ABSTRACT 1 Petrologic processes that generate the intermediate to felsic plutonic core of island...
1
1 10 100 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 1 10 100 Diorites, Tona & Leucotonalit (65% SiO2) (65% SiO2 (69% SiO2 (60% SiO2) (56% SiO2) 92-274 66% SiO2 (58% SiO2) 1 10 100 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 In exhumed arc sections worldwide, the upper mid-crust is composed dominantly of hornblende-bearing tonalite, quartz diorite, diorite, and gabbro (49-76 wt.% SiO 2 ) 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.% SiO 2 ) 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 SiO 2 . 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 arc’s history. In the Talkeetna arc, least squares calculations and REE Rayleigh fractionation modeling indicate that Type I tonalite/diorite (55-76 wt.% SiO 2 ) form via fractional crystallization from basalt to dacite. Type II tonalite/diorite (56-74 wt.% SiO 2 ) 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). 3 2 6 4 5 7 REE normalized to C1 chondrite (Sun & McDonough, 1989) 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 Volcanic and volcaniclasti c rocks Intermediate- felsic plutonic rocks Mafic plutonic rocks Upper- mid crust Upper crust 5-~7 km 10 kb 5 kb 0 (~30 km ) (~15 km ) D epth Interlayered interm ediate plutons and m etam orphic rocks (schist/ am phibolite) C rustatleast 60km thick ? 5 kb 0 + mingled m afic diorite tonalite volcanic rocks (~15 km ) am phibolite D epth c rus tal melting 10 kb pyroxenite dunite- harzburgite 5 kb 0 garnetgabbro layered 2-px gabbro volcanic rocks diorite tonalite M oho (~30 km ) (~15 km ) +mingled m afic D epth ? 10 kb 5 kb 0 layered 2-px gabbros pyroxenite, dunite +/-garnet tonalite,diorite 2-px gabbro volcanic rocks (~30 km ) (~15 km ) am phibolite garnet gabbro hornblende gabbro D epth granitoid batholith + mingled m afic c rus tal melting ? ? M oho • 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 >30 km (10 kbar)! • Crustal melting • Older oceanic basement • Int. bulk composition to ~30 km depth Questionable stratigraphy…… (bottom and top may actually be two sections based on ages) 1 10 100 La Ce Nd Sm Eu Gd Dy Ho Yb Lu Batch modal melt 37c (melanosom RESIDUE 35% amphibol 45% plagiocla 10% magnetit 37c - 'resi 1 - leucocratic model resul ~25% meltin 10% clinopyroxe Common processes observed in the mid crust of exposed arc sections 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 SiO 2 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 SiO 2 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. Seismic stratigraphy of modern arcs - more similarities than differences? • 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? Shillington et al (2004) Suyehiro et al. (1996) Crawford et al. (2003) Iwasaki et al. (2001) Nakanishi et al. (2007) Parsons et al. (1998) The crustal sections These sections have been color-coded to their expected seismic velocities based on lithology (velocities calculated using formulation of Behn and Kelemen, 2003) 1. Magma mingling (and mixing ) is pervasive in the middle crust in all arc sections. Extreme heterogeneity at all scales. 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 Close to homogenization? person for scale Fine-scale mingling Physical mixing of crystals Mingling on the large scale Break-up and mingling of basaltic sill Kohistan arc migmatite Bonanza arc migmatite Western Talkeetna Arc >54 wt.% SiO2 (diorite/tonal ite) Type 2 Type 1 Increasing SiO 2 In the Talkeetna Arc, the older plutonic rocks are Type 1. The youngest rocks are Type 2 (thicker, more mature crust?) 0 20 40 60 80 100 120 45 50 55 60 65 70 75 Sr/Y 1 10 100 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Gabbroic composition (5710J10B) Felsic end-m em ber (5711J01) 20% partial m elt of5710J10B com pared to 5711J01 0.8 0.9 1.0 1.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Mixing of the crustal melts with mantle-derived magmas can produce the Type 2 trend of decreasing REE with increasing SiO 2 Produce a Si-rich, Type 2 magma by crustal melting Concluding hypothesis : Type 1 : Increasing REE with increasing SiO 2 . Process is fractionation (young thin arcs?) Type 2 : Decreasing REE with increasing SiO 2 . Process is crustal melting and is often coupled with mixing with mantle- derived magmas and fractionation (older thicker arcs?) Both processes produce the non- Both processes produce the non- subductable nucleus of continental subductable nucleus of continental crust crust Bonanza Arc Type 2 also have high Sr/Y with A Cascade volcano with a similar pattern: Glacier Peak volcano Residual Cpx 10% Opx 5% Plag 45% Amphibole 35% Magnetite 5% ~20% fractional melting plutonicsam ples(observed) volcanic samples (observed) gabbroic rock (5710J10B) m odeled REE (melting) felsic end-member(5711J01) Example 1: Talkeetna arc (see Johnsen et al poster) 44.8 wt% SiO 2 71.2 wt% SiO 2 Example 2: Bonanza arc (field exposure of leucosome & melanosome) Type 1 Type 2 Increasing SiO 2 Decreasing SiO 2 65.8 wt % SiO 2 1 10 100 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Felsic end-m em ber (5711J01) Intermediate end-m em ber (5721J03) % offelsic end-m em ber 90 10 20% 5721J03 80% 5711J01 0.5 0.7 0.9 1.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu plutonic samples (observed) volcanic samples (observed) interm. end-member(5721J03) m odeled REE (m ixing) felsic end-member(5711J01) 60.4 wt % SiO 2 71.2 wt % SiO 2 Example: Talkeetna arc (see Johnsen et al poster) Type 2 (dacite) Mafic lavas that backmix with Type 2)
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Transcript of ABSTRACT 1 Petrologic processes that generate the intermediate to felsic plutonic core of island...
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La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
REE/Chondrite
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La Ce Nd Sm Eu Gd Tb Dy Ho Yb Lu
REE/Chondrite
Diorites, Tonalites& Leucotonalites
(65% SiO2)
(65% SiO2)
(69% SiO2)
(60% SiO2)
(56% SiO2)
92-27466% SiO2
(58% SiO2)
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REE/Chondrite
ABSTRACT1
Petrologic processes that generate the intermediate to felsic plutonic core of island arcsSusan DeBari & Michael Johnsen, Department of Geology, Western Washington University, Bellingham, WA
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 arc’s 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).
32
64 5 7
REE normalized to C1 chondrite (Sun & McDonough, 1989)
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
Volcanic and volcaniclasticrocks
Intermediate-felsic plutonicrocks
Mafic plutonicrocks
Upper-mid crust
Upper crust
5-~7 km
10 kb
5 kb
0
(~30 km)
(~15 km)
Depth
Interlayered
intermediateplutons
and
metamorphicrocks(schist/amphibolite)
Crust at least60 km thick
?
5 kb
0
+ mingledmafic
dioritetonalite
volcanicrocks
(~15 km)amphibolite
Depth
crustal melting
10 kb
pyroxenitedunite-harzburgite
5 kb
0
garnet gabbro
layered2-pxgabbro
volcanicrocks
dioritetonalite
Moho(~30 km)
(~15 km)
+mingledmafic
Depth
?
10 kb
5 kb
0
layered2-pxgabbros
pyroxenite,dunite+/- garnet
tonalite, diorite
2-px gabbro
volcanicrocks
(~30 km)
(~15 km)
amphibolite
garnetgabbro
hornblendegabbro
Depth
granitoidbatholith+ mingledmafic
crustal melting
??
Moho
•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 >30 km (10 kbar)!
•Crustal melting
•Older oceanic basement
•Int. bulk composition to ~30 km depth
Questionable stratigraphy…… (bottom and top may actually be two sections based on ages)
1
10
100
La Ce Nd Sm Eu Gd Dy Ho Yb Lu
Batch modal melting
37c (melanosome)
RESIDUE
35% amphibole45% plagioclase
10% magnetite
37c - 'residue'1 - leucocratic diorite
model results
~25% melting10% clinopyroxene
Common processes observed in the mid crust of exposed arc sections
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.
Seismic stratigraphy of modern arcs - more similarities than differences?
• 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?
Shillington et al (2004)
Suyehiro et al. (1996)
Crawford et al. (2003)
Iwasaki et al. (2001)
Nakanishi et al. (2007)
Parsons et al. (1998)
The crustal sections
These sections have been color-coded to their expected seismic velocities based on lithology (velocities calculated using formulation of Behn and Kelemen, 2003)
1. Magma mingling (and mixing) is pervasive in the middle crust in all arc sections. Extreme heterogeneity at all scales.
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
Close to homogenization?
person for scale
Fine-scale mingling
Physical mixing of crystals
Mingling on the large scale
Break-up and mingling of basaltic sill
Kohistan arc migmatite
Bonanza arc migmatite
Western Talkeetna Arc
>54 wt.% SiO2 (diorite/tonalite)
Type 2
Type 1
Increasing SiO2
In the Talkeetna Arc, the older plutonic rocks are Type 1. The youngest rocks are Type 2 (thicker, more mature crust?)
0
5
10
15
45 50 55 60 65 70 75
0
20
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60
80
100
120
45 50 55 60 65 70 75
Sr/Y
20% partial meltof 5710J10B
LaCePrNdSmEuGdTbDy
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100
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
concentration / C1 chondrite
Gabbroiccomposition(5710J10B)
Felsicend-member(5711J01)
Cc/Cm
20% partial meltof 5710J10Bcompared to5711J010.8
0.91.01.1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
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
Concluding hypothesis: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-Both processes produce the non-subductable nucleus of continental subductable nucleus of continental crustcrust
Bonanza Arc
Type 2 also have high Sr/Y with
A Cascade volcano with a similar pattern: Glacier Peak volcano
ResidualCpx 10%Opx 5%Plag 45%Amphibole 35%Magnetite 5%
~20% fractional melting
plutonic samples (observed)volcanic samples (observed)
gabbroic rock (5710J10B)modeled REE (melting)
felsic end-member (5711J01)
Example 1: Talkeetna arc (see Johnsen et al poster)
44.8 wt% SiO2
71.2 wt% SiO2
Example 2: Bonanza arc (field exposure of leucosome & melanosome)
Type 1
Type 2
Increasing SiO2
Decreasing SiO2
65.8 wt % SiO2
20% 5721J03 (int)80% 5711J01 (fel)
LaCePrNdSmEuGdTbDy
1
10
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La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
concentration / C1 chondrite
Felsicend-member(5711J01)
Intermediateend-member(5721J03)
% of felsicend-member90
10
Cc/Cm
20% 5721J0380% 5711J01
0.50.70.91.1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
comparedto 5712J07
plutonic samples (observed)volcanic samples (observed)
interm. end-member (5721J03)modeled REE (mixing)
felsic end-member (5711J01)
60.4 wt% SiO2
71.2 wt% SiO2
Example: Talkeetna arc (see Johnsen et al poster)
Type 2 (dacite)
Mafic lavas that backmix with Type 2)
