How granitic melts strengthen the crust - resolving the "pegmatite paradox"
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- Опубліковано 23 чер 2023
- Part of The Shear Zone Channel. Granitic melts are commonly proposed to act as weakening mechanisms for deforming continental crust, especially deep in mountain belts. And geologists finding pegmatite, coarse-grained granite sheets, in deformed rocks in outcrops cite these as evidence to support this idea. But many such deductions are misleading... Follow Rob to a spectacular field location on Scotland's north coast, building a counter-argument - in which the intrusion of melts that crystallise pegmatites have only served to strengthen deforming crust.
#granite #geology #NC500
Pegmatites are amazing rocks. I’ve been working on them for 10 years and I’ve still learned something. Thanks Rob 🙏
Yes indeed - there's always more to learn from the field... and digging into the microstructures should yield further insights! Thanks for the comment!
Watch 'hangman 1128'.
Superb lecture by an extraordinary professor. Thanks very much Dr. Butler
Thanks - glad you liked it.
Excellent. More videos please!
Big thank you for this lecture. Keep on rocking!
Another excellent video Rob. Cheers!!
Thanks again Rob for these video series; I knew the paper, but seeing the outcrops is (would) be way better!!!
They're great outcrops - well worth the trek to the far N of Scotland....
Thank you for this... I learned a lot :-)
Outstanding stuff Rob... I learned many new details of the mechanics etc. I found the competant boudinaged block rotational section particulaly re-enlightening, and was right back in the lab with thin sections and Mark Piasecki explaining how rotational garnets in schist were just like..." er err err...A chicken! Dropped into a large pot of porrige and rotated around, like stirring". On a more serious note: Is it likely that the competance of the country rock would be, locally at least and temporarily for sure, be made less relatively competant even more so, by the pegamatite loosing water via solid state diffusion into the country rock as the rind forms? A kind of 'hot steaming', but not on a truely metasomatic scale?
Thanks - glad you enjoyed the video and that it brought back some fond memories. Interesting idea about the possibility of metasomatic weakening of country rocks... there's no evidence of this at Torrisdale in the form of halos around the pegmatites as might be expected... The water (and other fluxes) apparently concentrates in the residual melt during fractional crystallisation - which facilitates the break-outs of apophyses ....
Very inspiring video. Just curious is it possible the deformation in pegmatite only occur in outcrop scale rather than go into mircoscopic scale?
@@tombear4597 Not sure I understand your question.... but glad you liked the video!
Excellent presentation...comprehensive! Thank you. However, one has a question: has there been studies of how much time it took to make the folds, individual folding as well as complex foldings...was folding rapid, say, in minutes to hours, or, in days to weeks to months? Thank you once again.
A critical question... personally I think we as geologists traditionally underestimate strain rates for ductile deformations... the implication here is that folds develop quickly - while residual melt remains... hard to know how long that is but presumably would (need to) be months... so this time-scale would also apply to the strain rate in the surrounding rocks too.... thanks for the question!
Accumulated strain is accommodated in discrete 'instantaneous' steps in the brittle surface zone of the crust by faults. What happens at the interface of the brittle and plastic zone of a fault? Surely the very top of the plastic zone is going to experience high strain rates, decreasing with depth until it matches the average movement rate.
Amazing natural art works !
Great video on these geological features.
How can you date the deformation of the Pegmatites? Key minerals?
Is there a general strike direction of these pegmatites? Even if they have been rotated 180 degrees?
Thanks again for all the wonderful geologic data.
There may be the odd monazite in the pegmatites (I've not looked) and the country rocks have garnet and micas. Need to take great care what "event" that's being dated. The pegmatites strike generally a few degrees anticlockwise of the strike of foliation in the country rocks. See map in the paper (Butler & Torvela) flagged in the outro. Thanks for the question!
Amazing video! I have a question - do the different cross-cutting relationships of the pegmatite stringers with the foliation of the country rock (i.e. earlier stringers are entrained and later stringers cross-cut the foliation) imply very very fast deformation rates? Especially with the rapid crystallization of the margins of the pegmatite to form those competent rinds - it seems like everything is happening very quickly.
It's a good question. My current thinking is - yes strain rate is fast, but the crystallisation can be pulsed as fluxes concentrate in the residual liquids - so maybe the lozenge pods remain part liquid for a while. It does mean that the pegmatites could act as a tectonic speedometer....
Simpler explanation, watch 'hangman 1128'.
Question? These Pegmatites show feldspar and quartz mineral assemblies. Are there micas and other minerals?
Pretty much everything is quartz and feldspar.
Thanks for the interesting video! How high deformation temperature you think was at the timing of intrusion? Was it not too high to affect the rheology of the contry rock?
In my study area, I also observed boudinaged but internally not deformed pegmatite surrounded by highly foliated crystalline marble and calc-silicate rock. Also this calc-silicate rock has grossular-rich layer, diopside, and wollastonite, which may show high-temperature contact metamorphism. These observations led to the following idea: pegmatite supplied enogh heat and fluid for HT metamorphism and deformation (evidenced by prism slip of Qtz) of the calc-silicate rock and it might have been near the timing of intrusion (before the intruded rock cools). But I'm not sure the pegmatite was hot enough. Sorry for suddenly asking about my research.
Well, if the pegmatite melts were undercooled, I'd reckon somewhere around 500C for the melt. The melt volumes are small compared with country rock so the thermal budget of country rock is, I'd a have thought, unlikely to be significantly impacted...
See previous comment... I think the observation of deformed intrusion shapes but undeformed internal textures is really common for pegmatites... Probable that in your case I'd reckon that fluid (water) diffusing from the melt into wall rocks is more likely as a mechanism promoting metamorphic reactions in the country rocks - rather than supplying excess heat.... is your reaction hydrating or dehydrating...?
@@robbutler2095 I also agree that it was a metasomatism caused by fluid from the melt. Actually I observed only minor talc and no tremolites. Most of these hydrous minerals might be changed to diopside + water (presumably under higher temperature condition). It seems necessary to further understand the role of fluid. Thank you for your kind comment!
Watch 'hangman 1128' . Simpler explanation.
Thank you, amazingly beautiful outcrops. What I saw, it seems to me that the pegmatite bodies are post-deformation. This explains why they develop at the intersection of deformation lines and fold locks. Those. boudinage and shear deformations earlier than the intrusion of pegmatites. It is unlikely that pegmatites formed at the peak of metamorphism.
Thanks for the reflections.. But I'm not sure what you're meaning here. The pegmatites and their apophyses are clearly deformed in shape... folded... regardless of how you may interpret the "boudins". Why is it unlikely that pegmatites were intruded at "peak" (?syn-kinematic?) metamorphism... Good to have these discussions...
Why do I say that pegematites were not formed at the peak of metamorphism? Very simply, when metabasites melt at the peak of metamorphism, tonalite melts are formed. Pegmatite magma is low-viscosity and transports well through polyphase environments. In addition, such compounds (quartz + feldspars + plagioclase) can already form metasomatic bodies. In general, I have not heard any arguments in favor of your formation scheme (i.e. that the pegmatites are syndeformational).@@robbutler2095
Pegmatites are usually the last gasp product of highly-fractionated post-orogenic granite intrusions. Shearing here has obviously been post-intrusion
Watch 'hangman 1128'.
excellent sir, i would like to know how rare earth elements are included in pegmatite vein . the pegmatites found in china are rich in rare eart elements, but not in many parts of the world . why ?
Thanks for the question. Actially rare earths are not uncommon in pegmatites (products of highly fractionated magmas) globally. Check out: ua-cam.com/video/OTFITiX3ev4/v-deo.html
@@robbutler2095 thank you very much for the link sir.
Watch 'hangman 1128'. Occam's razor.
انا لا افهم لغتك، لكنني مهتم وغير متخصص بعلم الجيولوجيا. هل هذه تكوينة بغماتيت. اريد ان استفيد من نظام الترجمة
Google translate will (approximately) translate from audio....
Watch 'hangman 1128'.
awesome. absoluetly interesting and enjoyed throughly