GEOL 2312 – Igneous and Metamorphic Petrology
Spring 2016
Name__________
QUIZ 5 – Chapters 16 and 17
Key ____________
Score _____/ 13
1) What is the most common volcanic rock type (basalt, andesite, or rhyolite) in island arcs? (0.5 pt)
Andesite or Basalt
2) In the diagram below of a typical island arc, label the tectonomagmatic feature with the appropriate letter from the list to the right. (2 pts)
D C B E
A
A.
Mantle
Wedge
B.
Volcanic Arc
C.
Forearc
D.
Trench
E.
Backarc
3) What minor element is commonly used to discriminate different magma series in island arc systems? Circle one, 0.5 pt
K - Potassium Si – Silica Ti - Titanium P - potassium
4) Which of these trace element spider diagrams is typical of arc magmas, A or B? Circle one (0.5 pt)
A B
Extra Credit – What magmatic environment does the other diagram represent? (0.5 pt)
Ocean Island Basalt (OIB)

5) Name two factors that control the extent to which the geotherms will be depressed due to subduction of oceanic lithosphere as shown in the figure below (1 pt)
1) Rate of Subduction
2) Age of the Oceanic Lithosphere
3) Age of the Volcanic Arc
4) Extent to which the subduction induces flow of the mantle wedge
6) In the diagram above, the numbers 1-5 show possible source areas for generating magmas in island arc systems. Which source is likely produce the most magma to the arc? ___ 2 ___; Which is likely to produce the least ? ___ 4 OR 5 ___ (1 pt)
7) The diagram to the right shows pressuretemperature paths of subducting oceanic lithosphere for different ages of the arc and different ages of the subducting lithosphere. Also shown are the wet and dry solidus curves for basaltic crust.
Based on these relationships, what is the ideal age of subducting ocean crust that will lead to it partially melting in an arc system? Young 5 Ma (0.5 pt)
Why is hornblende an important phase is allowing slab melting to occur? (0.5 pt)
Its melting liberates water which allows the slab to melt
8) Place the melting locations A & B shown in the figure on the left in their proper positions on the P-T diagram for mantle wedge melting on the right (1 pt)
B
A

9) What is the age distribution of Pacific ocean crust and how does this relate to the fact that subduction zones in the western Pacific are island arcs, whereas they are continental arcs in the eastern Pacific?
(1 pt) East is young, West is old; Young crust is buoyant and flexible and creates destructive margins like thick continental arcs. Old crust is dense and stiff and creates constructive margins by trench roll-back.
_
10) What is one possible explanation for the gaps in volcanic activity along the Andean arc? (0.5 pt)
Low angle of subduction or over-thickened ocean crust
11) The figure to the right shows the distribution of volcanic rock types corresponding to the Andean continental arc and SW Pacific island arcs. Why are mafic compositions more rare in continental arcs
(0.5 pt) Thick crust and strong compression acts as a filter to the passage of mafic magmas
12) The geochemistry of the central volcanic zone of the Andean continental arc differs from the northern and southern volcanic zones. State one way that the compositions of CVZ volcanics differs from the NVZ and SVZ - more felsic and andesitic compositions, few basalts, higher K compositions, more enriched incompatible trace elements, … (0.5 pt)
What is a possible reason for this difference? CVS composed of thick Precambrian crust (0.5 pt)
13) Continental arc are notoriously diverse in their magma compositions. One process that commonly produces this diversity is by the ponding of mantle melts at the base of the crust, which triggers the
“MASH” process. What does this acronym stand for? (At least describe the process in your own words) (1 pt) Melting Assimilation Storage Homogenization
14) Answer true or false to the following statements about the Cascade Arc of the NW US. (1.5 pts)
T F The Columbian embayment marks the edge of thick Precambrian crust
T F The subduction trench of the Juan de Fuca plate with the Columbia embayment has been migrating eastward creating a destructive continental arc margin.
T F Bimodal mafic and felsic magmatism dominates over andesitic volcanism in the Cascade arc because it has experienced extensional tectonics over much of its history