First there is a mountain

Then there is no mountain, then there is

About one meter wide

Celebrated in Donovan’s 1967 song, the Zen Buddhist saying about mountains suggests the impermanence of everything, but calls for the embracing of change and recognition of the oneness of things as both impermanent and eternal. In geology, it is also literally true.

The polite technical geological term for the outcrop above is "messed up." The field of view is about a meter wide. These rocks crop out near the crest of the Norris Hill, along US Highway 287 in southwest Montana USA, and are beautiful examples of multiply deformed metamorphic rocks.

The rocks in the southeastern Tobacco Root Mountains of Montana record a long tortuous past. Originally deposited as sediments around 3,300 million years ago (Archean time), they’ve been cooked and folded and re-cooked and refolded and faulted multiple times. A poorly understood early metamorphic event, high heat and pressure, probably culminated around 2,450 million years ago. Any mountains produced by that activity were eroded away, and 400 million years later there was an extensional rifting event marked by intrusion of dikes.

Map of the Wyoming Craton. Archean core, more than 2,500 million years old, in pink; Archean rocks affected by marginal collision and deformation belts (Big Sky Orogeny and others) about 1,800 million years ago in orange. From Harms and others, 2013, Keck Geology Consortium; see for example Harms and others, 2004, Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming Province: Geological Society of America Special Paper 377, p. 227-243.

For comparison here’s an interpreted magnetic basement terrane map I made in 1988 (revised a bit in 1993).

First there is a mountain

About 250 million years after that, the area that is now in the Tobacco Roots was on the edge of the Wyoming Craton when it collided with other old continental blocks to the northwest. That hundred-million-year-long collision is called the Big Sky Orogeny (Harms and others, 2004, GSA Special Paper 377) and as more or less a continent-continent collision, it probably created a significant large mountain range as well as superimposing another level of metamorphism on these rocks. That was about 1,750 to 1,850 million years ago (Paleoproterozoic time), and it probably caused most of the visible outcrop-scale deformation in the rock in the top photo, but without detailed geochemical and structural analysis it is challenging (or impossible) to know how much of that is related to the older activity about 2,450 million years ago. For comparison, the India-Eurasia collision began about 55 million years ago, has uplifted the Himalaya Mountains, and is far from over.

The small outcrop above is pretty much a “holy grail” rock in the Archean to Paleoproterozoic rocks of the northern Tobacco Roots. If you happen to find it, Please, No hammers!

The original sediments were deposited probably in late Archean (maybe as long ago as 3,300 million years) or early Paleoproterozoic time (perhaps around 2,400 million years ago). Those sediments were metamorphosed, probably during the Big Sky Orogeny (1,800 million years ago), to produce the brown-black-and-white hornblende gneiss in the photo above. The Big Sky Orogeny probably included at least one (possibly more) metamorphic and at least two deformational episodes. We know this because the foliation, created by the metamorphic event, was folded by a second deformational event.

Following that metamorphism and folding, there was an igneous intrusive event, when a basaltic dike (the brown rocks at left on the outcrop) cut across the older folded foliation. Because the rock is so fine grained, you have to look carefully in hand lens to see that the intrusive is also metamorphosed (that’s at least a second metamorphic event for the older gneiss) to amphibolite, indicated by its weak foliation or lineation, not parallel to that of the hornblende gneiss (which must also have that foliation in it, perhaps visible in thin section).

Within a few hundred yards there’s an outcrop that shows the folded foliation is refolded – another deformation, at least the third. It’s challenging in the field to say if that deformation preceded the intrusion mentioned above, but it’s probable; the intrusion seems to be metamorphosed but not locally folded.

You can’t tell from just this outcrop, but within a mile of it is evidence for a later (1,100 million years) intrusive event; that diabase dike is not metamorphosed, but it intrudes a probably older fault representing at least a fourth deformational episode, one of extension, rifting, and uplift in places, probably related to the formation of the Belt Basin about 1,450 million years ago.

After the 1,100-million-year-old intrusive and earlier rifting events, the region was relatively quiescent for 400 to 500 million years or so; any mountains that had survived from the earlier collisions were eroded to a relatively flat plain extending across most of North America. On that surface the sedimentary rocks of the Paleozoic and early to middle Mesozoic Eras were deposited, largely by marine waters transgressing across the continent over many millions of years.

Tobacco Root Mountains

Another collision and intrusive event began about 170 million years ago, the tectonic activity called the Laramide Orogeny that lifted up the present Rocky Mountains and culminated about 55 to 60 million years ago. It was a consequence of subduction and collision between western North America and diverse terranes. The cores of those uplifted ranges remain, but the modern topographic mountains and adjacent ranges are largely the result of extension beginning about 35 million years ago. The whole region was pulled apart and broken into the basins and adjacent mountain uplifts we see today. This extension continues, and much of today’s seismic activity in Montana is related to it and to the bulge resulting from North America’s passage over the Yellowstone Hot Spot.

In the outcrop near Norris (the top photo and another below), the metamorphic foliation (the light-dark alternating mineral banding) is folded at least once and probably twice. That probably indicates at least three periods of deformation, because the foliation is itself tight isoclinal (“same angle,” parallel bands) folding produced in the first metamorphic event. The more recent brittle deformation during the Laramide Orogeny is probably not reflected at this scale.

Western North America continues to interact tectonically with the Pacific oceanic plate. If and when the forces of uplift cease and erosion can continue unabated for perhaps 200 or 300 million years, then the present mountains will be in the category of “then there is no mountain” once again.

I picked up one rock at the Norris Hill outcrop because it was green and sparkly, and I thought it might be epidote, a common mineral in metamorphic rocks that can form pretty green micro-crystals. But under the microscope, it was evident that these were not epidote crystals.

Green, fairly soft, platy to flaky, blocky crystals with a silky luster on cleavage surfaces pretty much says chlorite. Chlorite is now a group of minerals, with the most common being a pair in a solid solution, from clinochlore, hydrous magnesium aluminosilicate, to chamosite, the iron-rich analogue and end member.

It’s pretty irrational (for me anyway) to expect to distinguish clinochlore from chamosite without analysis, since they are visually very similar. Having said that I think this is more likely toward the chamosite end because it seems to be a little harder (around 3 for chamosite vs around 2 for clinochlore), together with the fact that the only other abundant, visibly crystallized mineral in this rock is magnetite, iron oxide – so there’s plenty of iron to go around.

The chlorite-magnetite rock I picked up could marginally be called a schist, I guess, although it does not have the typical mica-based flaky partings because the chlorite crystals, flaky as individuals, are more blocky in terms of the whole rock. There is some white muscovite that I suspect is just altered chlorite, but not enough to give the rock much real schistosity.

Most of the rock in the outcrop is black to gray hornblende gneiss with some injected granitic seams. This location is not very far from the edge of the Cretaceous (about 76 million years old) Tobacco Root Batholith, which is probably the source of the granitic stringers as well as potentially the most recent (third) stage of metamorphism of these rocks (the second and I think most serious metamorphism was at least 1,700-1,800 million years ago, in the Big Sky Orogeny, when the Wyoming Craton collided with other ancient terranes to the northwest, after the earlier, enigmatic event around 2,450 million years ago).

Garnetite, Tobacco Root Mountains, Montana. FOV 5 mm. In places, the rock contains enough magnetite to easily deflect a Brunton compass needle.

There isn’t much of the chlorite-magnetite rock noticeable, although I didn’t do an exhaustive search. I speculate that it represents part of a volumetrically small body of material in the original sedimentary rocks (pre-metamorphism) that was rich in iron and magnesium. In other parts of the Archean-Paleoproterozoic metamorphic suite of the Tobacco Roots, small zones of layered garnetite – essentially nothing but garnet and magnetite – have been interpreted to possibly represent old deeply eroded soils, laterites. This might be a bit of similar material that happened to be perhaps silica deficient so that chlorite and magnetite formed and not garnets and magnetite.

Generally, we expect chlorite to develop in relatively low-grade metamorphism – lower temperatures and pressures. The rocks on Norris Hill have clearly reached quite high grades of metamorphism, so it’s interesting to speculate on the origin of the chlorite. One idea that comes to mind is that the original rock chemistry was (fortuitously) such that even at high grades, chlorite and magnetite were the minerals that formed.

Another idea is that there was some retrograde metamorphism (heating to a temperature lower than the stability of the first metamorphic rocks) so that high-grade metamorphic rocks reacted to create lower-grade metamorphic minerals (chlorite), perhaps in a more porous seam where water was more abundant. Or it could have formed much later, perhaps in a weak, low-temperature event related to the intrusion of the Tobacco Root Batholith.