GEOLOGY 285: INTRO. PETROLOGY

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1 Dr. Helen Lang Dept. of Geology & Geography West Virginia University Spring 2016 GEOLOGY 285: INTRO. PETROLOGY

2 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)

3 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 SiO 2, low incompatible elements) large volume erupted Daughters, Differentiates, Derivatives - the descendants

4 Changes displayed on Harker Diagrams (Metal Oxide vs. Silica (SiO 2 )) HL - Galapagos MgO vs SiO2 FeO* vs SiO2 Na2O Na2O+K2O

5 Trends on AFM Diagram Cascades and Galapagos Molar AFM F Mole% Skaergaard Layered Series F A 0 M A M

6 Some Differentiation Processes that can change magma composition are Crystal fractionation Magma mixing Assimilation of country rocks

7 Crystal Fractionation Crystals are removed from the liquid in which they formed Commonly by settling under the influence of gravity (olivine) = 3.22 g/cm 3 (Mg) g/cm 3 (Fe) (cpx) = g/cm 3 (opx) = g/cm 3 (plag) = g/cm 3 (magmas) = g/cm 3 (calculated)

8 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 Series

9 Bowen s Reaction Series (BRS) olivine orthopyroxene clinopyroxene Ca plagioclase NaCa plagioclase amphibole (Hb) biotite Na plagioclase alkali feldspar muscovite quartz By removing early-formed minerals from basalt, it is possible to get a small amount of granite

10 BRS 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)

11 M&M Magma Chamber February 2008

12 Layered 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, Montana

13 Skaergaard 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 1990s

14 Upper Zone Layered Series (Wager s 1930s photo) What do you notice?

15 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.

16 Photo: Kurt Hollocher, Union College Stillwater Intrusion Cumulate Mineral Texture Cumulate Opx, intercumulate Plag Poikilitic Augite

17 Layering (variation in mineral proportions and sizes)

18 Graded Bedding (coarsest at bottom) Best example from the Duke Island Complex in southeast Alaska

19 Cross-bedding, like in sandstones Trough crossbedding

20 The Skaergaard is an asymmetric lopolith N S

21 Skaergaard Geologic Map pc about 8 x 10 km Tbas Irvine, Andersen & Brooks, 1998, GSA Bull.

22 Crystallization Chilled margin Last Liquid was trapped at the Sandwich Horizon

23 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)

24 Rhythmic Layering

25 Original Skaergaard Magma was a Tholeiitic Basalt Layering and compositional changes mainly resulted from crystal fractionation by gravity settling (fractional crystallization)

26 Current Exposure (E-W)

27 The Layered Series Olivine is absent in the Middle Zone (MZ)

28 Why does Olivine disappear in Middle Zone? Pigeonite is an Enstatitelike Pyroxene Explained by Fo-En-SiO 2 diagram What happens when liquid X reaches a during fractional crystallization? Fo is replaced by Pigeonite, which (like Enstatite) is produced by reaction of SiO 2 -rich liquid with Mg-rich Olivine.

29 Olivine reappears in Upper Zone; Fe-rich Olivine is OK in SiO 2 - rich liquid Note that at bottom of Upper Zone, Olivine has only 40% Fo (Mg 2 SiO 4 ) and 60% Fa (Fe 2 SiO 4 ) Olivine changes from 67% Fo (Mg 2 SiO 4 ) at the base of the Layered Zone to 0% Fo, 100% Fa (Fe 2 SiO 4 ) at the top of the Layered Zone

30 Ab-An Diagram explains why plagioclase composition changes from bottom to top of Skaergaard Fractional Crystallization: liquid and Plag keep getting more Na-rich

31 Note other cumulus minerals Augite - more Ferich toward top

32 Photo: Kurt Hollocher, Union College Stillwater Intrusion Cumulate Mineral Texture Cumulate Opx, intercumulate Plag Poikilitic Augite

33 Granite in Upper Zone Quartz and micropegmatite with Fe-olivine in Upper Zone

34 Compare to M&M magma chamber MgOl FeOl CaPl Di Qtz Mt Ilm Kfsp NaPl

35 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

36 Crystal 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 basalt

37 Skaergaard Layered Series Continuous vertical lines indicate cumulus minerals, broken vertical lines relate to intercumulus minerals or those of indeterminate status