I collected some samples from an orogenic belt in China. These samples are located in different layers of a group. Zircon U-Pb dating by LA-ICP-MS reveals that they have same ages. I used to interprete them as strongly deformed granites, however, some researchers consider that they are volcaniclastic.
Geological and petrographic setting:
The samples were collected from Neoproterozoic Wudang Group which have underwent low-grade metamorphism. The group is very complex and need more research. Debates still exist on the formational ages of Wudangshan. Zircon U-Pb ages of the tuff and volcanic rocks in the Wudangshan Group are ca. 750 Ma. The group is comprised by two parts: (1)the lower parts of Wudangshan Group are mainly meta-vocanic rocks, (2)the upper parts of Wudangshan Group are meta-sedimentary. The samples were intruding into the lower and upper parts. Wudang Group was considered forming during the breakup of Rodinia supercontinent or oceanic subduction. The deformation occurred between 220-250M, which is the result of collision between South China Block and North China Block.
Dear Wang,
I think you are right with an interpretation of strongly deformed granites.
Your micrographs clearly show fractured relics of K-feldspar and plagioclase interspersed with elongate aggregates of recrystallized quartz and foliated aggregates of neocrystallized mica like muscovite and new biotite. The plagioclase and K-feldspar partly have resisted the deformation. Recrystallized quartz and mica flowed readily, forming a mylonitic foliation. Porphyroclasts of plagioclase and K-feldspar show finegrained quartz and mica particularly in their pressure shadows, locally forming recrystallized 'tails'.
An important argument against the interpretation of "volcaniclastic rocks" is:
1) the lack of typical mafic constituents such as amphibole, pyroxene and olivine, and
2) the lack of embayed phenocrysts of quartz, that are relatively good indicators of volcanic source rocks.
Best regards,
Guenter
Dear Wang
You have a right nomenclature of rock as deformed granite. In your thin-sections, k feldspar(may be orthoclase) and altered sericite have 2/3 of total feldspar conten. Recrystallied quartz and mica are strongly foliated as poited out by Guenter. One of the fact that you collected in orogenic belt where collision anatectic granite formed and mostly these granitic rocks are S type magnetite series.
An important argument for volcanic rock is:
1) None of juvenile fragments such as pumice or scoria and lithic fragments of volcanic origin.
2) lack of showing typical volcanic rock characteristic features such as fine-grained or porphyritic textures
Regards,
Myint Thein
Dear Dr. Ruirui,
the micrographs give an overview of the mineralogical composition of a Kf-Plag-qtz rock which in itself is by no means indicative of a strongly deformed granite. Neither mineral grain shows a post-mineralic deformation. There are quartz bands with pressure shadows and what might be called rotation (?). But for a clear-cut judgment in one or the other way you have to correlate the potential stress-strain pattern of the country rock/ wall rocks around with that of your samples or consider the potential volcaniclastic rock in context with the surrounding lithological units. Are there any signs and features which support the idea of a derivation from processes like air fall, flow or surge deposition?
The bottom line is, you cannot take a decision based on micrographs alone, you must view these samples looking at them from a different scale at outcrop of even wider.
Best regards
H.G.Dill
Dear Wang,
I totally agree with a granite with strong indications of deformation possibly related to metamorphic processes.
Regards,
Alexandrina
Dear Wang,
Let me add my own interpretation to an important point:
If quartz shows recrystallization structure, then overprint temperatures have been reached by at least 370 degrees Celsius. This means the deformed granites have been recrystallized by low grade metamorphism. In this case, you can call them meta-granites. And your meta-granites are metamorphic rocks.
Best regards,
Guenter
Dear colleagues,
we are going to discuss about the likelyhood of a metamorphosed granite which is neither placed within a regional stress-strain pattern nor do we know anything about the P-T conditions based on facies-critical minerals of the country rocks. There is nothing said about special features or structures playing in one way or the other. The micrographs did not tell us anything about the P-T conditions we have neither F.I. data nor chemical data which would allow for the applications of , e.g., chemical thermometers such as thta of Huang and Audetat (2012). I assume that those who advocate for a volcaniclastic origin have also had reasons to do so. We should have more information rather than embark on a journey of speculations.
Regards
H.G.Dill
As previous colleagues
I think that to disccuss properly about these rocks it is necessary know more data; what are the metamorphic conditions? what is the thickness of each layer?. Photographs have some aspects that can be assimilate to a deformed volcanic rock (porphyritic features and a fine-grained matrix could be interpreted in this way) but to a deformed granite too.
Dear Wang
The features in your photographs could be found in a deformed granite or in a volcaniclastic. As mentioned above, you should look for embayed quartz phenocrysts or hexagonal quartz phenocrysts. At high metamorphic grades a metamorphosed rhyolitic or dacitic welded pyroclastic can look very much like a granite in grainsize and mineralogy. At Broken Hill, Australia we found relict volcanic textures in one quartzo-feldspathic gneiss, but others had none and zircon dating showed those gneisses to be granites emplaced soon after the enclosing sediments were deposited.
Stevens BPJ and Barron LM 2002. Volcanic textures in the Palaeoproterozoic Hores Gneiss, Broken Hill, Australia. Geological Survey of New South Wales Quarterly Notes 113, 1-22.
Stevens BPJ, Page RW and Crooks A. 2008. Geochronology of Willyama Supergroup metavolcanics, metasediments and contemporaneous intrusions, Broken Hill, Australia. Australian Journal of Earth Sciences 55, 301-330.
Dear Wang,
As an example of the 'embayed quartz phenocrysts' in metamorphosed volcanic rocks, above mentioned, I want to show the figure 4.75, page 282, from the highly recommended world-class book by Ron Vernon (2004).
Ron H. Vernon (2004): A Practical Guide to Rock Micro Structure. Cambridge University Press, ISBN 0 521 89133 77 (paperback).
Best regards,
Guenter
Dear Dr. Ruirui,
I would like to recall my demand, why not disclosing facts as to the geological and petrographic setting. This is an awful back and fro with no real progress made in one or the other direction. You are the "master of the game" and must play your role.
Best regards
H.G.Dill
i agree with your describtion. this should be highly mylonitized granites
To :Prof. Grundmann and Stevens
It seem that there is no embayed quartz phenocrysts. Besides, the present research show that the country rocks have only underwent low-degree metamorphism. Therefore, we can rule out the possibility of high-degree metamorphism.
I agree with Brian. It could also be a volcaniclastic rock and what we see as a groundmass could also be a recrystallized matrix. I have in mind some welded rhyolitic or dacitic ignimbrites. However, I would like to see more features suggesting fragmentation as I would like to see some xenoliths. None of these features are absolutely necessary but in the absence of any features suggesting fragmentation, I would still diagnose the rock as a granite or meta-granite.
A key feature in any pyroclastic rock is the presence of broken phenocrysts, recording fragmentation during explosive eruption. You need to took for such texture over several thin sections. From the present images, I cannot see any strong evidence for a pyroclastic origin.
I don't see broken phenocrysts but in an ignimbrite, as an example, the breakage could be minimal. I strongly suggest to look at evidences. Also look at the geological setting. To me, these rock really look like slightly metamorphosed granite. Wang, could you add more photos?
In fig.d, fractured grain of k feldspar rotated and dextral shear movement of the grain and indicate brittle deformation in Riedel shear zone. So, rock was deformed in the crustal deformation and not ductile deformation related to metamorphism. If so it may be fault rock effected by cataclastic dynamic metamorphism in the upper supracrustal fault zone.
Dear Wang
The features of the photographs show a mild deformation under low to medium temperature conditioned on granite. As you already discuss that the surrounding rocks have similar conditions. You must check it from the filed observations if there is any shear plane occurring, then you may see such type of characters. Filed observations is more important to confirm it.
Good luck, Vinod
Dear Wang,
Most welcome, we found similar deformation in Himalayan region too. In Bundelkhand craton, India, we also found similar behavior where shear zones are prominent with very low degree of metamorphism. Means you should correlate the P-T conditions from the surrounding conditions.
Once again good luck, Vinod
Dear Wang,
According to me this is minor deformed granite. I think mild evidence of shearing is also present (try to see the effects of shear zone in the field, hand specimen if any). It better to cut the rock perpendicular to the shear plane before making thin section. In this case you can observe the features more clearly. I dont think this photos are representing perpendicular section to the shear plane!!
the Photos look like metamorphosed volcaniclastic rocks, granite ca not be easily deformed, but in clastic rocks each grain and layer acts as discontinuity surface ( or week zone), so they are easily deformed and metamorphosed
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