The Mids Problematics A B S E types Tagamite and Shock Veins

Right through to the beginning of the previous decade, many workers -especially in South Africa - had remained reluctant to accept the long proposed (e.g., Dietz 1960) impact origin of the Vredefort Dome. Within this context, the origin of pseudotachylite in Shand's type locality was not considered significant. It took the confirmation of bona fide shock deformation in quartz (Leroux et al. 1994) and zircon (Kamo et al. 1996), and the identification of a meteoritic component in the Vredefort Granophyre (Koeberl et al. 1996) to change the conviction of, by far, most of these sceptics. Several geochronological and field studies showed that most, if not all, of the so-called

"pseudotachylite" from Vredefort had to be impact-related (Trieloff et al. 1994; Spray et al. 1995; Kamo et al. 1996). The early 1990s then also saw the link made between the Vredefort Dome and the surrounding Witwatersrand Basin, and between the rock deformation (particularly large-scale brecciation) of the Dome and that in the Basin (e.g., Fletcher and Reimold 1989; Killick and Reimold 1990; Reimold and Colliston 1994). Therriault et al. (1997) and Henkel and Reimold (1998) showed that the Vredefort Structure encompasses the entire region known as the Witwatersrand Basin and indicated that the Vredefort impact structure could originally have been as large as 300 km in diameter (or even more - e.g., Phillips et al. 1999).

Martini (1978) demonstrated the existence of the high-pressure polymorphs of quartz, coesite and stishovite, within and in the immediate vicinity of narrow (millimeter-wide) veinlets of "pseudotachylite" (Fig. 7) from the collar of the Vredefort Dome (see also White 1993). Martini followed this up with a paper (Martini 1991), in which he proposed that these narrow veinlets represented a melt produced during the early compression phase of the impact cratering event. He termed this material "A-Type pseudotachylite", in contrast to the remaining occurrences of largely massive breccia (such as those described by Shand 1916), which he considered the result of "decompression melting" during the later modification stage of cratering and termed "B-Type pseudotachylite". Reimold et al. (1992) contested this classification into "A-and B-types" on the basis of lack of definitive recognition/separation criteria for such types.

A decade later Lambert (1981) proposed a classification scheme for breccias in impact structures, and he had also employed an A- and B-type terminology for dike breccias in impact structures, in fact further subdividing into A1, A2, B1, and B2 types. Lambert stated that A-type breccias formed under shock compression and, thus, would include Martini's A type; however, it is not clear which one of Lambert's sub-types would correspond to "pseudotachylite". B-type dikes would form during or after pressure release, with B1 dikes represented injections of material from the interior of the crater into openings in the crater floor, and B2 dikes formed "after, by, or during displacement of blocks" during crater modification. As it can be debated whether all "pseudotachylite" (i.e., all veins that have been termed such) has been formed in situ like tectonic friction melt would, it is not clear whether all that has been termed "B-type pseudotachylite" in impact craters could be categorized into the B1 or B2 types. In essence, we are not comfortable trying to make Martini's (1991) and Lambert's (1981) classifications compatible.

Detailed review of the breccia literature, specific to terminology employed by impact workers, by Reimold (1995, 1998) illustrated that a range of different breccia types has been indiscriminately classified as "pseudotachylite". This includes ultracataclasites and cataclasites, ultramylonites and mylonites, friction melt sensu stricto, impact melt rock, breccias of pre-, syn- or post-impact age in crater settings and involving both impact breccias and tectonic breccias, as well as the so-called "shock veins" of meteorites.

A good example of the confusion between impact melt rock and "pseudotachylite" is provided by the case of the term "tagamite". In the Russian literature, this geographic term originally referred to impact melt rock from the Popigai Structure, but then was applied to impact melt rock from a wide range of impact structures. However, it was also used for the description of narrow veinlets occurring in crater floors; for example those intersected to a depth of 4809 m (A. Deutsch, pers. commun.) in the deep VDW borehole into the central uplift of the Puchezh-Katunki impact structure (Russia) -irrespective of what these veinlets might actually represent. One of us (WUR) has personally seen thin sections of some such veinlets (kindly provided by Dr. Fel'dman, Moscow State University) that are nothing else than totally epidotized breccia veinlets that do not allow any speculations on the actual type of breccia that was present. Other veinlets from this impact structure studied by us and that were termed tagamite represent cataclasite or mylonite.

"Pseudotachylites" have also been described from the crater floor of the large Morokweng impact structure in northwest South Africa (e.g., Hart et al. 1997), but these veinlets were shown by Reimold et al. (1999a) and Koeberl and Reimold (2003) to also comprise a range of different breccias including impact melt injections and cataclasite, besides altered breccia of uncertain genesis. Most recently, Macdonald et al. (2003) described "pseudotachylite" from the old and deeply eroded Yarrabubba impact structure in the Archean Pilbara craton of western Australia, but, interestingly, these authors admitted themselves that at least some of these occurrences would be better termed mylonites.

So-called "shock veins" are known from numerous meteorites, especially from ordinary chondrites, and the fact that they often occur together with high-pressure polymorphs (ringwoodite, majorite, and others) of various rock-forming minerals (e.g., Langenhorst and Poirier 2000; Chen et al. 1996, 2003) has led many to believe that these breccia veinlets are the result of shock

Fig. 5. (a upper) Part of a vein system at Smilin Thru Resort, Vredfort Dome, that has a predominant subvertical vein that can be followed for several m and is visible here in the upper, middle part of figure (a). However, this image also demonstrates that prominent veins occur. It is, thus, basically impossible to determine whether a possible generation fault could have played a role, or whether these individual veins of highly irregular attitude are either the result of injection or of local generation. Compass for scale: ca. 9 cm sidelength. (b lower) A pseudotachylitic breccia exposure in medium-grained granitic gneiss of the core of the Vredefort Dome, indicating that there are many occurrences that show no evidence as to how these "pods" were formed or emplaced. Note the apparent flow structures that could provide a hint that flow took place in the third dimension. Knife for scale, 9 cm long. (c) Two more illustrations of breccia occurrences in granitoid of the core of the Vredefort Dome.

Fig. 5. (a upper) Part of a vein system at Smilin Thru Resort, Vredfort Dome, that has a predominant subvertical vein that can be followed for several m and is visible here in the upper, middle part of figure (a). However, this image also demonstrates that prominent veins occur. It is, thus, basically impossible to determine whether a possible generation fault could have played a role, or whether these individual veins of highly irregular attitude are either the result of injection or of local generation. Compass for scale: ca. 9 cm sidelength. (b lower) A pseudotachylitic breccia exposure in medium-grained granitic gneiss of the core of the Vredefort Dome, indicating that there are many occurrences that show no evidence as to how these "pods" were formed or emplaced. Note the apparent flow structures that could provide a hint that flow took place in the third dimension. Knife for scale, 9 cm long. (c) Two more illustrations of breccia occurrences in granitoid of the core of the Vredefort Dome.

Fig. 5. (c upper) Several apparently irregularly shaped veins occur in close proximity to a decimeter-wide vein (at bottom of image). Whether there is a link between these two occurrences is not clear. (d lower) This seemingly irregular breccia occurrence is, upon closer inspection, revealed to be linked to a N-S trending, millimeter-wide veinlet of variable - subvertical to near-horizontal - attitude. In both (c) and (d) knife for scale is 9 cm long.

Fig. 5. (c upper) Several apparently irregularly shaped veins occur in close proximity to a decimeter-wide vein (at bottom of image). Whether there is a link between these two occurrences is not clear. (d lower) This seemingly irregular breccia occurrence is, upon closer inspection, revealed to be linked to a N-S trending, millimeter-wide veinlet of variable - subvertical to near-horizontal - attitude. In both (c) and (d) knife for scale is 9 cm long.

compression. A comparison between naturally occurring shock veins in meteorites and the results of friction experimentation on meteorites by van der Bogert and Schultz (1997) and van der Bogert et al. (2000) resulted in the conclusion that a combination of shearing and shock, or either process could be responsible for these veins in meteorites. Our own recent study of the H4/5 meteorite Thuathe (Reimold et al. 2003a) also concluded that the narrow black veinlets that occur ubiquitously in specimens of this meteorite are the result of friction melting, without any indication that locally enhanced shock pressure could have contributed to their formation.

In addition to "tagamite", there are other local geographic names used for impact melt rock/breccia in the literature, for example "karnaite" for the impact melt rock occurring on the peninsula Karnansaari in the Lappajarvi meteorite crater in Finland, or "dellenite" for the impact melt rock of the Dellen Structure in Sweden. It appears to us that the generalized use of such local geographic nomenclature ought to be avoided in the interest of simplification and introduction of impact literature into the general geoscientific community. What is wrong with the general term "impact melt rock" that provides a clear genetic label? Reimold (1995, 1998) provided extensive evidence that this lack of discrimination continues to date! On the one hand, some workers have gone as far as to say that pseudotachylite can be used as a recognition criterion for impact structures (Fiske et al. 1995b) -disregarding the wealth of information about tectonic friction melts. And on the other, even the proposed IUGS Impactite Nomenclature (Stoffler and Grieve 1994) lists "pseudotachylite" as an impactite. Macdonald et al. (2003) recently cited "pseudotachylite" as a "shock metamorphic effect", in the face of a plethora of tectonic occurrences worldwide! Reimold (1995, 1998) acknowledged that it would be difficult to change long-time habits, but he recommended that anything not clearly identified as pseudotachylite (bona fide friction melt) should be separated and, at least, referred to as "pseudotachylite-like" or "pseudotachylitic breccia". It must be admitted that it is difficult to introduce conscientious application of this distinction even to close collaborators - but we believe that the reward of contributing to the understanding of the cratering process should be sufficient incentive to give it a try!

Much thought has gone into finding a term that would be completely independent from the association with "pseudotachylite", and many colleagues have also proposed alternative terms (including "frictionite" - which is, however, already used as a synonym for pseudotachylite; or "otavite" with regard to one of the type localities in the Vredefort Dome, the Otavi quarry northeast of Parys, but this term is already reserved for a Co mineral - apart from the general undesirable use of geographic names). One could have thought of another alphabetical term - such as the "X-breccia" - but as it is already impossible to properly distinguish the A, B, S and E breccias, and as a "H breccia" has recently been proposed by Ernstson et al. (2002) for a specific geometric development of dike breccias as observed by these authors in the Azuara structure of disputed impact origin, we refrain from introducing an X-, Y-, or Z-breccia concept.

One more example shall be given to illustrate the problems of indiscriminate use of the term "pseudotachylite": Fiske et al. (1995b), in an abstract, state: (1) "Pseudotachylite dikes and veins are important evidence for meteorite impact". This point has already been raised. Pseudotachylite by itself is not evidence for impact, as it occurs worldwide in tectonic deformation zones. Should Fiske et al. have referred to some "shock veins" that are widely considered as evidence of shock metamorphism in meteorites (Stoffler et al. 1991), this would have been a different matter, and yet - even the origin of shock veins by "simple" shock compression or a combination of shock and shear is now a matter of debate (as outlined above). However, even if found together with other diagnostic impact indicators (such as bona fide shock metamorphic effects), "pseudotachylite" in impact settings can not be considered diagnostic, and it must be proven whether this material indeed represents pseudotachylite (= friction melt) or another type of breccia, or whether it perhaps pre- or post-dates an impact event. (2) ". small pseudotachylite veins (Type A of Martini.) lacking any association with faults are also found. Commonly defining shatter cone surfaces". There are a number of problems with this statement: (a) Small pseudotachylite veins do not need to be of Martini's (1991) A-type (produced during shock compression) and, as, for example, discussed by Dressler and Reimold (2004), very large occurrences of breccia may also correspond to shock melt. Furthermore, shock experimentation by Kenkmann et al. (2000a) and Langenhorst et al. (2002) has shown that rock discontinuities such as lithological boundaries may provide appropriate locations for shock pressure excursions to high values that could lead to shock melting, or shock-cum-friction melting, at such discontinuities. Field evidence from the Vredefort Dome shows that existing zones of weakness, such as mylonitized fault zones, are preferred locations for pseudotachylitic breccia development. Thus, to differentiate a so-called "Type A pseudotachylite" from fault-association is wrong. Van der Bogert and Schultz (1997) and van der Bogert et al. (2000) also have shown that at least some so-called "shock veins" in meteorites may be associated with faulting/shearing as evidenced by displacements along them.

Finally, there is indeed evidence of thin melt films coating some shatter cone surfaces. Martini (1991) referred to this, and the work by both Gibson and Spray (1999) and Nicolaysen and Reimold (1999) has confirmed this. However, to make the assertion that such melt films correspond to the so-called "Type A pseudotachylite" is ill-advised. Temporal relationships between shatter cones and early formed pseudotachylitic breccia are not understood yet. Reimold and Colliston (1994), for example, showed that there is pseudotachylitic breccia in the Vredefort Dome that is overprinted by shatter cones, and Simpson (1981) recorded shatter cones overprinting a fault gouge on a presumed Vredefort-age fault. In addition, the process by which melt films on shatter cone surfaces are produced is not understood yet. Why would shock melting occur preferentially on shatter cone surfaces and, rather rarely, at intersections of Nicolaysen and Reimold's MSJS (multipli-striated joint surfaces that these authors linked unequivocally to the shatter cone phenomenon) in the interior of such a fractured specimen? One would expect that many other defect sites throughout a rock volume would render material susceptible to melting under strongly enhanced shock pressure. Nicolaysen and Reimold (1999) have shown that there are small displacements along MSJS throughout a rock specimen. Why could such a melt film on a shatter cone or MSJ surface not represent bona fide friction melt? What about the possibility that shatter cone surfaces represent extensional sites (extensional conditions developed after shock wave transition), in which case shock pressure would be low?

Fiske et al. (1995b) also claimed: (3) "High P polymorphs of quartz suggest that Type A pseudotachylite forms during compression and crater excavation". Apart from the controversial Type A classification (Reimold et al., 1992; Reimold 1995, 1998), there is no problem with shock veins containing high P polymorphs forming very early in the cratering process. The fact that some workers (Dressler and Sharpton 1997; also Dressler and Reimold 2004) have observed clasts in impact breccia that contain breccia veinlets resembling pseudotachylitic breccia is strong evidence that such a brecciation phase must occur early, likely during shock compression. And (4): "We have produced pseudotachylite-like material in shock experiments on quartz that may be analogous to Type A pseudotachylites". This statement may well be correct in that these authors produced similar melt to that experimentally generated by Kenkmann et al. (2000a) and Langenhorst et al. (2002). Whether they produced friction melt (pseudotachylite) or a shock melt, however, still remains to be proven.

Was this article helpful?

0 0

Post a comment