Spring 2014GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversityIgneous Rocks are grouped into SuitesRocks in a Suite might come from the same volcano (Kilauea), a group of island volcanoes (Hawaii, the Galapagos), a single intrusion (the Skaergaard 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 composition (hi MgO, low SiO2, low incompatible elements)– large volume erupted• Daughters, Differentiates, Derivatives - the descendantsChanges displayed on Harker Diagrams(Metal Oxide vs. Silica (SiO2))HL - Galapagos0.002.004.006.008.0010.0012.0014.0045.00 50.00 55.00 60.00 65.00 70.00 75.00MgO vs SiO2FeO* vs SiO2Na2ONa2O+K2OTrends on AFM DiagramCascades and Galapagos Molar AFM00.20.40.60.810 0.2 0.4 0.6 0.8 1 1.2Mole%FMASkaergaard Layered SeriesAFMSome Differentiation Processes that can change magma composition are• Crystal fractionation• Magma mixing• Assimilation of country rocksCrystal Fractionation• Crystals are removed from the liquid in which they formed• Commonly by settling under the influence of gravity (olivine) = 3.22 g/cm3(Mg) - 4.30 g/cm3(Fe) (cpx) = 2.96-3.52 g/cm3 (opx) = 3.21-3.96 g/cm3 (plag) = 2.63-2.76 g/cm3(magmas) = 2.4 - 2.8 g/cm3(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 was very important as a starting point• This is the origin of Bowen’s Reaction SeriesBowen’s Reaction Series (BRS)olivineorthopyroxeneclinopyroxenebiotiteamphibole (Hb)Ca plagioclaseNa plagioclaseNaCa plagioclasealkali feldsparmuscovitequartzBy removing early-formed minerals from basalt, it is possible to get a small amount of graniteBRS is inadequate for generating most granite• The amount of basalt in crust is approximately equal to the amount of granite• Bowen’s reaction series could only produce about 1/20 as much granite as the initial volume of basalt• Where are all the fractionated mafic minerals? (there would have to be a huge volume of ultramafic cumulate rocks hiding at the base of the continental crust)M&M®MagmaChamberFebruary 2008Layered Mafic Intrusions are the best examples of Crystal Fractionation• Palisades Sill along Hudson R. in NJ (see textbook)• Bushveld Intrusion in South Africa, pЄ, colossal!! 320 km in diameter• Skaergaard, Tertiary, E. Greenland**• Muskox, northern Canada• Great Dike, Zimbabwe• Stillwater, pЄ, MontanaSkaergaard 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• It was explored in 1930s (Wager), 1950s, 1970s and 1990sUpper Zone Layered Series (Wager’s 1930s photo)What do you notice?Evidence for Crystal Settling• Cumulate mineral textures, euhedral to subhedral grains piled up as if they settled in a liquid• Sedimentary-like structures– layering– graded bedding– cross-bedding– slump structures, etc.Cumulate Mineral TexturePoikilitic AugiteCumulate Opx, intercumulate PlagPhoto: Kurt Hollocher, Union College – Stillwater IntrusionLayering (variation in mineral proportions and sizes)Graded Bedding (coarsest at bottom)Best example from the Duke Island Complex in southeast AlaskaCross-bedding, like in sandstonesTrough cross-beddingSkaergaard Geologic 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 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 layers (hidden, you can’t see it, must have chemical analyses of minerals)Rhythmic LayeringOriginal Skaergaard Magma was a Tholeiitic Basalt• Layering and compositional changes mainly resulted from crystal fractionation by gravity settling (fractional crystallization)Current Exposure (E-W)The Layered SeriesOlivine is absent in the Middle Zone (MZ)Why does Olivine disappear in Middle Zone?Pigeonite is an Enstatite-like PyroxeneExplained by Fo-En-SiO2diagramWhat happens when liquid X reaches a during fractional crystallization? Fo is replaced by Pigeonite, which (like Enstatite) is produced by reaction of SiO2-rich liquid with Mg-rich Olivine.Olivine reappears in Upper Zone; Fe-rich Olivine is OK in SiO2-rich liquidNote that at bottom of Upper Zone, Olivine has only 40% Fo (Mg2SiO4) and 60% Fa (Fe2SiO4)Olivine changes from 67% Fo (Mg2SiO4) at the base of the Layered Zone to 0% Fo, 100% Fa (Fe2SiO4) at the top of the Layered ZoneAb-An Diagram explains why plagioclase composition changes from bottom to top of SkaergaardFractional Crystallization: liquid and Plag keep getting more Na-richNote other cumulus mineralsAugite -more Fe-rich toward topCumulate Mineral TexturePoikilitic AugiteCumulate Opx, intercumulate PlagPhoto: Kurt Hollocher, Union College – Stillwater Intrusion“Granite” in Upper ZoneQuartz and micropegmatite with Fe-olivine in Upper ZoneCompare to M&M®magma chamberQtzFeOlMgOlDiCaPlNaPlKfspMtIlmRemember, there are two kinds 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 layersCrystal Fractionation to an Extreme Degree• Mafic minerals are all Fe-richer toward the top of layered series (Fe-end members have lower melting/crystallization temperatures)• Plagioclase is more Na-rich toward the top (Na-plag crystallizes later and at lower temperature than Ca-plag)• Quartz and micropegmatite represent the little bit of “granite” that can result from crystal fractionation of a tholeiitic basaltSkaergaard Layered SeriesContinuous vertical lines indicate cumulus minerals, broken vertical lines relate to intercumulus minerals or those of indeterminate