Introductory Petrology GEOL 285
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Date Created: 09/12/15
Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 285 INTRO PETROLOGY Limestones and Dolomites Carbonate rocks usually deposited by or with the hel of bi010 ical 0r anisms Their mineralogy is simple Carbonate minerals Calcite CaCO3 gt Polymorphs Aragonite CaCO3 Dolomite CaMgCO32 Minor Quartz andor Clay Particle Types and Textures an Important for Classi cation Allochemical Particles allochems framewor gr m ically deposite limestone four main types some formed of calcite some aragonite Orthochemical Particles orthochems matrix and cement that ll spaces bind allowcms wcwwr w cl lithify the sediment Allcchems Fossils solid carbonate remains of organisms fossils and fragments of fossils Peloids ellipsoidal aggregates of microcrystalline CaCO3 lack internal structure mostly fecal pellets of worms fish etc Ooliths spherical polycrystalline carbonate particles of sand size with concentric or radial internal structure commonly have a nucleus fo precipitation Limeclasts fragments of earlierformed limestone mostly intraclasts from a local source T rm bk ng LXKJLJAAA 9 N are structureless composed mostly of micrite i quot ngAquot A Have radial and or concentric internal structures Orthochems Microcrystalline Calcite Micrite CaCO3 mud disarticulated algal material carbonate ooze S 4 nm diameter Note difference between lime mud and silicate mud Coarsely crystalline calcite Spauy calcite or Sparite calcite cement precipitated from pore uid inorganic ppt Usually one or the other not both A 39 r G JV V if 11 to Cunt Sb 444 y i3 6 u w 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 Limestcncs occur Where clastic input is minimal QC 1 0 Cwe39i c 01 quM exiw There are two commonly used limestone classi cation schemes Folk s Classi cation Based on major a o em Hard to use Without thin sections Dunham s Classi cation based on SU39UCIUI C and percent grams VS mud More useful in the eld Folk s Classi cation Major Allochem Major Orthochem pre x suf x Fossils bio micrite Peloids pel sparite Obliths 06 Limeclasts intra gt90 micrite is just called micrite 139 taxquot A V 39 1 b 39 a A uquot Af DE 9 I V gulixJF39L Folk Name O O S d Du T quotTo 9 FOL Name Mic te Dunham eld classi cation Original components not bound together during deposition Contains mud particles of clay and ne silt size Lacks mud Mudsupported Grain supported Less than More than 10 grains 10 grains M udstone Wackestone Packstone Grainstone Original components were bound together during deposition t as shown by intergrown skeletal matter lamination contrary to gravity or sediment oored cavities that are roofed over by organic or questionably organic matter and are too large to be interstices Boundstone Use names of fossils or other allochems as modifiers Dunham eld classi cation Original components not bound together during deposition Contains mud particles of clay and ne silt size Lacks mud Mudsupported Grain supported Less than More than 10 grains 10 grains M udstone Wackestone Packstone Grainstone Original components were bound together during deposition t as shown by intergrown skeletal matter lamination contrary to gravity or sediment oored cavities that are roofed over by organic or questionably organic matter and are too large to be interstices Boundstone Use names of fossils or other allochems as modifiers Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University Spring 2009 GEOLOGY 285 INTRO PETROLOGY igneous KocKs 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 Skaerv aard 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 compositiw m may WW WOz low incompatible elements large volume erul ted Daughters Differentiates Derivatives the descendants Changes displayed on Harker Diagrams Metal Oxide vs Silica Si02 HL Galapagos O MgO vs SiOZ l FeO vs SiOZ NaZO Na20K20 0 0 5000 5500 6000 6500 7000 7500 Trends on AFM Diagram Cascades and Gala a os Molar AFM I A Skaergaard F MOW Layered Series Some Differentiation Processes that can change magma composition are Crystal fractionation Magma mixing Assimilation of country rocks Crystal Fractionation Crystals are removed from the liquid in mJ iiiii m Commonly by settling under the in uence of gravity p olivine 322 gcm3 Mg 430 gcm3 Fe p ch 290532 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 Bowen s Reaction Series Olivine Ca plagioclase orthopyroxene clinopyroxene NaCa plagioclase amphibole Hb biotite Na plagioclase By removing alkali feldspar earlY39fOTmed minerals from basalt it is quartz possible to get a small amount muscovite 4 0 r7 quot 7 lt r1 2 7Q 1m r v I t u u i in7 M l v Mi l quot lthOi 7 l 4 id 39 7 zy 5 VJ 1 x i 36 I LV4 7 i 1 A K 7 774 67 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 ma c minerals there would have to be a huge amount of ultrama c cumulate rocks hiding at the base of the continental crust February 2008 Layered Ma c Intrusions are the best examples of Crystal Fractionation Palisades Sill along Hudson R in NJ see textbook Bushveld Intrusion in South Africa p oolossal 320 km in diameter Skaergaard Tertiary 11 ureenlandwlt Muskox northern Canada Great Dike Zimbabwe Stillwater 1 Montana Skaergaard in SE 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 Wager 1950s 1970s and 1990s ries Wager 19303 photo K What do you notice Evidence for Crystal Settling Cumulate mineral textures euhedral to subhedral grains piled up as if they settled in a liquid Sedimentarylike structures la erinv graded bedding crossbedding slump structures etc Layering variation in mineral proportions and sizes 1 j l 443 my In fax r V rs z 14 l 39 lb ta l v r w Vquot v V k uewUlil l Best example from the Duke Island Complex in southeast Alaska 1 1 7 A L 4 STLCCYCQL 1r0ugn cross Skaergaard Geologic Map about 8 X 10 km awew Irvine Andersen amp Brooks 1998 GSA Bull The Skaergaard is an asymmetric lopolith 393 399 0902 9 v goo Basalt ran7 A l 27777 PnesanfTogograph E y I I IruY 1quot v I Chilled margin Last Liquid was trapped at the Sandwich Horizon There are t V runds 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 la ers hidden 39 ou can t see it must have chemical analyses of minerals Original Skaergamc Magma was a Tholeiitic Basalt Layering and compositional changes mainly resulted from crystal fractionation by gravity settling fractional crystallization Current Exposure EW Sandwich Horizon 7 Nr39 n Q 5quot oquot quot VMIt lt may I IIIIImI gu 1 39i f lII I C ll p l I la I quot 7 l 011V1ne is absent in the Middle Zone MZ U Minis u SPECIMENS gt N130 C343M50F57 E I 0 I I L 4330 SHR 39 1 ml F422 A 3 IE39 ml Hm momma 2quot El 43154 Anssg CIMMgHFeW 1 43 I7 Ann 2 quot 5 Huh A Cszg11Fqg 9 I 2000 U1 quot35 m Z 239 434 M35 Lu CaasMguFus 639 1 quot m 1 5 E I I E SIBI 2quot X Gamma I NI I 4309 quotn g I I E I I 39 n 4305 c M F lt 4103 Anw I 315 35 m D 39 mo AMA CastsuFEze m CY Isoo a E 39 I I lt 39 39 w I I U z r I u u U 8 I P I o 5 o z w I w I 4m 5 Am E CassHsaeFexs E g E El I I M2 5 z n 1 lt lt1 I m a i I z I 300 4369 M51 6 C37M339Feu a I 8 239 gt I 1 I U E I t I An 1 I c 39 4453 39 Ans i U 39 438 I E HBSA Jr39ss HEM qu I I Sl I2 Ang I I E I I I o I I n I I I I I l I I I I I I I I I Why does Olivine disappear in Middle Zone f 39 CagMg45Fu CISMMFM OLIVINE INVERTED PIGEONITE Drr CistseFus OLIVINE Explained by FoEnSiO2 diagram Temperature degrees 0 Liquid Forsterile Liquid Crislobalile Furskarile Ensmme I I l l I MgZSiOA MgSiOS Forsleme Enslalile CIslobalile 0 MoleSi02gt 100 b Enslalile Cristobalite l v v What happens when liquid X reaches a during fractional crystallization F0 is replaced by Pigeonite which like Enstatite is produced by reaction of SiOzrich liquid with Mgrich Olivine in Upper V A quotHf up l M T if 4mm rst 7 ALIll Jliluxa quotVLF Qlv 239j fie liquid Note that at bottom of Upper Zone Olivine has only 40 F0 MgZSiO4 and 60 Fa FeZSiO4 Olivine changes from 67 F0 Mg2 i 4 at the base of the Layered Zone to 0 F0 100 Fa FeZSiO4 at the top of the Layered Zone AbAn Diagram explains Why plagioclase composition changes from bottom to to of Skaergaard SPECIMENS 4330 SHR IH 5131 4309 4306i 1 303 so FLAG IOC LAS E rLiquid QI Plagioclase solid solution 1 00 NaAISiaos Mole Anonhite ltgt CaA2Si205 Albite Ano hite Temperature degrees C Fractional Crystallization liquid and Flag keep getting more Narich SPECIMENS M E39I39l ES U I O I F t I W I x m m rs t W 6quot Mn more Fe 1 El II 4126 I a U c W39 nch toward E L 431 5 A n35 g I quotu F I7 I 41I7 Any top E 239 I C240Mg1F39 gt pm 2 3 kquot 1 I I 2000 U2 Am u Ca3gNg17Fe3 0 Few 539 I Z i w F031 I N I I Any x szsMgnFeu on I E I I Am 0 lt I I I 4306 gt CagsMnsFem Fm I D 4303 Amn O I U Castgvaem u CayMg45F246 4410 m E 39 w I O 5047 m A w t m S U g I 5 t 5 I w z 39 u I o III 2 lt I m I 9 An 2 CastszeFI E CagMg gFea u n I I 43 u 49 z 1 lt I a M Z 4 z n z E 39 A m 0 I n 3 c M F 39 l U An an 39 14 a I 4369 51 U m J looo J gt I lt 39 lt z I U u U I m E I I fl 39 4453 Anso I I t I 39 4m FW 4385A A955 CaggMg F911 CagMgg Fegs m F559 I I E 5 I2 Any I z I I g I 500 b I Z I I 39 I I LZ I 0 F06 I I I 4359 An I I 39 I 5036 I I F067 I I I I SINB Mes l I I oI o U Iquot O I l l 3 w SPECIMENS Aquot cmMchey Fan Q11 112 a d pl 1 MEIn ES 10 F0 I a m I 1590 rsH L 4330 SHR A 1 z gt Wm 31 Innr IIIl I IF c CameFesv r 3 I 447l n F Lm Mm 91 an w an wlt 1 Fe llVlI e g I n V U Ann 2 x 41m I I CzqoMsnF i Z 22 111 ppe Z39 I An F026 g I I F 10 2m UZ Am m Cm sv 3935 O n I I 2 Fe N l Ill 31 I I Ann gtlt ngsMgnFeal 7 14 n I Am 0 lt I E CazsmxsFem FM 3 I 43quot 430 Anm 9 F I I g CaJSngFega Ii CayMw 946 I 4410 m w U 500 m I z 1 I u lt u l I I 39 3 z Iquot I u I o u z lt E I o U CastszeFI CasMgaFea u n I 434i 5 A E 39 1 4 1 a I M2 4 D Z n d z 5 o I 39 A 2 CanMsastu a 3 I 4369 quot51 U I39 lt Im I gt I I lt Z u u I m F055 I 4 c 39 4453 5 I I t Fog z A An CaggMgMFen CastseFezs w F059 I I x 39 51 I2 57 I z I g I I 3 son I J I I Lz I 0 F05 I I 4359 Aug I I 39 39 5036 I I I Fo I 7 I j 5o7s quot65 I I Gamma a m aglrna rm 1 J 1 39gt a w 5 Jim J MIL Remember time we V was 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 was 1 o o 1 LSr Ail N 41 9le l a 1 illsl nJV KL if 5 i K Ma c minerals are all Fericher toward the top of layered series Fend 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 granite that can result from crystal fractionation of a tholeiitic basalt Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 285 INTRO PETROLOGY 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 Light sialic continental masses 30 are lower in volume than ocean basins 70 Deep trenches produced by subduction are near easily eroded highlands that ll trenches With sediment Shallow marine water commonly invades continental cratons epicontinental seas Terrestrial sediments are above sea level and are easily eroded gt many unconformities Sediments are deposited in Basins low places on Earth s surface There are several common Sedimentary Basin Types related to different Plate Tectonic Settings Maj or types of Sedimentary Basins Oceanic basins Arctrench system basins Continentalcollision basins Grabens along divergent continental margins lntracratonic basms Plate Tectonic Settings WWW rmsmw mvinazw anvEnuim commummmanz mmmw pmzmmm rmzammmnv mummy Lvuwawuvzsuwnmn mm vommo A ivmcw gt 41 5A Asmznawznz 45CVN mam Oceanic Basins On oceanic lithosphere Atlantic and Paci c Ocean basins Terrestrial muds near continents Away from continents sediments are the remains of planktonic oating usually within lightpenetration distance organisms that rain down from the surface Some planktonic organisms have carbonate remains some have siliceous leluaulb Carbonate Compensation Depth CCD CaCO3 is more soluble in cold deep water than in warm water There is a depth below which carbonate remains do not accumulate because they are dissolved by the cold water Called the Carbonate Compensation Depth CCD Below this depth only siliceous remains accumulate siliceous oozes turn into chert Carbonate Compensation Depth CCD Sealevel Siliceous andor carbonate sediments CCD is at 5000 meters near equator 5111060115 oozes become chert no carbonates in deep sea sediments 3000 meters near poles Arctrench System Basins Trench sediments in trench above subduction zone Forearc basins in front of relative o the trench the volcanic arc Intraarc basins Within the volcanic are between the volcanoes Ketroarc or bacKarc bas1ns ben1ncl the volcanic arc wawnf r F F39 AA HMM 5 Wl f JUquot Dnunn39 crust Trench Sediments Turbidites deposits from submarine ows of sediment water mixture the commonly develop from submarine landslides and are trans orted alonv the trench M langes chaotic tectonic mixtures of very large fragmws of older sediment J and crystalline rocks in a muddy matrix 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 JjacKarc Jjas1n SCOIIIICI IIS Fluvial delve wudor marine sediments Volcanic fragments from arc mixed with continental or terrestrial bUUllllUuLb 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 AlpsMediterranean Convergence of the Indian plate with the Eurasian 1 late in the Himalad as Grabens along Rifted Continental Margins like the east coast of US Cross section of Baltimore Lianyon 1rougn Note vertical Mesozoic Basinseastern US exaggeratlon Intracontinental Basins Epicontinental seas upon the continent Covered the interior of North America during most of the Paleozoic Relatively shallow water sediments Many unconformities Preponderance of nearshore sediments Abundant carbonates and evaporites eg the Michigan Basin unpqumg I mums umddxugxxgw m ultyqullmmd m ummmpjo ulrgunaoq I uvlumMsuund 1 uysng ma yqoyw Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 285 INTRO PETROLOGY Diagenesis of Sandstones All chanves 1 hi sical chemical and biological that occur in a sediment after deposition and before metamorphism lt1502000C These changes happen at the sediment Water interface and atter burial Two important processes Compaction decrease in volume largely by s Lueezinv 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 rook volume Permeability ability of a rock to transmit a uid water oil gas requires oonneoted porosity Compaction of Muds Modern muds contain gt 60 water which can be s Lueezed out by exertinv 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 grainto grain contacts Quartz and feldspar are not ductile at diagenetic 139 ClllU 1 Modern sands 45i5 porosity Compacted quartz sandstone 3 0 porosity Ductile lithic fra39ments can be s Aueezed into pore spaces so lithic sandstones can be compacted more j quotquot mm 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 0 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 01 gramgrain contact stick grains together and decrease pore space porosity Most common Cements are Quartz SiO2 Calcite CaCO3 Hematite Fe2O3 Clay kaolinite illite montmorillonite Chlorite not really a Clay mineral Quartz SiQZ Cement Quartz cement commonly nucleates on quartz grains is optically and crystallographically continuous with the detrital grain Quartz cement is most common Where quartz grains are abundant SiO2 must come from pore waters that move through the sandstone Quartz cemented quartz arenites Tuscarora Ss are very resistant to weathering Dust rings show detrital grain boundaries v E U a a E poros1 Note optically continuous and euhedral quartz overgrowths Calcite CaCO3 Cement Very common Reacts with acid Requires permeability for CaCO3saturated waters with Ca X CO3239 above a certain value Calcite is orders of magnitude more soluble than Quartz it may form and later dissolve Often discontinuous May form concretions locally cemented areas in friable Ss typically around fossils o A A l x x o A a J a 7 x A v A AA t wAj x 31 v 1 A A v A A AA U A 2A A SA SA AAon 943 AAA 1 1A a A AA A AA AA A AA A A Qw Ark 3 er 5 A PL r V t FOR AwK Axttcuu m t E A Hematite Fe203 Cement Forms in oxidizing environment Makes red beds red only about 1 FezO3 required to make red color Fe2 dissolved from ferromagnesian minerals during diagenesis gets oxidized to Fe3 and precipitated as insoluble hemat1te cement f 7 1 7 x w wirxlwax u C 3 f 1 J J Kit 4 4 x 94 Were d1 h Fe come from SM long imeh510n1um Clay Cement Some clay in sandstones is detrital Some clay is authigenic cement 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 the porosity 3503583086 m Clay Coatings a 393 5 391 Z S as 1 2 as U 3 gt1 15 3 5 E an HiMag SEM Spun Most common Cements are Quartz SiO2 Calcite CaCO3 Hematite Fe2O3 Clay kaolinite illite montmorillonite Chlorite not really a Clay mineral Sandstone 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 Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 285 INTRO PETROLOGY Metamorphic Mineralogy depends on Temperature Pressure Rock Composition but Metamorphic Rooks aren t as oom lioated as V ou mi ht think It has been observed that The number of different metamorphic mineral assemblages is relativeld small The number of essential minerals in each assemblage is relatiVuJ swan Certain assemblages in different rock types are repeatedly observed together 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 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 tne one 1n your textbook Boundaries are gradational These facies are found in low grade subduction zone rocks 26 o x a 5 6 u I E a Hornfels facies rocks are Iound in contact aureoles can be subdivided Medium Pressure Faeies Key Minerals in Ma c Rocks for each Facies Greenschist facies chlorite actinolite albite epidote Epidote Amphibolite facies Pressure lab ar hornblende actinolite epidote albite Amphibolite facies I hornblende 1 lauioclase garnet Temperature C Granulite Faeies very hi T 20 Minerals in Ma c Rocks Granulite facies hornblende auite orthoroxene plagloclase two different pyroxenes 1039 Pressure labar Rocks are dry otherwise they would have begun to melt U 200 430 500 39 800 mm Temperature 0C High Pressure Faeies 39 Minerals in Ma c Rocks Blueschist faeies r H laucohane blue amphibole lawsonite albite aragonite chlorite zoisite Pressure kb ar E010 gite facies V Vreen Na rich 1 clinopyroxene kyanite Temperature C Glaucophane pleochroic blue amphibole Metamorphic Facies Series Concept proposed by Miyashiro Show the progression of Facies across a large region Give a general idea of the change 111 temperature and pressure across a region Metamorphic Facies Series 0 Note AndKVSill elds 0 Low Pressure Facies Series contact and low Pressure regional metamorphism Medium Pr ure Pressure kbar Series typical of 1eional metamon hism High Pr ure Facies Series subduction zone metamorphism First hold pressure and temperature constant Eskola invented the 391quoti23crigggle to show minerals in Metamorphosed Ma o Rooks He eliminated uninformative minerals like 39cllDlLC quartz Kfeldspar magnetite ilmenite apatite He grouped elements that substitute tor one another FeO MgO and MnO ACF Diagram A A1203 A A1203 F6203 Na2OK20 amt A1203 in NaK feldsA ars C CaO b CaO 3 r205 amt CaO in apatite h rev F FeO MgO MnO Some minerals plot on top of each other some have a range or composition not all are stable at same P T conditions quartz albite magnetite ACF is especially good for Ma c Rocks because they plot near the center Vt 175 There are one 01 more different ACF diagrams for each met facies We ll look at ACFs for Greenschist Epidote Amphibolite and Amphibolite Facies E 0 x 2 3 m m 9 Q 400 600 Temperature 6C quartz albite magnetite The only signi cant quartz change is that alblte amphibcle gets more Alrich and becomes black Hornblende magnetite Hornblende gets even more Alrich quartz Calcic plagioclase magnetite replaces epidote garnet replaces Chlorite i Iquot if 1 We ve seen ACFs for 3 Facies in Medium P Facies Series E n x E 3 m U 9 D 200 600 Temperature C Amphibolite Facies represents range of conditions very important in regional metam In this range there are few changes in ma c rocks Many changes in metamorphosed shales metapelites Staurolite Kyanite and Sillimanite zones of Barrow s area There are good pressure indicators in pelites especially Andalusite Kyanite and Sillimanite not in ma c rocks More later 00 Temperature C Rock Compositions on the ACF Diagram ACF is especially good for Ma c Rocks because they plot near the center V a g A lllllh siailw l ll x 0 Calcite Diopside Dolo Actinolite Talc 0px Ol Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 285 INTRO PETROLOGY Diagenesis of Limestonw Diagenesis begins very early in limestones right on the sea oor Limestone Diagenesis Compaction and Cementation mostly calcite 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 dolomitization 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 ngers 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 no more I Ht J I teat A fag r 9 65 Ay hg Bruce Railsback U of Georgia Calcite is more stable at Earth Ar agonite surface conditions G29 than Aragonite but some organisms 5000 atm make their hard parts out or aragonite anyway G27 Calcite 400 200 Temperature 0C 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 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 Requires periodic ooding of an exposed tidal at or sabkha over a limestone Eva oration that causes eval orites es eciall gypsum CaSO4392HZO 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 Cadepleted Mgrich brine moves through the limestone and Calcite CaCOS is replaced by Dolomite CaMgC033 High tide line Supratidal area I Dolomitized zone Ocean Mgde Plat ne ed Limestone N Works for dolomites associated with evaporites Dolomites without Evaporites require a different model Mixing of fresh water and seawater called Dorag which means mixed blood in Persian A mixture with 5 to 70 seawater is under saturated with Calcite wants to dissolve Co and supersaturated with Dolomite wants to precipitate Do saturated undersaturated oversamrated o o U a 5 3 a H 70 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 Rainwater iii Falnyouth deose Formation zone 7 7 9 level gt gt r r uvu W7 e 0 5 06 9 4 Marine phreatxc zone oversaturated saturated undersaturated 0 5 0 100 freshwater seawater seawater Evaporative Re ux explains Dolomites associated with Evaporites Supratidal area I High tide line Mg depleted i m Limestone N Ocean Dolomitized gt Falni outh deose A Formation zone A I 4 0 level Meteoric phreatic zone Mia4 06 y A 39 4 e a t E f quot6 54 Marine phreatic zone Dorag Model north Jamaica Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 285 INTRO PETROLOGY Generation of Subduction related Andesites Calcalkaline Volcanisni occurs inboard of Subduction Zones at Convergent Plate Boundaries Island are Continental volcanic arc When mantle melts beneath MidOcean Ridges or at Hotspots uniform tholeiitic basalt is formed Why does melting of mantle at subduction zones produce Galealkaline magmas mostly andesites Things about subductionrelated magmatism that must be explained by any model The magmas and rocks are calcalkalinc not tholeiitic ie they show no Feenrichment Magmas and rocks are dominantly andesites with higher SiO2 than basalts More varied magma types than in Hawaii or at Midocean ridges Bimodal volcanism basalt rhyolite is common Eruptions are commonly explosive c Cwm mma mullng 6i Mamm wm g Mam cg Generation of Calealkaline volcanic rocks above Subduction Zones Continental crust Mantle Mantle II VI At 650 C 90 m depth In E11 QUE1 137 6 Density28 Density33 II VI 9 ag affix I 39 Mm II VI And the process continues Mechanisms Crystal Fractionation Contamination Magma mixing Assimilation and others lh Things about subduction related magmatism that must be explained by any model The magmas and rocks are calcalkalinc not tholeiitic ie they show no Feenrichment Magmas and rocks are dominantly andesites with higher SiO2 than basalts More varied magma types than in Hawaii or at Midocean ridges Bimodal volcanism basalt rhyolite is common Eruptions are commonly explosive Crosssection of a Subduction Zone Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2008 GEOLOGY 285 INTRO PETROLOGY Sandstones and Conglomerates Detrital or elastic sedimentary rocks made of solid products detritus from weathering of preeX1st1ng rocKs They make up 2025 of the 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 Maves Grain size Grain shape Grain sorting Grain mineralogy Sedimentary structures Grain size Detrital or elastic rocks have a huge range in grain size We need a 10V scale to re resent this Wide size range The Phi 1 Scale 1 log2 mm mm 239 l memorize bacn 1 step represents a ClOUDllng tsrnauer or more neg 0r halving larger d in size For example size in mm 239 4 6 26mm64mm O 20mm1mm 2 2392mm14mm025 mm 414 mm 116 mm00625 mm Size ranges are The UddenWentworth Grain Size Scale for Clastic Sediments S Name Millimeters Micromexers 4096 Boulder Cobble Pebble 256 Gravel gt 1 I gt2mm 64 GRAVEL 4 Granule 2 sand 4 I to 1 I Very coarse sand 1 to 1 Coarse sand 05 Medium sand Fine sand MUd lt 4 I Very ne sand lt00625mm Hm Medium silt Fine silt Clay lt 8 I Very ne silt lt 1256 Clay lt0004mm lt4 um Loose sediments can be separated by sieving lt3 21 4mm 11 2mm 14 05mm 2d 025mm 3d 0125mm 4d 0062mm Closed Grain size comparator for lithi ed Sandstones Transparent tape 50 25 mm 25 2 mm 711206 mm 06 03 mm V MEN 4 Coarse O l Sorting range in grain size Usually the size range in 1 that includes 23 of the grains m 8 C6 3 1 00 LH O o o of grains 4L 4 size size 1 oorld s0rted wellsorted Vch well 39 J Poorly ry puorly xorlcd surlcd wrlcd Grain Shape Sphericity relative equidirnensionality of three mutually perpendicular axes Roundness lack of sharp corners larger grains round faster because of more impacts High sphericity LOW sphericity Vcn I a 1 J 5 m uhr Angular Suhungular Suhrnundcd mundcd Mudrocks Compose mostly of detrital material smaller than 44 ie smaller than 0062 mm or 625 LLquot siltolay Non ssile stone or ssile shale Named by proportion of Silt and Clay gt 23 silt Siltstone Siltshale 13 to 23 silt Mudstone Mudshale gt23 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 We ll say gt30 gravel size material for a conglomerate Small amount of pebbles or oobbles is very noticeable so beware quot H j 1 4C L53 l H i a 39i 4 39 J 391 3 L ivrr m t 1 l l l i r i i m 7quot AL 39 i is the mos abundant sandsize grain very stable in sedimentary environment may be abundant may indicate rapid burial dry climate granite in source area Kfeldspar vs Plagioclase s are least abundant least stable but most informative Chert a stable lithic fragment goes with Quartz w Mioas may oat in water museovite es eeiall is ver stable Heavy minerals higher density than quartz and feldsA ar some are ver 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 heavies sink to the bottom These minerals can be quite informative Warmer h J J t a mgmi rm C39JEITEZ by Man LVN Lithic Fragments Sandstone Classi cation Naming of Sandstones is based on Percent of common sandsized grain types Quartz chert Feldspars both kinds Unstable lithic fragments rock fragments Percent or mud s e Sandstone Classi cation W from Folk 50 Quartz wacke 10 Subar ose Feldspathic Wacke Sandstones with less than 15 mud are called Arenites use the front triangle Feldspathic Litharenite 50 10 L 10 Feldspar Unstable Lithic Fragments QuartzChegt Naming Arenites 25 Subar 039 0 uanz arenite Su e Iitharenite Iy q 9 6 lt6 1 Y Lithic Feldspathic F Arkose Litharenite I l l l l l 10 50 10 L Feldspar Unstable Lithic Fragments Sandstone Classi cation WW was 50 Quartz wacke Q 10 QuartzChegt uartz arenite Sggar ose Su Ilthzagenlte A to A mud Feldspathic Lithic called Wackes use Wacke Wacke o mlddle trlangle 63 15 c229 4 Y5 Lithic Feldspathic F Arkose Litharenite 10 50 10 L Feldspar Unstable Lithic Fragments Quartz wacke 10 Feldspathic Wacke Textural Maturity of a Ss A measure of the progress of a elastic sediment in the direction of chemical mineralogical and textural stability Affected Uy processes hat take a long time Maturity increases with total input of kinetic energy time of transport distance of transport energy of medium Increasing Textural Maturity 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 or muddy matr1X poorly sorted poorly rounded fragments and grains lots of unstable lithios and unstable minerals mostly waokes deposited in convergent margin settings aretrench gap Supermature Sandstones Clean no mud matrix wellsorted wellrounded grains mostly quartz grains quartz arenites Cratonic typically recycled formed in beach or other hivh enerw environment Increasing Textural Maturity Wackes immature Litharenites Arkoses Subarkose and sublitharenite Quartz arenites supermature The Uddcn chlworih Grain Size Scale for Claslic SCdlrrlcntSu 3 mm Transparent tape 2 mm Name Millimeters Micrometers o JTL 4096 12 Boulder 256 8 L Cobble lt 64 6 m Pebble o 4 I 2 Granule 2 W 1 Coarse Medium Very coarse sand 1 0 0 Coarse sand 2 05 500 l 4 Medium sand m 025 250 2 Fine sand 1 3 crvwcil Well Mmlcrutcly nnrlv Vcn punrlv a very ne sand Norlcd I W nrlcnl 0i wrlcd 07 surlcd sorted I 0062 62 4 quot 39 Coarse silt 003l 31 5 Medium silt 0 0016 16 6 1 Fine silt 2 0008 8 7 Very ne silt 0004 4 quotquot 8 Clay 1 l 1 Jam 39 Angular 3 Suhungulur 3 Suhrnundcd J Rounded 5 fur3 Nuds e Sandstonv Classi cation was 50 Quartz wacke 10 QuartzChert Subar ose Feldspathic Wacke Feldspathic Litharenite 50 10 L 10 Feldspar Unstable Lithic Fragments Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2008 GEOLOGY 285 INTRO PETROLOGY 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 ofplants and animals Sediments are deposited in Basins low places on Earth s surface Six Maj or 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 lntracratonic 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 b biolo39ical or39anisms are made from ions that come from the weathering process Mechanical Weathering physical breakup of rocks esp frostwedging water gt ice causes 9 volume increase Chemical Weathering cnem1ca1 oreaKoown or minerals in the presence of water 5 0 8 Chemical Weathering is MUCH more important than mechanical weathering due to the extraordinary dissolving power of H20 Goldich s Weathering Series Susceptibility of common igneous minerals to weathering Olivine NIOSt susceptible Calcic Plagioclase Pyroxene Amphibole Biotite Sodic Plagioclase Potassium Feldspar Muscovite Quartz Least Susceptible Weathering Reactions of Orthoclase Step 1 3 KAISi3O8 2 H 12 H20 gt orthoclase KA13Si3010OH2 6 HAtSiO4 2 K illite muscovite soluble silica Step 2 2 KA13Si3OIOOH2 2 H 3 H2O 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 NaCaAlMg2Si4OloOH239nH2O Montmorillonites are EXPANDING Clays unlike illite and kaolinite Al is essential in all Clay minerals ie Al from weathered silicates goes into clay minerals Mg silicates also weather to form montmorillonite Iron in minerals weathers differently 0 Fe in most ferromagnesian minerals is reduced Fe2 because they re formed in reducing conditions low oxygen 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 mineralskaolinite illite montmorillonite Cations in solution Ferric hydroxides and oxides insoluble from the weathering of ma 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 Sedimentary Structures Sedimentary structures can tell a lot about the depositional environment Some sedimentary structures tell which way was UP during deposition Some indicate current direction You ll hear about lots more in StratSed quotPelagicquotshale Horixontal lamination Current ripple and TurbUICnt mixtures Of I convolute lamination sand gravel mud and Horizontal lamination Ripup claat water that are produced by submarine landslides deposits ne upward and basinward submarine H mud owsdebris ows Bouma Sequence Graded bedding Sui clan Proximalclose to source Distalfar from source m a S k C a r C d W Crossbeds tell current direction and sometimes UP direction Caused by avalanching of sand down the lee slope of wavelike structures DOWNSTREAM gt Note that crossbeds are cutoff on top asymptotic at bottom Crossbeds and laminar beds in M beach roclf 1 7 a dial F r39 YV LC 1 11 quot 7 s Asymmetric Ripplemarks Steeper slope is downstream Imbricated clasts W Elongate clasts dip upstream pushed like falling dominoes Flute Casts Spoonshaped sole markings on bottom of beds commonly caused by turbidity currents TOP View Side View deepest ZS DOMSTREAM 1 Flute casts are preserved on the bottom of overlying sand beds Examples of ute casts note ute on bottom of sand beds sole markings Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 285 INTRO PETROLOGY Metamorphisrn amp Metamorphic Rocks Because the Earth is a dynamic s stem rocks once formed ma be subjected to very different conditions Metamorphism means Change Changes in conditions cause changes in mineralogy and texture of rocks 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 l l 39 7quot i I l 71 H r 1 in fill 39 When metamorphic temperature gets very high the rocks begin to melt Partly melted rocks are called 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 gt200O T range If a rock gets mostly or completely melted it is considered igneous Protolith Any rock can be changed to become metamorphic KocK Irom wnicn a metamorphic rocK 1s Iormed is called its protolith Igneous protolith is indicated by the pre x ortho metaigneous Sedimentary protolith is indicated by pre x para metasedimentary Protolith is indicated by rock composition inherited textures often it s hard to determine Textures Characteristic of Metamorphic Koc1lts 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 lt Pressure Lithostatic Load Pressure increases With Depth p density 30 gcm3 g 981 msec2 103 msec2 h 1km 105 cm 0 AP kilometer 3 X 103 x 105 dynesom2 3 X 108 dynesom2 convert to bars pressure 0 AP kilometer 300 barskm or 03 kilobarskm 1000 bars l kilobar 33 kilometers deep 1ii39 af fireiai 1 jjzfiii at the base of a thick sedimentw nc V w V TmaX3OO C garnet grade metamorphic conditions may be reached 1 emperature lt Pressure D r 7 1 ivquot M ii I m m1 7 i 71 11 H t heat from a pluton may raise T of country rock mgh 0 o mtamorphi growth of new metamorphic minerals J JAJM T drops off rapidly away from pluton Highest grade or T nearest pluton Metamorphic effects are restricted to a narrow lt12 km contact aureole around the pluton i S 3 1R 7 2 1 3 xfr k l irluii git n a W H rietamcrol air crustal scale thrusting caused by continental l eT mch p ssures and temperatures to cause regional metamorphism l lt P depth Some causes of Metamorphism Subduction zone metamorphism when cold rocks are dragged down into a subduction zone temperatures are lower than normal for a given depth A Q l P amp depth De ne Isograd An isograd is a line on a map marking the rst appearance of a new metamorphic mineral De ned by Barrva we W9Os Interpreted to be a line of approximately equal metamOrpme grade or intensity equal T and P during metamorphism Metamorphic effects localized around a pluton are approximately concentric with pluton margin form a contact aureole Effects limited to a few kilometers from pluton Mineralogic chanUes re ect mostly changes in T Mostly low pressure minerals Few signs of deformation minerals lack preferred orientation rocks are called hornfels Contact Metamorphism of impure Limestone Gun 2 mar o 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 A ressure minerals mineral changes re ect changes in both T and P Rocks are generally deformed and have strong fabrics In 1893 Barrow showed progressive cnanges 1n a s1ng1e rOCK type and related them to an increase in metamorphic grade Dalradian Series in the Scottish Highlands Between the Great Glen Fault and the Highland Boundary Fault Regional metamorphism Metamorphosed shares pelitic rocks He de ned isograds The Great Glen Fault 0 Northern boundary of the Scottish Highlands is the Great Glen Fault Quite a lineamentll It is a wrench fault similar to the San Andreas fault in California Metainorphism in 39 Chlorite zone Scottish Highlands barely metamorphic look like shales Biotite isograd biotite zone Garnet isograd garnet zone StauroliteKyanite and Sillimanite isograds and zones Rocks in sillimanite V glgiZ39L im 39 G garnet zone look verv s staumm H Kyrkyani dltterent gneisses Smmm All racks also contain muscuvite quanz plaginc zse GENERALIZED MAP OF N a 7 I Lt Yellow lowest metamorphic grade barely metamorphic dark and light metamorphic grade 39 r int n itJ n l 3 H r w l 4 L U Quartz crystals in a quartz arenite Just get bigger recrystallize no other minerals can grow Sandstone becomes quartzite Pure calcite limestone becomes pure calcite marble Basalts form plagioclase 39cuupiuuuwb 39cuiu uuicI ma c minerals Shales form aluminous minerals like garnet biotite muscovite staurolite KyAnd Sill and quartz Contact Metamorphism Crestmore CA Cm acct6w L IJ Hmhzmlh39k cmgsxc Vrsuw ak unnmaumllxgoa 0 Camu cms ol Regional Metamorphism Scottish Highlands Izv r kyanite smimanne All rocks also contain mnscavile quamp1agioc1ase Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOL 285 INTRODUCTORY PETROLOGY Geol 285 Introductory Petrology The study of rocks Greek perm rook 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 s the difference between a rock and a mineral o 1 L41 Vivif 3 r yr VCR r 173 a i lt7 i all a H ml J A iquot A 3 J fx lt1 5 i We 1 375379 fix 1 i 4 K 4 L 2 Ti 172 lx ll K 13quot x 1 e n r x x 1 or v43 7K r 9 x 714 lt5 1 3 so i w rocks that solidi ed from molten 1 y 34 3x 0 WW Q x 7 13914 A f jn ill 9 l x or partially molten material magma rocks resulting from the l consolidation of loose sediment or chemical precipitation from solution at Earth s surface rocks formed from pre existing rocks by mineralogical chemical and textural changes in resl onse to change in conditions The Rock Cycle shows how rocks form and change from one type into another there are many different versions Why study petrology Learn about early history of Earth Learn about the 1nter10r or the Earth Only a small part of the Earth s crust is exposed or accessible to drilling Crust is less than 1 of Earth s volume 66 of crust is sedimentary other 34 is mostly igneous The Mantle is metamorphic The Core is liquid and solid metal 1 w u Thickness of Crust is just 12 of Earth s Diameter must is to scale here Why study Petrology continued We can learn about the Whole Earth only by study my ex osed rocks drill cores and geophysics Distribution of rock types at Earth s surface led to Plate Tectonic Theory We must cornpare rnodern processes with ancient rock record to infer processes that formed the rocks we see
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