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131.
贵州峨眉山玄武岩喷发,从动态的角度可以分为茅口期晚期和龙潭期(吴家坪期),龙潭期又可分为三个喷发旋回,对应于四个不同的岩相古地理环境,体现了东吴运动在造成贵州地区地壳抬升、下沉和接受最大海侵之后,又上升、拉张、沉陷带发生地裂(又称峨眉地裂)以及地幔物质喷溢等地质活动,具间歇性和多旋回性的特点。本文从研究海陆变迁入手,揭示峨眉山玄武岩喷发与沉积作用的内在联系,进而探讨其与金、锑等矿产的成因联系,提出该期各相区与成矿区的形成模式。通过对贵州峨眉山玄武岩不同喷发期岩相古地理的研究可以看到,茅口期晚期和龙潭期早期海域的沉积韵律和相带展布格局与玄武岩喷发的间歇性和多旋回性特征完全一致。玄武岩的喷发为成矿提供了物质基础,玄武岩喷发的间歇期又为沉积矿产的富集提供机遇。这种岩浆期后气液以富硅和二氧化碳为特征的玄武岩,本身富含铁、锰、铜、铅、锌、锑、砷、汞、金、银、氟、磷以及一些稀散和放射性元素等成矿组分。在喷发过程中,气液成分有一定变化,各阶段和离岩浆的远近距离不同以及喷发性质和环境差异,形成了火山气液矿床、火山沉积矿床和沉积矿床的不同成矿带。 相似文献
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133.
Valentina Rocchi Peter R. Sammonds Christopher R.J. Kilburn 《Journal of Volcanology and Geothermal Research》2003,120(1-2):25-42
Understanding how the strength of basaltic rock varies with the extrinsic conditions of stress state, pressure and temperature, and the intrinsic rock physical properties is fundamental to understanding the dynamics of volcanic systems. In particular it is essential to understand how rock strength at high temperatures is limited by fracture. We have collated and analysed laboratory data for basaltic rocks from over 500 rock deformation experiments and plotted these on principal stress failure maps. We have fitted an empirical flow law (Norton’s law) and a theoretical fracture criterion to these data. The principal stress failure map is a graphical representation of ductile and brittle experimental data together with flow and fracture envelopes under varying strain rate, temperature and pressure. We have used these maps to re-interpret the ductile–brittle transition in basaltic rocks at high temperatures and show, conceptually, how these failure maps can be applied to volcanic systems, using lava flows as an example. 相似文献
134.
James W. Head III Lionel Wilson 《Journal of Volcanology and Geothermal Research》2003,121(3-4):155-193
Submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur. Consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows, however, that significant pyroclastic activity can occur even at depths in excess of 3000 m. In order to document and illustrate the full range of submarine eruption styles, we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions: (1) no gas exsolution; (2) gas exsolution but no magma disruption; (3) gas exsolution, magma disruption, and hawaiian-style fountaining; (4) volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse; (5) magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity; and (6) volatile content builds up in the top of a dike leading to vulcanian eruptions. We also examine the role of bulk-interaction steam explosivity and contact-surface steam explosivity as processes contributing to volcaniclastic formation in these environments. We concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities, the most likely circumstance is the quiet effusion of magma with minor gas exsolution, and the production of somewhat vesicular pillow lavas or sheet flows, depending on effusion rate. The amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption. The most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity, due to the localized concentration of volatiles in magma that has a low rise rate; magmatic gas collects by bubble coalescence, and ascends in large isolated bubbles which disrupt the magma surface in the vent, producing localized blocks, bombs, and pyroclastic deposits. Another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions; these deposits, being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater, should be easily distinguishable from strombolian and hawaiian eruptions. However, we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles. In this case, a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian-style activity; the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile-induced hawaiian eruption. Specifically, typical pyroclast sizes might be smaller; fountain heights may exceed those expected for the purely magmatic hawaiian case; cooling of descending pyroclasts would be more efficient, leading to different types of proximal deposits; and runout distances for density flows would be greater, potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer. In addition, flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles, and thus extremely poor in vesicles relative to typical flows associated with hawaiian-style eruptions in the primary magmatic gas case. We examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid-ocean ridges. Furthermore, the latter stages of development of volcanic edifices (seamounts) formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth, but also to: (1) the presence of a summit magma reservoir, which favors the buildup of magmatic foams (enhancing hawaiian-style activity) and episodic dike emplacement (which favors strombolian-style eruptions); and (2) the common occurrence of alkalic basalts, the CO2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts. These models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles. 相似文献
135.
受玄武岩盖层的影响,石林地区的地下水在雨季和旱季对碳酸盐岩都具有侵蚀性,玄武岩盖层空气CO2呈现出低—高—低的双向变化梯度。溶蚀试验表明.地下0~0.6m,水平方向的溶蚀量大于垂直方向的溶蚀量,而随着深度的增加,垂直方向的溶蚀量大于水平方向。富含CO2的水通过具有最大渗透张量和较小主轴倾角的玄武岩裂隙下渗,对碳酸盐岩的溶蚀作用表现为一个脱钙、富铝铁、硅迁移的复杂过程,并在地下0~0.6m形成许多水平凹槽、穿洞等岩溶形态,而地表0.6m以下以垂向溶蚀为主,有利于石柱的形成与发育。 相似文献
136.
辽宁宽甸新生代火山岩和地幔包体He-Ar同位素组成 总被引:8,自引:1,他引:8
宽甸新生代碱性玄武岩、地幔包体及辉石巨晶的稀有气体同位素组成和流体含量分析表明,不同岩性稀有气体含量的差异反映了岩浆作用过程中轻、重稀有气体的分馏特性,较轻的稀有气体(He、Ne)比较重的稀有气体(Kr、Xe)具有更高的活动性和不相容性;该地区上地幔源区具有典型的MORB型源区特征,以辉石巨晶为代表;地幔包体的3^He/^4He值较低,可能与地幔上隆过程中富集地幔流体的交代作用或地幔塑性变形作用丢失了部分原始He有关;大陆碱性玄武岩具有与大洋玄武岩截然不同的He同位素组成,反映了大陆区地幔岩浆上升过程中受到了陆壳物质混染。地幔源区^40Ar/^36Ar值为350左右,二辉橄榄岩和碱性火山岩的^40Ar/^36Ar值比大气略高,可能有大气组分的混入。部分样品中有^21Ne、^22Ne、^129Xe、^134Xe和^136Xe相对于大气的过剩现象。 相似文献
137.
Petrogenesis of the Ulsan carbonate rocks from the south-eastern Kyongsang Basin, South Korea 总被引:1,自引:0,他引:1
Abstract The petrogenesis of the Ulsan carbonate rocks in the Mesozoic Kyongsang Basin of South Korea, which have previously been interpreted as limestone of Paleozoic age, is reconsidered in the present study. Within the Kyongsang Basin, a small volume of carbonate rocks, containing a magnetite deposit and spatially associated ultramafic rocks, is surrounded by sedimentary, volcanic and granitic rocks of the Mesozoic age. The simple cross‐cutting relationships and other outcrop features of the area indicate that the carbonate rocks are an intrusive phase and younger than the other surrounding Mesozoic rocks. The Ulsan carbonates have low concentrations of rare earth elements (REE) and trace elements with the carbon and oxygen isotope values in the range of δ13CPDB = 2.4 to 4.0‰ and δ18OSMOW = 17.0 to 19.5‰. Outcrop evidence and geochemical signatures indicate that the Ulsan carbonates were formed from crustal carbonate melts, which were generated by the melting/fluxing of crustal carbonate materials, caused by the emplacement‐related processes of alkaline A‐type granitic rocks. Compared to typical mantle‐derived carbonatites associated with silica‐undersaturated, strongly peralkaline systems, the relatively small size and geochemical characteristics of the Ulsan carbonates reflect carbonatite genesis in a silica‐saturated, weakly alkali intrusive system. Major deep‐seated tectonic fractures formed by the collapse of the cauldron or the rift system associated with the opening of the East Sea (Japan Sea) might have facilitated the ascent of the crustal carbonate melts. 相似文献
138.
139.
易门式大型铜矿床构造成矿动力学模型 总被引:5,自引:1,他引:5
针对金属矿床隐伏矿定位预测的关键问题,在提出构造成矿动力学的分支学术方向的基础上,以易门式大型铜矿床为例,从成矿地质背景、矿田构造、构造地球化学和成矿构造应力场等方面进行构造成矿动力学研究,阐述在构造应力场控制下成矿流体运移和聚集的规律。构造地球化学研究认为,构造地球化学异常反映矿体的原生晕;构造地球化学异常受构造控制,其分带特征可指示成矿流体的流向,并提供矿床成因和隐伏矿(化)体相对埋深的信息。构造应力场的控矿特征主要表现在:构造应力场导致控矿构造的形成,并驱动成矿流体的运移;构造应力场控制了成矿能量场;应力和能量的高值集中区分布特点可反映某些构造型式。在此基础上建立构造成矿动力学模型,概括隐伏矿成矿预测准则,据此提出若干重点找矿靶区和靶位,其中部分靶区得到工程验证,表明构造成矿动力学方法对隐伏矿定位预测和评价具有重要的指导意义。 相似文献
140.
攀西裂谷存在吗? 总被引:12,自引:0,他引:12
大陆裂谷以地幔上隆、岩石圈伸展、减薄、断陷和沉降为特征,伸展构造环境是大陆裂谷形成的必要条件和本质特征。中国学者以前所认为攀枝花-西昌裂谷的主要标志是海西期层状堆晶杂岩、晚二叠世峨眉山玄武岩、印支期环状碱性杂岩和晚三叠世裂谷盆地沉积。最近一系列研究成果表明攀西地区海西期-印支期构造岩浆热事件是地幔柱和岩石圈相互作用的结果,不是裂谷作用的产物。进一步对上扬子西缘二叠纪-三叠纪的沉积作用和构造特征综合分析表明攀西地区不存在裂谷盆地沉积。该区晚二叠世-中三叠世为古陆隆起遭受剥蚀,晚三叠世断陷型类磨拉石建造是前陆走滑复合盆地的产物。本文根据对攀西地区二叠纪-三叠纪的岩浆活动、沉积作用、构造特征和地球物理资料等方面综合研究对攀西裂谷的存在提出质疑,并以峨眉山地幔柱活动为主线探讨了攀西地区古生代和中生代的地质构造演化历史。 相似文献