Published on January 10, 2008
METAMORPHISM & ROCKS: METAMORPHISM & ROCKS © Marcia Anderson Metamorphic Rocks: Metamorphic Rocks Metamorphic rocks are highly deformed. Usually caused by intruded igneous masses. Metamorphism:: Metamorphism: Transformation of rocks by heat, pressure & chemically active fluids. Change is textural and mineralogical . Always occurs deep within the earth. Provides clues @ geologic processes w/in earth's crust Must remain solid, otherwise becomes igneous. Rock becomes unstable under heat & pressure & gradually changes. Calif.. Sin Nombre; ancient schist & gneiss Metamorphism:: Metamorphism: Occurs during mountain building; intense stress & temperatures associated with large scale deformation. This Large scale deformation is regional metamorphism. Regional Metamorphism: Regional Metamorphism Occurs during mountain building. Crust is squeezed and deformed, shortened and thickened. Thickening forms mountains. Cores of many mountains consist of intrusive igneous bodies surrounded by high-grade metamorphic rocks. Alaska Copper Mountain Regional Metamorphism: Regional Metamorphism Since they are formed under direct stress, metamorphic rocks are usually foliated. Shale slate mica schist feldspar/ hornblende gneiss Appalachian Mts.: Ohio-->Cent. Pa -->E. Pa.-->NY -->Vt../ N.H. Metamorphism & Plate Tectonics: Metamorphism & Plate Tectonics Most metamorphism occurs at convergent zones. Compressional forces squeeze & deform the converging plates & accumulated sediments along the continental margins. As lithosphere descends, pressure increases faster than temp. Material is continually transported to great depths at convergent plate boundaries. At only a few km., clay becomes unstable, it recrystallizes into minerals: (muscovite). Shields: Shields Extensive areas of metamorphic rocks & igneous plutons on every continent. Canadian shields forms the bedrock over much of no. Canada from Hudson Bay to No. Minnesota. Among oldest rocks on earth. Remnants of earlier mountain building. Contact Metamorphism: Contact Metamorphism Molten rock comes in contact with country rock Zone of alteration; aureole, or halo forms @ the magma body Near the magma body, high temp minerals form. le: garnet. Further away: low-grade minerals like chlorite are formed. Mineral composition and amount of water will affect the size of the aureole produced. Contact Metamorphism: Contact Metamorphism Clay minerals are 'baked' forming fine- grained, dense & hard rock. Since direct pressure is not a factor: usually not foliated. Hard, non-foliated metamorphic rock –hornfels. When water surrounds the crystal, it acts as a catalyst by aiding in ion migration. Minerals recrystallize to form more stable configurations. Sometimes ion exchange forms new minerals. Contact Metamorphism: Contact Metamorphism When near or touching magma intrusion: contact metamorphism. High temps "bake" the surrounding rock. Increased density. Larger crystals. Reorientation of mineral grains into banded appearance. Transformation of low temp. minerals into high temp. minerals. Introduction of ions generates new minerals. Dike at Deep Basin, N.C. Along Fault Zones: Along Fault Zones When faulting occurs: frictional heat is produced. Angular pieces of Rocks are broken off along the fault line & lithified: fault breccia. Heat as an AgentHigh grade Metamorphism: Heat as an Agent High grade Metamorphism Intrusive igneous body of ion-rich fluids enters host rock. If quartz sandstone: little alteration to become quartzite. Heat as an Agent: Heat as an Agent Recrystallization: encourages growth of larger crystals. Micas: sheet-like hornblende: elongated Minerals recrystallize with a preferred orientation. New orientation is I to compressional forces. Mineral alignment is horizontal, layered, banded : foliation Parallel alignment. Pressure as an Agent: Pressure as an Agent Directional stress can cause rocks to shear. Shear: similar to slippage. Thin slabs slide past one another. Slickenslide, N.Y. state Pa. Slate quarry Pressure as an Agent: Pressure as an Agent Confining pressure: stress exerted by the load above. Stress during Mt.. Building: applied force is directional, squeezing the material. At great depths, warm rocks behave plastically; they flow and bend into folds when subjected to shearing. Chemical Activity: Chemical Activity Fluids, usually water, with ions in solution, aid metamorphism. With pressure: water is forced out of mineral structures and aids in chemical reactions. Calcite and quartz crystals react chemically to form wollastonite: CaSi03 from limestone w/ quartz (sand) inclusions. C02 is liberated. Chemical / Hydrothermal Activity: Chemical / Hydrothermal Activity Country rock adjacent to magma body is altered by ion rich hydrothermal (water) solutions. Many metallic ore deposits formed from hydrothermal activity. Marble: coarse crystalline rock from limestone or dolomite. Quartzite: from quartz sandstone. A hard, compact rock. Texture: Texture Most metamorphic textures involve foliation. Foliation is generally caused by a preferred orientation of sheet silicates. If a rock has a slatey cleavage as its foliation, it is termed a slate. If it has a phyllitic foliation, it is termed a phyllite. If it has a shistose foliation, it is termed a schist. A rock that shows a banded texture without a distinct foliation is termed a gneiss. A rock that shows no foliation is called a hornfels if the grain size is small, and a granulite, if the grain size is large and individual minerals can be easily distinguished with a hand lens. Slide21: Meta- Rock Texture Meta- Environment Original Parent Rock Slate Foliated Low grade Shale (clay minerals) Phyllite Foliated Low to inter. Grade Shale Mica schist Foliated Intermediate to high grade Shale Gneiss Foliated High grade Granite, shale, andesite Marble Nonfoliated Low to high grade Limestone , dolomite Quartzite Nonfoliated Inter. to high grade Quartz sandstone Metaconglomerate Nonfoliated Intermediate grade Quartz-rich conglomerate Amphibolite Foliated Medium to high grade Basalt, gabbro Common Metamorphic rocks Foliated rocks: Foliated rocks Rocks subjected to heat and differential pressure during metamorphism have minerals arranged in a parallel fashion that gives them a foliated texture. Heat as an Agent - High grade Metamorphism: Heat as an Agent - High grade Metamorphism Extreme conditions closer to melting. Features such as fossils, bedding planes, inclusions, vesicles other rocks are completely destroyed or deformed. Under intense pressure & heat, rocks behave plastically, bend & fold. Mineralogy determines the degree of metamorphism. Folded gneiss. Low grade Metamorphism : Low grade Metamorphism Becomes more compact and dense. Clay particles from shale realign in a more compact arrangement and recrystallize into mica flakes. Flat surfaces are parallel. shale slate Slate beds at Camp Hoover, Newton, N.J. Low grade Metamorphism: Low grade Metamorphism Slate can be split easily along these layers of mica resulting in slatey cleavage. Mica grains are so minute that slate is not visibly foliated. Temp. & pressure only slightly greater than those associated with lithification of sediments. Pa. Slate mine Slate: Slate Slate: very fine grained & foliated. Minute mica flakes, results in excellent rock cleavage. Low-grade regional metamorphism. Both shale and slate split / cleave along bedding planes The grains cannot be seen w/o magnification; slatey cleavage. Black slate: carboniferous ; Red: iron oxide ; Green: chlorite Phyllite: Phyllite Platy minerals: larger than slate, but not large enough for ID. Similar to slate but glossy sheen (coarser grained) Rock cleavage & very fine crystals of muscovite or chlorite. slate phyllite schist High grade Metamorphism: High grade Metamorphism Under higher temp. & pressure mica grains In state will grow larger @ 1 cm. Rock looks platy or scaly: this foliation is Schistosity. This texture is schist. (most common: mica schist) Micro muscovite mica-bio-garnet-schist Schist: Schist Strongly foliated formed by regional metamorphism. Readily split into flakes or slabs. Contains more than 50% platy & elongated materials. (usually include micas & amphibole A wavy parting when broken. shale slate phyllite schist Almadine garnet in schist Vishnu schist -Grand Canyon Inner Gorge: Vishnu schist -Grand Canyon Inner Gorge Mica Schist: Mica Schist Mica schist: contains mostly micas & less quartz & feldspars Accessory minerals unique to metamorphic rocks: garnet, staurolite, sillimanite. Also garnet, mica schist; chlorite schist; talc schist. Blue Schist: Blue Schist Rock is a poor conductor of heat. Rock formed in high pressure relative to temp. is blue schist. Blue Schist: Blue Schist The Blueschist Facies develops in terranes in which the geothermal gradient is low or the overall P/T is moderate to high. Typically, these facies series form on the outer (trench) side of a paired metamorphic belt associated with a subduction zone. Gneiss: Gneiss Banded metamorphic rocks that contain mostly elongated and granular minerals. Most common minerals: quartz, K feldspar & Na Feldspar. Segregation of light & dark silicates. Many form from metamorphism of granite. shale slate phyllite schist gneiss Gneiss: Gneiss Granite gneiss: Granite gneiss Ion migration can cause minerals to segregate i.e.: gneiss. Gneiss: a banded appearance of dark & light minerals. Can form from metamorphosis of granite or gabbro. Non-foliated rocks: Non-foliated rocks Mineral grains do not show a preferred orientation. They show a mosaic of mixed minerals. They are from contact or regional metamorphism where no platy minerals were present. Fine grained limestone metamorphosed from small calcite crystals combine to form interlocking crystals with texture of coarse grained igneous rock. Marble Impurities: curved dark bands. Miqmatites: Miqmatites Partially melted & recrystallized rocks Migmitites – mixed rocks with both igneous and high grade metamorphic characteristics. With streaks or lenses of granite mixed; with high-grade ferromagnesian –rich metamorphic rocks. Wavy appearance. Greenstone: Greenstone Greenstone – any compact, dark green altered mafic igneous rock that formed under low high grade metamorphic conditions. Green is from chlorite, epidote and hornblende. Serpentine Hornfels: Hornfels Hornfels – a fine grained non-foliated metamorphic rock resulting from contact metamorphism. Usually from clays or clay – rich sediments. Coal: Review: Coal: Review Organic sedimentary rocks form from the build up and decay of plant and animal material. This usually forms in swamp regions in which there is an abundant supply of growing vegetation and low amounts of oxygen. The bacteria that decay the vegetation need oxygen to survive. Because these decaying layers are buried so fast the bacteria use up what oxygen there is available and can not finish the decomposition of the vegetation. The overlaying layers become so heavy that they squeeze out the water and other compounds that aid in decay. Anthricite Coal: Anthricite Coal The longer and deeper that coal is buried makes it of higher quality. Peat is the first stage of coal formation. Lignite is the next grade of coal followed by bituminous and the highest grade, anthracite. Anthracite forms during mountain building when compaction and friction are extremely high. This form of coal burns very hot and almost smokeless. It is used in the production of high grade steel. Thermal: Thermal Complete alteration of rock by hot mineral-rich water i.e.: Yellowstone National Park. Metaconglomerate: Metaconglomerate Conglomerate in which the quartz clasts have been stretched like plastic, from heat and pressure. Skarn Deposit: Skarn Deposit Skarns: Skarns Skarns are rocks that originate from contact metamorphism of limestone or dolostone. They show evidence of having exchanged constituents with the intruding magma. Skarns are generally composed of minerals like calcite and dolomite, from the original carbonate rock, but contain abundant Ca and Mg silicate minerals like andradite, grossularite, epidote, vesuvianite, diopside, and wollastonite that form by reaction of the original carbonate minerals with silica from the magma. The chemical exchange is that takes place is called metasomatism. Mylonite: Lincolnville Falls, N.C.: Mylonite: Lincolnville Falls, N.C. Mylonites are cataclastic metamorphic rocks that are produced along shear zones deep in the crust. They are usually fine-grained, sometimes glassy, that are streaky or layered, with the layers and streaks having been drawn out by ductile shear forces. Index Minerals: Index Minerals Certain minerals are known to form only within specific temperatures and pressure ranges. Chlorite biotite amphibole staurolite sillimonite Index minerals: good indicators of metamorphic environments in which they formed. Chlorite: formed at 200o C . Sillimanite forms at 600o C. Light colored silicates: melt first; (quartz & k-feldspar). Dark colored silicates (amphibole & boitite) will remain solid. Light crystalline igneous rock or unmelted dark material. Index Rocks: Index Rocks (textural type) Mineral Assemblage in Pelitic Rocks Chlorite (slates & phyllites) quartz, chlorite, muscovite, albite Biotite (phyllites & schists) biotite begins to replace chlorite, quartz, muscovite, albite Garnet (phyllites and schists) quartz, muscovite, biotite, almandine, albite Staurolite (schists) quartz, biotite, muscovite, almandine, staurolite, oligoclase Kyanite (schists) quartz, biotite, muscovite, oligoclase, almandine, kyanite Sillimanite (schists & gneisses) quartz, biotite, muscovite, oligoclase, almandine, sillimanite Index Mineral: Garnet: Index Mineral: Garnet Garnets as a group are relatively common in highly metamorphosed rocks and in some igneous formations. They form under the high temperatures and/or pressures that those types of rocks must endure. Garnets can be used by geologists as a gauge of how much temperature and pressure the rock has endured. Raspberry garnet Garnet: Garnet Garnets are greatly variable in colors and varieties, and many of these are both rare and beautiful, producing genuinely precious gemstones. Garnet: Garnet As a gemstone, garnets have had a mixed reputation. Garnets do possess high indices of refraction, are hard enough, have pretty colors, are wonderfully transparent, lack cleavage and are durable; thus making good candidates for gemstones. Many people consider garnets to be inferior to other colored gems due to garnet's relative abundance and widespread use, and therefore a (typically) low price. Almadine garnet in schist Slide53: In metamorphic rocks individual minerals may or may not be bounded by crystal faces. The crystalloblastic series: a listing minerals in order of their tendency to be idioblastic. In the series, each mineral tends to develop idioblastic surfaces against any mineral that occurs lower in the series. Those that are bounded by their own crystal faces are termed idioblastic. Those that show none of their own crystal faces are termed xenoblastic. Crystalloblastic Series Crystalloblastic Series: Crystalloblastic Series rutile, sphene, magnetite tourmaline kyanite, staurolite, garnet, andalusite epidote, zoisite, lawsonite, forsterite pyroxenes, amphiboles, wollastonite micas, chlorites, talc, stilpnomelane, prehnite dolomite, calcite scapolite, cordierite, feldspars quartz This series can enable us to determine the origin of a given rock. Slate Quarry: Slate Quarry A roofing slate quarry in Pa. Slate: Slate A low grade metamorphic material, that is easily split along lines of foliation, slate has long been a roofing, flooring and paving material. Slate Quarry: Slate Quarry Black and gray slate, such as this, is highly carboniferous. Marble: Marble These are rocks composed mostly of calcite, and less commonly of dolomite. They result from metamorphism of limestone and dolostone. Some foliation may be present if the marble contains micas. Washington Monument, Washington, D.C. , is made of white marble from Baltimore, Maryland encasing a Maine granite core. Marble: Marble Jefferson Memorial, Washington, D.C. Made of Danby marble from Vermont. The interior is Tennessee pink marble and the ring around the base is Missouri gray marble. Marble: Marble A highly used building stone throughout the ages, due to it’s beauty, ease of carving and abundance. The Leaning Tower of Pisa is made of Italian marble. Marble: Marble Vulc mine: meta-marble zone Serpentine QuarryAlbene Stone Division of the Georgia Marble Co.: Serpentine Quarry Albene Stone Division of the Georgia Marble Co. Serpentine: Serpentine Serpentinites are rocks that formed by hydro-thermal metamorphism of ultra-basic igneous rocks. Serpentines vary greatly in consistency and use: from asbestos to serpentine marble. Serpentine: Serpentine Serpentine is a major rock forming mineral and is found as a constituent in many metamorphic and weather igneous rocks. It often colors many of these rocks green. Most rocks that have a green color probably have serpentine in some amount. Slide65: Serpentine Uses: many industrial applications, including brake linings and fireproof fabrics and as an ornamental stone. Serpentines: Serpentines The following is a list of these minerals, their formulas and symmetry class: Antigorite; (Mg,Fe)3Si2O5(OH)4; monoclinic. Clinochrysotile; Mg3Si2O5(OH)4; monoclinic. Lizardite; Mg3Si2O5(OH)4; trigonal and hexagonal. Orthochrysotile; Mg3Si2O5(OH)4; orthorhombic. Parachrysotile; (Mg,Fe)3Si2O5(OH)4; orthorhombic. Their differences are minor and almost indistinguishable in hand samples. The chrysotile minerals are more likely to form serpentine asbestos, while antigorite and lizardite form cryptocrystalline masses sometimes with a lamellar or micaceous character. Asbestos was used for years as a fire retarding cloth in ceiling tiles, pipe insulation and in brake linings. Its links to cancer have led to the development of alternative materials for these purposes. Quartzite: Quartzite Quartz arenites & chert are both composed mostly of SiO2. Since quartz is stable over a wide range of pressures and temperatures, metamorphism of quartz arenites and cherts will result only in the recrystallization of quartz forming a hard rock with interlocking crystals of quartz. Quartz Sandstone --> Quartzite: Quartz Sandstone --> Quartzite Another example of sandstone and a comparable piece of quartzite. ULM Quartzite Quarries - NUQQ: ULM Quartzite Quarries - NUQQ Products from quartzite: Standard Coarse Aggregates Railroad Ballast Surface Seals Road Base Natural Filter Media Manufactured Sands, 100% Crushed Slide70: The NUQQ quarry company produces: · Four different sizes of concrete aggregate · Six sizes of bituminous aggregates including manufactured sands · Rip Rap · Railroad ballast · Crushed road base · Seal coat chips · Aggregate for concrete products such as pre-cast building panels and concrete pipe · Decorative landscape rock · Ganister and filtration media · Filter rock for sewage treatment plants The 2nd largest manufacturer of crystalline mineral poultry grit in the U.S., under the name Cherry Stone. The grit is also sold as a traction agent on ice and snow. Quartzite: Quartzite Trilobite fossil in quartzite from Morocco Ancient quartzite tool from Morocco Blue Schist as an Ancient Building Material: Blue Schist as an Ancient Building Material This 12th-century medieval citadel and a domes municipals (town hall) sits upon an ultramafic dome. Many of the stones used in the citadel are blue schist and other ultramafic rocks, derived from the Braganca Massif. Blue Schist: Blue Schist Blue Schist as an Ancient Building Material: Blue Schist as an Ancient Building Material Ancient Roman Bridge in Portugal Blue Schist: Blue Schist The rocks in this area have been interpreted as fragments of the mantle below an island arc that existed in NW Spain and Northern Portugal during the mountain building period called Variscan (sometimes called the Herycnian) orogeny. Detail of Ancient Roman Bridge. Blue Schist: Blue Schist This mountain building period is interpreted as a result of a late Paleozoic period of collision between the attached North America and European continents with Gondwana to form the major portion of the supercontinent Pangaea. Portuguese country home made of varied metamorphic rocks including blue schist.: Portuguese country home made of varied metamorphic rocks including blue schist.