igneous .winter (2001) an introduction to igneous and metamorphic petrology. ... to igneous and...

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  • Igneous

    Petrology

  • PETROGRAPHY

    The description and systematic classification of rocks, aided by

    the microscopic examination of thin sections.

    PETROLOGY

    The study of the origin, occurrence, structure and history of rocks,

    much broader process/study than petrography.

    PETROGENESIS

    A branch of petrology dealing with the origin and formation of

    rocks. Involves a combination of mineralogical, chemical and

    field data.

    Petrologic, petrographic, and petrogenetic studies can be applied

    to igneous, metamorphic or sedimentary rocks.

  • The Earths Interior Crust:

    Oceanic crust

    Thin: 10 km

    Relatively uniform stratigraphy

    = ophiolite suite:

    Sediments

    pillow basalt

    sheeted dikes

    more massive gabbro

    ultramafic (mantle)

    Continental Crust

    Thicker: 20-90 km average ~35 km

    Highly variable composition

    Average ~ granodiorite

  • The Earths Interior

    Mantle:

    Peridotite (ultramafic)

    Upper to 410 km (olivine spinel)

    Low Velocity Layer 60-220 km

    Transition Zone as velocity increases ~ rapidly

    660 spinel perovskite-type

    SiIV SiVI

    Lower Mantle has more gradual

    velocity increase

    Figure 1-2. Major subdivisions of the Earth.

    Winter (2001) An Introduction to Igneous

    and Metamorphic Petrology. Prentice Hall.

  • The Earths Interior

    Core:

    Fe-Ni metallic alloy

    Outer Core is liquid

    No S-waves

    Inner Core is solid

    Figure 1-2. Major subdivisions of the Earth.

    Winter (2001) An Introduction to Igneous

    and Metamorphic Petrology. Prentice Hall.

  • Figure 1-3. Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left,

    rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. Blackwell Scientific. Oxford.

  • NOMENCLATURE AND CLASSIFICATION

  • -Formation of minerals in an igneous rocks is controlled by

    the chemical composition of the magma and the physical-

    chemical conditions present during crystallization.

    - Mineralogical composition and texture are used to describe,

    name and classify rocks.

    - Both overall chemistry ( whole-rock chemistry) and the

    chemistry of constitute minerals offer clues to igneous rock

    origins.

    - Studies of rock chemistry reveal where magmas form and

    how they are modified before they solidify.

    - the problem in rock classification is the selection of a basis

    for classification.

    - proposed classifications use texture, mineralogy, chemistry,

    geographic location and rock associations.

  • -Systems of nomenclature and classification may reflect:

    genetic, textural, chemical or mineralogical features.

    GENETIC

    basic system which classifies rocks on the basis of where

    they form.

    plutonic - at depth

    hypabyssal - intermediate depth

    volcanic - on the Earth's surface.

    This system is not very practical, but it serves as a first

    approximation, it tells nothing about mineralogy, chemistry of

    the rocks and can not distinguish basalt from rhyolite.

  • -TEXTURAL

    relies on the grain size of individual minerals in the rock.

    aphanitic - fine grained < 1 mm

    phaneritic - medium grained 1 to 5 mm

    coarse grained (pegmatitic) > 5 mm

    This system has the same shortcomings as a genetic

    classification, however specific textures present may aid in

    classification, e.g., phenocryst, ophitic, coronas, but these are

    not indicative of a specific environment of formation or a

    specific lithology.

  • -CHEMICAL

    This type of classification requires a complete

    chemical analysis of the rock

    A chemical classification system has been

    proposed for volcanic rocks and a comparable

    scheme for plutonic rocks is not available.

    This leaves us with a system based on mineralogy.

  • MINERALOGICAL

    The one gaining application is the result of several

    years work by the IUGS Subcommission on the

    Classification of Igneous Rocks or Streckeissen Classification.

  • CLASSIFICATION SYSTEMS

  • Several aspects which historically have played and continue to

    play a role in the classification of igneous rocks should also be

    considered.

    GRADATION IN SILICA CONTENT

    - referred to as acid or basic, implying a range of silica content.

    Acidic > 66 wt% SiO2

    Granites ~ 72 wt% SiO2, granodiorites ~ 68 wt% SiO2

    Intermediate - 52 to 66 wt% SiO2

    Andesite 57 wt% SiO2

    Basic - 45 to 52 Wt% SiO2

    Basalts range from 48 to 50 wt%

    Ultrabasic - < 45 wt% SiO2

    peridotites 41 to 42 wt% SiO2

  • COLOUR GRADATION

    Felsic rocks are light coloured, contain felsic

    minerals (e.g. qtz, feldspar, feldspathoids) which are

    themselves light in colour and have a low density which contribute to the pale colour of the rock.

    Mafic Rocks are denser and dark coloured, the

    result of containing mafic minerals (pyroxene,

    amphibole, olivine, biotite). These minerals contribute to the green, brown and black colour of

    these rocks.

  • Chemistry of Igneous rocks

  • -Modern chemical analyses of igneous rocks

    generally include a major elements analyses and

    minor or trace elements analyses.

    - Earth is composed almost entirely of 15 elements, 12 of which are the dominant elements of the crust.

    - The crustal elements, considered to be the major

    elements, in order of decreasing abundance, are O,

    Si, Al, Fe, Ca, Na, Mg, K, Ti, H, P and Mn.

  • Composition of Earth shells Elements wt%

    Crust Mantle Core

    Continental Oceanic Upper Lower Outer Inner

    O 41.2 43.7 44.7 43.7 10--15

    Si 28 22 21.1 22.5

    Al 14.3 7.5 1.9 1.6

    Fe 4.7 8.5 5.6 9.8 80--85 80

    Ca 3.9 7.1 1.4 1.7

    K 2.3 0.33 0.08 0.11

    Na 2.2 1.6 0.15 0.84

    Mg 1.9 7.6 24.7 18.8

    Ti 0.4 1.1 0.12 0.08

    C 0.3

    H 0.2

    Mn 0.07 0.15 0.07 0.33

    Ni 5 20

    Cr 0.51

  • The chemical composition of rocks is determined by analyzing a powder

    of the rock.

    Routine geochemical analysis of geologic materials can be

    carried out using either or a combination of the following two

    techiques:

    X-ray Fluoresence Spectroscopy (XRF) to determine both major

    and trace elements

    Atomic Absorbtion Spectrometry (AAS) to determine both major

    and trace elements

    The composition of an igneous rock is dependant on:

    Composition of the source material

    Depth of melting

    Tectonic environment where crystallization occurs. e.g. rifting vs.

    subduction

    Secondary alteration

  • These are the 13 major oxide

    components which are reported as

    weight percent (wt%).

    Because these are reported as a

    percentage the total should sum to

    100 %, ideally, however acceptable

    totals lie in the range 98.5 to 101

    wt%.

  • A typical rock analysis

    For Major Oxides.

    Oxide Wt. %

    SiO2 49.20

    TiO2 1.84

    Al2O3 15.74

    Fe2O3 3.79

    FeO 7.13

    MnO 0.20

    MgO 6.73

    CaO 9.47

    Na2O 2.91

    K2O 1.10

    H2O+ 0.95

    (O)

    Total 99.06

  • Table 8-3. Chemical analyses of some

    representative igneous rocks

    Peridotite Basalt Andesite Rhyolite Phonolite

    SiO2 42.26 49.20 57.94 72.82 56.19

    TiO2 0.63 1.84 0.87 0.28 0.62

    Al2O3 4.23 15.74 17.02 13.27 19.04

    Fe2O3 3.61 3.79 3.27 1.48 2.79

    FeO 6.58 7.13 4.04 1.11 2.03

    MnO 0.41 0.20 0.14 0.06 0.17

    MgO 31.24 6.73 3.33 0.39 1.07

    CaO 5.05 9.47 6.79 1.14 2.72

    Na2O 0.49 2.91 3.48 3.55 7.79

    K2O 0.34 1.10 1.62 4.30 5.24

    H2O+ 3.91 0.95 0.83 1.10 1.57

    Total 98.75 99.06 99.3 99.50 99.23

  • Rare Earth Elements (REE or

    lathanides, atomic number

    57 to 71), are reported in

    ppm or mg/g. The REE are

    important for petrogenetic

    studies, because as a

    group the REE behave

    coherently.

  • SATURATION CONCEPT Used in reference to the SiO2 and Al2O3 which are the two most abundant

    components of igneous rocks. SiO2 Saturation

    SiO2 Saturation Minerals present in igneous rocks can be divided into two groups:

    Those which are compatible with quartz or primary SiO2 mineral (tridymite,

    cristobalite) these minerals are saturated with respect to Si, e.g feldspars,

    pyroxenes. Those which never occur with a primary silica mineral. These are

    undersaturated minerals, e.g. Mg-rich olivine, nepheline.

    The occurrence of quartz with an undersaturated mineral causes a reaction

    between the two minerals to form a saturated mineral. 2SiO2 + NaAlSiO4 ===> NaAlSi3O8

    Qtz + Ne ===> Albite SiO2 + Mg2SiO4 ===> 2MgSiO3

    Qtz + Ol ===> En

  • Rock Classification (Silica saturation)

    Oversaturated - contains primary silica mineral

    Saturated - contains neither quartz nor an unsaturated

    mineral Unsaturated - contains unsaturated minerals

  • Al2O3 Saturation

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