13 October 2011
The first stop on our pre-GSA field trip to the subprovince boundaries of the Superior Craton was a place just north of Virginia, Minnesota, where the Mesabi Iron Ranges are mined (same Proterozoic banded iron formations that were portrayed as the backdrop of the mining activity depicted in the film North Country). The pull-off is locally known (to geologists) as “Confusion Hill,” but marked on the roadside sign as the “Laurentian Divide.” Here’s a map to show where it is located on the North American continent:
(Map modified from an original version I scored from Wikimedia Commons)
The location is a drainage “triple point, with some water draining down into the Great Lakes watershed, other water heading south towards the Mississippi watershed, and still other water draining down a different slope towards the Hudson Bay watershed. But we weren’t there for the surficial hydrology. We were a group of structural geologists, and we wanted rock.
We stopped, and did what geologists do in these circumstances – immediately headed for the outcrops, where we clustered around to discuss:
Here, trip leaders Basil Tikoff and Peter Hudleston, give us an introduction to the geology of Minnesota, supplemented by a lovely new geologic map of the state:
The rocks exposed at “Confusion Hill” are rocks of the Giants Range Batholith, an interesting intrusive complex that includes a half dozen different compositional variations. Here, the evidence of the different compositions was manifest: there was evidence (inclusions, cross-cutting relationships) that suggested felsic intruded mafic, and also evidence that showed mafic intruding felsic. Hence the name, “Confusion Hill.”
Here’s an example of the complex relations, with a cm-demarcated pencil for scale:
A vertical train of what appear to be (older) mafic xenoliths, again with the cm-demarcated pencil for scale::
Here’s a particularly confusing expression, with a 55 mm diameter lens cap for scale:
There’s some crazy stuff going on in that previous image. Stare at it a while and see if you can figure out whether the felsic rock is older or younger than the mafic rock. It’s perplexing, and Peter attributed some of these complications to probable magma mingling during the emplacement of the batholith during the Archean.
Here, some geologists who participated in the trip eagerly examined the rocks for themselves: