Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring 2009 Differentiation of Igneous Rocks:Crystal Fractionation and Layered Mafic Intrusions Igneous Rocks are grouped into Suites Rocks 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 - different names for the descendantsChanges are displayed on Harker Diagrams (Metal Oxide vs. SiO2)Trends on AFM Diagrams Some Differentiation Processes that can change magma composition - Crystal fractionation (We will talk mainly about this.) - Magma mixing- Assimilation of country rocks Crystal 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 very important as a starting point - This is the origin of Bowen’s Reaction Series - see handout from before Bowen's Reaction Series (BRS) is inadequate for generating most granite- 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 amount of ultramafic cumulate rocks hiding at the base of the continental crust) Layered Mafic Intrusions are the best examples of Crystal Fractionation- Palisades Sill along Hudson River in NJ (see textbook) - Bushveld Intrusion in South Africa, pC, colossal!! 320 km in diameter - Skaergaard, Tertiary, E. Greenland**- Muskox, northern Canada - Great Dike, Zimbabwe - Duke Island Complex, SE Alaska- Stillwater, pC, MontanaWe did an imitation of a Layered Mafic Intrusion with the M&M® Magma Chamber Skaergaard in E. 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 - Explored in 1930s, 1950s and 1970s Upper Zone Layered Series (Wager 1930s photo) Evidence for Crystal Settling- Cumulus mineral textures, euhedral-subhedral grains piled up as if they settled in a liquid - Sedimentary-like structures - layering - graded bedding - cross-bedding - slump structures, etc. Cumulus Mineral Texture Layering (variation in mineral proportions and sizes)Graded Bedding (coarsest at bottom) Cross-bedding Trough banding Skaergaard is an asymmetric lopolith - see handout CrystallizationWhat is 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 throughthe layers (hidden, you can’t see it, must have chemical analyses of minerals) Original Skaergaard Magma was a Tholeiitic Basalt- Layering and compositional changes mainly resulted from crystal fractionation bygravity settling (fractional crystallization). Current Exposure (E-W cross section) The Layered Series - see handoutWhy does Olivine disappear in Middle Zone?Explained by Fo-En-SiO2 diagram - see previous handout Olivine reappears in Upper Zone; Fe-rich Olivine is OK in SiO2-rich liquid Ab-An Diagram explains why plagioclase composition changes from bottom totop of Skaergaard Note other cumulus minerals Remember, 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 layers Crystal Fractionation to an Extreme Degree- Mafic minerals are all Fe-richer toward the top of layered series (Fe-end membershave lower melting/crystallization temperatures) - Plagioclase is more Na-rich toward the top (Na-plag crystallizes later and at lowertemperature than Ca-plag) - Quartz and micropegmatite represent the little bit of "granite" that can result from crystal fractionation of a tholeiitic