Download and island arcs

Igneous activity related to subduction
(Chapters
(Ch
16
16, 17)
Subduction--related activity
Subduction
Igneous activity is related to convergent plate situations that result in the
subduction
bd i off one plate
l
b
beneath
h another
h
Ocean-ocean → Island Arc
Ocean-continent → Continental Arc
Image source: Winter, 2001
Island arc activity
Activity along arcuate volcanic island chains along
subduction zones i.e. island arcs
Distinctlyy different from the mainlyy basaltic provinces
p
thus far
• Composition more
diverse and silicic
• Basalt generally occurs
in subordinate
quantities
• Also more explosive
than the quiescent
basalts
• Strato-volcanoes are the
most common volcanic
landform
Augustine Volcano, Alaska
Image source: Wikipedia: http://commons.wikimedia.org/wiki/Image:Augustine_Volcano_Jan_12_2006.jpg
Island arc activity
Cross-section of an island arc subduction zone
Subduction Products
• Characteristic
igneous
associations
• Distinctive
patterns of
metamorphism
• O
Orogeny
n and
nd
mountain belts
Image source: Winter, 2001
Island arc activity
Cross-section of an island arc subduction zone
Subduction Features
• mantle flow
directions
(induced drag),
isolated wedge,
and upwelling to
→ back-arc basin
spreading
p
g system
y
• Benioff-Wadati
seismic zone (x
x x x)
• Volcanic Front
• Variable heat flow
Image source: Winter, 2001
Island arc activity
Cross-section of an island arc subduction zone
Subduction Features
• subduction dip
angles = 30-90°
(45° ave)
•d
depth
h to plate
l
below arc is
generally constant
at 110 km no matter
the dip angle
i.e. horizontal
distance of arc
ac
from trench
dependent on dip
• Volcanism
accounts for ~10%
of heat at arc
Image source: Winter, 2001
Island arc activity
Volcanic rocks of island arcs
Table 16-1. Relative proportions of Quaternary volcanic
island arc rock types
types.
Locality
Talasea, Papua
Little Sitkin, Aleutians
Mt. Misery, Antilles (lavas)
Ave. Antilles
Ave Japan (lava
Ave.
(lava, ash falls)
B
9
0
17
17
14
B-A
A
23
55
78
4
22
49
42
85
D
9
18
0
39
2
R
4
0
0
2
0
after Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite
A = andesite, D = dacite,
R = rhyolite
Image source: Winter, 2001
• Complex tectonic situation and broad spectrum of volcanic compositions
i.e. basalts to rhyolites = orogenic suite
• High proportion of basaltic andesite and andesite
• Most andesites occur in subduction zone settings
Island arc activity
Major elements and magma series
a. Alkali vs. silica - minor alkaline magmas
b. AFM - both
b
b h tholeiites
h l ii and
d calc-alkaline
l lk li
magmas exist
cc. FeO*/MgO vs.
vs silica - both tholeiites (more
in this diagram) and calc-alkaline magmas
exist
• Tholeiitic (MORB, OIT)
• Alkaline (OIA)
• Calc-Alkaline
C l Alk lin (~
( restricted
r tri t d to
t subduction
bd ti n zones)
z n )–
note subduction zones contain all three magma types
diagrams for 1946 analyses from ~ 30
island and continental arcs with emphasis
on the more primitive volcanics
Image source: Winter, 2001
Island arc activity
Sub-series
Sub
series of Calc
Calc-Alkaline
Alkaline magmas
The three andesite series
of Gill (1981)
K2O is an important
discriminator for the 3 subseries
i
Image source: Winter, 2001
Island arc activity
Major element chemistry of individual island arc systems
Calc-alkaline basalts are commonly high-alumina basalts (17-21% Al2O3)
Examples of actual island
arc magma series
• Tonga-Kemedic: Low-K
tholeiitic
h l ii i
• Guatemala: Medium-K calcalkaline
• Papu New Guinea: High-K
High K
calc-alkaline
Variations are controlled by
fractional crystallization
and possible magma
mixing
i i
Image source: Winter, 2001
Island arc activity
Major element chemistry of individual island arc systems
AFM diagram
g
distinguishing tholeiitic
and calc-alkaline series.
Arrows represent
differentiation trends
within a series.
Generally, the higher the
K2O, the less the Feenrichment
nri hm nt
Image source: Winter, 2001
Island arc activity
Major element chemistry of individual island arc systems
Fractionation features
Calc-alkaline trend shows
that with increasing SiO2
• there
h iis no ddramatic
i Fe
enrichment
• TiO2 decreases due to FeFe
Ti oxide
• The CaO/Al2O3 ratio
decreases due to
fractionation by Cpx
Image source: Winter, 2001
Island arc activity
Reasons for calc
calc-alkaline
alkaline differentiation
• Early crystallization of an Fe-Ti oxide phase
• Probably related to the high water content of
calc-alkaline magmas in arcs, dissolves → high
fO2
• High water pressure also depresses the
plagioclase liquidus and → more An-rich
I
Image
source: Winter,
Wi
2001
• As hydrous magma rises, ΔP → plagioclase liquidus moves to higher T
→ crystallization of considerable An-rich-SiO2-poor plagioclase
• The crystallization of anorthitic plagioclase and low-silica, high-Fe
hornblende is an alternative mechanism for the observed calc-alkaline
differentiation trend
Island arc activity
Petrography of island arc
volcanics
Major phenocryst mineralogy (Changes
with
ihK
K-type))
• Plagioclase is the most common
phenocryst (An50-70) - likely related to
high H2O depolymerizing the magma and
favors An plag.
• Mafic minerals (cpx, opx or ol) are
generally Mg-rich, and cpx is Al-rich
• Hornblende is common in med-high
K andesites - stable onlyy at elevated H2O
contents in the melt (may undergo later
dehydration due to sudden loss of H2O
or magma mixing)
• Also, biotite in more evolved magma.
• Disequilibrium textures are common
Image source: Winter, 2001
Island arc activity
Trace elements
Even the most primitive arc basalts
have low Ni (750-150 ppm), Cr and V
((200-400 pp
ppm)) - too low to be p
primaryy
mantle melts
REEs
Slope within series is similar, but
height varies with degree of
fractionation due to removal of Ol,
Ol
Plag, and Px
(+) slope of low-K similar to DM
HREE flat, so no deep garnet
peridotite or eclogite source
Image source: Winter, 2001
Island arc activity
Trace elements – spider
diagrams
Trace elements normalized to MORB
show two distinctions between rocks
from ocean island basalts (OIB) and
island arcs (IA).
• OIB shows enrichment of both LIL
(Sr to Ba) and some HFS elements
(Th to Yb)
• IA basalts show enrichment in LIL
but not HFS elements (decoupling
of LIL and HFS))
• Because LIL is hydrophylic, it
implies that H2O in subduction zone
systems helps concentrate the LIL
elements.
Image source: Winter, 2001
Island arc activity
Be and B data
10Be
created by cosmic rays + oxygen
and nitrogen in upper atmosphere
• falls to Earth by precipitation &
incorporates into clay-rich oceanic
sediments
• Half-life of only 1.5 Ma (long
enough to be subducted, but
quickly
i kl llost to mantle
l systems).
)
After about 10 Ma 10Be is no longer
detectable
Image source: Winter,
Winter 2001
•
10Be/9Be
averages about 5000 x 10-11 in the uppermost oceanic sediments
• In mantle-derived MORB and OIB magmas,
magmas & continental crust,
crust 10Be is below
detection limits (<1 x 106 atom/g) and 10Be/9Be is <5 x 10-14
Island arc activity
Be and B data
B is a stable element
p in
• Veryy brief residence time deep
subduction zones
• B in recent sediments is high (50150 ppm), but has a greater affinity
for altered oceanic crust (10-300
ppm)
• In MORB and OIB it rarely
exceeds 2-3 ppm
Image source: Winter,
Winter 2001
Conclusions based on Be and B data: there is participation of
young sediments and altered oceanic crust in the subductionbased production of igneous rocks
Island arc activity
Petrogenesis of Island Arc Magmas
Of many variables that can
affect isotherms in subduction
zone systems, the main ones
are:
1) rate off subduction
bd i
2) age of subduction zone
3) age of subducting slab
Image source: Winter, 2001
Other factors, now considered minor,
include:
1) dip of the slab
2) frictional heating
3) endothermic metamorphic reactions
4) metamorphic fluid flow
4) extent to which subducting
slab induces flow in mantle
wedge
Island arc activity
Petrogenesis of Island Arc Magmas
For typical thermal models of
subduction zone,, isotherms
will be higher (i.e. the system
will be hotter) if:
a)) convergence rate is
i slower
l
b) subducted slab is young and
near the ridge (warmer)
Image source: Winter, 2001
c) arc is young (<50-100 Ma)
Island arc activity
Petrogenesis of Island Arc Magmas
Principal source components
that may contribute to island
arc magmas
1. Crustal portion of subducted slab
1a. Altered oceanic crust (hydrated
1a
by circulating seawater, and
metamorphosed in large part to
greenschist facies)
Image source: Winter, 2001
2. Mantle wedge between slab and arc crust
3. Arc crust
4 Lithospheric mantle of subducting plate
4.
5. Asthenosphere beneath slab
1b. Subducted oceanic and forearc
sediments
1c. Seawater trapped
pp in p
pore spaces
p
Island arc activity
Petrogenesis of Island Arc Magmas
P-T-t paths for
peridotite in mantle
wedge
d as iit ffollows
ll
a
typical path
Included are some P-T-t
path range for subducted
crust in mature arc, and
wet and dry solidi for
peridotite.
Subducted crust
dehydrates, and water is
transferred to wedge
(arrow).
Image source: Winter, 2001
Amphibole-bearing hydrated peridotite should melt at ~ 120 km
Phlogopite-bearing hydrated peridotite should melt at ~ 200 km
Island arc activity
Petrogenesis of Island Arc Magmas
Trace element and isotopic data suggest that both subducted
crust and mantle wedge contribute to arc magmatism.
• Dry peridotite solidus too high for melting of anhydrous mantle to occur
anywhere
y
in thermal regime
g
– H2O must be involved in p
process
• LIL/HFS ratios of arc magmas imply water plays significant role in arc
magmatism
• LIL/HFS trace element
l
d
data underscore
d
the
h iimportance off slab-derived
l bd i d
water and a MORB-like mantle wedge source
• The flat HREE pattern argues against garnet-bearing (eclogite or garnet
peridotite) source
• Thus modern opinion has swung toward non-melted slab for
most cases
Island arc activity
Petrogenesis of Island Arc Magmas
Proposed model for
subduction zone
magmatism with
particular reference to
island arcs.
Dehydration of slab
crust causes hydration
of mantle (violet),
which undergoes partial
melting as amphibole
(A) and phlogopite (B)
d hd
dehydrate.
Image source: Winter, 2001
Island arc activity
Petrogenesis of Island Arc Magmas
A multi-stage, multisource process
1. Dehydration of slab
provides LIL, 10Be, B, etc.
enrichments.
i h
Th
These
components are transferred
to wedge in fluid phase (or
melt?)
2. Mantle wedge provides
HFS
FS and other
othe depleted
and compatible element
characteristics
Image source: Winter, 2001
Island arc activity
Petrogenesis of Island Arc Magmas
A multi-stage, multisource process
3. Phlogopite is stable in
ultramafic rocks beyond
conditions at which
amphibole
hib l b
breaks
k d
down
4. P-T-t paths for wedge reach
phlogopite-2-pyroxene
dehydration reaction at
about 200 km depth
Image source: Winter, 2001
g
for calc5. Parent magma
alkaline series is a high
alumina basalt, a type of
basalt that is largely
restricted to subduction
zone environments
Continental arc activity
Potential differences with respect to Island Arcs:
1. Thick sialic crust contrasts
greatly with mantle-derived
mantle derived
partial melts - and is likely
source of more
pronounced
d effects
ff t off
contamination
Arenal Volcano, Costa Rica.
Image source: arenal.net
2. Low density of crust may
retard ascent, and produce
stagnation of magmas and
more potential for
differentiation i.e. more evolved
magmas
Continental arc activity
Potential differences with respect to Island Arcs:
3. Low melting point of
crust
allows for partial melting
and crustally derived melts
4. Subcontinental
lithospheric mantle is likely
to be locally enriched,
enriched and
this will be reflected in
magmas that are derived
from this mantle
Arenal Volcano, Costa Rica.
Image source: arenal.net
Continental arc activity
Volcanism in South America
Map of western South America
• NVZ - Accreted Mesozoic and
Cenozoic oceanic crust and island
arcs, 30-45 km thick
• CVZ - Precambrian metamorphic
crust, 50-75 km thick
• SVZ - Accreted Mesozoic and
Cenozoic oceanic crust and island
arcs,, 30-45 km thick
These are separated by inactive gaps.
This belt of igneous rocks developed over last
500 Ma and is commonly termed an Andeantype margin.
Image source: Winter, 2001
Continental arc activity
Volcanism in South America
Schematic diagram to illustrate how
shallow dip of subducting slab can
pinch out the asthenosphere from
overlying mantle wedge.
• Active zones of Andes correspond to
more steeply-dipping (>25 ) slab i.e. mantle
wedge must be involved in melting process.
process
• Shallow dips are considered to be results
of thicker and less dense oceanic crust
e.g. Nazca Ridge.
Image source: Winter, 2001
Continental arc activity
Volcanism in South America
AFM and K2O vs. SiO2 diagrams
for Andes volcanics
Orange circles in the NVZ and SVZ are alkaline
rocks.
All rocks
k exhibit
hibit ttypical
i l calcl
alkaline trends
• NVZ and SVZ are mostly high
Al basaltic andesites and
andesites, but with some dacites
and
d rhyolites
h lit
Image source: Winter, 2001
• CVZ is generally more Si-rich
with a range of basalt to rhyolite
but most commonly
andesite/dacite
Continental arc activity
Volcanism in South America
Chondrite-normalized REE
diagram for selected
Andean volcanics.
• NVZ -lower
lower HREE (suggests
residual garnet in a deeper melt source).
• CVZ - enriched LREE likelyy
due to continental crustal
involvement.
• SVZ - low REE slope and high
HREE (suggests no garnet in melt
source due to shallower slab dip)
Image source: Winter, 2001
Continental arc activity
Volcanism in South America
MORB-normalized spider
diagram for selected
Andean volcanics.
Note decoupled LIL/HFS
patterns typical
i l off subduction
bd i
zone magmas i.e. there must be
dehydration of ocean crust.
• NVZ - pattern similar to island
arc magmas.
I
Image
source: Winter,
Wi
2001
• CVZ - pattern shows
h
more
enrichment due to continental
crustal involvement.
• SVZ - pattern similar to island
arc magmas.
Continental arc activity
P tr
Petrogenesis
n i off the
th Andes
And
1. Major and trace element
data consistent with origin
from fluid-fluxed and LILenriched mantle wedge
above
b
subducting
bd ti and
d
dehydrating Nazca plate.
Image source: Winter, 2001
2. Each of volcanic zones
indicate magmas interact
with local continental
crust as they pass through
crust. However, initial
melts
lt mustt come from
f
mantle wedge.
Continental arc activity
Cascade Magmatic Arc
• extends 1000 km and
corresponds to oblique
subduction of Juan de Fuca
plate that ends in strike-slip
f lti tto the
faulting
th N and
dS
• Current chain of
Quaternary andesitic
stratovolcanoes, but
volcanism goes back to
Phanerozoic
Image source: Winter, 2001
Continental arc activity
Cascade Magmatic Arc
• Bimodal mafic-silicic
volcanism due to mantle
melts and crustal melts,
however more mafic melts
than in Andes
• Basin and Range extension
lik l related
likely
l d to b
back-arc
k
extension and/or
Yellowstone hot-spot
p
interactions
Image source: Winter, 2001
Continental arc activity
C
Cascade
d M
Magmatic
ti Arc
Ar
Time-averaged rates of
extrusion of mafic (basalt and
basaltic andesite), andesitic,
and silicic (dacite and rhyolite)
volcanics
l
i and
d JJuan d
de F
Fuca
• North American plate
convergence rates for the past
35 Ma.
• Increase in mafic magmas at
7.5 Ma due to extension
Image source: Winter, 2001
Continental arc activity
C
Cascade
d M
Magmatic
ti Arc
Ar
Image source: Winter, 2001
REE diagram for mafic platform lavas of High Cascades.
Mafic magmas range from LREE-depleted MORB-like magmas to
variably enriched OIB-like magmas to subduction zone-type
magmas indicative of fluid alteration
Continental arc activity
Plutonism in Continental Arcs
Major plutons of North American Cordillera,
ap
principal
p segment
g
of a continuous
Mesozoic-Tertiary belt from Aleutians to
Antarctica.
• Cordilleran-type batholiths - these are generally
composite bodies with 100s-1000s of individual
intrusions over millions of years.
y
• Compositional range: gabbro-diorite- tonalitegranodiorite-granite -- this is similar to the
volcanic equivalent compositional range
• The transition is commonly from mafic magmas
d i extensional
during
i
l regimes
i
(b
(back-arc
k
extension)
i ) to
silicic magmas during compressional phase. This
oscillation may be related to variable spreading
Image source: Winter, 2001
rates
Continental arc activity
Plutonism in Continental Arcs
Major plutons of North American
Cordillera,, p
principal
p segment
g
of
continuous Mesozoic-Tertiary belt from
the Aleutians to Antarctica.
•C
Cordilleran-type
dill
b h li h - these
batholiths
h
are
generally composite bodies with 100s1000s of individual intrusions over
millions of years.
• Compositional range: gabbro-dioritetonalite-granodiorite-granite
li
di i
i
-- this
hi iis
similar to volcanic equivalent
p
range
g
compositional
Image source: Winter, 2001
Continental arc activity
Plutonism in Continental Arcs
Major plutons of South American
Cordillera,, a p
principal
p segment
g
of a
continuous Mesozoic-Tertiary belt
from Aleutians to Antarctica.
• Transition is commonly from mafic
magmas during extensional regimes
(back-arc
(back
arc extension) to silicic magmas
during compressional phase.
• Thi
This oscillation
ill i may b
be related
l d to
variable spreading rates.
Image source: Winter, 2001
Continental arc activity
Plutonism in Continental Arcs
Image source: Winter, 2001
Schematic cross section of Coastal batholith of Peru.
Peru
• Shallow flat-topped and steep-sided "bell-jar"-shaped plutons are stoped
into place.
pulses mayy be nested at a single
g localityy
• Successive p
• Flat sides and tops are consistent with cauldron subsidence after volcanic
eruptions
Continental arc activity
Plutonism in Continental Arcs
Harker-type and AFM
variation diagrams for
the Coastal batholith of
Peru.
Calc-alkaline trends
consistent with fractional
crystallization of
plagioclase and pyroxene
+/- magnetite, and later
+/
hornblende and biotite.
Image source: Winter, 2001
Continental arc activity
Plutonism in Continental Arcs
Chondrite-normalized REE
abundances for the Linga and
Tiybaya super-units of the
Coastal batholith of Peru and
associated
i t d volcanics.
l
i
Note similarity between
volcanics and plutonics.
Image source: Winter, 2001
Continental arc activity
Plutonism in
Continental Arcs
Schematic diagram of
(a) formation of gabbroic
crustal underplate (higher
density) at continental arc
(b) magma conduits get
shut off, basaltic magma
g
accumulates (magmatic
underplating) and
remelting of underplate
to generate tonalitic
plutons (not from
diff
differentiation
i i off b
basaltic
li
melt).
Image source: Winter, 2001
Continental arc activity
Plutonism in Continental Arcs
Schematic cross section
((1)) de
dehydration
yd o of
o
subducting slab
(2) hydration and melting of
heterogeneous mantle
wedge (including enriched
sub-continental lithospheric
p
mantle)
(3) crustal underplating of
mantle-derived melts
where MASH (melting,
Image source: Winter, 2001
assimilation, storage and
homogenization) processes may
occur.
Continental arc activity
Plutonism in Continental Arcs
Schematic cross section
• (4) Remelting
R
l i off
underplate to produce
g
and
tonalitic magmas
possible zone of crustal
anatexis.
Image source: Winter, 2001
• (5) As magmas pass
through continental
crust they
h may
differentiate further
and/or assimilate
continental crust.