ehaz convergent margins class adakites: 10 april 2008 1 constraints on adakite existence colin...

EHaz Convergent Margins ClassAdakites: 10 April 2008

1

Constraints on

Adakite Existence

Colin Macpherson

University of Durham


2

Hi! You have covered a fairly diverse range of topics

so far in this class that employ a wide range of

techniques to understand magmatism and

volcanism at convergent plate margins. I hope that

you will be up for some more geochemistry. I realise

that not everyone is conversant in trace element

ratios and isotopes but all arguments surrounding

adakites rely on their geochemistry. I have tried to

incorporate sufficient explanatory text or figures to

help you understand these.

The two papers you really need to read are:

1. Defant & Drummond (1990) from Nature, and

2. Macpherson et al (2006) from EPSL.

Several others are mentioned in this file for the

really keen!


3

“Normal” Arc Magmatism• subducted lithosphere releases hydrous fluids and, possibly, silicate melts,

• these infiltrate the overlying mantle wedge lowering the solidus of the mantle

peridotite there,

• partial melting of peridotite produces basaltic magma.

Hydration ofMantle Wedge

Fluid

+ Melt l

oss

Melts

Arc


4

• many processes operate within arc lithosphere to produce diverse

compositions from the primary basaltic flux.

• in general, though, most arc lavas lie along the basalt-andesite-dacite-rhyolite

differentiation trend as a result of differentiation at crustal pressures.


5

So, What is an Adakite?

Defant and Drummond (1990)

suggest that distinctive geochemical

trends in some dacites and andesites

cannot be produced by low pressure

differentiation. This group of rocks,

which they termed adakites, have:

• relatively high alumina content,

• intermediate silica content

• low concentrations of heavy rare earth elements, and

• elevated Sr/Y ratios


6Why Does This Geochemical

Signature Matter?

are exactly the characteristics that would be expected of a

magma extracted from hydrous basaltic crust by partial melting.

• relatively high alumina content,

• intermediate silica content

• low concentrations of heavy rare earth elements, and

• elevated Sr/Y ratios

The animation over the next few pages explains why Sr and the heavy

rare earth elements (to which Y is closely related) would be affected

in this way.


7

SrY

Plag

AmphGt

In subducted crust (mostly

greenschist facies rock)

amphibole hosts Y (+ HREE)

while plagioclase hosts Sr.


8

SrY

Plag

AmphGt

Phase changes occur as the slab

subducts. The transformation of

greenschist to blueschist has

relatively little effect on Y & Sr.


9

YY

SrPlag

AmphGt

As P increases plagioclase

destabilises, so Sr is

“homeless”. Garnet, in which Y

is very compatible, appears.


10

YY

SrPlag

AmphGt

If the (now eclogite facies) slab

melts then Y (+HREE) will be

retained in amphibole and garnet

but Sr will go into melt.


11

YY

Sr

Sr

Plag

AmphGt

Therefore:

residue = Y-rich Sr-poor

melt = Y-poor Sr-rich

i.e. melt has high Sr/Y and low Y


12

What Support for Slab Melting?

Defant and Drummond noted that all the rocks they called adakites were

associated with subducted slab that were young. Therefore, they claimed

that these slab were more likely to melt because they retained a lot of

heat from their (recent) formation. This diagram shows their evidence

base of ~ a dozen adakitic suites.

Adakites

Not Adakites

Of slab


13 So What?Why all the fuss?

Why does it matter if slab melts reach the surface (or arc crust)?

CA

Adakite

Phanerozoic Archean

Adakites show many similarities to

tonalite, trondhjemite, and granodiorite

(TTG) suites which are a defining feature

of Archean terranes. The two figures A

compare data for ordinary arc lavas (CA)

with adakites. The figures B show TTG

suites.

So, adakites are a potential analogue for

Archean TTG that would help

understand Archean tectonics and crust

generation (next slide).


14

Martin (1999)

Hot Slab follows geotherm 1Archean

Cool Slab follows geotherm 3Phanerozoic

Basalt slab melts

Basalt slab dehydrates

Hydrated peridotite melts


15

OK – Let’s Recap 1Adakites defined in 1990 based on their geochemical similarity to

expected slab melts and association with subduction of slabs <25Ma.

Adakitic rocks resemble important Archean crustal components

The model is nice – it is simple, quite intuitive and makes profound

predictions about how the early Earth operated.


16

Some things to discuss - 1

Are you happy that Defant & Drummond

(1990) exhausted all alternative explanations

for the geochemical signature of adakites?

Are Defant & Drummond’s adakites open or

closed systems (and does this matter)?

How would you recognise an adakite in the

field?


17 Post-1990Through the 1990s more rock suites with adakitic geochemistry were

found in modern and ancient convergent margins.

Not all of these suites were associated with young slabs.

Therefore, two possible alternatives were recognised:

1. The slab melting model is wrong.

2. Slabs can reach fusion point through other ways than just being

young.

Many of the workers studying the rocks chose to follow option 2 leading

to many slab-melting mechanisms being inferred. Some of these are

illustrated on the next few slides.


18 Flat Slab

Adakitic rocks have been found above some areas where the slab has a

shallow dip.

e.g. Gutscher et al (2000, Geology 28, 535-538)

Long time at low P therefore

different PTt path (~ path 1

in slide 14) and more chance

to heat slab above solidus

Normal PTt path (~ path 3 in

slide 13) for slab


19 Subduction Initiation

Adakitic rocks were found in the Philippines where the slab was old but

had not been subducting long. Numerical modelling (Peacock et al.,

1994, EPSL 121, 227-244) suggests that under these conditions the

mantle will be hotter than mantle adjacent to more mature slabs so may

provide sufficient heat to make the slab melt.

e.g. Sajona et al (1993, Geology 21, 1007-1010)

Mantle wedge not yet cooled

by ongoing subduction

High temperature adjacent

to slab, therefore slab

melting


20 Slab TearsSome adakitic rocks occur close to

“tears” or gaps in the slab. This is

assumed to expose the slab interior

to relatively high temperatures so

that it can melt.

e.g. Yogodzinski et al (2001, Nature

409, 500-504).

Mantle wedge cooled by

ongoing subduction

producing “normal” arc

lavas

High temperature adjacent

to slab, therefore slab

melting


21

OK – Let’s Recap 2Adakites defined in 1990 based on their geochemical similarity to

expected slab melts and association with subduction of slabs <25Ma.

Several occurrences defined from 1993 to 2001 use only geochemistry to

define slab melting.

By early 2000s three classes of exception to the rule (that I have listed

and others that I haven’t) have been introduced.

The model is not so nice now – it has lost its simplicity and can no

longer make specific predictions about how the early Earth operated.


22A Case Study

Many adakite “locations” based on relatively few rocks.

Subduction initiation study in Surigao, Philippines (slide 19) based on

three rocks.

Examine Surigao example in more detail with comprehensive dataset

collected in 1999 (Macpherson et al., 2006, EPSL 243, 581-593)

Thorough sampling of Surigao peninsula and wide array of geochemical

and petrological techniques used.


23

Philippine SeaPlate

55Ma 40Ma

Mindanao

Westwards subduction of

Philippine Sea Plate began in

north about 10Ma and has

propagated south

The Philippine Tectonic Context

This trench

propagating south


24

Philippine SeaPlate

55Ma 40Ma

Mindanao

Philippine Sea Plate is ~55Ma

where it is subducting so is too

old to melt under normal

geotherm



25

Philippine SeaPlate

55Ma 40Ma

Mindanao

SurigaoPeninsula

Adakitic rocks collected from

Surigao peninsula on Mindanao

island.



26

Trench is

over there

Large strike-slip fault

acts as western

graben-bounding

fault

Reactivated back-

thrusts act as eastern

graben-bounding

fault

Down-thrown area hosting volcanic peak

(star M) and shallow lake (surrounded by

very recent flat-bedded sediments).

Suggests crustal thinning.

Surigao Peninsula Geology

M


27

Northeasterly view from top of volcano M

(Maniayao) with eastern graben bounding

fault indicated


28

Surigao is nice, friendly place … for the most part!


29

In east, normal arc andesites, dacites and

rhyolites (ADRs).

In west, lots of adakites.

Perfect opportunity to test:

1 Relationship between adakites and ADRs

2 Relationship within the suite of adakites

3 Origin of an adakite suite


301 Relationship between

adakites and ADRs

All trace element ratios very

similar except that Y is depleted

in adakites. Also resemble

typical arc magmas.

Suggests similar sources and

processes of formation in

source for adakites and ADRs.

ADRs formed by normal arc

processes (slides 3 and 14). Y

depletion of adakites is main

difference to ADRs.


312 Relationship within the

suite of adakites

Significant contrast in behavior

of Y between adakites and

ADRs.

In adakites Y shows strong

negative correlation with SiO2.

Consistent with Y depletion

during differentiation, possibly

by fractional crystallisation of

amphibole or garnet. How can

we determine which is

involved?

Adakites (west): open circlesADRs (east): black circles


32

aa

La Dy Yb

DREE

DLa

<DDy

DDy

<DYb

Garnet

DLa

<DYb

Fractionation of middle (e.g. Dy) from heavy (e.g. Yb) REE is

different for removal of amphibole as opposed to garnet from

a melt.

Davidson et al. (2007)


33

aa

DLa

<DDy

DDy

>DYb

Amphibole

DLa

<DYb



a melt.



34

aa

Differentiation

LaYb

DyYbAmphibole

Amphibole

Garnet Garnet



a melt.



352 Relationship within the

suite of adakites

Adakite suite shows positive

correlation of Dy/Yb with SiO2.

This is consistent with

fractional crystallisation of

garnet from mafic melt.

ADRs are more consistent with

fractional crystallisation of low-

pressure crystal assemblage

(amphibole + plagioclase).


363 Origin of an adakite suite

If Surigao adakites are slab

melts then should have isotope

composition of Philippine Sea

Plate

ADRs and adakites

indistinguishable in isotope

ratios. Both different to PSP.

Suggests similar sources for

adakites and ADRs in

metasomatised mantle wedge

peridotite.


37OK – Let’s Recap 3

The geochemistry of Surigao adakites is very similar to many other

adakitic suites.

Adakitic rocks are largely indistinguishable from near-contemporaneous

ADRs. Trace element ratios and isotope ratios point to similar sources in

mantle wedge metasomatised by the same type of slab fluids found in

most subduction zones.

The simplest explanation for Surigao adakites is that they are the

produced by differentiation of hydrous basaltic magma at sufficient

pressure to include garnet in the fractionating assemblage.

In Surigao the crust is estimated to be ~ 25km thick while garnet require

pressures equivalent to ~33km or more. Differentiation must be sub-

Moho.


38Implications

On slide 17, I said the presence of adakites in subduction zones with old

slabs could be interpreted in one of two ways:

1. The slab melting model is wrong.

2. Slabs can reach fusion point through other ways than just being

young.

In 1993 the Surigao was interpreted as an “expectional” case of slab

melting.

The more thorough study outlined in slides 22 to 37 suggests that

Surigao adakites can be produced without slab melting.

Is Surigao an oddity, or are there other examples where high pressure

differentiation of basaltic arc magma can be invoked?


39Ecuador

Suites of different ages that appear to

be derived from similar parents but

differentiate at different depth.

Chiarardia et al (2004, Mineralium

Deposita 39, 204-222)


40Chile

Longaivi volcano. Early granet-

dominated phase (mafic enclaves)

followed by lower pressure

differentiation.

Rodriguiz et al (2007, Journal of

Petrology 48, 2033-2061)


41Other non-Slab-Melt Models

Differentiation:

Castillo et al. (1999, CMP) Camiguin Island, Philippines.

Garrison & Davidson (2003, Geology) Nothern Volcanic Zone, Andes

Prouteau & Scaillet (2003, J. Pet) Pinatubo 1991 dacite

Very low-degree partial melting of hydrous peridotite

Eiler et al. (2007, G-cubed)


42

Some things to discuss - 2

Does the slab melt?

What conditions would be required for those melts to reach the surface?

How can the deep differentiation model be reconciled with other

“classic” adakite occurrences?

What are the implications of the deep differentiation model for the

adakite - TTG analogue?

ehaz convergent margins class adakites: 10 april 2008 1 constraints on adakite existence colin...

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