Nagoya University
NagoyaUniversity

What

for metamorphic

is essential
without

40Ar

excesslithology with

andArgon dating?

on

Tetsumaru Itayaiand ChitaroGouzu2･3iResearch institute ofIVlaturai Sciences. OkaFzima
roO-Oe05,
2

Uhivet:sity ofScience, 1-1 Ridai-chq

Okayama,JapanCipen Research CentenyOkayarna V)iiversityofStiience,]-1 Ridai"cho,Okayama

700"OO05,{lapan3

HiruzenihstituteforGeologyand (]hronologM161"1 Sai,Okayama 703'8248,

cJhpanIntroduction
K'Ar systemdating method

However, discordant K"Arrocks.

(UHPM) andmetamorphic

from the Dora Mairaandis powerfu1

Ar/Ar

and

age

in

relations(HPM) rocks

High'pressuremetamorphic

et aL, 1996; Arnaud(Scaillet

massifageseoo]ingofmetamorphic

ultrahigh-pTessure
arewidelyreported

Kelley, 1995),the Su-Lu

and

China (Liet al., 1994; Giorgiset al., 2000),the Kaghan valley

Dabie areas,etal.,([ibnarini

Pakistantool to study1993), theGourmaMali (Jahn et

aTea,

area,

the

2001),

al,,

Sesia'Lanzozone, Italy(Ruffbt
et al., 1995, 1997; Inger et aL, 1996), the TavsanliZone,
Turkey (Sherlock
and Arnaud, 1999),and the BeticZone, Spain (DeJong,2001). In each
K'Ar and ArlAr analysis,

itis hard to identify the existence

ifk is identified,itis also difficult
to subtract

and

even

40Ar

to get the
40Ar

radiogenic

andlike Dora Maira

excessmassifbecause we

40Ar

radiogenic
coAr

in analysis.

in westernof

distinguishatmosphericcannotcontinental

have experienced

apparent

in the conventionalages

the coolingasthe metamorphosedmaterials

metamorphic

that wasfbr the Ar isotopic system

inherited
gives older

dating.This type of rock is so hard to datehypothesisthat the rockoceanic

This hypothesisis based on

Sanbagawa

40Ar

40Ar,

age.

We have a worcking
suchK'Ar systemcrustcomplicate

histories,
have provided the discordantages due to the excessfrom the protolith (Fig.
1). Imperfect resettingin the rocks,

quantitatively it from the total

Metamorphosed

Alps, which

40Ar

excessmetamoxphic

1988; Itaya androcks,

materials

withhas noa simplesignificant

history

metamorphic
amountof excess

40Ar.

K"hr dating on phengites from theJapan (Isozaki
andItaya,1990; Itaya and Takasugi,

the experienceof

Fujino,1999; Miyashita and Itaya,2002) and from the Otago schists,

New Zealand (Nishimuraet aL, 2000). The pelagic sediments

arecomposedoffine'grained-6NII-Electronic
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RadiometricAges from the Dora Malra Ma$sif

elO

30

50

7e

90

110

130

t50

17e

190

21D

230

250 (Ma}

O10

3D

5g

70

90

"e

130

t5e

UO

ISO

21e

230

25e

a70

29a

31g

3SO

37e

39e

41e

430

45e

470

490

510

530

550

570

590

610

630

650 (Ma)

Fig.1.Radioxnetricages from Dora Maira massif
in western

330

Alps.

Otago Schlst frorathe ChrystatlsBeaeh -Brightoncoastal section
TZ llBTex.t,:.r.ed
TexturalZonellA
Zone MATZ
ll!B

i,iili,ll,i,i,iliiiii"i",l,ii,l"rr

mmkewv.evtt-v.twwual --"W -'aF]

lne'gs,raAkatere Fault/･l)･;･{wt....ll....t4{::･:#IX"'
l.l.ll.l.l･l･l･l･/,Rt:in':I･
i

;

iO

:

ellil/illtlili'i'S'i'g'i'IS),#･illl.I/ll,l'I/,l'{/'i,ii/li'{'x/},iii'ii"iiis'iNt'5ftm

lpm

Pmp-Chl Zone

Pmp-ActZgne

l"t･++ -i
i
I

?"

e-e

g3.eeS

:

l

l

$l

1

E

s

/

l
f
+
t

eee

Seee

e

"X,iitw

18e
,,,

g.
Nl160

ur}

slso

-";i "t` g
i

xl40

3,sg<

l
l
l

3.4
e
¢ B"1

ige

F

i,

/4

BtZ

if
I

d

1

srighton""

i

:

l

l

i

Ep-Act Zone

P
r
F
:

6

3,]ovc:
A

e/

IQ

5
2

3

4

5G

18

9MM

l
l
20

15
l2 13

l

14ts

Fig. 2. K"Ar ages

1517

ee

ee-e}

130

l

25

30bistanceCkm)

le l92o21 2g3 24252fi 27 2S2D

3t
3g32

for phengite separated
from pelitic and semipelitic
metamorphic
rocks
materials
Otago schist, South Isiand, New Zealand. XRD data of earboRaceousalso plotted (after
Nishimura et aL, 2000).

inare-7NII-Electronic Mbrary
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Zolsige-Clinezoisite
schest
OIA
fioE

@

fif

23e,li2e,tTt".46e'oo',e'ox,"'

4Umos
30d

deuaNN37.6± O.6Ma･pt

2oza<

eoo･ton2o.oo

...
tttt.
.. ttt
tt..t. .t.

60
50

e

ee

?7-bfilrl.Rll,so

9.3"o 3,!%ao

.de.41.2

3e@64tv.7S-.iR-'

16"-

co

30
20

ny-

20

±

O.4Ma

tUm.t39.3±

10

oO04O.02V,OQ

og,"2O,Olo.oek..meveffd@

O.5Ma,'-tttttmuutttmrmmm.whwwvetxutsin
38,7± 1.3Ma

2oi:iiiiLm==.scw.tgrllll

i

60
50

so

rtv

T.oe,/
15E･

53e"

m

20
10

le

oo,a4O.02ooo

or･
o6O.03o.oo

3e

3gb4nt

o,4

o6

e.a

a

1.o

o.e

Fractlonot 39Ar released

Garnet-Phengitesehist
OIBk'4f,6 ± O.4Ma

"

s

o.4

4e

:o'ibebeninamal

o.6

o.s

1e

D.o

37t'e

watk4

O.3Mabe±1
wh.6

32.2 ±O.6

41

t

co

DO.6oso.o60

u

(IE)

±

40

se

4e8 O.5±ne3npm3s.O±

O.3Mawww.

20

le

10

o

o06ouoeaoaO.2o.o80soroao2010etetffesfEiif!-rEmemim..si@

Ltm

oeu

g,ua
omo

0

T5"/eS,se..･

12%22g/.,MTO!;i4e:F

39,O± O.7Ma

i.!lik:e.4o.ses
Fractionof 3!Ar releaseci
o.2

Fig. 3. Ar/Arage

spectrametamorphic

rocks

60

60

so

50

se
so
20
10

4V

e

±:=in

a.o

1.o

.g

so

ijs

37.9 D,7

37hossa{Aligffw.N･

16?.S4%2?%V.OSEb5A:.S.6e,,i

so

aeic

4.t%

17!'o

p
tl.za'

ocr)
o.eEs.o-aNco@

60sc40so2010

1.o

gss%
"22Sg.7.o,7de37.3

ODO

o.s

21k
soqose2010

10or

si

os

03A
AedEvorenrdan<orptxts"
so

2.P.r./dvps"22%S3e,,.cr.'e5sc-

20a<
oes04Ermeeto.4

Piemontiteschist

@

02A
eoE
3"Q

o.o

e.2

Fractienof 39Ar released

4omas.o,O,2010

pm

401.

stsve

mcudn<encoxpten

r-/!uEots

lots

tw-･rmimmbwwww.aj-

Fractlonof 39Ar released

O S

e=spt

o.2

O"-

12e.

to

o.2

b'6Eb48-'bt'

za%

20

e.e

¢

::fi-llit.",.

7.fi'･e'
1 1.Abe

40
3e
2o

M

h

lt,

ao

46,4±O,7rS..

...2fi,

ffO

30

sg

50 8eelo20ttnt.1":o45,9±O,4
AO
42,3 ±O,8se

8.el3mu±

kaf:ium-ma-J
io-?s--o ,O,fxoOPr-･
coooo

I32.5

wa11.6,.olegg

e%-31Esr

sowo a"btgls
4oa
!?e,.
3oasri±O.52e<CL± 13,9.932,4
i4
eggap.

su
40
30
?o

iovO

g

03B
A6oasX

6e
fixvmmcuma<otcos.ptpt

so

pa

jr

2S%S.2%S,,loatlaL".･.7,fiam'n.StS

suthwtdite.

es-37.3
±

O.4Ma

to

oO04nn2o.oo':,::o,2o,o"""`"i'f'i'..

e.4

e,6

o,s

to

Fractionot 39Ar released

o.o

o.2

o.a

o.6

as

le

Fractionef a9Ar released

phengites in ultrahigh'pressure
from the Lago di Cignana area, WesteTn Alps (after
Gouzu et a].,and

37Arca/3QArK

ratio

of

2006a).

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zone,
the
HP - UHP metamerphism
in a subduction
When they undierwent
resultingin complete
resettingin K"Ar
fine'grainedmaterials
ean
be recrysta]lized,
concept
arereportedfromthesystematie
K"ArsysteTn.
Wel]tdocumentedexamples
ofthisSW Japan and the Otago schists, New
in the Sanbagawa sehists,
phengite ana]yses
40AT,
i.e.discordantages and/or
significantly
Zealand.In these areas, no sign of excessofolder ages, are found because the metamorphic
gradient, doeumented by K"Ar analyses
the phengites, are
2).
(Fig.

materials.

"continuous"

ArfAr dating in three differentkiltdsof meeamorphic
Reeently,weandultrahigh'pressure

our

have

colieague

metamorphic

rocks

carriedout

sequences

AxtAr datingfrom the Lago di Cignana area,(GQuzu al., 2006a), eclogitic rocks in the Tso Morari Complex,etal., 2006),

We summarize

their results

fuomWestern

Alps

Himalaya, India

western(Gouzuet al., 2e06b) and BarToviantype roeks in the Danba area,rnicasofeasternetTibet(Itaya

here.

@

Age
[III]
:

of matrix

ee Agetof inclusien

amp : anagyzed po$ition$
Fig. 4. ArLAr spot
area,

ages

in the garnet-phengite

ofphengites

schistfrom the Lago di Cignanaasophiolite.

Westem Alps (aftex
Gouzu et al,, 2006a).

Lago di Cignana

area

The Zermatt-Saas
the ArlAr analyses

zone

ofphengites

in western

Alps is regarded

from the eoesite-bearingan

We carriedoutpeliticschist, piemontite schist

Ca"richschist. The protolithsof lattertwo were fbrmed by hydrothermalactivity andin pelagic setting, respectively.
Phengites in the rocks from the Lago di Cignana area
andprovide 37"42 Ma plateau ages3) that have very
(Fig.

littlevariation

those from Dora Maira massif

andSm"Nd and Rb'Sr mineralareconsistentwithin comparison

with
ages

(38

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Nagoya University
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Amato et aL, 1999) of eclogites

garnet give ages

43.2 ± 1.1 Ma and

of

and

phengite-quarzites. Phengite inclusions in

44.4 ± 1.5Ma, whichare(36.4 1.4 Ma) from thephengites, andInclusion(43and 44 Ma) are consistent with a zircon
spot

age

±

matrixages

Rubattoet(Fig.5). These resultsal.,1998)

a simpleunderwent

the peaksubduction

hypethesis mentioned
retrograde
relatedcrystals.

reaction

abeve,

We

or
also

that

of

rocks

and

This has leadto the apparent

SHRIMP
40Ar-freeages

significantof thematrix(Fig.4).
(44 1 Ma:

age

±

that

materials

phengite
ourconfirm

phengites

matrixcontemporaneously

releasingthan thethe plateau ages

littleolder, These analyses
suggestolder

that the oceanic

form excess

their ehemistrychanging

to the exhumation

UHPM,

relatedisca. 44Ma

metamorphism

singgest

significantly

and

that

workingexperieneed

deformation

with

40AT

radiogenicaphengites that arefrom thesignificantly

younger than the inclusion age.

Cooling Historyof the tago diCignana area

3.0

2.e

1

.0

200

300

400

500

600

700

P-T path:Relnecke (l998)
Sm-Nd <Amato
et al.,1999>: Grt -Omp - GIn -Czo - Pg
Hb-Sr (Arnato
et al.,1999): Phn -whole rockFig.5. P-T"tpath of ultrahigh'pressure
Western A!ps,

metamorphic

rocksfrom the Lago di Cignana area,-10NII-Electronic Library Service

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Tso Morari Complex
The Tso Morari Complex in westernof themarginIndian continent,

is eomposedrock

lenses(eclogites
andbasic schists),

andquartzin omphacite.

rods

lensesintercalated withstrongly

schists

the mineralandmeeamorphism.
re}ations
eclogiteshow

isthought to be originallythegneissesand schists including matlic
Eelogiteshave coesite pseudomorph in garnet

They occur

the peliticschists.

lenses have been

Himalaya, whichofpe}itic

only

in the coresof metertscale

mafic

rock

Small inafic lensesand the rim parts of large

deformed to form the fbliationpaTallel to that of the pelitic
assembiages

of

upper

greenschist to amphibolitefaciesThe garnet'omphacite thermometry
andthe univariant
reactionfor jadeite
formationgive 13 - 21 kb at 600"C and 16 - 18 kb at 750bC forthefbrmation (Fig.
6). Phengites in peliticschists

showvariable

SYAI and

Na/K

<kba

25

20

fi59g8tsIO

53eo
Fig. 6. P'T conditions

4eo

ofeclogite

500
600'Tbmperature

700

in the Tso Morari Complex,western(℃ )Himalaya, India(after

Gouzu et aL, 2006b)-11NII-Electronic Library Service

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grainsamong

ratioswell

as withinyielded K'Ar ages

peliticschist
andas

85.3 Ma (K

=

of

using

initial

40Arf36Ar

singl'ephengite grain and

ratio

of

reaction,

retrograde

associated40Ar

the eclogitic

of

which

from the crystals.
toretrogressionanisochron age

inverse isochron ageofanalyses

and

their

these rocksof

The argon

ofphengites

130± 39 Ma withduringchemistrywiththe release

stage

of

have experienced

intense dieformationof significant

took plaee in the schists

releaseanparagonites (Fig,the timing ofearlygreenschist faciesmetamorphisms

upperin ArfAr step-heating

132 Maofinterpreted to represent

modified

50-87 Ma. A

91 ± 13 Ma from the twoof

The phengites in pelitic schists

rocks,

the exhumation

with

an

434±90 in ArfAr spot
areofwt %) for paragonite-phengite mixture(K=4.983.5 Ma

gives a plateau age

7b),The Cretaceous isochron ages
exhumation

grains, anct give K'Ar ages

7.8wt %) ferphengite anddataset (Fig.7a).The eclogiteana]yses

singleradiogenicthat experienced

from thetheeclogitefaciesO.05

O.07

conditions.

(x104)

a.4

O.O03

1.2
1.0k<oco

O,O02L<g:o<opO.8)<X

O.6
o.oort

O.4
O.2

o.ooo

o.o
O

O,5

t.O

2.0

1.5

40K/36Ar

2.5

O

O.Ol

O.03
39Ar140Ar(xl06)'
Fig. 7 K'Ar isochron age (a)calculated
usmg
two data sets, phengite rich fractiQnand'from peliticsehist and Ar/Ar inverseisochron age (b)phengiteparagQnite mixture
calculated
using
fourdata sets ofphengites
andiparagonites from eclogite in the Tso
MoraTi Complex, western
Hima laya,India (after
Gouzu et al., 2006b).

Panba area
The Danba
consists

mainlyareaofin the central

Sinian to [[Itiassic
metasediments.

Barrovian type metamorphism
sillimanite-K

part of Longmenshan

feldsparschists

orogenie

The wholewithprogressive changes,

andestimatedpeakbeltin Eastern Tibet
has undergone

regionfrom chlorite"gradeeonditionsofcaslates8 kbar and

into
700 ℃

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Nagoya University
NagoyaUniversity(CHIME)monazite(Huanget al., 20e3), Electron

mieroprobeami

U'Pb dating of apatite

SHRIMP

give compatibleal., 2003).

Laser step heatingArfAr ana}ysis

extTemebiotiteages

oldbiotite grains withsuggestargon

moderate

ofbiotitezone

et(Wallisandmuscovite

withmetamorphic

apparent

ages

excessofzone.

gives, in

We

age.

40Ar

We refer to this as a trapped

46 to 94Ma in the lower grade zone

ages

with

have trapped excess

130 to 197Ma

in the sillimanite

Biotite grains withwave'.

than three times the metamorcphic

more

derived from degassing ofmuscovite`excess

crystalsin the sillimanite

40Ma

of ca

ages

of single

age

temperatures lower than 600CC. The higher kyanite zone

metamorphic

particular,of ea.

higherthan 600 ℃ and discordantages from 46 to 197Ma in the zones

temperaturespeakcoolingages

65Ma that is the metamorphic

zone

consistent

granodiorite both from thein the metamorphosedsillimanite

yiekdeddating in the metapelitetrappedduring the metamorphism.
inheritedfrom the host ],ithologies

argon

Conclusiong
The oeeanic
phengites

materials

withexcessargon.

without

in agegradient

metamorphic

andasingleThe

less than 50e ℃ havemetamorphism

metamorphic

materials

temperature. The metamorphic

sillimanite

with

40Ar

trapped the excess

wave

which

releasedduring the exhumation

zone}ithologies
experieneed
giving the reliable

of

the

roeks,

metamorphie

4eAr

the excessthat waslessthan 600 ℃ andi thatin theThe metamorphic

sequence.

significant

fromoriginatedby the decompositionof muscovite

higher than 600 ℃ have no

cooling

have the continuous

sequencesprehistory,have the micas withduring t･hemetamorphism
inheritedfrom the hest lithologies
continental

thein phengites,

40Ar

exeessage.

Aeknowledgmentg
We thank Simon Wallis andBarrevian typehis colleagues

for their valuablesuggestions

on

recks

in the Danba

area,

eastern

Tibet. The ArfAr geochronology af the

BaTrovian type rocks

in the Danba

area,

eastern

Tibet will be published in IslandArc

near

future.

References
Arnaud,N,O,andmetamorphism

Kelly, S., 1995. Evidence fbr excess

Arduringin the Dora Maira (western
Alps,Italy),
using

a

high pressure

Laser
Ultra"Violet

AblationMicroprobe 40Ar'39Ar technique. thntn'b. MineraL PetroL 121, 1'11.
Amato, M., Johnson, M., Baumgartner, L and Beard, B., 1999.Rapid exhumation

of

the

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Zermatt-Saas

deduced from

ophiolite

high'precision

Sm'Nd

Rb'Sr

and

geochronology, Ehrth Planet.Sbi.Lett.171, 425"438.

De Jong, K, F6raud, G.,Ruffet,G.,Amouric, M. and Wijbrans,J.R.,2001. Excess argon
incorporation in phengite

Complex: submicroscopic

the Mulhac6nof

fluidingress during late Miocene extension

andillitizationin the BetieZone, south'eastern

Spain. Chem. GeoL l78, 159'195.
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De Sigoyer, J., Guillot, S., Lardeaux, J.M. andin the Tso Morari dome (eastern
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ultrahigh'pressure

phengite infrom the Lago di Cignana area,rocks

Western

Alps. Lithos,92,418'430.

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K-Ar

dating of eclogitic

40Arf39Ar

and

Himalaya, India. Gondwana
Giorgis,D, Cosca, M. andin UHP

eclogite

ablationanalysis.

Ahmad, T.,2006b. Cretaceousisochronages

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Li, S,,2000. Distribution
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Wl, Cliff R.A.,Rex, D.C., 1996. Metamorphic

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Itaya, T. andand

Takasugi,H., 1988, Muscovite K-Ar ages

argon

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