Material mixing in shock-induced pseudotachylites, Vredefort impact structure, South Africa

Kovaleva, Elizaveta; Huber, M. S.; Dixon, Roger D.

doi:10.1016/j.lithos.2020.105621

Cited by 4 publications

(4 citation statements)

References 54 publications

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“…Such high-energy movements would be somewhat restricted at a depth of 8-10 km of the current erosional level due to the lithostatic pressure from the overlying masses of rock. Nevertheless, a degree of material mixing was observed even at the current erosional level [48]. A higher degree of melt mixing and melt transport at the upper levels might result in certain similarities between the impact melt (granophyre) and "in situ" melt (pseudotachylite), somewhat opposite to what was previously believed.…”

Section: Geochemical Properties Of Pseudotachylitecontrasting

confidence: 65%

“…Shocked monazite is also found in the vicinity of the PT melt vein and is composed of the twinned area (domains 1 and 2) and the re-crystallized area (domain 3). Impact-related mechanical twinning in monazite is not uncommon, with at least 12 distinct orientations of shock microtwins documented previously [54,55], with likely more twin orientations that are possible [48]. The twin orientation 180 • /[101] is among the previously documented orientations [55].…”

Section: Association Of the Pt Vein With Shocked Accessory Mineralsmentioning

confidence: 96%

“…On the other hand, the pseudotachylitic melt is advocated to form strictly in situ and correspond geochemically to its host rock [4,11]. Some pseudotachylites in granites were found to be slightly more mafic (i.e., enriched in Mg and Fe) than their host rocks [7,8] due to the initial melting of mafic and hydrous phases with lower fracture toughness in non-equilibrium conditions [47,48]. During the "flash" melting of granitic rocks, an excess of silica could survive as refractory quartz grains.…”

Section: Geochemical Properties Of Pseudotachylitementioning

confidence: 99%

“…For example, the formation of pseudotachylite may involve compositional mixing. Melt/clast transport to some distances and/or between different lithologies might have occurred [48].…”

Section: Geochemical Properties Of Pseudotachylitementioning

confidence: 99%

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Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

Kovaleva

Dixon

2020

Minerals

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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.

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Section: Geochemical Properties Of Pseudotachylitecontrasting

confidence: 65%

Section: Association Of the Pt Vein With Shocked Accessory Mineralsmentioning

confidence: 96%

Section: Geochemical Properties Of Pseudotachylitementioning

confidence: 99%

“…For example, the formation of pseudotachylite may involve compositional mixing. Melt/clast transport to some distances and/or between different lithologies might have occurred [48].…”

Section: Geochemical Properties Of Pseudotachylitementioning

confidence: 99%

See 2 more Smart Citations

Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

Kovaleva

Dixon

2020

Minerals

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On the role of shock melting and anatexis in breccia formation: Southern Sudbury impact structure, Canada

Généreux

Tinkham

Lafrance

2021

Precambrian Research

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Distinguishing friction- from shock-generated melt products in hypervelocity impact structures

Spray¹,

Biren²

2021

Large Meteorite Impacts and Planetary Evolution VI

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Field, microtextural, and geochemical evidence from impact-related melt rocks at the Manicouagan structure, Québec, Canada, allows the distinction to be made between friction-generated (pseudotachylite) and shock-generated melts. Making this distinction is aided by the observation that a significant portion of the impact structure’s central peak is composed of anorthosite that was not substantially involved in the production of impact melt. The anorthosite contrasts with the ultrabasic, basic, intermediate, and acidic gneisses that were consumed by decompression melting of the >60 GPa portion of the target volume to form the main impact melt body. The anorthosite was located below this melted volume at the time of shock loading and decompression, and it was subsequently brought to the surface from 7–10 km depth during the modification stage. Slip systems (faults) within the anorthosite that facilitated its elevation and collapse are occupied by pseudotachylites possessing anorthositic compositions. The Manicouagan pseudotachylites were not shock generated; however, precursor fracture-fault systems may have been initiated or reactivated by shock wave passage, with subsequent tectonic displacement and associated frictional melting occurring after shock loading and rarefaction. Pseudotachylites may inject off their generation planes to form complex intrusive systems that are connected to, but are spatially separated from, their source horizons. Comparisons are made between friction and shock melts from Manicouagan with those developed in the Vredefort and Sudbury impact structures, both of which show similar characteristics. Overall, pseudotachylite has compositions that are more locally derived. Impact melts have compositions reflective of a much larger source volume (and typically more varied source lithology inputs). For the Manicouagan, Vredefort, and Sudbury impact structures, multiple target lithologies were involved in generating their respective main impact melt bodies. Consequently, impact melt and pseudotachylite can be discriminated on compositional grounds, with assistance from field and textural observations. Pseudotachylite and shock-generated impact melt are not the same products, and it is important not to conflate them; each provides valuable insight into different stages of the hypervelocity impact process.

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Material mixing in shock-induced pseudotachylites, Vredefort impact structure, South Africa

Cited by 4 publications

References 54 publications

Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

On the role of shock melting and anatexis in breccia formation: Southern Sudbury impact structure, Canada

Distinguishing friction- from shock-generated melt products in hypervelocity impact structures

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