Can you give more information about the regional structural geology of the magmatic body?

Dear Professor Heikal,

am I right that the black patchy staining is caused by Fe-Mn limonite ?

If so, please try and compare the sculpturing of a granitic landscape under various climates as a function of the lithology and cooling behaviour of granititc rocks. But more information would be certainly helpful in limiting the field of speculation.

With kind regards

H.G.Dill

Thanks for all. the albite granite is mostly stained by impregnation of Mn-Fe, and highly intersected by shear zones, trending mainly NNW-SSE, associated with mineralized quartz veins enriched with tin minerals. In fact, this feature is very rare within granitic exposures. In addition, these rocks occur near Marsa Alam town, Red Sea region of Egypt.

To me, it seems a feature of spheroidal weathering of granites where white minerals may be the plagioclase and the melanocratic minerals are either biotite or pyroxene, but presence of biotite is little bit doubtful as there is no release of iron due to weathering. Similar features are not unusual in granite country, suffering tectonic dislocations. 

Dear Mohamed,

It looks to me like similar circular to elliptical, concentric to eccentric

structures "Ring schlieren" that show us Barrie Clarke during Eurogranites field meeting in Nova Scotia. However, these were a bit more "melanocratic" but I believe that process is identical... See his paper:

http://www.sciencedirect.com/science/article/pii/S0191814113000187

Best regards,

    Milan

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Dear Milan,

Thanks a lot for your positive reply. I strongly think that your contribution seems to bet= the best answer, because you explain completely this feature as mentioned   in the field. Again thank you very much. By the way, the melanocratic minerals referred to only Mn-Fe impregnation.

Dear Mohamed,

Thanks for reply. I see sometimes in field, especially within felsic aplitic rocks some kind of parallel structural arrangement in various scales. Indeed such circular structures are rare, but common plane-parallel structures are often, hence these structures remind “bedding and/or layering” better saying “bedding/layering-like structures” some people described “magmatic layering” in such felsic aplitic/granitic rocks, what is naturally wrong and it are just planar flow structures or common schlieren structures…! Magmatic layering exists only in mafic rocks.

Best wishes,

Milan

Dear Milan,

Thanks for your good field observation on both mafic-felsic plutons.

Dear Milan,

Good day. Please write in brief the field significance of ring structural relevant to albitic rocks (in minor) and did you find this feature in RMs-bearing albite granits??

Dear Mohamed,

It is hardly to say… mechanical / rheological features of these circular or "ring" structures are well described in Barrie Clarke paper. However, there is a question – what do you mean “field significance” = each one granitic body is more or less unique… we know some normal and reverse zoned granite plutons with fractionated – felsic material in upper or bottom part of granitic body… but everything is depend on real erosion level… Naturally, there exist some composite granitic plutons where standard fractionation processes play minor role, because the contacts of various granite types are intrusive and neighboring granitic rocks can have pretty different age and origin… Generally, albitic granitic rocks could be product of fractionation and often are on top (copula) of comagmatic body, but on the other hand nearly similarly look pockets of new partial melts within various lithologies (including trondhemites in basic stuff)… And when there comes to mixing or mingling you have no chance to identify – “what is what…” without fine isotope works… What do you mean “RMs-bearing albite granites” – Rare Minerals? or Rare Metals? – wow now I see “Radio-elements-bearing minerals hosted by A-type granites” – this explain your interest in “ring structures” but these are totally different mechanism. You must take care and don’t mix “mini- to meso-structures of turbulent flow” with mega-structures of A-type granite ring complexes that have different origin… there is no chance to explain all problems A-type magmatism and especially its mutual hybridization by products of common I/S-type granite magmatism that I discussed at the beginning!

Best regards, Milan

Dear Milan,

Thank you very much for your explanation.

Kind regards, Mohamed

Dear Milan,

Good day. Please send to me a copy of ring structural paper via my email: [email protected]

Kind regards

Mohamed

 I have not followed the question, or for that matter the comments. 

Are you talking about the Fe-Mg distribution or the ring-type structure in the lower middle? You have been provided good information by Prof. Harold Dill on the former.

The ring-type structure can be secondary, such as exfoliation, spheroidal weathering, conchoidal fractures or leaching rings. But it may, instead, be orbicules formed during solidification of the magma. You have to carefully re-examine them to decide. I have seen plutonic rocks displaying local layering as well as orbicular structures. The Kohistan batholith in northern Swat, Pakistan, has one of the best orbicular structures (please see Symes, Beaver and Jan, 1987, Mineral Mag, 51, 635-647, on the ResearchGate). 

Best, Qasim