Presentation on theme: "Metamorphism and Metasomatic Fluids Metamorphism is "changing the form" of rocks while they are still largely solid, including changes that happen while."— Presentation transcript:
Metamorphism and Metasomatic Fluids Metamorphism is "changing the form" of rocks while they are still largely solid, including changes that happen while strictly solid (simple phase changes, going from a low pressure to a high pressure crystal structure) and changes involving solutions and short-range transport. Metamorphism grades into diagenesis on the low end and generation of magmas on the high end. Metamorphism mainly involves changes in temperature, pressure, differential stress or chemistry, where the rock moves towards higher PT and presence of hydrothermal fluids. Hydrothermal/metasomatic fluids are high temperature aqueous liquids, usually very concentrated and often very acidic: they can be associated with metamorphism, igneous intrusions, loss of water from subducting rocks, or deep circulation of groundwater. v 0045 of 'Metamorphism and Metasomatic Fluids' by Greg Pouch at 2011-01-19 19:33:12 LastSavedBeforeThis 2011-01-19 19:31:02 C:\Users\GregAdmin\Documents\Geo101\06Metamorphism.ppt on 'GWPOUCHDELL1720'
Metamorphism and Metasomatic Fluids 3 Metamorphic Processes Products 4 Products > Metamorphic Textures 5 Products > Minerals and Metamorphic Grade Circumstances 6 Circumstances of Metamorphism 7 Circumstances of Metamorphism Figures 8 Metamorphism and Plate Tectonics 9 Map of Metamorphic Regions 10 Classification of Metamorphic Rocks Hydrothermal/Metasomatic Fluids 11 Hydrothermal or Metasomatic Fluids > Sources 12 Hydrothermal/Metasomatic Fluids > Processes and Products
Metamorphic Processes Changes in mineral stability at high P, T => new minerals Minerals formed under one set of conditions (P, T, and chemistry) might not be stable under a different set of conditions. Just as some minerals are more stable in the low-temperature, low-pressure, oxidized, slightly acidic conditions at the surface, other minerals are favored at high temperatures, pressures, and bizarre chemistry found at depth and near intrusions. Metamorphism is a kind of opposite to weathering. Re-crystallization => bigger crystals Pressure solution and re-deposition => orientation of minerals and foliated fabrics Hydrothermal alteration of chemistry and mineralogy => new minerals Partial melting, dissolution From http://www.geologyrocks.co.uk/forum/q_and_a/anybody_able_describe_how_diagenisis_occurs_sandstone%3Fhttp://www.geologyrocks.co.uk/forum/q_and_a/anybody_able_describe_how_diagenisis_occurs_sandstone%3F
Products > Metamorphic Textures Metamorphism involves growth or formation of new mineral grains, usually under high P,T conditions. Because these conditions often include differential stress, the minerals can develop preferred orientations and fabric. Planar Textures and Foliation ( an oriented fabric marked by the alignment of platy minerals, usually micas) –Slaty cleavage Slate breaks in parallel sheets, which have earthy luster Foliated –Phyllitic Phyllite breaks in parallel sheets, which have a more vitreous luster, sort of like silk F –Shistose Schist Doesn’t break into sheets, but is composed of aligned micas F –Gneissic Gneiss alternating bands of light and dark colored minerals –Migmatitic Mixed metamorphic rock and not-far-travelled magma. Lineation: oriented fabric marked by the alignment of needle-like (acicular) minerals, often amphibole. Mylonitic: "milled" rock. Rock is crushed and smeared out in a fault zone. Like foliation without big xtls. Massive none of the above. Hornfels, quartzite, marble, veins
Products > Minerals and Metamorphic Grade Metamorphic Grade is the assemblage of minerals stable under the conditions of metamorphism, usually the peak PT conditions. This is usually given as a hyphenated group of minerals that have a limited range, or a rock name. Biotite-garnet, biotite-staurolite, eclogite, blueschist, … The combination of grade (PT conditions + others) and protolith (precursor rock) determines the metamorphic facies (properties of actual rock, including fabric and mineralogy) ProtolithLowest grade Highest grade Igneous ShaleSlatePhylliteSchistGneissMigmatiteMagma BasaltGreenstone AmphiboliteGneiss Quartz sandstoneQuartzitequartziteQuartzite Dissolved LimestoneMarble Dissolved Granite SchistGneissMigmatiteMagma
Circumstances of Metamorphism Contact: Rocks altered by heat and fluid, with little or no differential stress, so no foliation, often rather hard and dense, unfoliated like brick. When associated with igneous intrusions or hydrothermal fluids, they typically form a halo around an igneous intrusion. Submarine volcanics can be metamorphosed immediately after/during eruption. The contact metamorphic rocks and veins are often important ore sites. Products: hornfels, quartzite, marble, skarn, greenstone [basalt altered to produce chlorite (a green, magnesian mica) + assorted green minerals], veins of almost any composition, clay-altered rocks, sulfide deposits. Common. Cataclastic: Mylonitic texture crushing and shearing of rock in a fault zone or shear zone. High differential stress, but low temperature and pressure. Associated with faults (where rock bodies slide past one another) or shear zones (where rock bodies slide internally). Products: are mylonites (they look a lot like schist but are very smeared out or abraded) and augen gneiss (gneiss with almond-shaped chunks of feldspar (augen, from the German for eyes) that result from the shearing of large feldspar crystals. Rare Regional: High temperature and pressure, with differential stress, resulting in a foliation. Very Common –Occurrence Mountain building Cores of mountain belts (fairly easy to map and apply grade concepts) Mid to lower crust probably has conditions that lead to regional metamorphism. Shields/basement Basement complex: igneous and metamorphic rocks under sediments or exposed at the surface. Usually complex enough to be very difficult/impossible to map. Includes most crust formed more than 1 billion years ago. Canada and upper Midwest, Africa, India… Very extensive –Distribution of metamorphic grade (Get rocks, find PT, plot on graph, and they often define trajectory in PT space called a geotherm) Normal continental Normal oceanic Batholith (Upper plate in collision zone) Blueschist (lower plate in subduction zone) Archean (older than 2.5 Ga, steeper increase of temperature with depth. Plate Tectonics and Metamorphism
Map of Metamorphic Regions From http://www.bbc.co.uk/news/science-environment-12249909 Report: Urgent action needed to avert global hunger (bad article but good map)http://www.bbc.co.uk/news/science-environment-12249909
Classification of Metamorphic Rocks Most metamorphic rock types are named for a texture, often with a string of hyphenated mineral names in front that describe the rock and indicate the metamorphic grade: Hornfels, Slate, phyllite, schist, gneiss, mylonite, migmatite Some are named based on mineralogy: Quartzite, Marble, Amphibolite You can prefix Meta-(rocktype) to emphasize the protolith meta- conglomerate, meta-basalt, meta-granite… Some get special names, like blueschist and greenschist, which are both fine-grained schists with distinctive mineralogy and colors.
Hydrothermal or Metasomatic Fluids > Sources Magmatic Segregation: A magma can become over-saturated with respect to water. When this happens, the magma separates into a silica-rich and a water-rich liquid. Each species of ion will tend to concentrate into one or the other liquid. (Same process can occur, very early on with a basaltic magma, yielding sulfide or oxide and silica-rich liquids) Process is called exsolution. –Exsolution is often accompanied by a volume increase or decrease (and thus pressure or stress increase) and shattering of surrounding rocks. –Porphyries usually are the result of segregation into a siliceous and aqueous phase, with large, early-formed crystals as phenocrysts; the groundmass usually forms when the water leaves and raises the melting point (all the minerals are now below their freezing point with the water gone, so they crystallize). –The resulting aqueous phase may have high concentrations of odd ions, or may have only quartz and feldspar dissolved. In either case, you get veins, sometimes pegmatites (common in Colorado), sometimes disseminated mineral deposits, such as copper porphyries from the Andes, Metamorphic: Clays => feldspar liberates water, Squeezing and loss of pore space liberates water, Result is much the same as for water coming off a melt. Deep circulation of groundwater
Hydrothermal/Metasomatic Fluids > Processes+Products Processes –Hydrothermal fluids are generally hot, acidic, and very ion-rich. The acidity makes it very easy for feldspars and mafics to be hydrolyzed, resulting in big clay-altered zones. The acids can leach any metal ions—especially ions that don’t quite fit into their crystal structure (substitutions)—out of minerals, so these can extract trace constituents, like Cu from surrounding rocks. –Where the circulating fluids undergo a sudden change of pressure, temperature, or chemistry, precipitates often form as the mineral-stability changes with PT. For example, where hydrothermal fluids are expelled into cold sea water, sulfides, that had been stable in solution, are suddenly over-saturated and precipitate, giving black smokers. Where boiling occurs due to decrease in pressure, many ions will partition into the vapor phase and others in the liquid become over-saturated, and you get precipitates. Products –Clay (and other) alteration zones –Veins –Disseminated ores –Smokers, black and white –Porphyries –Geysers, fumaroles, paint pots, etc.
Metamorphic Rocks Metamorphism is the result of temperatures and pressures intermediate between diagenesis and melting, associated with –mountain-building, –igneous intrusions and extrusions, and –deep crustal conditions. Metamorphic rocks develop new textures and minerals in response to their metamorphic grade. Metamorphic rocks are classified based on texture and somewhat on mineralogy and origin. Hydrothermal/metasomatic fluids are associated with metamorphism, igneous activity, and even sedimentary diagenesis. They are hot fluids that contain lots of ions and alter surrounding rocks and can deposits ores and other vein-fillings.