m apart from the intrusion. The granodiorite displays a lower grade of differentiation, low content of SiO2 and high contents of mafic components. There are three types of fluid inclusions in the wolframite‐quartz vein systems, i. e. aqueous, CO2‐H2O and CO2‐rich. The homogenization temperature of aqueous inclusion ranges from 140 to 380d?C and their salinities from 6.4 to 17.4 equivalent wt% NaCl. Laser Raman spectroscopy shows that the inclusions contain a relatively high content of CO2. The δ34S values of skarn type sulfides range from +8.1 to +12.7 per mil and those of quartz vein sulfides from +9.3 to +14.9 per mil, similar to sulfides of the granodiorite with from +6.0 to +11.7 per mil. The δ18O values of quartz are between +10.5 and +13.3 per mil and those of wolframite between +3.4 and +5.1 per mil. The δ18O water values of ore forming fluids range from +0.6 to +6.4 per mil and suggest the mixture of magmatic fluids with meteoric water formed the ore‐forming fluids. It has been proved that Precambrian strata in the west sector of North Qilian region are enriched in tungsten. We propose the strata were remelted to be tungsten‐granitoid during subduction. The polymetallic tungsten was gradually accumulated into the roof pendants of the granite intrusion by fractional crystallization and then was deposited by hydrothermal fluids during metasomatism and infilling along fractures. On the other hand, the granite intrusion also acted as “heating machine” to make hydrothermal fluids leach out the metals from Precambrian strata and these metals joined the ore‐forming hydrothermal system. 相似文献
83.
M. Sans J. Vergs E. Gomis J. M. Pars M. Schiattarella A. Trav F. Calvet P. Santanach A. Doulcet 《Tectonophysics》2003,372(1-2):85-104
Cleavage-fissility perpendicular to bedding is a common feature in the external part of fold-and-thrust belts. Three techniques were used to determine the internal distortion in the frontal Southern Pyrenees: the analysis of strain markers such as burrows and rain drops, the measurement of fissility, and the measurement of anisotropy of magnetic susceptibility (AMS). The comparison of the three techniques showed a good fit although they differ in sensitivity to penetrative strain variations in the range of deformation values explored in the study case. On the regional scale, the values of layer parallel shortening (LPS) derived from the markers analysis are very constant and account for 16–23% of shortening. These values are two to three times larger than the shortening values calculated from the restoration of the macroscopic scale structures and indicate a good decoupling above the Cardona salt Formation. This study permitted an accurate restoration of the low-amplitude el Guix detached anticline. 相似文献
84.
Valery A. Vernikovsky A. E. Vernikovskaya A. B. Kotov E. B. Sal'nikova V. P. Kovach 《Tectonophysics》2003,375(1-4):147
The geological, structural and tectonic evolutions of the Yenisey Ridge fold-and-thrust belt are discussed in the context of the western margin of the Siberian craton during the Neoproterozoic. Previous work in the Yenisey Ridge had led to the interpretation that the fold belt is composed of high-grade metamorphic and igneous rocks comprising an Archean and Paleoproterozoic basement with an unconformably overlying Mesoproterozoic–Neoproterozoic cover, which was mainly metamorphosed under greenschist-facies conditions. Based on the existing data and new geological and zircon U–Pb data, we recognize several terranes of different age and composition that were assembled during Neoproterozoic collisional–accretional processes on the western margin of the Siberian craton. We suggest that there were three main Neoproterozoic tectonic events involved in the formation of the Yenisey Ridge fold-and-thrust belt at 880–860 Ma, 760–720 Ma and 700–630 Ma. On the basis of new geochronological and petrological data, we propose that the Yeruda and Teya granites (880–860 Ma) were formed as a result of the first event, which could have occurred in the Central Angara terrane before it collided with Siberia. We also propose that the Cherimba, Ayakhta, Garevka and Glushikha granites (760–720 Ma) were formed as a result of this collision. The third event (700–630 Ma) is fixed by the age of island-arc and ophiolite complexes and their obduction onto the Siberian craton margin. We conclude by discussing correlation of these complexes with those in other belts on the margin of the Siberian craton. 相似文献
85.
U–Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet–biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U–Pb and Pb–Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50–100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet–biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800–850 °C at 12–14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east. 相似文献
86.
Natalia M. Levashova Kirill E. Degtyarev Mikhail L. Bazhenov Adam Q. Collins Rob Van der Voo 《Tectonophysics》2003,377(3-4):249-268
In order to test different hypotheses concerning the Paleozoic evolution of the Ural–Mongol belt (UMB) and the amalgamation of Eurasia, we studied Middle Devonian basalts from two localities (11 sites) and Lower Silurian volcanics, redbeds, and intra-formational conglomerates from three localities (20 sites) in the Chingiz Range of East Kazakhstan. The Devonian rocks prove to be heavily overprinted in the late Paleozoic, and a high-temperature, presumably primary, southerly, and down component is isolated at only four sites from a homoclinal section. Most Silurian redbeds are found to be remagnetized in the late Paleozoic; in contrast, a bipolar near-horizontal remanence, isolated from Silurian volcanics, is most probably primary as indicated by positive tilt and conglomerate tests. Analysis of paleomagnetic data from the Chingiz Range shows that southward-pointing directions in Ordovician, Silurian, and Devonian rocks are of normal polarity and hence indicate large-scale rotations after the Middle Devonian. The Chingiz paleomagnetic directions can be compared with Paleozoic data from the North Tien Shan and with the horseshoe-shaped distribution of subduction-related volcanic complexes in Kazakhstan. Both paleomagnetic and geological data support the idea that today's strongly curved volcanic belts of Kazakhstan are an orocline, deformed mostly before mid-Permian time. Despite the determination of nearly a dozen new Paleozoic paleopoles in this study and other recent publications by our team, significant temporal and spatial gaps remain in our knowledge of the paleomagnetic directions during the middle and late Paleozoic. However, the paleomagnetic results from the Chingiz Range and the North Tien Shan indicate that these areas show generally coherent motions with Siberia and Baltica, respectively. 相似文献
87.
Blade算法的产生是根据地震点的分布寻找地震断层,其优势在于能从较稀疏的点分布中找出线型条带模式,但不足的是完全不考虑地震的震级屑性。针对Blade算法的几个不足之处逐步改进,提出了震级加权四指标Blade算法。算法主要的改进体现在:①引入以震级为变量的权重函数,体现了高震级地震的控制作用,通过模拟与实例比较了三种权重函数的效果;②改进原方法中所挑出的Blade中心必须有地震点,引入四指标评价体系代替原方法的二指标评价体系,同时还根据指标值将挑选出的线型条带修正到更淮确的位置。将原算法与改进算法应用于华北地区4级以上的地震数据集以按寻地震带,结果表明,震级加权四指标Blade算法较Blade算法有显改进,取得了较好的应用效果。 相似文献
88.
位于川西地区的奔子栏——唐克深地震测深剖面以NNE走向穿越松潘——甘孜造山带.根据人工地震记录分析得到的震相走时和相关的振幅信息,确定了该剖面二维P波地壳速度结构.剖面的地壳结构可分为5层,其中第1,2,3层为上地壳;第4,5层为下地壳.上地壳中部普遍存在低速异常带,但龙日坝以北,这一低速带与其上覆的低速基底合为一体.同时,沿剖面的地壳速度结构具有较强的横向变化.据此,可将剖面分为4段,即甘孜——理塘断裂以南、甘孜——理塘断裂至鲜水河断裂、鲜水河断裂至龙日坝断裂和龙日坝断裂以北. 这与区域构造划分基本一致. 地壳厚度沿测线从南西向北东逐渐减薄,即从金沙江畔的62 km减小到黄河附近的52 km. 根据PmP震相分析,莫霍界面深度在鲜水河断裂两侧没有明显变化.全剖面的地壳平均速度较低,为6.30 km/s.奔子栏——唐克剖面揭示了该地区的造山带型地壳上地幔结构特征.鲜水河断裂带位于剖面的中部,该地区的上地壳速度为正异常,而下地壳和上地幔顶部存在负异常.笔者认为,这是一类有利于强震孕育和发生的深部构造环境. 相似文献
89.
2002年南海季风建立及其雨带变化的天气学研究 总被引:12,自引:1,他引:12
利用南海海 气通量观测试验资料结合NCEP ,GPCP以及GMS - 5云图资料 ,综合分析了 2 0 0 2年 5~ 6月南海西南季风建立过程及其雨带变化 ,确定 5月 14日西沙及北部海区西南季风爆发 ,5月 15日整个南海季风爆发 ,季风爆发时间属于正常年 ;季风爆发时风向、风速、云量、降水、湿度、辐射及海面温度等要素都发生突变。这种突变是由大气环流的突变造成的。季风爆发前后大气环流变化过程是 :80~ 90°E越赤道气流加强 ,同时印缅低压加深 ,孟加拉湾南北向气压梯度增大 ,而后东亚大陆上气旋发展东移 ,副热带高压东撤 ,孟加拉湾低压槽前的赤道西风突然加强越过中南半岛 ,南海北部首先出现强西南风 ,继而南海季风迅速地全面爆发。孟加拉湾西南风加强到南海季风爆发是一个连续的过程 ,大陆冷空气南下起了重要的作用。南海季风爆发时呈现单雨带型 ,而后由单雨带型转变为双雨带型 ,雨带受副热带高压和季风系统共同影响 ,并且随着副热带高压移动位置变化。 相似文献
90.
对下扬子与华南边界结合带东延问题的地球物理探讨 总被引:2,自引:1,他引:2
以江山-绍兴、铅山-宜春等断裂带作为华南与扬子块体之间的边界结合带,这种认识目前已基本为大家所接受.但是,这条边界结合带向东延伸入海之后的位置与去向一直是多年来研究的热点.本文基于黄、东海研究区的地球物理数据(空间重力数据、布格重力数据和地震层析成像结果),利用方向导数等处理方法,对研究区的地球物理数据进行处理,并对研究区的地球物理场进行了分析,划分出不同的区块.结合研究区的磁力与地质资料,利用各种成图、成像技术,形成一系列分析图件.在此基础上对华南与扬子块体之间的边界结合带进行了追踪.研究结果认为:该结合带的位置有可能比传统认识中自长江口至大黑山群岛的位置更向南一些,在杭州湾-长崎、对马海峡一线,并呈现向北略微凸出的弓形.边界结合带在深度上属于深大断裂带,一直可以追索到上地幔的顶部.与中朝与扬子块体之间的边界结合带相比较,本条结合带的踪迹不是十分清晰.表明加里东运动之后,该结合带的运动明显减弱. 相似文献