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Most modern works (e.g. Nichols 2009; Allaby & Allaby 1999) seem to restrict the definition of ‘concretion’ specifically to formations in sedimentary rocks. I have recently come across a taxonomy of concretions in Crimes (1966), which includes concretions ‘formed as a result of metamorphism’. He gives little detail about formation processes but writes that ‘[m]etamorphic concretions have also received much attention and have been reviewed briefly by Barth and Ramberg.’ (I don’t have access to the cited references.) It seems plausible to me that the concretionary processes that operate in sedimentary rock would also be possible at very low grades of metamorphism, provided that some pore space remains. However, Crimes’ concretions are apparently formed as a result of metamorphism.

Do such metamorphic concretions have a place in currently accepted taxonomies of sedimentary features, or have they been reclassified as a different phenomenon altogether?

  • Allaby, A. & Allaby, A., 1999. A dictionary of earth sciences, 2nd ed. Oxford University Press.
  • Barth, T. F. W., 1952. Theoretical Petrology. New York.
  • Crimes, T. P., 1966. The relative age of some concretions in Cambrian sediments of St. Tudwal's Peninsula, North Wales. Geological Journal, 5(1), 33-42.
  • Nichols, G., 2009. Sedimentology and Stratigraphy, 2nd ed. Wiley-Blackwell.
  • Ramberg, H., 1952. The origin of metamorphic and metasomatic rocks; a treatise on crystallisation and replacement in the earth’s crust. Chicago.
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    $\begingroup$ I can understand this as being put on hold for "too general", "too lazily formulated" or similar, but I am honestly baffled as to why it's been closed for being off-topic. As Wikipedia puts it, "A concretion is a hard, compact mass of sedimentary rock formed by the precipitation of mineral cement within the spaces between the sediment grains." Please could somebody explain to me on what grounds sedimentary diagenesis would fall outside the scope of earth science? $\endgroup$ – Pont Apr 19 '14 at 21:53
  • $\begingroup$ I agree. While it could do with a bit more effort, this kind of close is going to scare amatuers away. Upvoting and voting to re-open. $\endgroup$ – naught101 Apr 19 '14 at 22:31
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    $\begingroup$ @naught101 if you want to re-open this, do more than just vote, please edit it into something worth opening. $\endgroup$ – casey♦ Apr 20 '14 at 0:33
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    $\begingroup$ @casey I have rewritten the question to focus it on the most promising part of the original (whether concretions are strictly confined to sedimentary rocks) and to provide some more thorough background. I hope that it will be considered on-topic in this form. As I said in my previous comment, I had no objection to closing the question, but a strong objection to deeming it off-topic. $\endgroup$ – Pont Apr 20 '14 at 8:19
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Not really. Concretions are features of sedimentary rocks almost by definition. Here's my go at a definition; it essentially presupposes a sedimentary rock:

Concretions are spatially discrete zones of above-average (compared to the rest of the rock) cementation by authigenic minerals. Common cements include calcite, quartz, siderite, and pyrite.

Concretions form during the process of diagenesis. However, diagenesis and metamorphism are really on a continuum of 'burial effects' — increasing temperature and pressure, compaction, changing porewater chemistry, and so on. One could almost call diagenesis a metamorphic facies. Concretions can start to form immediately after burial, and grow during burial as long as geo- and electro-chemical conditions allow, or until the pores are completely occluded. According to some workers (e.g. Bjoerkum & Walderhaug), total occlusion usually happens somewhere between about 150 °C and 200 °C. By this time, clay minerals have started altering and we are into low-grade metamorphism (zeolite facies).

Conversely, I think most so-called soft-rock geologists just lump all alteration into 'diagenesis' (I know I did until I started working on geothermal reservoirs). So, if you're prepared to explain yourself because it sounds weird, you could claim that concretions form in metamorphic rocks.

Footnotes

  • I've never seen one, but I don't see why you couldn't have concretions in volcaniclastic rocks (which I count as sufficiently like sedimentary rocks to warrant no more than a footnote).
  • If you were desperate to call something else a concretion — a mass of phenocrysts in a granite, say — you could always broaden your definition of 'concretion'. We geologists love arguing about the definitions of words!
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  • $\begingroup$ +1 Great answer. Concretions are indeed in sedimentary rocks by definition. A similar feature in another rock would probably have a different name. $\endgroup$ – Gimelist Nov 8 '14 at 11:55

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