What’s the Difference?

Recent posts have discussed metamorphic rocks found along the banks of the Potomac River in Northern Virginia, buried and deformed during closing of the Iapetus ocean, between 1000 and 500 Ma (million years ago). Those rocks are schist, which formed at depths of about 15 km (10 miles), and temperatures of approximately 500 C (about 1000 F). The original mudstone was ductile under these conditions and the sedimentary layers (bedding) were folded like taffy, while low-melting minerals like quartz were squeezed out and filled cracks and voids, to form veins and irregular, rounded bodies. As extreme as these conditions sound, these are intermediate-grade metamorphic rocks.

I took a trip to Lynchburg, Virginia (Fig. 1), to see some higher grade metamorphic rocks that were formed at about the same time.

Figure 1. Geologic map of central Virginia, showing the location of previous posts (square at top of image) and the area discussed in this post (circle at bottom). Note that the schists observed along the Potomac River are shown in a darker shade than the rocks we will see near Lynchburg. Note also that the ribbon of rocks discussed in this post are not evident in the northern study area. The distance (as the crow flies) between the two study areas is about 140 miles.

I looked at three exposures of Precambrian rocks on this trip. I would love to show all of the photos I took of the metamorphic and igneous structures I saw, but I will have to restrain myself. (I’ll try anyway…)

The first locale we visited was Ivy Creek Park (Fig. 2), where there was an exposure of the Lynchburg Group that revealed one of its many facies.

Figure 2. Plate G shows the location of a manmade pond on Ivy Creek, blocked by an earthen dam (Plate B). It is a fairly steep incline and there is no evidence of erosion along the silty shore (Plate A). About 100 feet above the lake level, we found an exposure of dark rock that weathered to red (indicating a high iron content), as seen in Plate C. This rock was foliated and weathered to reveal a fissile texture reminiscent of schist (Plate D). I estimate the orientation of the foliation to have a strike of about 40 degrees east of north (Plate E) with a dip of approximately 70 degrees. A close-up of this exposure (Plate F) reveals a fine-grained texture.

The rocks seen in Fig. 2 (especially Plate F) are probably either biotite schist or graphite schist. Without petrological analyses that are unavailable at this time, there is no way to tell–not that it would matter. The protoliths of these metamorphic rocks were fine-grained sediments, probably deposited in either a back-arc basin or continental subduction zone. They were buried to at least 10 miles and compressed by colliding tectonic plates; however, this small outcrop showed no evidence of ductile deformation (e.g., folds or quartz inclusions). Note that the strike of the foliation in these rocks (Fig. 2E) is consistent with the orientation of geologic trends seen in Fig. 1. So far, so good…

Our next stop took us to Candler Mountain, where a road cut exposed metamorphic rocks for more than 100 m along a narrow road, still part of the Lynchburg Group (Fig. 3).

Figure 3. Typical exposure of Lynchburg Group metamorphic rocks on Candler Mountain. Note the irregular “bedding” and delineation of grey and tan rocks due to weathering.

This exposure revealed many fascinating metamorphic textures–too many to share here. I’m going to focus on extensional features, which might not be expected since I just said that these rocks were deformed during collision of solid land masses, at least island chains like Japan of the Philippines, if not continents.

The context for this situation–stretched crust in a tectonic plate collision–is best illustrated in a cartoon (Fig. 4), which shows how the crust can actually stretch to release the stress caused by volcanism, as subducting crust melts under increasing pressure and temperature.

Figure 4. Schematic view of subduction with an island (instead of a continent). Note location (1), where the crust is being torn apart because of the immense compressional stress at location (2), which is released episodically as the subducting oceanic crust melts and creates vast quantities of magma which rise to form the extensional “back-arc basin.”

Keeping Fig. 4 in mind, let’s look at the rocks we found on Candler Mountain.

Figure 5. Photo of a loose sample from the road cut in Fig. 3. The classic “phyllite” sheen couldn’t be observed on the rocks that were in place because of their orientation. Phyllite often consists of muscovite, quartz, and chlorite, although biotite and plagioclase can also occur. All mineral grains are microcrystalline, which explains the sheen. No minerals can be seen with the naked eye.

Phyllite is a low-grade metamorphic rock that forms from mud stone at fairly low temperature and pressure (Fig. 6). It is associated with convergent tectonic plate boundaries, and is found in both subduction zones (see Fig. 4) and continental collisions.

Figure 6. Schematic of metamorphic facies in a subduction zone. Note that phyllite is formed at lower temperature and pressure than schist (~300 C or ~600 F). This figure would be representative of Location 1 in Fig. 4, although it could also be found more seaward (to the right in Fig. 4).

Focusing on extensional tectonics for this post, we can see some of the textures we’ve seen elsewhere on Candler Mountain.

Figure 7. Extensional forms like the boudins seen in the center of the photo were dominant in this road cut.

Figure 8. Large quartz/feldspar boudin.

The extensional features seen in Figs. 7 and 8 are consistent with these rocks being deformed as suggested in a back-arc basin (Site 1 in Fig. 4) but not buried too deeply (Site e in Fig. 6). However, they are not horizontal, as indicated by the steep dip seen in Fig. 8 (actually they are dipping at more than 60 degrees).

After being heated in the back-arc basin environment, they would have been crushed by the imminent collision of continental land masses, which cannot be subducted because of the low density of the granitic rocks that comprise them. During this compressional stress, they were folded and probably transported many miles along thrust faults, without being buried deeply enough to transform them into schist.

Figure 9. This image shows a quartz/feldspar boudin that has been deformed during compression, leading to partial melting of the low-temperature minerals (light colored), filling gaps in the surrounding phyllite rock. Think of this rock as being mangled between an irresistible object (N. America) and an irresistible force (Europe). They escaped deep burial to become schist and gneiss because of the presence of horizontal weak zones (photo-thrust faults), as they slid over the rocks that made up proto-North America.

The tilting of these rocks is due to large-scale folding during this later compressional period. The rocks of this area are actually part of an anticlinorium–a region filled with anticlines of every scale, from inches to miles in width. There are a couple of additional superimposed structures I would like to mention before we move on.

Figure 10. This photo shows almost vertical striations in the shiny rock, superimposed on the folded structure created during compression (fine-scale, gray rock at top of photo). Note that this structure is orthogonal to the layering seen in the tan rock to the upper-right. I don’t know what this represents, but it came later than extension or compression and was expressed only in small areas. The photo is a couple of feet across.
Figure 11. The yellow lines indicate a set of joints that resulted from decompression of these rocks during exhumation. This is the last phase of deformation. Recall that the beds are steeply dipping to the ESE (~40 degrees south of east).

There is one more piece of the puzzle that we observed on our field trip. Before we finish with a wave of our hands (and a good bit of conjecture), let’s review.

Figure 12. Summary of rocks observed in the Lynchburg area. Site A is where we saw an example of the Lynchburg Group (Fig. 2), including gneiss and schist (both high-grade metamorphic rocks). Site B is where we observed phyllite with superimposed metamorphic textures (Figs. 3-11). Site C is discussed below. The dashed line represents the approximate division between rocks of the Blue Ridge (high grade) and low-grade rocks seen in Fig. 1. Location is approximate and this is a wide transition zone.

The dashed line in Fig. 12 is oriented approximately in line with the general lithological trends in Fig. 1, as well as several faults (solid lines in Fig. 1) that have been identified. This line is meant to delineate the schist and phyllite zones (~300 C isotherm in Fig. 6) within a back-arc basin. Schist is high grade. But what do we expect to find at Site C?

The last location we visited was a slope, the edge of a bulldozed lot covered by an apartment complex. None of the rocks we saw were in place, but they hadn’t been moved far, maybe a few yards. We weren’t looking for orientation data, so that wasn’t a problem. What we found was very interesting.

Figure 13. This was a loose boulder pushed over the edge of the development. All of the other rocks were the same, including some that were in place (they were covered with grass and biological material). Individual mineral crystals can be seen as lumps from a distance (Plate A). A closer look (Plate B) reveals well-formed crystals (circled) of a dark mineral, probably hornblende. The lighter areas between the hornblende are plagioclase feldspar. This combination of minerals, and the large crystals, suggests that this is either gabbro or dolerite.

The exposure faced a ravine that had been partly filled to support a road, so we followed the slope until we found more loose boulders with a different lithology.

Figure 14. The small boulder in Plate A reveals a fine-grained, layered rock which, upon closer examination, was found to contain a distinct contact zone with the gabbro we saw in Fig. 13 (Plate B). Plate C shows this contact zone in detail (image width is ~6 inches). The circles enclose large crystals of (probably) hornblende.

I cannot say, from the available photographs, which lithology in Fig. 14 is intrusive. However, those details are not the purpose of this post; between the high-grade (schist and gneiss) and low-grade (phyllite) metamorphic rocks we’ve observed, there is a line of intrusive and possibly volcanic rocks with the same general orientation (about 40 degrees east of north). We cannot say anything about post-emplacement deformation of these magmatic rocks, but it is curious that they lie between metamorphic rocks of different grade, in an area with several identified faults (see Fig. 1) having the same orientation.

I have followed one interpretation of the rocks discussed in this post, that they were deposited as muddy sediments about a billion years ago and deformed when volcanic processes stretched the crust. The subduction zone that created this tectonic environment was subsequently caught between opposing continental plates and crushed like a beer can. At some point–probably continuously until the final cataclysm–basaltic magma collected in a subterranean chamber and periodically emerged, creating submarine basaltic flows.

I had a lot of fun with this post (and crushed a few beer cans myself)…

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