I think it is an alteration structure. See attached paper

The greenish yellow lichens on the surface of the rock clearly prove that an alteration has occurred by (rain or dew) water. The ocher yellow alteration products are very likely to be iron hydroxides.

thanks for your response, ¿Will this alteriacion can be as continuous? 

Martin Adrian Menzies: I have not found any evidence for rheomorphic ignimbrite, but I've found, under these outcrops, current ripple marks; Perhaps, the presence of water modifies its behavior.How I can show this?

here I upload another picture...

Similar round-shaped forms are common of silicic volcanics of Kazakhstan belt (Devonian), Balkhash-Ili belt (Carboniferous to Permian) and Okhotsk-Chukotka belt (Cretaceous). They occur both in lavas and ignimbrites, and sometimes host voids with nice quartz/chalcedony/zeolites (see photo attached). Indeed, they may be spatially related to contacts between volcanic and epiclastic rocks, hence water could contribute to their formation.

Unfortunately, I never saw any publications on these matters. In my opinion, such forms may be related with the initial stages of devitrification. At least, flow banding penetrates these spherules, suggesting that the latters were formed in a solid rock. The hydrothermal origin is unlikely because glass is too unstable, and should not survive in an altered rock.

Hi Juan--

If you observed this outcrop at the base of your ignimbrite, this is the basal vitrophyre of the ignimbrite deposit. A vitrophyre forms when a hot volcanic deposit (e.g. ash-flow tuff, rhyolite flow) comes into contact with the relatively cold surface of Earth. This sharp temperature gradient results in rapid cooling or quenching of the lowest part of the volcanic deposit, forming a glassy basal vitrophyre. I have observed vitrophyres up to 1-2 m-thick for ash-flow tuffs that were 100's of m thick. I have observed more chaotic and thicker vitrophyres (5-10 m-thick) in rhyolite flows, which can sometime cannibalize their own basal material during viscous flow.

The spherical feature you show in your photograph is called 'spherulitic texture'. This is common to the lower portions of ash-flow tuffs (ignimbrites). I have observed spherulitic texture in the black basal vitrophyre, but have also observed it higher up in an ignimbrite deposit, above the basal vitrophyre.

I suggest looking at this book for detailed discussion of both 'vitrophyres' and 'spherulitic texture':

Cas, R. A. F. and J. V. Wright (1987). Volcanic successions, modern and ancient.

Here are 2 papers I have authored about an extensive Miocene ignimbrite in the Gulf of California, which commonly displays a beautiful vitrophyre:

Bennett, S. E. K., et al. (2013). "Transtensional rifting in the proto-Gulf of California, near Bahía Kino, Sonora, México." Geological Society of America Bulletin 125(11/12): 1752-1782.

Bennett, S. E. K. and M. E. Oskin (2014). "Oblique rifting ruptures continents: Example from the Gulf of California shear zone." Geology 42(3): 215-218.

I also attach 2 photographs. The first photo shows a typical vitrophyre as the base of the Tuff of San Felipe. The second photo shows a close up of small spherulites in the vitrophyre of this tuff. Other locations of this tuff displayed spherulites up to ~5 cm in diameter.

I hope this information is useful for your interpretation of your ignimbrite!

Cheers,

Scott Bennett

U.S. Geological Survey

Article Oblique rifting ruptures continents: Example from the Gulf o...

Article Transtensional Rifting in the Proto-Gulf of California Near ...

Hello, thank you very much everyone for your answers.

Petr Tikhomirov: I thought it was a litofisa at first, but, inside these "balls", i found the same ignimbrites. That left me puzzled.

Scott Edmund Kelsey Bennett: I agree with your interpretation, I see in your picture that the spherulites are very small. In my case, if these "balls" were spherulites, are 10 cm in diameter. This seems very strange, besides I do not see a radial growth. Anyway I think you're right. 

look at this picture that attached, they are quite large.

note: thank you, I hope to get the degree of geology, Greetings from Patagonia.

Stefan Kraus: indeed, it is not the dominant part. Perhaps, these "balls" are formed only by differential cannibalization. 

Where I can read about the process of cannibalizing?

Thanks!!

Your photo resembles some pillow  structures I observed years ago in similar rocks in the Trans-Pecos volcanic field, West Texas, United States.  You might try Googling "Trans-Pecos Volcanics" and "Pillow Stuctures" and see if any references pop up.  Good luck!

Hi Juan--

The process of 'rheomorphic flow' occurs as viscous flow of the ignimbrite during or shortly after deposition. Rheomorphism can recycle (or 'cannibalize') any stratified levels of the deposit that initially formed through late-stage folding and even brecciation. This process usually requires hot and/or thick ignimbrites. One observation that could support rheomorphic flow is to see contorted or folded zones of pumice fiamme, indicating initial compaction/flattening/stretching, followed by folding via rheomorphic flow. Rheomorphism is usually observed near the base of an ignimbrite deposit.

Your new photograph (with the green backpack) appears to show a large boulder of vitrophyre material, rather than a huge spherulite. This could be due to rheomorphism. Were you able to observe and document chaotic pumice fiamme or brecciation of the welded ignimbrite near this intriguing feature?

It is important to note that a vitrophyre does not always form as a continuous, tabular sheet (e.g., as I show in my first photo). Depending on local conditions of temperature and paleo-topography, vitrophyre formation can be quite heterogeneous in thickness along strike.

Other spherical features that form in tuffs are 'accretionary lapilli'. However, you would want to see fine/laminated concentric ('accretionary') rings within the spherical features. I am not familiar with reports of accretionary lapilli forming within (or near the base of) a pyroclastic density current or densely welded ash-flow tuff. The black/red concentric zonation in your first photograph appears to be differential alteration of spherulites, rather than accretionary lapilli or a mix of non-vitrophyre and vitrophyre components of the ignimbrite.

Chapter 8 in the 'Volcanic Successions' book by Cas and Wright discusses the process of rheomorphism and provides additional papers to look into.

Saludos,

Scott

Excellent points about accretionary lapilli, Stefan. I completely agree. I have never seen or heard of accretionary lapilli in an ash-flow tuff. The points you bring up are all excellent reasons why!

Cheers,

Scott

Hi everybody,

Here are examples of spherulites from rhyolitic lavas of the Okhotsk-Chukotka belt (NE Asia). Actually, the size of spherulites may vary widely, from ca. 0.1 mm to as large as 0.5 m. Their radial texture is sometimes clearly visible but commonly obscured by later re-crystallization. I do not insist the round-shaped fragments on Juan's photo have the same nature but they look quite similar to what I observed.

The large spherulites which do not affect flow banding are definitely post-magmatic. Their relation to the process of devitrification is most likely because they are frequently present in rocks which still contain a lot of fresh glass. The question is, what controls the devitrification textural patterns which may be so different (spherulitic, felsitic, poikiloblastic etc.). Perhaps quenching rate and water content could play some role.

Huan,  ask  this question about ignimbrites to good specialist, as I know:

Cyril Pshenichny,   [email protected]

                                    [email protected]

Hi to all,

Sorry to interupt...

Stefan thanks for the great answer. Just a short questuion, is it still possible that such formed lapillis are deposited into sea. If the volcanic vent is close to shore?

And can they be transported without breaking to deeper areas?

Thanks,

best to all,

Duje