Zircon with polycrystalline or polygranular appearance is either produced in the magmatic environment through crystallization, or due to deformation in metamorphic settings (including regional ...metamorphism and ductile shear zones), or as a result of shock-induced recrystallization. All three types can be easily confused and potentially lead to incorrect interpretations, especially if the crystallographic orientation analyses of zircon are not conducted. It is particularly important to establish the difference between tectonically-deformed polygranular zircon and shock-induced polygranular zircon because the latter serves as an indicator of shock event and is often used for dating asteroid impacts. In this paper, a series of polycrystalline zircon grains from ductile shear zones and metamorphic rocks are analyzed using a combination of techniques (BSE, CL, orientation contrast, EBSD, and microprobe mapping), and their properties are compared to reported polycrystalline zircons from magmatic and impact settings. This work shows how appearance, crystallographic orientation, and CL signature of “granules” differ between the different types of deformed zircon.
The Vredefort impact structure in South Africa is deeply eroded to its lowermost levels. However, granophyre (impact melt) dykes in such structures preserve clasts of supracrustal rocks, transported ...down from the uppermost levels of the initial structure. Studying these clasts is the only way to understand the properties of already eroded impactites. One such lithic clast from the Vredefort impact structure contains a thin pseudotachylite vein and is shown to be derived from the near-surface environment of the impact crater. Traditionally, impact pseudotachylites are referred to as in situ melt rocks with the same chemical and isotopic composition as their host rocks. The composition of the sampled pseudotachylite vein is not identical to its host rock, as shown by the micro-X-ray fluorescence (µXRF) and energy-dispersive X-ray (EDX) spectrometry mapping. Mapping shows that the melt transfer and material mixing within pseudotachylites may have commonly occurred at the upper levels of the structure. The vein is spatially related to shocked zircon and monazite crystals in the sample. Granular zircons with small granules are concentrated within and around the vein (not farther than 6–7 mm from the vein). Zircons with planar fractures and shock microtwins occur farther from the vein (6–12 mm). Zircons with microtwins (65°/{112}) are also found inside the vein, and twinned monazite (180°/101) is found very close to the vein. These spatial relationships point to elevated shock pressure and shear stress, concentrated along the vein’s plane during impact.
The Vredefort impact structure is among the oldest and largest impact structures preserved on Earth. An understanding of its key features can serve as a guide for learning about the development of ...basin-sized impact structures on Earth and other planetary bodies. One of these features is the so-called Vredefort granophyre dikes, which formed when molten material from the impact melt sheet was emplaced below the crater floor. The importance of these dikes has been recognized since the earliest studies of the Vredefort structure, nearly 100 years ago. The present study is a systematic literature review to determine the extent to which peer-reviewed scientific publications have generated unique data regarding the granophyre dikes and to investigate how scientific methods used to investigate the granophyre have changed over time. In total, 33 unique studies have been identified. Of those, more studies have been performed into the core-collar dikes than the core dikes. The majority of the studies have focused on field analyses, bulk geochemistry, and the studies of mineral components. The granophyre has long been recognized as a product of post-deformational processes and thus has been a target of age dating to constrain the minimum age of the impact event. In the last 25 years, studies of stable isotopes and shock deformation of minerals in lithic clasts within the dikes have taken place. A small number of geophysical studies relevant to the granophyre dikes have also been undertaken. Overall, there has been a relatively small number of studies on this important rock type, and the studies that have taken place tend to focus on two particular dikes. Several of the dikes have only been investigated by regional studies and have not been specifically targeted. The use of modern techniques has been lacking. More fieldwork, as well as geophysical, isotopic, microstructural studies, and application of novel techniques, are necessary for the granophyre dikes to be truly understood.
Better characterization features borne from long-term crustal modification processes is essential for understanding the dynamics of large basin-forming impact structures on Earth. Within the deeply ...eroded 2.02 Ga Vredefort Impact Structure in South Africa, impact melt dikes are exposed at the surface. In this study, we utilized a combination of field, remote sensing, electrical resistivity, magnetic, petrographical, and geochemical techniques to characterize one such impact melt dike, namely, the Holfontein Granophyre Dike (HGD), along with the host granites. The HGD is split into two seemingly disconnected segments. Geophysical modeling of both segments suggests that the melt rock does not penetrate below the modern surface deeper than 5 m, which was confirmed by a later transecting construction trench. Even though the textures and clast content are different in two segments, the major element, trace element, and O isotope compositions of each segment are indistinguishable. Structural measurements of the tectonic foliations in the granites, as well as the spatial expression of the dike, suggest that the dike was segmented by an ENE–WSW trending sinistral strike-slip fault zone. Such an offset must have occurred after the dike solidified. However, the Vredefort structure has not been affected by any major tectonic events after the impact occurred. Therefore, the inferred segmentation of the HGD is consistent with long-term crustal processes occurring in the post-impact environment. These crustal processes may have involved progressive uplift of the crater floor, which is consistent with post-impact long-term crustal adjustment that has been inferred for craters on the Moon.
Mechanical microtwins in zircon is a characteristic shock feature and a potential tool for estimating shock pressure and/or shear stress. However, so far, zircon with microtwins has rarely been ...described in situ. The present study of pseudotachylite-bearing granitic gneisses from the Vredefort impact structure focuses on the spatial distribution of zircon with microtwins and other deformation features.
Zircon hosted by granitic gneiss does not reveal extensive shock deformation. With decreasing distance to pseudotachylite, zircon progressively develops (sub)planar fractures, planar deformation bands, low-angle boundaries, rarely thin {112} microtwin lamellae and, closest to the vein, it is fragmented. In contrast, zircon hosted by pseudotachylites contains thick and closely-spaced microtwin lamellae, which form up to three sets within individual grains. Microtwin lamellae are associated with planar {112} low-angle boundaries, and with overprinting crystal-plastic and brittle deformation. We suggest that planar {112} low-angle boundaries in zircon are indicative of shock deformation; and that microtwins in zircon within pseudotachylite veins are an indicator of the impact origin for the latter, as opposed to endogenic, seismically-induced friction melts. Locally-elevated shock pressure and shear stress induce the formation of pseudotachylites in the target rock as well as shock microtwins in zircon.
•Zircon in impact-related pseudotachylite veins has abundant shock microtwins.•Zircon in the host granite gneiss has scarce {112} shock microtwins.•Zircon with microtwins indicates an impact origin of a host pseudotachylite vein.•Shock deformation is associated with crystal-plastic and brittle deformation.•Zircon deformation style changes with distance to pseudotachylite in the host rock.
The author analyses the works of the prominent politician and memoirist Vasily Shulgin published in the emigre periodicals to examine his views on the origin of the Slavic people and the Ukrainian ...question. Trying to prove the “Russianness” of the Ukrainian people, Shulgin often referred to historical sources as a journalist, not as a scholar (although Shulgin remarked ironically, “Sure, I am not a historian, but I have an affinity for history - a sort of the disease.”) Speculating about the origin of Rus, Shulgin repeatedly told the theory championed by General Grigory Yanushevsky based on the legend of three brothers - Czech, Lech, and Rus. According to this legend, they came from the Roman Pannonia in the 3rd century to settle down in Czechia, Poland, and Galicia. They became “the progenitors of the Czech, Polish (Lachian) and Russian people” Since there was no “brother Ukr” in the legend, Shulgin considered it a strong argument supporting the artificial construction of the Ukrainian nation. In his opinion, Poles invented the term “Ukrainians” in the 18th century, and before that there had been no nation with such name, so the term had been used to denote the residents of the borderlands. Thus, the term referred to the territory and did not mean a nation. In his journalism, Shulgin argued against the “political Ukrainism” that was striving for separatism and imposition of “Grushevsky's language ”, though he never denied the local cultural tradition. He saw the solution to the Ukrainian question in recognizing that the path chosen once by Bohdan Khmelnitsky was the right one.
Impact-generated pseudotachylites (PTs) have been extensively studied in the Vredefort Impact Structure, South Africa, and have been shown to correspond chemically and isotopically to their host ...rocks. Such melts are frequently described as forming strictly in situ with lithic clasts derived from the immediate wall rocks. Mixed compositions are rare, where PTs are observed directly along a lithological contact. In this study, we document the presence of vast numbers of granitic clasts within dolerite-hosted PT veins within the Vredefort impact structure. The groundmasses of dolerite-hosted PT veins have two distinct geochemical compositions: (1) mafic, and (2) intermediate. The two compositional melt phases have a sharp contact, indicating immiscibility and/or mingling. Shocked zircon and monazite within the granite clasts enclosed in a dolerite-hosted PT vein indicate shock deformation prior to clast transport into the dolerite. In monazite, we report a new shock microtwin geometry 180°/104.
The mechanisms by which granitic clasts were emplaced in the dolerite-hosted veins are complex. Previous experimental work has suggested that varying shock velocity in different lithologies may be responsible for material mixing. In addition, the thermal pulse accompanying the shock wave would have caused quartz in the granite to transform into volumetrically larger high-temperature polymorphs, significantly increasing the granite volume compared to the adjacent dolerite. Together with fluctuation of stresses (post-impact reverberations) after the passage of the main shock wave, this volume adjustment initiated brittle fracturing of the dolerite. A negative pressure gradient was generated within the dolerite that allowed granitic material to be injected into the dolerite-hosted melt veins.
•Dolerite-hosted pseudotachylites contain exotic clasts of granite•Pseudotachylites have intermediate composition due to material mixing•Material mobilized due to fracture dilation in dolerite•Quartz phase transition allowed material mixing to take place•Impact pseudotachylites are not strictly in situ melts
Drawing on previously unpublished archival sources and new materials of periodicals of the Volhynian province, the article examines the history of The Pochaev-Volhynian People's Loan Society and the ...role of Archimandrite Vitaly (Maksimenko) in the creation of the Pochaiv Bank. Particular attention is paid to the analysis of historical realities and the problems Father Vitaly had to face during the implementation of this project. In the early 20th century, several factors influenced the agrarian question in the Volhynian Governorate. The price of the fertile lands of this region was constantly growing, and the Volhynian peasants could not compete with the expansion of the non-native population (Jews, Poles, Germans). Having no alternative and suffering from a lack of land, the peasants were forced to borrow money either at an extremely high percentage from the Jews, or on very difficult terms from the landowners. To alleviate the situation of the Volhynian peasantry, Vitaly (Maksimenko) created a credit institution to give loans to the needy at a lower interest rate than those offered by usurers and landowners. Those who own excess funds could get a high interest rate on deposits. Having concentrated in his hands the editorial of the Pochaev Lavra and the leadership of the Pochaev Department of the Union of the Russian People, Archimandrite Vitaliy (Maksimenko) achieved the main goal of alleviating the situation of land-poor peasants. Despite the difficulties and obstacles that arose from both ideological opponents and local authorities, managed to create a large-scale banking institution with an extensive network of branches.
The Vredefort impact structure, South Africa, is comparable to the Sudbury impact structure, Canada, in size, age, and target rock composition. Both impact structures feature impact melt dikes. The ...melt sheet of the Sudbury impact (Sudbury Igneous Complex; SIC) is genetically linked to the Sudbury offset dikes in the underlying target rock. At Vredefort, the melt sheet was eroded so that only the granophyre dikes retain compositional melt sheet characteristics. XRF analyses of 43 samples from four granophyre dikes are similar to previous studies, but identify an anomalous mafic phase within one of the dikes. The results from the Vredefort granophyre dikes are compared to the Sudbury offset dikes and shown to follow similar geochemical trends, controlled by crystallization of feldspar and pyroxene. The mafic granophyre phase is compositionally remarkably similar to the offset dike compositions. The program Rhyolite‐MELTS was used to test possible melt sheet compositions. Modeling results are broadly consistent with the overall chemical and mineral composition of the dikes. Modeling is consistent with offset dikes being derived from the basal mafic layer of the SIC, and the granophyre dikes being derived from alkali‐depleted bulk continental crust. For all modeled compositions, crystallization primarily occurred at temperatures between 1150°C and 1000°C. The emplacement of the felsic granophyre dikes from a homogenized crustal melt suggests emplacement within tens of years after the impact event. The presence of the mafic phase in one of the granophyre dikes is explained by its emplacement following some differentiation of the Vredefort melt sheet.
Granular neoblastic zircon (ZrSiO4) with systematically oriented granules has been proposed as evidence for extreme shock pressures (>30 GPa) and subsequent high temperatures (>1200°C). It is widely ...agreed to reflect the solid-state phase transition from zircon to its high-pressure polymorph reidite and subsequent reversion to zircon. This model is based on crystallographic relationships between granules of a single type of granular zircon and does not explain the formation of other types of granular zircon textures, for example, grains with randomly oriented granules or with large, often euhedral granules. Here we report the first nano-scale observations of granular neoblastic zircon and the surrounding environment. We conducted combined microstructural analyses of zircon in the lithic clast from an impact melt dike of the Vredefort impact structure. Zircon granules have either random or systematic orientation with three mutually orthogonal directions of their c-axes coincident with 110 axes. Each 1-2 μm zircon granule is a mosaic crystal composed of nanocrystalline subunits. Granules contain round inclusions of baddeleyite (monoclinic ZrO2) and amorphous silica melt. Tetragonal and cubic ZrO2 also occur as sub-μm-sized inclusions (<50 nm). Filament-like aggregates of nanocrystalline zircon are present as “floating” in the surrounding silicate matrix. They are aligned with each other, apparently serving as the building blocks for the mosaic zircon crystals (granules). Our results indicate shock-related complete melting of zircon with the formation of immiscible silicate and oxide melts. The melts reacted and crystallized rapidly as zircon granules, some of which experienced growth alignment/twinning and parallel growth, causing the characteristic systematic orientation of the granules observed for some of the aggregates. In contrast to the existing model, in which this type of granular zircon is considered to be a product of reversion from the high-pressure polymorph reidite, our nano-scale observations suggest a formation mechanism that does not require phase transition via reidite but is indicative of instant incongruent decomposition, melting and rapid crystallization from the melt.
•Granular neoblastic zircon is found in a small clast in Vredefort impact melt.•Zircon granules with systematic or random orientations retain the age of impact.•Nano-structural study shows inclusions of Zr oxide and silicate glass in zircon.•Each zircon granule is a mosaic crystal composed of nano-sized crystals.•Nano-scale features point at crystallization from two immiscible melts.
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