GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 16
Continental Arc Magmatism
March 2, 2016
CONTINENTAL ARCS VS ISLAND ARCS
AFFECTS OF THICK DIFFERENTIATED
CONTINENTAL CRUST
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Thick sialic crust contrasts
greatly with mantle-derived
partial melts may produce more
pronounced effects of
contamination
Low density of crust may retard
ascent causing stagnation of
magmas and more potential for
differentiation
Low melting point of crust allows
for partial melting and crustallyderived melts
Subcontinental lithosphere may
be more compositionally diverse
that suboceanic lithosphere,
especially if crust is old
WHY ISLAND ARCS IN THE WESTERN PACIFIC AND
CONTINENTAL ARC IN THE EASTERN PACIFIC?
CONTINENTAL ARCS FORM WHERE THE
SUBDUCTING OCEAN CRUST IS YOUNG
Young Oceanic Crust is:
Warmer – Dehydrates and melts more readily
Lower Density - Bouyant
More Ductile – Flexible, Deformable
TYPES OF CONTINENTAL ARCS
Destructive
more common where
Continental crust is older
e.g. Andean Margin
Constructive
more common where
Continental crust is younger
e.g. Pacific NW
ANDEAN
CONTINENTAL ARC
Gaps in volcanic activity
• shallow subduction
• overthickened slab
ANDEAN VOLCANIC COMPOSITIONS
DISTRIBUTION OF ROCK TYPES
Melting of Lower
Crust generates
Felsic Magmas
Lower Crust traps
Mafic Magmas
ANDEAN VOLCANIC COMPOSITIONS
MAJOR ELEMENTS
Northern Volcanic Zone
• more andesitic to felsic
• K-rich comps to east
Alkaline Rocks
Central Volcanic Zone
• more andesitic to felsic
• basalts rare
• more staging beneath
Precambrian crust
Southern Volcanic Zone
• broad range of comps
• K-rich comps to east
• shallower subduction
angle
• Young continental
crust especially
to south
• Most similar to island arc magmatism
Island Arcs
ANDEAN VOLCANIC COMPOSITIONS
TRACE ELEMENTS
CVZ – Assimilation of
Precambrian crust
and SCLM
SVZ - Shallower
subduction angle 
melting of Gt-free
mantle
ANDEAN VOLCANIC COMPOSITIONS
TRACE ELEMENTS
CVZ – Assimilation of
Precambrian crust
and SCLM
Nb-Ta depletion
similar to island arc
dehydration of
subducted slab
ANDEAN VOLCANIC COMPOSITIONS
ISOTOPIC COMPOSITIONS
CONSTRUCTIVE
CONTINENTAL
ARC
PACIFIC NW
Juan de Fuca Plate –
Young, hot, bouyant;
dehydrates quickly upon
subduction
Columbia Embayment area of young crust
and arc construction by
rollback or trench
jumping
CASCADE MAGMA TYPES OVER TIME
Mafic Volcanism
Bimodal Volcanism
Mafic Underplating/Crustal Melting
CalcAlkaline
Volcanism
Crustal
Assimilation
Crustal Extension
= Back-arc?
= Northward
Migration of Basin
and Range?
CONTRASTING THE ANDEAN AND CASCADE ARCS
Andean Arc
Strong compressional tectonics
promotes staging and
contamination of mantle melts
in thick, old crust 
Intermediate volcanics
dominate
Cascade Arc
Roll-back extension
promotes dominance of
mafic volcanism. Silicic
magmatism related to
underplating/melting of the
lower crust.
GENERAL MODEL FOR
CONTINENTAL ARC MAGMATISM
MASH – Melting - Assimilation – Storage - Homogenization