Dielectric anisotropy as a petrofabric indicator

A sensitive instrument for measurement of the angular dependence of dielectric constant in a rotating rock specimen is described and the precautions necessary to obtain meaningful results are summarized. An important problem is the choice of length/diameter ratio for cylindrical specimens, so that complete three-dimensional information can be obtained from a single specimen; this ratio averages 0.86, but is found to be a function of dielectric constant and of electrode geometry. The measurements reported here have been concerned with weakly anisotropic rocks which lack visible fabrics, especially sandstones and basalts. A sandstone was used in a study of the statistical “noise” or scatter in anisotropy axes of a set of 52 similar specimens. The anisotropy was found to be much greater than expected from the finite numbers of grains in a sample. It is concluded that, in spite of careful drying of specimens, the anisotropy is dominated by thin grain coatings which have very high dielectric constants or may even be conducting. A basalt has been found in which the dielectric and magnetic anisotropies have different axes, apparently reflecting different grain alignment events in the history of the rock. However, this is the only example that has been found in which all of the information obtainable from dielectric anisotropy could not have been obtained more easily from magnetic anisotropy.     

Experimental shear zones and magnetic fabrics

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.     

柴达木盆地西部新生界磁组构特征及其构造意义

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。      

Magnetic fabrics in garnet peridotites–pyroxenites and host felsic granulites in the South Bohemian Granulites (Czech Republic): Implications for distinguishing between primary and metamorphism induced fabrics

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.     

Magnetic fabric characteristics of bioturbated wave-produced grain orientation in the Bridport-Yeovil Sands (Lower Jurassic) of Southern England

There is little visible primary hydrodynamic lamination preserved in the Bridport-Yeovil Sands as a result of intense bioturbation. Where lamination is present, it exhibits wave-produced characteristics, although current ripple lamination is also found. The grain orientation of a variety of bioturbated and non-bioturbated fine-grained sandstones has been determined by measuring the magnetic susceptibility anisotropy. The magnetic fabric is of a primary style and preserves two lineation directions approximately 90° apart in azimuth. These lineation directions are interpreted as the result of grain long-axis orientations produced by wave and current processes. The magnetic fabric is dominantly carried by a small proportion of paramagnetic minerals, thought to be largely detrital chlorite and micas. This magnetic fabric has been acquired by depositional alignment of the detrital phyllosilicates and by reorientation of the phyllosilicates during the early stages of compaction. The magnetic fabric of the intensely bioturbated sandstone is not significantly different in magnitude characteristics or in the preservation of lineation directions from that of the non-bioturbated sandstone.     

Fabric and seismic properties of carrara marble mylonite

Shear deformation in calcite-rich rocks can produce strong lattice preferred orientations (LPO), which result in a high anisotropy of bulk seismic properties because of the high elastic anisotropy of calcite (32% Vp anisotropy). Deformed rocks often show also strong shape preferred orientations (SPO). Theories for averaging the elastic properties have not yet satisfactorily predicted the contribution to the seismic anisotropy caused by the SPO alone.A calcite mylonite from Carrara (Italy) was investigated, which is characterised by a strong SPO and a weak LPO. It was composed of about 80% calcite, then white mica, quartz and hematite. Flattening of mica and of calcite grains defined the mylonitic foliation, and elongation of calcite grains defined the lineation. On average calcite grains have aspect ratios of about 2.5:1.6:1, and grain sizes of about 10 μm. At 400 MPa confining pressure, the measured Vp (km/s) parallel to the lineation (X direction) was highest (6.63), lower in the intermediate Y direction (6.47); the Vp normal to the foliation (Z direction) was lowest (6.30). This yielded a Vp anisotropy of 5%. The LPO, determined by automated electron backscatter diffraction (EBSD), was very weak (texture index 1.1), with intensities between 0.6 and 1.6 m.r.d. in the c-axis pole figure. Extrapolation of the texture index to an infinite number of orientation measurements indicated that the observed variations were mostly random noise in the orientation distributions and that the bulk rock texture was random. The Vp anisotropy of the Voigt, Reuss and Hill averages calculated from this calcite LPO is predicted to be close to zero. Adding 5% of muscovite with (001) perfectly aligned parallel to the foliation, we calculated a total anisotropy of 2.8%. The anisotropy calculated for the special directions X, Y and Z remained at 2.6% only.It was concluded that the measured seismic anisotropy cannot be explained by the LPO of calcite and by 5% of mica alone. It is also attributed to the strong SPO and to further grain boundary effects.     

湖南道县辉长岩包体的组构与波速各向异性研究及其意义

辉长岩包体的主要造岩矿物斜长石、单斜辉石的组构测定结果表明 :条带状辉长岩的斜长石组构为结晶b轴的优选定向 ,推测其成因是在重力作用占主导并伴有低应力的情况下形成的。宏观呈块状的辉长岩 ,在微观组构上仍有定向线理存在。经由组构、岩石密度、矿物组成计算得到的理论波速各向异性与实测波速得到的一致 ,从而表明地震波的各向异性对矿物组构的强烈依赖性。推测道县辉长岩包体的波速各向异性反映了下地壳地震波的一般特征。其组构特征反映了该区中、新生代下地壳的变形特征。     

A magnetic fabric study of the Shap region in the English Lake District

The magnetic susceptibility anisotropy technique was applied to 50 sites, comprising 280 core samples from the Shap Granite and the surrounding country rocks from both within and outside the thermal aureole. The magnetic fabrics of individual cores from the granite pluton indicate preferred orientations of non-equi-dimensional magnetite grains. This fabric developed during the slow ascent and consequent cooling of the magma. The sites from the metamorphic rocks from the immediate contact with the Shap Granite show weak axial shortening directions which are subnormal to the granite contact. Core samples from the surrounding Ordovician and Silurian rocks within the thermal metamorphic aureole but further from the granite have oblate magnetic- fabric ellipsoids with subvertical magnetic foliations trending approximately NE-SW, parallel to the regional Caledonian cleavage planes. This suggests that the magnetic fabric observed in the granite is primary with respect to the time of emplacement of the magma, and that this emplacement post-dates the main compressive phase of the Caledonian Orogeny. The effect of the granite on the country rock is mainly thermal metamorphism with mechanical deformation limited to a very narrow strip around the granite.     

Current-independent paleomagnetic declinations in flysch basins: a case study from the Inner Carpathians

Paleomagnetic declinations from the Inner Carpathian Paleogene Basin imply that the area rotated counterclockwise about 60°, during the Miocene[1]. The question may arise if the paleomagnetic declination could have been biased by the W-E directed turbidity currents prevailing in the basin causing an apparent counter-clockwise rotation of the paleomagnetic direction.

The paleomagnetic results were obtained for fine grained strata, deposited in relatively calm water. Nevertheless, to confirm the paleomagnetic rotation, we needed evidence that flow activity on the magnetic grains was indeed insignificant in the beds yielding paleomagnetic results. Therefore, we carried out magnetic anisotropy measurements.

Results of AMS (representing para and ferromagnetic minerals together) measurements, compared with paleomagnetic observations, demonstrate that well-clustered lineations at locality level and failure to define a paleomagnetic direction are coupled. Lineation, when observable, is flow parallel, suggesting that magnetic lineation in the Inner Carpathian flysch basins may be regarded as a good proxy for turbidity current direction. It is remarkable, however, that the well-defined paleomagnetic directions are observed for localities, where the magnetic fabric is not showing lineation on locality level. Moreover, the lineation direction of the ferromagnetic minerals alone (obtained by measuring the anisotropy of the remanence) is independent of that of the turbidity currents. Thus we can safely conclude that the Inner Carpathian flysch basin indeed was affected by 60° tectonic rotation, and the paleomagnetic vectors were not biased by paleocurrents.     

Experiments on the production of transverse grain alignment in a sheared dispersion

ABSTRACT A series of experiments was designed to investigate grain orientation in flows with concentrations of granular solids in the range from 0.5 to 15%. Fourteen experiments were carried out in which sand-sized material was dispersed in liquid plaster of paris and the dispersion allowed to flow down a slope until it came to rest. The preferred orientation of the grains was then estimated by measuring the anisotropy of magnetic susceptibility of specimens cut from the solidified flows.
Five of the experiments showed signs of deformation, both in the gross characteristics of the flow and in the grain alignment. The remaining nine had grain alignment dependent on grain concentration. At concentrations of 1.2% and less and 11.1% and more, by volume, alignment of long axes was parallel to flow. These observations were consistent with the existing theories for low concentrations, in which preferred alignment is shown to result from the varying rate of rotation of grains in a shearing flow, and for high concentrations, in which preferred alignment results from the transfer of angular momentum between colliding grains.
At some intermediate concentrations transverse alignment was observed. A theoretical explanation combining the theories of the extreme cases is suggested. The observation of transverse alignment is compared with a similar observation in some turbidites.     

Emplacement of the Ardara pluton (Ireland): new constraints from magnetic fabrics, rock fabrics and age dating

The Ardara pluton as part of the Donegal batholith was intruded into Neoproterozoic metasediments and metadolerites at mid-crustal levels. The emplacement mechanism of the Ardara granite is very controversial, and mechanisms ranging from diapirism, ballooning and stoping followed by nested diapirism have been proposed. Magnetic fabrics, rock fabrics and K/Ar dating of micas are used here to constrain the emplacement history. The compositional zoning of the Ardara pluton is clearly reflected in the different bulk magnetic susceptibilities between the outer quartz monzodiorite and the central granodiorite, whereas the intervening tonalite is of intermediate nature. The magnetic carriers are characterized by the anisotropy of the magnetic susceptibility (AMS), thermomagnetic measurements and through high field analyses (HFA). The separation of the ferrimagnetic and paramagnetic contributions revealed that biotite and magnetite control the AMS in the quartz monzodiorite. Both minerals are oriented in such a way that their summed contribution is constructive and originates from the shape fabric of magnetite and the texture of biotite. Biotite is responsible mainly for the AMS in the tonalite and granodiorite. The magnetic foliation can be directly related to the macroscopic foliation and also to the D4 structures in the country rocks. The foliation is consistent with the geometry of the roughly circular shape and has a mostly steep to vertical dip. Towards the central granodiorite the magnetic foliation dies out, although plagioclase texture measurements indicate a weak magmatic shape fabric. With the exception of the tail, the K_max axes (magnetic lineation) vary from steeply to gently plunging. The so-called lineation factor is approximately 1.01 and therefore points to a less significant axial symmetry. These observations coincide with strain estimates on mafic enclaves that show a very consistent pattern of K ∼0 flattening strain. Texture analyses of biotite and quartz additionally support the observations made by the strain analyses and the magnetic fabric data. Microstructural investigations give evidence that the fabrics are associated with the emplacement over a range of temperatures from truly magmatic to high-temperature solid-state conditions. The age of the intrusion is still under discussion, but a new cooling age was determined by K/Ar dating of biotite at 403.7±8 Ma corresponding to a temperature range between 450 and 300°C. For a mylonite along the southern contact between the Ardara pluton and the country rock a K/Ar muscovite age of 378.8±7 Ma indicates a minimum age for the shear zone when the Ardara pluton must have already been cooled down below 350±50°C. Received: 28 January 1999 / Accepted: 28 December 1999     

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.     

Strain distribution across a partially molten middle crust: Insights from the AMS mapping of the Carlos Chagas Anatexite,Araçuaí belt (East Brazil)

The easternmost part of the Neoproterozoic Araçuaí belt comprises an anatectic domain that involves anatexites (the Carlos Chagas unit), leucogranites and migmatitic granulites that display a well-developed fabric. Microstructural observations support that the deformation occurred in the magmatic to submagmatic state. Structural mapping integrating field and anisotropy of magnetic susceptibility (AMS) revealed a complex, 3D structure. The northern domain displays gently dipping foliations bearing a NW-trending lineation, southward, the lineation trend progressively rotates to EW then SW and the foliation is gently folded. The eastern domain displays E–W and NE–SW trending foliations with moderate to steeply dips bearing a dominantly NS trending lineation. Magnetic mineralogy investigation suggests biotite as the main carrier of the magnetic susceptibility in the anatexites and ferromagnetic minerals in the granulites. Crystallographic preferred orientation (CPO) measurements using the electron backscatter diffraction (EBSD) technique suggest that the magnetic fabric comes from the crystalline anisotropy of biotite and feldspar grains, especially. The delineation of several structural domains with contrasted flow fabric suggests a 3D flow field involving westward thrusting orthogonal to the belt, northwestward orogen-oblique escape tectonics and NS orogen-parallel flow. This complex deformation pattern may be due to interplay of collision-driven and gravity-driven deformations.     

福建中甲锡矿及其外围岩石磁组构特征

磁组构成分析是利用岩石磁化率各向异性研究构造变形特征及其应力作用方式和方向的方法,研究表明,中甲地区岩石各向异性度Ｐ值比较小,反映本区总体变形较弱,但变质石英砂岩相对变形较强。变质石英砂岩磁面理发育,磁线理较弱,显示压扁变形,变形主压应力方向是ＮＷ－ＳＥ向。火山（碎屑）岩具有明显的磁线理,反映流纹构造特征;最大磁化率轴方向屡示本区火山岩流体构造为ＮＷ－ＳＥ向。矿化蚀变岩和矿石的磁各向异性度Ｐ值明显     

Understanding the Mesozoic kinematic evolution in the Cameros basin (Iberian Range,NE Spain) from magnetic subfabrics and mesostructures

Analysis of anisotropy of magnetic susceptibility (AMS) and brittle mesostructures (hydroplastic synsedimentary faults and tension gashes) is applied in this study in order to characterize the Mesozoic tectonic events in the Cameros basin (NW Iberian Range), formed between Tithonian and Albian times. Low-field AMS at room and low temperature (LF-AMS at RT and LF-AMS at LT, respectively) together with high-field AMS (HF-AMS) measurements allow separating ferro- and paramagnetic fabrics. The combination of LF-AMS at LT and HF-AMS torque measurements confirms the reliability of both procedures in terms of isolating the paramagnetic contribution to the AMS. Magnetic fabric results combined with the analyses of synsedimentary faults indicate a NW–SE extension direction during Aptian (and probably Barremian) times. This extension direction is perpendicular to the main extension direction (NE–SW) prevailing during early and late stages of basin evolution. It is also consistent with extension direction deduced from large-scale bending folds and tension gashes, developed after partial lithification. Cleavage development during Albian enhanced the orientation of the magnetic fabric in lithologies where the previous extensional magnetic lineation is coaxial with the expected one for compression.     

Composite detrital and thermal remanent magnetization in tuffs from Aeolian Islands (southern Tyrrhenian Sea) revealed by magnetic anisotropy

This paper reports on the complex relation between rock emplacement and remanence acquisition in tuffs deposited by pyroclastic density currents, disclosed by systematic measurements of the anisotropy of magnetic susceptibility and natural remanent magnetization (NRM). Thermal demagnetization shows that the NRM consists of two components with different blocking-temperature spectra. The direction of the low-temperature component is consistent with the geocentric axial dipole value, whereas the high-temperature component has dispersed directions. The magnetic fabric is oblate, the magnetic foliation is close to the bedding and the lineations are generally dispersed along a girdle within the foliation plane. The directions of the magnetic lineation and the high-temperature remanence component of individual specimens are close to each other. This correspondence suggests that the high blocking-temperature grains acquired a remanence aligned to their long dimension before deposition, while cooling within the explosive cloud and the moving pyroclastic current. Thereafter, during deposition, the traction processes at the base of the current oriented the grains along the flow direction and affected both fabric and high-temperature remanence. This NRM component results from mechanical orientation of previously magnetized grains and is thus detrital in origin. A second, thermal component was then acquired during the cooling of the low blocking-temperature grains after deposition. These results show that NRM in fine-grained pyroclastic rocks is affected by the Earth’s magnetic field as well as the emplacement processes and that magnetic fabric data are essential to unravel its complex nature.     

Diamagnetic anisotropy of sheetsilicates

The diamagnetic properties of sheetsilicates are studied by measuring the magnetic orientation of micron-sized crystals. In the case of synthetic phlogopite with the average diameter of 0.65 μm and the thickness of 0.2 μm, the alignment of the grains is achieved in the magnetic field below 35 kOe. The planes of the platy single-crystal grains were alignned parallel to the field when the alignment was achieved. The alignment of the grains is realized because the field-induced anisotropic energy, caused by the diamagnetic anisotropy in the mineral's crystal structure, exceeds thermal agitation energy in the applied field. By analyzing the field dependence of grain alignment, the diamagnetic anisotropy per formula unit, Δ_χ, of sheetsilicate mineral is estimated even if a large single crystal is not provided. The field-induced anisotropic energy is proportional to NΔ_χ where N is the number of molecules in the grain. A linear correlation has been found between the Δ_χ value and the F/OH mole ratio among the measured sheetsilicates. This correlation supports the assumption that the hexagonally packed oxygen-layer in the crystal lattice induces the anisotropies of the sheetsilicates.     

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.     

南京下蜀土的岩石磁学特征

对南京附近的下蜀土进行了岩石磁学测定，通过磁化率各向异性测量研究了它的沉积磁性组构特征，下蜀土的各异向性很不明显，各向异性废Ｐ小于水成岩的Ｐ值（＞１．０２），且与困面理度Ｆ高度相关，与磁线理度Ｌ相关不明显，这种特征与中国中部风成黄土极相似。对新生圩剖面进行的磁化率测量发现用化率曲线上有７个旋回的波峰、波谷变化，这与剖面中的古土壤－黄土系列相一致，说明下蜀土的剖面磁化率同样可以作为地层划分与对比，反映古气候冷暧变化的物理参数。对典型下蜀土样品进行的低温磁化率测量揭示出古土壤和黄土的成颗粒的构成不同，古土壤以超顺磁质为主，黄土以顺磁质为主。细小磁颗粒在古土壤中的积聚可能与古土壤的发育及古气候的暧湿程度有关。     

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.