About 45
years ago, together with several students and colleagues, I undertook a
delightful study of what we referred to as the Central Montana Igneous
Province. The result was published in 1980
in two “letters” to the journal Geophysical
Research Letters. There is no reason
for anyone to dig up these articles (Diehl et al, 1980; Jacobson et al, 1980);
they together are the brightest star on the NA APW path (I will brook no
disagreement on this issue!), but they have no significance to the present subject.
The present subject is: why are they there? Over beer
on many a balmy summer evening we pondered that question. They were roughly 1000 km east of the nearest
active plate margin, and moreover 100 km or so east of the Rocky Mountain
Front. They seemed to have derived from
volcanic plug-like intrusions into much older, flat-lying cratonic sediments. Moreover they were lithologically peculiar,
consisting of highly alkaline rock types, the names of which I have long since
forgotten. Well, as some of you know,
recently I made an Amtrak journey to Wisconsin and back. The route (the Empire Builder) passes
directly through the Central Montana Igneous Province and so, for the first
time in 45 years, I was reminded of those enigmatic little buggers and the
question once again arose: why are the there?
Some of my
Facebook friends are geologists and all younger and vastly more au currant than me.
Please: Somebody fill me in on
the latest hypothesis for the origin of my old igneous friends.
Maybe a topic for. A Nick Zentner lecture
ReplyDeleteMyrl! Challis magmas. Will try my best this winter. The Crazy Eocene!
DeleteRob Thomas (Roadside Geology of Montana, 2020), who stays on top of these things says "The professionals are still sorting out the origin of the Central Montana alkalic rocks and the Paleogene arc to extensional volcanism in western Montana, but several of them have suggested that they are related to processes associated with shallow subduction and the sinking of the Farallon Plate into the mantle." I agree yet I've mused over Peter Bird's contribution many times (http://peterbird.name/publications/1984_Laramide/1984_Laramide.htm); the heat transport idea is interesting.
ReplyDeleteThanks for your contribution. It might be too much to ask you to summarize Bird’s ideas, but could at least give me a good citation? Multi García. Myrl
DeleteHi Myrl,
DeleteRob has a facebook group Montana Geology that you could join. Lots of pretty pictures and mostly good information. Rob answers a lot of questions on there.
https://www.facebook.com/groups/200453861048301
Try this:
ReplyDeletehttps://science.sciencemag.org/content/239/4847/1501
I think that's probably it but it comes with a caveat, I haven't followed it through into recent literature. Moving upper crustal heat producing elements deep really cranks dT/dz and I think that's necessary for the igneous activity we see.
Ski season ended early, damn climate change...
Cheers,
Steve