Mastodawn

John Faithfull🌍🇪🇺🏴󠁧󠁢󠁳󠁣󠁴󠁿🧡 ✊🏼✊🏿Jul 25, 2023

#DogWalkingGeology from this morning. Here's a lovely wee layer of pale blue elongate kyanite crystals, pale pink elongate andalusite, plus some wee brown round garnets, in schist from Ardalanish Bay. Although the andalusite and kyanite look happy together, they are not actually in equilibrium: this layer has "frozen" during the slow process of converting kyanite to andalusite. Reactions are all about the kinetics! #Metastable #Mull #Moine #RossOfMullGranite #Geology #Minerals #Al2SiO5 #Kinetics

Show thread

Bob Downie Jul 25, 2023

@FaithfullJohn My undergraduate field trip to Glen Clova is coming back to me. Barrovian?
https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095448739;jsessionid=94DD686DAD1B9CDADBBE9200EB03E4F7

Show thread

John Faithfull🌍🇪🇺🏴󠁧󠁢󠁳󠁣󠁴󠁿🧡 ✊🏼✊🏿Jul 25, 2023

@geomannie Yes and no. Kind of a tiny bit, in that "Barrovian" refers to the kind of regional metamorphic minerals that grow in "normal" pressure-temperature profile thickened crust (as in the eastern Dalradian rocks in Glen Clova). The kyanite here grew under that kind of metamorphism. But in the "Moine" package of rocks, not the "Dalradian" package of rocks, and probably a bit later, and at higher pressure than the Clova Dalradian. .../more

Show thread

John Faithfull🌍🇪🇺🏴󠁧󠁢󠁳󠁣󠁴󠁿🧡 ✊🏼✊🏿

@geomannie .../cont'd... But the later andalusite overprint here in Mull is due to the thermal aureole of the Ross of Mull Granite pluton. This is quite restricted in extent, and not "Barrovian". Although lots of metamorphism is neither purely thermal from magma, or regional without magma, so the "Barrovian"/"non-Barrovian" distinction can be a bit blurred sometimes. 🤓 I'll stop now 😁

Show thread

Bob Downie Jul 25, 2023

@FaithfullJohn As a softy these ideas are scouring the dregs of my student days but thanks for the explanation. Andalusite did seem a bit off beat for Barrovian metamorphism so thanks for the explanation.