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.

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

• The Ophiolite Suite (pp. 177 - 180)
• What rock types comprise an ophiolite?
• What is an ophiolite thought to represent?
• Where were these rocks formed?
• How have the compositions of these rocks been modified? (e.g., spillite)

• Obduction vs Subduction
• Why is the oldest oceanic lithosphere younger than the Triassic?
• What is the significance of the age of the oceanic lithosphere and depth of the ocean floor?
• What drives subduction? (i.e., ridge push vs slab pull)
• Why do we have ophiolite suites?

• Subduction of Oceanic Lithosphere
• What significant phase changes occur in the oceanic lithosphere during subduction?
• Why are these changes important? (e.g., changes in density)
• What is the stability limit of amphibole?
• Why is this phase change important? (e.g., release of a fluid phase)

• Melting the Slab vs Melting the Overlying Mantle Wedge (pp.170-177)
• Typically the overlying mantle wedge (asthenosphere) melts. Why?
• Under what circumstances may it be possible to melt the subducted slab?
• What triggers melting of the overlying mantle wedge, and why?

• Trace Element Signature (The infamous "Spiderdiagrams") (p. 78)
• LILEs (Large Ion Lithophile Elements)
• What are they?
• Why are they enriched in igneous rocks originating from a subduction zone environment?
• REEs (Rare Earth Elements) (LREEs Light... MREEs Middle...HREEs Heavy....)
• HFSE (High Field Strength Elements)
• What are they?
• Why are they depleted in igneous rocks originating from a subduction zone environment?

• Spatial Variation of Magma Composition. (pp.172-174)
• What is the correlation between K₂O content and distance from the trench?
• What are the possible causes of this correlation?
• How are suites classified using K₂O content and what might this signify?
• Alkali-lime Index or "Peacock Index" (p. 78 & 176)
• Classification of suites using this relationship.
• Variation with regards to distance from the trench.
• Variation with regards to tectonic setting.
• AFM Ternary Diagram (p. 77-78, & 172-174)
• Tholeiite Trend (Fe-enrichment, or Skaergaard Trend)
• Typical of what tectonic setting?
• Calc-Alkaline Trend (BADR Basalt-Andesite-Dacite-Rhyolite)
• Typical of what tectonic setting?
• What are two possible explanations for this trend?
• Mixing, and fractionation of a magma with an intrinsically higher Oxygen fugacity.

Part I. Experimental Phase Relationships (pp. 113-116 but pay more attention to your notes)

• The Albite-Orthoclase Binary System (see also pp. 203-206)
• Understand the difference between a minimum and a eutectic.
• (e.g., during heating what is the composition of the first melt to appear?)
• Understand the significance of the solvus.
• (e.g., the formation of perthite textures in alkali feldspar)
• Low Pressure Relationships and "Hypersolvus Granites"
• What are the characteristics of Hypersolvus Granites?
• What do these characteristics imply? (especially regarding P, T, & H₂O)
• High Pressure Relationships and "Subsolvus Granites"
• What are the characteristics of Subsolvus Granites?
• What do these characteristics imply? (especially regarding P, T, & H₂O)
• The Quartz-Albite-Orthoclase Ternary System (QABOR or the Haplogranite system)
• Low pressure vs High Pressure Phase Relationships
• The transition from a minimum to a eutectic.
• The correspondence between the normative Qtz-Ab-Or compositions of granitoid rocks and minimums and eutectics in this system "Petrogeny's Residua".
• Two possibilities to consider: 1) Extreme Fractionation, 2) Partial Melting.

• Fractional Crystallization of Basalt vs Partial Melting of the Crust
• Yield from extreme fractional crystallization of basalt is ~10%.
• What is the significance of this in terms of generating large granitic batholiths?
• What tectonic settings may be appropriate to find granites formed from fractional crystallization of basalt?
• What determines the "fertility" of crustal source materials in terms of potential felsic melt production?
• Consider normative Q-Ab-Or contents as well as H₂O contents.
• What is the role of H₂O in crustal melting?
• Consider the effect on the Temperature at which melting takes place.
• Consider the types of heat sources needed (e.g., crustal thickening vs intrusion of basaltic liquids)
• Consider the effect on the potential level to which granitic magma may rise in the crust before solidifying.
• Dehydration Melting.
• 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?

• Source Material vs Tectonic Setting.
• The Alumina Saturation Index (ASI) (p. 76)
• Understand the this chemical classification: peraluminous, subaluminous, metaluminous, peralkaline.
• Relationship to normative minerals (e.g., corundum normative vs acmite normative)
• Relationship to real minerals present in the rock (e.g., the significance of primary cordierite, or amphibole to rock composition)
• How may the characteristics of the source material determine the ASI composition of the felsic melt.
• Consider which minerals are undergoing dehydration melting and the ASI value of those minerals.
• S-type, I-type, and A-type granites
• What are their general characteristics and how might they reflect the composition of their source materials.
• Trace Element Contents of Granites
• How are trace elements used to characterize the tectonic setting of granites?