Arc magmatism (volcanic/plutonic) - University of Washington

Arc magmatism (v

•Arcuate/linear chains of volcanoes and/or pluton
Why are the chains arcuate?

•The arc magma paradox. Why is there magmat
slabs are being subducted into the mant

•Three main arc types:
Continental Arcs (oceanic lithosphere su
Island Arcs I (oceanic lithosphere subdu
Island Arcs II (oceanic crust subducted b
continental fragments or peninsulas)

•Examples: Active Continental Arcs: Cascades, A
Active island Arcs I: Marianas, Tonga-Ke
Active island arcs II: Japan, Kamchatka,
Inactive Plutonic Arcs: Sierra Nevada, P
Coast Range (BC) batholiths,

•Compositionally more diverse, more silicic than
Tholeiitic series (aka low K series): mos
Calc-alkaline series (aka medium- to hig
Both lava series show complete spectru
Most abundant rock type is ANDESITE

Global perspective—regions where new continen

volcanic/plutonic)

ns developed above active subduction zones.

tic activity in regions where cold lithospheric
tle?

ubducted beneath mature continental lithosphere)
ucted beneath oceanic lithosphere):
beneath thinner, commonly immature,

Andes, Eastern Aleutiams
ermadec, Scotia, Western Aleutians…
, New Zealand…
Peninsular Ranges, Idaho batholith,
Peruvian batholith…

MORB, OIB or CFB, more explosive
stly island arcs
gh-K series): mostly continental arcs
um of rock types from basalt through rhyolite.

ntal crust is generated

Ocean crust Ocean
Ocean crust Contine

Principal subduction zones associated with orogenic volc
PBS = Papuan-Bismarck-Solomon-New Hebrides arc. Afte

n crust → Island Arc (IA)
ental crust → Continental Arc

canism and plutonism. Triangles are on the overriding plate.
er Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.

Schematic cross section

nal view of an Island Arc

After Gill (1981), Orogenic Andesites and Plate Tectonics.
Springer-Verlag. HFU= heat flow unit (4.2 x 10-6 joules/cm2/sec)

Summary of tectonic fea

Spacing: Inactive zone
60-70 km

Volcanic front

Volc

trench

Depth from arc volcanoes to •Volcanis
Benioff-Wadati zone: •Volcano
•Low dip
• Main arc: 100-120 km
• Back arc: up to 200 km
• Forearc (rare): <100 km

Volcanoes behind the main arc: usually m
Cessation of volcanism ~ 5 Ma after cessa

atures of magmatic arcs

canic arc Arc width:
25-250 km

Trench-arc gap:
160 ± 60 km

Alkalic, e.g., Granada

trench—trench
transform

sm may migrate inward from trench with time
o spacing averages ~60 - 70 km, as low as 20 km

and/or low convergence angle: amagmatic arc

more K-rich
ation of subduction

Mariana Island Arc

Exten
axis

Trench

Magma

Remnan

Martinez & Taylor 2003

Island arc volcanoes:
largely submarine

nsion
h
atic arc

nt arc

Development of remnant a

arcs and back arc basins

Mt. Mayon stratocone

Examples of activ
arc volcanoes

Strombolian eruption
at Cerro Negro,
Nicaragua, 1968

Block-and-ash flow, Mon

ve

ntserrat Andesite lava flow on
Bagana, Papua New Guinea

Continental Arc Magmatism

The dip of the subducting slab has an
important role in controlling the amount
of magmatic activity (if any) in a
continental arc. A shallow dipping slab
pinches out the asthenosphere in the
overlying mantle wedge and also
restricts most of the slab dehydration
processes to the lithospheric mantle

Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.



Continental Arc
Magmatism

Map of the Juan de Fuca plate-
Cascade Arc system, after McBirney
and White, (1982) The Cascade
Province. In R. S. Thorpe (ed.),
Andesites. Orogenic Andesites and
Related Rocks. John Wiley & Sons.
New York. pp. 115-136. Also shown
is the Columbia Embayment (the
western margin of pre-Tertiary
continental rocks) and approximate
locations of the subduction zone as
it migrated westward to its present
location (after Hughes, 1990, J.
Geophys. Res., 95, 19623-19638).
Due to sparse age constraints and
extensive later volcanic cover, the
location of the Columbia Embayment
is only approximate (particularly
along the southern half).

Reproduced from Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.



Continental Arc
Magmatism

Map of western South America showing
the plate tectonic framework, and the
distribution of volcanics and crustal
types. NVZ, CVZ, and SVZ are the
northern, central, and southern volcanic
zones. After Thorpe and Francis (1979)
Tectonophys., 57, 53-70; Thorpe et al.
(1982) In R. S. Thorpe (ed.), (1982).
Andesites. Orogenic Andesites and
Related Rocks. John Wiley & Sons.
New York, pp. 188-205; and Harmon et
al. (1984) J. Geol. Soc. London, 141,
803-822.

Reproduced from Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.



Continental arc magma

Major plutons of the North American Cordill
principal segment of a continuous Mesozoic
Tertiary belt from the Aleutians to Antarctica
Anderson (1990, preface to The Nature and
of Cordilleran Magmatism. Geol. Soc. Amer
Memoir, 174. The Sr 0.706 line in N. Americ
after Kistler (1990), Miller and Barton (1990
Armstrong (1988).

Reproduced from Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall

atism

lera, a
c-
a. After
d Origin
r.
ca is
0) and

Continental Arc Magmatis

Major plutons of the South
American Cordillera, a principal
segment of a continuous
Mesozoic-Tertiary belt from the
Aleutians to Antarctica. After
USGS.

Reproduced from Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.

sm

Volcanic Rocks o

Table 16-1. Relative Proport
Island Arc R

Locality B

Talasea, Papua 9
Little Sitkin, Aleutians 0
Mt. Misery, Antilles (lavas) 1
Ave. Antilles 1
Ave. Japan (lava, ash falls) 1

After Gill (1981, Table 4.4) B = basalt

A = andesite, D = dacite, R = rhyolit

Note the preponderance of basaltic an

of Island Arcs

tions of Quaternary Volcanic
Rock Types

B B-A A D R

9 23 55 9 4
0 78 4 18 0
17 22 49 12 0
17 42 39 2
14 85 2 0

B-A = basaltic andesite

te

ndesite and andesite in island arcs

Arc Magma

Relative frequency of rock types in the An
from 397 Andean and 1484 SW Pacific an

From: R. S
(2001) An

atism

ndes vs. SW Pacific Island arcs. Data
nalyses in Ewart (1982)

S. Thorpe (ed.), Andesites. Wiley. New York, pp. 25-95 and Winter
Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Classification of

The most useful single discriminator is K2O wh

f arc volcanics

hich Gill used to define 3 sub-series as shown

The three andesite series. Contours
represent the concentration of 2500
analyses of andesites.

Andesite: grey, porphyritic rocks
with abundant plagioclase +
pyroxene phenocrysts (no quartz,
sanidine of feldspathoids)

4 R

3 shoshonite
High K

2 A D

B Med K

1

Low K

48 53 57 63 68

Island arc tholeiite

After Gill (1981) Orogenic Andesites and
Plate Tectonics. Springer-Verlag.

K2O-SiO2 diagram distinguishing high-K

Large symbols = averages. Differentiation within a series
by arrow. Different primary magmas (to the left) are distin

After Gil

K, medium-K and low-K island arc series.

s (most likely due to fractional crystallization) is indicated
nguished by vertical variations in K2O at low SiO2.

ll, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.

Distinction between tholeiitic and c

This is also known as a Miyashiro diagram.

From: Winter (2001) An I

calc-alkalic series in island arc lavas

Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

AFM diagram is also used to distinguis

From: Winter (2001) An I

shing tholeiitic and calc-alkaline series.

Arrows represent
differentiation trends
within a series. Note
the FeO enrichment
trend in the tholeiitic
series relative to the
calc-alkalic series.

Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Six andesite su

Low K, medium K and high K
series exist in both the tholeiitic
and calc-alkalic trends giving a
total of six sub-series.

The low-K series is almost
exclusively tholeiitic and
largely restricted to island
arcs, the medium-K series is
primarily calc-alkalic while the
high-K series is mixed.

Note that the points plotted are
based on a fit to the trends in a K2O
vs SiO2 plot and a FeO/MgO vs.
SiO2 plot. The value on the trends
at 57.5 SiO2 is then plotted in this
diagram.

Note also that there is a continuum of
compositions and the pigeon holes are
largely as an aid to communication. It
is not uncommon for a series at a
single volcano to pass from tholeiitic to
calc-alkalic or vice versa.

FromGill (198

ub-series

81) Orogenic Andesites and Plate Tectonics. Springer-Verlag.

Mineral assemblage

Volcanic: Basalt Andesite
Plutonic: Gabbro Diorite

SiO2 <53 wt% 53-63

Plagioclase An70-80 An80-9
Commonly z
Orthoclase
~30%
Quartz ~50% ~2
Pyroxene

Hornblende

Biotite

Olivine ~50%
Total mafic ~50%
Total felsics

es in arc rocks

e Dacite Rhyolite

Granodiorite Granite

63-68 >68%

90 0 An20-30 ~30
0 ~30
zoned ~30

20% <2%
<10%
<5%
<5%

<10%
>90%