Hello Ajay, one potential way of distinguishing between the meta-shale and meta-tuff could be a rather simple petrographic analysis of zircons in these rocks. A volcanic tuff would usually contain sharp-edged, fresh zircon crystals whereas I'd rather expect a certain portion of transport-rounded zircon grains in a fine-grained clastic rock. How strong is the metamorphic overprint? If that's just greenschist facies or so, the original petrographic characteristics of the protolith zircons might still be visible under the optical microscope and/or SEM (e.g., rounded versus angular zircons), even if they have acquired metamorphic rims. Just an idea, though.

Hi,  Depending on metamorphic overlapping this could be a very difficult issue. At lower grades check for corroded  quartz crystals, partially fractured quartz crystals , flame-like groups of filosilicates (old glass...)... Looking for the most grainy sections of the shales may help in your research... good luck!

It may help if you can do geochemical analyses; shales should have a higher aluminium saturation index (Al2O3/(Na2O+K2O+CaO)) than tuffs, and likely higher K2O/Na2O ratios too. If your tuffs are weathered before being metamorphosed, this trick won't work anymore though... The zircon approach suggested by Martin sounds like a good idea, but it might be difficult to find zircons of sufficient size in a real shale (which you could then use to argue it's a shale rather than a tuff, perhaps).

Dear Ajay,

The easiest way to distinguish between metamorphosed shale and volcanic tuffs is to study thin sections of these rocks, to study the mineral paragenesis, the microstructures and the relationship between crystallization and deformation. The interpretation of thin sections of course requires some experience. Nevertheless, an excellent overview of the main metamorphic rocks and their structures can be found in the book

'Atlas of metamorphic rocks and their textures' Paperback – 1990

by B. W. D Yardley. This book is highly recommended.

If you like, I can also show you some representative photomicrographs of thin sections from my collection, and I will deliver some explanations.

Best regards,

Guenter

Thanks Dear Guenter Grundmann 

I would like your valuable of your suggestion, if you would send me some representative photomicrographs  of thin section that will be more helpful for my research and that is good for me.

Dear Ajay,

As promised, I am sending you a few photomicrographs of shale, tuff and the metamorphosed equivalents from my own thin section collection. In addition, I have included some captions.

If you still have any further questions, do not hesitate to tell me about it.

Best regards,

Guenter

Dear Mr. Singh,

there have been some remarks as to the discrimination between shale and metatuffaceous rocks or tuffs. I see one basic problem in your question. A shale is an argillaceous sedimentary rock which has not yet reached the very-low to low-grade stage of regional metamorphism, e.g., Jurassic Posidonia Shale, otherwise we have to speak of a slate or schist. So the metatuffaceous part of the story is no longer of impact on the denomination of the pyroclastic /clastic rock. All the hallmarks of fall, flow and surge deposits which are shown in textbooks like Fisher and Schmincke (1984), and with regard to claystones, mudstones, and siltstones dealt with in Dietrich & Skinner (1979), Potter, P. E., Maynard, J. B. & Pryor, W. A. (1980), Stow, D. A.V. (1981), Weaver, C.E. (1990), Blatt & Tracy, (1996), Schieber, J.; Zimmerle, W.; Sethi, P. (1998), MacQuaker, J.H.S. & Adams, A.E., (2003) Potter, P.E.; Maynard, J.B.; Depetris, P.J. (2005), Aplin, A.C. & MacQuaker, J.H.S. (2010) will be applicable and will help you come to grips with your problem. At the moment it is hard to give you a more specific answer;  neither a chemical nor a mineralogical approach taken at his stage will help you get out of the drink. You must fine-tune your question so that the partner of the discussion will find out what type and chemical composition can we expect in terms of the magmatic source and what is the environment of deposition like.

Best regards

Harald G. Dill

Very wise reply form Harald!

In old volcanis rocks, with some metamorphism, I've found very usefull  the book  "Volcanic Textures: a guide to the interpretation of textures in volcanic rocks", from McPhie et al 1993 (but I guess a 2005 edition is also around).

Good luck!

Juan

Indurated shale partings visible under micro-photographs are the best way to differentiate.

The inputs and citations provided by Harald Dill are rather comprehensive.

From your question and background I assume you are in a dilemma in context of the horizons from a Proterozoic metasedimentary sequence in India, mostly affected by lower greenschist metamorphism.

I have also encountered a similar problem in several of these sequences which were traditionally marked as 'shales' or meta-argillites. The keys to identifying the original rock as being a shale v/s tuff is (a) the accessory mineral crystallanity & paragenesis [zircons help, but so does rutile and palagonite - if present]; (b) bedding plane patterns (different from the foliation superimposed on the rock); (c) development or absence of chloritic flakes along the fissile planes would indicate a sedimentary rather than volcanic origin (d) whole-rock chemistry in terms of higher Al2O3 presence in the sedimentary rocks (e) trace element chemistry.

Having said that, the toughest problem comes when one looks at reworked argillites derived from a tuffaceous / volcanic source rock. The interpretation can then be really biased by individual choices, with no clear answers available as far as my knowledge goes.

Multivariate Geo-statistical property of shale and tuff are quite different and your goodself  can take advantage of this.

Dear all 

I am getting high Al2O3 and CAI ~77-80 and ASI (Aluminum saturation index ) also high but it is giving 2.9Ga age, three population present is there. Because of so old meta-tuff/shale, therefore it could not preserved pronounced features. That three population age may be due to tectonic activities and metamorphism. Tuff would also high CIA because of so long time has been spend in open system.

Now, anyone can clear the doubts what it may be?  

You can consult the book ''Pyroclastic Rocks''

Authors: Fisher, Richard V., Schmincke, Hans-Ulrich.

By petrographic study you can easily differentiate between shale and tuff.. 

Do you have any plagioclase laths in the rock? Can you provide a description of them. If present, their form, geometry, borders of crystals and reaction rims if any would be reliable inputs. Compare with the descriptions given in Fisher et al text for further clarification.

Dear Ajay,

How high is your 'high' ASI?

How are the zircon age populations distributed over the morphological zones of the zircons? If younger ages are a result of metamorphism, then you may expect to see them as rims on the older crystals. More importantly: do other rocks from the same area show the same three age populations, so you can be sure that they represent metamorphic events?

In petrographic analysis of shale and tuff my advise to do MSG (Maximum Grain Size Analysis). It will help a lot. It is diagnostic property of shale. 

Buena fran! creo que la textura te puede ayudar caleta. Esperaría que una toba metamorfizada tenga su masa fundamental con alguna textura de desvitrificación, dependiendo de la composición (felsica generaría felsofídica mientras que un vidrio basaltico generaría colores pardos asociado al cambio textural puedes revisar McPhie). También buscaría concentraciones de minerales metamórficos, estas podrian ser vesiculas del protolito. En metamorfismo de rocas clásticas de grano fino esperaría sólo cristales equigranulares poligonales, también me fijaría en la composición y grado metamórfico. Podriai mandar fotos [email protected]