Posted: Nov 07, 2016 6:10 pm
So, what the hell is going on with these dramatically differently grained rocks right next to each other? Unsurprisingly, we aren't the first people to note this and ask that question.
I think their assessment is reasonable. A border phase would have been at the edge of the Sherman when it was formed. The edge would cool first, and fastest, resulting in less time for the crystals to grow and a finer grain structure. I think.
Houston, R. S., & Marlatt, G. (1997). Proterozoic geology of the Granite Village area, Albany and Laramie Counties, Wyoming, compared with that of the Sierra Madre and Medicine Bow Mountains of southeastern Wyoming (Vol. 2159). US Government Printing Office.
In the map area, rocks mapped as Sherman Granite include a fine-grained facies, a medium-grained facies, a porphyritic facies, and a coarse-grained facies. These rocks are mapped as Sherman because they intrude all other rock units except younger dikes and are generally nonfoliated. The fine to medium-grained to porphyritic facies occur along the margin of the coarse-grained facies, which is the major rock type of this area. We believe that the finer grained to porphyritic facies is a border phase of the Sherman and is approximately the same age as the coarse-grained (typical) Sherman (Houston & Marlatt, 1997, p. 16).
I think their assessment is reasonable. A border phase would have been at the edge of the Sherman when it was formed. The edge would cool first, and fastest, resulting in less time for the crystals to grow and a finer grain structure. I think.
Houston, R. S., & Marlatt, G. (1997). Proterozoic geology of the Granite Village area, Albany and Laramie Counties, Wyoming, compared with that of the Sierra Madre and Medicine Bow Mountains of southeastern Wyoming (Vol. 2159). US Government Printing Office.