Olivine-rich units in the Fongen–Hyllingen Intrusion, Norway: implications for magma chamber processes     

Gurli B. Meyer  J. Richard Wilson 《Lithos》1999,47(3-4):157-179

The Fongen–Hyllingen Intrusion (FHI) is considered to have crystallised from stratified magma residing in a bowl-shaped magma chamber. Seven olivine-rich units, representing the most primitive cumulates in the central part of the intrusion, are associated with compositional reversals and are interpreted as having formed at the lowest part of the magma chamber floor. Based on phase-relationships, the crystallisation order is explained in terms of magma mixing and fractional crystallisation. Repeated influxes of small volumes of dense, primitive magma at the base of the chamber had a major impact on the crystallising assemblage on the local floor and a decreasing effect towards the flanks of the chamber. This was due to the small volume of replenishing magma, the geometry of the chamber and the consequent restriction of magma mixing to the deepest part of the chamber where the new magma was emplaced. It is estimated that the chamber floor sloped as little as 1–2°, but this was sufficient to give widely different cumulate sequences near the bottom of the chamber and on the flanks.  相似文献   

Does the Ulleung eddy owe its existence to β and nonlinearities?     

Wilton Z. Arruda  Doron Nof  James J. O&#x;Brien 《Deep Sea Research Part I: Oceanographic Research Papers》2004,51(12):2073-2090

A nonlinear theory for the generation of the Ulleung Warm Eddy (UWE) is proposed. Using the nonlinear reduced gravity (shallow water) equations, it is shown analytically that the eddy is established in order to balance the northward momentum flux (i.e., the flow force) exerted by the separating western boundary current (WBC). In this scenario, the presence of β produces a southward (eddy) force balancing the northward momentum flux imparted by the separating East Korean Warm Current (EKWC).It is found that, for a high Rossby number EKWC (i.e., highly nonlinear current), the eddy radius is roughly 2R_d/ε^1/6 (here ε≡βR_d/f₀, where R_d is the Rossby radius), implying that the UWE has a scale larger than that of most eddies (R_d). This solution suggests that, in contrast to the familiar idea attributing the formation of eddies to instabilities (i.e., the breakdown of a known steady solution), the UWE is an integral part of the steady stable solution. The solution also suggests that a weak WBC does not produce an eddy (due to the absence of nonlinearity).A reduced gravity numerical model is used to further analyze the relationship between β, nonlinearity and the eddy formation. First, we show that a high Rossby number WBC which is forced to separate from the wall on an f plane does not produce an eddy near the separation. To balance the northward momentum force imparted by the nonlinear boundary current, the f plane system moves constantly offshore, producing a southward Coriolis force. We then show that, as β is introduced to the problem, an anticyclonic eddy is formed. The numerical balance of forces shows that, as suggested by the analytical reasoning, the southward force produced by the eddy balances the northward flow force imparted by the boundary current. We also found that the observed eddy scale in the Japan/East Sea agrees with the analytical estimate for a nonlinear current.  相似文献   

京西房山岩体南侧青白口系下马岭组构造片岩中红柱石的变形特征及其地质意义     

万永平王根厚  郭春影于海亮 张静 《地质通报》2007,26(2):200-205

在房山岩体南侧下马岭组构造片岩中发现红柱石横截面上发育一组共扼裂隙和“环带状”构造,环带形状与红柱石变斑晶横截面的形状相似,均压扁为长方形,红柱石的变形是同构造递进变形的结果。裂隙及环带均被流体包裹体所充填,流体包裹体的热爆温度为604～685℃,利用黑云母-石榴子石温度计得出变质温度为592℃。计算出变质发生深度为2.69～3.11km。  相似文献   

单轴压缩条件下房山花岗岩超声波速度成像实验研究     

和泰名  刘政一  李世愚 《CT理论与应用研究》2015,24(5):657-665

在实验室对房山花岗岩进行单轴压缩加载,通过布设在岩石四周的26个超声波探头,在不同变形阶段,观测了岩石的超声波速度,反演了P波速度结构。速度成像结果显示,在压实和线弹性变形阶段,P波速度逐步全局升高。最终失稳破裂前,高速区域范围逐渐收缩,同时,形成条带低速带,最终失稳破裂发生在低速带上。   相似文献   

北京房山北岭向斜形成与形变综合解析   总被引：1，自引：0，他引：1  

李东旭 《现代地质》2015,29(3):481-493

弯月状北岭向斜是北京房山周口店地区的主体构造形迹,也是一个非常特殊的构造现象。它的原型是晚古生代和侏罗纪两套陆相盆地叠加而成的复合盆地,其特征与整体华北板块的构造演化一致;而其晚期(燕山期）的褶皱变形是由于房山花岗闪长岩体侵位造成的局部动力作用结果。特别是不同于教科书中所讲的褶皱形成机制和构造叠加的一般概念。当我们采用系统论即整体论的思考方式会将一般看来互不相关的局部构造细节联系起来思考,其成因就会迎刃而解。再有,由于这里的地质构造与地层建造、岩浆活动、变质作用等构成相关的复杂系统,因此必须采用综合构造解析,才可能对北岭向斜的形成与形变的复杂过程获得较全面的理解。  相似文献   

2010年智利8.8级地震在北京房山岩体附近的动态触发活动          下载免费PDF全文

龚萱  陈棋福  彭志刚  王伟君  吴春泉  吴晶 《地球物理学报》2014,57(1):115-128

本文采用波形分析和β统计方法,基于首都圈地区数字测震台网和部分流动地震台的观测资料,对2010年2月27日M_w8.8智利大地震在北京房山岩体附近地区的动态触发活动进行了分析讨论.结果表明,在背景地震活动相对较弱的北京房山岩体附近,我们检测出至少5次小震活动事件在智利地震面波到达时发生.这些触发活动显著地改变了房山岩体地区这一地震活动相对平静地区的地震活动性,但是对北京地区地震活动性的影响并不明显.NKY地震台记录的智利地震触发活动的最大动态应力与之前的研究结果相比要小,约为7 kPa.这可能与房山岩体附近地区的背景地震在智利地震前一直较为平静有关.此外,相对有利的面波入射方向,以及在对蹠点上的前两组面波叠加的效应等因素综合作用,使得本研究能在约2万公里极远处观测到远震触发小震活动.然而,在随后的多次面波叠加期间并没有观测到明显的触发地震活动.在智利地震10 h前发生的琉球M_w7.0地震也未在房山地区触发相关的微震活动,这可能与它们引起的动态应力变化太过微弱有关.  相似文献   

A model for radial dike emplacement in composite cones based on observations from Summer Coon volcano,Colorado, USA   总被引：1，自引：0，他引：1  

Michael P. Poland  William P. Moats  Jonathan H. Fink 《Bulletin of Volcanology》2008,70(7):861-875

We mapped the geometry of 13 silicic dikes at Summer Coon, an eroded Oligocene stratovolcano in southern Colorado, to investigate various characteristics of radial dike emplacement in composite volcanoes. Exposed dikes are up to about 7 km in length and have numerous offset segments along their upper peripheries. Surprisingly, most dikes at Summer Coon increase in thickness with distance from the center of the volcano. Magma pressure in a dike is expected to lessen away from the pressurized source region, which would encourage a blade-like dike to decrease in thickness with distance from the center of the volcano. We attribute the observed thickness pattern as evidence of a driving pressure gradient, which is caused by decreasing host rock shear modulus and horizontal stress, both due to decreasing emplacement depths beneath the sloping flanks of the volcano. Based on data from Summer Coon, we propose that radial dikes originate at depth below the summit of a host volcano and follow steeply inclined paths towards the surface. Near the interface between volcanic cone and basement, which may represent a neutral buoyancy surface or stress barrier, magma is transported subhorizontally and radially away from the center of the volcano in blade-like dikes. The dikes thicken with increasing radial distance, and offset segments and fingers form along the upper peripheries of the intrusions. Eruptions may occur anywhere along the length of the dikes, but the erupted volume will generally be greater for dike-fed eruptions far from the center of the host volcano owing to the increase in driving pressure with distance from the source. Observed eruptive volumes, vent locations, and vent-area intrusions from inferred post-glacial dike-fed eruptions at Mount Adams, Washington, USA, support the proposed model. Hazards associated with radial dike emplacement are therefore greater for longer dikes that propagate to the outer flanks of a volcano.  相似文献   

北京房山岩体的磁组构特征及其 对岩体侵位的约束   总被引：3，自引：0，他引：3       下载免费PDF全文

纪新林  王磊  潘永信 《地球物理学报》2010,53(7):1671-1680

岩浆岩的磁组构与岩浆流动、侵位时应力和冷凝后遭受后期构造改造作用有关.本文分析了北京房山岩体东山口-凤凰亭剖面岩石磁化率各向异性(AMS)变化.热磁曲线和磁滞回线分析显示,岩石的主要载磁矿物为多畴磁铁矿颗粒.AMS磁化率椭球体呈压扁状,磁面理发育且产状陡倾,磁化率各向异性度P值平均值高达1.189.自边缘相(东山口)到中心相(凤凰亭),岩石磁化率椭球的扁率E值和形状因子T值逐渐减小,说明磁组构主要反映岩浆流动和岩体侵位时近NNE-SSW方向的挤压作用,在侵位冷凝后并没有遭受到明显的构造改造.  相似文献   

The age and petrology of the Chimbadzi Hill Intrusion, NW Zimbabwe: first evidence for early Paleoproterozoic magmatism in Zimbabwe     

Tawanda D. Manyeruke  Thomas G. Blenkinsop  Peter Buchholz  David Love  Thomas Oberthür  Ulrich K. Vetter  Donald W. Davis 《Journal of African Earth Sciences》2004,40(5):281

The mafic-ultramafic Chimbadzi Hill intrusion in the NW of the Zimbabwe craton is a dyke with inward-dipping margins comprising magnetite peridotite, troctolite and magnetite melatroctolite. The magnetite peridotite is composed of about equal amounts of V- and Ti-bearing magnetite and olivine (Fo₆₀). The troctolite is composed of about 50% olivine (Fo_50-54), 40% plagioclase (An_53-58), 7% clinopyroxene and minor apatite and magnetite with ilmenite lamellae. Geochemical trends suggest that the Chimbadzi Hill Intrusion formed by fractional crystallisation from a single initial magma. However, the more primitive magnetite peridotite overlies the more evolved troctolite in the intrusion. This ‘apparent’ inverted stratigraphy may be due to emptying of a fractionated magma chamber from the top, or to floor subsidence during intrusion.U–Pb dating on baddeleyite reveals that the age of the Chimbadzi Hill Intrusion is 2262 ± 2 Ma. This age does not correspond to any known tectono-thermal event in the Zimbabwe Craton or adjacent metamorphic belts. It is 300 Ma younger than the late Archean Great Dyke, and 230 Ma older than other Paleoproterozoic events in and around the craton. Therefore, it may represent a so far undocumented very early Proterozoic igneous event in the Zimbabwe Craton. The intrusion represents a vanadium resource for Zimbabwe, with titanium potentially being mined as by-product.  相似文献   

Relationship between the Layered Series and the overlying evolved rocks in the Bjerkreim-Sokndal Intrusion, southern Norway   总被引：1，自引：0，他引：1  

J. Richard Wilson  Gitte Overgaard 《Lithos》2005,83(3-4):277-298

The Bjerkreim-Sokndal layered intrusion (BKSK) consists of a > 7000-m-thick Layered Series comprising anorthosites, leuconorites, troctolites, norites, gabbronorites and jotunites (hypersthene monzodiorites), overlain by an unknown thickness of massive, evolved rocks: mangerites (hypersthene monzonites; MG), quartz mangerites (QMG) and charnockites (CH). The Layered Series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma. We have studied a ca. 2000-m-thick section that straddles the sequence from the uppermost part of the Layered Series to the QMG in the northern part of the intrusion. Mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the MG-unit (plagioclase An37-18; olivine Fo40-7; Ca-poor pyroxene Mg#57-15; Ca-rich pyroxene Mg#65-21). Above this compositions are essentially constant in the upper part of the MG-unit and in the QMG (An21-13; Fo6-4; Mg#opx17-13; Mg#cpx25-20). The amount of interstitial quartz and the amount of normative orthoclase, however, both increase systematically upwards through the QMG-unit, implying that these rocks are cumulates. There is no evidence of a compositional break in the MG-QMG sequence that could reflect influx of relatively primitive magma.

Two types of QMG/CH are known in the uppermost part of BKSK. Olivine-bearing types are comagmatic with the underlying Layered Series; the studied stratigraphic sequence belongs to this suite. Two-pyroxene QMG and amphibole CH define a separate compositional lineage related to jotunites. An intrusive unit of dominantly two-pyroxene QMG is discordant to the olivine-bearing jotunite-MG-QMG sequence near Rapstad, confirming the presence of two compositionally distinct suites of QMG and related lithologies in the upper part of BKSK.

A xenolith-rich unit near the olivine-bearing MG-QMG boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation.  相似文献