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.