Metamorphic Rocks Lect 14QuestionsSedimentary featuresFossilsPowerPoint PresentationRipple marksSlide 7Cross beds (can occur at many scales)Slide 9Graded Beds (indicate a rapid event)Slide 11Slide 12MudcracksSlide 14Slide 15Metamorphic rocksClassification of Meta RocksHow do rocks alter without melting?Slide 19Slide 20Slide 21The results of lithostatic or directional stress:The Effects of TemperatureThe role of fluidsHow does metamorphism occurAl2O5 PolymorphsSlide 27Limits of metamorphismSlide 292 main categories of metamorphismSlide 31Slide 32Regional metamorphism Foliated RocksSlide 34Slide 35Pressure creates FoliationsControls on Composition/Texture of Foliated rocks (Regional Meta)Slide 38Slide 39PhylliteSlide 41SchistSlide 43GneissSlide 45Amphibolite:Slide 47Slide 48At the upper end of MetamorphismSlide 50Metamorphic RocksMetamorphic RocksLect 14Lect 14Questions•How and where do the various types (names) of meta rocks form?•What are the controls of meta rock texture and composition?•What are Index Minerals and what do they tell us about metamorphic processes?Sedimentary features•Fossils–Environment– how habitable an area may have been•Ripple marks–Symmetric v Assymetric–Direction of wind or water flow•Cross bedding•Graded Beds•Mud cracks•Rain drop impressions•And many others….FossilsRipple marksCross beds (can occur at many scales)From large Dunes to…Graded Beds (indicate a rapid event)Graywacke Sandstone often forms from deep sea turbidity currentsIn a single depositional bed, the grain size changes from coarse to fining upwardsMudcracksRock CycleTemperatureAnd PressureMetamorphic rocksMeta = “changed”; Morph = “form”Changes in mineralogy and/or internal structure while still in solid stateThese are altered rocks–Recrystallize without melting!–Temperature; Pressure and Fluids–ANY rock can be metamorphosed–Type of Meta Rock is dependent upon:•Original rock type•The combinations of T and PClassification of Meta Rocks•Texture and Composition•These are crystalline rocks but the interlocking crystals did not form from cooling magma (Igneous) or precipitation (Chemical sedimentary)•Texture–Directed pressures create diagnostic Textures•Mineralogy–New and different minerals begin to form at Higher Temps and Higher PressureHow do rocks alter without melting?1. Pressure–Increasing burial increases pressure–Lithostatic pressure (equal in all directions) is from the weight of overlying rocks–Directional (differential) pressure is unequal and deforms and/or distorts rocks2. Temperature–Burial: Geothermal gradient ~ 25˚C per km depth–Magma bodies: Country rock can be “cooked”Lithostatic pressure is generated by burial. It is equal in all directions1. Compressive stress: forces are opposing2. Shearing stress: forces are sliding past each otherDifferential or directionalStress is unequal in one directionDirectional pressure makes elongate mineralsalign in preferential directionsCompressive stress makeselongate crystalsalign perpendicular to stressShear stress makes elongate crystalsalign parallel to stressThe results of lithostatic or directional stress:•New mineral phases will develop•Denser, more compact mineral forms(Polymorphs)•Or old minerals will dissolve and the ions recrystallize into new forms stable at those pressuresThe Effects of Temperature•Temperature Effects–As T changes minerals become unstable–Below 200oC, reaction rates are slow–As T increases, reaction rates increase–New atomic structures form in equilibrium with new environment–Rocks begin to partially melt at ~700oC, complete melting may begin at ~1300 oCThe role of fluids3. Fluids increase the rates of chemical reactions•Fluids remove and transport Ions creating new mineral species•Fluids can contain a variety of components facilitating reactions•Fluids originate from:–Dewatering from pore spaces–Volatiles from magmas–Release from hydrous mineralsHow does metamorphism occur•The Heat and Pressure breaks bonds–Water lowers the temperature necessary for this to occur•Ions migrate to new sites–Water transmits ions•New minerals are formed–Unique mineral species that form under certain T and P conditions are called:INDEX MINERALSAl2O5 PolymorphsSeveral Index minerals that form only underCertain T and P conditionsKYANITEKYANITESILLIMANITESILLIMANITEANDALUSITEANDALUSITELimits of metamorphism•Metamorphism begins when–Temperature exceeds 200OC–Pressure exceeds 300 M Pa (approx. 10,000 ft deep)•Metamorphism ends when melting begins (~ 1300OC)2 main categories of metamorphism1. Regional metamorphism–Large scale of affected rocks–Usually a consequence of High P; and Med – High T•Can be from either DEEP BURIAL, or TECTONIC Forces•Results in minerals becoming aligned•Alignment called FOLIATED TEXTURE2 main categories of metamorphism2. Contact metamorphism–Result of Med - High T, Low – Med P–Usually narrow zone of country rock near magma bodies–Smaller scale of affected rocks–No Alignment of minerals–NONFOLIATED textureFoliated v Non-foliated1. Regional metamorphismFoliated RocksRegional metamorphismPressure creates FoliationsControls on Composition/Textureof Foliated rocks (Regional Meta)•Parent Rock or PROTOLITH helps determine the mineralogy•Degree of metamorphism controls texture•Ex) Sedimentary rock (Shale)Sed Metamorphic rocksSHALE > SLATE > PHYLLITE > SCHIST>GNEISSIncreasing metamorphic conditionsThe surface shows The surface shows ““slaty cleavageslaty cleavage”Original beddingOriginal beddingPhyllite•Aligned Clays in the rock begin to grow•Rock develops a sheen or luster•Protoliths include:Shale, Claystones, and the Meta rocks such as SlatePhylliteSchist•Clays grow to a visible size•Foliations apparent to unaided eye•Protoliths include:•A Variety:–Shales, Carbonates, Mafic Igneous rocks–Plus the meta rocks Slate and PhylliteA Garnet-Muscovite SchistA muscovite mica schistGneiss•Minerals segregate into distinct “bands”•Gneissic banding = foliations•Can be formed by highly metamorphosed shale•But also, other Protoliths can produce gneiss–Sandstone > Gneiss–Granite > GneissGneiss with characteristic“Gneissic Banding”Amphibolite:•Metamorphosed Mafic Rocks•Contains the mineral Amphibole•May be foliated or non-foliated•Often found as the dark bands in gneiss (though the dark bands are often other minerals)•Can be