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Welcome to Durant Nature Preserve!
It's located on the East side of North Raleigh, NC, in the Piedmont physiographic region. It is located on the edge of a granitic body, gives us some very nice outcrops of injected gneiss.
This is your final stop of this field trip, so you are encouraged to use the skills you learned in all the previous stops to figure out what’s going on here!
Durant1.mp4Durant 2(before outcrop).mp4View 1: As we enter the park, we follow the trail labeled with green diamonds. About 0.25 miles down the trail we reach a river, and a split in the trail. The majority of the rocks we're interested in are to the right, and straight across the river (in the boulders). But watch out for snakes!
View 2: Look closely and interact with the photos. Notice the relationships you see between different rocks. There's some really cool stuff going on here!
Durant 2.mp4Durant3.mp4View 3:
View 4:
View 5:
Okay! Have you finished having a look around?
What is the host rock (the rock that is being intruded)? Start by looking at what minerals are present and the grade of metamorphism.
What kind of rocks are the intrusions? Look at what minerals are present, and their sizes.
How does the country rock change as it approaches a dike? How does the dike vary from the contact to the center?
Look at some of the pictures that have multiple dikes. Which ones intruded first? How can you tell?
Also, there are no papers (formal or informal), websites, or posts that I've found that discuss any of the geology of this area. The only thing you can find is the proposal for the park, which cites "there are no unique geologic features" and we would beg to differ! So, everything you're about to learn regarding the geologic history is coming from a student, just like you. Observations are key to geologic history!
This Durant outcrop is an unmapped part of a very small series of plutonic bodies. In this area, we have Fall Lake leucogniess and the Raleigh gneiss. Inside of these units, there are several small (1000 m2 at the greatest) plutonic bodies. These plutons are high in biotite (as can be observed above). The intrusions range from aplitic (a very fine-grained felsic texture) to nearly pegmatitic, and also range between being quartz-dominated, to K-spar-dominated, to plagioclase-dominated. And that's all about its history. If you want more, I suggest you get into research, and go collect yourself some samples! I can offer you no ages, nor any samples (they're hard rocks, not easy to fracture a piece off).
Let's talk about what we can actually see here.
Here we have a good example of a migmatic texture. There is clearly some sort of compositional separation, but the bands no longer have a clear structure, almost as if they were melted.
Outlined here, we can see an igneous section (which is actually many dikes intruding over each other, as well as a possible plutonic body), but we also see a gneiss, that is likely country rock (meaning the basement rock, or rock present before intrusion). We can also see a thin igneous intrusion cutting across the gneissic banding.
This shows a contact between a large igneous dike, and the host gneiss the dike is intruding.
Outlined here is a faint aplitic dike. It's more difficult to see because of its fine-grained minerals.
Compared to the fine-grained dike to the left, this intrusion is extremely coarse grained, like a pegmatite would be.
When dikes intrude, there are two primary methods, as shown to the left. In this case, yellow represents the intruding dike, while purple is marking an existing dike.
The dike intrudes through some sort of assimilation of the surrounding material, and there is no offset.
The dike intrudes either along an existing fault (causing displacement) or the dike intrudes and pushes the wall on either side out, which causes the apparent offset.
Ravensridge(last).mp4One final note. There are no chilled margins here (a chilled margin is a crisp clean contact that occurs when a hot intrusion enters a cold rock and cools quickly). But, there are several small-scale baked contacts, where you can see:
A change in the foliation direction from 325° in non-intruded areas to 35° along the margins of dikes
A gradational change in biotite abundance and size as you move away from the igneous bodies.
This tells us that this was a hot intrusion that cooled slowly and radiated heat into the surrounding rocks.
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