Exploring the emplacement of Cretaceous arc granites and dextral shear zones in northwestern Nevada, USA, using the StraboSpot data system

ABSTRACT This field trip traverses the Sahwave and Nightingale Ranges in central Nevada, USA, and northward to Gerlach, Nevada, to the Granite, northern Fox, and Selenite Ranges. Plutonic bodies in this area include the ca. 93–89 Ma Sahwave nested intrusive suite of the Sahwave and Nightingale Ranges, the ca. 106 Ma Power Line intrusive complex of the Nightingale Range, the ca. 96 Ma plutons in the Selenite Range, and the ca. 105–102 Ma plutons of the Granite and Fox Ranges. Collectively these plutons occupy nearly 1000 km2 of bedrock exposure. Plutons of the Sahwave, Nightingale, and Selenite Ranges intrude autochthonous rocks east of the western Nevada shear zone, while plutons of the Granite and Fox Ranges intrude displaced terranes west of the western Nevada shear zone. Integrated structural, geochemical, and geochronological studies are used to better understand magmatic and deformation processes during the Early Cretaceous, correlations with Cretaceous plutons in adjacent areas of Idaho and California, and regional implications. Field-trip stops in the Sahwave and Nightingale Ranges will focus on: (1) microstructure and orientation of magmatic and solid-state fabrics of the incrementally emplaced granodiorites-granites of the Sahwave intrusive suite; and (2) newly identified dextral shear zones hosted within intrusions of both the Sahwave and Nightingale Ranges. The Sahwave intrusive suite exhibits moderate to weak magnetic fabrics determined using anisotropy of magnetic susceptibility, with magnetic foliations that strike NW-NE and magnetic lineations that plunge moderately to steeply. Microstructural analysis indicates that these fabrics formed during magmatic flow. The older Power Line intrusive complex in the Nightingale Range is cross-cut by the Sahwave suite and contains a N-S–trending solid-state foliation that reflects ductile dextral shearing. Field-trip stops in the plutons of the Gerlach region will focus on composition, texture, and emplacement ages, and key differences with the younger Sahwave suite, including lack of evidence for zoning and solid-state fabrics. The field trip will utilize StraboSpot, a digital data system for field-based geology that allows participants to investigate the relevant data projects in the study areas.

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.

Paleowind directions from the magnetic fabric of loess deposits in the western Chinese Loess Plateau and implications for dust provenance

The aeolian loess-paleosol sequences in the Chinese Loess Plateau (CLP) are an excellent archive of variations in atmospheric circulation in the geological past. However, there is no consensus regarding the roles of the East Asian winter monsoon and westerly winds in transporting the dust responsible for loess deposition during glacial and interstadial periods. We conducted detailed measurements of the anisotropy of magnetic susceptibility (AMS) on two parallel loess profiles covering the most recent 130 ka in the western CLP to determine paleowind directions. Results show that the magnetic lineations of the loess and paleosol units in both sections are significantly clustered along the northwest to southeast direction. These observations demonstrate that the prevailing wind system responsible for dust transport in the western CLP was the northwesterly winter monsoon, rather than the westerly winds. The AMS-derived dust-bearing wind direction was relatively stable during the last glacial and interglacial cycle in the western CLP, consistent with sedimentary and AMS evidence from the eastern CLP. Accordingly, it is reasonable to conclude that large areas of deserts and Gobi deserts areas located in the upwind direction were the dominant sources for the aeolian deposits of the Loess Plateau.

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.

Emplacement mechanism of Late Triassic granitic Dushan pluton, North China and its tectonic implications

<p>To better understand the Late Triassic tectonic setting in the northern North China Craton (NCC), a multidisciplinary investigation, including structural geology, geochronology, anisotropy of magnetic susceptibility (AMS) and gravity modeling, has been carried out in the Dushan pluton. The Dushan pluton consists of monzogranite and biotite-rich facies along the pluton margin without sharp contact between them. The granite varies southwestwards from isotropic texture to arcuate gneissic structures, with locally mylonitic structures. The intensity of solid-state deformation increases southwestwards across the pluton, leaving preserved magmatic fabrics in the northeastern part. The compatible outward dipping magmatic and solid-state magnetic fabrics, together with mesoscopic fabrics, define an elliptic dome-like pattern with a NE-SW oriented long axis, despite the fabrics dip inwards in the southeastern margin of the pluton. Combining gravity modeling, the Dushan pluton presents an overall tabular or tongue-like shape with a northeastern root. The magnetic lineations nearly strike NE-SW, concordant with the stretching lineations observed in the mylonitic zones. We propose the emplacement mode that the Dushan pluton emplaced southwards through the feeder zone in its northeast, beginning probably with a sill. The later successive magma batches may laterally and upwardly inflate, deform and even recrystallize the former cool-down magma. This inflation forms an arcuate, gneissic to mylonitic foliation in the southwestern margin. The Dushan pluton is considered as typically post-tectonic in emplacement, recording a Late Triassic post-tectonic setting of the northern NCC.</p>

Emplacement of Jurassic dolerite sills in Tasmania: Implications for Australia-Antarctica connections and Gondwana breakup

<p>The ca. 182 Ma Jurassic dolerite sills of Tasmania, SE Australia, overlap in age with dolerite sills and basaltic lavas in the Ferrar province, Antarctica, and the Karoo, South Africa. Hence, the Tasmanian dolerites have long been considered to be part of a major Large Igneous Province that extended parallel to the Jurassic margin of Gondwana from what is now southern Africa, the Transantarctic Mountains, to Tasmania and South Australia. Two hypotheses have been proposed for the Ferrar and Tasmanian dolerites. 1) They are related to a mantle plume emplaced in the present-day Wedell Sea region, implying long-range, shallow-crustal transport of magmas in sills and dykes over distances of up to 4,000 km. 2) They are sourced from the mantle below Tasmania and Antarctica, implying only short-range lateral transport at the level of emplacement. We report results from a combined structural and anisotropy of magnetic susceptibility (AMS) study of the Tasmanian dolerites conducted to evaluate these hypotheses by differentiating between flow patterns and structural architectures in sills that are indicative of local versus distal sources.</p><p>Detailed structural mapping and 3D modelling indicate that no more than a few individual large sub-horizontal dolerite sheets were emplaced parallel to bedding in Permian sedimentary host rocks. They are offset by map and outcrop scale steps that we interpret to be NW-SE-trending, steeply dipping broken bridges.</p><p>The AMS of dolerite was measured in oriented samples collected from 126 sites across Tasmania. Their mean bulk magnetic susceptibility is ~0.01 SI units, which together with high-temperature susceptibility measurements indicate that the AMS is carried by magnetite, which occurs as skeletal grains with morphologies controlled by the petrofabric of plagioclase and pyroxene. These observations, and scant microstructural evidence for solid-state deformation, indicate that the AMS records a magmatic fabric that formed during emplacement and crystallization of the dolerite sheets. Magnetic lineations are dominantly subhorizontal, trending mostly NW-SE. Steeply-moderately inclined magnetic lineations are rare and mostly plunge SE. Subsets of shallow N-S and NE-SW lineations are associated with sites with subvertical E-W and NW-SE striking magnetic foliations. Magnetic foliations are dominantly subhorizontal, parallel to bedding in the surrounding sedimentary rocks, and the upper and lower contacts of subhorizontal dolerite sheets. Anomalous subvertical E-W and NW-SE striking magnetic foliations are associated with steps or broken bridges observed in the field and cross sections.</p><p>The AMS results are consistent with dominantly NW-SE magma flow within subhorizontal sheets, which is supported by the NW-SE orientation of steps and broken bridges. The architecture of segmented sheet fronts indicates that the polarity of sill propagation was from SE to NW. This finding is inconsistent with a magma source immediately below Tasmania and implies lateral transport from another location. However, the magma flow vector does not point back to the Ferrar dolerites in Antarctica, and therefore does not support the long-range Ferrar-Tasmania LIP hypothesis. Rather fabrics in the Tasmania dolerite are consistent with lateral flow from the present SE, perpendicular to the Gondwana margin with a source in the back-arc of the associated subduction zone</p>

Magnetic fabrics in Portuguese Variscan granites: structural markers of the Variscan orogeny

<p>This work focuses on the magnetic fabric of 20 variscan granitic massifs from northern and central Portugal and considers the Anisotropy of Magnetic Susceptibility (AMS) results obtained in about 750 sampling sites. In the northern and central Portugal, three main ductile deformation phases were recognized and described: D<sub>1</sub>, D<sub>2</sub> and D<sub>3</sub>, being the variscan magmatism events mainly related to D<sub>3</sub> phase. D<sub>3</sub> produced wide amplitude folds with NW-SE subhorizontal axial plane and subvertical dextral and sinistral ductile shear zones, forming obtuse angles with the maximum compression direction, σ1, NE-SW oriented. The post-D<sub>3</sub> brittle phase was responsible for the development of conjugate faults (NNW-SSE, NNE-SSW and ENE-WSW), related to a N-S maximum compression. The studied granites were subdivided according to U-Pb dating, field observations and considering the chronology of their emplacement relative to the D<sub>3</sub> phase of Variscan orogeny. Therefore, the studied granites are subdivided into: (1) syn-D<sub>3</sub> two-mica granites, ca. 311 Ma; (2) late-D<sub>3</sub> monzogranites, biotite-rich and two-mica granites, ca. 300 Ma; (3) post-D<sub>3</sub> monzogranites and biotite-rich granites, ca. 299 – 297 Ma. Magnetic fabric gives two types of directional data, magnetic foliations and magnetic lineations, which provide important information regarding the orientation of the magmatic flow, feeder zone location, relationship between the magma emplacement and tectonics and, also, the stress field. The data obtained for the magnetic fabric, based on AMS technique, allowed concluding: (i) syn-D<sub>3</sub> granites show magnetic foliations and lineations consistent with the syn-D<sub>3</sub> variscan structures ca. N110°-120°E, related to a NE-SW maximum stress field . The foliations are, mainly, subvertical (> 60º), which may indicate a high thickness of the granitic body and deep rooting; on the other hand, the magnetic lineations exhibit variables plunges. (ii) Late-D<sub>3</sub> granites are characterized by foliations and lineations, dominantly NNW-SSE to NNE-SSW oriented. The foliations are subvertical dips (> 60º) and the lineations have, generally, soft plunges. (iii) Post-D<sub>3</sub> granites have, in general, magnetic foliations and lineations associated with important regional post-D<sub>3</sub> brittle structures, which display NNE-SSW and ENE-WSW trending. The subhorizontal fabric may suggest a small thickness of the granitic bodies. In all granite sets under study there is a dominance of weakly dipping lineations (slope <60º), indicating that the feeding zones are deep, which supports the idea of an emplacement at high structural levels.</p><p>Acknowledgments: The authors thank Department of Geosciences, Environment and Spatial Planning at Faculty of Sciences of the University of Porto and the Earth Sciences Institute (Porto Pole, Project COMPETE 2020 (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690).</p>

Magnetic fabric in brittle faults and ductile shear-zones: Examples from cataclasites from the Iberian Peninsula

<p>Shear zones, or their counterparts in near-surface conditions, the brittle fault zones, constitute crustal-scale, narrow, planar domains where deformation is strongly localized. The variation with depth of deformation conditions (P-T), rheology and strain rates entails a wide range of fault rock types, characterized by different petrofabrics and classically grouped into mylonitic (fault rocks undergoing crystalline plasticity) and cataclasitic (fault rocks undergoing frictional deformation) series. Magnetic fabric methods (most frequently anisotropy of magnetic susceptibility, AMS) have been established as a useful tool to determine fault rock petrofabrics in shear/fault zones, being interpreted as kinematic indicators with a considerable degree of success. However, mylonites and cataclasites show remarkable differences in magnetic carriers, shape and orientation of the fabric ellipsoid. Here, we present a study of ten brittle fault zones (one of them at the plastic-brittle transition) located in various locations in the Iberian Plate, with an aim  to interpret patterns of AMS in cataclasites.</p><p>Reviewing AMS studies dealing with SC mylonites, three fundamental features can be drawn: i) the presence of composite magnetic fabrics with shape and lattice-preferred orientations, ii) the fabric is carried predominately by ferromagnetic minerals and iii) surprisingly in composite fabrics, the absolute predominance of magnetic lineations parallel to (shear) transport direction (88% of the reviewed sites), independently of fabrics being defined by paramagnetic or ferromagnetic carriers. Based on our study, magnetic fabrics in cataclasites: i) are mainly carried by paramagnetic minerals and ii) show a strong variability in magnetic lineation orientations, which in relation with SC deformational structures, are either parallel to transport direction (44% of sites) or parallel to the intersection lineation between shear (C) and foliation (S) planes (41%). Furthermore, changes between the two end-members can be frequently observed in the same fault zone. Sub-fabric determinations (LT-AMS; AIRM and AARM) also indicate that the type of magnetic lineation cannot be consistently related with a specific mineralogy (i.e. paramagnetic vs ferromagnetic minerals).</p><p>The wide range of deformation conditions and fault rocks covered in our study allowed us to analyse the factors that control these different magnetic lineation orientations, especially in brittle contexts. Plastic deformation results into a mineral stretching parallel to transport direction which can be directly correlated with the development of transport-parallel magnetic lineation. In brittle fault zones, the degree of shear deformation can be directly correlated with the type of magnetic lineation. The fault cores, where strain and slip are localized, show a predominance of transport-parallel magnetic lineations, most probably related with the development of lineated petrofabrics. Furthermore, the minor development of shear-related petrofabrics enhance the frequency of intersection-parallel magnetic lineations, also contributing the presence of inherited, host rock petrofabrics in the fault rocks.</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>

Ballooning emplacement and alteration of the Chah-Musa subvolcanic intrusion (NE Iran) inferred from magnetic susceptibility and fabric

The porphyritic diorite Chah-Musa subvolcanic intrusion is located in the Toroud-Chah Shirin magmatic arc in the northern Central Iranian structural zone. The elliptical Chah-Musa body hosts a copper deposit and intrudes an Eocene sequence of volcanic breccia, agglomerate and red tuffaceous sediment. High magnetic susceptibility values are attributed to the presence of magnetite as a magnetic carrier. Changes in bulk magnetic susceptibility correlate with zonation of alteration in the intrusion. Although the degree of anisotropy of magnetic susceptibility decreases due to hydrothermal alteration, the field observations confirm that this parameter can be used as a strain marker. Strongly oblate magnetic ellipsoids are found in the eastern half of the intrusion where isolated outcrops of flat-lying tuffaceous host cover dioritic rocks (roof zone). Stations with prolate ellipsoids mostly belong to the centre of the intrusion where the magnetic lineations plunge steeply. They are interpreted as indicating the main feeder zone. The concentric fabric pattern at the periphery of intrusion, the oblate magnetic ellipsoids at the roof, the highest anisotropy degree along the small diameter of the intrusion, and an intense deformation of the host rocks, especially at the western margin, all are evidence that the intrusion was ballooning during the late stages of its emplacement. Ascent and emplacement of the Chah-Musa body is ascribed to the tensional space provided by a dextral shear zone created by the regional left-lateral movement on the bounding Anjilow and Toroud strike-slip faults.