祁连地块北缘前寒武基底早古生代再活化:变基性岩相平衡模拟和锆石年代学证据     

毛小红  张建新  路增龙  周桂生 《岩石学报》2021,37(10):3095-3117

北祁连造山带是典型的早古生代增生型造山带，具有典型增生型造山带的"沟-弧-盆"体系。祁连地块位于北祁连造山带南部，由深变质的前寒武纪基底和新元古代到中生代的沉积盖层组成。在连祁地块北缘门源宝库河附近延伸数千米的变基性岩（主体为斜长角闪岩和角闪岩）中识别出了基性麻粒岩，峰期矿物组合为石榴子石+斜长石+钾长石+普通辉石+角闪石+钛铁矿+石英，相平衡模拟计算结果显示峰期温压条件为P=10.6kbar、T=800℃，峰期之后经历了近等温降压的P-T轨迹。锆石LA-ICP-MS U-Pb年代学结果显示变基性岩的原岩结晶年龄分别为1110±18Ma和1140±30Ma，变质年龄分别为478.2±3.9Ma和469±4.6Ma，结合近等温降压的P-T轨迹，469~478Ma的变质年龄应近似代表了麻粒岩相变质作用时代。结合区域资料，我们认为早古生代北祁连洋存在南北双向俯冲极性，祁连洋南向俯冲使得祁连地块前寒武纪基底在早古生代再活化。变基性岩的原岩年龄与祁连地块南部欧龙布鲁克地块（全吉地块）中发现的变基性岩和花岗质片麻岩原岩年龄及副片麻岩的变质年龄一致，均为~1100Ma，表明祁连地块与欧龙布鲁克地块一起卷入了格林威尔造山事件。  相似文献   

有效全岩成分的计算方法及其在变质相平衡模拟中的应用     

田作林  张泽明  董昕 《岩石学报》2020,36(9):2616-2630

变质相平衡模拟是变质岩领域近几十年最重要的进展之一，它已经成为确定变质作用P-T-t轨迹和探索变质演化过程的有力工具。变质岩的矿物组合不但与其形成的温度（T）和压力（P）条件有关，而且受控于岩石的全岩成分（X）。但是变质岩通常是不均匀的并且往往保留两期以上的矿物组合，因此计算不同成分域或不同变质演化期次的有效全岩成分是模拟P-T视剖面图的核心问题之一。在中-低温变质岩中，石榴石变斑晶的生长会不断地将其核部成分"冻结"而不参与后续变质反应，这导致根据实测全岩成分计算的P-T视剖面图无法有效地模拟石榴石幔部或边部生长阶段的变质演化过程。"瑞利分馏法"和"球体积法"利用电子探针实测的石榴石成分环带可以模拟计算石榴石各个生长阶段所对应的有效全岩成分，本文推荐使用这两个方法来处理石榴石变斑晶的分馏效应问题。相比较而言，石榴石在高温变质岩中通常无法保留生长阶段的成分环带特征，这是因为石榴石成分在高温条件下会发生扩散再平衡，并同时与多数基质矿物达到热力学平衡，这时一般不需要考虑石榴石的分馏效应。但是高温变质岩通常会发生部分熔融并伴随熔体的迁移，进而改变岩石的有效全岩成分。因此，通过P-T视剖面图模拟熔体迁移前后的变质演化过程需要使用"相平衡法"计算迁移的熔体成分以及熔体迁移前后岩石的有效全岩成分。此外，后成合晶与反应边是变质岩中最常见的退变质反应结构，但是后成合晶或反应边中的矿物之间并未达到热力学平衡。这种情况需要结合岩相学观察和矿物成分，利用最小二乘法确定后成合晶或反应边中发生的平衡反应方程式，进而获取变质反应发生时的有效全岩成分并通过计算P-T视剖面图来估算退变质的温压条件。除此之外，岩石体系中三价铁（Fe₂O₃）和H₂O含量的估算一直以来都是相平衡模拟研究中的难点，本文推荐使用P/T-X（Fe³⁺/Fe^tot，MH₂O）视剖面图来确定这两个组分的含量，这是因为P/T-X图可以估算各个变质演化阶段或特定矿物组合的Fe₂O₃或H₂O含量。  相似文献   

Liquid immiscibility and its role at the magmatic–hydrothermal transition: a summary of experimental studies     

Ilya V. Veksler 《Chemical Geology》2004,210(1-4):7-31

Several types of fluid immiscibility may affect the evolution of volatile-rich magmatic systems at the magmatic–hydrothermal transition. The topology of silicate–salt–H₂O systems implies that three-fluid immiscibility (silicate melt+hydrosaline melt+vapour) should be stable in a broad range of compositions and P–T conditions. The most important factor controlling the immiscibility appears to be the Coulombic properties (electric charges Z and ionic radii r) of the main network-modifying cations and the capacity for immiscibility appears to decrease in the following sequence: Mg>Ca>Sr>Ba>Li>Na>K. Liquid immiscibility is enhanced in peralkaline compositions and in the presence of nonsilicate anions such as F⁻, Cl⁻, CO₃²⁻ and BO₃³⁻. In volatile-rich magmatic systems, the H₂O is likely to react with the chloride, fluoride, borate and carbonate species and the chemical effects of high-temperature hydrolysis may be greatly enhanced by phase separation in systems with multiple immiscible fluid phases. Natural granitic magmas can thus exsolve a range of chemically and physically diverse hydrosaline liquids and the role of these fluid phases is likely to be especially significant in pegmatites and Li–F rare-metal granites.  相似文献   

A counterclockwise PTt path of high-pressure/low-temperature rocks from the Coastal Cordillera accretionary complex of south-central Chile: constraints for the earliest stage of subduction mass flow     

A. P. Willner  J. Glodny  T. V. Gerya  E. Godoy  H. -J. Massonne 《Lithos》2004,75(3-4):283-310

Within the metamorphic basement of the Coastal Cordillera of central Chile, the Western Series constitutes the high-pressure (HP)/low-temperature (LT) part (accretionary prism) of a fossil-paired metamorphic belt dominated by metagreywackes. In its eastern part, blocks derived from small lenses of garnet amphibolite with a blueschist facies overprint are locally intercalated and associated with serpentinite and garnet mica-schist. Continuously developed local equilibria were evaluated applying various independent geothermobarometric approaches. An overall anticlockwise PT path results. The prograde path evolved along a geothermal gradient of 15 °C/km, passing the high-pressure end of greenschist facies until a transient assemblage developed within albite-epidote amphibolite facies transitional to eclogite facies at peak metamorphic conditions (600–760 °C, 11–16.5 kbar; stage I). This peak assemblage was overprinted during an external fluid infiltration by an epidote blueschist facies assemblage at 350–500 °C, 10–14 kbar (stage II) indicating nearly isobaric cooling. The retrograde equilibration stage was dated with a Rb–Sr mineral isochron at 305.3±3.2 Ma, somewhat younger (296.6±4.7 Ma) in an adjacent garnet mica-schist. Localized retrograde equilibration continued during decompression down to 300 °C, 5 kbar. The retrograde evolution is identical in the garnet amphibolite and the garnet mica-schist.

The counterclockwise PT path contrasts the usual clockwise PT paths derived from rocks of the Western Series. In addition, their ages related to stage II are the oldest recorded within the fossil wedge at the given latitude. Its “exotic” occurrence is interpreted by the path of the earliest and deepest subducted material that was heated in contact with a still hot mantle. Later accreted and dehydrated material caused hydration and cooling of the earliest accreted material and the neighbouring mantle. After this change also related to rheological conditions, effective exhumation of the early subducted material followed at the base of the hydrated mantle wedge within a cooler environment (geothermal gradient around 10–15 °C/km) than during its burial. The exotic blocks thus provide important time markers for the onset of subduction mass circulation in the Coastal Cordillera accretionary prism during the Late Carboniferous. Continuous subduction mass flow lasted for nearly 100 Ma until the Late Triassic.  相似文献   

Biot固结理论在地面沉降计算中的应用   总被引：8，自引：0，他引：8  

陈杰  朱国荣  顾阿明  王彩会 《水文地质工程地质》2003,30(2):28-31

在回顾地面沉降研究现状的基础上,提出采用Biot固结理论进行由降水形成的渗流场与应力场的耦合分析,并运用二维Biot固结平面有限元程序计算因开采地下水而引起的地面沉降问题,最后通过工程实例验证了该方法的可行性。  相似文献   

Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia   总被引：16，自引：2，他引：16  

B. A. Marmo  G. L. Clarke  R. Powell 《Journal of Metamorphic Geology》2002,20(1):151-165

Chemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards. An effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock. To examine this effect on mineral assemblages, the Bence–Albee matrix correction was applied to X‐ray intensity maps collected using eclogite samples from northern New Caledonia in order to determine the chemical composition of all parts of the sample. The manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix. A series of calculated equilibrium‐volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high‐P conditions. Pressure–temperature pseudosections are calculated for these compositions, in the CaO–Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O system. Assemblages, modal proportions and mineral textures observed in the New Caledonian eclogites can be closely modelled by progressively ‘removing’ elements forming garnet cores from the bulk rock composition. The pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage, prograde textures and the development of retrograde assemblages.  相似文献   

低渗油气储层物理参数解释模型——以宝浪油田煤系粗粒低渗储层为例   总被引：3，自引：2，他引：1  

周锋德  姚光庆  魏忠元  刘斌  黄郑 《地质科技情报》2006,25(6):67-71

为了建立宝浪油田粗粒低渗储层的精确渗透率解释模型,从储层物性的影响因素分析入手,确定影响此类储层物性的关键属性参数.从表征岩石结构参数的流动带指数的角度建立了孔隙度和渗透率的关系模型,逐步建立以测井数据为基础的流动单元的判别函数.在流动单元判别的基础上,根据孔隙度和渗透率的关系模型进行了从测井数据到渗透率参数的计算.宝浪油田三工河组储层Ⅲ油组划分出5类流动单元类型,在流动单元控制下解释的储层渗透率值,经实际应用能够满足储层精细描述的要求.  相似文献   

Metamorphism,fluid behavior and magmatism in oceanic subduction zones     

Wei  Chunjing  Zheng  Yongfei 《中国科学:地球科学(英文版)》2020,63(1):52-77

Based on the updated results of experimental petrology and phase equilibria modelling and combined with the available thermal structure models of subduction zones, this paper presents an overview on the dehydration and melting of basic,sedimentary and ultrabasic rocks that occur in the different stages during oceanic subduction processes and their influences on magmatism above subduction zones. During the subduction at the forearc depth of <90–100 km, the basic and ultrabasic rocks from most oceanic slabs can release very small amounts of water, and significant dehydration may occur in the slab superficial sediments. Strong dehydration occurs in both basic and ultrabasic rocks during subduction at the subarc depth of 90–200 km. For example, more than 90% water in basic rocks is released by the successive dehydration of chlorite, glaucophane, talc and lawsonite in the subarc depths. This is diversely in contrast to the previous results from synthetic experiments. Ultrabasic rocks may undergo strong dehydration through antigorite, chlorite and phase 10 ? at the subarc depth of 120–220 km. However,sediments can contribute minor fluids at the subarc depth, one main hydrous mineral in which is phengite(muscovite). It can stabilize to ~300 km depth and transform into K-hollandite. After phengite breaks down, there will be no significant fluid release from oceanic slab until it is subducted to the mantle transition zone. In a few hot subduction zones, partial melting(especially flux melting) can occur in both sediments and basic rocks, generating hydrous granitic melts or supercritical fluids, and in carbonates-bearing sediments potassic carbonatite melts can be generated. In a few cold subduction zones, phase A occurs in ultrabasic rocks, which can bring water deep into the transition zone. The subducted rocks, especially the sediments, contain large quantities of incompatible minor and trace elements carried through fluids to greatly influence the geochemical compositions of the magma in subduction zones. As the geothermal gradients of subduction zones cannot cross the solidi of carbonated eclogite and peridotite during the subarc subduction stage, the carbonate minerals in them can be carried into the deep mantle.Carbonated eclogite can melt to generate alkali-rich carbonatite melts at >400 km depth, while carbonated peridotite will not melt in the mantle transition zone below a subduction zone.  相似文献   

Three-dimensional solutions of the magnetohydrostatic equations for rigidly rotating magnetospheres in cylindrical coordinates     

F. Wilson  T. Neukirch 《地球物理与天体物理流体动力学》2018,112(1):74-95

We present new analytical three-dimensional solutions of the magnetohydrostatic equations, which are applicable to the co-rotating frame of reference outside a rigidly rotating cylindrical body, and have potential applications to planetary magnetospheres and stellar coronae. We consider the case with centrifugal force only, and use a transformation method in which the governing equation for the “pseudo-potential” (from which the magnetic field can be calculated) becomes the Laplace partial differential equation. The new solutions extend the set of previously found solutions to those of a “fractional multipole” nature, and offer wider possibilities for modelling than before. We consider some special cases, and present example solutions.  相似文献   

Piecewise uniform potential vorticity pancake shielded vortices     

Jean N. Reinaud 《地球物理与天体物理流体动力学》2017,111(1):32-64

Shielded vortices consist of a core of potential vorticity (PV) of a given sign surrounded (or shielded) by a layer of opposite-signed PV. Such vortices have specific properties and have been the focus of numerous studies, first in two dimensional geometries (where PV is just the vertical component of the vorticity vector) and in geophysical applications (mostly in layered models). The present paper focuses on three-dimensional, spheroidal shielded vortices. In particular, we focus on vortical structures whose overall volume-integrated PV is zero. We restrict attention to vortices of piecewise uniform PV in the present research. We first revisit the problem within the quasi-geostrophic model, then we extend the results to the non-hydrostatic regime. We show that the stability of the structure depends on the ratio of PV between the inner core and the outer shield. In particular it depends on the polarity of the core and of the wavenumber of the azimuthal mode perturbed.  相似文献