IgneousIgneous && MetamorphicMetamorphic PetrologyPetrology LectureLecture NotesNotes

ByBy DavidDavid T.T. AllisonAllison EarthEarth’’ss InternalInternal LayersLayers

ƒƒ 22 CriteriaCriteria –– CompositionComposition (i.e.(i.e. mineralogymineralogy andand geochemistry)geochemistry) –– SeismicSeismic (mechanical(mechanical behavior)behavior) EarthEarth’’ss LayeringLayering byby CompositionComposition ƒƒ DepthDepth ThickThick LayerLayer Diorite (2.7) 7-50km Crust Basalt (3.0)

650km Upper mantle Peridotite (3.3) 700km

2200km Lower mantle Garnet Lherzolite (3.5-5.5)

2900km

2200kmOuter core Ni-Fe-S Alloy (10-12)

5100km 1300kmInner core Ni-Fe Alloy (13) 6400km EarthEarth’’ss LayeringLayering byby SeismicSeismic PropertiesProperties ƒƒ DepthDepth ThickThick LayerLayer Brittle 70-150km Lithosphere

550km Asthenosphere Ductile 700km

2200km Mesosphere Brittle

2900km

2200kmOuter core Liquid (very ductile)

5100km 1300kmInner core Brittle 6400km PlatePlate TectonicsTectonics andand PetrologyPetrology

ƒƒ PlatePlate BoundariesBoundaries –– Divergent:Divergent: platesplates movemove directlydirectly awayaway fromfrom boundaryboundary –– Convergent:Convergent: platesplates movemove directlydirectly towardtoward boundaryboundary –– Transform:Transform: platesplates movemove parallelparallel toto boundaryboundary –– Collisonal:Collisonal: continentalcontinental lithospherelithosphere inin eacheach plateplate collide,collide, therefore,therefore, subductionsubduction isis terminatedterminated SeafloorSeafloor SpreadingSpreading

ƒƒ AlfredAlfred WegenerWegener (1915):(1915): continentalcontinental driftdrift theorytheory ƒƒ ArthurArthur HolmesHolmes (1931):(1931): mantlemantle convectionconvection ƒƒ HarryHarry HessHess (1962):(1962): ““conveyorconveyor beltbelt”” modelmodel forfor seafloorseafloor ƒƒ VineVine andand MathewsMathews (1963):(1963): paleomagnetismpaleomagnetism discoverydiscovery definitivelydefinitively provedproved thatthat thethe seafloorseafloor waswas spreadingspreading awayaway fromfrom oceanocean ridgesridges PetrologyPetrology ofof PaleomagnetismPaleomagnetism

ƒƒ PaleomagnetismPaleomagnetism isis thethe preservationpreservation ofof thethe EarthEarth’’ss magneticmagnetic fieldfield magnitude,magnitude, orientationorientation andand polaritypolarity inin mineralsminerals

- Magnetite crystal + PaleomagneticPaleomagnetic ReversalsReversals

ƒƒ PeriodicallyPeriodically thethe EarthEarth’’ss magneticmagnetic fieldfield reversesreverses polarity,polarity, anan eventevent thatthat isis recordedrecorded inin igneousigneous magneticmagnetic mineralsminerals

Reversal of polarity ++-- S --++ N --+ + PaleomagneticPaleomagnetic SeafloorSeafloor ““StripesStripes””

ƒƒ EachEach sectionsection ofof oceanocean lithospherelithosphere inheritsinherits thethe EarthEarth’’ss magneticmagnetic polaritypolarity GlobalGlobal PaleomagneticPaleomagnetic PatternsPatterns

ƒƒ WidthWidth ofof paleomagneticpaleomagnetic stripesstripes indicatesindicates thethe relativerelative raterate ofof spreadingspreading OphioliteOphiolite SuiteSuite

ƒ 1. Sediment layer (1-100m) – Radiolarian chert, Fe oxides, turbidites ƒ 2. Pillow basalt layer (1 km) – Submarine extrusion of basaltic lava ƒ 3. Sheeted dike layer (2 km) – Diabase dikes ƒ 4. Gabbro layer (4 km) – Batholiths of gabbro with layering controlled by fractional crystallization ƒ 5. Peridotite layer (63 km) – Ol+Opx+Cpx mantle FormationFormation ofof OphiolitesOphiolites

ƒ Formation of ophiolites is driven by magma chamber under divergent ocean ridge systems ƒ Basalts produced at ocean ridge divergent boundaries have a geochemical signature that is termed tholeiitic (MORB) ContinentalContinental RiftRift ZonesZones

ƒ Initial developing phase of a divergent plate boundary ƒ Magma is enriched in alkali (K, Na, Rb) because of high pressure source area (Alkali basalt, Nepheline syenite; Bimodal volcanics) ƒ Transition from continental to ocean lithosphere is termed a passive continental margin ConvergentConvergent PlatePlate BoundariesBoundaries

ƒ Always marked by ocean trench physiographic features ƒ One of the converging plates, always ocean lithosphere, is consumed by subduction ƒ Volcanic and magmatic arcs are associated with subduction CharacteristicsCharacteristics ofof ConvergentConvergent BoundariesBoundaries Volcanic Arc Back- Accretionary Fore-arc basin arc Trench Prism (eugeocline) basin

Melange

1200 deg C isotherm Greenschist Blueschist (miogeocline) Amphibolite Granulite Benioff Zone seismic Focal points Calc- Alkaline Eclogite magma

Magma source region TransformTransform BoundariesBoundaries

ƒƒ FaultFault actionaction producesproduces aa typetype ofof metamorphicmetamorphic faultfault--zonezone rockrock termedtermed mylonitemylonite ƒƒ ““LeakyLeaky transformstransforms”” erupterupt alkalialkali basaltbasalt similarsimilar toto continentalcontinental riftrift zoneszones

Example: Death Valley along the San Andreas fault system CollisionalCollisional ZonesZones

ƒƒ IgneousIgneous activityactivity associatedassociated withwith convergenceconvergence ceasesceases ƒƒ SeismicSeismic activityactivity becomesbecomes intenseintense ƒƒ EclogitesEclogites maymay becomebecome exposedexposed ƒƒ ExtremeExtreme topographictopographic reliefrelief ƒƒ OphiolitesOphiolites areare preservedpreserved inin suturesuture zoneszones betweenbetween collidingcolliding continents/Islandcontinents/Island ArcsArcs