Igneous Petrology: Classification, Phase Diagrams, and Magma Evolution

What diagram is used for quartz-bearing feldspathic plutonic rocks?

QAP

What diagram is used for feldspathoid-bearing rocks?

FAP

What schemes are used for gabbroic and ultramafic rocks?

Pl-Cpx-Opx-Ol schemes

Should TAS be used for a coarse-grained modal problem?

No

What is the first step when normalizing QAP data?

Calculate S = Q + A + P

How do you calculate normalized quartz (Qn)?

Qn = 100Q/S

How do you calculate normalized alkali feldspar (An)?

An = 100A/S

How do you calculate normalized plagioclase (Pn)?

Pn = 100P/S

What should Qn + An + Pn equal?

100%

What minerals should be included during normalization?

Only minerals used by the selected diagram

What defines anorthosite in the study guide?

More than 90% plagioclase

When should modifiers be added to a rock name?

Only when supported

What two percentages should you be able to determine from a T-X diagram?

% melt and % solids

Besides percentages, what must you determine from a T-X diagram?

Composition of the melt and solids

When describing a cooling path, what should you identify first?

The first crystal to form

During cooling, what compositional changes should you track?

Changes in crystal and liquid compositions

What should you identify when another phase appears during cooling?

When the phase joins or reacts

What should you identify about the final liquid?

Its composition

What should you identify at the end of a cooling path?

The final mineral assemblage

Diopside-anorthite crystallization is associated with what texture?

Ophitic texture

Plagioclase disequilibrium is associated with what texture?

Zoning

A solvus is associated with what process?

Exsolution

What texture/mineral feature can result from a solvus?

Perthite

What broad rock compositions should you compare?

Mafic, intermediate, and silicic

What compositional properties should be compared among mafic, intermediate, and silicic rocks?

Silica, alkalis, and Fe-Mg-Ca abundance

What mineral-related features should be compared among rock compositions?

Typical minerals and plagioclase composition

What physical magma properties should be compared?

Temperature and viscosity

What volcanic behavior should composition be connected to?

Likely eruption style

What two main branches of Bowen's Reaction Series should you trace?

Discontinuous branch and plagioclase branch

How does plagioclase change along Bowen's Reaction Series?

Ca-rich to Na-rich

What should you know at the low-temperature end of Bowen's Reaction Series?

The low-temperature minerals

Is Bowen's Reaction Series a universal crystallization path?

No

How should Bowen's Reaction Series be treated?

As a model of crystallization

What are the three ways to generate melt?

Add heat, lower pressure, or add volatiles

How does adding heat generate melt?

It moves the rock across the solidus

How does lowering pressure generate melt?

It moves the rock across the solidus

How does adding volatiles generate melt?

It moves the rock across the solidus

What factors affect magma ascent?

Buoyancy, fractures, viscosity, warm crust, volatile expansion, cooling, crystallization, and host-rock strength

What can happen to early mafic crystals during fractional crystallization?

They can form cumulates

How does residual melt change during fractional crystallization?

It becomes more evolved

What happens to incompatible elements in the residual melt?

They become enriched

What processes provide evidence of magma evolution?

Crystal settling, assimilation, xenolith/xenocryst transport, recharge/mixing, and liquid immiscibility

What textures can provide evidence of disequilibrium?

Zoning and reaction textures

What are Harker diagrams used for?

Identifying trend direction and possible fractionating minerals

What does an AFM diagram help distinguish?

Tholeiitic vs. calc-alkaline trends

What characterizes the tholeiitic trend on an AFM diagram?

Fe enrichment

How does the calc-alkaline trend move on an AFM diagram?

More directly toward alkalis

Do smooth geochemical trends prove samples belong to a common suite?

No

What can smooth geochemical trends indicate?

They are consistent with a common suite

What major compositional variable affects polymerization and viscosity?

SiO2

What happens to volatiles as pressure changes?

Volatiles exsolve and bubbles expand

What affects the ability of gas to escape magma?

Magma viscosity

What volcanic rock is associated with mid-ocean ridges?

MOR basalt

What volcanic compositions are associated with subduction?

Andesite-dacite-rhyolite

What volcanic rock is associated with hot spots and rifts?

Basalt

What is a possible bimodal volcanic association?

Basalt-rhyolite volcanism

What is a dike relative to surrounding structures?

Discordant

What is a sill relative to surrounding structures?

Concordant

What do chilled margins indicate?

Rapid cooling

What do batholiths represent?

Multiple plutons

What does a volcanic neck record?

Erosion of a volcanic conduit

How are deep plutons exposed?

Uplift and erosion

Why are pegmatites exceptionally coarse?

They are volatile-rich late-stage rocks

What may pegmatites concentrate?

Incompatible elements

What are aplites?

Fine-grained, light-colored late-stage dikes or veins

What processes are associated with layered mafic intrusions?

Repeated intrusion, fractional crystallization, crystal settling/flotation, and cumulate formation

What materials can layered mafic intrusions concentrate?

Chromite, sulfides, and platinum-group elements

______ is dominated by potassium feldspar

granite

______ is mostly plagioclase with some potassium feldspar

granodiorite

_______ is almost entirely plagioclase with little to no potassium feldspar

tonalite

Classified as a mafic rock, sitting as the intrusive equivalent of basalt. Contains calcic plagioclase, dominated by pyroxene, and often olivine

Gabbro

Classified as an intermediate rock, intrusive equivalent of andesite, contains sodic to intermediate plagioclase, dominated by amphibole (hornblende) and/or biotite mic

diorite