SHRIMP U Pb dating of zircons in the sedimentary rocks

from the Akiyoshi and Suo zones,Southwest Japan

Yukiyasu TSUTSUMI,Kazumi YOKOYAMA ,Kentaro TERADA

and Yuji SANO

Department of Earth and Planetary Sciences,Graduate school of Science,

Hiroshima University,Higashi Hiroshima,Hiroshima,739 8526,Japan

National Science Museum,Department of Geology,

Shinjuku ku,Tokyo 169 0073,Japan

Radiometric ages of zircons in psammitic rocks from the Nishiki and Tsuno groups were determined from

U/ Pb ratio and isotopic compositions of Pb which are analyzed by Sensitive High Resolution Ion

Microprobe(SHRIMP II).Detrital zircons in the Nishiki sandstones show a continuous distribution from

320 Ma to 240 Ma,which is similar to those from the Tsuno psammitic schists;about 380 220 Ma.The

youngest zircons from the Nishiki and Tsuno groups are 238.7±5.9 Ma and 218.6±11.4,respectively.It

was estimated as upper limits of the depositional ages.

Detrital zircon age in the psammitic rocks shows continuous variation from 380 Ma to 220 Ma,whereas

K Ar age of white mica corresponding to metamorphic age is around 220 Ma in both the groups.Hence,

it is realistic to conclude that the youngest age is close to the depositional age and clastic rocks in the Tsuno

Group is Triassic in origin.Probability distributions for zircon ages in psammitic rocks from both groups,

calculated by a multi peak Gauss fitting,indicate that provenances for the clastic materials in both the

groups overlap widely and are similar to those in the Mino and Hida belts.

Acidic tuff in the Nishiki group occurs in a hemi pelagic unit different from the sandstone of trench

fill sediment.Zircons in the tuffs show apparently two distinctive U Pb ages;about 2700 2400 Ma and

320 260 Ma.The former is considered to be accidental materials derived from Archean basement which

will be a part of the Sino Korean craton with 2700 2500 Ma.Least squares fitting for the latter younger

cluster yields a concordia intercept age of 270.3±8.5 Ma(2σ)which probably represents a depositional age

for the tuff.

Introduction

Western part of the Japanese Islands has four major

metamorphic terranes:from north to south,the Hida,

Sangun,Ryoke and Sanbagawa belts(Fig.1).These

belts and associated non metamorphosed accretionary

terranes run nearly parallel to the Nankai trough where

the Phillipine Sea Plate underthrusts with a low dip

angle.The Hida and Ryoke belts belong to low to

medium pressure type metamorphic belts,whereas the

other two belts to high pressure type.There have been

many debates about the depositional ages of these

metamorphics.The Ryoke belt is considered to be the

metamorphic equivalent of the Mino Tanba belt which

is an accretional complex with abundant microfossils of

the Jurassic.On the other hand,there is no reliable

data of depositional ages for the other two high pressure

metamorphic belts.The Sangun belt is subdivided by

the K Ar ages into three zones:the Renge zone;around

300 Ma,Suo zone;220 Ma and Chizu zone;around 180

Ma(Fig.1;Nishimura et al.,1989).The Suo zone is

associated with non or weakly metamorphosed ac-

cretional complex and huge limestone blocks,Akiyoshi

zone,where mega and micro fossils have been studied

in detail.

In this paper,we studied acidic tuff and sandstone

in the Nishiki group,a part of the Akiyoshi zone,and

psammitic schist in Tsuno group,a part of the Suo zone.

Journal of Mineralogical and Petrological Sciences,Volume 95,page 216 227,2000

Y.Tsutsumi，堤 之恭，ytsutsu＠hiroshima u.ac.jp Corre-sponding author

K.Yokoyama，横山一己，yokoyama＠kahaku.go.jp

K.Terada，寺田健太郎，terada＠sci.hiroshima u.ac.jp

Y.Sano，佐野有司，ysano＠hiroshima u.ac.jp

The acidic tuffs collected for this study are close to the

locality where radiolarian fossils of the latest Middle

Permian to earliest Late Permian were reported(Ni-

shimura et al.,1989).Both the Nishiki and Tsuno

groups had undergone a single metamorphic episode

with white mica showing K Ar age around 220 Ma

(Nishimura et al.,1989).Fossil in the Tsuno group

was not been found due to strong metamorphic recrystal-

lization.We separated zircons from the samples,and

measured U Pb ages by SHRIMP II installed at Hiro-

shima University.As U Pb system of zircons remain

closed under low to medium grade metamorphism,

(Cherniak et al.,1991;Sano et al.,1999)the age of the

Nishiki and Tsuno groups by their U Pb are not

disturbed.Measurement of zircons in acidic tuffs are to

decipher the eruption and/or upper limit of depositional

age.On the other hand,measurement of zircons in

psammitic rocks is to presume upper limit of dep-

ositional ages.It will permit to compare the prove-

nance of sediments in between the Nishiki and the

Tsuno groups.

Geological setting

In the Chugoku district,Southwest Japan,the Sangun

metamorphic rocks are associated commonly with non

metamorphic or very low grade metamorphic rocks of

the Permian accretionary complexes.The boundary

between them is a fault or thrust.This relationship

exists also in the rocks of the eastern part of Yamaguchi

Prefecture where the Nishiki group and Tsuno group are

widely distributed.As shown in Figure 2,the Nishiki

group is in contact with the Tsuno group in the south by

a low angle thrust dipping to the north(Nishimura and

Nureki,1966;Nishimura,1971).The Nishiki group

consists mainly of volcanoclastic sandstone,mudstone,

acidic tuff and hemi pelagic siliceous rocks.Although

there is no clear schistose texture,they contain locally,

near the contact with Sangun metamorphics,prehnite

and pumpellyite.The occurrences of these minerals are

explained by the regional low grade metamorphism

corresponding to the prehnite pupellyite facies(Ni-

shimura,1971).

The Nishiki group is divided into two units,A and

B.Unit A is subdivided into lower,middle and upper

parts.The lower part is composed mainly of siliceous

rock and fine acidic tuff,and sediments of pelasic

characters.The middle part is composed of an alternat-

ing bed of terrigeneous shale and acidic tuff which are

interpreted as hemi pelagic sediments.The upper part

consists of trench fill sediments such as terrigeneous

clastics,sandstone,mudstone,turbidite and acidic tuff.

Therefore,Unit A is inferred to be an ancient subduc-

tion complex.In contrast,Unit B is composed of

sandstone and mudstone,and is considered to have

deposited at middle fan setting(Hara and Kiminami,

1989).The latest Middle Permian to earliest Late

Permian radiolarian fossils such as Folicucullus scholas-

ticus and Folicucullus monacanthus were found from

acidic tuffs and pelitic rocks(Naka and Ishiga,1985;

SHRIMP U-Pb dating of zircons

Figure 1.Generalized geological map of inner zone of Southwest Japan(after Nishimura et al.,1989).

Ishiga et al.,1986).Samples,98062403,NSK1 A,

NSK1 C and NSK2 collected for the present study is

located close to the point where radiolarian assemblages

were found(Nishimura et al.,1989).This age is 250±20 Ma,when we apply a geologic time scale of Harland

et al.(1990).The Nishiki Group contains only small

limestone bodies in contrast to huge bodies in members

of the Akiyoshi zone.However,the group is consid-

ered as a part of the Akiyoshi zone because of their

lithology and of the matrix age(radiolarian fossil age).

The Tsuno group is a part of the Sangun belt and

belongs to the Suo zone with K Ar white mica age

around 220 Ma(Nishimura et al.,1989).The group

experienced a regional metamorphism of the pumpel-

lyite actinolite to glaucophane schist facies.The

Tsuno group is composed mainly of well recrystallized

pelitic and green schists containing small amounts of

psammitic schist and serpentinite.No fossil has been

discovered in this group due to the extensive deforma-

tion and metamorphic recrystallization.Since the data

of K Ar white mica ages of the Nishiki and Tsuno

groups are concentrated in a range of 220±7 Ma,metamorphisms of the Nishiki and Tsuno groups are

considered to be contemporaneous(Nishimura et al.,

1989).

Samples and separation of zircons

Ten acidic tuff samples were collected from the Nishiki

group.Two sandstones(IW21 and IW36)from the

Nishiki group and two psammitic schists(IW18 and

IW19)from the Tsuno group were selected from the

collection of the National Science Museum.

All of the acidic tuff samples from the Nishiki

group belong to the middle part of unit A.They were

interpreted as hemi pelagic sediments(Hara and

Kiminami,1989).The acidic tuffs are light green and

very fine grained.Their volcanic glass texture are well

preserved.

Two sandstone samples of the group belong to the

upper part of unit A.They are interpreted as trench

fill sediments(Hara and Kiminami,1989).Chlorite

Yukiyasu Tsutsumi,Kazumi Yokoyama,Kentaro Terada and Yuji Sano

Figure 2.Geological map of the western part of Yamaguchi Prefecture(after Hara and Kiminami,1989).Sampling points of acidic

tuffs show only zircon bearing ones.NSK 1 A,NSK 1 C and NSK 2 were collected near the point of 98062403.

and sericite occur locally in the sandstones.Other

metamorphic minerals were scarcely observed under the

microscope.Schistose texture is not found in the sam-

ples.

Psammitic schists from the Tsuno group were

highly deformed to produce a well developed schistose

texture.Chlorite,muscovite and epidote are observed

in the samples.

Zircon grains are separated by following proce-

dures at the National Science Museum.At first,small

pieces of samples were crushed into fine fragments using

a stainless steel stamp mill.Sieved fractions of more

than 250μm were washed in running tap water to

remove fine dusty particles.Zircons are separated from

dried coarse grained fraction using methylene iodide

with specific gravity around 3.30.Finally,zircon grains

are picked up by hand under stereo microscope.

From only 5 of 10 acidic tuff samples,zircons were

obtained.Zircon is very rare,about 1 grain per 1 kg

sample in each acidic tuff.On the other hand,zircon is

abundant in sandstones,roughly 1 grain per 2 g sample.

Zircon in a psammitic schist from the Tsuno group is

also abundant,more than 100 grains in a separated

fraction,as well as that in a sandstone from the Nishiki

group.All of zircons found in the heavy fractions from

tuffs were used in the age analyses.We selected large

and clear grains,in the psammitic rocks,for radiometric

age determinations.

Experimental techniques

Selected zircon grains and several grains of standard

zircons QGNG,whose TIMS U/Pb age is 1850±2 Ma(2σ)(Sano et al.,1999;Sano et al.,2000),are cemented

in an epoxy resin and polished until their center are

exposed roughly on a flat surface.The surface is

polished with 1μm diamond paste and then coated by

carbon.Before the isotope analyses all the zircon

grains are examined under a back scattered electron

image(BEI)to make sure the absence of metamict state

and overgrowth.All the measured points for isotope

analyses were selected from the BEI photos.

After EPMA analysis,the surface was polished by

0.25μm diamond paste to remove the thin carbon

coating.They were then gold coated for SHRIMP

analysis.

U Pb measurements of zircons were carried by

using SHRIMP II installed at the Department of Earth

and Planetary Science,Faculty of Science of Hiroshima

University.As primary beam,we used mass filtered

O beam,whose intensity is about 3 to 4 nA when it

is focused to sputter an area of 30μm in diameter.In

order to clean up the sample surface from possible

contaminations,we rastered the surface for 2 minutes.

We measured 9 mass peaks:196 for Zr O,back-

ground at the mass numbers 204.1,Pb isotopic mass

number at 204,206,207,and 208, U at the number

238, ThO peak at the number 248 and 254 for

U O.

We adjusted the width of source and collector slits

to 100μm.Mass resolving power of 5800 at 1%peak

height was attained to completely separate Pb from

HfO with an adequate flat top.Under these

conditions about 70%transmission of secondary ions

and about 20 counts/sec/ppm Pb/1nA O sensitiv-

ity was achieved(Sano et al.,2000).

Calibration of the Pb/ U ratios in the sample

was carried out using standard zircons of QGNG by

following method.The Pb/ U ratio in the sample

were calibrated using an empirical relationship(Claoue

Long et al.,1995)as follows:

Pb/ U＝A×( Pb/ U) /( U O/ U)

where A,( Pb/ U) ,and( U O/ U)denote a constant,observed secondary Pb/ U and

U O/ U ratios,respectively.The constant A

was obtained by repeated measurements of QGNG

standard zircon.This quadratic relation is confirming

that relative sensitivity factors of secondary ions are

controlled by a fundamental physical process.

The uncertainly in the measurement of Pb/ U

ratio was dominated by the external reproducibly of

relation between Pb/ U and U O/ Uratios in measuring standard QGNG.It propagated to

the individual sample measurements as shown in the

above equation above.

Concentration of U in samples calibrated against a

standard zircon QGNG.

Concentration of Th is calibrated using U concen-

tration and ThO/UO ratio of samples.(Williams,

1998)

Subtraction of common Pb from measured Pb is

required to estimate the accurate age.In this study,a

measured Pb/ Pbratio was used for the correction

of common Pb whose isotopic composition were

assumed by a single stage model with U/ Pb＝8.8(Compston et al.,1984).

Results

Table 1 lists zircon data on U and Th concentration,

Pb/ Pb, Pb/ Pb, Pb/ Pb and U/ Pb

ratios,radiometric Pb U ages and Pb Pb

ages for the Nishiki group acidic tuff samples.Data for

SHRIMP U-Pb dating of zircons

the Nishiki group sandstones and the Tsuno group

psammitic schists are presented in Tables 2 and 3,respec-

tively.Sub numbers such as IW19.05.1 and IW19.05.2

in Table 3 indicate different pit positions in a single

grain.Zircons in acidic tuffs are small,clear,and have

sharp edges compare with zircons in psammitic rocks

(see Fig.3).Errors assigned to the isotopic and elemen-

tal ratios are one sigma,and those for the radiometric

ages are two sigma.

Nishiki group acidic tuff

Figure 4 shows a total Pb/U diagram of zircons for the

Nishiki group acidic tuff samples(Table 1).There are

apparently two distinctive ages:about 2700 2500 Ma

and 320 260 Ma(Fig.4).Older zircons are observed

only in a sample,98062403.Least squares fitting by

Isoplot/Ex(Ludwig,Berkeley Geochronology Center

Special Publication No.1)to younger cluster,except for

two samples excluded by F test,yields common Pb and

the concordia intercept age of 270.3±8.5 Ma(2σ).

Nishiki group sandstone

Figure 5 shows a Tera Wasserburg concordia diagram

of zircons for the Nishiki group sandstone samples

(Table 2).Zircon age ranges from about 320 to 240 Ma

with apparently continuous distribution on the diagram.

Ages older than these two limits were not obtained in

the selected zircons in the Nishiki group sandstones.

Tsuno group psammitic schist

Figure 6 shows a Tera Wasserburg concordia diagram

of zircons for the Tsuno group sandstones(Table 3).

Most of zircons have radiometric ages from about 380 to

220 Ma,slightly wider in range than those in the Nishiki

group sandstones.Two positions analyzed of the zir-

cons from IW19 show far older ages:1740 and 1310 Ma.

Discussion

Zircons from the Nishiki group acidic tuffs are bimodal

in age;about 2700 2500 Ma and 320 260 Ma(Table 1).

The older ones are clearly accidental materials derived

from Archean basement rocks,as the presence of Per-

mian radiolarians and younger zircons in the same tuffs

indicate.However,the older cluster gives a valuable

information about eruption site for the tuffs.The

Sino Korean massif were evolved mainly 2700 2500 Ma

(Jahn and Ernst,1990)and there is no other candidate

for the cluster in Far East Asia.Hence,it is considered

Table 1.U Pb analysis of zircons in

Yukiyasu Tsutsumi,Kazumi Yokoyama,Kentaro Terada and Yuji Sano

that the tuffs erupted in the Sino Korean massif and

they brought with the essential younger zircons,the

accidental older ones from wall or cap rock.Sano et

al.(2000)reported that zircons from clastic rocks in the

Mino and Hida belts indicate several geochronological

episodes.One of them indicates around 2550 Ma,and

the youngest episode is 350 220 Ma.Therefore,it is

probable that the accidental older zircons in Nishiki

tuffs have derived from the provenance similar to those

for detrital zircons in the Hida and Mino belts.

Least squares fitting to the younger cluster except

for two analysis yields an age of 270.3±8.5 Ma(2σ).There are two possibilities for the origin of the young

zircons in the tuffs:essential or accidental minerals.If

they were crystallized from a magma which produced

the acidic tuffs,the fitting age approximates a dep-

ositional age.Zircon with an age younger than 260 Ma

is not found in the acidic tuffs,whereas it is common in

the associated sandstones described below.Therefore,

we rather favor the assumption of the essential origin.

Radiolarian fossils were found from acidic tuffs close to

the sampling sites of the tuffs studied here,indicating the

latest Middle Permian to earliest Late Permian,i.e.

255±20 Ma as an absolute age(Harland et al.,1990).Taking account of the error range,the present estimate of

the depositional age is consistent with that from the

microfossils.

Except for two old zircons in Tsuno group,data of

zircon ages of psammitic rocks in the Nishiki and Tsuno

groups show almost continuous distribution.

Although it is not always possible to obtain the dep-

ositional age directly from the detrital zircons,the

youngest zircon data represent an upper limit of the

depositional age of the rock:the youngest Pb U

ages of Nishiki and Tsuno groups are 238.7±5.9 Ma and

218.6±11.4 Ma,respectively.As reported by Sano et

al.(2000),zircons with Permian to Triassic age are

common in the psammitic rocks in Mino and Hida belts,

probably indicating that volcanism was active during the

period in Far East Asia.Supposing at least,a part of

zircons produced by volcanism had always entered into

a sedimentary basin,the youngest age obtained here

should be close to the depositional age.The Nishiki

group was weakly suffered from the same metamorphism

as the Tsuno group,with a white mica K Ar age of

220±7 Ma(Nishimura et al.,1989).The age is near the

upper limit of zircon age of the Tsuno Group,suggesting

that sedimentation and metamorphism of the group

occurred within a Triassic and a time lag between them

was very short.

In addition to the upper limit of depositional age,

present data give us information about the provenance

of psammitic rocks.Probability distributions for zir-

con ages in both groups are calculated by a multi peak

Gauss fitting (Williams,1998).To estimate the

provenances,sorting out the detrital zircon ages into

several groups is important.Sediments are the“mix-

ture”of detrial zircons from various rock mass.But it

is impossible,in our case,to sort out the data using

histograms,because ages are too close each other.

Therefore,we try to apply the multi peak Gauss fitting

to the distribution curves.As shown in Figure 7,four

peaks are recognized in the distributions of both the

Nishiki and Tsuno groups.Although the number and

position of the peaks are arithmetical,it is noteworthy

that three peaks are consistent each other within a

margin of error(2σ).It probably indicates that the

provenances for the clastic materials in both the groups

overlap widely.

In spite of the presence of Archean zircons in acidic

tuffs,zircons older than 400 Ma are not observed in the

Nishiki group sandstone.It may be due to far less

abundance of such old zircon or to selection of zircon,

i.e.almost all grains in tuff picked up,in contrast to

selected coarse grained and clear zircons in sandstones.

Old zircon is rarely obtained in the psammitic schist in

the Tsuno group.Only two grains show ages of 1311

and 1741 Ma.One of them shows core age of 1311 Ma

and rim of 267 Ma(Table 3 and Figure 6).It is not

uncertain whether the overgrowth rim represent

Nishiki group acidic tuffsamples

SHRIMP U-Pb dating of zircons

Table 2.U Pb analysis of zircons in Nishiki group sandstone samples

Yukiyasu Tsutsumi,Kazumi Yokoyama,Kentaro Terada and Yuji Sano

Table 3.U Pb analysis of zircons in Tsuno group sandstone samples

SHRIMP U-Pb dating of zircons

Table 3.(Continued)

Yukiyasu Tsutsumi,Kazumi Yokoyama,Kentaro Terada and Yuji Sano

metamorphic or igneous in origin.

Conclusion

On the basis of the analysis of zircons in the Nishiki and

Tsuno groups,the following conclusion is obtained.

1.The depositional age of hemi pelagic sediments of

the Nishiki group is about 270 Ma,while upper

limit age of about 240 Ma in clastic rocks which are

considered trench fill sediments.

2.The provenance for clastics of the sandstone in

Nishiki group is similar to that for psammitic schists

in Tsuno group.

3.The depositional age of psammitic schist in the

Tsuno group is Traiassic,about 220 Ma,close to the

metamorphic age,but is younger than those of

Nishiki tuff and sandstone.

4.Acidic tuffs contain accidental old zircons with

2700 2500 Ma which were derived from the Sino

Korean massif.

5.As far as younger zircons are concerned,zircon ages

in psammitic rocks from the Nishiki and Tsuno

groups are roughly similar to those in clastics of the

Hida and Mino belts.

Acknowledgements

We wish to express our sincere thanks to Mrs.M.

Shigeoka and Dr.S.Kiyokawa of the National Science

Museum for their help in sample preparations and

EPMA analysis.This is contribution No.9 of the

Hiroshima SHRIMP laboratory.

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SHRIMP U-Pb dating of zircons

Figure 4.Total Pb/U diagram of zircons in the Nishiki group acidic tuffbased on 19 analysis.Least square fitting to filled characters

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Manuscript received;19 July,2000

Manuscript accepted;4 December,2000

Figure 7.Probability distribution diagram of ages of zircon in

psammitic rocks from the Nishiki and Tsuno groups.Bold

broken lines indicate multi peak Gauss fitting calculated from

the distributions.Three peaks are consistent each other

within a margin of error(2σ).

SHRIMP U-Pb dating of zircons