广西南部钦州地区早—中三叠世火山事件——台马碎斑熔岩     

杨丽贞  刘融涛  白艳萍 《中国区域地质》2011,(1):95-100

通过对前人划分的十万大山岩带的主要岩体——台马岩体的野外地质特征、岩石学特征的研究,认为原台马岩体主要由碎斑熔岩组成,是火山侵出相的产物而不是侵入相的花岗斑岩。结合台马碎斑熔岩与周边火山岩、侵入岩、地层的关系,认为它是早—中三叠世中心裂隙式火山构造中央部位侵出的穹状岩脊。因此,十万大山岩带是火山-侵入杂岩带,台马碎斑熔岩是该火山-侵入杂岩带的一个相。  相似文献   

长白山区二道白河流域早更新世玄武质熔岩的成因   总被引：2，自引：1，他引：1  

马晗瑞  杨清福  盘晓东  武成智  陈聪 《岩石学报》2015,31(11):3484-3494

采用岩石化学和同位素分析方法,研究了二道白河流域早更新世玄武质熔岩的成因。玄武质熔岩由钠质拉斑玄武岩和钾质粗面玄武岩、玄武质粗面安山岩组成。它们的REE分配形式比较相近,表明它们来自共同的源区。Sr、Nd、Pb同位素示踪表明,二道白河流域早更新世玄武质熔岩岩浆源区接近于似原始地幔。它们的Mg#=100Mg O/(Mg O+Fe O)低于中国东部新生代玄武岩原始岩浆的Mg#(60~68),Ni(27.76×10-6~200.6×10-6)低于原始地幔,Rb/Sr(0.05~0.09)、Ba/Rb(15.64~264)高于原始地幔,说明这些岩石不是源自原始地幔。玄武质熔岩的DI变化于42~67,具有高Ca、高Sr、Eu正异常,微量元素图解显示玄武岩保留部分熔融趋势,粗面玄武岩、玄武质粗安岩具有结晶分异趋势,岩浆上升过程中发生了不同程度的地壳混染作用。玄武质熔岩的Nb/Ta之比为14.8~15.8,与勘察加半岛深俯冲带火山类似。Nb/Ta-(Na2O-K2O)关系图解显示研究区玄武质岩浆的形成与俯冲板片的部分熔融有关。  相似文献   

Magma yield stress and permeability: Insights from multiphase percolation theory     

Stuart D.C. Walsh  Martin O. Saar 《Journal of Volcanology and Geothermal Research》2008

Magmas often contain multiple interacting phases of embedded solid and gas inclusions. Multiphase percolation theory provides a means of modeling assemblies of these different classes of magmatic inclusions in a simple, yet powerful way. Like its single phase counterpart, multiphase percolation theory describes the connectivity of discrete inclusion assemblies as a function of phase topology. In addition, multiphase percolation employs basic laws to distinguish separate classes of objects and is characterized by its dependency on the order in which the different phases appear. This paper examines two applications of multiphase percolation theory: the first considers how the presence of bubble inclusions influences yield stress onset and growth in a magma's crystal network; the second examines the effect of bi-modal bubble-size distributions on magma permeability. We find that the presence of bubbles induces crystal clustering, thereby 1) reducing the percolation threshold, or critical crystal volume fraction, ?_c, at which the crystals form a space-spanning network providing a minimum yield stress, and 2) resulting in a larger yield stress for a given crystal volume fraction above ?_c. This increase in the yield stress of the crystal network may also occur when crystal clusters are formed due to processes other than bubble formation, such as heterogeneous crystallization, synneusis, and heterogeneity due to deformation or flow. Further, we find that bimodal bubble size distributions can significantly affect the permeability of the system beyond the percolation threshold. This study thus demonstrates that larger-scale structures and topologies, as well as the order in which different phases appear, can have significant effects on macroscopic properties in multiphase materials.  相似文献   

Dating lava flows of tropical volcanoes by means of spatial modeling of vegetation recovery   总被引：1，自引：0，他引：1       下载免费PDF全文

Long Li  Lien Bakelants  Carmen Solana  Frank Canters  Matthieu Kervyn 《地球表面变化过程与地形》2018,43(4):840-856

The age of past lava flows is crucial information for evaluating the hazards and risks posed by effusive volcanoes, but traditional dating methods are expensive and time‐consuming. This study proposes an alternative statistical dating method based on remote sensing observations of tropical volcanoes by exploiting the relationship between lava flow age and vegetation cover. First, the factors controlling vegetation density on lava flows, represented by the normalized difference vegetation index (NDVI), were investigated. These factors were then integrated into pixel‐based multi‐variable regression models of lava flow age to derive lava flow age maps. The method was tested at a pixel scale on three tropical African volcanoes with considerable recent effusive activity: Nyamuragira (Democratic Republic of Congo), Mt Cameroon (Cameroon) and Karthala (the Comoros). Due to different climatic and topographic conditions, the parameters of the spatial modeling are volcano‐specific. Validation suggests that the obtained statistical models are robust and can thus be applied for estimating the age of unmodified undated lava flow surfaces for these volcanoes. When the models are applied to fully vegetated lava flows, the results should be interpreted with caution due to the saturation of NDVI. In order to improve the accuracy of the models, when available, spatial data on temperature and precipitation should be included to directly represent climatic variation. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Large-scale silicic alkalic magmatism associated with the Buckhorn Caldera, Trans-Pecos Texas, USA: comparison with Pantelleria, Italy     

Don F. Parker  John C. White 《Bulletin of Volcanology》2008,70(3):403-415

Three major rhyolite systems in the northeastern Davis and adjacent Barrilla Mountains include lava units that bracketed a large pantelleritic ignimbrite (Gomez Tuff) in rapid eruptions spanning 300,000 years. Extensive silicic lavas formed the shields of the Star Mountain Formation (37.2 Ma-K/Ar; 36.84 Ma ³⁹Ar/⁴⁰Ar), and the Adobe Canyon Formation (37.1 Ma-K/Ar; 36.51-³⁹Ar/⁴⁰Ar). The Gomez Tuff (36.6 Ma-K/Ar; 36.74-³⁹Ar/⁴⁰Ar) blanketed a large region around the 18×24 km diameter Buckhorn caldera, within which it ponded, forming sections up to 500 m thick. Gomez eruption was preceded by pantelleritic rhyolite domes (36.87, 36.91 Ma-³⁹Ar/⁴⁰Ar), some of which blocked movement of Star Mountain lava flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. Adobe Canyon rhyolite lavas then covered much of the region. Star Mountain Formation (~220 km³) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types form a total of at least six major flows in the northeastern Davis Mountains. Adobe Canyon Formation (~125 km³) contains fewer flows, some up to 180 m thick, of chemically homogenous, mildly peralkalic comendite, extending up to 40 km. Gomez Tuff (~220 km³) may represent the largest known pantellerite. It is typically less than 100 m thick in extra-caldera sections, where it shows a pyroclastic base and top, although interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections contain one cooling unit; two sections contain a smaller, upper cooling unit. Chemically, the tuff is fairly homogeneous, but is more evolved than early pantelleritic domes. Overall, although Davis Mountains silicic units were generated through open system processes, the pantellerites appear to have evolved by processes dominated by extensive fractional crystallization from parental trachytes similar to that erupted in pre- and post-caldera lavas. Comparison with the Pantelleria volcano suggests that the most likely parental magma for the Buckhorn series is transitional basalt, similar to that erupted in minor, younger Basin and Range volcanism after about 24 Ma. Roughly contemporaneous mafic lavas associated with the Buckhorn caldera appear to have assimilated or mixed with crustal melts, and, generally, may not be regarded as mafic precursors of the Buckhorn silicic rocks, They thus form a false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.  相似文献   

Hazards from pyroclastic density currents at Mt. Etna (Italy)     

Boris Behncke   《Journal of Volcanology and Geothermal Research》2009,180(2-4):148

Despite the recent recognition of Mount Etna as a periodically violently explosive volcano, the hazards from various types of pyroclastic density currents (PDCs) have until now received virtually no attention at this volcano. Large-scale pyroclastic flows last occurred during the caldera-forming Ellittico eruptions, 15–16 ka ago, and the risk of them occurring in the near future is negligible. However, minor PDCs can affect much of the summit area and portions of the upper flanks of the volcano. During the past ~ 20 years, small pyroclastic flows or base-surge-like vapor and ash clouds have occurred in at least 8 cases during summit eruptions of Etna. Four different mechanisms of PDC generation have been identified during these events: (1) collapse of pyroclastic fountains (as in 2000 and possibly in 1986); (2) phreatomagmatic explosions resulting from mixing of lava with wet rock (2006); (3) phreatomagmatic explosions resulting from mixing of lava with thick snow (2007); (4) disintegration of the unstable flanks of a lava dome-like structure growing over the rim of one of the summit craters (1999). All of these recent PDCs were of a rather minor extent (maximum runout lengths were about 1.5 km in November 2006 and March 2007) and thus they represented no threat for populated areas and human property around the volcano. Yet, events of this type pose a significant threat to the lives of people visiting the summit area of Etna, and areas in a radius of 2 km from the summit craters should be off-limits anytime an event capable of producing similar PDCs occurs. The most likely source of further PDCs in the near future is the Southeast Crater, the youngest, most active and most unstable of the four summit craters of Etna, where 6 of the 8 documented recent PDCs originated. It is likely that similar hazards exist in a number of volcanic settings elsewhere, especially at snow- or glacier-covered volcanoes and on volcano slopes strongly affected by hydrothermal alteration.  相似文献   

Kaguyak dome field and its Holocene caldera,Alaska Peninsula     

Judy Fierstein  Wes Hildreth 《Journal of Volcanology and Geothermal Research》2008

Kaguyak Caldera lies in a remote corner of Katmai National Park, 375 km SW of Anchorage, Alaska. The 2.5-by-3-km caldera collapsed ~ 5.8 ± 0.2 ka (¹⁴C age) during emplacement of a radial apron of poorly pumiceous crystal-rich dacitic pyroclastic flows (61–67% SiO₂). Proximal pumice-fall deposits are thin and sparsely preserved, but an oxidized coignimbrite ash is found as far as the Valley of Ten Thousand Smokes, 80 km southwest. Postcaldera events include filling the 150-m-deep caldera lake, emplacement of two intracaldera domes (61.5–64.5% SiO₂), and phreatic ejection of lakefloor sediments onto the caldera rim. CO₂ and H₂S bubble up through the lake, weakly but widely. Geochemical analyses (n = 148), including pre-and post-caldera lavas (53–74% SiO₂), define one of the lowest-K arc suites in Alaska. The precaldera edifice was not a stratocone but was, instead, nine contiguous but discrete clusters of lava domes, themselves stacks of rhyolite to basalt exogenous lobes and flows. Four extracaldera clusters are mid-to-late Pleistocene, but the other five are younger than 60 ka, were truncated by the collapse, and now make up the steep inner walls. The climactic ignimbrite was preceded by ~ 200 years by radial emplacement of a 100-m-thick sheet of block-rich glassy lava breccia (62–65.5% SiO₂). Filling the notches between the truncated dome clusters, the breccia now makes up three segments of the steep caldera wall, which beheads gullies incised into the breccia deposit prior to caldera formation. They were probably shed by a large lava dome extruding where the lake is today.  相似文献   

A Miocene coarse volcaniclastic mass-flow deposit in the Shimane Peninsula,SW Japan: product of a deep submarine eruption?     

Kazuhiko Kano 《Bulletin of Volcanology》1996,58(2-3):131-143

 A subaqueous volcaniclastic mass-flow deposit in the Miocene Josoji Formation, Shimane Peninsula, is 15–16 m thick, and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non- to poorly vesiculated rhyolite. It can be divided into four layers in ascending order. Layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0.3–6 m thick. Layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6–11 m thick. This layer bifurcates laterally into minor depositional units individually composed of a massive, lithic-rich lower part and a diffusely stratified, pumice-rich upper part with inverse to normal grading of both lithic and pumice clasts. Layer 3 is 2.5–3 m thick, and consists of interbedded fines-depleted pumice-rich and pumice-poor layers a few centimeters thick. Layer 4 is a well-stratified and well-sorted coarse ash bed 1.5–2 m thick. The volcaniclastic deposit shows internal features of high-density turbidites and contains no evidence for emplacement at a high temperature. The mass-flow deposit is extremely coarse-grained, dominated by traction structures, and is interpreted as the product of a deep submarine, explosive eruption of vesicular magma or explosive collapse of lava. Received: 10 January 1996 / Accepted: 23 February 1996  相似文献   

大别造山带北缘西部中生代晚期火山、沉积地层的时代     

丁莉  杜晓冉  周世全 《华南地质与矿产》2006,(2):48-54

大别造山带北缘西部的一套火山岩及其上的"红层",依据叶肢介、同位素年龄值和较丰富的恐龙蛋和少量介形虫化石,从下而上划分为陈棚组、周家湾组,时代归属早、晚白垩世.陈棚组角度不整合于上侏罗统朱集组或段集组之上,向东与安徽省境内的早白垩世毛坦厂组、白大畈组为相同层位.周家湾组角度不整合于陈棚组之上,含较丰富的恐龙蛋化石,始-古新世李庄群不整合覆于其上.  相似文献   

新疆西天山艾肯达坂组火山岩的地球化学特征     

姬红星 《物探与化探》2007,31(3):218-220

新疆西天山艾肯达坂组火山岩的岩石学和稀土元素的地球化学等研究证明，本区火山岩由基性火山岩和中性火山岩组成，火山岩均属于钙碱性系列，二者均具有低钾、富钠的特点。这些火山岩的稀土元素配分曲线均属于轻稀土富集型，明显地表现为轻稀土元素配分曲线陡倾，而重稀土元素配分曲线相对平坦的特征。这些岩石学和地球化学的特征证明本区火山岩形成于裂谷环境。  相似文献