Nagoya University
NagoyaUniversity

Lu-Hf dating of eclogite from the Sanbagawa belt,JapanWALLIS, S.R.
Ndgaya Uitiversity,.ltipan
The results

phengiteAr andfbrthe Sanbagawa metamorphic
offission-track
datinghave establishedzirconbeltfrom around

4500C to 250eC duringthe periodbetween
has been much

85 Ma to 65 Ma. However, the timing of the peak of metamorphism
to many
problem common
importantto relate metamorphism
elusive;aprocesses,Determining
belts.Garnetisa common

orogenic
orogenic

terranes.The peak age

metamorphic

to large-scale
tectonic eventsandmoreis

efmetamorphism

to constrain

rates

of

this ageis,therefbre, an importantgoal in studies ofmineralinmediumto high-grade
metamorphic

rocks

andfeatures
make
preservesprogradechemical zoning. [[Ihese
garneta suitable target
for dating projectsdesignedto establish an older limit on the age of peakcommonlymineral

Sm-Nd dating
isa widely

metamorphism.

Sm-Nd datingdoes notcases

historya coolingtechnique. However, in manygarnet-datingisochrons.
One of the main reasons fbr
produce well-defined
appliedthisisthe common
canwithin
the garnetsuch as epidote and sphene that
presenceof inclusions
high amountsof both Sm and Nd and which
can dominate their budget.
Thiscontainproblem iscompounded
the amountofby the very

spreadsimilarbehaviourof Sm and Nd which

chemicallikely to develop in SmfNd

ratios.

Despite several

resnicts

Sm-Nd

attempts,

datinghas failedto givea clear age fbrgarnetgrowth in the Sanbagawabelt.
Lu-Hf datingisan alternativeand more
effective
methodto datingeclogite. The
particularlywell-suitedelements

Lu

gamet

resultingassociatedandHf have contrasting chemical

behaviourwith Lu strongly

in a wideratios

spreadof

LufHf

Applicationof this methoderrors.

andintopartitioned
high-qualityisochronswith low

to Sanbagawa rocks

age
giveshigh-precision

determination
fbreclogite fbrrnationand fi11s
a major gap inunderstanding
the Cretaceouseast Asian convergent

margin.

ProgradeP-T pathsof the subduction-typesuggest the metamorphism
spreadingofthe ridgeridge

andSanbagawa beltand therrnalmodeling

represents unusually warm

thatthese conditions

at the convergent

margin.

werewith

respect

due to the approachstudiesof afew million yearsbefbre
the arrival
kinematicand radiometric age datafrom

attained

In addition,conditions
athe Sanbagawa belt
suggest the associated platemovement
componentof the teconticsof

vector had a largesinistralobliqueto the CretaceousEast Asian convergent

margin.

A re-examination

of-19NII-Electronic Library ServiceNagoya University
NagoyaUniversity

platereconstructions

fbrthe Cretaceousto Tertiaryfbrthis region

that the onlyshows

candidatewith such motion isthe Izanagi
Plateplausible
platecompatibleplate.
reconstructions
implythatprogressively
younger sections ofthe Izanagiplatewere

beneaththe CretaceousEastAsian convergent

margin:This impliesthe approach

thatcontinued until the remainingof a spreadingridge

thisperiodisone

platebecamefusedto the Pacificplateand ceased to move
periodrepresents a major reorganization

ridgeapproach.
partofthe

independently
at 85 - 83 Ma. Thisof plates
and associatedinmovementchanges

inthe Pacificrealm and islikelyto be the age of large-scaleinteraction
between thevectors
ridgeofsubduetedandconvergent

margin.

thatthe peak metamorphism

Thermal modellingshouldinteraction,
Lu-Hfdatingofgarnet
excellent agreement
metamorphismof cmfyr.

withof P-T conditions

andP-T pathspredicts

be a few million yearsolder than the age of ridge
and omphacite from eclogite givesages of 88 - 89 Ma inthemodel prediction.
Combiningthisestimate forthe peak age of
publishedthermochronological results impliesvertical exhumation rates
withThis high rate ofexhumation

inthe Sanbagawa beltduringsubduction

canof

explainthe lackofa

significant

thermal overprint

the ridge,--20 NII-Electronic Library Service