首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 359 毫秒
1.
The anisotropy of magnetic susceptibility has been proven to be an excellent indicator for mineral fabrics and therefore deformation in a rock or sediment. Low-field anisotropy is relatively rapid to measure so that a sufficient number of samples can be measured to obtain a good statistical representation of the magnetic fabric. The physical properties of individual minerals that contribute to the observed magnetic fabric include bulk susceptibility and intrinsic anisotropy of the mineral phase, its volume concentration, and its degree of alignment. Several techniques have been developed to separate magnetic subfabrics arising from magnetization types, i.e., ferrimagnetism, antiferromagnetism, paramagnetism, and diamagnetism. Susceptibility anisotropy can be measured in low or high fields and at different temperatures in order to isolate a particular subfabric. Measuring the anisotropy of a remanent magnetization can also isolate ferrimagnetic fabrics. A series of case studies are presented that exemplify the value of isolating magnetic subfabrics in a geological context. It is particularly useful in rocks that carry a paramagnetic and diamagnetic subfabric of similar magnitude, such that they negate one another. Further examples are provided for purely paramagnetic subfabrics and cases where a ferrimagnetic subfabric is also identified.  相似文献   

2.
During the ascent, emplacement and post-emplacement deformation of igneous rocks, two or more phases of deformation that overprint each other are often depicted. These overprints, when magnetic minerals are present, are recorded in magnetic fabric. In this contribution, overprints are studied by means of numerical modeling, following several basic scenarios common to igneous rocks. Biotite and amphibole that occur often together in igneous rocks are considered as carriers of the anisotropy of magnetic susceptibility. Modeling shows that (1) a constrictional fabric with a low degree of anisotropy as commonly recorded in magmatic rocks may result from a deformation overprint and not necessarily from an extensional/transtensional regime, and (2) that the constrictional AMS fabrics originates from orthogonal superimposition of a deformation event on an AMS fabric inherited from earlier magma emplacement history. Therefore, the interpretation of a constrictional fabric must be performed with caution. Numerical modeling may provide a suitable help in strengthening the interpretation of real magnetic fabric data.  相似文献   

3.
The progressive development of mylonitic fabrics in a series of Torridonian sandstones and shales has been studied along traverses across the Kishorn Nappe. The fabrics developed have been investigated using the following techniques.
1. 1. Optical examination of thin sections.
2. 2. Measurements of the anisotropy of magnetic susceptibility.
3. 3. X-ray texture goniometry.
The results are used in support of a proposed deformation history of the area and the relative advantages of the techniques used are discussed.The early deformation was well lubricated with layer-parallel sliding and little internal deformation of the rocks, except for development, in the east, of a layer-parallel penetrative fabric with an extension direction to the ESE. This deformation produced a westward facing isoclinal anticline and a recumbent syncline in the Torridonian rocks which became at least partly decoupled from the basement.The important phases of fabric development post date this folding. In the west the sandstones developed a spaced, pressure solution cleavage, but in the east the grain shape fabric has been produced by both dislocation and diffusion processes. The shales reveal more details of the deformation episodes than do the sandstones and thus show different fabric intensities and orientations when measured by magnetic and X-ray techniques.The magnetic anisotropy technique of fabric analysis gives a rapid method of mapping the deformation domains formed by different deformation mechanisms and intensities. However, the rocks carry several magnetic components and these have different anisotropy tensors and different responses to deformation, also, measurements made at high fields (5 kOe) give magnitudes and orientations of the magnetic anisotropy tensor which are different from those made at low fields. It is concluded that it is not possible to relate variations in the magnitude and shape of the magnetic anisotropy ellipsoid quantitatively to the deformation.Chlorite and muscovite fabrics measured by X-ray techniques show variations in intensity and orientation similar to those of the magnetic anisotropy ellipsoid due to paramagnetic minerals. However, the data demonstrate the difficulty of correlating this fabric intensity with deformation intensity where there has been a change in deformation mechanisms with time and space.  相似文献   

4.
Besides granites of the ilmenite series, in which the anisotropy of magnetic susceptibility (AMS) is mainly controlled by paramagnetic minerals, the AMS of igneous rocks is commonly interpreted as the result of the shape-preferred orientation of unequant ferromagnetic grains. In a few instances, the anisotropy due to the distribution of ferromagnetic grains, irrespective of their shape, has also been proposed as an important AMS source. Former analytical models that consider infinite geometry of identical and uniformly magnetized and coaxial particles confirm that shape fabric may be overcome by dipolar contributions if neighboring grains are close enough to each other to magnetically interact. On these bases we present and experimentally validate a two-grain macroscopic numerical model in which each grain carries its own magnetic anisotropy, volume, orientation and location in space. Compared with analytical predictions and available experiments, our results allow to list and quantify the factors that affect the effects of magnetic interactions. In particular, we discuss the effects of (i) the infinite geometry used in the analytical models, (ii) the intrinsic shape anisotropy of the grains, (iii) the relative orientation in space of the grains, and (iv) the spatial distribution of grains with a particular focus on the inter-grain distance distribution. Using documented case studies, these findings are summarized and discussed in the framework of the generalized total AMS tensor recently introduced by Cañon-Tapia (Cañon-Tapia, E., 2001. Factors affecting the relative importance of shape and distribution anisotropy in rocks: theory and experiments. Tectonophysics, 340, 117–131.). The most important result of our work is that analytical models far overestimate the role of magnetic interaction in rock fabric quantification. Considering natural rocks as an assemblage of interacting and non-interacting grains, and that the effects of interaction are reduced by (i) the finite geometry of the interacting clusters, (ii) the relative orientation between interacting grains, (iii) their heterogeneity in orientation, shape and bulk susceptibility, and (iv) their inter-distance distribution, we reconcile analytical models and experiments with real case studies that minimize the role of magnetic interaction onto the measured AMS. Limitations of our results are discussed and guidelines are provided for the use of AMS in geological interpretation of igneous rock fabrics where magnetic interactions are likely to occur.  相似文献   

5.
Experimental shear zones and magnetic fabrics   总被引：1，自引：0，他引：1  
Magnetic fabric analysis has been used as a non-destructive means of detecting petrofabric development during experimentally produced multi-stage, transpressive deformations in ‘shear zones’. Artificial, magnetic-bearing silicate sands and calcite sands, bonded with Portland cement, were deformed at room temperature and at 100 and 150 MPa confining pressure. The slip-rate for the shear zone walls was 0.73 × 10−4 mm s−1 and the maximum shear strains were about 0.38, across zones that were initially about 5 mm thick. The magnetic fabric ellipsoid rapidly spins so that the maximum and intermediate susceptibilities tend to become parallel to the shear zone walls throughout the sheared zone. The ellipsoid becomes increasingly oblate with progressive deformation. However, in all cases, the anisotropy is strongly influenced by the pre-deformation magnetic fabric. During deformation the cement gel collapses so that cataclasis of the mineral grains is suppressed. In the quartz-feldspar aggregates the magnetite's alignment is accommodated by particulate flow (intergranular displacements) of the grains. In the calcite aggregates stronger magnetic fabrics develop due to plastic deformation of calcite grains as well as particulate flow. However, the calcite grain fabrics are somewhat linear (L ≥ S) whereas the magnetic fabrics are planar (S >L). The preferred dimensional orientations of magnetite are weak and it is possible that the magnetic fabrics are due to intragranular rearrangements of magnetic domains.The transpressive shear zones are much more efficient than axial-symmetric shortening in the increase of anisotropy of the magnetic fabrics, especially in the case of the calcite aggregates. This suggests that flow laws derived for axial-symmetric shortening experiments may not be appropriate for non-coaxial strain histories such as those of shear zones.  相似文献   

6.
The fabric elements in pelitic rocks of Brioverian age in South Finistere (France) consist of three main components; planar, linear and discrete, corresponding to schistosity, elongate mineral alignment and porphyroblast distribution respectively. Each component plays a role in the initiation, propagation and evolution of crenulations which deform the planar anisotropy.Strain associated with the initiation of crenulation is heterogeneous on the thin section scale, producing spatially associated, contemporaneous microshears and kink-structures. The influence of porphyroblasts becomes increasingly evident as crenulation develops and microshearing becomes the dominant mode of deformation. Linear fabric appears to influence the orientation of crenulation structures and slip directions on microshear planes.  相似文献   

7.
In the high-grade Moldanubian Zone of the European Variscides, numerous bodies of ultramafic rocks occur embedded in granulite. The anisotropy of magnetic susceptibility and its low-field variation as well as the anisotropy of magnetic remanence were used to investigate magnetic fabrics of some ultramafic bodies and host granulite. In granulite, the magnetic foliation is roughly parallel to the metamorphic foliation and the magnetic lineation is near the mineral alignment lineation. In ultramafite, the magnetic foliation is relatively scattered spatially, but mostly oriented in a different way than that in granulite. The magnetic lineation is also scattered, but still relatively well defined spatially. Again, its orientation is mostly different than that of granulite. The magnetic fabric in ultramafic rocks is therefore different from that in the host granulite even though both rock types underwent at least partially common structural history. The componental movements forming the granulite fabric, mostly during amphibolite facies retrograde metamorphism, were evidently not intensive enough to strongly overprint the magnetic fabric of ultramafite. The ultramafite is therefore strong enough to maintain its pre-metamorphism fabric even at such high temperatures and pressures that are characteristic of high amphibolite facies retrograde metamorphism.  相似文献   

8.
The anisotropy of magnetic susceptibility (AMS) was systematically measured for samples collected across the Lachlan Transverse Zone in the Eastern Subprovince of the Lachlan Orogen, New South Wales. Although the degree of anisotropy is usually moderate to low, it can be shown that the origin of the magnetic fabric is generally composite. Many localities are witness to a tectonic influence in addition to a magnetic foliation preserved from the time of rock formation (compaction). Furthermore, some localities indicate the presence of superimposed magnetic fabrics, potentially associated with a Silurian east–west direction of shortening, and a younger north–south (?) direction of shortening. Finally, the progressive southwards change in orientation of the magnetic lineation in the Molong area from north–south to east–west and then back to north–south again south of the Lyndhurst–Neville Fault suggests that the Lachlan Transverse Zone coincides with, and reflects, a major cross-structure in the Eastern Subprovince. AMS is thus a powerful tool to help map the fabric of Paleozoic rocks in the Tasmanides. Additional data will be required to help obtain a comprehensive picture of the tectonic history of the region.  相似文献   

9.
柴达木盆地西部狮子沟一带新生代沉积岩磁组构分析结果显示, 岩石磁组构具有磁面理发育、磁线理不发育、磁化率量值椭球呈压扁状的特点; 磁化率各向异性度P值不大, 反映总体构造变形相对较弱。岩石磁组构反映的应力状态总体为以NE向挤压为主, 与轴向NW的背斜构造发育相一致。该区岩石磁组构大多具有原始沉积磁组构特征, 磁面理产状大体上反映沉积岩层的层理, 同时也记录了受NE向挤压作用的痕迹。根据岩石磁组构与地层层理之间的关系分析, 柴西地区两翼不对称的狮子沟背斜具有断展褶皱性质, 其形成与下部的花土沟逆冲断层向南西方向的仰冲有关。   相似文献   

10.
Magnetic fabric and rock-magnetism studies were performed on the four units of the 578 ± 3-Ma-old Piracaia pluton (NW of São Paulo State, southern Brazil). This intrusion is roughly elliptical (~32 km2), composed of (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), which is predominant in the pluton, (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the units. Several rock-magnetism experiments performed in one specimen from each sampled units show that for all of them, the magnetic susceptibility and magnetic fabrics are carried by magnetite grains, which was also observed in the thin sections. Foliations and lineations in the units were successfully determined by applying magnetic methods. Most of the magnetic foliations are steeply dipping or vertical in all units and are roughly parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for the whole pluton. However, for eight sites, they are steep up to vertical. Thin-section analyses show that rocks from the Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state fabric in the north of the pluton, indicating that magnetic fabrics in this area of the pluton are related to this strain. Otherwise, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the SW of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlie this area.  相似文献   

11.
Abstract

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82·10?5 S.I. and 28 to 75·10?5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6 %, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (Kmax) and the pole to the magnetic foliation (Kmin) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry. © Elsevier, Paris  相似文献   

12.
《Geodinamica Acta》1998,11(6):271-283
The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82 · 10−5 S.I. and 28 to 75 · 10−5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6%, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (Kmax) and the pole to the magnetic foliation (Kmin) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry.  相似文献   

13.
This paper examines the geological implications of an analysis of constraints on the orientation of fold axes in orthotropic materials. We argue that rocks with penetrative linear and planar shape fabrics may have orthotropic (anisotropic) properties during deformation. Two forms of anisotropy (rheological and structural) could be potentially important in the control of fold axial directions. We discuss a model of deformation of rocks with linear/planar fabrics where, in a single deformation event, major fold axes need not be parallel to minor fold axes and neither need be perpendicular to the principal compressive stress direction. Geological and model examples of anistropic control on fold axial directions are given.  相似文献   

14.
The 616 ± 3 Ma (Ediacaran) Egersund doleritic dike swarm cuts across the Rogaland anorthosite province and its granulitic country rocks, in SW Norway. The structure of eight out of eleven main dikes of the swarm was investigated using the anisotropy of magnetic susceptibility (AMS) technique. Thermomagnetic data and values of the bulk magnetic susceptibility reveal a magnetic mineralogy dominated by Ti-poor titanomagnetite. Magnetic fabric and global petrofabric are coaxial, except in sites strongly affected by hydrothermal alteration, as demonstrated through image analysis. Asymmetrical dispositions of the magnetic foliation and lineation support the existence of a syn-emplacement, sinistral strike-slip shearing resolved on dike walls. Such asymmetrical fabrics are attributed to a transtension tectonic regime, in a context of oblique extension during the continental rifting phase which preceded the opening of the Iapetus Ocean along the SW margin (present-day orientation) of Baltica.  相似文献   

15.
Clay fabrics in relation to the burial history of shales   总被引：1，自引：0，他引：1  
MANUEL SINTUBIN 《Sedimentology》1994,41(6):1161-1169
Quantitative appraisal of compaction strain is essential for the study of the burial history of shales in sedimentary basins. The results of a preliminary fabric analysis of Westphalian and Zechstein shales in the Campine Basin (Belgium) show that clay fabric analysis, using an X-ray pole figure goniometer, is suitable for this purpose. Clay fabrics, in the range studied, are independent of depth and therefore cannot be used as depth indicators. This suggests that in the early stages of the burial history a stable clay fabric has to develop, which will basically remain unchanged during the subsequent burial history. The degree of clay particle preferred orientation not only reflects the compaction strain, but is also determined by mineralogical parameters: the presence of non-platy particles and the relative concentrations of the different clay minerals. This degree of preferred orientation furthermore determines the degree of fissility of the shales. These mineralogical factors limit the use of clay fabrics as truly quantitative strain markers. Their use as semi-quantitative strain markers remains advantageous, mainly because of the common occurrence of clay fabrics in the geological record. Moreover, the relative ease of measurement and the possibility of distinguishing compaction from tectonic strains favour the use of clay fabrics in the quantitative strain analysis of argillaceous rocks.  相似文献   

16.
玲珑花岗岩体的变形磁组构特征及其与金矿的关系   总被引：4，自引：1，他引：3       下载免费PDF全文
周伟新  万天丰 《地质科学》2000,35(4):385-395
本文利用磁组构的方法对玲珑花岗岩体的磁组构特征及其与金矿的关系进行研究.玲珑岩体的磁化率各向异性度P的平均值为1.2872,具有典型的构造变形成因特点;磁面理围绕其西北侧的北截花岗闪长岩展布,倾角平缓;磁线理呈SW向近水平展布.玲珑岩体的变形磁组构的特征表明玲珑岩体的变形磁组构是由于后期侵入的北截郭家岭花岗闪长岩体侵入造成的.研究还发现除了断裂的控矿作用以外,位于玲珑岩体内部和边缘的金矿和磁化率各向异性间存在密切的空间对应关系.金矿往往位于磁化率各向异性度P值高的区域,而磁化率各向异性度P值低的区域金矿很少.  相似文献   

17.
在胶东地区招远-平度断裂带上的大磨曲家金矿区选取了典型的构造区域进行岩石磁组构研究。沿着断裂带在不同构造部位的36个采样点钻取了112个定向岩心样品,所有样品均沿勘探线布置。磁组构研究显示,研究区以弱磁性岩石为主,总体上,胶东群变质岩的磁化率值较高,而玲珑黑云母花岗岩的磁化率值较低,尤以碎裂程度高的强蚀变岩的磁化率值最低;磁化率各向异性方位主要为NE向挤压,磁组构所指示的构造应力场与大磨曲家矿区的区域挤压应力方向是相同的;对具不同程度矿化的81线的Au含量与80线磁组构各向异性度(P值)进行对比研究发现,P值与金品位呈负相关关系;弱矿化糜棱岩中的金矿化在磁面理发育的岩石中较为发育,成矿晚期,Au元素含量随着岩石磁性的减弱而增加,特别是在强应变后弛豫阶段矿液充填于相对张性的石英脉中Au含量最高。  相似文献   

18.
Gentoso, M. J., Evenson, E. B., Kodama, K. P., Iverson, N. R., Alley, R. B., Berti, C. & Kozlowski, A. 2012 (January): Exploring till bed kinematics using AMS magnetic fabrics and pebble fabrics: the Weedsport drumlin field, New York State, USA. Boreas, Vol. 41, pp. 31–41. 10.1111/j.1502‐3885.2011.00221.x. ISSN 0300‐9483. Thick, relatively homogeneous basal tills exposed in the drumlins and flutes of the Weedsport drumlin and flute field in New York State exhibit anisotropy of magnetic susceptibility (AMS) and pebble fabrics that are consistently oriented parallel to the streamlined bedforms. The pebble fabrics and AMS fabrics are concordant. In this study, six drumlins and five flutes were sampled. Thermally induced, incremental reduction of isothermal remanent magnetization indicates that AMS is caused by primarily elongate maghaemite grains. The orientations of principal axes of maximum susceptibility (k1) are generally parallel to pebble long‐axis orientations, and tend to plunge mildly up‐glacier. Fabric directions are generally parallel to drumlin long‐axis orientations, but deviate by 12°–23° from flute directions. Fabrics of the flutes are stronger and more unidirectional than those of the drumlins. These results support the use of AMS as a fast and objective method for characterizing fabrics in tills, and suggest hypotheses about basal processes linked to glacially streamlined landforms.  相似文献   

19.
A regional analysis of the anisotropy of the magnetic susceptibility on low-grade metamorphic, chloritoid-bearing slates of the Paleozoic in Central Armorica (Brittany, France) revealed very high values for the degree of anisotropy (up to 1.43). Nonetheless, high-field torque magnetometry indicates that the magnetic fabric is dominantly paramagnetic. Chloritoid's intrinsic degree of anisotropy of 1.47 ± 0.06, suggests that chloritoid-bearing slates can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic ferromagnetic (s.l.) minerals. To validate this assumption we performed a texture analysis on a representative sample of the chloritoid-bearing slates using hard X-ray synchrotron diffraction. The preferred orientation patterns of both muscovite and chloritoid are extremely strong (∼38.6 m.r.d. for muscovite, 20.9 m.r.d. for chloritoid) and display roughly axial symmetry about the minimum magnetic susceptibility axis, indeed suggesting that chloritoid may have a profound impact on the magnetic fabric of chloritoid-bearing rocks. However, modeling the anisotropy of magnetic susceptibility by averaging single crystal properties indicates that the CPO of chloritoid only partially explains the slate's anisotropy.  相似文献   

20.
The Apiaí gabbro-norite is a massive fine-grained Neoproterozoic intrusion emplaced in a core of synformal structure that deforms low-grade marine metasedimentary rocks of the Ribeira Belt of south-eastern Brazil. The lack of visible magmatic layering or any internal fabric has been a major limitation in deciding whether the emplacement occurred before or after the regional folding. To assist in the tectonic interpretations, we combine low-field anisotropy of magnetic susceptibility (AMS) and silicate shape preferred orientation (SPO) to reveal the internal structure of the mafic intrusion. Magnetic data indicate a mean susceptibility of about 10−2 SI and a mean anisotropy degree (P) of about 1.08, essentially yielded by titanomagnetite. The magnetic and silicate foliations for P ≥ 1.10 are parallel to each other, while the lineations tend to scatter on the foliation plane, in agreement with the dominant oblate symmetry of the AMS and SPO ellipsoids. For lower P values, the magnetic and silicate fabrics vary from coaxial to oblique, and for P ≤ 1.05, their shapes and orientations can be quite distinct. The crystal size distribution (CSD) of plagioclase for P > 1.05 is log linear, in agreement with a bulk simple crystallisation history. These results combined show that for a strong SPO, corresponding to a magnetic anisotropy above 1.10, AMS is a reliable indicator of the magmatic fabric. They indicate that the Apiaí gabbro-norite consists of sill-like body that was inclined gently to the north by the regional folding.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号