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Metamorphic rocks Geology 115 Metamorphic rocks • Unlike what you may have heard, it’s not just “heat and pressure” applied to existing rocks • Also, not due to partial melting of rocks • What it is: “the solid-state reaction of minerals within the rock to produce new minerals and thus new rocks” Metamorphism • Metamorphism is a series of chemical reactions that occur to stabilize minerals in relatively high temperature and/or pressure conditions • Notice this is not freezing minerals, like in igneous rocks Conditions for metamorphism Besides heat and pressure, time is needed to complete the chemical reactions and fluids (either water, or more rarely, carbon dioxide) are needed to transport ions Recognizing metamorphic rx • Metamorphic rocks subjected to directed pressure typically result in foliated rocks • The rock fabric gives a sense of the pressure direction Recognizing metamorphic rx • Metamorphic rocks subjected to confining (or lithostatic) pressure do not show foliation • However, because of metamorphic reactions, the rock tends to be the same hardness all the way through; cracks in a rock go through grains, rather than around them – intragrain fracture Three types of metamorphism Contact metamorphism • Contact metamorphism occurs when a hot body (pluton or lava flow) cools in contact with preexisting cold “country rock”. • Relatively low pressures (high T, low P) Blueschist metamorphism • Also known as “dynamothermal” or “subduction zone” metamorphism • Relatively low temperatures (high P, low T) Regional metamorphism • “Standard” metamorphic conditions - both temperature and pressure rise due to increasing depth of burial Regional metamorphism • Characteristic facies – an index mineral or combinations of minerals - are found that indicate a more precise maximum pressure and temperature • For instance, garnets are geobarometers PT diagrams Metamorphic grade diagram Facies diagrams Units are °C and kb (kilobars) where 1 bar is roughly 1 atmospheric pressure So what rocks do you find? Sed/met rx boundary • If there is very hot (>200°C) water flowing through rocks, minerals may be hydrothermally altered, or, in some cases, deposited • Metasomatism creates ore deposits North Cascades NP (1968) The entire North American west coast is a series of accreted terranes, which are volcanic island arcs, sea floor sediment, parts of other continents, or the sea floor itself that has been brought by plate tectonics to this coast and, instead of subducting, fused to the existing North American craton. Wrangellia and Sonoma are the names of two such terranes. Terrane accretion is due to a subduction-type convergent bounda which leads to the formation of volcanic arcs Within the North Cascades, two stories: Terrane accretion Volcanic arc orogenies Combining the two stories (terrane accretion and volcanic arc orogenies), you get the quite complicated story of the North Cascades. In essence, there are two accreted terranes surrounding a volcanic arc. The bit of the North Cascades that we are interested in this week is the Northwest Cascades Thrust System, which was directly pushed against the side of North America during subduction at a shallow depth, and thus led to blueschist (low T, high P) metamorphism. Rocky Mountain NP (1915) These Rocky Mountains were uplifted by the Laramide orogeny (starting 70 million years ago), but the part of the story of interest this week occurred in the Archean Eon The Archean rocks at the foothills of Rocky Mountain NP show a distinctive set of minerals, typified by these index minerals microcline feldsparstaurolite sillimanite garnet increasing temperature and pressure that the rocks were subjected This progression is a characteristic of regional metamorp Just east of the park, there are a series of metamorphic rocks, each with a key index mineral, indicating the degree of metamorphism Yellowstone NP (1872) but none of this is relevant this week About 53 to 43 million years ago, just at the northeastern edge of Yellowstone, a series of 13 volcanoes erupted at different times; these are the Absaroka Volcanic Province. The products of the eruption were mostly basalt and basaltic andesite. Since then, the volcanoes have all mostly eroded Breccia from the Absaroka field, which contains some clasts of the pre-existing limestone “country rock”, which was then selectively mineralized by sulfide minerals pyrite replacing skarn and gold. Skarn is a metamorphosed limestone that was subject to high temperatures but not high pressures – something that might happen near the surface of a volcano. This is contact (low P, high T) metamorphism. In 1989, a Canadian firm wanted to develop claims on the New World Mine just outside of the park boundary. This causes concern among environmentalists and park conservationists. Eventually, the mineral rights were purchased by the US Forest Service from the original owner, ending the prospect of mining in this area.