Fabric Analysis in Upper Crustal Post-Collisional Granitoids from the Serre Batholith (Southern Italy): Results from Microstructural and AMS Investigations

The Serre Batholith in Central Calabria (southern Italy) represents the intermediate portion of a continuous cross-section of late Variscan continental crust. The various granitoid units of the batholith were emplaced at depths between 23 and 6 km through an overaccretion mechanism that, at its upper levels, was marked by the emplacement of two-mica granodiorites and granites (MBG) at c. 295 Ma, followed by weakly peraluminous granodiorites (BAG) at c. 292 Ma. These upper crustal granitoid rocks have recorded tectonic stresses, which affected the batholith during cooling of the magmatic bodies, exhibiting a range of deformation microstructures from submagmatic to low-temperature subsolidus conditions, but without developing an evident meso/micro-structural fabric. Anisotropy of magnetic susceptibility (AMS) was employed to identify a possible “internal” fabric of the Serre upper crustal granitoids, revealing a magnetic foliation represented by a mainly oblate AMS ellipsoid. Magnetic foliations and lineations are consistent with a stress field characterized by a shortening axis roughly oriented NW–SE. Further studies are in progress to investigate more in depth the relationships between regional tectonic structures and the emplacement of the late-Variscan Serre Batholith granitoids.

Magmatic stock emplacement and its constraints on the localization of related skarn orebodies: an example from the Tongguanshan stock, Tongling district, eastern China

Study of constraints of stock emplacement and geometry on associated skarn orebodies is significant for the understanding of the epithermal deposit system. We have chosen the typical Tongguanshan skarn ore deposit (eastern China) as our target area. The Tongguanshan stock was emplaced at the NE–SW-striking Tongguanshan anticline and is characterized by macroscopically homogeneous quartz–monzodiorite. The magnetic parameters show that the stock is dominated by oblate magnetic ellipsoids and a high degree of anisotropy (> 1.1), and this value is higher at the stock margin. The strike of magnetic foliation at the stock margin is parallel to the stock boundary with sub-horizontal magnetic lineations. A vertical NE–SW-striking magnetic foliation, which is parallel to the regional structures, is revealed inside the stock. The three-dimensional geometric modelling shows that the stock has a tongue-like geometry and the contact surface in both eastern and western sides dips to the NW, but the western side is steeper. Nevertheless, the orebodies are almost developed at the eastern side. Accordingly, we propose that the Tongguanshan stock was constructed by multiple magma pulses, initiated at the SW part of the stock, and ascended along inherited NE–SW extended fractures in the Tongguanshan anticline. The successive magma pulses either accreted by a unilateral E-wards trend or by bilateral magma accretion, which resulted in a deformation difference in the contact zone and caused uneven orebody development. Our study also shows that the strike, dip angle and curvature situation of contact surface, which affects the water–rock reaction process and distribution of the dilation zone, are important ore-controlling factors.

Magnetic fabric of the Parashi stock and related dyke swarm, Alta Guajira (Colombia): The Caribbean-South American plates oblique convergence

Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) were used to evaluate the emplacement history of the Parashi stock and related dyke swarm situated in NW Colombia. The average magnetic susceptibility of 4.5×10-2 SI, in conjunction with low-coercivity components provided by the isothermal remanence and thermomagnetic curves with net Verwey and Curie transitions, indicates that multidomain magnetite records the anisotropy directions. The similar orientation and shape of the AMS and AAR ellipsoids indicate the absence of very fine magnetite with an inverse fabric. The magnetic foliation is the best-defined fabric element in these rocks and outlines a concentric structure, elongated parallel to the NE-SW direction of the pluton. Crystallisation age of the stock and dykes (51-47 Ma), along with pressure of emplacement determination indicate that the stock and the dyke swarm probably formed simultaneously, and they were emplaced in the shallow crust (

Magnetic anisotropy reveals Acadian transpressional fabrics in an Appalachian ophiolite (Thetford Mines, Canada)

SUMMARY Magnetic anisotropy has proved effective in characterizing primary, spreading-related magmatic fabrics in Mesozoic (Tethyan) ophiolites, for example in documenting lower oceanic crustal flow. The potential for preservation of primary magnetic fabrics has not been tested, however, in older Palaeozoic ophiolites, where anisotropy may record regional strain during polyphase deformation. Here, we present anisotropy of magnetic susceptibility results from the Ordovician Thetford Mines ophiolite (Canada) that experienced two major phases of post-accretion deformation, during the Taconian and Acadian orogenic events. Magnetic fabrics consistent with modal layering in gabbros are observed at one locality, suggesting that primary fabrics may survive deformation locally in low strain zones. However, at remaining sites rocks with different magmatic origins have consistent magnetic fabrics, reflecting structurally controlled shape preferred orientations of iron-rich phases. Subhorizontal NW-SE-oriented minimum principal susceptibility axes correlate with poles to cleavage observed in overlying post-obduction, pre-Acadian sedimentary formations, indicating that the magnetic foliation in the ophiolite formed during regional NW-SE Acadian shortening. Maximum principal susceptibility axes plunging steeply to the NE are orthogonal to the orientation of regional Acadian fold axes, and are consistent with subvertical tectonic stretching. This magnetic lineation is parallel to the shape preferred orientation of secondary amphibole crystals and is interpreted to reflect grain growth during Acadian dextral transpression. This structural style has been widely reported along the Appalachian orogen, but the magnetic fabric data presented here provide the first evidence for transpression recorded in an Appalachian ophiolite.

Mapping of Pyroclastic Density Currents Hazards and Assessment of Related Risks by AMS Technique in the West-Cameroon Highlands: Case of Bambouto and Bamenda Volcanoes

Ignimbritic flow deposits which derived from pyroclastic density currents (PDCs) are mostly observed in West-Cameroon Highlands located in the central portion of the Cameroon Volcanic Line (CVL), especially in Bambouto (21.12 - 0.50 Ma) and Bamenda (27.40 - 0 Ma) volcanoes. These deposits covering approximately 27% (≈195 km2) of the volcanoes surface with thickness ranging from 30 to 200 m representing a total volume estimated at 20 km3. Because of the intense weathering of the ignimbritic formations after their setting up and being buried by basaltic and trachtytic flows, the initial volume of these pyroclastic deposits is really much larger. Soil fertility has fostered an important population growth (more than 1,200,000 people) in these volcanoes. The economic and agropastoral activities on the flanks and inside the caldera of the volcanoes are estimated at about $US7.5 billion. In this paper, we evaluate and realize cartography of the hazards associated to ignimbritic eruptions which are most disastrous in term of volcanic process in this region. Magnetic studies, specifically, Anisotropy of Magnetic Susceptibility (AMS) method has been utilized for the determination of flow directions in visually nearly isotropic ignimbritic deposits outcrops. The AMS data reported from the Bamenda and Bambouto volcanoes ignimbrites produced significant informations about the depositional scheme of the PDCs. In most sites, magnetic lineations and principally magnetic foliation are reliably parallel to downhill directions, frequently with an upslope imbrication. Inferred palaeoflow directions based on the field indicators, orientation of minerals and other objects in oriented thin sections and the directional AMS data show that Bambouto caldera, Oku crater and Santa-Mbu caldera are the sources of main PDCs of Bambouto and Bamenda volcanoes. These AMS results have aided us to produce a hazard and risks maps related to potential future pyroclastic flows on these volcanoes. The assessment of risks in these volcanoes was based on populations in the study area, infrastructures (houses and roads) and average income of breeding activity.

Deformation and magnetic fabric of the Capinha granite (Fundão, Central Portugal): ascent and emplacement mechanisms during the late-Variscan crustal thinning

<p>The Capinha area is located in the Central Iberian zone and is characterized by several Variscan granites intruded in the Neoproterozoic–Cambrian metasedimentary rocks. The main goal of the study is to identify the deformation patterns and provide crucial information to investigate the evolution of the magnetic fabrics in a post-Variscan granite emplaced during the crustal thinning, at the end of the Variscan orogeny. In order to achieve these purposes, fieldwork, petrography, microstructures and anisotropy of magnetic susceptibility (AMS) analysis were undertaken. The AMS was measured in 160 oriented cores, collected from 20 sampling sites homogeneously distributed, allowing the quantification of scalar (magnetic susceptibility, K; paramagnetic anisotropy, P<sub>para</sub>; magnetic ellipsoid shape, T) and directional data (magnetic lineation, //K<sub>1</sub>; magnetic foliation, perpendicular to K<sub>3</sub>). The Capinha granite (CG), exposed over an area of about 7 km<sup>2</sup>, is a small circular circumscribed outcrop in the NE-SW contact between the regional Belmonte–Caria granite (301.1±2.2 Ma) and the metasedimentary sequences. The CG is cut by two main fracturing systems: N30º-40ºE and N110º-120ºE, both subvertical. The contact is sharp, intrusive and discordant with the general trending of the D<sub>1</sub> and D<sub>3</sub> Variscan structures registered in the metasedimentary rocks. The CG is homogeneous in the whole area and consists of a fine- to medium-grained, muscovite-biotite leucogranite. The CG exhibit a paramagnetic behaviour with a K mean of 73 µSI, belonging to the ilmenite-type granites. At several scales, the CG does not show any magmatic flow or ductile deformation patterns displaying P<sub>para</sub> of about 1.6%, which corresponds to dominant magmatic to submagmatic microstructures. The P<sub>para</sub> highest values are concentrated in the NE border associated to prolate ellipsoids (linear fabric). Based on the interpretation of the magnetic fabric, is possible to observe that the orientation of the magnetic foliation is variable ranging from NNW-SSE to NNE-SSW. Generally, the magnetic foliations are sub-horizontal, being the vertical dips observed in the NE border, near the intersection of the N100º-120ºE and the N30º-40ºE fractures. The arrangement of the magnetic foliations follow concentric trajectories, with the symmetry axe parallel to the major axis of the outcrop (roughly NNE-SSW). The magnetic lineations are mainly sub-horizontal NNE-SSW parallel to the granite major axis; although, in the SW border the lineations tend to be parallelized to the contact. The magnetic lineation arrangement develops linear trajectories converging to the NE zone, where the dip is strong. The common gently magnetic fabric suggests the roof of the CG intrusion. During the late stages of the Variscan orogeny (D<sub>3</sub>, 321-300 Ma), ductile extensional detachments promoted the thinning of a previously thickened crust, providing the opening of pre-existing structures and the production of new ones. These structures act as conduits for a passive magma ascending and emplacement at shallow levels. Therefore, it is suggested that the CG magma ascent and emplaced in the intersection of pre-existing fractures, located in the NE zone, and flowed to the SW, developing a small asymmetric laccolith, poorly eroded, with a tongue-shaped body.</p>

Magnetic fabric of Lamas de Olo Pluton: AMS and AARM fabrics comparison

<p>The Lamas de Olo Pluton (LOP) is a small outcrop located in the Northern part of Central Iberian Zone from the Iberian Variscan belt. The LOP is a post-tectonic (ca. 297.19 ± 0.73 Ma) pluton composed of different granites: Lamas de Olo (LO; medium to coarse-grained porphyritic granite, ilmenite, and magnetite-type), Alto dos Cabeços (AC; medium to fine-grained porphyritic, ilmenite-type granite), and Barragem (BA; leucocratic fine- to medium-grained, slightly porphyritic, ilmenite-type granite). The magnetic fabric was characterized by measurements of anisotropy of magnetic susceptibility (AMS), and anisotropy of anhysteretic remanent magnetization (AARM). Both techniques are based on the magnetic properties of rock minerals, but while AMS consider the contribution of all rock minerals (paramagnetic, diamagnetic and ferromagnetic s.l.), in the AARM, the fabric is exclusively given by the ferromagnetic s.l. minerals. A correlation between AMS and AMR tensor was established, in order to compare both fabrics. The magnetic lineation is K<sub>max</sub> or AARM<sub>max</sub> and the magnetic foliation is perpendicular to K<sub>min</sub> or AARM<sub>min</sub>. Considering the global magnetic fabric for all samples from all the granite set, the magnetic foliations (AMS: N166°, 82°NE; AARM: N167°, 83°NE) and the magnetic lineations (AMS: 23°- N166°; AARM: 68°- N163°) are coaxial in both tensors. On the other hand, the analysis of each site sampling shows some differences in the ilmenite-type granites. Magnetic lineations and foliations given by both tensors (AMS and AARM) are coaxial in the magnetite-type granites, meaning that the magnetite and paramagnetic (or diamagnetic) minerals have the same orientation. The coaxial AMS and AARM magnetic foliations are due to magnetite grains imitating the fabrics of paramagnetic phases, through preferred collage, or crystallization of magnetite along grain boundaries, or exsolutions of magnetite along biotite cleavage planes. However, in the ilmenite-type granites, the AMS and AARM foliations are parallel, but the AMS and AARM lineations are not coaxial. Previous magnetic mineralogy studies (e.g. thermomagnetic experiments and isothermal remanent magnetization) pointed out the presence of magnetite/Ti-poor magnetite in all LOP granites, even in the ilmenite-type, but in different proportions. The petrographic observations also showed that, in the ilmenite-type granites, the magnetite is often oxidized to hematite (martite). The presence of martite may justify non-coaxility linear fabrics. Regarding the LOP emplacement, WSW-ENE opening structures provided the space for magma ascending, with an NNW-SSE magmatic flow controlled by regional structures, as shown by the magnetic foliations and lineations ca. N170º trending. The absence of outcrop deformation and the lack of solid-state microstructures precludes the substantial deformation after full crystallization of LOP.</p>

Simultaneous Free Flow and Forcefully Driven Movement of Magma in Lamprophyre Dykes as Indicated by Magnetic Anisotropy: Case Study from the Central Bohemian Dyke Swarm, Czech Republic

A composite lamprophyre dyke from the Central Bohemian Dyke Swarm (Czech Republic) shows both indications of magma free flow (normal magnetic fabric with magnetic foliation and lineation parallel to the dyke plane) as well as those of forcefully driven magma movement (intermediate and inverse magnetic fabrics with magnetic foliation perpendicular to the dyke plane). The overall characteristics of the magnetic parameters across the dyke indicate the existence of at least two slightly differing parts that probably represent two magma pulses. The marginal part of the dyke is formed by kersantite, while toward the axial part, the composition gradually changes to spessartite, and obtains an increasing degree of amphibolization. The rocks of the dyke are inhomogeneous, both compositionally and structurally. It is likely that some portions of ascending magma were more viscous than the others, and the magnetic minerals in the more viscous magma portions may have oriented according to their longer dimensions perpendicular to the dyke, creating an inverse fabric. The lengthening perpendicular to the dyke was compensated by the vertical escape of neighboring more fluid magma, creating a normal magnetic fabric. The frequent oblique magnetic fabrics may represent transitions between the above two mechanisms.

ANIZOTROPIE MAGNETICKÉ SUSCEPTIBILITY HORNIN NA KONTAKTU METABAZITOVÉ A DIORITOVÉ ZÓNY BRNĚNSKÉHO MASIVU V OKOLÍ VELKÉ BABY U JINAČOVIC

An anisotropy of magnetic susceptibility and temperature dependence on magnetic susceptibility were used to reveal an evolution history along the contact of the Metabasite and Diorite zones of Brno massif north of Brno-Řečkovice. The analysis of temperature dependence of magnetic susceptibility indicated that magnetic properties of all rocks in this area are essentially controlled by magnetite with a very small contribution of pyrhotite and hematite. These minerals were formed later than the primary magmatic minerals. Therefore we assume that magnetic fabrics in studied rocks reflect deformational processes which affected these rocks. There are three patterns in anisotropy of magnetic susceptibility (AMS) in studied rocks. In the first pattern detected in diorites, the magnetic foliation is striking NE–SW, dipping to the NW and there is subvertical magnetic lineation. The second planar magnetic indicates a rotational movement of microgranite rocks along the contact of the Metabasite and Diorite zones. The last pattern found in rocks of the Metabasite zone is magnetic foliation striking NNE–SSW dipping on the NWW and magnetic lineation trending to the SW with plunge of 42° and it shows normal faulting of studied area.