Question: was Laurentia, the "ancient core of north America" a craton then, like the Midwest is now?-- A very quiet place, no volcanoes, (except for wannabe Hicks dome in S. Illinois.)
Rolf Leutke directed me to Gallagher mine, years ago, in Az, and there I found the only campylite I have. Thanks for the background info. Yes, like so many xls in collections, it's a group situation. I'm a lumper not a splitter. Are you, Richard, a lumper?
I think the real core of Laurentia, the Archean Canadian Shield plus Grenville, plus maybe the Archean Wyoming and other small cratons to the west (except their western margins), was mostly without volcanic activity after the Grenville Orogeny and Mid-Continent Rifting ended about 1.0 to 1.1 billion years ago. There are plenty of younger intrusives (I’m not aware of any extrusive volcanics) of late Proterozoic age in the west, 700 to 900 million years or so, probably related to the breakup of Rodinia. And there’s a carbonatite intrusive in southeastern Nebraska dated to 460 to 540 million years. I have no idea what that very young date means, but again I think no extrusive rocks of that age are known in the core of the craton or its mid- to late Proterozoic additions, except where much later tectonic activity happened (like the Rio Grande Rift, and related igneous activity into Colorado and NE New Mexico, almost to Kansas, but that’s Tertiary, Miocene and later).
By the time of the Alleghenian-Hercynian collisions in Devonian to Carboniferous time, I think any real action (tectonic and igneous) was indeed limited to the margin where the Appalachians were developing. Far-flung effects within the craton likely did warp and rejuvenate some basins and arches in what is now the US Midwest, but I think no extrusive (and little intrusive) igneous activity there at that time.
Hicks Dome and probably a few other igneous bodies are very much enigmatic in terms of their tectonic framework (at least for me). Dated to 270 million years (Permian), there’s no obvious tectonic reason for the igneous rocks at Hicks Dome, although it is near some ancient tectonic weak zone intersections. I’ll try to research it more in connection with something about the fluorite (don’t hold your breath!).
Take all that with some grains of salt. I’m not an expert on that.
I’d say I’m usually more interested in the big picture, continent-scale and regional geology and tectonics, so I’m necessarily one who lumps small-scale things to arrive there. I love taking one little strange micromineral and trying to get to the understanding of a mineral district, a geological province, and the overall tectonic story of a setting – I guess I’m a lumper there too. I value the splitters and the details they discover, but I’m personally not too interested in doing that!
One of my favourite mineral habits, although for me, the classic Caldbeck Fells ones are THE ones to have! 😁
Question: was Laurentia, the "ancient core of north America" a craton then, like the Midwest is now?-- A very quiet place, no volcanoes, (except for wannabe Hicks dome in S. Illinois.)
Rolf Leutke directed me to Gallagher mine, years ago, in Az, and there I found the only campylite I have. Thanks for the background info. Yes, like so many xls in collections, it's a group situation. I'm a lumper not a splitter. Are you, Richard, a lumper?
Great questions, Mike, thanks!
I think the real core of Laurentia, the Archean Canadian Shield plus Grenville, plus maybe the Archean Wyoming and other small cratons to the west (except their western margins), was mostly without volcanic activity after the Grenville Orogeny and Mid-Continent Rifting ended about 1.0 to 1.1 billion years ago. There are plenty of younger intrusives (I’m not aware of any extrusive volcanics) of late Proterozoic age in the west, 700 to 900 million years or so, probably related to the breakup of Rodinia. And there’s a carbonatite intrusive in southeastern Nebraska dated to 460 to 540 million years. I have no idea what that very young date means, but again I think no extrusive rocks of that age are known in the core of the craton or its mid- to late Proterozoic additions, except where much later tectonic activity happened (like the Rio Grande Rift, and related igneous activity into Colorado and NE New Mexico, almost to Kansas, but that’s Tertiary, Miocene and later).
By the time of the Alleghenian-Hercynian collisions in Devonian to Carboniferous time, I think any real action (tectonic and igneous) was indeed limited to the margin where the Appalachians were developing. Far-flung effects within the craton likely did warp and rejuvenate some basins and arches in what is now the US Midwest, but I think no extrusive (and little intrusive) igneous activity there at that time.
Hicks Dome and probably a few other igneous bodies are very much enigmatic in terms of their tectonic framework (at least for me). Dated to 270 million years (Permian), there’s no obvious tectonic reason for the igneous rocks at Hicks Dome, although it is near some ancient tectonic weak zone intersections. I’ll try to research it more in connection with something about the fluorite (don’t hold your breath!).
Take all that with some grains of salt. I’m not an expert on that.
I’d say I’m usually more interested in the big picture, continent-scale and regional geology and tectonics, so I’m necessarily one who lumps small-scale things to arrive there. I love taking one little strange micromineral and trying to get to the understanding of a mineral district, a geological province, and the overall tectonic story of a setting – I guess I’m a lumper there too. I value the splitters and the details they discover, but I’m personally not too interested in doing that!
Thanks again!