http://www.sciencedirect.com/science/article/pii/S1674987112000618

The Moyar Shear Zone(MSZ) of the South Indian granulite terrain hosts a prominent syenite pluton (～560 Ma) and associated NW-SE to NE-SW trending mafic dyke swarm(～65 Ma and 95 Ma). Preliminary magnetic fabric studies in the mafic dykes,using Anisotropy of Magnetic Susceptibly(AMS) studies at low-field,indicate successive emplacement and variable magma flow direction.Magnetic lineation and foliation in these dykes are identical to the mesoscopic fabrics in MSZ mylonites,indicating shear zone guided emplacement.Spatial distribution of magnetic lineation in the dykes suggests a common conduit from which the source magma has been migrated.The magnetic foliation trajectories have a sigmoidal shape to the north of the pluton and curve into the MSZ suggesting dextral sense of shear.Identical fabric conditions for magnetic fabrics in the syenite pluton and measured field fabrics in mylonite indicate syntectonic emplacement along the Proterozoic crustal scale dextral shear zone with repeated reactivation history.     

Anisotropy of magnetic susceptibility (AMS) studies of Campanian–Maastrichtian sediments of Ariyalur Group,Cauvery Basin,Tamil Nadu,India: An appraisal to Paleocurrent directions

Oriented samples of sediments from Ariyalur Group, Cauvery Basin, south India, were studied for low field anisotropy of magnetic susceptibility (AMS) measurements to unravel the magnetic fabrics and paleocurrent directions. The results of AMS parameters of the sediments indicate primary depositional fabrics for Sillakkudi, Ottakovil and Kallamedu sandstone formations and secondary fabric for Kallankurichchi limestone formation. The obtained low degree of anisotropy (P _j ), oblate shape AMS ellipsoid and distribution of maximum (K ₁) and minimum (K ₃) susceptibility axes on equal area projection confirm the primary sedimentary fabric for Sillakkudi, Ottakovil and Kallamedu Formations. In the case of ferruginous, lower arenaceous, Gryphaea limestone and upper arenaceous limestone beds of Kallankurichchi Formation have recorded more than one fabric. The observed AMS parameters like shape factor (T) (prolate to oblate), q value and random distribution of minimum (K ₃) and maximum (K ₁) susceptibility axes are supported for secondary fabrics in Kallankurichchi Formation as a result of post-depositional processes. Based on petrographic studies, it can be established that K ₁ AMS axis of biotite mineral could represent the flow direction. The established paleocurrent direction for Sillakkudi is NW–SE direction while Ottakovil and Kallamedu Formations recorded NE–SW direction. Overall the paleoflow directions observed for Ariyalur Group is NE–SW to NW–SE.     

Magnetic fabrics and rock magnetism of Proterozoic dike swarm from the southern São Francisco Craton, Minas Gerais State, Brazil

Magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a Proterozoic dike swarm from the southern São Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10⁻³ (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes.Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two dikes, from both non-hydrothermalized and hydrothermalized dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR_0–30, pAAR_30–60 and pAAR_60–90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of K_max inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes (K_max<30°). Intermediate AMS fabric is the most important (41%) in the investigated swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. ARM determinations for these sites also remained intermediate except for two dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after dike emplacement. InverseInverse AMS fabric is a minority (four dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric.     

Anisotropy of magnetic susceptibility and rock magnetic applications in the Deccan volcanic province based on some case studies

Anisotropy of Magnetic Susceptibility (AMS) as a tool has been explored here to investigate the nature of petrofabrics in Deccan Volcanic Province (DVP) of west-central Indian region by representative sampling in typical pahoehoe and rubbly pahoehoe lava flows, dykes within flows, shear zone and the impact crater units. The rock magnetic analysis indicate varying degree of concentration of titanomagnetite compositions dominated by multi domain (MD) to pseudo single domain (PSD) grains favoring shape anisotropy of minerals that form primary fabrics. The pahoehoe type lava flows shows planar oblate fabrics without any preferred orientation of principle susceptibility axis (K₁) depicting crystal settling (of magnetic grains) as chief mechanism of fabric development. The rubbly pahoehoe type lava flow exhibit prolate fabrics with well clustered maximum susceptibility axis within horizontal to sub-horizontal planes depicting their response to viscosity shear. The dykes show well clustered K₁ parallel to it’s plane locked during rapid contractional cooling. The sampling at Lonar impact crater was unable to trace any clear fabric due to impact/shock induced deformation and rather preserve the primary fabrics. Further, the shear zone depict random fabrics demanding more detailed and systematic sampling in both the cases. The present investigation infer that the magnetic mineralogy and magnetic fabric variations in the DVP are controlled by the flow mechanism and style of cooling that is characteristic of the given flow unit or dyke and any secondary or superimposed fabric needs to be examined by critical sampling strategy. While more detailed attempts are required to establish the AMS as a tool to record various aspects including the flow dynamics and rate of effusion in the vast terrain of DVP; the present approach is useful to characterize and correlate the lava flow units and dyke occurrences.     

Distinctive diamagnetic fabrics in dolostones evolved at fault cores,the Dead Sea Transform

We resolve the anisotropy of magnetic susceptibility (AMS) axes along fault planes, cores and damage zones in rocks that crop out next to the Dead Sea Transform (DST) plate boundary. We measured 261 samples of mainly diamagnetic dolostones that were collected from 15 stations. To test the possible effect of the iron content on the AMS we analyzed the Fe concentrations of the samples in different rock phases. Dolostones with mean magnetic susceptibility value lower than −4 × 10⁻⁶ SI and iron content less than ∼1000 ppm are suitable for diamagnetic AMS-based strain analysis. The dolostones along fault planes display AMS fabrics that significantly deviate from the primary “sedimentary fabric”. The characteristics of these fabrics include well-grouped, sub-horizontal, minimum principal AMS axes (k₃) and sub-vertical magnetic foliations commonly defined by maximum and intermediate principal AMS axes (k₁ and k₂ axes, respectively). These fabrics are distinctive along fault planes located tens of kilometers apart, with strikes ranging between NNW-SSE and NNE-SSW and different senses of motion. The obtained magnetic foliations (k₁–k₂) are sub-parallel (within ∼20°) to the fault planes. Based on rock magnetic and geochemical analyses, we interpret the AMS fabrics as the product of both shape and crystallographic anisotropy of the dolostones. Preferred shape alignment evolves due to mechanical rotation of subordinate particles and rock fragments at the fault core. Preferred crystallographic orientation results from elevated frictional heating (>300 °C) during faulting, which enhances c-axes alignment in the cement-supported dolomite breccia due to crystal-plastic processes. The penetrative deformation within fault zones resulted from the local, fault-related strain field and does not reflect the regional strain field. The analyzed AMS fabrics together with fault-plane kinematics provide valuable information on faulting characteristics in the uppermost crust.     

Magnetic fabrics and rock magnetism of Archaean and Proterozoic dike swarms in the southern São Francisco Craton, Brazil

Magnetic fabric and rock magnetism studies were performed on three mafic dike swarms (total of 38 dikes) from the southernmost part of the São Francisco Craton (SFC) (Minas Gerais State, SE Brazil). They cut Archaean granite–gneiss–migmatite and paleoprototerozoic terranes. These swarms are classified as basic–noritic (Sm–Nd age  2.65 Ga), basic (Rb–Sr age  1.87 Ga) and metamorphic (Rb–Sr age  1.87 Ga) suites, in which the second is the most important. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). In most sites magnetic susceptibility is dominantly carried by ferromagnetic minerals, however, in some sites the paramagnetic contribution exceeds 70% of bulk susceptibility. Mainly coarse to fine-grained Ti-poor titanomagnetite up to pure magnetite carry the magnetic fabrics.Three primary AMS fabrics are recognized which are all coaxial with the AARM fabric. Normal AMS fabric is dominant in the basic suite (16 of 20 analyzed dikes) and occurs in 4 and 3 dikes from the basic–noritic and metamorphic suites, respectively. This fabric is interpreted as a result of magma flow in which the analysis of K_max inclination permitted to infer that the majority of dikes were fed by inclined flows (30° < K_max < 60°), although 44% of dikes from the basic suite were fed by horizontal or sub-horizontal flows (K_max < 30°). Intermediate AMS fabric was found in 50% of dikes from the basic–noritic and metamorphic suites, but in only 2 dikes from the basic suite. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. Inverse AMS fabric is a minority (2 dikes from each suite). The parallelism between AMS and AARM tensors for dikes with abnormal fabrics suggests a primary origin for them. Gyroremanent magnetization (GRM) effect was negligible for the majority of dikes, but it was found in two dikes from the basic suite with normal AMS fabric.Magnetic fabrics recognized for the three studied swarms do not depend on magnetic mineralogy, geochemical composition, dike strikes, nor the age of the swarms since the same magnetic minerals and magnetic fabric types are found in dikes from all suites. Inclined and horizontal flows allow us to infer the relative position of at least three magma sources (or magma chambers) from which the dikes were fed.     

Grain to outcrop-scale frozen moments of dynamic magma mixJing in the syenite magma chamber,Yelagiri Alkaline Complex,South India

Magma mixing process is unusual in the petrogenesis of felsic rocks associated with alkaline complex worldwide. Here we present a rare example of magma mixing in syenite from the Yelagiri Alkaline Complex, South India. Yelagiri syenite is a reversely zoned massif with shoshonitic (Na₂O + K₂O=5–10 wt.%, Na₂O/K₂O = 0.5–2, TiO₂ <0.7 wt.%) and metaluminous character. Systematic modal variation of plagioclase (An_11–16 Ab_82–88), K-feldspar (Or_27–95 Ab_5–61), diopside (En_34–40Fs_11–18Wo_46–49), biotite, and Ca-amphibole (edenite) build up three syenite facies within it and imply the role of in-situ fractional crystallization (FC). Evidences such as (1) disequilibrium micro-textures in feldspars, (2) microgranular mafic enclaves (MME) and (3) synplutonic dykes signify mixing of shoshonitic mafic magma (MgO = 4–5 wt.%, SiO₂ = 54–59 wt.%, K₂O/Na₂O = 0.4–0.9) with syenite. Molecular-scale mixing of mafic magma resulted disequilibrium growth of feldspars in syenite. Physical entity of mafic magma preserved as MME due to high thermal-rheological contrast with syenite magma show various hybridization through chemical exchange, mechanical dilution enhanced by chaotic advection and phenocryst migration. In synplutonic dykes, disaggregation and mixing of mafic magma was confined within the conduit of injection. Major-oxides mass balance test quantified that approximately 0.6 portions of mafic magma had interacted with most evolved syenite magma and generated most hybridized MME and dyke samples. It is unique that all the rock types (syenite, MME and synplutonic dykes) share similar shoshonitic and metaluminous character; mineral chemistry, REE content, coherent geochemical variation in Harker diagram suggest that mixing of magma between similar composition. Outcrop-scale features of crystal accumulation and flow fabrics also significant along with MME and synplutonic dykes in syenite suggesting that Yelagiri syenite magma chamber had evolved through multiple physical processes like convection, shear flow, crystal accumulation and magma mixing.     

Magnetic fabric of a basaltic dyke swarm associated with Mesozoic rifting in northeastern Brazil

The anisotropy of magnetic susceptibility (AMS) has been studied in a 120 km long, Early Cretaceous tholeiitic dyke swarm emplaced during the early stages of rifting and opening of the equatorial Atlantic Ocean. The vertical dykes filled a set of E-trending fractures that cut the structural grain of the Precambrian basement of northeastern Brazil at a high angle. These strongly magnetic rocks contain pseudo-single domain, Ti-poor magnetite and secondary maghemite as revealed by thermomagnetic and hysteresis data. The contribution of the paramagnetic and the high coercivity antiferromagnetic fractions to the bulk susceptibility is less than 1.2%. The dykes generally show well-clustered AMS principal directions. The plunge of the magnetic lineation varies from nearly subvertical in the center of the swarm to horizontal in the west. The strike of the magnetic foliation is generally oblique to the dyke wall and exhibits a curved trend at the regional scale. This fabric pattern suggests that the magma source that fed the dykes was situated in the center of the swarm, which is presently below Tertiary sandstones.     

Magnetic fabrics in the Jurassic–Cretaceous continental basins of the northern part of the Central High Atlas (Morocco): Geodynamic implications

The aim of this work is to study the Anisotropy of the Magnetic Susceptibility (AMS) in two Jurassic–Cretaceous synclines located in the northern border of the Central High Atlas (Morocco): the Aït Attab and Ouaouizaght basins. AMS is used in order to obtain the magnetic fabric and its relationship with the kinematic evolution of both basins. The tectonic evolution of the basins, still under discussion, is mostly considered as the result of inversion during Tertiary and perhaps since Bathonian, of extensional and/or strike-slip Jurassic basins. Both basins are filled with Upper Jurassic to Lower Cretaceous silts and sandstones, with less frequent marine marly limestones.The bulk magnetic susceptibility (km) generally shows higher values in the red facies (163.2 E−6 in AT and 168.6 E−6 in WZ) than in the yellowish marly limestones (97.88 E−6 in AT and 132 E−6 in WZ). Most sites show an oblate magnetic fabric. The rock magnetic analyses indicate that the main carrier of the magnetic susceptibility for the red facies is hematite, whereas in the yellowish facies there is a dominance of paramagnetic minerals. In both basins, the magnetic lineation (long axis of the ellipsoid, k_max axes) shows a predominant E–W direction. The overlapping of the stress fields during the Atlasic basins evolution, in both compressional and extensional regimes and hinder the straightforward interpretation of the magnetic fabrics. However, a coeval N–S compression during the times of sedimentation with an E–W transtension can explain the magnetic lineation found in many of the sites analyzed in the present work. There are also other less frequent directions of k_max axes (NE–SW and NW–SE) are interpreted as the result of local change of the stress field during the early extensional stage of basin formation.     

A record of ductile syn-intrusional fabrics to post solidification cataclasis: Magnetic fabric analysis of neoproterozoic Mirpur and Mt. Abu Granitoids,NW India

The Mirpur granite body represents a relatively small (10 km²) pluton intruded along the northern margin of the adjacent Mt. Abu batholith (∼125 km²) in NW India. It is a visibly undeformed alkali feldspar rich pink granite; in contrast, the Mt. Abu is a composite granitoid body and variably deformed. Both are intruded by rhyolitic dykes and the terminal magmatic events in both the cases are mafic dykes. The AMS (Anisotropy of Magnetic Susceptibility) data identify the Mt. Abu with SE-dipping foliations and subvertical lineations as a single structural domain while the Mirpur granite body shows two domains characterized by predominantly E — W trend of magnetic foliation in the eastern part (domain I) and N — S orientations in the western part (domain II). The domain I shows magmatic fabrics, typical for the peraluminous granites of Malani Igneous Suite (MIS). Change in fabric orientation in the domain II has resulted from cataclasis wherein the samples show destruction of the original E — W fabric and complete transposition by N — S trends. The foliations in the Mt. Abu granites have been related to SE orientation of maximum horizontal stress. The same maximum stress direction can be inferred from dyke orientation in the Mirpur granite, which is interpreted as continuation of the tectonic imprint in this region during emplacement of both the granites. Age of the cataclastic overprint with a predominant N — S orientation is not yet constrained but corresponds with the trend of the nearby Sindreth basin within the Malani Igneous Suite. The Neoproterozoic tectonic scenario for the region has been interpreted in terms of an ongoing crustal convergence and granitic magma emplacement against the back stop offered by the rigid Delhi Fold Belt.     

Fabric analysis of quartzites with negative magnetic susceptibility – Does AMS provide information of SPO or CPO of quartz?

We ask the question whether petrofabric data from anisotropy of magnetic susceptibility (AMS) analysis of deformed quartzites gives information about shape preferred orientation (SPO) or crystallographic preferred orientation (CPO) of quartz. Since quartz is diamagnetic and has a negative magnetic susceptibility, 11 samples of nearly pure quartzites with a negative magnetic susceptibility were chosen for this study. After performing AMS analysis, electron backscatter diffraction (EBSD) analysis was done in thin sections prepared parallel to the K₁K₃ plane of the AMS ellipsoid. Results show that in all the samples quartz SPO is sub-parallel to the orientation of the magnetic foliation. However, in most samples no clear correspondance is observed between quartz CPO and K₁ (magnetic lineation) direction. This is contrary to the parallelism observed between K₁ direction and orientation of quartz c-axis in the case of undeformed single quartz crystal. Pole figures of quartz indicate that quartz c-axis tends to be parallel to K₁ direction only in the case where intracrystalline deformation of quartz is accommodated by prism <c> slip. It is therefore established that AMS investigation of quartz from deformed rocks gives information of SPO. Thus, it is concluded that petrofabric information of quartzite obtained from AMS is a manifestation of its shape anisotropy and not crystallographic preferred orientation.     

A paleomagnetic and magnetic fabric study of the Illapel Plutonic Complex,Coastal Range,central Chile: Implications for emplacement mechanism and regional tectonic evolution during the mid-Cretaceous

The Illapel Plutonic Complex (IPC), located in the Coastal Range of central Chile (31°–33° S), is composed of different lithologies, ranging from gabbros to trondhjemites, including diorites, tonalites and granodiorites. U/Pb geochronological data shows that the IPC was amalgamated from, at least, four different magmatic pulses between 117 and 90 Ma (Lower to mid-Cretaceous). We present new paleomagnetic results including Anisotropy of Magnetic Susceptibility (AMS) from 62 sites in the plutonic rocks, 10 sites in country rocks and 7 sites in a mafic dyke swarm intruding the plutonic rocks.Remanent magnetizations carried by pyrrhotite in deformed country rock sediments nearby the intrusive rocks indicate that tilting of the sedimentary rocks occurred prior or during the intrusion. The paleomagnetic study shows no evidence for either a measurable tilt of the IPC or a significant rotation of the forearc at this latitude range. Moreover, new ⁴⁰Ar/³⁹Ar ages exclude any medium- to low-temperature post-magmatic recrystallization/deformation event in the studied samples. AMS data show a magnetic foliation that is often sub-vertical. Despite an apparent N–S elongated shape of the IPC, the large variations in the orientation of the AMS foliation suggests that this plutonic complex could be made of several units distributed in a N–S trend rather than N–S elongated bodies.Previous works have suggested for this area a major shift on tectonic evolution from highly extensional during Lower Cretaceous to a period around 100 Ma, associated with exhumation and compressive deformation to conform the present day Coastal Range. The low degree of anisotropy and the lack of evidence for a tectonic fabric in the intrusive rocks indicate that the shift from extensional to compressional should postdate the emplacement of the IPC, i.e. is younger than 90Ma.     

Magnetic fabric variations in Mesozoic black shales, Northern Siberia, Russia: Possible paleomagnetic implications

A 28-m-long section situated on the coast of the Arctic Ocean, Russia (74°N, 113°E) was extensively sampled primarily for the purpose of magnetostratigraphic investigations across the Jurassic/Cretaceous boundary. The section consists predominantly of marine black shales with abundant siderite concretions and several distinct siderite cemented layers. Low-field magnetic susceptibility (k) ranges from 8 × 10^− 5 to 2 × 10^− 3 SI and is predominantly controlled by the paramagnetic minerals, i.e. iron-bearing chlorites, micas, and siderite. The siderite-bearing samples possess the highest magnetic susceptibility, usually one order of magnitude higher than the neighboring rock. The intensity of the natural remanent magnetization (M₀) varies between 1 × 10^− 5 and 6 × 10^− 3 A/m. Several samples possessing extremely high values of M₀ were found. There is no apparent correlation between the high k and high M₀ values; on the contrary, the samples with relatively high M₀ values possess average magnetic susceptibility and vice versa. According to the low-field anisotropy of magnetic susceptibility (AMS), three different groups of samples can be distinguished. In the siderite-bearing samples (i), an inverse magnetic fabric is observed, i.e., the maximum and minimum principal susceptibility directions are interchanged and the magnetic fabric has a distinctly prolate shape. Triaxial-fabric samples (ii), showing an intermediate magnetic fabric, are always characterized by high M₀ values. It seems probable that the magnetic fabric is controlled by the preferred orientation of paramagnetic phyllosilicates, e.g., chlorite and mica, and by some ferromagnetic mineral with anomalous orientation in relation to the bedding plane. Oblate-fabric samples (iii) are characterized by a bedding-controlled magnetic fabric, and by moderate magnetic susceptibility and M₀ values. The magnetic fabric is controlled by the preferred orientation of phyllosilicate minerals and, to a minor extent, by a ferrimagnetic fraction, most probably detrital magnetite. Considering the magnetic fabric together with paleomagnetic component analyses, the siderite-bearing, and the high-NRM samples (about 15% of samples) were excluded from further magnetostratigraphic research.     

Rock texture and magnetic lineation in dykes: a simple analytical model

We develop two simple models for simulating the combination of magnetic sub-fabrics related to magma flow in dykes. The basic assumptions are (i) the petrofabric is representative of the flow fabric, and (ii) the petrofabric is composed of S/C-type structures related to flow. The first model consists of summing the magnetic tensors of two identical sub-fabrics, differing solely by their relative rotation. This model accounts for the possible change of the macroscopic magnetic lineation from a flow-related fabric to a lineation situated at the geometric intersection between the two sub-fabrics. Such a result is obtained in the case of oblate to highly oblate sub-fabric ellipsoids. The second model integrates the effect of very oblate grains of variable orientations into calculating the shape controlled magnetic tensor of each sub-fabric, and emphasizes the possible under-estimation of fabric superposition due to microscopic disordering. The magma fluxes along the East Greenland volcanic margin are illustrated by the flow pattern within the major dyke swarm. The magmatic flow vectors inferred from the imbrication of magnetic foliation at the dyke margins are primarily horizontal. The classic use of magnetic lineation can lead to contradictory results, giving flow vectors perpendicular to the flow directions. The magnetic lineation is situated close to the zone axis of magnetic foliation planes over a wide range of scales throughout the dyke swarm, suggesting that the contradiction may arise from the association of several textural domains at the sample scale. Forward modelling of macroscopic magnetic fabrics using the first model yields good agreement with the measured magnetic fabric of the East Greenland dykes. Our results, which are applicable to strained sedimentary rocks, highlight the possible misuse of the magnetic lineation due to combination of magnetic textures. The exchange between a microscopic lineation, i.e. mineralogical lineation, and a macroscopic lineation, i.e. intersection lineation, is particularly expected for dykes that generally bear oblate magnetic textures.     

吕梁—晋北地区晚前寒武纪镁铁质岩墙群侵位方式的磁组构证据

华北克拉通区内的吕梁-晋北地区广泛发育晚前寒武纪镁铁质岩墙群。该地区近EW向和NW-NNW向岩墙的磁组构测量结果表明,岩墙群磁各向异性与岩墙侵位的关系密切,磁化率的长轴能指示其岩浆侵位的流向。该区的岩墙群的岩浆源位于东侧的燕辽拗拉槽处,岩浆沿岩墙走向以一定仰角由ESE(或E)朝NW-NNW(或W)向流动,具板内裂谷模式。     

华北晚前寒武纪镁铁质岩墙群的流动构造及侵位机制

华北克拉通中部广泛发育晚前寒武纪镁铁质岩墙群。这些岩墙群未变形和未变质,保存了清晰完好的流动构造,完整地反映了前寒武纪岩浆活动的特征和流动构造,这在世界上是罕见的。通过对晚前寒武纪镁铁质岩墙群的形态和流动构造研究,如:流动线理、矿物组构和磁组构等,提出岩墙群的侵位方向和侵位方式。结合本区岩墙群与燕辽—中条拗拉槽系的关系以及岩墙群的力学性质,探讨本区岩墙群的侵位机制。     

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     

Structural geometry and kinematics of the Ailao Shan shear zone: insights from integrated structural,microstructural, and fabric studies of the Yao Shan complex,Yunnan, Southwest China

ABSTRACT

The Yao Shan complex, a massif near the southern segment of the Ailao Shan–Red River (ASRR) shear zone, bears important information on the structural framework of the massif and the kinematics of ductile shearing along the ASRR shear zone. In this contribution, structural, microstructural, quartz c-axis fabric, magnetic fabric, and geochronologic data are used to determine the structural framework of the Yao Shan massif and its tectonic implications for the ASRR shear zone. The Yao Shan complex is characterized by an overall linear A-type antiform that contains a core of high-grade metamorphic rocks with Palaeoproterozoic to Mesozoic protoliths and a mantle of Permo-Triassic low-grade rocks. Both the high-grade metamorphic core and low-grade Permo-Triassic rocks have experienced progressive ductile shearing. Anisotropy of magnetic susceptibility (AMS) results from 17 samples collected along the Xinjie–Pingbian section across the complex show that magnetic lineation (K_max) and foliation (K_max–K_int) are generally subparallel to the corresponding structural elements in the sheared rocks. The shape parameter E values of the magnetic ellipsoids are indicative of dominantly oblate and plane strain, but vary with protolith type and degree of strain among the various rock types. In agreement with the field and microstructural observations, the corrected degree of anisotropy (Pj) values reflect high shear strain in the core rocks and relatively low shear strain in the low-grade strata. A kinematic analysis based on structural and magnetic fabric data shows that both left- and right-lateral shear occurred during the deformation of the Yao Shan complex. Therefore, instead of being an element of the ASRR shear zone, the Yao Shan complex constitutes a crustal-scale inharmonic A-type fold with a fold axis parallel to the stretching lineation. Geochronologic data reveal that the folding occurred coevally with ductile shearing of the middle to lower crust between ca. 30 and 21 Ma.     

Palaeocurrent and environmental implications from anisotropy of magnetic susceptibility (AMS): a case study from Talchir and Barakar formations,Raniganj Basin,West Bengal,India

Oriented cylindrical cores of rock samples were collected from the Talchir and Barakar formations of the Lower Gondwana Supergroup of the Raniganj Basin exposed in and around Kalyaneshwari and Maithon areas. The cores (2.54 cm diameter and 2.2 cm height) were studied in the low field anisotropy of magnetic susceptibility (AMS) measurement to determine the nature of magnetic fabrics, to correlate it with the sedimentological characteristics and to determine the palaeocurrent patterns. The results derived from the statistical parameters (especially the q-factor), the shapes of the susceptibility ellipsoids and directional data of the AMS indicate that the magnetic fabrics within the studied units are primary (depositional) and are correlatable form the palaeoenvironmental features. The orientation of the maximum (K₁), intermediate (K₂) and minimum (K₃) susceptibility axes is dispersed on the lower hemisphere equal area diagram rather than strong clusters which is not because of secondary (tectonic) influence but due to the moderate to high-energy environment of deposition of the sediments in the studied units. Based on the q-factor (which is 0.581 for Barakar Formation and 0.565 for Talchir Formation which are both <?0.7), it is suggested the AMS indicates that the imbrication of the K₁ axis is the indicator of palaeocurrent. Also, the magnetic foliation (average value?=?1.255) exceeds the magnetic lineation (average value?=?1.107) and the shape parameter exceeds 0 in most cases pointing towards an oblate fabric. The palaeocurrent in the present study as indicated by the K₁ axis imbrication is very similar in both the units under study and is due SW. However, apart from this precise palaeocurrent direction, there exists a certain degree of randomness of the susceptibility axes which are very clear indication of corresponding depositional environments.     

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.