Slide 1DefinitionEffects of Increasing Temperature The Primary Factor in MetamorphismEffects of Pressure Secondary Effect on Mineralogy, Major Effect on TextureLithostatic vs. Directional PressureDeviatoric Stress Generation of Foliation and Lineation TexturesImportance of Fluids in Metamorphic ReactionsImportance of the Parent Rock ProtolithMineralogical Response to MetamorphismTextural Response to MetamorphismOther Textural Responses PorphyroblastsProgressive MetamophismTypes of MetamorphismSlide 14Types of Metamorphism Regional MetamorphismTypes of Metamorphism involving Fluids Ocean-floor Metamorphism, Hydrothermal Metamorphism, and MetasomatismSlide 17Slide 18GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 20Introduction to Metamorphic PetrologyMarch 23, 2009DEFINITION“Metamorphism is a subsolidus process leading to changes in mineralogy and/or texture and often in chemical composition in a rock. These changes are due to physical (P & T) and/or chemical conditions* that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.” IUGS-SCMR *Chemical conditions commonly involve a fluid phasePressure and Temperature conditions of Metamorphic GradesLower Limit - T >100-200°C, P not constrained; depends or reactability of starting rock; zeolite type an accepted indicator.Upper Limit - onset of “SIGNIFICANT” meltingEFFECTS OF INCREASING TEMPERATURETHE PRIMARY FACTOR IN METAMORPHISMIncreasing temperature has several effects:1) Promotes recrystallization  increased grain size2) Drive reactions (endothermic)3) Overcomes kinetic barriersEFFECTS OF PRESSURESECONDARY EFFECT ON MINERALOGY, MAJOR EFFECT ON TEXTUREPressure usually follows “Normal”gradients, but may be perturbed in several ways, typically:Low T/P geotherms in subduction zonesHigh T/P geotherms in areas of plutonic activity or riftingWinter (2001) Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.LITHOSTATIC VS. DIRECTIONAL PRESSUREStress is an applied force acting on a rock (over a particular cross-sectional area)Strain is the response of the rock to an applied stress (= yielding or deformation) Lithostatic P  Uniform StressDirectional P Deviatoric StressFoliationCleavageSchistosityGneissic BandingSimple ShearPure ShearDEVIATORIC STRESSGENERATION OF FOLIATION AND LINEATION TEXTURES-1 > 2 = 3  foliation and no lineation (Flattening Strain)-1 = 2 > 3  lineation and no foliation -1 > 2 > 3  both foliation and lineation1 – Maximum Stress2 – Medial Stress3 – Minimal Stress1223312233IMPORTANCE OF FLUIDS IN METAMORPHIC REACTIONSEvidence for the existence of a metamorphic fluid:Fluid inclusionsFluids are required for hydrous or carbonate phasesVolatile-involving reactions occur at temperatures and pressures that require finite fluid pressuresIMPORTANCE OF THE PARENT ROCK PROTOLITHMINERALOGICAL RESPONSE TO METAMORPHISMMinerals that form depend on: T and P conditionsBulk composition of the source rockProgressivemetamorphismof a graywacke(dirty sandstone)TEXTURAL RESPONSE TO METAMORPHISMReflects the intensity, directionality and duration of pressure (or stress).Increased grain size - During prograde metamorphism or at a particular grade that is maintained for a long period of time, minerals will tend to increase in size. Foliation - As new platy minerals grow, they will align themselves perpendicular to the maximum stress direction. For clay mineral and fine-grained micas, the planar fabric that results is referred to as a slaty cleavage. In higher grade rocks, coarser grained mica minerals are said to impart a schistosity to the rock. Gneissic Banding - In very high grade rocks, the dark minerals tend to segregate from the lighter colored minerals (feldspar and quartz) resulting in banded rock..OTHER TEXTURAL RESPONSESPORPHYROBLASTSSome metamorphic minerals have strong growth habits, meaning they will develop as large well-formed crystals called porphyroblastsAlkali FeldsparStaurolite GarnetPROGRESSIVE METAMOPHISM•Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions•Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event•Prograde reactions are endothermic and easily driven by increasing T•Devolatilization reactions are easier than reintroducing the volatiles•Geothermometry indicates that the mineral compositions commonly preserve the maximum temperatureTYPES OF METAMORPHISM1. Based on principal process or agentDynamic MetamorphismThermal MetamorphismDynamo-thermal Metamorphism2. Based on settingContact MetamorphismRegional MetamorphismoOrogenic MetamorphismoBurial MetamorphismoOcean Floor MetamorphismHydrothermal MetamorphismFault-Zone Metamorphism Impact or Shock MetamorphismTYPES OF METAMORPHISMContact (or Thermal) metamorphism oGrades up to very high temperatures, low-mod. lithostatic pressuresoForms a metamorphic aureole adjacent to igneous intrusions that is best developed in the cooler upper crust (epizone)oCommonly develops granoblastic texture creating a rock called HORNFELSGranoblastic Texture – equant grains, 120° jctsTYPES OF METAMORPHISM REGIONAL METAMORPHISMBurial MetamorphismFollows normal geothermal gradient with lithostatic pressureOrogenic MetamorphismBroad range of P-T paths with differential pressure dominantTYPES OF METAMORPHISM INVOLVING FLUIDSOCEAN-FLOOR METAMORPHISM, HYDROTHERMAL METAMORPHISM, AND METASOMATISMGenerally low temperatures and pressures Distinguished from other forms of metamorphism by the loss and/or gain of material (usually transported by a fluid). Other types of metamorphism are thought to occur as nearly closed processes (except for water loss)Hydrothermally Altered Basalt – Spilite / GreenstoneTYPES OF METAMORPHISMFault Zone Metamorphism (Cataclasis)Variable temperature, very high directed PTypically localized to narrow zones of intense mechanical deformation (shear zones). Rock types formed breccia (fragmental), gouge (clay), mylonite (strongly foliated)Summary of Metamorphic Rock