In my opinion those amphibole needles are formed by advanced metamorphic segregation of porphyrytic basalts in extensional or in shear zones. Porphyritic texture of protolith facilitates synmetamorphic grouping of mafic material in small-scale aggregates. Try to find any shear markers in the rock (maybe any sigma or delta clasts etc.).

Dear colleagues:

I came across your  Q &A relation and I am forced to say that this is a good or bad example depending upon the point of view that any remote-controlled image analysis without a proper database is meaningless. That is why I always try and convince those who raise such questions should provide the audience which as much as possible they can. Unless this is done it will be wasted time and the worse it will also discredit geosciences as a "picture puzzle". I think nobody can be really interested in a description like that.

There is a strong implication of nucleation and growth rate but anything further needs a proper database.

Best regards from

Harald G. Dill

Dear Prof. Dill

Thank you so much for your advice. I edited the description of the question. I hope that new information to be useful. 

All the best

Dear Mr. Miri:

a very good round-up now. In this case our colleague from Poland would have come to a different explanation.

Do not find fault with my critical comment but I am some kind of an "old ironside" in terms of geosciences and I feel we should do everything to strenghten the position of our discipline.

Best regards

Harald G. Dill

Dear Gentlemen,

I have specific habit studying amphibolites namely if the detailed description of the rock is not provided by the author, it means typical layered or banded amphibolite with dark amphibole+garnet-bearing and light plagioclase+quartz-bearing laminae. This is obviously wrong approach and such estimations are simply inappropriate.

Now, after modification of your question Mohammad, my point of view seems to be wrong because if you have marked no specific orientation of the needles, you have to go back to meso-scale observations in the field (if you have not made them yet), compare them to regional geological setting in the studied area and thereafter you can just guess the origin of those structures, however as Prof. Dill said, without any databases of f.e. mineral chemistry, you cannot say more.

Dear Maksymilian

I am sorry for previous incompelet description and thank you so much for your new suggestion. I have followed your researches and know that your are studying banded amphibolites.  You are right, but  unfortunately these samples are as debris and I can not find their relationship with deformation stages of the area. Chemical analysis show that they are ferro-hornblende just like the other samples in the area. Do you have any suggestion about effect of recrystalizatoin of usual hornblendes  to create the fan-shaped amphiboles?

Kind regards

Dear Mohammad,

Basically, each crystal (artificial and natural) can form in radially fibrous, sheaf-shaped aggregates. Even diamond and pyrite forming radiating aggregates. This requires a fast growth rate from one point in the direction of the fastest material supply through the fluid phase. Amphibole sheaves are characteristic of 'Garbenschiefer'. 'Garben' beeing the German for sheaves. Fan-shaped (sheaf-structure) amphiboles form frequently during greenschist-facies regional metamorphism.

The growth of sheaf-structure is not caused by recrystallization but formed post-tectonic with respect to the foliation of the host rock.

Best regards,

Guenter Grundmann

I agree with Mr Guenter that such highly elongated crystals/blasts had to be formed quickly and fluids played important role. However greenschist facies for fe hornblende is arguable . Greenschist facies is characterized by Si-bearing amphiboles(actinolite or tremolite Si > 7 apfu). Fe-hornblende is typical for amphibolite facies regional metamorphism. 

Your question about origin of those structures remains open for wider audience but saturation proposed by prof. Dill coupled with regional metamorphic segregation sounds possible. What is geological setting of the studied area? Does the unit undergo regional metamorphism? Under which conditions? Please search literature of the studied unit. This will be very useful to further study.

To be a little more precise, the crystallization of amphibole-sheaves would appear during greenschist-amphibolite facies transition, resulting in the replacement of the albite+actinolite pair by the plagioclase+hornblende pair.

Albite dissapears and is replaced by plagioclase, typically An17 (oligoclase) and actinolite disappears and it is replaced by alkali-bearing aluminous hornblende.

Please check the anorthite content of the plagioclase and the presence or absence of actinolite.

Exactly, plagioclases and their An content play crucial role in recognizing metamorphic grade if they are paired texturally with amphiboles. Concluding, further study is needed to provide valid interpretation about those structures.

Dear Pro.  Grundmann and Maksymilian

Thank you so much for you very useful comments.

You are right about composition of amphiboles and plagioclases, if fact these hornblendes have higher SiO2 and lower Al2O3 than the prismatic ones (more actinolite content). The plagioclases are mostly oligoclase in these samples also. 

The study area was metamorphosed during late Jurassic regional metamorphism event of west Iran because of Arabian plate-Iran microplate collision. After that, the metamorphic rocks were affected by intrusion of some igneous bodies and changed to hornfles near them. These samples are far from the igneous bodies but are in vicinity of some pegmatite veins. Overall all, the regional metamorphism sample in the area endured 580 oC and 5 kbar condition.

According to your suggestion, now I wonder if pegmatite fluids could have role here? 

Best regards

Dear Dr Miri,

Here is my answer to your possible "role of pegmatite fluids":

A hornblende garben genesis, that includes the formation of the remarkably thick hornblende garben schists, driven only, or influenced by the heat and fluids given off by the crystallization of pegmatites is unlikely in view of their very small size and very low heat capacity.

Best regards,

Guenter Grundmann

Dear Dr. Pintea

Thank you for your suggestion but spinifex texture can be found in ulteramaphic rock while the samples are meta-sedimentary.

Regards.

Hi Mohammad,

Your rock is reminiscent of the "garbenschiefer" described in the Alps.  In my experience, garbenschiefer fans can be seen parallel to foliation (XY) planes but not in any other orientation (i.e., they do not form fans in the XZ direction).  The fact that the amphiboles can splay in a fan at all suggests that there is not a strong shape-preferred orientation developing (i.e., no S-C fabric), and likely this is a thermal overprint with in-situ (retrograde?) growth unrelated to structural kinematics.  Typical greenschist metamorphic fluids are capable of growing such actinolitic grains.  Finally, I would not expect this texture to be the result of being cross-cut by pegmatites, which tend to be water-rich and quite low temperature (< c.600°C)--in the transition between magmatic and hydrothermal systems.  I agree with what Grundmann states above: pegmatites have small size and low heat capacity and they do not, i.e., cause extensive hornfels.

cheers,

Jason

Dear Prof. Grundmann and Prof.  Price

Thank you for your suggestions. I think that thermal overprint was a good way to form it as this phenomenon has been proposed for andalusite-fibrolite transformation in aluminosilicate bearing schist of the area. Now I suggest that there is an igneous bodies beneath this amphibolite layers with no outcrop.

Would you please introduce me a good reference for such a thermal overprint crystallization?

Best regards

Hi Mohammad,

When I say "thermal overprint", I'm imagining a static situation where there is no attendant deformation because the actinolite fibrolites are growing in multiple directions (no shape-preferred orientations). 

You can do some easy searches of this using scholar.google.com  Type in "thermal overprint andalusite" or "thermal overprint actinolite" and read through those papers. Some of the papers are even on Researchgate. 

You do not need to have a buried intrusion to get andalusite or actinolite; moreover, if you map out the zones of andalusite and actinolite, you should be able to tell if it's a regional metamorphic feature or a localized halo around an intrusion or, like in the Sierra Nevada, California, a screen (high-angle zone) of metamorphic rocks caught up within the larger batholith. in one location near Lake Isabella, California, I have observed andalusite partially replaced by sillimanite because of proximity to granodiorite intrusions. If there are buried intrusions in your map area (that are important for the thermal story) your rocks could show similar features.

cheers,

Jason 

Dear Prof.  Price

Thanks a lot for your new suggestions.

Best reagrds