The structure is mylonite, and it's formed by progressive deformation.
Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals, resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a classification based on the textural appearance of the rock.
Dynamic metamorphism is associated with zones of high to moderate strain such as fault zones. Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture.
The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the temperature and confining pressure determine the deformation mechanisms which predominate. Within depths less than 5 km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of breccia or cataclasite is formed, with the rock milled and broken into random fragments. This generally forms a mélange. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones.
Within the depth range of 5–10 km, pseudotachylyte is formed because the confining pressure is enough to prevent brecciation and milling and thus energy is focused on discrete fault planes. Frictional heating, in this case, may melt the rock to form pseudotachylyte glass.
Within the depth range of 10–20 km, deformation is governed by ductile deformation conditions and hence frictional heating is dispersed throughout shear zones, resulting in a weaker thermal imprint and distributed deformation. Here, deformation forms mylonite, with dynamothermal metamorphism rarely observed as the growth of porphyroblasts in mylonite zones.
The three pictures not only show a certain ("this") structure but a large number of different structures. The images represent two-dimensional sections through deformed and undeformed mineralization.
But, a detailed analysis of the relationship between crystallization and deformation of the individual sub-areas is only possible by examining representative thin sections and polished sections, including the third dimension!
The thin sections and polished sections must be taken from both the deformed and the undeformed mineralization. It is important that the entire mineral content must be determined, and then the internal (deformed or undeformed) structure of each individual mineral species must be clarified!
The statement of Khaliq Hussain "The structure is mylonite, and it's formed by progressive deformation" is by far too general. It may apply to certain small areas. But, as I said, that can only be clarified with thin section and polished sections.
Best regards,
Günter Grundmann
Dear Xuan,
It looks to me that your pictures document common deformation in carbonate rocks (limestones or dolomites) rather in shallow crustal conditions. Naturally, there was shear deformation connected with transtensional regime. Mineralized veins are filled by calcite ±siderite and sulfides. However, concerning σ1-σ2-σ3 - it is obvious that σ1 - maximal extension is parallel with veins orientation and σ3 - compression is perpendicular to the course of the veins, but keep in mind that it was mostly transtensional (± transpressional?) regime... and recent horizontal foliations can be result of subsequent tectonics... = there were operate multiple tectonic processes. Yes, one can say simply - there is "mylonitization" and/or cataclasis + brecciation. P-T conditions - hardly to say without microscopy + EPMA but something between 175 - 275°C and 100 - 250 MPa; since carbonates "flow" under lower conditions in some places it could be in the brittle/ductile transition.
Definitively, this deformation is pretty far from presudotachylite origin, because of different host rocks (here carbonates, and not granites or crystalline basement), and system was saturated by fluids what contradict friction...!
Cheers,
Milan
Dear Xuan,
Yes, generally it is "fault-fill vein" connected with extension, but probably this was last process. I guess that this deformation has a regional charater, and not only local one. You must know original bedding of these carbonates, and its relation to this last extensional foliation. All these structures you must see in the field.
Cheers,
Milan
******************
Dear Mr. Wang,
please find attached my sequential interpretation.
In conclusion, it is a complex retrogressive mineralizing process to be seen in some metamorphosed massive sulfide deposits,
with folding and emplacement of tight folds /foliation. The structures move from sheared mineralized structures into cut-and fill structures. The order of labeling marks the sequence of the structural and mineralizing development of the ore zone.
Glück auf!
H.G.Dill
Kind regards
H.G.Dill
Dear Mr. Wang,
as a field and economic geologist I used to describe what I can see at outcrop, in this case textures and structures and nothing else. That is what I did. I have no idea of the mineralogy in detail , the type of deposit etc. For such a tectonic analysis you want me to hand down a full-blown verdict, I have not the full-blown overview and my assessment could simply be called poking around in the fog.
This would be against my working ethics to drift away into speculation.
H.G.Dill
Dear Dr. Dill,
You did the right thing. Please forgive me for being so reckless. I apologize to you. I don't quite understand that' Tight isoclinal recumbent folds with quartz and Cu-Fe sulfides, footwall limb is eaten away by younger brecciation '. Can you further explain this sentence, or how do you judge the basis?
I apologize again and look forward to your reply.
Cheers,
Xuan.
My previous post has obviously not shown up. Therefore, I raise some questions again concerning the P-T regime and local differences related in time and space to the tectonic structures. A more precise statement about the mineralogy and lithology should also been posted. Everything what I could distil out of the images has been done.
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