Introductory Petrology GEOL 285
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Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2009 Weathering When you talk about sedimentary rocks you have to talk about weathering Weathering Clastic rocks conglomerates sandstones mudrocks are composed of fragments and solid weathering products of preeXisting rocks Even carbonate rocks and cements whose constituents are precipitated from seawater by biological organisms are made from ions that come from the weathering process Two aspects to Weathering Mechanical Weathering physical breakup of rocks Chemical Weathering chemical breakdown of minerals in the presence of water Mechanical Weathering is trivial compared to chemical weathering due to the extraordinary dissolving power of H20 Mechanical Weathering Abrasion by wind or watercarried fragments Frostwedging is most important agent of mechanical weathering water freezes to ice 9 volume increase most important where water is liquid in daytime and freezes every night most important mechanism for increasing surface area of rocks Chemical Weathering H20 is polar therefore good at dissolving ions H20 dissociates into H and 0H39 H reacts readily with minerals Weathering is probably aided by organic acids and microorganisms Different minerals weather at different rates Weathering of minerals depends on their chemical stability in the weathering enVironment low T low P high H20 oxidizing What factors probably control the weathering rates of minerals Consider oliVine biotite quartz and plagioclase Goldich s Weathering Series 7 see handout Note similarity to Bowen s Reaction Series Why Summarizes susceptibility of minerals to weathering Weathering Reactions of Orthoclase Step 1 3 KAISi308 2 Ht 12 H2O gt KAIgSi3010OH2 6 H4SiO4 2 Kl orthoclase illitemuscovite soluble silica Step 2 2 KAIgSi30100H2 2 H 3 H20 gt 3 A12Si205OH4 2 K illite kaolinite All feldspars weather similarly React with H20 and H Release silica in solution and cations Produce clay minerals sheet silicates Albite H20 H Sodium montmorillonite H4Si04 Na Anorthite H20 H Calcium montmorillonite H4i04 Ca2 Montmorillonite Montmorillonite formula NaCaAlMg2Si40100H2nH20 Montmorillonites are EXPANDING clays unlike illite and kaolinite Al is essential in all clay minerals ie Al in weathered silicates goes into clay minerals Mg silicates also weather to form montmorillonite Iron in minerals weather differently Fe in most ferromagnesian minerals is reduced Fey because they re formed in reducing conditions low oxygen Surface waters are very oxygenrich ie oxidizing Fe2 released during weathering immediately oxidizes to Fe Fe precipitates rapidly as EXTREMELY INSOLUBLE FeOH3 and other hydroxides Weathering of pyroxene for example CaFeSi205 Fe part of augite H20 H Calcium montmorillonite H4SiO4 Ca2 FeOH3 The most common products of weathering are Quartz Clay mineralskaolinite illite montmorillonite Cations in solution Ferric hydroxides and oxides insoluble from the weathering of ma c minerals Which will weather more rapidly basalt or granite Why Weathering of Basalt What will be the most common sandsized fragments What will be the most common mineral weathering products Weathered ash deposits form bentonite a mixture of clay minerals mostly montmorillonite expand when wet very slippery Weathering of Granite Grus Where does alteration first occur Disaggregation of grains forms quotgrusquot Surface esp of feldspars gets soft and punky why What are the mineral products of granite weathering disaggregated grains of quartz and feldspar from Granite Geol 285 Petrology Dr Helen M Lang West Virginia University Fall 2005 Generation of Subductionrelated Andesites J K Gill s Model Different from model presented in textbook When mantle melts beneath MidOcean Ridges or at Hotspots uniform tholeiitic basalt is formed Why does melting of mantle at subduction zones produce calcalkaline magmas mostly andesites Things about subductionrelated magmatism that must be explained by any model The magmas and rocks are calcalkaline not tholeiitic ie they show no Feenrichment Magmas and rocks are dominantly andesites with higher SiOz than basalts More varied magma types are produced Generation of Subductionrelated Andesites JK Gill s Model See handout FOAM fractionation key to Gill s model fractionation of the following minerals PPlagioclase OOliVine and or Orthopyroxene AAugite MMagnetite fractionation of some phase with high Fe and low SiOz is necessary to explain SiOz rich andesites and lack of Feenrichment of subduction related magmas could be Mt or Hornblende or Garnet and to lower density of trapped melt pk1152 Subductionrelated volcanoes commonly have bimodal volcanism Cinder cones rhyolite domes and zoned ash ows at Mt MazamaCrater Lake Cinder cones rhyolite domes at Mt Shasta Abundant andesite lava ows at Mt Rainier Explosive rhyolitic ash ows at many subductionrelated volcanoes Liquid Segregation in Shallow Chamber can lead to Bimodal Volcanism copy sketch Ash eruption could produce layered ash ows like at Crater Lake Pinnacles Crater Lake Ash Flow Pinnacles Gentle gaspoor eruption could form rhyolite domes central and basalt cinder cones anks Also at Crater Lake after the big ash ows Bimodal Volcanism at Mount Shasta Things about subductionrelated magmatism that must be explained by any model and Gill s explanation The magmas and rocks are calcalkaline not tholeiitic ie they show no Fe enrichment Magmas and rocks are dominantly andesites with higher SiOz than basalts Explanation POAM fractionation at base of crust More varied magma types are produced Explanation Liquid segregation in shallow chamber variable fractionation contamination on way through crust Geol 285 Dr Helen Lang West Virginia University Spring 2008 Metamorphism of Shales Pelitic Rocks So far we have talked about metamorphism of Ma c Rocks metabasalts amp metagabbros Metamorphic Facies ACF diagram Note One of the major effects of metamorphism is dehydration or decarbonation Next we ll look at Metapelites metamorphosed shales They were the rst metamorphic rocks to be understood in a modern way by Barrow in about 1890 Probably the most studied metamorphic rocks They form index minerals that are porphyroblastic and easily identi ed in the eld garnet staurolite kyanite etc so eld mapping of isograds is possible They provide more Pressure information than any other common rocks based mostly on Alei05 polymorphs We ll need a new Triangle We need a diagram that separates FeO and MgO Different minerals have different preferences for Fe and Mg Some minerals of metapelites are high in Fe some are high in Mg We need to be able to show this Separates minerals like Garnet and Chlorite which plot on top of eachother on ACF Important chemical components in metapelites from clay and Qtz silt in shales I K20 F60 MgO A1203 SlOz H20 CaO and NazO are mostly in Plagioclase assume it s present Quartz is always present so we can delete SiOz Assume H20 is present pure or at constant fraction so we can delete H20 The four remaining components can be represented on corners of a tetrahedron see handout JB Thompson in 1957 suggested projecting from Muscovite onto the AFM triangle Plotting positions of common Metapelite Minerals on AFM diagram see handout A128i05 Polymorphs occur in metapelites and are very important as Pressure indicators Properties ofA128i05 Polymorphs Kyanite blue bladed different hardness in different directions one good cleavage Andalusite white to pink stubby square prisms may be Chiastolite with cross of graphite inclusions Sillimanite long thin prisms or needles clear and colorless Metamorphism of pelitic rocks in the Snow Peak area northern Idaho Dr Lang s PhD project Location and General Geology Snow Peak Best outcrops are on ridgetops During eld work we stayed at Fly Flat Campground My advisor Dr Jack Rice University of Oregon taught me to cook in a dutch oven Pseudomorphs are prominent in many samples Lowest Grade rocks ChlBt zone rocks are still pretty shalelike First isograd Garnet isograd Next isograd Staurolite isograd Staurolite is euhedral twinned and porphyroblastic Staurolite Garnet in thin section First Kyanite found along Snow Peak Trail Kyanite is most abundant in Pseudomorphs At the next two isograds Chlorite and Staurolite nally disappear Highest grade rocks contain Garnet Biotite Kyanite Muscovite Quartz Plagioclase Ilmenite and Graphite Highest grade rocks are in the southwestemmost comer of the area Metamorphic grade increases toward the nearest exposures of the Idaho batholith Ithink this is an example of regionalcontact metamorphism From progression of assemblages and isograds we can say that metamorphic conditions in all zones were generally in this region I analyzed minerals on an electron microprobe to estimate T and P From minerals analyzed with the electron microprobe I calculated the following temperatures for each zone called geothermometry Garnet zone 470 C Staurolite zone 490 C Transition zone 530 C StauroliteKyanite amp Kyanite zones 560 C Pressure estimate for Transition St Ky and Kyanite Zones is 665 kbars called geobarometry A little bit about the larger region AS K rocks at Goat Mountain Andalusite clearly replaces Kyanite crystals so Kyanite iSillimanite formed rst and Andalusite formed later due to a Pressure decrease Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2008 Volcanism Volcanism is a Planetary Thermal regulatory Mechanism Interior temperature is much higher than T at surface Heat left over from planetary accretion Radioactive decay of long lived isotopes 40K 235U 238U 232Th gives off heat Earth and all planets lose heat to their external environment Upward movement of magmas is a very effective mechanism for moving heat toward the surface Distribution of Active Volcanoes on Earth see Simkin el 01 2006 This Dynamic Planet USGS Smithsonian Institution interactive map Oceanic Volcanism Midocean ridge basalt MORB volcanism More or less continuous volcanism along the midocean ridges Mainly quiescent because of low viscosity and low gas content of lowK tholeiite Occasionally above sea level as in Iceland 0 Island arc volcanism Caused by oceanocean subduction Some highAl basalts but Mostly andesites Also some dacites and rhyolites Higher in SiOg and Viscosity and more explosive than MORBs Common in the West Pacific 0 Oceanic island volcanism Related to hotspots or hot localized plumes of rising mantle Hawaii and other oceanic islands Paci c Ocean Floor Topography Linear AgeDistance Relation indicates constant rate of motion of Paci c Plate over the Hawaiian Hot Spot The Hawaiian Islands Five Volcanoes on the Big Island of Hawaii show earliest stages Classic Shield Volcanoes 510O slope Mauna Loa and Kilauea show shield building stage subaerial substage 2c Kilauea Caldera from the air Recent eruptions of Kilauea A a lava ows A a is blocky rough jagged with a spiny surface Pahoehoe Flows smooth billowy ropy surface Maps and Photos of Recent Kilauea Eruptions 1983 7 present Pu u O o eruptions are most voluminous from the East Rift of Kilauea in 500 years Mauna Kea represents the Capping Stage 3 Review Volcanic History The Hawaiian Islands Haleakala Volcano East Maui renewed volcanism stage 5 after erosional stage 4 Rocks at Haleakala Summit renewed volcanism Stage 5 Continental Volcanism 0 Continental Arc Volcanism Continental Margin Volcanism When oceanic crust is subducted beneath continental crust Continental Margin Volcanism Stratovolcanoes or Composite Volcanoes lnterlayered Pyroclastics amp Lavas Typical Platy Andesite Continental Volcanism Continental Flood Basalts Tholeiites Columbia River Basalts CRBs Miocene Deccan Flood Basalts NW India Bombay CretaceousEocene Continental Rift Basalts East African Rift Alkali Basalts Columbia River Basalts Distribution of Active Volcanoes on Earth Eruptive Styles Plinian Large explosive events that form enormous dark columns of tephra and gas high into the stratosphere described by Pliny the younger in relation to the disastrous 79 AD eruption of Vesuvius Eruptive Styles Phreatic Eruptions Phreatic eruptions are steamdriven explosions that occur when water beneath the ground or on the surface is heated by magma causing water to boil and ash to steam thereby generating an explosion of steam water ash blocks and bombs Nu e Ardente ash ow or pyroclastic ow Pel ean eruption 0 A groundhugging avalanche of hot ash pumice rock fragments and volcanic gas that rushes down the side of a volcano as fast as 100 kmhr39 temperature within the ow may be greater than 500 C Eruptive Styles Strombolian Intermittent explosion or fountaining of basaltic lava from a single vent or crater produces cinder cones like Stromboli in the Mediterranean lower photo Controls of Eruption 0 Composition and viscosity Si02 and to a lesser extent A1203 control viscosity of magmas because they form polymers in magma basalt viscosity at eruption T 102104 pascalsec andesite viscosity at eruption T 104106 pas rhyolite Viscosity at eruption T 1010 pas up to onehundred million X higher than basalt Dissolved magmatic gases H20gtgtgtC02gtgtF2C12 N2 802 or H28 Solubility of gases decreases with decreasing Pressure Boiling formation of bubbles caused by magma rise and crystallization Boiling of Magma Evolution of gas produces bubbles which rapidly expand and rise to the top of the chamber Volume eXpansion of bubbles fragments the surrounding liquid into tiny fragments and clasts and provides the driving force for eruption Rapid and continuous magma ascent Plinian eruption Continuing gas evolution and magma fragmentation at low to moderate ascent rates Cinder cones Stagnation and gradual outgassing of magma below a vent plug Strombolian eruption Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 285 INTRO PETROLOGY Weathering When you talk about sedimentary rocks you have to talk about weathering Weathering Clastic rocks conglomerates sandstones mudrocks are composed of fragments and solid weathering products of preeXisting rocks Even carbonate rocks and cements whose constituents are precipitated from seawater b biolo39ical or39anisms are made from ions that come from the weathering process TWO aspects to Weathering Mechanical Weathering physical breakup of rocks Chemical Weathering chemical breakdown of minerals in the presence of water Mechanical Weathering is trivial compared to chemical weathering due to the extraordinary dissurvmg power UL 1120 Mechanical Weathering Abrasion by Wind or watercarried fragments Frostwedging is most important agent of mechanical weathering water freezes to ice 9 volume increase most important Where water is liquid in daytime and freezes every night most important mechanism for increasing surface area of rocks a 1 x r if t W rv r lHnl 39 L J l llllll x J a 1 ll r39 H0 H20 is polar therefore good at dissolving ions H20 dissociates into H and OH 5 8 H reacts readily with minerals 5C weathering is aided by organic ac1ds and microorganisms Different minerals weather at different rates Weathering of minerals depends on their Chemical 111 L11 our iauu Wuauiui 1115 environment at low T low P high H20 XllZlIlO nriin Goldich s Weathering Series Summarizes susceptibility of minelam LU WUaulUl mg Olivine NIOSt susceptible Calcic Plagioclase Pyroxene Amphibole Biotite Sodic Plagioclase Potassium Feldspar Muscovite Quartz Least Susceptible Weathering Reactions of Orthoclase Step 1 3 KAISi308 2 H 12 H20 gt orthoclase KA13Si3010OH2 6 HAtSiO4 2 K illite muscovite soluble silica Step 2 2 KA13Si3OIOOH2 2 H 3 H20 gt illite 3 A128i205OH4 2 K kaolinite All feldspars weather similarly React with H20 and H Release silica in solution and cations Produce clay minerals sheet silicates Albite H20 H Sodium montmorillonite HZlSiO4 J Anorthite H2O H Calcium montmorillonite HZlSiO4 Ca2 Montmorillonite Montmorillonite formula NaCaAlMg2Si4OloOH239nHZO Montmorillonites are EXPANDING Clays unlike illite and kaolinite Al is essential in all Clay minerals ie Al in weathered silicates goes into clay minerals Mg silicates also weather to form montmorillonite Iron in minerals weather differently Fe in most ferromagnesian minerals is reduced Fe2 because they re formed in reducing low oxygen conditions Surface waters are very oxygenrich ie oxidizing Fe2 released during weathering immediately oxidizes to Fe3 Fe3 precipitates rapidly as EXTREMELY INSOLUBLE FeOH3 and other hydroxides Weathering of pyroxene for example babe51206 Fe part of augite HZU 11 2 Calcium montmorillonite HALSiO4 La2 FeOH3 FeOH3 and other Ferric oxyhydroxides precipitate oranve or rust and eventuall deh39 drate to Hematite Which gives subaerial soils and sediments red beds their red color The most common products of weathering are Quartz Clay minerals kaolinite illite montmorillonite Cations in solution Ferric hydroxides and oxides insoluble from the weathering ofma c minerals Which Will weather mow Lapidly basalt or granite Why Weathering of Basal What will be the most common sandsized fragments 0 What will be the mom common mineral weathering products Weathered ash deposits form bentonite a m1Xture or clay mmerals mostly montmorillonite expand when wet very slippery can crack foundations Weathering of Granite Where does alteration rst occur Disaggregation of grains forms grus Surface esp of feldspars gets soft and punky Why What are the mineral products of granite weathering Grus is a common rst step in granite weathering Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2009 Sandstones and Conglomerates Clastic Sedimentary Rocks 2025 of stratigraphic record but receive much more attention from sedimentary petrologists than 25 What geologists want to learn from Sandstones Source area rock type direction weathering environment Transport medium energy distance Depositional environment marine or nonmarine physical environment beach river delta etc What clues are present in Sandstones Grain size Grain shape Grain sorting Grain mineralogy Sedimentary structures Grain size Detrital 0r clastic rocks have a huge range in grain size We need a log scale to represent this wide size range The Phi 1 Scale I logz mm mm 2quot memorize Each 4 step represents a doubling smaller or more neg 0r halving larger in size For example size in mm239 626mm64mm 0 20mmlmm 2 2392mm14mm025mm 4 2394 mm 116 mm 00625 mm Size ranges are given names See handout of UddenWentworth scale Gravel larger than l gt 2 mm Sand 44 to l 00625 mm to 2 mm Mud smaller than 44 lt 00625 mm Clay smaller than 84 lt 0004 mm lt 4 pm Loose sediments can be separated by sieving 2 4mm l 2mm 11 05mm 24 025mm 34 0125mm 44 0062mm closed Grain size comparator for lithi ed Sandstone Sorting range in grain size Usually the size range in 1 that includes 23 of the grains Sorting by comparison Grain Shape Sphericity relative equidimensionality of three mutually perpendicular axes Roundness lack of sharp comers larger grains round faster because of more impacts Mudrocks Composed mostly of detrital material smaller than 41 ie smaller than 0062 mm or 625 um mudsiltclay Non ssile stone or ssile shale Named by proportion of Silt and Clay gt 23 silt Siltstone Siltshale 13 to 23 silt Mudstone Mudshale gt2 3 clay Claystone Clayshale Conglomerates No agreement about the of gravel sized material required to make a sediment a GRAVEL or a sedimentary rock a CONGLOMERATE Let s say gt30 gravel size material for a conglomerate Small amount of pebbles or cobbles is very noticeable Classi cation of Sandstones is based on Detrital Minerals Quartz is most abundant sandsize grain very stable in sedimentary environment Feldspar may be abundant may indicate rapid burial dry climate granite in source area Kfeldspar vs Plagioclase Unstable Lithic Fragments are least abundant but most informative Chert a stable lithic fragment goes with Quartz Accessory Minerals other minerals Micas may oat in water muscovite especially is very stable Heavy minerals higher density than quartz and feldspar some are very stable zircon tourmaline rutile hornblende garnet ilmenite magnetite apatite pyroxene etc If loose sand or disaggregated sandstone is put in a heavy liquid sg 2830 quotheaviesquot sink to the bottom Can be quite informative 0 Quartz o Chert is a stable lithic fragment grouped with Quartz o Feldspars distinguished from quartz by alteration twinning and perthite 0 What s the large grain o Lithic Fragments o Volcanic and Plutonic Lithic Fragments in a Sandstone Sandstone Classi cation Sandstone Classi cation after Folk see handout based on of detrital QuartzChert Q Feldspar F plagioclase orthoclase Lithics L unstable lithic fragments and mud matrix Naming Arenites Sandstones with lt15 mud are called Arenites and are Plotted on the front triangle Plot Q quartz chert F feldspar L unstable lithic fragments on the front triangle Know names and their positions Naming Wackes Sandstones with 15 to 50 mud are called Wackes and are Plotted on the middle triangle Plot Q QQFL F FQFL L LQFL on the middle triangle Know names and their positions quotTextural Maturityquot of a Ss A measure of the progress of a clastic sediment in the direction of chemical mineralogical and textural stability Affected by processes that take a long time Maturity increases with total input of kinetic energy time of transport distance of transport energy of medium Increasing quotTextural Maturityquot is indicated by clay removal increased sorting increased rounding breakdown absence of unstable fragments breakdown absence of unstable minerals Immature Sandstones limited transport rapid deposition and burial Lots of muddy matrix poorly sorted poorly rounded fragments and grains lots of unstable lithics and unstable minerals mostly wackes formed in convergent margin settings arctrench gap Supermature Sandstones Clean no mud matrix wellsorted wellrounded grains mostly quartz grains quartz arenites Cratonic typically recycled formed in beach or other high energy environment In order of increasing Textural Maturity Wackes immature Litharenites Arkoses Subarkose and sublitharenite Quartz arenites supermature Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2009 Classi cation of Igneous Rocks Plutonic Igneous Rocks are classi ed on the basis of their Mineral Content Minerals are usually large enough to see and identify Mineral Content is expressed as the Mode where Mode of each mineral present Mode is plotted in IUGS classi cation It is called the IUGS or Streckeisen Classi cation and is based on the Percent ofthe felsic minerals Q Quartz A Alkali Feldspar P Plagioclase F Feldspathoids Q A P and F are plotted on the diamondshaped diagram see handout What are the properties diagnostic and otherwise of Quartz Alkali Feldspar PlagioclaseFeldspathoids are SiOgde cient feldsparlike minerals also framework silicates Streckeisen QAPF Diagram with Plutonic Igneous Rock names see handout How to plot on the Streckeisen QAPF Diagram Ignore ma c minerals if lt90 Calculate and plot QQAP 100 counting up from AP side Or FFAP 100 counting down from AP side Igneous rocks will either have Quartz Q or Feldspathoids F but not both Q amp F incompatible Calculate and plot PAP 100 counting from A at the left toward P at the right connect diagonal line to Q comer Plotting see handouts There is a Streckeisen QAPF Diagram for Volcanic Rocks but it is hard to use because most mineral grains in volcanic rocks are too small to identify therefore Volcanic rocks are most commonly classi ed or named on the basis oftheir chemical compositions Igneous Rocks have a limited range of chemical compositions Major Elements generally gtl0 wt SiOz 3078 wt A1203 Fe203 FeO MgO CaO NazO 010 wt K20 015 wt Minor Elements generally 01 to 10 wt HzO structural water bound in minerals HzO39 adsorbed water driven off at 110 C TiOz P205 MnO C02 Cl I F Trace Elements in Igneous Rocks Present in amounts less than 01 wt or lt1000 ppm parts per million Very low amounts But they tell a lot about the rock s history They include Rare Earth Elements REES or lanthanides radioactive elements and other heavy elements Review Periodic Table Si02 is the most important and variable major element Primary subdivision of volcanic rocks is on the basis of SiOz content picrobasalt 4145 wt Si02 basalt 4552 basaltic andesite 5257 andesite 5763 dacite 6369 rhyolite gt69 dep on wt NazOKzO Alkali Content is also Important Some groups of igneous rocks are relatively low in alkalis NazO and K20 called subalkaline Some groups of igneous rocks are relatively high in alkalis called alkaline These may be either sodic or potassic depending which is dominant IUGS Volcanic Classi cation is Graphical plot Na20K20 vs Si02 TAS total alkali silica diagram It is Traditional and useful to Classify Volcanic Rocks as follows Subalkaline Rocks Tholeiitic Series relatively common Calcalkaline Series Alkaline Rocks AlkaliOlivine Basalt Series Nephelinites Leucitites Analcitites rare Each of the common series has different chemical and mineralogical characteristics and occurs in different tectonic environments The Tholeiitic Series Subalkaline Midocean Ridge Basalts MORBs Iceland plateau or ood basalts Columbia River Basalts some oceanic islands Hawaii major component of Archean greenstone belts Not porphyritic may have a few Olivine or Pyroxene phenocrysts Basalt is dominant The Calealkaline Series Subalkaline Subductionrelated volcanoes and plutons CircumPaci c andesitic stratovolcanoes Strongly porphyritic dominantly Plagioclase phenocrysts also Olivine OpX or Homblende phenocrysts Andesite is dominant Distinguish Tholeiitic from Calealkaline Series on AFM diagram see handout The AlkaliOlivine Basalt AOB Series Alkaline Cap Hawaiian volcanoes dominate some oceanic islands Tahiti St Helena Azores occur in interior of island arcs associated with continental rifting East African rift Plagioclase Augite or Olivine phenocrysts may be dominant Felsic rocks are trachytic Silica minerals are rare Feldspathoids are common in the groundmass IUGS Volcanic Classi cation is Graphical plot Na20K20 vs Si02 and read off the rock name Geol 285 Dr Helen Lang West Virginia University Spring 2009 Limestones and Dolomites Carbonate rocks usually deposited with the help of biological organisms Mineralogy is simple Carbonate minerals Calcite CaC03 Aragonite CaC03 Dolomite CaMgCO3z Minor Quartz and0r Clay Therefore Particle Types and Textures are Important for Classi cation Allochemical Particles allochems framework grains of a mechanically deposited limestone four main types some formed of calcite some aragonite Orthochemical Particles orthochems matrix and cement that ll spaces bind allochems together and lithify sediment Allochems Fossils solid carbonate remains of organisms fossils and fragments of fossils Peloids ellipsoidal aggregates of microcrystalline CaC03 lack internal structure mostly fecal pellets of worms fish etc Oquotliths spherical polycrystalline carbonate particles of sand size with concentric or radial internal structure commonly have a nucleus for precipitation Limeclasts fragments of earlierformed limestone mostly intraclasts from a local source Fossils Brachiopod Shell Fossils Peloids Ooids Orthochems Microcrystalline Calcite Micrite CaCO3 mud disarticulated algal material carbonate ooze l 4 pm diameter Note difference between lime mud and silicate mud Coarsely crystalline calcite Sparry calcite or quotSparitequot calcite cement precipitated from pore uid inorganic ppt Usually one or the other not both Micrite Sparitecoarse crystalline calcite Cc is stained pink Noncarbonate Minerals Typically less than 5 terrigenous detritus quartz clay chert Limestones form only where input of terrigenous detritus especially mud is minimal fresh water changes salinity organisms are killed or buried by mud Chert is intrabasinal from siliceous organisms or is diagenetic Gulf of Mexico Limestones occur Where elastic input is minimal There are two commonly used classi cation schemes Folk s Classi cation 7 Based on major allochem and major orthochem 7 Hard to use Without thin sections 7 Not useful in the eld Dunham s Classi cation 7 Based on structure and percent grains vs mud 7 More useful in the eld Folk s Classi cation Major Allochem pre x Major Orthochem suf x Fossils bio micrite Peloidspel sparite Ooliths oo Limeclasts intra gt90 micrite is just called Micrite Folk Name Biomicrite Folk Name Oosparite Folk Name Micrite Dunham eld classi cation see handout Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2009 Geol 285 Introduction to Petrology The study of rocks Greek petra rock logos discourse or explanation study Petrology is central to Geology and is based on Mineralogy A rock is a naturallyoccurring aggregate of minerals or mineraloids What is the difference between a rock and a mineral Rocks are conveniently divided into 3 categories Igneous rocksrocks that solidi ed from molten or partially molten material magma Sedimentary rocksrocks resulting from the consolidation of loose sediment or chemical precipitation from solution at Earth s surface Metamorphic rocksrocks formed from preeXisting rocks by mineralogical chemical and textural changes in response to change in conditions The Rock Cycle shows how rocks form and change from one type into another Why study petrology Learn about early history of Earth Learn about the interior of the Earth Only a small part ofthe crust is exposed or accessible to drilling o Crust is less than 1 of Earth s volume 0 66 of crust is sedimentary 0 other 34 is mostly igneous Mantle is metamorphic Core is liquid and solid metal FeNi Thickness of Crust is 12 of Earth s Diameter Why study Petrology continued We can learn about the whole Earth only by studying exposed rocks drill cores and geophysics Distribution of rock types at Earth s surface led to Plate Tectonic Theory Compare modern processes with ancient rock record infer processes and explain differences We ll start with Igneous Rocks Outcrop Characteristics see Table 11 in textbook Volcanoes and related lava ows Crosscutting relations to surrounding rocks dikes veins stocks and batholiths Thermal effects on adjacent rocks Chilled negrained borders against adjacent rocks Lack fossils and strati cation Generally structureless massive and composed of interlocking grains Typical Igneous Textures Porphyritic Glassy Vesicular Pyroclastic Interlocking crystalline Typical Igneous Minerals Plagioclase Feldspars CaNaAlSi408 Alkali Feldspars NaKAlSi308 Quartz SiOz OliVine MgFezSiO4 Pyroxene CaMgFezSi205 Hornblende Micas biotite and muscovite Glass not a mineral but a mineraloid Initial Classi cation based on Percent ma c and felsic mineral content re ects chemistry of magma more FeO MgO more mafic minerals darker color more SiOz NaZO K20 more felsic minerals lighter color Grain size relates to cooling rate negrained or glassy cooled rapidly coarsegrained cooled slowly See handout for General Classi cation of Igneous Rocks and their properties The amount of Si02 in igneous magmas is quite variable and signi cant Magmas with enough SiOz to crystallize quartz pure free SiOz are said to be Oversaturated with SiOz Magmas with so little SiOz that they crystallize minerals that are incompatible with quartz are said to be Undersaturated with SiOz Geol 285 Dr Helen Lang West Virginia University Spring 2009 Metamorphism amp Metamorphic Rocks Because the Earth is a dynamic system rocks once formed may be subjected to very different conditions Metamorphism means Change Changes in conditions cause changes in mineralogy and texture of rocks Because minerals that were stable at original conditions are no longer stable at new conditions Changes that take place in the solid state between diagenesis lower limit and melting upper limit are called metamorphism Diagenesis vs Metamorphism Gradational boundary Metamorphism begins with the formation of new minerals not observed in any sediments at Earth s surface muscovite chlorite epidote albite paragonite pyrophyllite DiagenesisMetamorphism boundary is at about 15002000C 2 kilobars but P is not critical Metamorphic Igneous boundary When metamorphic temperature gets very high the rocks begin to melt Partly melted rocks are called migmatites mixed rocks and are considered metamorphic melting T depends on rock composition granite and shale begin to melt at 650 C basalts begin melting at 800 C liquidsolid mixture for gt2000 T range If a rock gets mostly or completely melted it is considered igneous Protolith Any rock can be changed to become metamorphic Rock from which a metamorphic rock is formed is called its protolith Igneous protolith is indicated by the pre x quotorthoquot metaigneous Sedimentary protolith is indicated by pre x quotparaquot metasedimentary Protolith is indicated by rock composition inherited textures often it s hard to determine Textures Characteristic of Metamorphic Rocks Deformation causes anisotropic fabrics foliation any planar texture or structure in a rock schistosity alignment of platy minerals thin aky layers gneissosity mineral segregation thicker layers lineation alignment of elongate minerals Metamorphic rocks are commonly folded They commonly contain porphyroblasts Agents of Change in Metamorphism Mainly temperature T and pressure P Both temperature and pressure increase with depth in the Earth The rate of increase of temperature with depth in the Earth is called the geothermal gradient The geothermal gradient varies with tectonic setting Continental Shield vs Oceanic geotherm Lithostatic Load Pressure increases with Depth P pgh p density N30 gcm3 g 981 cmsec2 N 103 cmsec2 h 1 km 105 cm AP kilometer N 3 X 103 X 105 dynescm2 3 X 108 dynescm2 convert to bars pressure AP kilometer N 300 barskm or 03 kilobarskm 1000 bars l kilobar N 33 kilometers depth Some causes of Metamorphism Burial metamorphism at the base of a thick sedimentary sequence very low grade TmaXN3000C garnet grade metamorphic conditions may be reached Contact metamorphism heat from a pluton may raise T of country rocks high enough to cause metamorphism growth of new metamorphic minerals Regional metamorphism crustal scale thrusting caused by continental collisions exposes rocks to high pressures and temperatures to cause regional metamorphism Subduction zone metamorphism when cold rocks are dragged down into a subduction zone temperatures are lower than normal for a given depth De ne Isograd An isograd is a line on a map marking the rst appearance of a new metamorphic mineral De ned by Barrow in 1890s Interpreted to be a line of approximately equal metamorphic grade or T and P during metamorphism Characteristics of Contact Metamorphism Metamorphic effects are localized around a pluton obVious association with a pluton Isograds are approximately concentric with pluton margin form contact aureole Very limited extent at most a few km wide Mineralogic changes re ect mostly changes in T Minerals are mostly low pressure minerals Minerals lack preferred orientation rocks are generally undeformed called homfels Contact Metamorphism of impure Limestone Characteristics of Regional Metamorphism Metamorphic effects are not clearly associated with a pluton Effects are regional extending over 10s to 100s of kilometers Rocks contain moderate to high pressure minerals Like what Rocks are generally deformed and have strong fabrics lineation and foliation Mineral changes re ect changes in both T and P Regional Metamorphism in Scottish Highlands see handout The Great Glen Fault Intermediate Cases between Contact and Regional Metamorphism are Common Low Pressure Regional or RegionalContact Metamorphism Mineralogic Changes depend on original rock composition because ingredients needed to make minerals must be present in the protolith Quartz crystals in a quartz arenite just get bigger recrystallize nothing else can grow Ss becomes quartzite Pure calcite limestone becomes pure calcite marble Basalts form plagioclase amphiboles and other ma c minerals Shales form aluminous minerals like garnet biotite muscovite staurolite KyAnd Sill and quartz Barrow 1893 was the rst to show progressive changes in a single rock type and relate them to an increase in metamorphic intensity grade Metamorphosed shales pelitic rocks Dalradian Series in the Scottish Highlands Between the Great Glen Fault and the Highland Boundary Fault De ned iso grad Metamorphism in Scottish Highlands see handout Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2008 Crystallization of Magmas and Phase Diagrams Igneous Rocks form by the Crystallization of Magmas at or beneath the Earth s surface Effects of cooling rate Magma cooled very suddenly freezes to glass basalt must cool more quickly than rhyolite to form glass Magma cooled pretty quickly has many small crystals microscopic or sub microscopic called aphanitic Magma cooled very slowly has big crystals called phaneritic How might a porphyritic texture develop Porphyritic means What happens to a magma as it cools Pure liquids quotfreezequot crystallize at a single temperature melting T freezing T Water to ice at 0 C SiOz liquid to cristobalite at 1740 C Impure liquids like magmas crystallize over a range of T crystals are different composition than liquid Different minerals form at different temperatures as the magma cools Sequence of mineral crystallization and T at which different minerals begin to crystallize depends on Magma composition Pressure Water pressure Cooling rate and many other factors Bowen s Reaction Series see handout is a generalization about quottypicalquot Crystallization order of basalts To really understand igneous crystallization we need to go beyond Bowen s Reaction Series In the last 50 years geochemists have done laboratory experiments on many magmalike liquids One Method Melt a rock cool liquid to known T quotfreezequot it View it under the microscope identify minerals called quotcookandlookquot Better Method Start with simple systems e g various proportions of pure Anorthite Caplag CaAlei208 and pure Diopside pyroxene CaMgSi205 examine results at different Ts Results are displayed on phase diagrams What is a phase a homogeneous portion of a system that is physically distinct and mechanically separable from the other parts of the system p phases We also need to know about chemical composition components the minimum number of chemical constituents necessary to describe the compositions of all solids liquids and gases phases in a system c components Ifthe System ofinterest is Quartz Cristobalite Tridymite Stishovite Coesite and liquid SiOz All phases can be described with the single chemical formula SiOz 1 component SiOz I 01 For more complex systems nding the components is a trialanderror process for Olivines and corresponding liquids MngiO4Ol or liq FezSiO4Ol or liq MgFeSiO4Ol or liq Mg15Fe05SiO4Ol or liq etc try elements Mg Fe Si 0 try oxides MgO FeO SiOz 3 try mineral endmembers MgZSiO4 Fe2SiO4 2 2 is the smallest number therefore MgZSiO4 and Fe2SiO4 are the components and c2 For Olivines Mg28i04 Fe28i04 and Orthopyroxenes MgSiO3 FeSiO3 and corresponding liquids try mineral endmembers MngiO4 FezSiO4 MgSiOg FeSi03 4 try oxides MgO FeO SiOz 3 try elements Mg Fe Si O 4 3 is the smallest number so MgO FeO and Si02 oxides this time are the components and c3 The Gibbs Phase Rule A generalization about phase diagrams by J Willard Gibbs in the 1870s f degrees of freedom the number of parameters that can be varied independently without changing the phases present f 2 c p f degrees of freedom 0 components p phases Examples will show what this means f0 invariant point fl univariant line f2 divariant eld or area f3 trivariant One component system Si02 see handout Real Magmas have many components 8 or 9 We can t draw diagrams that represent all those components Requires too many dimensions But we can learn a lot about crystallization of real magmas like basalts by examining several different 2component systems 2component Binary Systems We d need 3 dimensions to represent one phase elds c2 pl f22l3 trivariant But we can only draw diagrams in 2 dimensions easily Hold Pressure P constant lose one degree of freedom only need 2 dimensions Phase rule at constant P becomes fp l c p Simple 2component system Diopside CaMgSi206 Anorthite CaAlgSi208 Di An See handout This system is a pretty good simple model for basalts How do we use this diagram to tell about crystallization of basalt follow on handout Crystallization ofa Diopside Di rich liquid Crystallization of an Anorthite An rich liquid Tielines and the Lever Rule View Animated DiAn Diagram Professor Ken Windom Iowa State University Textures of rocks in DiAn System See handout Plagioclase and Pyroxene in Diabase slightly coarsergrained quotbasaltquot Flag and Pyroxene Phenocrysts in negrained groundmass porphyritic basalt Perfect Equilibrium Crystallization Crystals that have formed remain in contact with the liquid and continually equilibrate with it Constant bulk composition Makes little difference in DiAn system Contrasted with fractional crystallization where crystals are physically separated from the liquid in which they were formed unable to equilibrate More Complex 2component system Forsterite MggSiO4 Enstatite MgSiO3 Silica SiOg System Fo En Si02 See handout Olivine in this Basalt reacted with Liquid to form Orthopyroxene 2component system with Complete Solid Solution Albite NaAlSi3Og Anorthite CaAlQSlQOgg System AbAn See handout See Windom Animated phase diagram Zoned Plagioclase Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2008 Sedimentary Rocks Rocks resulting from the consolidation of loose sediment or chemical precipitation from solution at or near the Earth s surface or organic rocks consisting of the secretions or remains of plants and animals Sediments are deposited in Basins low places on Earth s surface Six Major Basin Types occur in different Plate Tectonic Settings Oceanic basins Arctrench system basins Continentalcollision basins Basins in displaced or exotic terranes Grabens along continental margins Intracratonic basins Weathering Clastic rocks conglomerates sandstones mudrocks are composed of fragments and solid weathering products of preeXisting rocks Even carbonate rocks and cements whose constituents are precipitated from seawater by biological organisms are made from ions that come from the weathering process Two aspects to Weathering Mechanical Weathering physical breakup of rocks esp frostwedging water gt ice causes 9 volume increase Chemical Weathering chemical breakdown of minerals in the presence of water which is polar and good at dissolving ionic minerals Chemical Weathering is MUCH more important than mechanical weathering due to the extraordinary dissolving power of H20 Goldich s Weathering Series see handout Summarizes susceptibility of minerals to weathering Weathering Reactions of Orthoclase All feldspars weather similarly React with H20 and H Release silica in solution and cations Produce clay minerals sheet silicates Albite H20 H Sodium montmorillonite H4SiO4 Na Anorthite H20 Ht Calcium montmorillonite H4SiO4 Ca2 Montmorillonite Montmorillonite formula NaCaAlMg2Si4010OHz nHzO Montmorillonites are EXPANDING clays unlike illite and kaolinite Al is essential in all clay minerals ie Al in weathered silicates goes into clay minerals Mg silicates also weather to form montmorillonite Iron in minerals weathers differently 2 Fe in most ferromagnes1an m1nerals 1s reduced Fe because they re formed 1n reducing conditions low oxygen Surface waters are very oxygenrich ie oxidizing Fe2 released during weathering immediately oxidizes to Fe Fe3 precipitates rapidly as EXTREMELY INSOLUBLE FeOH3 and other hydroxides and oxyhydroxides Weathering of pyroxene for example The most common products of weathering are Quartz Clay mineralskaolinite illite montmorillonite Cations in solution Ferric hydroxides and oxides insoluble from the weathering of mafic minerals Which will weathers more rapidly basalt or granite Why Weathering of Basalt What will be the most common sandsized fragments What will be the most common mineral weathering products Weathered ash deposits form bentonite a mixture of clay minerals mostly montmorillonite expand when wet very slippery can crack foundations Weathering of Granite Where does alteration first occur Disaggregation of grains forms grus Surface esp of feldspars gets soft and punky why What are the mineral products of granite weathering Sedimentary Structures 0 Sedimentary structures can tell a lot about the depositional environment 0 Some sedimentary structures tell which way was UP during deposition 0 Some indicate current direction 0 You ll hear about lots more in StratSed Graded Bedding Graded bedding is typical of turbidites Distal Turbidites Washington coast Load casts or balland pillow structures tell WAY UP Mud cracks are concave UP Crossbeds tell current direction and sometimes UP direction Crossbeds in windblown dunes Supai Sandstone Sedona AZ Crossbeds in unlithified sands Crossbeds and laminar beds in beach rock Other Current direction indicators Flute Casts Spoonshaped sole markings on bottom of beds commonly caused by turbidity currents Examples of ute casts note ute shape Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2009 The Cascade Volcanoes A good example of the CircumPaci c ring of re of subductionrelated andesitic volcanoes Calealkaline Magrnas Subalkaline Flat trend on AFM diagram no Feenrichment Strongly plagioclase porphyritic Andesitedominated stratovolcanoes Wider variety of rock types basaltandesitedaciterhyolite suite than in tholeiitic suites Much more likely to have explosive eruptions than Hawaiian volcanoes Complex Tectonics of Western North America Cascades Tectonic Setting Called Stratovolcanoes or Composite Volcanoes Steepsided slopes up to 36 Typically explosive violent eruptions Composed of lava ows interlayered with pyroclastic material composite Pyroclastic material any volcanic material that is ejected from volcanic vents as loose or fragmental material includes many speci c terms that refer to shapes or sizes of particles ash bombs pumice cinders etc Only 11ooth of the volume of a large shield Mount Rainier Mount Hood Eroded Cascade Volcanoes Mt Washington and Three ngered Jack Eroded Volcanoes like Three ngered Jack show the Composite Stratiform nature of Cascade volcanoes South Sister of the Three Sisters with Obsidian Flow Once upon a time there was a big volcano called Mount Mazama in southern Oregon Climactic Ash Flows lled valleys surrounding the Volcano Back to the Cascades Map Mount Shasta Mount St Helens 1980 Eruption Small earthquakes Small steam and phreatic ash eruptions in March and April 1980 USGS monitoring station set up north of the mountain on Coldwater Ridge May 1980 North side of mountain began to bulge many microearthquakes eruption was imminent redzone closed See USGS Professional Paper 1250 1981 for much more information and pictures 832 AM 51880 Magnitude 51 earthquake triggered the Big Eruption The unstable bulge on the North side collapsed and exposed magma in a shallow chamber to air Gas bubbles formed instantaneously in the magma causing expansion a shock wave and a big ash eruption Most of the force of the eruption was directed horizontally to the North Only 57 people died in the eruption Vancouver Vancouver This is it David Johnston Gary Rosenquist photos made reconstruction of the eruption sequence possible Rosenquist 4 and 5 Rosenquist 6 amp 8 Rosenquist 10 Reconstructed Eruption Sequence After the rst few minutes the ash erupted upward and drifted ENE with the prevailing winds Plinian eruption Ash ows Dome growth may eventually ll the crater Effects of May 18 1980 eruption Before and After May 18 1980 We think Mount St Helens 1980 was a big deal Volcanic Hazards Directed blast Hot ash ows lava ows Airfall ash threatens airplanes Mud ows lahars Mixture of melted ice debris water and ash Threaten people cities and towns far away Lahars from Mt Rainier could threaten Seattle andor Tacoma LaharsMud ows are the main Volcanic Hazards at Mt Rainier USGSUSAID Volcano Disaster Assistance Program VDAP httpvulcanwrus gs govVdapdescription7vdaphtml Established after volcanic disaster in Columbia Armero in 1985 23000 killed in mud ow VDAP is a mobile well equipped team of experienced volcanologists who can respond whenever a volcano crisis threatens anywhere in the world VDAP monitoring and successful prediction of 1991 eruption at Pinatubo saved thousands of lives Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2009 Sedimentary Rocks Rocks resulting from the consolidation of loose sediment or chemical precipitation from solution at or near the Earth s surface or organic rocks consisting of the secretions or remains of plants and animals Sedimentary Rocks are mostly marine Why Sediments are deposited in Basins low places on the Earth s surface There are several common Sedimentary Basin Types related to different Plate Tectonic Settings Six Maj or types of Sedimentary Basins Oceanic basins Arctrench system basins Continentalcollision basins Grabens along continental margins Intracratonic basins Plate Tectonic Settings Oceanic Basins On oceanic lithosphere Atlantic and Pacific Ocean basins Terrestrial muds near continents Away from continents sediments are the remains of planktonic floating usually within lightpenetration distance organisms that quotrainquot down from the surface Some planktonic organisms have carbonate remains some have siliceous remains Carbonate Compensation Depth CCD see handout Arctrench System Basins Trench sediments in trench above subduction zone Forearc basins in front of relative to the trench the volcanic arc Intraarc basins within the volcanic arc between the volcanoes Retroarc or Backarc basins behind the volcanic arc ArcTrench System Basins see handout Trench Sediments Turbidites deposits from submarine ows of sediment water mixture they commonly develop from submarine landslides and are transported along the trench Melanges chaotic tectonic mixtures of very large fragments of older sedimentary and crystalline rocks in a muddy matrix The Aleutian Trench Forearc Basin Sediments Sediment from volcanic arc mainly volcanic and plutonic source rocks Lithic sandstones and wackes common Sandstones rich in volcanic rock fragments and calcic plagioclase grains Continental Collision Basins Low places where sediments accumulate when two continental blocks collide Convergence of the southeastern part of North America and the northeastern part of South America in the Caribbean Convergence of Africa and Europe in the Alps Mediterranean Convergence of the Indian plate with the Eurasian plate in the Himalayas Himalayan Collision Zone high erosion rate enormous quantities of sediment many sites for sediment accumulation Grabens along Rifted Continental Margins like east coast of US Intracontinental Basins Epicontinental seas upon the continent Covered the interior of North America during most of the Paleozoic Relatively shallow water sediments Many unconformities P r J of Abundant carbonates and evaporites eg the Michigan Basin 139 Michigan Basin Geol 285 Dr Helen Lang West Virginia University Spring 2009 Diagenesis of Sandstones All changes physical chemical and biological that occur in a sediment after deposition and before metamorphism lt1502000C These changes happen at sedimentwater interface and after burial Two important processes Compaction decrease in volume largely by squeezing out of water Cementation introduction of chemical precipitates between grains Together these result in lithi cation the change from a loose sediment into a cohesive rock Compaction Spaces between grains of sediment are usually lled with water Porosity void volume total rock volume Permeability ability of a rock to transmit a uid water oil gas requires connected porosity Compaction ofMuds Modern muds contain gt 60 water which can be squeezed out by exerting little pressure Muds can be compacted because grains are ductile exible and can pack easily Compaction of Sands Sands are not easily compacted because they are supported by graintograin contacts Quartz and feldspar are not ductile at diagenetic P and T Modern sands 45i5 porosity Compacted quartz sandstone 30 porosity Ductile lithic fragments can be squeezed into pore spaces so lithic sandstones can be compacted more Can compaction alone convert sand into sandstone Sometimes Quartz i Feldspar water squeezed to the limit of sedimentary conditions still loose grains 80 Quartz 20 schist or mudstone fragments yields multigrain aggregates 100 Mud yields mudrock Compaction alone can produce a rock from a sediment with high content of ductile lithic fragments or mud Cementation Growth of new authigenic minerals from pore uids Authigenic grown in the sediment after deposition as opposed to detrital Cements precipitate in pores usually coat grains increase areas of graingrain contact decrease pore space porosity Most common Cements are Quartz SiOz Calcite CaCO3 Hematite Fe203 Clay kaolinite illite montmorillonite chlorite not really a clay mineral Quartz SiOZ Cement Quartz cement commonly nucleates on quartz grains is optically and crystallographically continuous with detrital grain Quartz cement is most common where quartz grains are abundant SiOz must come from pore waters that move through the sandstone Quartz cemented quartz arenites Tuscarora Ss are very resistant to weathering Dust rings may show detrital grain boundaries Calcite CaCO3 Cement Very common Reacts with acid Requires permeability for CaCO3 saturated waters with Ca X Cng39 above a certain value Calcite is orders of magnitude more soluble than Quartz may form and later dissolve Often discontinuous May form concretions locally cemented areas in friable Ss typically around fossils Calcitecemented Sandstone Calcite and dolomite Cement stained Hematite Fe203 Cement Forms in oxidizing environment Makes red beds red only about 1 Fe203 required to make red color Fe2 dissolved from ferromagnesian minerals during diagenesis gets oxidized to Fe and precipitated as hematite cement Hematite Cement Clay Cement Some clay in sandstones is detrital Some clay is authigenic Clay cement coats sand grains Clay plates grow perpendicular to surface and form honeycomb texture Clay coatings can prevent quartz cement from growing and preserve porosity Clay Coatings Authigenic clay is perpendicular to grain boundaries Diagenesis is complex Compaction depends on mud content sorting ductile fragments angularity of grains depth of burial pressure Cementation depends on chemistry and amount of pore uid Geol 285 Dr Helen Lang West Virginia University Spring 2009 Metamorphism of Ma c Rocks and Metamorphic Facies Metamorphic Mineralogy depends on Temperature Pressure m Rock Composition but Metamorphic Rocks aren t as complicated as you might think It has been observed that The number of different metamorphic mineral assemblages is relatively small The number of essential minerals in each assemblage is relatively small Certain assemblages in different rock types are repeatedly observed in association around the world and throughout geologic time Based on these observations Pentii Eskola 1915 originated the Metamorphic Facies Concept A metamorphic facies is a set of metamorphic mineral assemblages one for each common rock type that are commonly associated in space and time and seem to have formed reached equilibrium at similar metamorphic conditions Each metamorphic facies has been associated with a certain range of metamorphic conditions P and T Metamorphic facies can therefore be represented on a PressureTemperature P T diagram My diagram from Spear 1993 is slightly different from the one in your textbook Boundaries are gradational See handout Names of Metamorphic Facies are based on assemblages in Ma c Rocks Medium Pressure Facies Minerals in Mafic Rocks for each Facies Greenschist facies o chlorite actinolite albite epidote Epidote Amphibolite facies o hornblende actinolite epidote albite Amphibolite facies o hornblende plagioclase garnet Granulite Facies very hi T Minerals in Mafic Rocks Granulite facies o hornblende augite orthopyroxene plagioclase two different pyroxenes o Rocks are dry otherwise they would have begun to melt at these temperatures High Pressure Facies Minerals in Ma c Rocks Blueschist facies o glaucophane blue amphibole lawsonite albite aragonite chlorite zoisite Eclogite facies o Mgrich garnet Omphacite bright green Na rich clinopyroxene kyanite Blueschist Eclogite Metamorphic Facies Series Concept proposed by Miyashiro Shows the progression of Facies across a large region Give a general idea of the change in temperature and pressure across a region Metamorphic Facies Series Note AndKySill elds Low Pressure Facies Series contact and low Pressure regional metamorphism Medium Pressure quotBarrovianquot Facies Series typical regional metamorphism High Pressure Facies Series subduction zone metamorphism Eskola recognized the need to show the effect of Rock Composition on Mineral Assemblage at a given Pressure and Temperature To use a triangle we need to reduce important chemical components to three Eskola invented the ACF diagram to show minerals in Metamorphosed Ma c Rocks He eliminated uninformative minerals albite quartz Kfeldspar magnetite ilmenite apatite He grouped elements that substitute for one another FeO MgO and MnO ACF Diagram A quotA1203quot A A1203 Fe203 Na20K20 amt A1203 in NaK feldspars C quotCaOquot C CaO 33 P205 amt CaO in apatite F quotFeOquot F FeO MgO MnO Minerals on the ACF Diagram see handout Rock Compositions on the ACF Diagram Different minerals at different P and T eg in the Greenschist facies see handout at higher T Epidote Amphibolite Facies see handout still higher T Amphibolite Facies see handout We ve seen ACFs for 3 Facies in Med P Facies Series Amphibolite Facies iEpAm represents range of conditions very important in regional metamorphism Few changes in ma c rocks Many changes in metapelites metamorphosed shales Staurolite Kyanite and Sillimanite zones of Barrow s area There are good pressure indicators in pelites esp And Ky and Sill More later Geol 285 Dr Helen Lang West Virginia University Spring 2009 Diagenesis of Limestones Diagenesis begins very early in limestones right on the sea oor Limestone Diagenesis Compaction and Cementation similar to that in sandstones Pressure solution dissolution caused by pressure of one grain on another Replacement of Aragonite by Calcite Local replacement of limestone by chert Replacement of limestone by dolomite called quotdolomitizationquot Pressure Solution Load pressure causes some calcite to dissolve In some limestones as much as 40 of original carbonate may have dissolved Insoluble things clay organic matter get concentrated or left behind and may form stylolites Stylolites Irregular surface of interpenetrating quot ngersquot marked by concentrations of insoluble clay or organic matter In cross section they look like the writing of a stylus See walls of bathroom stalls in White Hall Stylolites Calcite and Aragonite are polymorphs of CaCO3 Calcite is more stable at surface conditions 7 see handout Aragonite changes to Calcite during diagenesis Exposure to fresh water speeds up aragonite to calcite conversion Paleozoic limestones don t have any aragonite left Dolomitization Dolomite is rare in modern carbonates Makes up about 14 of Paleozoic limestones Makes up about 34 of Precambrian limestones Why When and where does dolomite form Observations about Dolomite Almost all dolomite forms by replacement of preeXisting carbonates Dolomite rhombs crosscut allochems Dolomite obscures ne structures in limestones Dolomite crosscuts bedding planes Dolomite is commonly associated with evaporites Dolomite Rhombs in Thin Section To form dolomite by replacement of calcite or aragonite you need Water of the right composition and A mechanism to move that water through the limestone There are two proposed mechanisms Evaporative Re ux Model Requires periodic ooding of an exposed tidal at or quotsabkhaquot over a limestone Evaporation that causes evaporites especially gypsum CaSO42HZO to precipitate Two effects increased density of brine so it sinks through the limestone increase in the Mg Ca ratio of brine Evaporative Re ux as the Ca depleted brine moves through the limestone Cc CaC03 is replaced by Dolomite CaMgC03z Evaporative Re ux explains Dolomites associated with Evaporites Dolomites Without Evaporites require a different model Mixing of fresh water and seawater called quotDoragquot for mixed blood in Persian A mixture with 5 to 70 seawater is undersaturated with Calcite wants to dissolve it and supersaturated with Dolomite wants to precipitate it Calcite and Dolomite Saturation see handout Dorag Dolomite Landward of the shoreline there is a zone of mixing of fresh groundwater and seawater Should be a dolomitizing zone Zone migrates landward as sealevel rises during transgression Zone migrates seaward as sealevel falls during regression This model is attractive for dolomites with no eVidence of evaporites Dolomite formation on the north side of Jamaica Dorag model 7 see handout Geol 285 Petrology Dr Helen M Lang West Virginia University Spring 2009 Differentiation of Igneous Rocks Crystal Fractionation and Layered Ma c Intrusions Igneous Rocks are grouped into Suites Rocks in a Suite might come from the same volcano Kilauea a group of island volcanoes Hawaii the Galapagos a single intrusion the Skaergaard intrusion Greenland a chain of volcanoes the Cascades Different magmas rocks in a Suite must be related by some process Parental magma the one from which others are descended highest liquidus temperature most primitive composition hi MgO low SiOz low incompatible elements large volume erupted Daughters Differentiates Derivatives different names for the descendants Changes are displayed on Harker Diagrams Metal Oxide vs SiOZ Trends on AFM Diagrams Some Differentiation Processes that can change magma composition Crystal fractionation We will talk mainly about this Magma mixing Assimilation of country rocks Crystal Fractionation Crystals are removed from the liquid in which they formed Commonly by settling under the in uence of gravity p olivine 322 gcm3 Mg 430 gcm3 Fe p cpx 296352 gcm3 p opx 321396 gcm3 p plag 263276 gcm3 p magmas 24 28 gcm3 calculated Norman L Bowen popularized Crystal Fractionation He thought all igneous rocks came from a basaltic parent mainly by crystal fractionation His idea was too extreme but very important as a starting point This is the origin of Bowen s Reaction Series see handout from before Bowen s Reaction Series BRS is inadequate for generating most granite Amount of basalt in crust is approximately equal to the amount of granite Bowen s reaction series could only produce about 120 as much granite as the initial volume of basalt Where are all the fractionated mafic minerals there would have to be a huge amount of ultramafic cumulate rocks hiding at the base of the continental crust Layered Ma c Intrusions are the best examples of Crystal Fractionation Palisades Sill along Hudson River in NJ see textbook Bushveld Intrusion in South Africa pC colossal 320 km in diameter Skaergaard Tertiary E Greenland Muskox northern Canada Great Dike Zimbabwe Duke Island Complex SE Alaska Stillwater pC Montana We did an imitation of a Layered Ma c Intrusion with the MampM Magma Chamber Skaergaard in E Greenland Perhaps the most studied rock body on Earth Best example of an igneous body that has fractionated to an extreme degree through crystal fractionation Bowen s idea Most of its thickness is exposed Explored in 1930s 1950s and 1970s Upper Zone Layered Series Wager 1930s photo Evidence for Crystal Settling Cumulus mineral textures euhedralsubhedral grains piled up as if they settled in a liquid Sedimentarylike structures layering graded bedding crossbedding slump structures etc Cumulus Mineral Texture Layering variation in mineral proportions and sizes Graded Bedding coarsest at bottom Crossbedding Trough banding Skaergaard is an asymmetric lopolith see handout Crystallization What is Sandwich Horizon There are two kinds of Layering in the Layered Series Rhythmic Layering changes in the identity and proportion of minerals Cryptic Layering changes in chemical composition of minerals upwards through the layers hidden you can t see it must have chemical analyses of minerals Original Skaergaard Magma was a Tholeiitic Basalt Layering and compositional changes mainly resulted from crystal fractionation by gravity settling fractional crystallization Current Exposure EW cross section The Layered Series see handout Why does Olivine disappear in Middle Zone Explained by FoEnSi02 diagram see previous handout Olivine reappears in Upper Zone Ferich Olivine is OK in SiOgrich liquid AbAn Diagram eXplains why plagioclase composition changes from bottom to top of Skaergaard Note other cumulus minerals Remember there are two kinds of Layering in the Layered Series Rhythmic Layering changes in the identity and proportion of minerals Cryptic Layering changes in mineral compositions upwards through the layers Crystal Fractionation to an Extreme Degree Mafic minerals are all Fericher toward the top of layered series Feend members have lower melting crystallization temperatures Plagioclase is more Na rich toward the top Naplag crystallizes later and at lower temperature than Caplag Quartz and micropegmatite represent the little bit of quotgranitequot that can result from crystal fractionation of a tholeiitic basalt
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