This remarkable stop exposed beautifully and complexly layered granodiorite. The proposed model is for magma emplacement at the top of a detached, rotating and sinking block of solidified or semi-solidified granodiorite at the magma chamber roof. As the block slowly and episodically subsided, new magma from below was emplaced via the composite "logjam dike" into the potential space above the block. Crystal settling over distances of several centimeters produced the layering. There are normal layers, bifurcated layers, apparent "soft sediment" deformation, and erosional scour and fill structures. I never thought I would see structures like these in a granitoid rock. Many of the structures look like those in the Skaergaard intrusion.
The broom tip rests right on the lower contact of the layered section, which is only 7 m thick. The knife-sharp contact clearly shows that the surrounding rock must have been rather solid at the time of emplacement of the layered section.
This man's feet are standing on the host rock to the layered section. Right in front of his knees is the lowermost part of the layered section, with its knife-sharp contact. To the left, where the layers turn a corner and become vertical, is the "log jam dike" that obviously carried the magma up through a roof fracture into the space above the subsiding block. Multiple thin layers of obviously different texture and possibly composition can be seen, strongly suggesting that layered section emplacement was episodic.
Modally graded layers within the layered section. Lens cap is 5 cm across, so the layers range from ~6 to ~20 cm thick here. Layer bottoms are rich in biotite.
Modally graded layers that are cut by a particularly phenocryst-rich layer. Note that in cross section the K-feldspar phenocrysts are sub-parallel to the layer base.
On the top surface of the phenocryst-rich layer seen above. The euhedral K-feldspar phenocrysts are randomly oriented in the plane of the layer. The longest K-feldspars are ~6 cm, and many of the K-feldspars have euhedral concentric zoning. This surface put to rest, in my mind, the idea that big K-feldspar phenocrysts have to be metasomatic, or at least a secondary feature.
Layered exposure with probable erosional scour and deposition indicated by the layering. A crosscutting K-feldspar-rich dike is visible at left center.
The same outcrop as seen above, but from a different angle. Truncations of and deposition over lower layers are more obvious here. The phenocryst-rich dike is visible at the far left center. White spots are K-feldspar phenocrysts.
Closeup of the phenocryst-rich dike seen in the preceding two images. The dike contains light K-feldspar phenocrysts and dark objects that look like xenoliths. It is interesting that the lower part of the dike is nearly free of these coarse components.
Layered section with an impressive scour structure visible in the center, that cuts through at least two underlying layers.
A view to the left of the photo above, showing that the scour feature continues through the outcrop from the right side (previous image and here) to the left. Note the irregular topography at the scour base, and internal layering in the scour fill. Irregular layers to the lower left are suggestive of a "soft-sediment" style of deformation.
This image was taken above the top of the layered section. The central body of phenocryst-free rock is thought to represent a "diapir" or perhaps a residual liquid "pipe" that rose out of the layered section through the overlying roof rock. Wall-parallel layering suggests that this is a composite feature.
Not very good view of the "log jam dike" that apparently fed the layered section. The nearly vertical layering in this composite dike can be seen in the center and lower right. Some of the layered section, northeast (right) of the dike can be seen just to the right of top center. All images above were taken on the southwest (left) side of the dike.
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Schenectady, NY 12308 U.S.A. |