Carmichael ultramafics
Bits of oceanic crust in Montana
As I have probably said repeatedly, I like beautiful sharp crystals worthy of an art museum as much as anyone. But for me as a scientist, it’s the deep stories contained in rocks – all rocks – that are the real goals in pursuing specimens. I really pursue those stories more than the minerals themselves. If you’ve been here any length of time, you also know I like to go from the tiny (an individual specimen) to the big picture. Today I offer an example of both.
I collected today’s minerals from some isolated knobs in the wooded area west of the South Boulder River in the northern Tobacco Root Mountains of Montana USA, where the Carmichael Fault brings Archaean (more than 2,500 million years old) and Paleoproterozoic (probably 1,700 to 1,900 million years) rocks to the surface in the Pole Canyon (Carmichael) Anticline.
These rocks have been significantly metamorphosed at least twice and deformed three or four times, which makes it difficult to determine what they were originally (the original rock is called the protolith, a word that means “first rock”). Most of the rocks in the vicinity are consistent with original sedimentary materials (sandstone, siltstone, shale) that contained a lot of impurities, especially iron and magnesium, and overall they have a geometry that suggests layering like sedimentary strata (albeit highly deformed). I described some of those metamorphosed sediments in this post.
But the knobs the rocks in my photos are from are not conformable with the overall picture. They are more like isolated little intrusions that cut across everything else.
There isn’t much in these rocks besides fibrous amphibole and reddish pyroxene plus some mica. We call such rocks ultramafic (“beyond mafic,” where mafic means MAgnesium + a contraction of ferric, for iron). The original rock, the protolith, might have been something like a dirty dolomitic shale (dolomite is a magnesium-calcium carbonate), heated and under enough pressure for the chemicals to reorganize extremely, but it’s more likely that the protolith was some igneous rock that was already pretty rich in magnesium and iron silicates like olivine, probably something like an olivine basalt. Although other interpretations are possible, by far the greatest volume of olivine basalt is in oceanic crust, and these bodies are probably slices of such rocks.
The amphibole is probably anthophyllite, whose formula is Mg2Mg5Si8O22(OH)2. Cummingtonite has the same formula but with iron substituting for some of the magnesium, which makes cummingtonite monoclinic in its crystallography rather than orthorhombic for anthophyllite. It’s nearly impossible to distinguish between these two minerals without x-ray or other analysis, but anthophyllite tends to be more fibrous and I suspect that’s what I have here, or at least toward the anthophyllite end of the solid solution series with cummingtonite.
Likewise, there is a series in the pyroxenes, from enstatite (MgSiO3) to ferrosilite (FeSiO3), and they have ortho- and clino- polymorphs too. The large reddish crystals are probably enstatite, but they might be clinoenstatite. Clinoenstatite tends to be more pleochroic (displaying color changes depending on the angle of incidence of light) than enstatite, and the mineral here is essentially non-pleochroic, which supports but certainly does not prove my thought that it’s enstatite.
Clinoenstatite also tends to form in lower-temperature metamorphic grades than enstatite; that’s also not definitive, but these rocks probably reached upper amphibolite or even granulite levels of metamorphism, very high temperatures and high pressures as you can see in the chart above. Ferrosilite and its analog clinoferrosilite are pretty rare, but there could certainly be (probably is) some iron in this inferred enstatite. Older geologists like me may remember intermediate or undetermined members of the enstatite-ferrosilite series as hypersthene, a term that is officially discredited by the International Mineralogical Association.
Anthophyllite with the (OH) at the end of its formula is a little unexpected in high-grade metamorphic rocks, because the heat and pressure of metamorphism tend to drive water out (here as the hydroxyl ion, OH). This suggests that the later metamorphic event, while still at temperatures and pressures high enough to seriously metamorphose most rocks, was actually at conditions lower than the temperatures and pressures under which the original ultramafic rock formed and stabilized. We call that situation retrograde metamorphism – it isn’t necessarily cooling, but can produce mineral assemblages that are somewhat unexpected. That could explain older anhydrous (no water) enstatite crystals (big ones, up to 3 inches, 8 cm, long) rimmed by younger hydrous anthophyllite masses, as seems to be the case here (although I have not looked at any thin sections). In other words, rocks that were originally really hot, almost melting, cooled, and were later cooked again to a temperature any other rock would consider high, but for these rocks was just a comfortable early spring day.
“Younger” and “older” metamorphic events here are relative—both metamorphic/mineralogic developments probably occurred more than 1,700 million years ago. The probable Archean oceanic crust that became the metamorphosed ultramafic rocks may have been sliced into these rocks during the Big Sky Orogeny about 1,800 million years ago, but I think it’s easier to explain the two major metamorphic events if the oceanic crust was emplaced earlier.
The map above at left shows the context of these rocks. It is from Vuke and others, 2014, Geologic Map of the Bozeman Quad, MBMG Open-File Report 648. In the map above, XAah means Paleoproterozoic (“X”) to Archean (“A”) amphibolite and hornblende gneiss. XAqfg is for quartzo-feldspathic gneiss. The ultramafic bodies I’m discussing here are too small to show at this 1:100,000 scale, but the Precambrian mapping in there is based on Vitaliano and Cordua, 1979, Geologic map of the southern Tobacco Root Mountains, Madison County, Montana: Geological Society of America Map and Chart Series MC-31, scale 1:62,500. Their map, at right above, does show these bodies in the green and white pattern, labeled “Au” for Archean ultramafics. (Disclaimers: Susan Vuke is a good friend of mine; Charles Vitaliano was my professor of optical mineralogy, igneous and metamorphic petrology, and field geology; Bill Cordua was my office mate at Indiana University; and I taught at the IU geologic field station for 16 summers and lived there year-round for four years. So I might be a little prejudiced about all this.)
The modern anticline (arched uplift) that exposes these Precambrian rocks was formed much, much later than the time of the metamorphism described above. The Carmichael Anticline developed along a fault during the Laramide Orogeny (mountain-building event) about 55 to 80 million years ago, at least 1,620,000,000 years after the metamorphism. We know this because the fold, the anticline, involves all the Paleozoic and Mesozoic rocks that overlie the Precambrian rocks, and therefore the folding came later than all of them. The Paleozoic and Mesozoic rocks are not metamorphosed, so the metamorphism took place before they were deposited.
The Precambrian rocks of the Carmichael Anticline are in the hill at right behind the Indiana University Geologic Field Station in this circa 1950 photo. The ultramafic rocks are about a half mile beyond the ridge top at upper right. Carmichael Mountain, at left above, is the pointy peak at left in the photo below of the Tobacco Root Mountains seen from the west, looking across the Jefferson River Valley. In the photo below, the Precambrian rocks described here are in the low hills just left (north) of Carmichael Mountain.
Amphibole, the name of a large group of common mafic minerals in igneous and metamorphic rocks, is from Greek words meaning “to throw on both sides,” words that also lead to “ambiguous.” If all the chemical and crystallographic stuff I said above doesn’t make you believe amphiboles are ambiguous, I don’t know what will.
Pyroxene means “stranger to fire,” the name given to these minerals in the erroneous belief that they were unusual or alien to igneous rocks that crystallized from magmas, but pyroxenes are actually early-forming minerals that are hallmarks of many igneous rocks. They ought to be called “pyrophils,” lovers of fire, but old nomenclature often sticks. No one said it would be straightforward.
Anthophyllite (meaning “clove leaf” for its common clove-brown color and platy leaf-like habit) is sometimes even more fibrous, more asbestiform, than my example here, and it is regulated as asbestos when it is. Enstatite is from a Greek word meaning “adversary,” for the mineral’s refractory nature, which in turn is a reflection of its origin in high-temperature igneous and metamorphic rocks.
I know there was a lot of jargon and detail in this. But hey, you were not required to read it and it won't be on the final exam. And while I’d be happy if you learned things, mostly I hope you just enjoyed sharing the chase. Cat. No. 1806
Sorting the multiple metamorphosis of Archean there in MT seems similar to the challenges East Coast geologists face in the Blue Ridge and western Piedmont. Throw in the understanding that many of these rocks are accreted terranes and you have what we call "a dad-gum mess".
Nice piece, and a nice look at the IU field station. Rather more scenic than U of MT's field station on campus at Southwest Montana College, but it was fun for Easterners to be in such a big cow town in the 1970s.
I learned that hypersthene is no longer a thing!