The Creighton Pluton: Postscript

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

A Note on Nomenclature and National Styles of Geology

Canadian geologists have a penchant for premature naming of things. The Canadian literature is saddled with the clunky terms Aphebian for the early Proterozoic (2500-1750 m.y.), Helikian for middle Proterozoic (1750-950 m.y.) and Hadrynian for late Proterozoic (950-570 m.y.). The deformation of the Archean rocks was long called the "Kenoran orogeny," although the deformation was probably not a single event. Around 1400 m.y. there was supposedly an "Elsonian orogeny" although almost everything with that age is intrusive. And it still goes on. The presumptive pre-impact deformation and metamorphism has now been given the name "Blezardian" although the event remains poorly documented to this day, and Blezard Township lies almost entirely within the Sudbury Basin.

Americans, on the other hand, like to take well established names and replace them with bland, generic terms. Thus, the Churchill Province (named after Churchill, Manitoba) has been relabeled the Trans-Hudson Orogen, though almost all of it lies west of Hudson's Bay. Likewise the Keeweenawan Rift (named for the Keeweenaw Peninsula, where its rocks are well exposed) is now either the Mid-Continent Rift System or the Central North American Rift. Americans also have a wholly irrational fear of geophysics, based on what I call the "Gaussian myth." The Gaussian myth is the notion that because there are an infinite number of ways to model a given potential field, therefore we can't conclude anything useful. People who buy this reasoning apparently believe a round gravity anomaly 5 kilometers across can somehow be modeled by a square rock body 10 kilometers across.

Regional Setting

A great deal has been learned about the Proterozoic of North America since 1976. When I began reading the literature on the Canadian Shield, I assumed that the oldest block, the Superior Province, was made up of whatever had not yet been deciphered. I soon discovered that the Superior Province, if not well understood, was at least a pretty clearly defined entity, and that the Churchill Province was the grab bag of leftovers. The Churchill Province (now Trans-Hudson Orogen) is now known to be the product of numerous accretion events involving small terranes as well as large blocks of reworked Archean crust. (The Superior Province is probably also the result of multiple accretion events, but that's another story.)

The implications of terrane accretion were only beginning to become apparent in 1976. Partly for that reason and partly for sheer lack of evidence to the contrary, I treated the early Proterozoic rocks of the Great Lakes as a single subduction complex. In fact, Wisconsin is now known to consist of three terranes: a sliver of the Superior Province in the far north, a volcanic arc complex (Central Wisconsin Magmatic Terrane) and a southern gneiss complex (Marshfield Terrane) that may correlate with the ancient gneisses of southern Minnesota. The hope that the Huronian rocks might have correlatives around Lake Superior has been pretty well dashed; the shelf and platform sequences of Michigan and Minnesota are quite a bit younger.

Virtually nothing was published on the Precambrian of Wisconsin in 1976. That was one reason I took a job in Wisconsin. Unfortunately, I didn't stop to think that there was a good reason why the last glacial advance is called the Wisconsin. I knew I was in serious trouble when I got to Fond du Lac before seeing my first outcrop. The Geologic Map of the United States showed northern Wisconsin as mostly early Proterozoic granite with inclusions of metavolcanic rocks. It's actually the other way around: metavolcanic rocks predominate.

Something else not fully appreciated in 1976 was tectonic burial. The Sudbury area may well have been buried by kilometers of nappes following the Penokean Orogeny, and was almost certainly buried by kilometers of nappes following the Grenvillean Orogeny.

Changes in Sudbury Geology

The lowermost volcanic rocks, called the Stobie Formation in my thesis, were subdivided shortly after I did my field work. The lower part of my Stobie Formation is now called the Elsie Mountain Formation. The contact is defined by the appearance of appreciable metasedimentary interbeds in the upper part of the sequence. The subdivision does not affect any of my conclusions.

Radiometric ages tend to increase as better analytical techniques evolve and more samples are dated. The Nipissing Diabase is now dated around 2200 m.y., rather than the 2150 given in my thesis. The Murray Pluton has been dated at 2470 m.y. and the Creighton Pluton at 2330. On the presumption that the Murray Pluton is orogenic, we can probably account for the term "Blezardian." The northern end of the Murray Pluton extends into Blezard Township, whereas the southern end lies in McKim Township, and "McKimian" grates on the ears even worse that "Blezardian." The refinements in dating also do not affect my conclusions.

The early date for the Murray Pluton really does inspire speculation that the volcanic and granitic rocks, and maybe the overlying McKim Formation, might be latest Archean. They may be the very last of the Archean greenstone belts. Considering the reverence

that Sudbury geologists accord early workers who postulated multiple tectonic events, it may pay to re-examine some of their early ideas about great discontinuities in the lower part of the Huronian.

Advances in Impact Geology

In 1976, it was generally assumed that the Sudbury Irruptive (a term apparently used nowhere else, and meaning something that bursts into something else) was intrusive. Sudbury geologists who didn't (and some still don't) believe in impact thought in terms of a violent collapse of a magma chamber. Those of us who did believe in impact pictured something like the ring fracture around the Manicouagan impact structure forming a conduit for magma. In any event, much of the funnel shape of the intrusive complex was considered primary.

That changed radically when R. A. F. Grieve, D. Stoffler, and A. Deutsch suggested in 1991 that the igneous complex was actually an impact melt sheet. It turns out to be remarkably easy to get the Sudbury igneous complex - and the ores - by melting the basement rocks and allowing the melt to differentiate. This origin also handily explains the otherwise anomalous granitic upper portion of the complex.

Unlike magmas, which can never be far above their melting points, there is virtually no limit to how hot an impact melt can be. Some studies suggest temperatures as high as 2,000 C. Unlike magmas, which are invariably a mush of crystals and melt, impact melts can be pure liquids. At 2,000 C, the melt would splash like water (the viscosity would be maybe that of light oil, but the density would make it easier to flow downhill). The implications are profound. The melt sheet has to have been perfectly horizontal. Conical initial geometries simply cannot work. Since the lower Huronian and the igneous complex face structurally in opposite directions with no intervening discontinuity, the lower Huronian has to have been completely inverted when the melt was emplaced.

Fortunately, there's a way to do that. Very large impacts create peak-ring craters. The compressed floor of the crater rebounds into a central peak, but unlike normal craters with central peaks, this central peak is so large it collapses and spreads outward to make a ring of peaks. It is also becoming clear that the present Sudbury Basin is far smaller than the original crater.

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Created 4 August 2004, Last Update 11 January 2020

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