ABSTRACT

1
Petrologic processes that generate the
intermediate to felsic plutonic core of island
arcs
Susan DeBari Michael Johnsen, Department of
Geology, Western Washington University,
Bellingham, WA
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Seismic stratigraphy of modern arcs - more
similarities than differences?
The crustal sections
ABSTRACT
Talkeetna arc (Alaska) Jurassic x-section
Bonanza arc (Vancouver Island) Jurassic x-section
S. Coast Plutonic Cplx (Washington) Cretaceous
x-section
Kohistan arc (Pakistan) Cretaceous
x-section
In exhumed arc sections worldwide, the upper
mid-crust is composed dominantly of
hornblende-bearing tonalite, quartz diorite,
diorite, and gabbro (49-76 wt. SiO2) whose
compositions would correspond to an in situ Vp in
the range of 6.0-6.3 km/s. This is in contrast to
a more mafic, cumulate lower crust composed
dominantly of two-pyroxene gabbro (hornblende,
garnet) and pyroxenite, (43-52 wt. SiO2) whose
compositions would correspond to an in situ Vp
7.0 km/s. This grossly simplified crustal
structure is surprisingly similar to many modern
arcs whose seismic velocity structures have been
determined (IBM, Tonga, Kurile, Aleutians, North
Honshu, Cascades). In all of these modern arcs,
an upper mid-crust with Vp 6.0-6.5 km/s is
present, corresponding to velocities calculated
for exhumed arc upper mid-crust lithologies. If
we presume that modern arcs and exhumed arcs all
contain upper mid-crust with intermediate to
felsic plutonic rocks (an unsubductable nucleus),
we must be able to model how these rocks are
generated. In general, we have discerned two
chemically distinct groups of tonalite/diorite in
the exhumed arc sections. The first compositional
group (Type I) typically has flat to slightly
LREE enriched rare earth element patterns where
REE abundances increase with increasing SiO2. The
second compositional group (Type II) shows trends
of LREE enrichment and HREE depletion, where both
LREE and HREE abundances decrease with increasing
SiO2. They are also depleted in Y and enriched in
Sr. The more felsic members of this group
generally exhibit concave-up patterns of HREE
depletion. Most exhumed arcs show one or the
other of these trends, but some, including
Talkeetna, show both, but at different times in
the arcs history. In the Talkeetna arc, least
squares calculations and REE Rayleigh
fractionation modeling indicate that Type I
tonalite/diorite (55-76 wt. SiO2) form via
fractional crystallization from basalt to dacite.
Type II tonalite/diorite (56-74 wt. SiO2) must
be produced by more complicated means that
involve some component of cannibalization of
lower crust, either by partial melting, or by
assimilation. Type II tonalites in the Talkeetna
arc can be effectively modeled as a result of
magma mixing between an andesitic parental liquid
(presumably formed by fractional crystallization)
and felsic partial melts of hornblende-bearing
mafic rock (amphibolite, hornblende gabbro
cumulates). In the Talkeentna arc, these Type II
rocks post-date the Type I rocks, and were formed
after the arc had matured and (presumably)
thickened. These mechanisms provide a
testable hypothesis for modern arcs. If the arcs
are relatively young and thin, then
tonalite/diorite should have geochemical
characteristics of Type I (fractionation only).
If the arcs are more mature and thicker, then
tonalite/diorite may have geochemical
characteristics of Type II (some component of
lower crustal melting).
Crawford et al. (2003)
Suyehiro et al. (1996)
Questionable stratigraphy (bottom and top may
actually be two sections based on ages)
Iwasaki et al. (2001)
Nakanishi et al. (2007)
Shillington et al (2004)
Parsons et al. (1998)

• Modern arcs in a gross sense have similar seismic
  velocity structures
• Even Izu Bonin and Aleutians are not that
  different in the upper crust.
• What lithologies make up this upper crust? Can
  we make generalizations based on arc crustal
  sections?

• Intermediate plutonic layer
• No obvious exposure of crustal melting
• No oceanic basement
• Mafic bulk composition

• Intermediate plutonic layer
• Crustal melting
• Older oceanic basement
• Unknown bulk composition

• Intermediate plutonic layer
• Crustal melting
• Older oceanic basement
• Unknown bulk composition

• Intermediate to gt30 km (10 kbar)!
• Crustal melting
• Older oceanic basement
• Int. bulk composition to 30 km depth

These sections have been color-coded to their
expected seismic velocities based on lithology
(velocities calculated using formulation of Behn
and Kelemen, 2003)

• How is the intermediate-felsic mid crust
  generated?
• Using geochemical signatures to discern magmatic
  processes
• The intermediate to felsic plutonic rocks of the
  mid crust display distinct REE patterns that fall
  into two categories
• Type 1 - REE abundances increase with increasing
  SiO2 content (and decreasing Mg). This can be
  easily modeled as fractional crystallization,
  typically involving cpx plag amphibole
  Fe-Ti oxide (see Johnsen et al. poster for
  Talkeetna detailed example).
• Type 2 - REE abundances decrease with increasing
  SiO2 content (and decreasing Mg). This cannot
  be modeled as fractional crystallization (even
  taking into account observed quantities of
  apatite). This is best modeled as melting of a
  distinct low LREE source (cumulates?) coupled
  with mixing.

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Mixing of the crustal melts with mantle-derived
magmas can produce the Type 2 trend of decreasing
REE with increasing SiO2
Produce a Si-rich, Type 2 magma by crustal melting
Common processes observed in the mid crust of
exposed arc sections
1. Magma mingling (and mixing) is pervasive in
the middle crust in all arc sections. Extreme
heterogeneity at all scales.
Example 1 Talkeetna arc (see Johnsen et al
poster)
Example Talkeetna arc (see Johnsen et al poster)
44.8 wt SiO2
Physical mixing of crystals
Residual Cpx 10 Opx 5 Plag 45 Amphibole
35 Magnetite 5
20 fractional melting
71.2 wt SiO2
Fine-scale mingling
Break-up and mingling of basaltic sill
person for scale
Mingling on the large scale
Close to homogenization?
2. Crustal melting - snapshots of this process
observed in the Bonanza arc and the Kohistan arc
in the mid-crust to upper parts of the lower crust
Western Talkeetna Arc
Bonanza Arc
Type 1
Kohistan arc migmatite
65.8 wt SiO2
Example 2 Bonanza arc (field exposure of
leucosome melanosome)
A Cascade volcano with a similar pattern Glacier
Peak volcano
Increasing SiO2
Type 1
In the Talkeetna Arc, the older plutonic rocks
are Type 1. The youngest rocks are Type 2
(thicker, more mature crust?)
Bonanza arc migmatite
Increasing SiO2
Concluding hypothesis
Mafic lavas that backmix with Type 2)
Type 1 Increasing REE with increasing SiO2.
Process is fractionation (young thin arcs?) Type
2 Decreasing REE with increasing SiO2. Process
is crustal melting and is often coupled with
mixing with mantle-derived magmas and
fractionation (older thicker arcs?) Both
processes produce the non-subductable nucleus of
continental crust
Type 2
Type 2
Decreasing SiO2
gt54 wt. SiO2 (diorite/tonalite)
REE normalized to C1 chondrite (Sun McDonough,
1989)
Type 2 (dacite)
Type 2 also have high Sr/Y with