Todays installment of the perhaps-overly-nerdy Stories Rocks Tell: Enclaves Yesterday, I spoke about Rapakivi texture as an indicator for reduction of pressure in an actively crystallizing granitic body, using the type locality of the Knob Lick Granite as an example. Todays factoid brings us back to the Butler Hill Caldera in SE Missouri to examine another consequence of the calderas collapse... Enclave is the generalized term for any rock enveloped by another rock type that cant be immediately explained by a commonly occurring process...just so were clear on that point, igneous or metamorphic clasts in sedimentary rock and dikes of former melt in metamorphic complexes ARENT enclaves (generally, the enclosing rock must be actively forming to begin to qualify...sed gets disqualified at the start). Enclaves are evidence of the dynamic interactions between at least two different rock bodies and have many forms, depending on chemistry and the rock types involved; most common are inclusions of a pre-existing rock within an igneous body. For an example, please see the first three images of this abandoned quarry in Knob Lick Granite. From yesterday, the Knob Lick Granite is one unit that has been demonstrated to have been emplaced during the collapse of the Butler Hill Caldera. Overlying is the Grassy Mountain Ignimbrite AKA Lake Killarney Rhyolite, the surficial deposit of ash from the Butler Hill volcano. At the contact, the heat from the granite has baked the Ignimbrite, devitrifying and recrystallizing the unit. Small bodies of rhyolite can be found within the granite near the contact with the GMI. These enclave bodies are called xenoliths or foreign rocks and have a story to tell. The collapse of the Butler Hill Caldera in upon itself resulted in displacement of the granite plutons underneath to higher levels through opening fissures (a la rapakivi formation). Pieces of rhyolite became entrained within the cooling granite and began to slowly sink downward. Heat from the crystallizing granite fuses the rhyolite tephra, while the colder rhyolite cools the granite immediately surrounding it, giving rise to chilled margins. A different type of enclave can also be found here. Bands of fine grained, amphibole-rich granite can be found stacked with alternating layers of large-grained and fine-grained granite with a composition typical of the Knob Lick Granite. While open to interpretation, these have been suggested to be the result of cycling heat input to the area: small crystals of Fe and Mg containing minerals followed by fine-grained feldspar and quartz may have formed a chilled margin against the pluton wall and peeled away as heat input to the body increased, wafting downward and becoming sandwiched between portions of granite with larger crystals that were not in contact with the wall. The death of a volcano, as told by granite. (Wow, this one seems long-winded...Jo Schaper I bet this is familiar :) )
Posted on: Sat, 10 Jan 2015 01:31:41 +0000
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