Petrology & Petrography of Igneous Rocks
(Geology and Geophysics 234 & 433)
Final Exam
The philosophy of this course is to meld observations
of real rocks (textures, mineralogy, etc.) with the results of compositional
analysis (variation diagrams, chemical classification schemes, geochemical
properties of groups of elements etc.) and the results of experimental
petrology (phase diagrams, mineral stability's) to build comprehensive
petrogenetic models for the origin and diversity of igneous rocks that
are amenable to testing.
Any petrogentic model for igneous rocks must consider
the following:
1) Igneous rocks represent past thermal anomalies.
Therefore, a heat source/mechanism is required to induce melting.
2) Something melted. What was the modal mineralogy
and composition of the source material that underwent melting (How will
this determine the intrinsic properties of the melt).
3) Processes operating during melting (e.g., fractional,
batch, equilibrium). How can we modify the composition of melts during
melting?
4) Processes operating after melt segregation either
during ascent or emplacement or both (e.g., fractional crystallization,
mixing, assimilation, restite unmixing). How can we modify the compositions
of melts after they have left the site of partial melting?
5) Finally, the role of the Tectonic Setting. What
processes, specific to different tectonic settings, leave distinct imprints
on the final compositions of igneous rocks (e.g., Trace element signatures,
Oxygen fugacity,), and when and how does this occur.
We can then use the mineralogical and compositional diversity
of igneous rocks to address the redistribution of energy and material
throughout the evolution of the Earth. This is a very exciting topic....for
another semester.
The above themes will be implicit in the questions
on the final (that will constitute the "review" portion). However,
the questions on the final will specifically cover the topics below.
It has been a pleasure having you all in class. Do
well!
Magmatism at Convergent Margins
Part I. Structure of the Oceanic Lithosphere
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The Ophiolite Suite (pp. 177 - 180)
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What rock types comprise an ophiolite?
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What is an ophiolite thought to represent?
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Where were these rocks formed?
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How have the compositions of these rocks been modified? (e.g.,
spillite)
Part II. Destruction of the Oceanic Lithosphere
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Obduction vs Subduction
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Why is the oldest oceanic lithosphere younger than the Triassic?
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What is the significance of the age of the oceanic lithosphere
and depth of the ocean floor?
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What drives subduction? (i.e., ridge push vs slab pull)
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Why do we have ophiolite suites?
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Subduction of Oceanic Lithosphere
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What significant phase changes occur in the oceanic lithosphere
during subduction?
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Why are these changes important? (e.g., changes in density)
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What is the stability limit of amphibole?
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Why is this phase change important? (e.g., release of a fluid
phase)
Part III. Generation of Basaltic Magma at Convergent Margins
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Melting the Slab vs Melting the Overlying Mantle Wedge (pp.170-177)
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Typically the overlying mantle wedge (asthenosphere) melts.
Why?
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Under what circumstances may it be possible to melt the subducted
slab?
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What triggers melting of the overlying mantle wedge, and
why?
Part IV. Compositional Variation in Igneous Rock Suites
from Convergent Margins
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Trace Element Signature (The infamous "Spiderdiagrams") (p.
78)
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LILEs (Large Ion Lithophile Elements)
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What are they?
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Why are they enriched in igneous rocks originating from a
subduction zone environment?
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REEs (Rare Earth Elements) (LREEs Light... MREEs Middle...HREEs
Heavy....)
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HFSE (High Field Strength Elements)
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What are they?
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Why are they depleted in igneous rocks originating from a
subduction zone environment?
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Spatial Variation of Magma Composition. (pp.172-174)
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What is the correlation between K2O content and
distance from the trench?
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What are the possible causes of this correlation?
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How are suites classified using K2O content and
what might this signify?
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Alkali-lime Index or "Peacock Index" (p. 78 &
176)
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Classification of suites using this relationship.
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Variation with regards to distance from the trench.
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Variation with regards to tectonic setting.
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AFM Ternary Diagram (p. 77-78, & 172-174)
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Tholeiite Trend (Fe-enrichment, or Skaergaard Trend)
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Typical of what tectonic setting?
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Calc-Alkaline Trend (BADR Basalt-Andesite-Dacite-Rhyolite)
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Typical of what tectonic setting?
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What are two possible explanations for this trend?
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Mixing, and fractionation of a magma with an intrinsically
higher Oxygen fugacity.
Generation and Crystallization of Granitic Magma
Part I. Experimental Phase Relationships (pp. 113-116
but pay more attention to your notes)
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The Albite-Orthoclase Binary System (see also pp. 203-206)
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Understand the difference between a minimum and a
eutectic.
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(e.g., during heating what is the composition of the first
melt to appear?)
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Understand the significance of the solvus.
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(e.g., the formation of perthite textures in alkali feldspar)
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Low Pressure Relationships and "Hypersolvus Granites"
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What are the characteristics of Hypersolvus Granites?
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What do these characteristics imply? (especially regarding
P, T, & H2O)
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High Pressure Relationships and "Subsolvus Granites"
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What are the characteristics of Subsolvus Granites?
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What do these characteristics imply? (especially regarding
P, T, & H2O)
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The Quartz-Albite-Orthoclase Ternary System (QABOR or the
Haplogranite system)
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Low pressure vs High Pressure Phase Relationships
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The transition from a minimum to a eutectic.
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The correspondence between the normative Qtz-Ab-Or compositions
of granitoid rocks and minimums and eutectics in this system "Petrogeny's
Residua".
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Two possibilities to consider: 1) Extreme Fractionation,
2) Partial Melting.
Part II. Crustal Melting (pp. 185-189 & 203-206)
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Fractional Crystallization of Basalt vs Partial Melting of
the Crust
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Yield from extreme fractional crystallization of basalt is
~10%.
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What is the significance of this in terms of generating large
granitic batholiths?
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What tectonic settings may be appropriate to find granites
formed from fractional crystallization of basalt?
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What determines the "fertility" of crustal source materials
in terms of potential felsic melt production?
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Consider normative Q-Ab-Or contents as well as H2O
contents.
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What is the role of H2O in crustal melting?
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Consider the effect on the Temperature at which melting takes
place.
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Consider the types of heat sources needed (e.g., crustal
thickening vs intrusion of basaltic liquids)
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Consider the effect on the potential level to which granitic
magma may rise in the crust before solidifying.
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Dehydration Melting.
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The vast majority of H2O in the crust is stored
in hydrous mafic silicates (e.g., muscovite, biotite, amphibole). What
is the significance of this to the generation of granitic magmas?
Part III. Composition of Granitic Magmas (pp. 185-189
& 203-206)
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Source Material vs Tectonic Setting.
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The Alumina Saturation Index (ASI) (p. 76)
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Understand the this chemical classification: peraluminous,
subaluminous, metaluminous, peralkaline.
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Relationship to normative minerals (e.g., corundum normative
vs acmite normative)
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Relationship to real minerals present in the rock (e.g.,
the significance of primary cordierite, or amphibole to rock composition)
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How may the characteristics of the source material determine
the ASI composition of the felsic melt.
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Consider which minerals are undergoing dehydration melting
and the ASI value of those minerals.
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S-type, I-type, and A-type granites
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What are their general characteristics and how might they
reflect the composition of their source materials.
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Trace Element Contents of Granites
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How are trace elements used to characterize the tectonic
setting of granites?