geochemical indicators of plate tectonic processes in old rocks julian pearce (cardiff university)
TRANSCRIPT
Geochemical Indicators of Plate Tectonic Processes in Old Rocks
Julian Pearce
(Cardiff University)
Bob’s Smoking Guns for Archean Subduction(Blueschists, UHP rocks, Ophiolites)
However, Subduction Fluxes are Forever
Even if the arc is overwritten by collision or eroded away, inherited subduction signals remain in the mantle lithosphere
and can be reactivated later
Finding Evidence of Subduction is the Key to Knowing when Plate Tectonics started
But present-day subduction is a complicated processes; essentially mantle flow and subduction input are the geochemical indicators of
plate tectonics, while crustal interactions tend to mask these indicators. The key to identifying arc lavas in the Archaean is separating
subduction signals from crustal signals.
Pearce & Peate (1995)
Geochemical Indicators of Plate Tectonic Processes
•Indicators of Plate-driven Mantle Flow•Indicators of Subduction
At subduction zones,
these two processes act
together.
rear-arcseamount
intraplatevolcano
volcanicarc
asthenosphere
lithosphere
back-arcridge
mantle depletion by episodicmelt extraction towards arc front
A’
B
B ’
C
F
A
LT
HT
UHT
Geochemical Tracing of Subduction Input
rear-arcseamount
intraplatevolcano
volcanicarc
asthenosphere
lithosphere
back-arcridge
mantle depletion by episodicmelt extraction towards arc front
A’
B
B ’
C
F
A
Progressive subduction leads to sequential release
of:
LT elements (Rb, Ba etc)
HT elements (LT elements plus L-
MREE, Th, P)
UHT elements ( HT elements plus Nb,
Ta, Zr, Hf). In the Archean, we would expect this sequence to take place at shallower depths than at present
LT
HT
UHT
Geochemical Tracers for Subduction Input
Shallow subduction components cannot be investigated in Archaean rocks
because of alteration-sensitivity
Deep subduction components are
more robust. Th/Nb (an indicator of
negative Nb anomalies also an effective tracer of deep subduction
input that is robust to upper amphibolite
facies
ZrBa U Ta La Pb Sr Nd Eu Gd Dy Ho Tm
Rb Th Nb K Ce Pr P SmHf
Ti Tb Y Er Yb
100
10
1
0.1
rock
/MO
RB
a
a: Nb/Yb (mantle fertility and % melting)
f
b
b: Ba /Nb (total subduction component)
c
c: Th/Nb (de ep subduction component)d d: Ba/Th (shallow subduction component)
e (U/Th) and f (Nb /Ta) have complexproperties as discussed in the text e
proxies:
mantle componentfocu s of
this paper
deep subduction component
shallow s ubduction component
Geochemical Tracing of Mantle Flow
Flowing mantle undergoing decompression can drastically change its chemical composition
Pearce (2005)
rear-arcseamount
intraplatevolcano
volcanicarc
asthenosphere
lithosphere
back-arcridge
mantle depletion by episodicmelt extraction towards arc front
A’
B
B ’
C
F
A
Geochemical Tracing of Mantle Flow
Tonga-LauSystem:
collabration w.Pam Kempton, Jim Gill
Results indicate that mantle entering subduction systems progressively loses incompatible elements by
melt extraction while flowing to the sub-arc region
Geochemical Tracers for Mantle Flow
ZrBa U Ta La Pb Sr Nd Eu Gd Dy Ho Tm
Rb Th K Ce Pr P SmHf
Ti Tb Y Er
100
10
1
0.1
rock
/MO
RB
e nriched M O RB(North F iji Basin)
depleted M O RB(Southern Lau B asin)
Nb/Y b acts a proxy for m antle fertility
Nb/YbThus Nb/Yb acts as a
Good proxyfor mantle fertility
Nb/Yb gradients provide
a means of tracing mantle flow
Isotope ratios reach plateaus, so trace element ratios are more effective for
mapping. Most effective for
subduction systems are VICE/MICE ratios based on immobile
elements
Th/Yb-Nb/Yb Fingerprinting
Pearce and Peate, 1995
At the present day, MORB and IOB plot in a well defined array, along the mantle flow axis (Nb/Yb); arc lavas are
displaced to higher Th/Nb ratios. The overall dispersion of arc lavas is parallel to the MORB array indicating the importance
of melt extraction during mantle flow in magma genesis.
Th/Yb-Nb/Yb Fingerprinting:Role of Mantle Flow
1010.1
0.1
0.01100
1
10
Th/Yb
MO
RB-O
IB ar
ray
Nb/Yb
N-MORB
HTsubduction
meltextraction
two components to a first approximation
Th/Yb-Nb/Yb Fingerprinting:Interaction of Mantle Flow and SZ input
1010.1
0.1
0.01100
1
10
Th/Yb
MO
RB-O
IB ar
ray
Nb/Yb
N-MORB
Location of data and shapes of trends indicate
process
1= Add SZ component before melt extraction
2= Add SZ component during melt extraction
3. Add SZ component without or after melt
extraction
4. UHT SZ component adds Nb as well as Th
12
34
Types of Subduction Zone
North Tonga: Oceanic plume-subduction interaction
Cascades: Continental plume-subduction interaction
IBM Eocene: Intra-oceanic subduction initiation
Japan Miocene: Intra-continental subduction initiation
Various Localities: Ridge subduction
Taiwan, SE Indonesia: Syn-collision
Mariana: Arc Rifting
Anatolia: Subduction component reativation
plussteady state subduction
Each type of Subduction Zone shows a different topology on the Th/Yb-Nb/Yb diagram
Example: W. Pacific Eocene
1010.1
0.1
100
1
10
MOR
B-O
IB a
rray
Nb/Yb
N-MORB
Boninites at Subduction Init iation(Data of Pea rce et al., 1992)
Acoje, PhilippinesGuam
Chichijima
Bonin Forearc
Saipan
Mariana Forearc
Th/Yb
Earliest Arc
Example: North Tonga
1010.1
0.1
0.01100
1
10
MO
RB-OIB
arra
y
Nb/Yb
N-MORB
11
Data of Pearceet a l. (In press)
Samoa
W Boninites
E Boninites
N Tofua arc
Th/Yb
N. Tonga(Plume-arc interaction)
Example: Kamchatka
1010.10.01
100
1
10
MO
RB-OIB
arra
y
Nb/Yb
N-MORB
11
0.1
Data compliation
Distal
Proximal
Th/Yb
Kamchatka(flow from rear-arc to arc front)
Misinterpretation of Subduction Zones: Amphibolite-facies metamorphism
HT fluids/melts from sediments impregnated and metamorphosed the lavas.
The result is that some normally-immobile LIL elements (e.g. Th) are
enriched
1010.10.1
1
10
Th/Yb
Ta/Yb
N-MORB
increasingmelt addition
Broken Hill cores
Misinterpretation of Subduction Zones: granulite facies metamorphism
The lower crust loses a melt fraction (which can be seen
in places ‘escaping’ from the rock) leaving a granulite
residue
This depletes the residue in LIL elements.
David Waters
1010.10.1
1
10
Th/Yb
Ta/Yb
N-MORB
increasingmelt addition
increasing meltdepletion
Misinterpretation of Subduction Zones: Crustal Contamination
1010.1
0.1
0.01100
1
10
Th/Yb
MO
RB-O
IB a
rray
Nb/Yb
N-MORB
Greenland Margin - Iceland
1010.1
0.1
0.01100
1
10
Th/Yb
MO
RB-O
IB ar
ray
Nb/Yb
N-MORB
SE Anatolia Post-Collis iondata from Pearce et al. (1990)
The crustal contamination vector is typically parallel to the UHP subduction vector.
Crustal Contaminationis also part of continental arc dispersion
1010.10.01
100
1
10
MO
RB-OIB
arra
y
Nb/Yb
N-MORB
11
0.1
Data compliation
Distal
Proximal
Th/Yb
B
A
D
R
Kamchatka(AFC trend)
Proposed Archean Subduction-Related Rocks
Komatiites
Boninites
BADR volcanic series
Adakite lavas and TTGs
Aim is to assess these subduction signals
Testing Archean Komatiite Subduction Models
1010.1
0.1
0.01100
1
10
Th/Yb
MO
RB-O
IB ar
ray
Nb/Yb
N-MORB
Komati Formation, Barberton (3.5Ga)(Data of Chavagnac (2004) & Parman et al. ( 1997)
Parman et al. (1997, 2001), following the work of Grove, controversially argue that Barberton lavas
were wet rather than hot – i.e. subduction related. However, crustal contamination is more
consistent with the data
1010.1
0.1
0.01100
1
10
MO
RB-OIB
arra
y
Nb/Yb
N-MORB
11
Data of Jochumet al. (1991) for
Onverwacht, Kambalda, Monro, Alexo, Belingwe
Th/Yb
Komatiite/tholeiitesuites
Testing Archean Boninite Subduction Models
1010.1
0.1
0.01100
1
10
Th/Yb
MO
RB-O
IB ar
ray
Nb/Yb
N-MORB
Iringora
Khi zovaara
Finnish Greenstone Bet (2.8Ga)
Data of Shchipansky et al. (2004)
1010.1
0.1
0.01100
1
10
Th/Yb
MO
RB-O
IB ar
ray
Nb/Yb
N-MORB
Isua Greenstone Belt
Data of Polat & coworkers
Central Domain (boninites)
Other Domains
Dispersion is along a crustal contamination vector, not mantle flow vector; however the Isua boninites do require source
depletion.
Alternative Model for Archean Boninites and Related Rocks
Arculus et al. (1992)from IODP Leg 125
Phanerozoic boninites result from shallow, wet melting(opx = ol + siliceous melt)
Alternative Model for Archean Boninites and Related Rocks
Arculus et al. (1992)from IODP Leg 125
But Archean boninites could be explained by komatiite-crust interaction
Komatiite
crust
Testing Archean BADR Subduction Models
The proposed BADR series shows increasing Th/Nb with increasing silica content and trend parallel to
crustalcontamination trends. Wawa lavas do not however have an end member in the mantle array: a difficult call.
1010.1
0.1
0.01100
1
10
MO
RB-OIB
arra
y
Nb/Yb
N-MORB
Pilbara: Whundo‘arc’ 3.12Ga
Data of Smithies et al. (200 5)
Thol. Basalts
CA Basalts-And.
Bonin ites
Nb-enriched bas.
Th/Yb
1010.1
0.1
0.01100
1
10
MO
RB-OIB
arra
y
Nb/Yb
N-MORB
Super ior: Wawa 2.7Ga
Data of Polat et al.
Basalts
Andesites
High-Mg andesites
Nb-enriched bas.
Th/Yb
Turkish Analogue
0.1
0.1
1
10
Th/Yb
Average UpperCrust
AFC
Foreland
S of Pontides
N of Pontides
AFC
SZ
Average N-M ORB
P. Meltin
g
(gt.
facie
s)
MO
RB-OIB
arra
y
Ta/Yb0.05 1 5
Post-collision magmas erupted at the site of the
‘dead’ arc have subduction signatures
Post-collision magmas erupted where there was
no arc have intraplate signatures ( with crustal
assimilation)
Reactivation of sub-arc lithosphere following collision
can be evaluated using Nb anomalies
Testing Archean Adakite/TTG Subduction Models
Maybe all four! But subduction not essential.
All agree on melting of mafic material but how?
Flat subduction (lower crust melting)
Hot subduction (slab melting)
Delamination (lower crust melting)
Magma chambers (mid-crust melting)
Crustal Processing
With no plate tectonics, could Archean volcanic terranes have undergone
intracrustal reprocessing like present-day Collision Zones, so explaining subduction-like chemical signatures?
Conclusions
Many Archean boninites and other evolved high-Mg magmas could be explained by interactions between komatiite (and related) magmas and crust rather than by subduction.
Archean basalt-andesite-dacite-rhyolite (BADR) series require substantial magma-crust interaction and may not all additionally have subduction components.
Adakites could (as is well known) involve melting of mafic rocks in the crust as well as in subduction zones.
Unlike modern arc lavas, Archean ‘arc’ lavas do not exhibit a geochemical indication of plate-driven flow into and within a mantle wedge.
If there was subduction in the Archaean, it must have involved variable ‘adakitic’ addition to a homogneous mantle
Personally, I would start subduction around 2.7 Ga but work still needs to be done separating crustal and subduction signals.