The Variscan Millares granite (central Pyrenees): Pluton emplacement in a T fracture of a dextral shear zone

This work deals with the magnetic susceptibility and its anisotropy (AMS) in the Variscan Millares pluton in the Central Pyrenees. The zonation of low-field magnetic susceptibility is consistent with the concentric arrangement of rock-types, with more basic compositions at the external areas. Magnetic foliations defined from AMS strike NE-SW and dip gently towards the NW. Magnetic foliations are mainly perpendicular and oblique to the elongation of the pluton in map view (NW-SE) and show a concentric pattern at the central part, where the more acid rocks crop out. Magnetic lineations are scattered between NW-SE and NE-SW and plunge shallowly to the N. In map view magnetic lineations are distributed in domains normal to the elongation of the pluton. The contours of P' (degree of magnetic anisotropy) are oriented NE-SW and bands of oblate and prolate ellipsoids alternate perpendicular to the elongation of the pluton in map view. P' is between 1.009 and 1.055 in 93% of the specimens. Such low values are currently recorded in granites having magmatic fabrics and for which the anisotropy is mainly carried by biotite. The attitude of the magnetic foliation and the magnetic lineation, the geometry of the pluton, and their relationship with the host-rock structure suggest an intrusion contemporary with a transpressional regime, syntectonic with the late stages of the Variscan orogeny.     

白银厂矿区劈理成因的磁组构分析

将磁性组构方法用于甘肃白银厂矿区的构造分析.结果表明该区岩石的磁性组构与岩石组构具有同一分布规律.岩石的磁线理与岩石的拉长线理L~a近于一致,磁面理与第一期劈理S_1基本平行,并垂直于岩石的磁化率椭球的最小主轴,也垂直于应变椭球的最短主轴方位,故推断第一期劈理S_1的成因主要属压扁成因.同时受到后期的多次变形与褶皱迭加影响.S_1轴面的优选面状分布发生变位不大,表明该区多次构造变形主要来自早期的同一主应力场作用所致.矿区岩石的磁面理与应变椭球拉长轴面或第一期劈理面S_1存在某些的角度差,表明该区岩石的劈理面由于受后期多次构造变形影响,可能发生剪切运动或旋转机制.     

Structural and magnetic fabric study of the Marimanha granite (Axial Zone of the Pyrenees)

The structural and magnetic fabric study of the Marimanha granite, Axial Zone of the Pyrenees, provides new data to characterize the zonation and the internal structure of the pluton. The Marimanha granite intrudes Cambro–Ordovician clastic rocks and Silurian–Devonian limestones and slates. The zonation of the low field magnetic susceptibility, consistent with the petrological zonation of the igneous body, indicates a concentric arrangement of rock types, with more basic compositions at the external areas. This pluton is characterized by a low susceptibility, and rock–magnetic studies indicate a majority of “paramagnetic” samples. Magnetic foliations strike parallel to petrographic contacts and to contours of zonation of magnetic susceptibility, and show a dominant NE–SW strike, steeply dipping towards the North. Locally, in the northern border of the pluton foliation directions become perpendicular to petrographic contacts and depict sigmoidal trajectories. Magnetic lineations are characterized by the predominance of NE–SW trends with shallow plunges to the NE and SW. These foliations and lineations are parallel to the slight elongation of internal petrographic zonation. Magnetic fabric within the granitoid body and internal elliptical shape of petrographic zonation, suggest an intrusion contemporary with a transpressional regime and NNW–SSE shortening, syntectonic with the late stages of the Variscan orogeny. These results are in accordance with anisotropy of magnetic susceptibility studies of others plutons in the Pyrenees. To explain the origin of the Marimanha granite we propose magma ascent through faults at depth and emplacement by ballooning in situ at the rheological boundary between Cambro–Ordovician and Siluro–Devonian metasediments.     

Pervasive horizontal fabric and rapid vertical extrusion: Lateral overturning and margin sub-parallel flow of deep crustal migmatites, northeastern Bohemian Massif

In the West Sudetes, northeastern Bohemia Massif, geochronometry provides evidence for repeated episodes of rapid cooling that contrasts sharply with an absence of structural evidence for significant tectonic exhumation by crustal extension. Instead, high-grade assemblages of the Orlica–Snieznik Complex have a regional sub-horizontal foliation and sub-horizontal lineations that trend parallel to narrow sub-vertical shear zones containing exhumed high-pressure assemblages. Mesoscopic petrofabrics combined with anisotropy of magnetic susceptibility (AMS) data from amphibolite facies to migmatitic meta-sedimentary and meta-igneous rocks reveal remarkably consistent average lineations that plunge shallowly to the SSW on both steep and sub-horizontal NNE-trending planar fabrics. The dominant SSW–NNE fabric orientation is parallel to the Bohemia–Brunia suture, which marks a major boundary along the eastern margin of the massif. The shape of the AMS ellipsoid is predominantly oblate, revealing flattened fabrics, with only local prolate ellipsoids. We envisage that the continental Brunian indentor operated as a rigid backstop and allowed the migmatized lower crustal orogenic root to be exhumed along the Bohemian margin shortly following terminal arc collision. Sub-vertical extrusion of the orogenic root was arrested in the mid-crust, where the lower ductile crust was laterally overturned at the base of rigid upper crustal blocks. Upon reaching the crustal high-strength lid the exhumed ductile mass of continental material laterally spread sub-parallel to the margin, underwent subsequent supra-Barrovian metamorphism, and quickly cooled. The application of AMS techniques to high-grade metamorphic rocks in concert with macroscopic structural observations is a powerful approach for resolving the deformation history of a terrane where visible rock fabrics can be tenuous.     

Composite magnetic anisotropy fabrics: experiments, numerical models and implications for the quantification of rock fabrics

Magnetic fabrics from rocks with multiple mineral-preferred orientations may have anisotropy ellipsoids whose shape and orientation arise from the addition of two or more component fabrics. Our numerical models and experiments demonstrate that such composite magnetic fabrics do not directly reflect the shapes and/or orientations of the individual mineral fabrics and we provide criteria for the recognition and interpretation of composite fabrics in natural rocks. These criteria include:

1. (1) the orientation of the maximum susceptibility axis is located at the intersection of two planar fabrics, and

2. (2) the shape of the susceptibility ellipsoid changes from oblate to prolate and the degree of anisotropy decreases, as the relative intensity of two planar component fabrics becomes equal and as the angle between the planar fabrics increases.

Composite magnetic fabrics are observed in the shales and slates of the Martinsburg Formation, Lehigh Gap, Pennsylvania. Modeling of the AMS (anisotropy of magnetic susceptibility) and ARMA (anhysteretic remanent magnetization anisotropy) behavior constrains the relative degree of anisotropy of the bedding-parallel and cleavage-parallel fabrics. In particular, ARMA model results allow a good estimate of magnetite fabric strength.

We conclude that, in the presence of composite magnetic fabrics, quantitative measures of finite strain in deformed rocks are limited by the ability to accurately determine the degree of anisotropy and relative susceptibility of each component fabric. Such determinations require knowledge of the mineral(s) that are responsible for the measured magnetic fabric and their behavior during deformation.     

The Chinamora batholith, Zimbabwe: structure and emplacement-related magnetic rock fabric

The origin of dome-and-keel structural geometries in Archean granite–greenstone terrains appears to lack any modern analogues and is still poorly understood. The formation of these geometries is investigated using structural and anisotropy of magnetic susceptibility (AMS) data for the Chinamora batholith in Zimbabwe. The roughly circular-shaped batholith is surrounded by ca. 2.72–2.64 Ga greenstones. The batholith granitoid suites have been divided on the basis of their ages and fabric relationships into four distinct units: (i) banded basement gneisses; (ii) granodioritic gneisses; (iii) equigranular granites; and (iv) central porphyritic granites. In the gneissic granites a partial girdle (N–S) of poles to the magnetic foliation is developed that has been folded around a consistent, flat lying magnetic lineation plunging at shallow angles to the E or W. In the equigranular granites, the magnetic lineation generally plunges to the NW. The magnetic foliation has a variable strike, no clear trends can be distinguished. The AMS measurements of the porphyritic granite revealed a NW–SE striking foliation and showed subhorizontal magnetic lineations. The magnetic foliation is subparallel to the macroscopic foliation. Wall rocks are moderately inclined and show radial or concentric lineations, triaxial strain ellipsoids and kinematics that demonstrate off-the-dome sliding and coeval pluton expansion. The results of the observations do not point to a single emplacement process. Neither the observed structural data nor the magnetic fabric support a model envisaging spherically ‘ballooning’. It is argued that pluton diapirism played a major part in the formation of the fabrics in the gneisses, whereas the fabrics in the porphyritic granites reflect emplacement as laccolith-like sheets.     

Using AMS combined with mineral shape preferred orientation analysis to understand the emplacement fabrics of the Apiaí gabbro-norite (Ribeira Belt,SE Brazil)

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.     

太原西山上二叠统陆相地层的磁组构学研究

对太原西山剖面上石盒子组 1段至孙家沟组的 111块定向标本所进行的磁组构学研究表明 ,其磁化率各向异性参数的主体特征为 :磁化率介于 2 0 0— 35 0 ( 10 - 6 SI)之间 ;各向异性度≥ 1.0 3,各向异性明显 ;压扁率 >1,磁化率各向异性数值椭球呈弱压扁型 ;磁面理度 >磁线理度 ,磁面理发育 ;磁面理倾角∠ 2 4°,倾向多变 ,但沿纵向具向南趋势 ;磁面理置信角∠ 2 5°。上述磁化率各向异性参数的沉积地层学意义为 :自上石盒子组 1段至孙家沟组 ,沉积相为水流方向自由迂回摆动的曲流河相 ,沉积环境为地势平坦的冲积平原 ;孙家沟组成岩之末 ,地形略呈北高南低之势 ,当时最近的海岸线位于太原之南。     

AMS fabric and tectonic evolution of Quaternary intramontane extensional basins in the Picentini Mountains (southern Apennines, Italy)

In this work, we report the results of combined geological, structural, and anisotropy of magnetic susceptibility (AMS) studies carried out on Quaternary deposits in the Picentini Mountains, southern Apennines (Italy). The study concerns four small continental basins, Acerno, Tizzano, Iumaiano, and Piano del Gaudo, related to fluvial–lacustrine depositional environments, ranging in altitude from 600 to 1,200 m a.s.l. and strongly incised during recent time. Stratigraphic and structural analyses, integrated by low- and high-field anisotropy of magnetic susceptibility (AMS), show that the formation of these basins has been controlled by extensional and transtensional tectonics. Most of the AMS sites exhibit a well-defined magnetic foliation parallel to the bedding planes. A well-defined magnetic lineation has also been measured within the foliation planes. In the Iumaiano, Tizzano, and Piano del Gaudo basins, magnetic lineations cluster around NNE–SSW trend and are parallel to the stretching directions inferred by structural analysis of faults and fractures. On the basis of structural, sedimentological, and high-field AMS data, we suggest a tectonic origin for the magnetic lineation, analogously to what has been observed in other weakly deformed sediments from Neogene and Quaternary extensional basins of the Mediterranean region. Our results demonstrate that onset and the evolution of the investigated basins have been mainly controlled since lower Pleistocene by NW–SE normal and transtensional faults. This deformation pattern is consistent with a prevalent NE–SW extensional tectonic regime, still active in southern Apennines, as revealed by seismological and geodetic data.     

Magnetic fabrics and their relationship with the emplacement of the Piracaia pluton,SE Brazil

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.     

Pan-African strike–slip tectonics in eastern Cameroon—Magnetic fabrics (AMS) and structure in the Lom basin and its gneissic basement

Field, microstructural, and anisotropy of magnetic susceptibility (AMS) or magnetic fabric studies were applied to identify the sequence and character of the Pan-African structures in the basement of Eastern Cameroon at both sides of the regional scale Bétaré-Oya Shear Zone (BOSZ). The NE-SW trending BOSZ separates older gneisses and migmatites towards SE (domain I) from the younger rocks of the Lom meta-volcano-sedimentary basin towards NW (domain II). In domain I, early, ductile compressional deformation occurred in two events, D1 and D2, under relatively high T conditions. During subsequent cooling, strain partitioned between the competent basement gneisses with only mild compression and the bordering shear zone (BOSZ) with intense simple shear-wrenching (D3). Strain in the less competent rocks of domain II is dominated by simple shear, strike-slip wrenching (D3), with an earlier stage of compressional deformation preserved only in some low strain pods.Magnetic fabrics (AMS) document a progressive change from oblate ellipsoids towards prolate ellipsoids in domain I, when proceeding from the south towards the BOSZ. Foliations are mostly steep but define a girdle with a pole plunging gently towards WSW. The magnetic lineations also plunge mostly towards WSW at shallow angles. These fabrics indicate a compression approximately normal to the BOSZ, which is also the SE margin of the Lom Basin. In the Lom metasediments (domain II), AMS ellipsoids are typically oblate. Foliations trend NE-SW with mostly steep dips. Magnetic lineations plunge gently NE or SW. This fabric with foliations mostly steep and subparallel with the major BOSZ, combined with generally subhorizontal lineations implies the BOSZ as a Pan-African strike–slip shear zone with a subordinate component of compression.At a larger scale, the area is part of a continent-scale shear zone, separating external Pan-African domains of compression along the northern margin of the Congo craton from internal domains dominated by high-angle strike–slip and transpressional deformation. Together with published data, the present study thus demonstrates that transpression is a regional phenomenon in the Pan-African orogen of central and eastern Cameroon.     

Anisotropy of magnetic susceptibility as an indicator for palaeocurrent analysis in folded turbidites (Outer Western Carpathians,Poland)

The anisotropy of magnetic susceptibility is a well-known geological proxy in revealing the directional tectonic and sedimentological features of rocks, although it can be ambiguous in situations where these two factors co-occur. This paper demonstrates the usefulness of the anisotropy of magnetic susceptibility in determining palaeotransport directions in turbiditic rocks that underwent subsequent thrusting and folding. This study demonstrates that the magnetic lineation is largely unsuitable as a palaeocurrent direction proxy, and suggests that the imbrication of magnetic foliation is better in such cases. Moreover, the anisotropy of magnetic susceptibility results were analyzed in reference to a joint and fold study within the framework of the regional structural geology. Magnetic fabric investigations were conducted in the eastern part of the Outer Western Carpathians (south-east Poland). During the study, a total of 191 oriented palaeomagnetic samples were collected from three outcrops (Nasiczne, Dwernik and Hoczew) in the Krosno Beds, Silesian Unit. For the purpose of sedimentological analysis, 121 m of turbidite successions were documented and 126 directional sedimentary structures were measured. The magnetic anisotropy of sandstones revealed typical sedimentary fabrics, often overprinted by variably intense tectonic deformation. Oblate susceptibility ellipsoids from Nasiczne showed tilt coherent with the palaeoflow direction, whereas the rocks from Dwernik and Hoczew contained triaxial magnetic fabric developed during compressional palaeostress. This paper suggests that medium-grained and coarse-grained sandstones, preferably with high mica content, are the most suitable for palaeotransport reconstructions among the studied lithologies.     

西昆仑山前晚新生代沉积岩磁组构及构造意义

西昆仑山前晚新生代沉积岩磁组构特征表明，沉积岩原生磁组构受后期构造活动改变。磁组构测试结果表明晚新生代沉积岩生较明显变形，岩石磁化率椭球体指示磁面理较发育，反映岩石受压扁型变形为主。磁化率椭球体最小轴方向为NW，指示该区最大主压应力为NW，与区域构造分析结果相一致。     

Anisotropy of magnetic susceptibility of eclogites: mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal,Spain)

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 (K_max) and the pole to the magnetic foliation (K_min) 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     

Anisotropy of magnetic susceptibility of eclogites: Mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal,Spain)

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 (K_max) and the pole to the magnetic foliation (K_min) 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.     

藏东昌都地区侏罗纪岩石磁组构研究及其构造意义

本文对藏东昌都地区侏罗纪汪布组、东大桥组和小索卡组红层共71个采点开展了磁组构(AMS)研究。磁组构测试结果表明,早侏罗世汪布组岩石磁线理较磁面理发育,磁化率各向异性度较高,磁化率椭球最小轴K3散布于层面缩短方向,代表了与构造成因相关的磁组构;中侏罗世东大桥组和晚侏罗世小索卡组岩石则磁面理较磁线理发育,磁化率各向异性度较低,磁化率椭球最小轴K3与层面近垂直,指示了原生沉积磁组构。早侏罗世汪布组地层的磁组构揭示了其构造应力场方向为NE-SW向。中侏罗世东大桥组的磁组构指示了其沉积时的古水流方向为SE向(138.3°),而晚侏罗世小索卡组磁组构指示了其沉积时的古水流方向为NNW向(328.3°)。古水流方向的明显变化揭示了昌都地区从中侏罗世到晚侏罗世沉积物物源发生了相应的转变,表明昌都地区南早北晚的隆升过程。     

Magnetic fabric study of the SE Rhenohercynian Zone (Bohemian Massif): Implications for dynamics of the Paleozoic accretionary wedge

The progressive deformation recorded in the magnetic fabric of sedimentary rocks was studied in the SE Rhenohercynian Zone (RHZ), eastern margin of the Bohemian Massif, Czech Republic. Almost 800 oriented samples of the Lower Carboniferous mudstones and graywackes were collected from the SSE part of the Czech RHZ, so-called the Drahany Upland. The anisotropy of magnetic susceptibility (AMS) is predominantly controlled by the preferred orientation of paramagnetic phyllosilicates, mainly iron-bearing chlorites. A regional distribution of the magnetic fabric within the Drahany Upland revealed an increasing deformation from the SSE to the NNW. In the SE, the magnetic fabric is bedding-parallel with magnetic lineation scattered in the bedding plane or trending N–S to NNE–SSW. Further to the NW, the magnetic foliation rotates from the bedding-parallel orientation to the orientation parallel to the evolving cleavage. This rotation is accompanied by a decrease of the anisotropy degree and the prolate nature of the anisotropy ellipsoids. The magnetic lineation is parallel to the strike of the bedding, bedding/cleavage intersection, pencil structure or the fold axes on a regional scale. In the NW part of the Drahany Upland, the magnetic foliation becomes parallel to the cleavage accompanied by an increase of the anisotropy degree and the oblate nature of the anisotropy ellipsoids. The increasing trend of deformation corresponds to the SSE–NNW increase in the degree of anchimetamorphism; both trends being oblique to the main lithostratigraphic formations as typically observed in the sedimentary rocks of the accretionary wedges. The SSE–NNW increase in deformation and anchimetamorphism continues to the Nízký Jeseník Mts., representing the northern part of the same accretionary wedge. The kinematics of deformation could not be unambiguously assessed. The observed magnetic fabric may reflect either lateral shortening or horizontal simple shear or a combination of both mechanisms. Regarding the subduction process, it seems that the sedimentary sequences of the Drahany Upland were subducted, partly offscraped and accreted frontally or partly underplated as opposed to the Nízký Jeseník Mts. where some return flow must have occurred.     

岩石的磁性组构及其在岩石变形分析中的应用

文本总结了对采自巴彦乌拉山韧性剪切带的部分标本进行岩石组构数据测定的分析结果,并将其与用常规应变分析方法所得结果予以比较,表明了岩石磁化率各向异性椭球体能够反映总的应变椭球体,证明岩石磁化率各向异性技术做为一种岩石组构因素的研究是有发展前景的。岩石磁性组构资料可应用于变形岩石的应变分析,特别是在通常的岩石组构分析技术太费时间和粗糙的情况下更显示了其优越性。     

Magnetic fabric study across the Ailao Shan–Red River shear zone

The anisotropy of magnetic susceptibility (AMS) of 351 specimens from 51 sites across the Ailao Shan–Red River shear zone (ASRR) was measured to determine its magnetic fabric. Rocks range westward from core schistose gneiss, through low-grade schist, to Triassic sediment. Magnetic ellipticity analysis shows that 41 of 51 sites have an oblate compressional fabric and the other 10 sites have a prolate fabric. P_J value drops by 22.4% in the low-grade schist and by 27.4% in the Triassic sediment on average with respect to the gneiss, suggesting a rapid decrease of deformational intensity. The directions of principal susceptibilities are closely related to the deformation of the Ailao Shan–Red River shear zone. The susceptibility plane always coincides with the schistosity or cleavage plane. Most of the maximum susceptibility axes trend NW–SE. In the shear zone, the maximum susceptibility axes (K_max) are parallel to the lineation within the foliation plane. With increasing distance from the shear zone, there is a trend that they become parallel to the down-dip of reverse faults or cleavage. This indicates changes in deformation mode, inside and outside the shear zone. Within the shear zone, horizontal movement is dominant. Outside, shortening prevails. The overall minimum magnetic axes align NE–SW with subhorizontal to low dip angles, suggesting that the dominant shortening is NE–SW directed. Caution should be exercised when AMS is used to determine shear sense in strong shear zones because the angle between the minimum susceptibility axis (K_min) and pole of foliation is small, and also because the attitude of foliation varies from place to place. They result in unreliable or even wrong shear sense. Another important result is the axial ratio of magnetic susceptibility ellipsoid along the study section. With these data, it is possible to establish an axial ratio relationship between the finite strain ellipsoid and magnetic susceptibility ellipsoid for quantitative calculation of offset.     

玲珑花岗岩体的变形磁组构特征及其与金矿的关系

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.