Emplacement Mechanisms Research Articles

Geospatial Insights into Ophiolitic Complexes in the Cimmerian Realm of the Afghan Central Block (Middle Afghanistan)

Ophiolites are remnants of oceanic crust that have been thrust onto continental crust due to tectonic processes. They are composed of mostly mafic and ultramafic rocks, which are genetically associated with gold, silver, platinum group element (PGE), chrome, manganese, titanium, cobalt, copper, and nickel deposits. The main objective of this research was to identify the spatial distribution of Mesozoic ophiolitic complexes within the Central Afghan Block in Middle Afghanistan using optical remote sensing data and spectral analyses. Distinct algorithms, such as false color composite (FCC), proposed band ratios (PBR), principal component analysis (PCA), and spectral angle mapper (SAM), were used to map the targeted ophiolitic complexes. New band ratios were proposed in this study based on the spectral properties of mafic-ultramafic minerals and rocks, which showed high efficiency. Based on the results, four different ophiolitic complexes were delineated within this study area. These complexes are consistent with previous studies. The accuracy assessment of this study showed an overall accuracy of 72.2%. The findings of this study can significantly contribute to further studies on the emplacement mechanism and paleo-Tethys history of Middle Afghanistan. Also, the spatial distribution of the ophiolitic complexes identified in this study can be used to constrain models of the tectonic evolution of the Central Afghan Block. Additionally, the identification of new band ratios for mapping ophiolitic complexes can be used in future studies of other ophiolite-bearing regions.

The effect of fault systems on volcanic activity: Insights from the subduction-related, Quaternary Villamaría-Termales monogenetic volcanic field in Colombia

Structural control of volcanic activity is a topic of great significance around volcanic fields, mostly associated with risk management near cities. Structural geology provides valuable information for understanding the magmatic processes associated with the rise of magma and the presence of volcanoes. The Villamaría-Termales Monogenetic Volcanic Field (VTMVF) is a group of Quaternary monogenetic volcanoes settled in an active fault system, which allows us to propose the role that these faults play in the volcanic activity. The field is located near the city of Manizales (∼475.000 inhabitants), Colombia, on an active fault system at the western flank of the Cordillera Central in the northern Andes. The VTMVF is characterized by andesitic lava domes and flows, which individually reach volumes of up to 2.5 km3. This work uses 1) geometry and kinematics of the faults that affect the VTMVF to define the structural features associated with the emplacement of the volcanoes, and 2) new and published ages to evaluate periods of volcanic activity. Three trends of fault systems associated with the location of the volcanoes were identified: 1) NW-SE (Villamaría-Termales), 2) NE-SW (Sancancio, El Perrillo, Santa Rosa and Palestina), and 3) N-S to NNE-SSW (San Jerónimo and Samaná-sur). The fault kinematics are dominantly normal, with dextral (NE) and sinistral (NW) movements. Analysis of ages shows that the field has temporally evolved since 2 Ma in different time intervals (2.0–1.2 Ma, 0.18–0.045 Ma, and 0.045 Ma – Present). This supports the presence of different emplacement mechanisms related to both the movement in the strike of some faults and the intersection between faults. The pauses of volcanic activity could coincide with the absence of structural conditions (interseismic cycles for faults, where the sealing of the faults is generated) to facilitate the rise of magma; this is related to temporary compressive conditions. Despite the apparent structural controls, there is no clear spatial or temporal pattern of volcano distribution within the field. There is evidence of seismic activity in the area. Thus, the future formation of dilatating zones that allow the magma to rise and the emplacement of new monogenetic volcanoes is feasible not only in the area defined for the volcanic field but also in the continuation of the fault lines around it.

Rheological context of emplacement of the Dori, Gorom-Gorom and Touka Bayèl granitic plutons (northeast Burkina Faso, West African Craton)

The reconstitution of the geodynamic processes of the Eburnean orogeny in the time span around 2.1 Ga in Dori area is here carried out through the examination of the internal structures and microstructures of the Dori, Gorom-Gorom and Touka Bayèl granitic plutons. The radiometric ages of these plutons vary between 2164 Ma and 2148 Ma. The internal structures have been evidenced using the Anisotropy of Magnetic Susceptibility (AMS) method and the examination of microstructures through thin sections under optical microscope.The organization of the internal fabrics strongly suggests a diapiric emplacement for the Dori and the Touka Bayèl plutons in a still soften crust. The pluton of Gorom-Gorom seems to be constituted of four small plutons. Three of them, coalescent in the southern part, are emplaced by the diapirism and the fourth in the northern part can be interpreted as emplaced in a tension gash.The overall disposition of the fabrics lets suggest a synchronous emplacement in accordance with the dextral sense of the Tiébélé-Dori-Markoye (TDM) shear zone in a regional E-W shortening context responsible of a crustal thickening.The two emplacement mechanisms (diapirism and tension gash) identified through the internal fabrics testify a rheological contrast of the crust during this early period of the Eburnean Orogeny.

High-Precision Geochronology of LIP Intrusions: Records of Magma–Sediment Interaction

Reconstructing the tempo and emplacement mechanisms of large igneous provinces (LIPs) and establishing potential links to environmental change and biological crises requires detailed and targeted high-precision geochronology. Contact metamorphism during LIP intrusive magmatism can release large volumes of thermogenic gas, so determining the timing of these events relative to global climate change is crucial. The most reliable age information comes from U-Pb geochronology; however, LIP mafic igneous rocks do not commonly crystallize U-bearing minerals, such as zircon or baddeleyite. Recent work has shown that U-rich minerals can crystallize in fractionated melt pockets in intrusive components of LIPs after contamination of the melt by sedimentary rocks at emplacement level. Zircon and baddeleyite from these pockets make high-precision U-Pb geochronology of LIPs possible, but these unique mechanisms add other complexities.

American Psychiatry and the Interminable Exigence of Blackpain

This essay analyzes the American Psychiatric Association’s (APA’s) responses to George Floyd’s death to critically illuminate the ways that reconciliatory discourses of racial healing deploy blackpain, or representations of black suffering, as a mechanism of emplacement, or temporal orientation. I argue that for American psychiatry, blackpain functions as an interminable exigence that ceaselessly reorients and revivifies its biopolitical project of identifying, measuring, and understanding black bodies. Additionally, I examine how these racialized temporal rhetorics aim to constitute the American Psychiatric Association as a legitimate, progressive public health organization in the face of renewed attention to its historical racism.

The Sea Point contact, Cape Town, South Africa: a geological site made famous by Charles Darwin's visit

Abstract The Sea Point intrusive contact on the Atlantic coast, South Africa, is a site of historical significance that continues to be useful for studying the details of granite–wall rock relationships, some of which have not been properly evaluated yet. At a time when the origin of granites was still controversial, and the concept of contact metamorphism not yet properly developed, Charles Darwin visited the Sea Point contact to make some geologically important observations. He concluded that the granite originated as a magma deep in the Earth, intruded a country rock of sedimentary origin and changed this country rock's appearance owing to the thermal action of the hot granite magma. He also contemplated the long periods of time required to expose the rocks by erosion. Besides its historic value, the Sea Point heritage site exposes unique features giving rare insights into the physical and chemical interaction between intruding granitic magma and a regionally low-grade metamorphosed sedimentary sequence. Questions regarding mechanisms of magmatic emplacement may be answered from this outcrop, including emplacement dynamics and interrelationships of various granitic phases, pre- versus syn-intrusive deformation and metamorphism, assimilation of country rock by a granitic magma and the origin of feldspar phenocrysts seemingly ‘stranded’ in the country rock.

An overview of sedimentary volcanism on Mars

Abstract. Extensive fields of sub-kilometre- to kilometre-scale mounds, cones, domes, shields, and flow-like edifices cover large parts of the martian lowlands. These features have been compared to structures on Earth produced by sedimentary volcanism – a process that involves subsurface sediment/fluid mobilisation and commonly releases methane to the atmosphere. It was proposed that such processes might help to explain the presence of methane in the martian atmosphere and may also have produced habitable, subsurface settings of potential astrobiological relevance. However, it remains unclear if sedimentary volcanism on Earth and Mars share genetic similarities and hence if methane or other gases were released on Mars during this process. The aim of this review is to summarise the current knowledge about mud-volcano-like structures on Mars, address the critical aspects of this process, identify key open questions, and point to areas where further research is needed to understand this phenomenon and its importance for the Red Planet's geological evolution. We show here that after several decades of exploration, the amount of evidence supporting martian sedimentary volcanism has increased significantly, but as the critical ground truth is still lacking, alternative explanations cannot be ruled out. We also highlight that the lower gravity and temperatures on Mars compared to Earth control the dynamics of clastic eruptions and surface emplacement mechanisms and the resulting morphologies of erupted material. This implies that shapes and triggering mechanisms of mud-volcano-like structures may be different from those observed on Earth. Therefore, comparative studies should be done with caution. To provide a better understanding of the significance of these abundant features on Mars, we argue for follow-up studies targeting putative sedimentary volcanic features identified on the planet's surface and, if possible, for in situ investigations by landed missions such as that by the Zhurong rover.

Granitoid characteristics of basement to the U-mineralized Bhima basin: Implications for crustal epizone processes

The Dharwar Craton (DC) is an archive containing a good proportion of ancient records of crustal evolution in its rocks. The greenstone-gneiss-migmatite and granitoid complex show significant variation in terms of time, space and mechanism of emplacement according to tectonics and geodynamic set up. The plutons of the northernmost part of Eastern Dharwar Craton (EDC) are especially recognizable from typical litho-structural attributes. The intensity of regional/burial metamorphism is negligible in these granitoids with versatile nature. Simple and composite plutons are demarcated on the basis of field association with distinguishable petro-mineralogy and geochemical characters. K-metasomatism in pluton is established from petrography with supportive geochemistry. Simple pluton is syenitic in nature unlike the composite plutons with lithological variation from Tonalite-Trondhjemite-Granodiorite (TTG) to alkali feldspar granite. The granitoids are developed from crustal melting during the plate convergence. Post-orogenic nature of syenite is indicated from geochemical data. The sharp contact between the plutons and country rocks indicates significant temperature difference between country rocks and emplaced magma. The characteristic features like brittle deformation dominated structures, sharp contact between emplaced plutons and country rocks, very low grade of regional metamorphism, intensely differentiated nature and absence of diatexitic migmatite indicate a shallow crustal depth of plutonic emplacement. Higher concentration of large ion incompatible elements (e.g., U, Th, K) and associated high heat production imply shallow crustal segment with rocks derived from evolved differentiated magma. The study indicates a depth less than 8 km and temperature condition less than 250°C which correspond to epizone, where uranium remobilization is facilitated.

New insights from an emplacement model for the Santa Eulália Plutonic Complex (SW Iberian Peninsula)

The Santa Eulália Plutonic Complex is formed by two main granites: G0 and G1. The G0 granite hosts metasedimentary carbonate and pelitic rocks (roof pendants) and elongated masses of mafic–intermediate rocks (the M-group). The host rocks form a diverse sequence of igneous, metasedimentary and metamorphic rocks. The study area is constrained by Variscan structures formed in a transpressional/transtensional sinistral tectonic regime (the Tomar–Badajoz–Córdoba sinistral shear zone) and is cut by the Alter do Chão and Assumar faults. The geological complexity of the area makes it difficult to determine the emplacement mechanism of the pluton. We propose a model for the ascent and emplacement of the pluton that can also be applied to similar post-collision Variscan granites. The gravity anomalies suggest that the pluton is slightly asymmetrical and extends to the SSE beneath the host rocks; its main root is >8 km thick in its deepest areas. The available radiometric data for an extended area embracing several regional plutons suggest a west–east magmatic alignment. The Variscan structures probably formed efficient crustal discontinuities enabling the generation and ascent of magma. Our model involves a west–east magmatic axis for magma spreading along extensional fractures (T-fractures) related to the Tomar–Badajoz–Córdoba shear zone. The opening movement along these fractures created divergent forces that allowed the ascent and emplacement of the plutonic rocks. The importance of these fractures is represented well by outcrops of porphyritic biotite granites (the Ervedal, Fronteira and G1 granites).

The ‘ gigantic boulders ’ of the Torino Hill (NW Italy): geohistorical significance and geoheritage value

Abstract Large boulders, up to several metres in diameter and mainly made up of metamorphic rocks, are scattered throughout the Torino Hill in NW Italy. These boulders were originally included in Oligocene–Miocene conglomerate beds and were released by alteration of the host rock at the surface. In the past two centuries, their origin and mechanisms of emplacement in the conglomerate bodies were largely debated by both Italian and foreign geologists. Some of the boulders had been given proper names by the local people and had become the object of legends and rituals. Moreover, in the past, the boulders had an economic value, being used as construction stones or exploited to produce aggregates, lime and millstones. This, together with the progressive urbanization of the area, caused the disappearance of many of the large boulders once present on the Torino Hill. The remaining ones are presently not appreciated and are almost forgotten, even though they represent an invaluable geoheritage and deserve rediscovery and protection.

On the Emplacement of the Impact Melt at the Dhala Impact Structure, India

AbstractWe investigate the magnetic fabrics and magnetic mineralogy of the impact melt rock at the Dhala impact structure to understand its emplacement mechanism. Pseudo‐single domains of Ti‐poor magnetite and Ti‐hematite are the prime magnetic carriers in the impact melt rock. The magnetic foliations show a range of dip amounts. The overall trend of the magnetic foliation of the impact melt rock draws a resemblance with the flood basalts or lava flows. A well‐developed magnetic lineation (K1) indicates the strong alignment of Ti‐poor magnetite grains. Therefore, the magnetic carriers may have crystallized and aligned themselves along the flow direction before the emplacement. It may be possible that after the crystallization of the magnetic carriers, the impact melt moved in a semi‐molten state similar to lava flows with temperatures below c. 1,500°C, which is the melting point of Ti‐magnetite and was emplaced as crater‐fill deposits. Among the three principal magnetic susceptibility axes, K1 aligns best with the mesoscopic flow indicators. K1 of individual specimens' trends between NW and SW, while the mean K1 at all the sites trends westward. Thus, at the studied sites, the impact melt flow was dominantly eastward. In the sites located to the NW and W, the eastward flow could be due to gravity‐driven crater inward flow toward the center. At site IM2, which is located to E, the eastward flow of the impact melt is possibly due to crater outward excavation flow.

Thermobarometric and structural constraints on multistage emplacement mechanism of the Garagheh pluton, Sistan Suture Zone (SE Iran)

Thermobarometric and structural constraints on multistage emplacement mechanism of the Garagheh pluton, Sistan Suture Zone (SE Iran)

Physical and chemical characteristics of active sulfur flows observed at Lastarria volcano (northern Chile) in January 2019

Molten sulfur is found in various subaerial volcanoes. However, limited records of the pools and flows of molten sulfur have been reported: therefore, questions remain regarding the physicochemical processes behind this phenomenon. A suite of new sulfur flows, some of which active, was identified at the Lastarria volcano (northern Chile) and studied using satellite imagery, in situ probing, and temperature and video recording. This finding provides a unique opportunity to better understand the emplacement mechanisms and mineral and chemical compositions of molten sulfur, in addition to gaining insight into its origin. Molten sulfur presented temperatures of 124–158°C, with the most prolonged sulfur flow reaching 12 m from the source. Photogrammetric tools permitted the identification of levees and channel structures, with an estimated average flow speed of 0.069 m/s. Field measurements yielded a total volume of 1.45 ± 0.29 m3 of sulfur (equivalent to ∼2.07 tons) mobilized during the January 2019 event for at least 408 min. Solidified sulfur was composed of native sulfur with minor galena and arsenic- and iodine-bearing minerals. Trace element analysis indicated substantial enrichment of Bi, Sb, Sn, Cd, as well as a very high concentration of As (>40.000 ppm). The January 2019 molten sulfur manifestations in Lastarria appear to be more enriched in As compared to the worldwide known volcanoes with molten sulfur records, such as the Shiretoko-Iozan and Poás volcanoes. Furthermore, their rheological properties suggest that the “time of activity” in events such as this could be underestimated as flows in Lastarria have moved significantly slower than previously thought. The origin of molten sulfur is ascribed to the favorable S-rich chemistry of fumarolic gases and changes in host rock permeability (fracture opening). Molten sulfur in Lastarria correlates with a peak in activity characterized by high emissions of SO2 and other acid species, such as HF and HCl, in addition to ground deformation. Consequently, molten sulfur was framed within a period of volcanic unrest in Lastarria, triggered by changes in the magmatic-hydrothermal system. The appearance of molten sulfur is related to physicochemical perturbations inside the volcanic system and is perhaps a precursor of eruptive activity, as observed in the Poás and Turrialba volcanoes.

Emplacement dynamics of a complex thick mafic intrusion revealed by borehole image log facies analyses: Implications for fluid migration in the Parnaíba Basin petroleum system, Brazil

Sill intrusions may play a significant role in the development of petroleum systems in sedimentary basins by promoting maturation of source rocks, influencing fluid migration and acting as reservoirs, traps, and seals for hydrocarbon accumulations. Within this study, we present a novel approach to interpreting geophysical borehole data for a ca. 174 m thick sill from the Parnaíba Basin revealing new insights into both the emplacement mechanisms and impacts of the sill on hydrocarbon migration and trapping. High-resolution ultrasonic and microresistivity borehole image logs were integrated with conventional log data, petrography of eight side wall cores and total gas data recorded during drilling. The petrographic analysis revealed finer crystalline dolerite at the top and bottom layers of the sill while the upper central portion revealed coarser crystalline gabbroic facies, associated with anomalies in gamma ray, neutron porosity, sonic and geochemical log responses. From borehole image logs we interpreted key facies units including massive, fractured, cooling jointed and layered dolerite along with clear evidence for layers containing diverse inclusions interpreted as enclave structures. The integration and correlation of these data with analogue studies allowed the interpretation of gradational cooling with magmatic differentiation, partially melted host rock linked to sulphide production and organic matter maturation, mafic microgranular enclaves related to mingling processes of magma recharge, banding layers related to cumulate zones, and intense cooling fractures in the central sill body. An interval of ca. 44 m at the top of the sill with notably less jointing appears to be responsible for sealing the gas underneath, whilst the fractured central portion of the sill has enabled gas migration evidenced by increased gas signatures. In addition, this work also reveals for the first-time image log evidence for magma mingling and indications of assimilation and anatexis processes within the Parnaíba Basin. This study has important implications for improving the understanding of emplacement mechanisms of thick parallel layer intrusions and their implications for fluid flow in sedimentary basins worldwide.

Two-stage emplacement mechanism and symmetrical differentiation process of mafic-ultramafic sills in an arc setting: A case study of the Middle Tonian rock assemblage from the Fanjingshan area

The emplacement and differentiation mechanisms are significant parts of understanding the evolution of a magmatic system. There is debate regarding the emplacement mechanism of long-distance lateral transport of magma. And the differentiation mechanisms mainly target the rock assemblage in mantle plume-related or rift-related settings. As yet, few studies had been followed in the arc setting. In this contribution, the mafic-ultramafic sills formed by long-distance lateral transport of magma in the Fanjingshan area in South China have been selected to decode the magmatic emplacement and differentiation mechanism in an arc system. Voluminous basaltic lava and mafic-ultramafic sills are emplaced, concordant to bedding, within the lower part of the Middle Tonian Fanjingshan Group and laterally extend for tens of kilometers. Some mafic-ultramafic sills display symmetrical lithology and geochemical distribution in vertical sections. We provide a detailed stratigraphy and geochemical analysis of the mafic-ultramafic sills of the Fanjingshan Group. The mafic sills within the underlying strata (the Taojinhe Formation) of the lower part of the Fanjingshan Group were intruded at 832 Ma, which is >8 Ma younger than the basaltic eruptions in the overlying strata (the Huixiangping Formation). Moreover, these mafic sills within the Taojinhe Formation have a positive ɛNd(t) value (+0.9 − +1.6), the lowest Platinum Group Elements (PGE) content and highest Rare Earth Elements (REE) content. Compared with other mafic sills and basalts within different formations (the Huixiangping, Xiaojiahe, and Yujiagou formations), mafic sills within the Taojinhe Formation display a separate field on the principal component analysis diagram, suggesting two stages of mafic magma emplacement with a short interval for the Fanjingshan Middle Tonian volcanic plumbing system. The magma channel of the first stage magma is subsequently used by the second stage magma as suggested by ɛNd(t) values and Nb-Ti anomalies. Although island arc magmas differ greatly from mantle plume magmas in volume, temperature and composition, the calculated dimensionless temperature and dimensionless flux of magma imply that the solidification can affect the formation of mafic-ultramafic sills in the island arc systems. At the same time, the difference in rigidity of the upper and lower strata is not the main reason to control the formation of the mafic-ultramafic sills as suggested by the statistical analysis of the mafic-ultramafic sills occurrence between different wall-rock lithology associations. Through the comparative study of the sills formed by the long-distance lateral transport of magma, it is proposed that the clastic wall-rock provides the first-order control for the sills formed by the long-distance lateral transport of magma. According to the detailed geochemical study of an M-shaped geochemical profile within the Huixiangping Formation, two steps of in situ crystallization model have been suggested. In the first step, minerals crystallizing from the inflowing magma on the floor and roof of mafic-ultramafic sills will become more primitive inwards. In the second step, the magma evolves both downward and upward by uninterrupted in situ crystallization in a closed system that results in the whole-rock becoming more evolved in composition inward.

Friction Experiments on Lunar Analog Gouges and Implications for the Mechanism of the Apollo 17 Long Runout Landslide

AbstractThe Light Mantle landslide is a hypermobile landslide on the Moon. Apollo 17 astronauts collected a core sample of the top 60 cm of the Light Mantle deposit, which is currently being analyzed as part of the NASA's Apollo Next Generation Sample Analysis program. The origin of its hypermobility remains undetermined, as the proposed mechanisms are difficult to prove because of the lack of theoretical and experimental support and the scarcity of field data related to the internal structures of its deposit. Regardless of the emplacement mechanisms, it has been proposed that localized dynamic frictional weakening is responsible for the early stage instability that leads to catastrophic failure. Here, we conduct friction experiments under vacuum to investigate the viability of dynamic friction weakening in lunar analog anorthosite‐bearing gouges (i.e., rock powders). Our results show that localized dynamic friction weakening does not occur in these gouges at loading conditions where, instead, weakening is observed in other materials on Earth. Therefore, possibly other fluidization‐related mechanisms contributed to the initiation of the hypermobile Light Mantle landslide. Finally, we describe the microstructures formed in the experiments, including the presence of clast cortex aggregates. Preliminary investigation of the Light Mantle core samples (73001/73002) shows the presence of similar microstructures. Therefore, our microstructural observations will help the analysis and interpretation of the Apollo 17 core samples, as keys to insights about internal processes occurring during the emplacement of the landslide.

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

The Campinas-Jaguariúna Sill, NE Paraná Magmatic Province, Brazil: Insights on the mechanisms of emplacement and differentiation from geochemical and magnetic data

The Paraná Magmatic Province (PMP), one of the largest magmatic provinces on Earth, is composed of lava flows, dike swarms and sills that occupy thousands of kilometers across central-eastern South America. The Campinas-Jaguariúna Sill (CJS) is a large (>50 m thick and ±150 km2) diabase intrusion showing frequent medium- to coarse-grained gabbroic segregations and was emplaced at the interface between the Precambrian basement and sedimentary rocks in the northeastern Paraná Basin. This paper applies airborne magnetic and geochemical modeling to help understand the emplacement mechanisms and petrological evolution of the CJS. Magnetic field filtering techniques demonstrate that the CJS continues for tens of kilometers in the subsurface; analogous features are shown to be associated with other neighbor diabase occurrences, creating lobe-shaped structures that seem to freeze their original geometry and are not fully exposed by the current erosion levels. Petrological modeling indicates that the parental magma of the CJS was emplaced at ∼1 kbar, 1000 ppm H2O, and oxygen fugacity near FMQ-1. The gabbroic segregations, exposed as cm- to dm-sized sheets and eye-shaped lenses, are more evolved than the host diabase and can be shown by geochemical modeling to correspond to residual melts formed by crystal fractionation in a closed system. The parent magma of the CJS is akin to the more primitive Paranapanema magma type of the PMP, and we tested the possibility that sills intruding upper sedimentary layers may be part of a single episode of intrusions. For this purpose, we used geochemical data from the well-studied Limeira Sill, which is akin to the more evolved Pitanga magma type. Our modeling shows that, under crystal fractionation in the representative system condition, the CJS starting compositions cannot generate the Limeira rocks.

Comparison of Topographic Roughness of Layered Deposits on Mars

Impact craters with layered ejecta deposits are widespread on Mars. Prevailing views suggest that such ejecta were formed due to the involvement of target water and/or water ice in the impact excavation and/or the post-deposition movement of the impact ejecta. The long-runout landslides and lobate debris aprons that are likely formed due to the involvement of water ice are used as analogs to compare roughness at multiple scales, considering that these three landforms share some similarities in their geomorphology. Analog studies of the morphological similarities and differences of layered ejecta deposits with different emplacement mechanisms are an important approach to untangling how layered ejecta deposits might form on Mars and beyond. Earlier morphological comparisons were usually based on qualitative descriptions or one-dimensional topographic roughness characteristics at given azimuths; however, the emplacement processes of layered deposits are recorded in two-dimensional topography and at multiple scales. In this study, we designed a multiwavelet algorithm to characterize the multi-scale topographic roughness of different forms of Martian layered deposits. Our comparisons show that the inner facies of the layered ejecta deposits and long-runout landslides exhibited similar roughness characteristics, and the outer facies of the layered ejecta deposits were more similar in roughness to lobate debris aprons. This study highlights the importance of the spatial resolution of digital terrain models in characterizing fine topographic fluctuations on layered ejecta deposits, providing additional insights into the possible emplacement mechanisms of different parts of layered ejecta deposits.

The emplacement mechanism of the Early Carboniferous Nepluyevka batholith according to petromagnetic data (the Southern Urals)

In this article we present the first data on the anisotropy of magnetic susceptibility (AMS) for the rocks of the Early Carboniferous Nepluyevka polyphase pluton, which is situated in the southern part of the East Urals megazone. We consider the properties of magnetic mineralogy of all phases of the batholith, and show that the leading role in determining the magnetic properties of the rocks belongs to an isotropic grain population of MD magnetite and multiple populations of SD magnetite, which are the chief contributors to AMS. We consider the characteristic parameters of AMS for each phase, and show that the AMS is magmatic (non-deformational) in nature. An emplacement mechanism of “magmatic duplex” developing in a kinematic setting of sinistral transtension is proposed for the batholith, based on the analysis of the characteristic magnetic fabrics.