Generation, mobilization and crystallization of impact-induced alkali-rich melts in granitic target rocks: Evidence from the Araguainha impact structure, central Brazil

Machado, Raphael C. S.; Lana, Cristiano; Stevens, Gary; Filho, Carlos Roberto Souza; Reimold, W. U.; McDonald, Iain

doi:10.1016/j.gca.2009.08.029

Cited by 27 publications

(60 citation statements)

References 51 publications

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“…This is in contrast to melt observed in small‐scale veins in the rock volume proximal to larger melt zones; here, melt probably originated from larger melt zones. This is consistent with the work of Machado et al. (2009) and Lafrance et al.…”

Section: Discussionsupporting

confidence: 93%

Formation of pseudotachylitic breccias in the central uplifts of very large impact structures: Scaling the melt formation

Mohr‐Westheide¹,

Reimold²

2011

Meteorit & Planetary Scien

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Abstract– The processes leading to formation of sometimes massive occurrences of pseudotachylitic breccia (PTB) in impact structures have been strongly debated for decades. Variably an origin of these pseudotachylite (friction melt)‐like breccias by (1) shearing (friction melting); (2) so‐called shock compression melting (with or without a shear component) immediately after shock propagation through the target; (3) decompression melting related to rapid uplift of crustal material due to central uplift formation; (4) combinations of these processes; or (5) intrusion of allochthonous impact melt from a coherent melt body has been advocated. Our investigations of these enigmatic breccias involve detailed multidisciplinary analysis of millimeter‐ to meter‐sized occurrences from the type location, the Vredefort Dome. This complex Archean to early Proterozoic terrane constitutes the central uplift of the originally >250 km diameter Vredefort impact structure in South Africa. Previously, results of microstructural and microchemical investigations have indicated that formation of very small veinlets involved local melting, likely during the early shock compression phase. However, for larger veins and networks it was so far not possible to isolate a specific melt‐forming mechanism. Macroscopic to microscopic evidence for friction melting is very limited, and so far chemical results have not directly supported PTB generation by intrusion of impact melt. On the other hand, evidence for filling of dilational sites with melt is abundant. Herein, we present a new approach to the mysterium of PTB formation based on volumetric melt breccia calculations. The foundation for this is the detailed analysis of a 1.5 × 3 × 0.04 m polished granite slab from a dimension‐stone quarry in the core of the Vredefort Dome. This slab contains a 37.5 dm3 breccia zone. The pure melt volume in 0.1 m3 PTB‐bearing granitic target rock outside of the several‐decimeter‐wide breccia zone in the granite slab was estimated at 5.2 dm3. This amount can be divided into 4.6 dm3 melt (88%), for which we have evidenced a limited material transport (at maximum, ≈20 cm) and 0.6 dm3 melt (12%) with, at most, grain‐scale material transport, which we consider in situ formed shock melt. The breccia zone itself contains about 10 dm3 of matrix (melt). Assuming melt exchange over 20 cm at the slab surface, between breccia zone and surrounding melt‐bearing host rock volume, the outer melt volume is calculated to contain the same amount of melt as contained by the massive breccia zone. Meso‐ and microscopic observations indicate melt transport is more prominent from larger into smaller melt occurrences. Thus, melt of the breccia zone could have provided the melt fill for all the small‐scale PTB veins in the surrounding target rock. Extrapolating this melt capacity calculation for 1 m3 PTB‐bearing host rock shows that a host rock volume of this dimension is able to take up some 52 dm3 melt. Scaling up 1000‐fold to the outcrop scale reveals that exchange between a host roc...

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Section: Discussionsupporting

confidence: 93%

Formation of pseudotachylitic breccias in the central uplifts of very large impact structures: Scaling the melt formation

Mohr‐Westheide¹,

Reimold²

2011

Meteorit & Planetary Scien

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“…Stratigraphic and structural reconstructions of the pre-impact target stratigraphy indicate that this core-collar contact was approximately 2 km below surface, consistent with bore-hole core logging some 100 km north of the impact crater (Engelhardt et al, 1992;Lana et al, 2006Lana et al, , 2007. More recently, Machado et al (2009) showed field and petrographic observations that this contact was within or substantially close to the zone of impact melting during the cratering event, which explains the presence of sandstone-derived melt around the central uplift (Engelhardt et al, 1992; this study).…”

Section: Introductionsupporting

confidence: 81%

“…Both optical and scanning electron images indicate that the groundmass in the matrix and in the shear zones share identical textural homogeneity, similar in many ways to an impact glass phase (e.g., cooled from an impact melt). However typical impact-melt quench textures suggestive of fast cooling (e.g., Machado et al, 2009) have not been observed. Detailed SEM images show instead that it consist of submicroscopic clay minerals (predominantly kaolinite and allophane) with a dominant Si-Alrich composition and minor amounts (<1%) of K, Fe.…”

Section: Groundmassmentioning

confidence: 98%

“…Similarly, siliciclastic rocks have rarely developed pseudotachylites in deformation experiments (e.g., Logan et al, 1973;Friedman et al, 1974) but tend towards brittle failure and subsequent cataclastic deformation in response to high stress and strain rates (e.g., Wong et al, 1997). The lack of extensive shear zones or fractures filled with pseudotachylites in the Furnas sandstone, such as found in the granitic core of the Araguainha central uplift (Machado et al, 2009), is a result of their very different responses to the impact. In the sandstones, impact energy was dissipated by pervasive microbrecciation, associated with heating, boiling and expansion of pore fluids, microbrecciation and creation of the incohesive, liquefied mass.…”

Section: Pore-fluid Expansion Sandstone Microbrecciation and The Abmentioning

confidence: 99%

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Liquefaction of sedimentary rocks during impact crater development

Hippertt

Lana

Weinberg

et al. 2014

Earth and Planetary Science Letters

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“…Target rocks of, for example, the Vredefort, Sudbury, Araguainha and Slate Islands impact structures are characterized by the pervasive presence of fragment-rich pseudotachylite bodies (Reimold, 1991;Dressler and Sharpton, 1997;Spray et al, 2004;Machado et al, 2009). These bodies form geometrically complex systems of veins, dikes and pods that, in the case of Sudbury, can be hundreds of metres wide.…”

Section: Introductionmentioning

confidence: 99%

Petrographic and geochemical evidence for an allochthonous, possibly impact melt, origin of pseudotachylite from the Vredefort Dome, South Africa

Lieger

Riller

Gibson

2011

Geochimica et Cosmochimica Acta

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Generation, mobilization and crystallization of impact-induced alkali-rich melts in granitic target rocks: Evidence from the Araguainha impact structure, central Brazil

Cited by 27 publications

References 51 publications

Formation of pseudotachylitic breccias in the central uplifts of very large impact structures: Scaling the melt formation

Formation of pseudotachylitic breccias in the central uplifts of very large impact structures: Scaling the melt formation

Liquefaction of sedimentary rocks during impact crater development

Petrographic and geochemical evidence for an allochthonous, possibly impact melt, origin of pseudotachylite from the Vredefort Dome, South Africa

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