Dr. Helen LangDept. of Geology & GeographypgygpyWest Virginia UniversitySpring 2008GEOLOGY 285:INTRO. PETROLOGYINTRO. PETROLOGYIRk ditIgneous Rocks are grouped into SuitesRocks in a Suite might come from the gsame volcano (Kilauea), a group of island volcanoes (Hawaii, the (,Galapagos), a single intrusion (the Skaergaard intrusion, Greenland), a g)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 SiO2,most primitive composition (hi MgO, low SiO2, low incompatible elements)–large volume eruptedgp• Daughters, Differentiates, Derivatives - the descendantsdescendantsChanges displayed on Harker Diagrams(Metal Oxide vs. Silica (SiO2))HL-GalapagosHL Galapagos14.0080010.0012.00MgO vs SiO24.006.008.00FeO* vs SiO2Na2ONa2O+K2O0.002.004.0045.00 50.00 55.00 60.00 65.00 70.00 75.00Trends on AFM DiagramTrends on AFM DiagramCascades and Galapagos Molar AFMSkaergaardpg1Mole%FSkaergaard Layered Series0.60.8F0.20.400 0.2 0.4 0.6 0.8 1 1.2MAAMAMSomeDifferentiationProcesses thatSomeDifferentiationProcesses that can change magma composition are•Crystal fractionation or fractional ycrystallization•Magma mixingMagma mixing• Assimilation of country rocksCrystal Fractionation• Crystals are removed from the liquid in whichtheyformedwhich they formed• Commonly by settling under the influence of gravityof gravityρ (olivine) = 3.22 g/cm3(Mg) - 4.30 g/cm3(Fe)()296352 /3ρ(cpx) = 2.96-3.52 g/cm3ρ (opx) = 3.21-3.96 g/cm3(l)263276 /3ρ(plag) = 2.63-2.76 g/cm3ρ (magmas) = 2.4 - 2.8 g/cm3(calculated)Norman L Bowen popularizedNorman L. Bowen popularized Crystal Fractionation•He thought all igneous rocks came from a•He thought all igneous rocks came from a basaltic parent; mainly by crystal fractionationHi id b i•His idea was too extreme, but very important as a starting point• This is the origin of Bowen’s Reaction SeriesBowen’s Reaction SeriesolivineorthopyroxeneCa plagioclaseorthopyroxeneclinopyroxene NaCa plagioclasebiotiteamphibole (Hb)Na plagioclasebiotiteNa plagioclasealkali feldsparBy removing early-formed minerals frommuscovitequartzminerals from basalt, it is possible to get ll ta small amount of graniteBRS is inadequate for generating imost granite•Amount of basalt in crust is approximatelyAmount of basalt in crust is approximately equal to the amount of granite•Bowen’s reaction series could only produce•Bowen’s reaction series could only produce about 1/20 as much granite as the initial volume of basaltvolume of basalt• Where are all the fractionated mafic il?(h ldh bhminerals? (there would have to be a huge amount of ultramafic cumulate rocks hiding hb fh i l )at the base of the continental crust)M&M®MM&M®MagmaChamberChamberFebruary 2008Layered Mafic Intrusions are the best examples of Crystal Fractionation• Palisades Sill along Hudson R. in NJ (see textbook)•Bushveld Intrusion in South Africa pЄcolossal!!Bushveld Intrusion in South Africa, pЄ, colossal!! 320 km in diameter•Skaergaard Tertiary E Greenland**•Skaergaard, Tertiary, E. Greenland**• Muskox, northern Canada• Great Dike, Zimbabwe• Stillwater, pЄ, Montana,p,Skaergaard in SE Greenland• Perhaps the most studied rock body on Earth•Best example of an igneous body that hasBest example of an igneous body that has fractionated to an extreme degree through crystal fractionation(Bowen’s idea)crystal fractionation(Bowen s idea)• Most of its thickness is exposedE l d i 1930 (W ) 1950 1970 d•Explored in 1930s (Wager), 1950s, 1970s and 1990sUpper Zone Layered Series (Wager 1930s photo)What do you notice?Evidence for Crystal Settling• Cumulate mineral textures, euhedral to bh d l i il d if h l dsubhedral grains piled up as if they settled in a liquid• Sedimentary-like structures–layeringyg– graded beddingcrossbedding–cross-bedding– slump structures, etc.Cumulate Mineral Texturentrusiontillwater Inollege – StUnion CoHollocher, to: Kurt HPoikilitic AugiteCumulate Opx, intercumulate PlagPhotLayering (variation in mineral idi)proportions and sizes)Graded Bedding (coarsest at bottom)Best example fromBest example from the Duke Island Complex inComplex in southeast AlaskaCross-bedding, like in sandstonessandstonesThTrough cross-beddingSkaergaard gGeologic MappCTbasabout 8 x 10 kmIrvine, Andersen & Brooks, 1998, GSA Bull.The Skaergaard is an asymmetric lopolithNSCrystallizationChilled marginLast Liquid was trapped at the “Sandwich Horizon”There are two kinds of LayeringThere are two kinds of Layering in the Layered Series• Rhythmic Layering - changes in the identity and proportion of minerals•Cryptic Layering -changes in chemical yp y ggcomposition of minerals upwards through the layers (hidden, you can’t see it, must y( ,y ,have chemical analyses of minerals)Rhythmic LayeringOriginal Skaergaard Magma wasOriginal Skaergaard Magma was a Tholeiitic Basalt• Layering and compositional changes mainly ldf lf i i bresulted from crystal fractionation by gravity settling (fractional crystallization)Current Exposure (E-W)Current Exposure (EW)The Layered SeriesOlivine is absent in the Middle Zone (MZ)Why does Olivine disappear in Middle Zone?ElidbFESiOdiPigeonite is an Enstatite-like PExplained by Fo-En-SiO2diagramPyroxeneWhat happens when liquid X reaches a during fractional crystallization? Fo is replaced by Pigeonite, which (like s ep ced by geo e, w c ( eEnstatite) has a reaction relationship with Mg-rich Olivine.Olivine reappears in Upper Zone; Fe-rich Olivine is OK in SiO2-rich liquidqNote that at bottom of Upper ZoneNote that at bottom of Upper Zone, Olivine has only 40% Fo (Mg2SiO4) and 60% Fa (Fe2SiO4)Olivine changes from 67% Fo (Mg2SiO4) O v e c a ges o 67% o ( g2SO4)at the base of the Layered Zone to 0% Fo, 100% Fa (Fe2SiO4) at the top of the Layered ZoneLayered ZoneAb-An Diagram explains why plagioclase composition changes from bottom to top of SkaergaardgpgFractional Crystallization: liquid and lk iihPlag keep getting more Na-richNote other cumulus mineralsAugiteAugite-more Fe-rich toward toptopCumulate Mineral Texturentrusiontillwater Inollege – StUnion CoHollocher, to: Kurt HPoikilitic AugiteCumulate Opx, intercumulate PlagPhotNote other cumulus mineralsQuartz and micropegmatite with Fe-olivine iU Zin Upper ZoneCompare to ®M&M®magmamagma