Transcript Slide 1

The felsic plutonic core of the western Talkeetna island arc crustal section, Alaska:
Its formation and implications for crustal growth along continental margins
8
24
4
Al2O3
22
K 2O
20
3
med-K
18
16
14
10
MgO
8
4
4
0
The Talkeetna island arc is an exhumed and tilted section
where deep levels of arc lower crust as well as middle and upper
crust arc lithologies are now exposed at the surface.
4
CaO
TiO2
0
45
50
55
60
65
70
75
45
50
55
SiO2 (wt %)
100
60
65
70
75
Whole-rock major element variation diagrams for
western arc rocks with published data from the
eastern arc (Clift et al., 2005; Greene et al., 2006).
RARE EARTH ELEMENTS
Eastern Arc
Cs Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy
100
Volcanic and
volcaniclastic
rocks
5
Petrographic Characteristics
Volcanic and
volcaniclastic rocks;
Talkeetna Formation
(53.6 - 72.7 wt % SiO2)
Samples include fine-grained massive volcanic rocks and rounded
volcanic clasts within volcaniclastic sediment. Predominantly
porphyritic, samples range from basaltic-andesite to rhyolite.
Phenocrysts are mostly subhedral plagioclase and cpx (when present).
Plagioclase is commonly concentrically zoned and intergrown, many
contain seive textures. Amphibole phenocrysts are largely broken up
and surrounded by reaction rims of Fe-Ti oxides + pyroxene. Trace
minerals: quartz. Alteration minerals: chlorite.
Intermediate to felsic
plutonic rocks
(54.3 - 76.1 wt % SiO2)
Subhedral to anhedral granular texture. Samples include diorite, quartz
diorite, granodiorite, tonalite, and trondhjemite. Subhedral to anhedral
plagioclase, commonly corroded and concentrically zoned. Poikilitic
anhedral to subhedral amphibole containing plagioclase and/or biotite.
Consertal and interstitial anhedral quartz. Some Mg-rich samples from
the suite contain trace amounts of clinopyroxene and/or orthopyroxene
(Mg-no. 61-65). Trace minerals: K-felspar, apatite, titanite, and zircon.
Alteration minerals: chlorite, epidote, calcite, and pyrite.
Anhedral granular texture dominant. Samples are predominately
gabbroic. Anhedral to subhedral plagioclase, faint concentric zoning
and corroded cores common. Anhedral poikilitic amphibole containing
plagioclase. Clinopyroxene and orthopyroxene commonly cored in
amphibole. Trace minerals: apatite and titanite. Alteration minerals:
chlorite.
87.0 %
4,860 km2
0.4 %
24 km2
1
100
1
100
10
10
Int-Felsic Plutonic
avg. [La/Yb]N = 4.0
Plutonic and volcanic exposures in the western Talkeetna
island arc and the locations of rock samples used in this study.
Western arc exposures are from Detterman & Reed (1980),
Riehle et al. (1993), and Reed et al. (1982).
58.2
~20-30% xtal
Step 1
1.1
0.9
0.7
0.5
cpx, opx,
plag, hbl, mag
~15-25% xtal
Step 2
La Ce
plag, hbl,
mag, ilm
Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
1
1
100
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
Mafic Plutonic < 54 wt % SiO2
10
Compositional
Group II (blue)
avg. [La/Yb]N = 2.1
1
Cs Rb Ba Th U Nb Ta
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
1
REE normalized to C1 chondrite (Sun & McDonough, 1989)
60.4 wt
% SiO2
Intermediate
end-member
(5721J03)
10
10
90
71.2 wt
% SiO2
1
La Ce
Pr
% of felsic
end-member
Felsic
end-member
(5711J01)
plutonic samples (observed)
volcanic samples (observed)
modeled REE (mixing)
interm. end-member (5721J03)
felsic end-member (5711J01)
Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
20% 5721J03
80% 5711J01
compared
65.8 wt
to 5712J07
% SiO2
1.1
0.9
0.7
0.5
melting assemblage
cpx, opx, plag,
hbl, mag
Pr
44.8 wt
% SiO2
Gabbroic
composition
(5710J10B)
10
Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
Several gabbroic samples collected from
the western arc can be used to produce
felsic liquids. On average, ~15-20%
La
melting of gabbroic
compositions can
Ce
produce the observed
felsic, HREEPr
Nd from group II.
depleted samples
Sm
Eu
Gd
Tb
Dy
Felsic
end-member
(5711J01)
plutonic samples (observed)
100.0 samples (observed)
volcanic
modeled REE (melting)
gabbroic rock (5710J10B)
10.0end-member (5711J01)
felsic
100
Compositional Group I REE
patterns are relatively flat
like the eastern arc, while
10
Compositional Group II
samples exhibit trends of
concave-up HREE depletion.
La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd
100
0.1
100
71.2 wt
% SiO2
1
La Ce 1.0
Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
1.1
0.9
0.1
0.7
Tb Dy Ho Er Yb Y Lu
0.5
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
20% partial melt
of 5710J10B
compared to
5711J01
How does western arc chemistry,
petrologic modeling, and age
correlation relate to the development
of the arc crustal section?
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
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Cpx
Mineral Modes
40-70% Matrix (plag dominated)
Phenocrysts:
50-95% Plagioclase
0-20% Cpx (Mg-no. 73-84)
0-5% Opx (Mg-no. 65-67)
0-5% Amphibole (Mg-no. 67-73)
0-5% Fe-Ti oxide
40-70% Plagioclase (An 51-75)
10-30% Amphibole (Mg-no. 46-70)
5-20% Quartz
5-15% Biotite
1-5% Fe-Ti oxide
Zoned cpx phenocryst (volcanic
sample 5713J05)
Typical magnesio-hornblende
(intermediate-felsic sample 2727M01)
Cpx
45-65% Plagioclase (An 73-92)
25-45% Amphibole (Mg-no. 60-69)
0-15% Cpx (Mg-no. 68-82)
0-10% Opx (Mg-no. 54-69)
0-5% Biotite
2-5% Fe-Ti oxide
Cpx cores in amphibole (mafic
sample 2729M10A)
TWO COMPOSITIONAL GROUPS
The arc crustal section studied in the east comprises a calc-alkaline suite where
all lithologies display consistent major and trace element trends. In contrast, two
compositionally distinct groups can be defined in the western arc using
geochemistry of plutonic and volcanic rocks.
100
Sr/Y
100
Early Stages (compositional group I)
• Arc volcanism (starting ~198 Ma)
60
Compositional Group I (75 samples)
Compositionally similar to rocks from the eastern arc:
relatively low K at a given SiO2 and flat REE patterns
([La/Yb]N < 5; average 2.6) where REE abundances increase
with increasing SiO2. Rocks cover a wide range of
compositions (43.0 – 76.1 wt % SiO2), including mafic and
intermediate-felsic plutonic as well as volcanic samples.
20
• Older plutons (~185-170 Ma) emplaced in relatively thin, immature arc crust
10
• Fractional crystallization and accumulation dominant
Arc Maturation (compositional group II)
• Younger plutons (~170-160 Ma) intrude into mature, thicker arc crust
1
0
• Conductive heating of gabbroic lower crust produces felsic partial melts
100
Y
15
[La/Yb]N
10
• Mixing of felsic and intermediate magmas produces the observed range of
compositions
10
Compositional Group II (21 samples)
Compositional Group I
rocks show trends similar
to eastern arc samples.
Compositional Group II
samples show an increase
in [La/Yb]N and Sr/Y with
increasing SiO2.
[La/Yb]N
80
40
Show an entirely different trend of LREE enrichment and HREE
depletion ([La/Yb]N > 5; average 7.3) where REE abundances
decrease with increasing SiO2. Samples exhibit higher Al2O3, Sr,
and Ba, and lower TiO2, FeO, and Y compared to group I at the
same silica contents. Define a more restricted range of
compositions (56.1 – 73.6 wt % SiO2), including intermediatefelsic plutonic and volcanic samples.
9 ARC SECTION DEVELOPMENT
Age Correlation
120
 Fractional crystallization, partial melting, and magma mixing
may all have contributed to the formation of the western
Talkeetna island arc crustal section.
5
1
200
0
190
180
170
160
Age (Ma)
45
50
55
60
65
SiO2 (wt %)
70
75
Group I rocks are older (~185 – 175 Ma)
Group II rocks are younger (~170 – 160 Ma)
150
Pr
Nd
Eu
Gd
Partial Melting of Mafic Compositions
avg. [La/Yb]N = 1.7
> 54 wt % SiO2
Compositional
Group I (green)
Magma mingling of
intermediate and
felsic compositions,
north shore Lake
Grosvenor
(5710J05)
61.8
1.0
concentration / C1 chondrite
Int-felsic
avg. [La/Yb]N = 2.0
Ce
Tb
Dy
Ho
1.1
0.9
0.7
0.5
La Ce
Cc/Cm
700 km2
Total = 5,584 km2
A mp h
Mafic plutonic rocks
(43.0 - 53.7 wt % SiO2)
12.6 %
Mafic
MINERALOGY AND PETROGRAPHY
Lithology
Volcanic
Several intermediate and felsic endmember pairs from the group II suite
seem to predict the observed range
Ho
Yb Y Lusamples. Presumably,
of Er
observed
100.0
the intermediate
liquid formed via
fractional crystallization, but how
ppm
The Talkeetna magmatic
arc was created by
northward dipping
subduction (present-day
coordinates) of the oceanic
Farallon plate beneath the
Peninsular terrane (part of
the WCT) (Plafker et al.,
1994; Clift et al., 2005).
Exposed Area
10
Volcanic
avg. [La/Yb]N = 4.6
concentration / C1 chondrite
Mafic
plutonic
rocks
Unit
Lower Cook Inlet region study area and other Talkeetna island arc plutonic and volcanic
exposures. Western arc exposures from Detterman & Reed (1980) and Riehle et al. (1993).
Terranes from Barker et al. (1994), Siberling et al. (1994), and Wilson et al. (1998).
10
concentration / C1 chondrite
Uppermid
crust
Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
La
Sm
Mixing Intermediate and Felsic End-member Magmas
did the felsic10.0liquid form?
~7 km
Intermediatefelsic plutonic
rocks
Cc/Cm = 1 is a
perfect match
Simple fractional crystallization (FC) and
assimilation fractional crystallization (ECAFC developed by Bohrson & Spera,
2001) do not explain REE trends for
compositional group II rocks, specifically
decreasing REE abundances with
increasing SiO2 (fractionation) and HREE
depletion. So what processes formed these
rocks?
4
Western Arc
Upper
crust
Measured
(observed)
concentration
(Cm)
Compositional Group II
8
0.0
Schematic section showing
rocks exposed in the western
arc (Lower Cook Inlet region)
Calculated
concentration
(Cc)
Increasing REE abundances
with increasing SiO2 (proxy
for fractionation) and REE
pattern shape are matched well
by modeling fractional
crystallization from andesitic
to dacitic compositions.
12
0.5
The western arc section is composed predominantly of intermediate-felsic
plutonic rocks (diorites and tonalites), minor amounts of gabbroic rock, and a
significant component of overlying Talkeetna Formation volcanic and
volcaniclastic rocks. The western arc is a small sub-section of the entire arc crustal
section exposed in the east.
La Ce
Step 1 Daughter
(5710J02)
Parent
(81AR193)
64.5
0
1.5
EXPOSED ARC CRUSTAL SECTION
10
concentration / C1 chondrite
3
0
Fractional Crystallization
8
6
1.0
GEOLOGIC SETTING
Compositional
Group I
68
12
FeO
2
Continental Growth: what implications do
the chemical trends and magmatic
formation of western arc rocks have on
models for the growth of continental crust
at convergent margins?
1
low-K
12
Processes of magma differentiation: what
magmatic processes are responsible for the
formation of the exposed intermediatefelsic plutonic and volcanic rocks in this
region?
48
(5721J04)
Cc/Cm
Along strike compositional variation: is
there significant geochemical variability
along arc strike (compared to the eastern
arc)?
2
concentration / C1 chondrite
MAJOR ELEMENTS
Research presented here addresses three
main questions yet to be resolved for the
intermediate-felsic core of the western
Talkeetna arc:
MAGMATIC PROCESSES
What magmatic processes are responsible for the formation of intermediate-felsic plutonic and volcanic rocks
exposed in the western arc? Were these rocks formed by simple fractional crystallization or did assimilation of
existing crust and/or magma mixing play an important role? Greene et al. (2006) modeled fractional crystallization
from basalt to andesite compositions in the
100
step 2 daughter (observed)
eastern
arc.
The
research
presented
here
Le Maitre et al. (1989)
step 2 daughter (calculated)
attempts to model the formation of more
step 1 daughter (observed)
step 1 daughter (calculated)
wt % SiO2
48 compositions starting with an
felsic
parent liquid (81AR193)
68
andesitic
composition.
Step 2 Daughter
weight %
The accretion of island arc crust is believed to be a major contributor to the growth of continents. A
particularly important question in arc evolution is the origin of felsic plutonic rocks in island arcs. Felsic rocks
represent the nucleus of continents, yet there is no clear consensus on how these rocks originate. The Jurassic
Talkeetna island arc in south-central Alaska is an exhumed and titled arc section where middle and upper crust
lithologies are now exposed at the surface, providing us with the rare opportunity to directly model processes
responsible for the formation of the felsic core of an island arc.
Studies in the eastern Talkeetna arc (Talkeetna and Chugach Mountains) indicate that the arc crustal section
comprises a calc-alkaline suite where all lithologies display consistent major and trace element trends, each
group of rocks forming by processes of fractional crystallization and accumulation. In contrast, two chemically
distinct groups can be defined in the western Talkeetna arc (Lower Cook Inlet region). Compositional group I
(n = 75; 43.0-76.1 wt % SiO2) shares many of the same trends as rocks from the eastern arc: relatively low K at
a given SiO2 and flat REE patterns ([La/Yb]N < 5; average 2.6) where REE abundances increase with
increasing SiO2. Compositional group II (n = 21; 56.1-73.6 wt % SiO2) shows trends of LREE enrichment and
HREE depletion ([La/Yb]N > 5; average 7.5), where REE abundances decrease with increasing SiO2. The most
silicic members of this group exhibit concave-up patterns of HREE depletion.
Of particular interest is the formation of the voluminous felsic core of the western arc, exposed over more
than 4800 km2 of the region. Major element and REE modeling indicate the majority of these rocks formed
through fractional crystallization and accumulation (compositional group I). In addition, modeling results for
compositional group II rocks indicate that the observed range of intermediate-felsic compositions can be
produced by variable mixing of an andesitic parent liquid (presumably formed by fractional crystallization)
with a felsic end-member magma (formed by ~15-25% partial melting of mafic crust). This study provides
important insights into the range of processes responsible for the formation of the felsic core of island arcs and
potentially the nucleus of continental crust.
6
WHOLE-ROCK CHEMISTRY
Michael Johnsen, Department of Geology, WWU, Bellingham, WA
Susan DeBari, Department of Geology, WWU, Bellingham, WA
Matthew Rioux, Department of Earth Science, UCSB, Santa Barbara, CA
Cc/Cm
2
RESEARCH
QUESTIONS
weight %
1
ABSTRACT
Paper No. 30 - 8
 Both compositional groups are components of the buoyant
intermediate-felsic nucleus of the western arc that accreted
to the continental margin.
Er