In three of my earlier posts I discussed the Billy Goat Trail at Great Falls National Park in Maryland. On Wednesday (quickly becoming Geology Day on my calendar) the team (Alan and I) trekked back to the BGT to further explore the geologic features. This time around the water level of the Potomac was considerably lower than previous, and exposed more outcrops for us to observe.
The bedrock of Great Falls was metamorphosed in multiple events, and evidence for these events is seen throughout the park. One piece of evidence is the presence of porphyroblasts found near the end of Mather Gorge in the heavily metamorphosed metagraywhacke. Porphyroblasts are large mineral crystals that form within a metamorphosed rock during periods of high temperature and pressure; one example is a garnet, the state mineral of Connecticut. When we first went through the BGT in April we came across a small sampling of porphyroblasts in the same vicinity. But, this time with more exposed rock we were in our own smorgasbord of porphyroblasts, boudinage, and kinematic porphyroclasts. Alan and I found five separate areas with the porphyroblasts, and could have found more if it weren’t for the blazing heat melting me to the rocks. One particularly good example is pictured below (pasty-pale hand for scale).
The question now arises: what are the porphyroblasts made of? Well, at this point they are composed of sericite, but only after they experienced retrograde metamorphism. In other words, after the intense pressure and temperature that created the porphyroblasts began to diminish the composition of them changed with the environment. Well then, what were they before they were sericite? Well, I don’t know, and I am not alone. There is some level of debate for this topic as some parties believe it to be kyanite and others sillimanite. The confusion arises out of both being polymorphs of each other along with andalusite. Polymorphs are minerals of the same chemical composition, but differing crystal structures depending on temperature and pressure levels. All three of our polymorphs discussed here have the chemical composition Al2SiO5, in relatively high pressure kyanite is formed while in high temperature sillimanite is formed. According to the USGS paper, “The River and the Rocks: the Geologic Story of Great Falls and the Potomac River Gorge” (1970) the porphyroblasts are sillimanite and suggests that they may have been kyanite and andalusite originally, but changed due to the heat from granite intrusions. George Fisher’s paper “The Piedmont crystalline rocks at Bear Island” from 1971, and discussed by Callan Bentley in his blog entry: “Crystal Ghosts” (April 2010), suggest evidence that all three polymorphs are found within the park, and that this could imply the conditions were near the aluminosillicate triple point in terms of pressure and temperature. I like this idea, and not just because it presents a more interesting scenario, but because there is solid evidence for both sides of the argument. With the triple point hypothesis both scenarios are explained.
I hope to have a better understanding once I complete the Igneous Metamorphic Petrology class, and will possibly explore the topic again then. For now I would have to consider the metamorphic environment of Great Falls during the Taconian Orogeny. Which would have been greater between temperature and pressure, or were both at the precise levels to generate all three polymorphs?