Anisotropy Of Magnetic Susceptibility Research Articles

Multi-stage tectonic evolution of the Tatra Mts recorded in the para- and ferromagnetic fabrics

The Tatra Mts form the highest part of the Carpathian mountain chain; however, their tectonic and thermal evolution is still debatable. Previous magnetic fabric studies have primarily focused on the crystalline basement and its autochthonous cover. We investigate the magnetic fabrics of Cretaceous marly limestones from a Mesozoic nappe unit and post-thrusting Oligocene shales and mudstones to unravel the most recent tectonic evolution of the Tatra massif. In addition to standard petromagnetic measurements such as the acquisition of the Isothermal Remanent Magnetization or temperature-dependent susceptibility analyses, we investigated the paleotemperature of the Tatra region because high temperatures are known to significantly affect the magnetic mineralogy. The most common minerals in the studied units are paramagnetic phyllosilicates which govern the in-phase Anisotropy of Magnetic Susceptibility. The ferromagnetic fraction is represented by fine-grained magnetite with a minor contribution of hematite. Measured and counted vitrinite reflectances document an eastward increase in maturity, which is also reflected in the magnetite–hematite grain size ratios. Because the paleotemperatures recorded in the Cretaceous rocks follow the same increasing trend as the post-thrusting shales, it appears that both units were affected by a single major thermal event linked presumably to the Late Oligocene/Early Miocene burial. We propose that magnetic fabrics carried by phyllosilicates document the impact of crucial tectonic phases such as Miocene uplift and Cretaceous thrusting, whereas the out-of-phase Anisotropy of Magnetic Susceptibility and Anisotropy of Anhysteretic Remanent Magnetization fabrics most likely record the stress orientation during major burial episodes. Finally, the conspicuous vertical ferromagnetic lineation present in some Cretaceous sites documents the transpression-controlled tectonic regime in the Oligocene–Early Miocene.

Mineral magnetism and palaeoenvironment recorded in loess in southern England

ABSTRACTExtensive research has focused on the loess deposits in southern England, aiming to unravel their stratigraphic and palaeoenvironmental significance. However, no systematic mineral magnetic study on these deposits has yet been undertaken. Here we address this by investigating the magnetic mineral composition and alignment of loess at two sites in southern England, Lowland Point (LP) on the Lizard Peninsula of Cornwall, and Pegwell Bay (PB) in eastern Kent on the Isle of Thanet. A set of rock magnetic analyses was conducted to understand mineral magnetism within the studied sections. In addition, the primary depositional origin of the magnetic fabrics as well as their overprinting caused by various possible post‐depositional processes were examined through anisotropy of magnetic susceptibility (AMS). The magnetic mineral composition is similar at both sites, derived from both sedimentary (aeolian) and post‐depositional processes. The AMS results show magnetic foliation that varies between the two sites, aligning with the bedding plane, indicating deposition from gravitational dust fall at the PB site, and a stronger wind transport energy at the LP site. Although the magnetic lineation of bulk AMS is relatively weak, there is a noticeable tendency towards alignment in most samples, with flow directions from the SE indicated at both sections if a strong wind is assumed at LP, although with a possible SW direction if a weaker wind is assumed. These observations may imply the preservation of palaeowind directions during dust transport, with a substantial southerly component, which contradicts previous assertions of dominant northerly or northeasterly winds. Indeed, a possible dominant southeasterly wind direction between 25 and 18 ka would suggest a limited influence of katabatic, westerly or polar northeasterly winds during dust transport, and rather may imply the effect of low‐pressure systems passing through the English Channel during that period.

Rotation of crustal domains related to Gondwanide deformation in the Neoproterozoic–Cambrian Sauce Chico Complex (Argentina), southwestern margin of the Río de la Plata Craton: Insights from AMS data

The tectonic evolution of the Ventania System fold and thrust belt and the origin of its curvature have been the subject of debates for decades. The Gondwanide Orogeny generated the mylonitization of its Tonian–middle Cambrian basement (i.e., Sauce Chico Complex), accompanied by an intense hydrolysis which produced secondary muscovite (i.e., sericite), and the folding of the overlying Paleozoic sedimentary cover. To evaluate the structural and tectonic evolution of the Ventania System as a result of the interaction of its basement with the Río de la Plata Craton, we studied the anisotropy of magnetic susceptibility (AMS) and rock magnetism on samples of the Sauce Chico Complex. Rock magnetic studies suggest that paramagnetic minerals are the main carriers in the entire Sauce Chico Complex. We suggest that our results reflect secondary muscovite generation due to mylonitization. AMS data reveal that the orientation of the magnetic foliation coincides to a large extent with the rock foliation. However, magnetic and rock foliation orientations are not uniform among the different sampled units, displaying strikes between N 320° (NW) and N 345° (NNW) on average. Localities with different orientations are separated by regional strike-slip structures, referred to as tear faults, which have also been reported and interpreted by previous authors. According to this, we suggest that the variations detected between the orientation of the magnetic and rock foliations of the different basement units may be the result of the rotation of crustal blocks bounded by tear faults. We also propose the existence of a new tear fault not previously recognized. Furthermore, a tear fault of regional character, known as the Sauce Chico Tear Fault, juxtaposes two distinct crustal domains. The differential displacement of such domains could have contributed not only to different structural styles between blocks on both sides of the Sauce Chico Tear Fault, but also to the curved shape of the Ventania System during the progression of the Gondwanide deformation.

Emplacement and flow dynamics in a small volcanic dyke swarm: The example of Mount Calanna (Etna, Italy)

The emplacement mechanism and flow dynamics of small dyke swarms associated with monogenetic eruptive centres are less well studied than those of large swarms associated with shield volcanoes and flood basalts. This is due to their limited exposure and ease of reworking by later events. However, the study of small dykes helps to better constrain the forces controlling dyke propagation and thus reconstruct the emplacement history of the volcanic system. This study presents a multidisciplinary analysis of Mount Calanna (Mount Etna, Italy) where we sampled a total of 11 dykes among the two groups: 1) sub-vertical dykes from the lower part of the system, and 2) sub-horizontal dykes from the upper part. To infer the magma flow direction, we used Anisotropy of Magnetic Susceptibility (AMS) and compared the results to the Shape Preferred Orientation (SPO) of the plagioclase crystal fabric. To quantify the relative timing of dyke emplacement, and to characterise the magma plumbing system, we also used the palaeomagnetic and geochemical signature of the sampled dykes.The fabric analysis highlights a NE-SW flow direction in the sub-vertical dykes, and a NE-SW and NW-SE direction in the sub-horizontal dykes. Dykes emplaced at the lowest levels in the swarm show sub-vertical to inclined magma flow, whereas those towards the top of the Mount Calanna show inclined to sub-horizontal flow. The combined structural and geochemical analyses suggest the Mount Calanna swarm is a coherent intrusive complex associated with a phase of activity of Ancient Alkaline centre (AAC), with Mount Calanna representing an example of the uppermost portion of a shallow plumbing system in which intrusion orientations are controlled by regional tectonics.

Evaluating the Role of Titanomagnetite in Bubble Nucleation: Rock Magnetic Detection and Characterization of Nanolites and Ultra‐Nanolites in Rhyolite Pumice and Obsidian From Glass Mountain, California

AbstractWe document the presence, composition, and number density (TND) of titanomagnetite nanolites and ultra‐nanolites in aphyric rhyolitic pumice, obsidian, and vesicular obsidian from the 1060 CE Glass Mountain volcanic eruption of Medicine Lake Volcano, California, using magnetic methods. Curie temperatures indicate compositions of Fe2.40Ti0.60O4 to Fe3O4. Rock‐magnetic parameters sensitive to domain state, which is dependent on grain volume, indicate a range of particle sizes spanning superparamagnetic (<50–80 nm) to multidomain (>10 μm) particles. Cylindrical cores drilled from the centers of individual pumice clasts display anisotropy of magnetic susceptibility with prolate fabrics, with the highest degree of anisotropy coinciding with the highest vesicularity. Fabrics within a pumice clast require particle alignment within a fluid, and are interpreted to result from the upward transport of magma driven by vesiculation, ensuing bubble growth, and shearing in the conduit. Titanomagnetite number density (TND) is calculated from titanomagnetite volume fraction, which is determined from ferromagnetic susceptibility. TND estimates for monospecific assemblages of 1,000 nm–10 nm cubes predict 1012 to 1020 m−3 of solid material, respectively. TND estimates derived using a power law distribution of grain sizes predict 1018 to 1019 m−3. These ranges agree well with TND determinations of 1018 to 1020 m−3 made by McCartney et al. (2024), and are several orders of magnitude larger than the number density of bubbles in these materials. These observations are consistent with the hypothesis that titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation.

Matrix deformation of marls in a foreland fold-and-thrust belt: The example of the eastern Jaca basin, southern Pyrenees

In this study, we used the Anisotropy of Magnetic Susceptibility (AMS) to investigate the matrix strain record of two calcareous shale formations, the Eocene Larrès and Pamplona Marls, along the eastern Jaca foreland fold-and-thrust belt (Southern Pyrenees). More than 1000 unoriented fragments, collected from 62 sites along 4 sub-parallel sections in the footwall of the regional Oturia thrust and through local Yebra anticline, were measured. The analysis of the degree of anisotropy (P’) and shape parameter (T) allowed to identify four types of magnetic fabrics. Type II fabrics associated with poorly deformed rocks are characterized by a relatively high anisotropy and an oblate shape. In contrast, type III fabrics, associated with strongly fractured rocks are characterized by the lowest anisotropy and a triaxial shape. Type IV and type V fabrics are characterized by increasing anisotropy and shape parameters, and are associated with the development of a weak to a slaty cleavage in rocks. The distribution of the magnetic fabric is roughly similar along the four studied sections. In the footwall of the Oturia thrust, magnetic fabrics evolve from the type V to type II over a 1000 m-long interval. By contrast, the distribution of magnetic fabric is roughly symmetric across the Yebra anticline, evolving from a dominating type II fabric in both limbs to mixed type III-type V fabrics within the 1 km-large hinge zone. The succession of the magnetic fabrics is interpreted as recording various degrees of matrix strain in response to thrusting and folding. The correlation of magnetic fabrics between the four sections highlights some along-strike variations in the extension of fabric domains that are interpreted as reflecting the local influence of 2nd-order factors, such as the syn-tectonic sedimentation. Results are integrated within the tectono-sedimentary framework of the studied area to propose a model of matrix strain related to the tectonic and sedimentary evolution of a foreland fold and thrust belt.

New palaeocurrent analysis approach from two‐dimensional trough cross‐strata using photographs and anisotropy of magnetic susceptibility

AbstractPalaeocurrent analysis is vital for basin analysis and helps in the interpretation of depositional environments (along‐slope or downslope). For that, it is crucial to have multiple measuring methods at hand to apply palaeocurrent analysis with a wide range of different datasets (outcrops, cores and photographs). Here, two relatively underexploited palaeocurrent measurement techniques are assessed when applied to trough cross‐stratification observed in the Arenazzolo Formation at Eraclea Minoa (Sicily). The first technique is a novel design of a qualitative approach to infer palaeocurrent directions from photographs of two‐dimensional sedimentary structures. The second technique involves measurements of the anisotropy of magnetic susceptibility from drilled samples. A broad agreement, with overlapping uncertainty boundaries, is observed between results from both techniques. This agreement validates the use of trough cross‐strata to infer palaeocurrent directions. Moreover, the addition of photographs improves reproducibility and prevents a bias towards the best‐exposed troughs. The application of both techniques to outcrops and sedimentary cores provides new opportunities for palaeocurrent analysis in any type of sedimentary environment.

Deciphering Iberian Variscan Orogen Magmatism Using the Anisotropy of Magnetic Susceptibility from Granites

In this paper, we have synthesized the information derived from more than 20 papers and PhD theses on the anisotropy of the magnetic susceptibility (AMS) of 19 Variscan granite plutons, spanning the period between 320 Ma and 296 Ma. The AMS data are obtained from 876 sampling sites with more than 7080 AMS measurements and a re-interpretation is proposed. The studied granites exhibit a magnetic susceptibility (Km) ranging from 30 to 10,436 × 10−6 SI units. Most granites typically exhibit Km values below 1000 × 10−6 SI, indicative of paramagnetic behavior. Biotite serves as the main carrier of iron (Fe), emphasizing the reduced conditions prevalent during the formation of granite melts in the Variscan orogeny. The AMS fabrics of the studied granite plutons record the magma strain, expressing the chronologic evolution of the stress field during the orogeny. This chronologic approach highlights the magmatic events between around 330 and 315 Ma, occurring in an extensional regime, in which the Borralha pluton is an example of a suite that recorded this extensional AMS fabric. Plutons with ages between 315 and 305 Ma show AMS fabrics, pointing out their emplacement in a compressional tectonic regime related to the Variscan collision. The plutons, younger than 305 Ma, record AMS fabrics indicating that the tectonic setting for emplacement changes from a wrench regime to an extensional one at the end of the collision stage. This is evident as there is a chronological overlap between the granites that exhibit AMS fabrics indicating extension and the ones that have AMS fabrics indicating a wrench regime.

Postmortem 7Li NMR analysis for assessing the reversibility of lithium metal electrodes in lithium metal batteries

Despite the proficiency of lithium (Li)-7 NMR spectroscopy in delineating the physical and chemical states of Li metal electrodes, challenges in specimen preparation and interpretation impede its progress. In this study, we conducted a comprehensive postmortem analysis utilizing 7Li NMR, employing a standard magic angle spinning probe to examine protective-layer coated Li metal electrodes and LiAg alloy electrodes against bare Li metal electrodes within Li metal batteries (LMBs). Our investigation explores the effects of sample burrs, alignment with the magnetic field, the existence of liquid electrolytes, and precycling on the 7Li NMR signals. Through contrasting NMR spectra before and after cycling, we identified alterations in Li0 and Li+ signals attributable to the degradation of the Li metal electrode. Our NMR analyses decisively demonstrate the efficacy of the protective layer in mitigating dendrite and solid electrolyte interphase formation. Moreover, we noted that Li+ ions near the Li metal surface exhibit magnetic susceptibility anisotropy, revealing a novel approach to studying diamagnetic species on Li metal electrodes in LMBs. This study provides valuable insights and practical guidelines for characterizing distinct lithium states within LMBs.

The effect of rotatability of measuring directions design on the precision of the determination of the anisotropy of magnetic susceptibility: Mathematical model study

The precision of the measurement of the anisotropy of magnetic susceptibility (AMS) depends, in addition to other factors like accuracy of measurement of directional susceptibility and number of measuring directions, also on the orientation of the measuring design with respect to the specimen's anisotropy. The last factor can be characterized by rotatability coefficient of the measuring design. We investigated through mathematical modelling of measuring process the effect of the rotatability coefficient on the error in the determination of degree of AMS, shape factor and principal directions. The effect is conspicuous in strongly non-rotatable designs with low number of measuring directions. It decreases with increasing rotatability and with increasing number of measuring directions. In designs with high number of directions this effect is small, virtually negligible from the practical point of view.

The Pleistocene ice-sheet dynamics in north-central Poland based on magnetic fabrics of tills and landform analysis

Abstract This research was carried out in north-central Poland, which was occupied by the ice sheet of the Saalian (Marine Isotope Stage [MIS] 6) and Upper Stadial of the Weichselian (MIS 2) glaciation. The application of the anisotropy of magnetic susceptibility (AMS) method supported by a digital elevation model (DEM) analysis of the orientation of glacial landforms allowed for the reconstruction of ice-sheet extent and ice movement directions of these two Pleistocene glaciations. The research used an innovative method of collecting AMS till samples from the glacial plateau areas. Based on the research, it was found that during the general recession of the ice sheet of the Saalian glaciation, a previously undescribed glacial transgression occurred, characterized by a different direction of ice-sheet movement. On the basis of detailed geomorphological studies of the area of terminal moraines, previously described in fragments, the maximum extent of the ice sheet during the Weichselian glaciation was clarified. The recession of the ice sheet of the Weichselian glaciation from the area of north-central Poland took place in four regressive or transgressive–regressive stages with variable directions of ice-sheet movement. The results obtained indicate the great potential of the AMS method in paleoenvironmental studies of glaciated areas.

Magnetic fabric of Ocean Plate Stratigraphy mélanges: a tool for unravelling protracted histories of oceanic plates from seafloor spreading to tectonic emplacement into accretionary wedges

Multiple magnetic fabrics, referred to as F1–F5, were revealed through the anisotropy of magnetic susceptibility in an Ocean Plate Stratigraphy (OPS) mélange of the Neoproterozoic–Cambrian Blovice accretionary complex, Bohemian Massif. The fabrics post-date the formation of the mélange and the rotation of basalt blocks within the matrix and are interpreted in terms of a complex structural history of the mélange. Excluding local fabrics, the F1 fabric formed earlier along the mélange belt, recording shortening of the accretionary wedge front, whereas the higher grade F4 fabric pervasively overprinted both blocks and matrix in the SW part of the belt, recording shearing and vertical shortening at deeper structural levels closer to a megathrust surface. The preservation of angular relationships between the F1 and F4 fabrics across different parts of the mélange suggests that the blocks were only strained and not rotated during deformation, exemplifying the notion that the OPS mélanges may be a product of deformation at very shallow levels. The F1–F5 fabrics may be viewed as snapshots in a protracted evolution of OPS, where the earlier fabrics in basalt blocks may record the travel path of an oceanic plate from mid-ocean ridge towards the trench, before being overprinted in the accretionary wedge. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists Supplementary material: Thin-section photomicrographs, anisotropy of magnetic susceptibility data and summary diagrams for magnetic fabric types are available at https://doi.org/10.6084/m9.figshare.c.7008173

Identification of magnetic mineralogy and paleo-flow direction of the Miocene-quaternary volcanic products in the north of Lake Van, Eastern Turkey

Abstract One of the major challenges facing geochemistry, petrology, and volcanology researchers is the difficulty in determining the origin and paleo-flow directions of igneous and volcanic rocks. It is not possible to clearly determine origins and paleo-flow directions in areas with numerous volcanic centers. Anisotropy of magnetic susceptibility (AMS) is a valuable method that provides insights into the origins and paleo-flow directions of lavas that are complex to study. The presence of volcanic materials with thicknesses up to 1 km, coming from different sources at varied time intervals in the north of Lake Van, makes this area an ideal setting for implementing AMS in establishing paleo-flow directions. This study presents the magnetic mineralogy and AMS analysis of volcanic rocks from the Miocene to the Quaternary in the Lake Van region. We conducted isothermal remanent magnetization (IRM) and high-temperature susceptibility (HTS) studies to determine the magnetic mineralogy. IRM studies revealed that (titano)magnetite is responsible for the magnetization in most samples, while both (titano)magnetite and hematite are responsible for the rest. Alteration degrees and Curie temperatures of the rock samples were also determined through HTS measurements. There is good agreement between the anticipated directions of lava flow and our findings for nearly all volcanic rocks.

Magnetic Fabric of Freshly Consolidated Lacustrine Mudstones Constrains the “Present‐Day” Strain Field

AbstractThe anisotropy of magnetic susceptibility (AMS) has great potential in deciphering weakly deformed fabrics that may be related to tectonic stress. Previous studies have suggested that magnetic lineation is a good indicator of paleostrain direction. It is unclear whether the magnetic fabric can also be used to indicate the present‐day strain field. To verify this idea, we measured the AMS of freshly consolidated lacustrine fine‐grained sediments at 11 locations in the Qaidam and Chaka‐Gonghe basins of the northeastern Tibetan Plateau and compared it with the present‐day strain field deduced from the global position system (GPS) velocity field. The magnetic lineations of both room‐temperature and low‐temperature AMS are roughly perpendicular to the GPS‐derived tectonic shortening direction within the error range, suggesting that the AMS of freshly consolidated muds is an effective indicator of the present‐day strain field, even if the sediments appear undeformed at the outcrop scale.

Magnetoelastic effect of kimberlite host rocks (Yakutsk diamondiferous province)

The purpose of the research is to conduct petro- and paleomagnetic studies of Early Paleozoic rocks of the carbonate basement of a number of diamond deposits in the Yakutsk diamondiferous province in order to study the changes in petrophysical parameter values in the dynamic influence zone of a kimberlite pipe. It is shown that the formation of kimberlite diatremes accompanied by pulsating explosions shifting upwards brings about thermoelastic stress fields in the kimberlite-bearing medium, which are characterized by epigenetic changes and associated petrophysical heterogeneities (petrophysical anomalies). Petromagnetic heterogeneities of burning and stress are, therefore, some of these petrophysical anomalies, within which kimberlite-bearing rocks have contrastingly changed their original magnetic characteristics under the action of thermodynamic processes. Primarily, petromagnetic anomalies are reflected in the changed nature of the anisotropy of magnetic susceptibility: from sedimentary to dyke geotype. In addition, petromagnetic anomalies of magnetic susceptibility can be accompanied by the formation of metachronous natural residual magnetization vectors in kimberlite host rocks. The dimensions of petromagnetic anomalies (petromagnetic heterogeneities) may significantly exceed the size of the kimberlite pipe itself, which facilitates identification and delineation of the most promising areas. Besides, the magnetoelastic effect can create zones close to the kimberlite bodies that are hardly permeable for relatively viscous, protocrystal-rich mafic magmas. This is the reason for their wedging out along petrophysical barriers that is presented by splitting into thin tongues, formation of trap-free windows and corridors, toroidal shafts with sharply increasing thickness in intrusions, etc. Having relatively elevated values of magnetic and density parameters, such forms of igneous formations will be reflected in the observed geophysical fields. Thus, it is reasonable to consider petromagnetic anomalies as an important petrophysical search criterion for the detection of bedrock kimberlite bodies.

Transport of Magma in Granitic Mush Systems; an Example From the Götemar Pluton, Sweden

AbstractGranitic magma bodies form in the ephemeral part of magma mush systems and are emplaced by a variety of mechanisms in different tectonic settings. This study investigates how granitic magma emplacement processes and tectonomagmatic interactions assert control over the architecture of mush state pluton‐scale magma transport pathways. The 1.45 Ga shallow‐crustal Götemar pluton is a 4.5 km diameter circular pluton that consists of three granite units: a coarse‐grained red granite, a medium‐grained pale to red granite, and fine‐grained pale microgranite sheets. We employed geological mapping supported by Anisotropy of Magnetic Susceptibility (AMS) to examine the magmatic and regional tectonic controls on late‐stage magma transport in the Götemar granitic magma mush system. Multiple parallel arcuate subhorizontal microgranite and medium‐grained granite sheets (from 0.1 to 10s of meters thick) were mapped within the pluton. The arcuate sheets pinch out from the northern part of the pluton toward the SE inferring magma propagation direction. A dominant set of vertical granitic sheets within the granite body strikes NW‐SE. The AMS fabrics are contact‐parallel in the main medium‐grained granite body and indicate inflation. Within the microgranite sheets, the AMS fabrics are parallel to the sheet strike and support a sheet propagation direction to the SE. The Götemar pluton displays a clear link between arcuate (concentric) magma‐transporting sheets and concentric strain‐partitioning related to the intrusion of medium‐grained granite magma. The vertical magma sheet orientations are consistent with an NE‐SW extensional stress field that is associated with the extensional back‐arc stress regime of the contemporary Hallandian Orogen.

Unstructured Grid Finite Element Modeling of the Three-Dimensional Magnetotelluric Responses in a Model With Arbitrary Conductivity and Magnetic Susceptibility Anisotropies

Unstructured Grid Finite Element Modeling of the Three-Dimensional Magnetotelluric Responses in a Model With Arbitrary Conductivity and Magnetic Susceptibility Anisotropies

Emplacement of the paleoproterozoic Itareru tonalite (NE São Francisco Craton, Brazil) during arc-continent collision revealed by magnetic fabrics

The Itareru tonalite is one of the Paleoproterozoic plutonic rock units that intruded the Serrinha block, an Archean-Paleoproterozoic crustal fragment of the São Francisco Craton (Bahia, Brazil). It is characterized by a ca. 80 km long, NW-SE elongated leaf-shaped body, mainly comprised of tonalite-diorite to granodiorite rocks with numerous dioritic enclaves. Magnetic studies were performed on 26 sites of the Itareru pluton (including country rocks). Anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) are coaxial, indicating that all minerals have the same orientation, partially controlled by regional strain. Rock-magnetism measurements reveal that paramagnetic–matrix minerals contributed significantly to the magnetic susceptibility and AMS while magnetite grains in the range of 2–5 μm are responsible for AARM. Magnetic fabrics in the central region of the pluton are probably primary (magmatic), which was acquired when the rocks were crystallized, reflecting magma flow linked to a magma feeder zone (magma chamber). The moderated magnetic lineation plunges in the northern part of the pluton suggest that this region was close to the feeder zone. In contrast, the southern region should be far from it, since the magnetic lineations are horizontal to sub-horizontal. The Itareru tonalite emplacement was also controlled by regional strains related to the collision of the Paleoproterozoic Rio Itapicuru greenstone Belt with the Archean basement.

Magnetic fabric data on partially molten granitic rocks from the Florianópolis Batholith, Southern Brazil

Contact thermal aureoles around mafic intrusions provide information about the emplacement and segregation of melts. The Garopaba-Silveira region is characterised by intrusions with large heat input responsible for the formation of a thermal aureole in the granitoids of the Paulo Lopes Suite (PLS), which extends along the coast to more distant locations in this region. Such melting processes are observed from meso-to microscale, along the granitoid-mafic dike interface and in areas as distant as 500 m from the mafic dikes. It is suggested that these melting processes have been aided by the breakdown of hydrated minerals, such as biotite, through dehydration-melting processes, as commonly found in migmatites and partially molten rocks. On the other hand, the contacts between the granitoids and the mafic dikes are predominantly sharp and straight, as well as, in some dikes, they are sinuous, diffuse, and present back veining, attesting to mutual intrusion. Given the complexity of these melting processes along the granitoid-mafic dike interface, this study applies magnetic susceptibility anisotropy (AMS), anisotropy of anhysteretic remanent magnetisation (AARM) and rock magnetism techniques to melting processes in a thermal aureole context. The magnetic mineralogy of the mafic dikes comprises ferrimagnetic minerals, magnetite, and Ti-poor titanomagnetite. In the granitoids, the ferrimagnetic contribution is dominant, although there are paramagnetic contributions, allowing for direct lateral variations of size of the magnetic grains at different distances (profiles), which may indicate crystallisation time as well as thermal effects in these rocks due to the mafic dike intrusions.

Restoring a syntectonic transtensional pluton by integrating structural geology, anisotropy of magnetic susceptibility, and gravimetric modelling

The kinematic analysis of magmatic structures is key evidence for the tectonic settings and deformation mechanisms related to the emplacement of intrusive bodies. The Dom Feliciano Belt (DFB), southernmost Brazil, registers voluminous post-collisional magmatism during the Neoproterozoic and the Arroio do Silva Pluton (ASP) is a diorite-monzonite-syenite body emplaced in this tectonic setting. The ASP structural investigation and magnetic fabric analysis showed the coexistence of flat-lying and steeply-dipping planar structures in most rock types. Heterogeneous shearing along the steeply-dipping planes led to heterogeneities in the fabric, providing individual analyses of zones affected and unaffected by shearing. The magnetic fabric analysis corroborated the interpretation of dextral kinematics based on field structural data. Complementary gravimetric modelling of the ASP demonstrates its continuity in the subsurface along an NNE-SSW axis. The integration of structural and magnetic fabric analyses combined with gravimetric modelling indicates that the older ASP rock types register a syntectonic emplacement within a transtensional setting, whereas younger magmatic activity in the pluton occurred under extensional conditions. The integrative approach allowed a comprehensive characterisation of the magmatic structures developed under different tectonic stresses and their association with emplacement conditions for the pluton.