Is this thin section volcanic or plutonic?

Dear Ms Hamideh,

        Does the rock break with a conchoidal fracture in the non mineral portions? If so,it is an indication of volcanic character due to the glassy matrix. If the rock is visibly altered it may give off a clayey odor when dipped in distilled water. Please check the mineral to glass proportions. It may indicate the rock to either be volcanic or subvolcanic (merocrystalline).Was the thin section photo taken in PN or XN condition? Glass, if present is distinctly extinct (isotropic) in XN condition.  It is also important to reexamine the outcrop for further details, for example, the wall rock - igneous rock contact are commonly merocrystalline with earlier formed mineral clusters.

Hope this helps.

M.Jayabalan.

Dear Mr Baghban

Thanks a lot for your valuable replay.

Regards

Dear Mr Manickam

There is not glass in the Matrix and matrix is granular with very small grains.Also this rock is pebble.

Thanks for very valuable replay.

regards

Ms Hamideh

  You are welcome.

        M.Jayabalan.

Dear Ms Hamideh,

             Have you identified the type or species of matrix minerals present?  If the matrix is confirmed anisotropic with fine grained granular minerals,this rock 'pebble' is from a weathered plutonic rock. 

                         Regards,

                         M.Jayabalan.

Hello Miss Zeinalian

Field observation is very important to discriminate between plutonic and volcanic rocks. Based the handspeciment and this section I believe this is subvolcanic rock.

with best regards

MH karimpour

Ferdowsi University of Mashhad

University of Colorado Boulder

Dear Ms Hamideh 

first of all, the quality of both photos is evil. So can you show other images?                  

In any case, the hand specimen is very altered and it is difficult for me recognize which kind of crysts (or suspected crysts) are present on the surface of the sample.                

However, the presence of that aggregate of crystals and the presence of another xenocryst visible in the upper left side of the photo, make me think that your sample is a volcanic rock made up by a "likely" crystalline groundmass, while the xenocrysts were inherited from the magma chamber host rock.

For this reason, I think that Dr. Saeid Baghban could be right. 

Best Regards

Fabio Mazzeo

porphyritic in texture, but altered or weathered

In my opinion it is a plutonic rock. No doubts

Hi Hamideh,

It is a porphyritic igneous (volcanic) rock containing different phenocrysts (quartz, feldspar,  pseudomorphs of seladonite after amphibole and / or pyroxene) in a fine-grained groundmass. In this case it may be a strongly altered andesitic porphyry.

Please see and compare the attached illustration of a rhyolitic porphyry.

Best regards,

Guenter

Hi I will not be able to identify all those minerals mentioned by Dr. Guenter from a photo micrograph. As the scale you have mentioned is 4.2 mm (base of the photomicrograph) It appears to be a volcanic rock.  If you can send a thin section I might be able to identify and also name it.  knrao

Hi Hamideh, The basic aspect if your specimen is the drastic difference is the grain size of the minerals, It looks like a porphyry and is probably a hypabyssal rock that formed close to the surface.  It differs from granite porphyries in its mineralogical composition. The fine-grained groundmass is more than the amount of so-called "phenocrysts". The groundmass is is a "chilled" portion of the entire rock. You need to determine the mineralogical nature of the groundmass and comnpare it with the so-called "phenocrysts.". If they are about the same, then it is probably a porphyry. Best wishes, Paul.

The two photos give me the possibility to presume that it is a volcanic to subvolcanic acid to intermediate rock trachydacite to trachyandesite. The rock is altered. All the mafic minerals in the agglomeration of small phenocrysts (each one less than 1 mm) are replaced by chlorite or seladonite (supposed by G. Grundmann). The ground mass is holocrystalline, but it could be the result of a recrystallization during the alteration process. Around the agglomeration of phenocrysts you can observe a halo of fine grained groundmass that probably is the result of a reaction of the agglomeration (not in equilibrium) with the host melt. If the secondary green pale mineral is seladonite it will be most probably a volcanic rock altered in a subaqual milieu.

Best regards

R. Nedialkov

It looks like a plutonic xenolith in a volcanic rock to me.

This groundmass texture is certainly most typical of a volcanic rock, but just a reminder that mineralogy and texture relate to the competing affects of composition, cooling rates and volatile content. This texture could still emerge from magma intruded into cold wet sediment at depths of up to 10 km... It is most likely volcanic, but only field relations can make that clear.

I agree with James S. Beard's opinion

Dear Hamideh,

I agree with David Kitchen that only field relationships can make it clear. Such porphyritic rocks can be found either as volcanic or subvolcanic, i.e intrusive. The thin section is suggesting what Jemes S. Beard mentioned, a plutonic xenolith is a portphyritic groundmass. Also Guenter Grundmann is right saying that the thin section who looks green is "a fine-grained groundmass. In this case it may be a strongly altered andesitic porphyry". Normally the the condition for rock alteration suggest an intrusive environment, but sometimes this can be seen in volcanic rocks close to a hydrothermal system. So, my suggestion to you is: try to find the place the rock was collected and you will know the correct answer.

Ioan S.

To me, it looks as an intrusive xenolith (or magmatic cogenetic enclave, by "remote sensing" from Parma, Italy, it's not possible to distinguish between these two hypotheses), trapped within a melt. Unfortunately, I'm not allowed to enlarge two times the first photo (clicking on it several times, the system always enlarges, and overlaps, the example of porphyry posted later). Also the microstructures point to a magmatic "in depth" sequence of crystallization. It seems that the surrounding matrix (not able to enlarge again the photo) gets finer towards the margin of the bigger crystals, suggesting something very similar to true "chilled margins". So, i will prefer the hypothesis of a xenolith l.s. (causing thermal consequences in around), rejecting that of a glomerocryst. Whether it should be a cogenetic enclave or a xenolith, I guess that only microprobe work can give answers

The rock really seems volcanic, but as said by David, check field relations carefully. The sample could also represent marginal, rapidly cooled part of a pluton. Cluster of crystals in the micro-photo seems as an inclusion of plutonic rock, may-be a xenolith as James said. Its composition (altered mafic minerals, plagioclases, biotite, minor quartz-if it is not secondary?) points toward granodioritic to monzodioritic rock. You have to compare its composition to the surrounding volcanic rock to see, if it is the same. If there are some bigger inclusions or xenoliths in the rock, you can compare their composition to the inclusion in your figure to get an idea what it represents. Some geochemistry would be of help.

It appears that many have missed the information provided by Ms.Hamideh Zeinalian  that the sample is that of a pebble.   In the light of the "field (mode of) occurrence" of the sample all possible interpretations like a xenolith of plutonic rock in a volcanic rock, attempting to compare its composition with surrounding volcanic rocks etc., etc., etc. are interestingly amusing.  However, the hand specimen photograph does not look like a pebble.  Hope some more hilarious discussions will be forthcoming as to whether it is a pebble or not  and whether it is picked up from a beach or from mountain foot hills etc. Dr. K.N. RAO

The hand specimen tends to be a fault breccia, most probably, was collected from shear and/or fault zones, I believe that!!

It seems to be a volcanic sample, and phenocryts in thin section was possibly reacted with its surrounding (magma?), and giving a reaction zone around phenocryst

I agree with James S. Beard's opinion.

Dear Ms Hamideh,

         I am assuming that the 'pebble' specimen was previously rounded prior to breaking for sectioning. It would be best to go back to the field and properly assess the provenance of the 'pebble' or 'fragment' (in the upstream direction). If the 'pebble' was originally angular, it might suggest the parent rock to be nearby or may it may be a residual fragment above or around the parent rock. I think we may get a factual conclusion in the field.

                                                Regards,

                                                  M.Jayabalan.

As shown by the hand specimen, the rock is fine-grained and porphyritic. Therefore, it is most likely a hypabyssal rock. The thin section was prepared from a part of the rock in which there is high concentration of phenocrysts.

Dear Mrs. Zeinalian,

both images are somewhat contradictory as to the outward appearance of the hand specimen and the thin section. The thin section shows a glomeroporphyritic texture as far as the feldspar is concerned. If the dark parts of the mineral aggregates are different from feldspar and not only a type of feldspar altered by hydrothermal processes, the texture has to be described as cumuloporphyritic. These textures are common to basalts. The reddish rock slap hardly can be classified as a basalt. It has a color common to dacites or andesites. So the matrix should be investigated in more detail to get a full-blown picture of the mineralogical composition. There are dark green mineralisations filling in a stockwork-like manner fractures in the rock. I suspect of being an epidote-enriched end member of the epidote-clinozoisite series. This mineralisation is indicative of post-depositional fracturation and hydrothermal alteration. I am reluctant to go any further as it would be mere speculation, in view of the resolution of the images, which does not allow any precise mineral identification.

Best regards

H.G.Dill

Dear Hamideh

In my opinion it is an altered  volcanic rocks based on its glomeroporphyritic texture. I recommended that you if it possible prepare a new thin section from unaltered part of it.

with best regard

Dr. Morovvat Faridazad

Sahand University of Technology, Tabriz, Iran

i think it is volcanic rock.

It is a volcanic rock with glomeroporphyritic texture or with an inclusion

Regards

It is a volcanic rock with an inclusion

Many thanks for all answers.

It is rather volcanic

     

Seems to be volcanic with chilled margins (no crystals) and crystalline center.

Volcanic rock

volcanic rock may be cut by another vein

The thin section appears to be that of a volcanic. Probably the peripheral portions of the photograph are out of focus.