Studies of melt inclusions in some basalts from eastern China

rot. 5N0. ~ G E O C N E M I S T R Y i086

Studies of Melt Inclusions in Some Basalts

from Eastern China

Ltu Y~NGJON (~J~:(~), L~ ZHAOLtN ( ~ $ 6 ~ ) , WU Qizm ( ~ ) ,

ZHAO MEIFANG ( ~ ) , AND JIANG HAOSHEN ( ~ ' ~ )

(Department of Geology, Nan]ing University)

Abstract

Cenozoic basaIts widespread in eastern China constitute an important sector of the circum- Pacific Cenozoic basalt belt. Basalt samp,res were collected from Wudalianchi (HeiIongjiang Pro- vince), Nushan (Anhui Province), Fangshan (Jiangsu Province), Zhuji (Zhejiang Province), and Mingxi(Fujian Province). These basalts, for the most part, belong to the alkali basaltic series, and :partly to thoIeiites.

A variety of inclusions commonIy occurs in the rock-forming minerals of these basalts. The physicochemical conditions of basalt formation in different areas have been reviewed in special re- ference to the inclusion data. Our studies have shown that there is a close relationship between the features of the inclusions and the physicochemica~ conditions of basalt formation, which can, therefore, b: regarded as a guide to the mechanism of basaltic petrogenesis. The results of research in this aspect are presented in thc present paper.

Introduction

Source materials of basa.lts, which are considered to be closely related to the chemistry of the deep crust and t~p,per mantle, have attracted increasing attention of many geologists. Cenozoic basalts widespread in ea~slern China constitute an important sector of the eiremn-Pac~fie Cen'ozoic basaltic belt '~'~. In this paper the physie,oehemical conditions under which the basalts were Nr~ned. in different localities have been reviewed on the basis of melt inclusion studies. I~epresentative basalts were collected for this purpose from Wudeti,anchi (tteil, ongjia~g Pro~fince), Nush.an, Jiashan County (An'hui), Fangshan, Liuhe County (Ji,angsu), Zhuji (Zhejiang) and Mingxi (Fujian) (Fig. 1). Most of the basalts belong ~o. the alkali series and only a small p,orti.on to the tholeiite series.

General Geologic Setting

The Wt~dalia.nchi basalts in Hei[orLgjiang Pro,Ance c,onstitute one of the well ka, own Cenozoic volcanic clusters in China. Frequent volcanic activities have proceeded since Quaternary time with the last eruptions registered in 1719 and 1720--1721 A. D. Con- trolled by NE- and, to a lesser extont, NW-striking faults in the basement rocks, alkali- rich magma erupted through vents at the intersection of ~lle two se,ts of faults, resulting in eruptions elf the central a~d fissure types as well as a combination of the two.

The Nush~m basal0s constitute a f~amo~us ancient volcano on the Huaihe plain, 40 km

No. 2 G E O C H E M I S T R Y 109

f _.

, . . : .. ) I Harb n / lll~'%

-</, li .(I

Beij ing

W.h~n ~.- ) ..!°

Nane hang

¢'"'-'z f ,I r * % ' ~ . . . .

• . / , ( d ~ H a i n a n Island

Fig. l , Sketch map of eastern China, showing the geographic distribution of Cenozoic basalts studied in this paper.

northeast of Jiashan County, Anhui Province, and their distributi,on is controlled by the Taneheng-Lujiang fault zone. Abmldant xenoliths of great varieties, v~hich are composed of olivine and pyroxene, were found in b.asalts o.f Pleistocene age.

The Fangshan b,asMts are distr~bated 17 km so.uth@ast of Li.~he County, Jiangsu. They erupted at Plic~cene time, forming a volcanic cone w~h a ring-fracture structure. Alkaline olivine-b,asalts are predominant, and pyroxene and olivine inclusions are pres- e];t.

B, estrieted by NNE-NWW faults, the Z h u j i basalts, consisting mostly o.f black olivine basalt, occur in central Zhejiang. They are stratigraphicMly correlated with the Teriiary-Plioeene Shengxian Formation.

Loc

alit

y

Wud

alia

nchi

,

Hei

long

jian

g

Sam

ple

Tab

le 1

. C

bem

|eai

e6m

posi

t|en

t of

the

sel

ecte

d ba

salt

s fr

om e

a,te

en C

hiaa

1.

Poro

us

alka

li

basa

lt

2.

Com

pact

alk

ali

basa

lt

SIC

), 1

ic),

A

I .... O

~ Fe

,()~

Fe

O

MnO

M

g(3

C

aO

Na2

0 K

,()

P,O

, C

r~O

3 N

iO .

C

O 2

i54.

0~

2.35

13

.79

1.22

o.

58

~J.

ll

5.69

5.

73

3.55

5.

21

0.99

9.78

! 2.

47

12.6

8 4.

22

5.22

0

.i4

6.

95

7.75

3.

71

4.88

1.

08

3.

Ear

ly-s

tage

alk

ali

basa

lt*

52.4

2 2.

02

14.3

4 4.

06

4.15

0.

1 6.

37

5.9

i 3.

81

5.2~

4.M

idd

le-s

tag

ealk

alib

asah

*5

3.1

8

2.16

14

.55

.~.8

4

5.I

6

0.1

6 42

5.

69

4.08

4.

94

5.

Lat

e-st

age

alka

li

basa

lt*

51.4

6 2.

6 13

.51

2.64

6.

12

0.17

6.

8 6.

36

4.18

5.

91

HaO

÷ L

oss

on T

otal

ig

niti

on

0.63

99

.88

0.64

99

.52

98.4

4

99.1

2

99.7

5

6.

Nus

han

basa

lt,

Anh

ui

48.1

1 1.

37

9.45

4.

08

4.78

0.

14

12.8

3 16

.22

0.9

0.04

0.

13

1.99

98

.05

Anh

ui-a

-ian

gsu

7.

Bas

anit

e, N

usha

n, A

nhui

42.

87

2.00

14

.63

7.55

5.

37

0.15

8.

28

8.25

4.

84

1.03

1.

34

0.02

3.

103

0.11

3.

42

8.

Alk

alin

e ol

ivin

e-ba

salt

, L

ower

F

angs

han,

Jia

ngsu

9.

Alk

alin

e ol

ivin

e-ba

salt

, U

pper

Fa

ngsh

an~

J'ia

ngsu

10.

Alk

alin

e o

liv

ine-

bas

alt,

U

pper

F

angs

han,

Jia

ngsu

11.

Alk

alin

e ol

ivin

ebas

alt,

L

ower

F

angs

han,

Jia

ng

nln

g,

Jian

gsu

12.

Alk

alin

e ol

ivin

e ba

salt

U

pper

F

angs

han,

Jia

ng

nin

g,

Jian

gsu

47.1

0 2.

33

14.9

3 1.

91

8.68

0.

15

9.78

7.

04

4.76

2.

24

0.53

0.

0380

.043

1.

01

48.9

9 2.

39

17.3

9 13

.00

0.54

0.

12

3.85

6.

20

3.40

1.

03

0.42

0.

0370

.028

0.

11

1.86

51.4

2 1.

97

15.0

9 6.

72

4.06

0.

14

5.63

8.

23

3.74

1.

28

0.35

0.

0310

.026

0.

10

1.13

47.3

2 2.

27

15.0

3 8.

15

3.60

0.

137

5.78

9.

05

4.07

1.

13

0.75

0

.03

1).

02

3

0.02

3 2.

85

i 15.82

l

1

99.9

{

99.9

4

99

.3;

99.9

~

100.

15

©

c'~

o e,a

3.

Bas

alt,

W

uxie

-Mei

chij

ie ~

di

stri

ct,

Zbu

ji,

Zhe

jian

g 0.

92

4.

Bas

alt,

X

iken

g-S

imin

g [

dist

rict

, Z

huji

, Z

heji

ang

t9"4

211

0. 1

2

5.

Zhuji

,Bas

alt' Zh

ejiang

QUan

ce dist

rict

, 19

.42'

0.9

4

Z, hej

iang

-Fuj

ian

6.

Fel

dspa

thoi

d-be

arln

g 50

.47

0.86

ba

salt

, Ji

nyun

, Z

heji

an8

7.

Fel

dspa

thoi

d-be

arin

g i

basa

lt,

Jiny

un,

Zhe

jian

g t9

.45

0.61

J

8.

Bas

alt,

Z

heji

ang

50.5

8;

2.51

9.

Bas

alt,

F

ujia

n 52

.781

1.

3:2

~ver

age

47. l

'

4.3

!0.

Alk

alin

e ol

ivin

e-ba

salt

**

2.7

13.5

!1.

Oce

anic

th

olei

ite*

* ~9

.3

I 1.

8 t5

.2

I 2.

4

!2.

Con

tine

ntal

th

olei

ite*

* 50

.7

]

.~3.

Bas

alts

in

eas

tern

Chi

na

10.4

2

10.3

5

8.28

6.65

8.3

8.0

L29

7.

07

L33

I

8.63

!.96

i

8.75

).36

11

4.9

118.

89

..

..

1.

17

14.8

7 18

.96

1.15

7.

50

8.89

I

).14

I

6.93

I

9.01

I

).I7

I

7.0

I 9.

0

).17

I

8.3

* L

iu

Zhe

nhua

n,

1979

.

:* D

. W

. H

ydm

an,

1972

.

].72

2.

26

--

j

!.11

]0

.77

I

~.6

6

i .

).41

0.35

).50

0.

06

!.80

1.

06

LT

0 0

.46

2.6

1.0

3.6

2.44

,.28

.38

.30

.39

.17

1.41

b.21

w ).67

3.96

....

2.9

---~-

I

3.38

.18

1.33

.02

2

0.03

1 0.

30

1.07

I

~.03

4 0.

023

0.17

1.

07

02.9

5

O1.

26

02.9

6

01.4

3

)9.9

0

}9.8

3

)9.8

6

< ) 0

112 GEOOHEMISTt~Y Vol. 5

0

Z + ©

6 ,7

4

. / f

1" 2 /

6o

o 8 A . ~

1~ t . o . ' t " ~ A X _ ~ /

9 /~13 / / H i g h - a l u m i n a b a s a l t

) / ~ 1 9

@ 2 °4 R

A l k a l i ba sa l t ~ ~ ' ( ~ ~

T h o l e i i t e

o l6 o17

45 55 65 75

Si20 wt. (%)

Fig. 2. Variation diagram of K20 + Na20 vs. SiO2 (after Kuno, 1966t3]).

1--19. Basalts from eastern China (1--5 from Wudalianchi; 6--12 from Jiangsu and Anhui~ 13--19 from Zhejiang and Fujian); the dashed lines refer to the rock series boundaries; the average compositions of tavas from North America indicated by B (basalt), BA (basaltic andesite),

A (andeslte), D (dacite), and R (rhyolite).

Located in western Fujian, the Mingxi basalts, Pliaeene in age, are generally slrb- alkaline. Their distribution is controlled by NEE-, EW- and NNE-striking faults.

Petrochemistry

The ~bove studied basalts are compositionally similar t,o the average basalts thro.u- gh.out the world as shown in Table 1. However, the data availa~ble in the literature show that the basalts in eastern C'hina are characterized by low FeO and high K contents. According to Peacock's classification a~d ,by using Rittmann index ~sl, the Wu- da]i.a'nehi basalts should ,'be designated to the .alkaline series; ~he Nusl~an basalts ,a~,d Fangsh,an basa.lts to the alkali-caleie ov calc-alka]ine series, with a small p~ortion being alkaline ~r calcic; and the Ztrt~ji and Mingxi ~basalts are ealc-~alka~ine and calcie (Fig. 2). As can be seen, the b,asalts of Heilongjiang have an apparently higher alkali index th.an those of Jiangsu, Anhui, Zhejiang and Fujian.

As irrdica*ed by geological a~.d geophysical data, the K,O content af volcanic l~v~s beats a definite rel.ativn to the depth of m,agma source in deep recto.hie zones. General- ly, K,O increases with the depth o.f magma source, indicating a tnead o~f the increase of alkali c omp,o~ems towards deep povtio~s 'beneath the crust. I t is, tlmrefore, obvious from Table 1 that the Wudalianchi basalts must have originated from a deep source re- lative to the other b~salts. According to Zhou Xinmin et .al. (1978) tl'~, the depth o~ basaltic magma increases frown east to west in noTtb Jian..~:u, accompanied by a parallel increase in alkali contents of the rocks.

The consolid,~tien Ludices range frena 25 to 27 for the WuSalianehi ~b,asa]ts, 17 to 36 for the Nushan and Fangshan basalts, and 19 to 33 for the Zhuji and Mi~gxi basa]ts.

No. o GEOCHEMISTRY 113

Thi.~ indieates th,at the conso]ida.tion index tends to decrease as a result of the decrease of Mg due to the incveasir~g influence of fractio.n,ation, cons~li~atio.n, and contamination by the surrounding rocks when the ma~o~n,a came from a deeper sqm'ee ,a~d traveled over a longer distance before eruption. On the contrary, high consolidation indices are e~pect- ed for a shallow magma source. In general, a re~a£ively r~stvicted variation range e~n always be nloticed with respect to p,etrochemistry and consoli,d'atio'n index for basaits within the same area, s~ges t ing , to some extent, the eom,a,gm, atic n~atur4e of these r~cks.

Characteristics of Inclusions in Minerals

The basalts studied in this paper commonly ean~ain a variety of inclusions. These ine~u.sions e~n be divided into the following types E4-gJ according toi ~heir phase asse~nb- ]age, phase ratio and c omp.osition:

(Crystal l ine solid inclusions CFe, Asi* Solid

inclusi°nS[AmorphousJ solid inclusions Asi

CF¢ + Csi

Crystall ine melt inclusions~C -}- A -t- G

[C -}- A -+- nG Melt inclusions~

One phase Asi, AF¢

• , . [Two or more phases A ~ - G , Amorphous melt mcmslons~ nA + G, A + nG

|

[Mixed phase Csi -b AFe

* G refers to gas phase; A to amorphous phase; and C to crystalline phase.

Solid inclusions ore co,mposed mainly of ibaenite, magnetite, and a~narphous glass, and commonly observed in rock-forming minerals such as olivine, pyroxene and feldspar

in basalts from all the areas studied.

Melt inclusions resulted from consolidation of the original magma trapped c~uring the crystallization o~ ho~t minerals in it. Two types, crystalline and am,orpkous, can be distinguished on the ~bas]s of .erystallin~ty.

1. Crystalline melt inclusions are found in the Nush.an and Zhuji bas~lts. In Nu- shun, stuch inclusi'ons, made up o,f crysSa;l+gas, crystal+glass+gas, ox' of more than one distinct crystalline ph,ases, axe ge,nerally distributed in pyroxene xenoerysts (Table 2). which are very eommc~n in the Nushan basalts and are beliezved to ~b.e of upper mantle or ion. Crystalline mel.t inclusions in the pyroxene xenoerystaJs were formed in the process of slow crystallization of the mantle magma trapped in the d~fects of pyraxene xerL~crystals a.~. great depths. Measuring 50×20 ~, such inc~l~sions are distributed in a recVan4gle pat tern in the features parallel to the long axis of pyroxene. As a result of immiscibility, melt in the inclusion broke down into a ferruginous phase (.d~rk brown, amo,u~ting to 60--90%) and a calorless, transp,aren¢ silic'ate phase (aceo0~ting for 40-- 10%, of the ~otal volume of the inclusion). The contact between the two phases is irregular and sometimes exhibits a zigzag pattern. A 10°--15 ° difference is noticed in ex-

114 G E O C t I ] ~ M I S T R ¥ Vol. 5

Table Z. Phase a s s e m b l a g e s in m e l t inclusions in the se lec ted hasa l t s from eas tern China

Sample No. Rock type Locali ty Host mineral Type of melt Phase ratio in inclusion melt inclusion*

G = 29 - 25 Pyroxene Amorphous A = 80 -- 75 V - l - 6

V - l - a

V-2

V-3

LS-I

LS-PY[

LS-PYII

LS-PYII1

jsx-I

z J x - I

z j x - 2

FJX-1

Alkali basalt

Basanite

Pyroxen 9 xenocryst in basalt

A lka l i basalt

Ol ivine basalt

Ol ivine basal t

Wudal ianchi ,

Hei longj iang

Nushan, Anhui

Liuhe, Jiangsu

Zhuj i , Zhe j iang

Mingxi , Fuj ian

Ol iv ine Amorphous G = 20 A = 8 0

G -= 30 -- 35 Ol ivine Amorphous A = 70 -- 65

Pyroxene Amorphous G = 4 0 A = 7 5

G = 2 5 A = 6 0

Ollvine Amorphous

Pyroxene Crystalline CFe = 70 CS, = 30

Pyroxene Crystall ine CFe ---~ 70 Csl = 30

Pyroxene Crystall ine

Ol ivine

Olivine

Amorphous

Amorphous

Crystall ine

Amorphous

Olivine

Ol ivine

CF¢ = 65 Cs: = 35

G = 1 5 A = 8 5

G = 2 O - - 25 A = 80 -- 75

G = 2 0 -- 30 C = 6 0 -- 40 A = 3 0 -- 20

G - - 1 5 - - 4 0 A = 83 -- 60

* G - - g a s phase; A - - - a m o r p h o u s phase; C - - c r y s t a l l i n e phase.

tinetion a~lgle between the silicate phase and the host mineral. In the features perpen- dieul, ar to the ~ong axis of pyroxene, the inclusions, measuring 10×20 to nXn~, are round or irregular in shape. Commonly they cluster in a ciraular,..a~ea with lar=m~r inclusions at the center a~a,d ~ore .and ,more increasingly finer ones distributed outwaxds (Photo 1). Further s~udi,es of these .crystall'me melt inc]~us4ons in pyro.~ene, which were known ~ S,ehiller's stamcture in the previous ]iteratu, re, are helpful far .a better under- st'aa~ding of the cemposi,ti~nal characteristics and ehe temperature and pressure conditions in the mantle.

Crystalline melt ine~usions are also seen in olivine from the Zhuji basalts o,f Ztle- ji,ang Pro~illce. They are coanposed of erysta~+g}ass+gas or of crystal+gas. In the f.orm~r ease, they measu,re 10×20--20×40 ~, $ake o~al to irregazl~r folmas, and colrsi~t of 20--30% gas phase, 50--5% crystal a~d 30--65% glass. The refractive index of the solid phase is generally lower tha3a that of olivine. The presence of erystallirre melt in- clus}ons indicates that the trapped basaltic rrragana h'as t~I~devgo,ne crystallization under s~o,w coo,ling conditions. The hos~ mineral, olivine, may have crystallized in the &eep upper mantle or under thiel: overlying lava layers (Photo 2).

Sa

mp

le

No

. 1

.6ca

li~

ty

V-l

-6

= Z

V

-l-a

w

.-2

V-2

V-3

LS

-i

.~

LS-P

yl

e-

LS -Py

ll

LS -PYlI[

=

Z2

-2

"~"

.=

FJX

- 1

.. "~

7 ,~

, ¢-

FJ

X-

2

Ho

st

m~n

I

ock

typ

e er

al

2Z

W

Ba

san

ite

©

Sta

rtin

g

sta

tus

G=2

~%

..

..

..

Ill

Va

ria

tio

n

in

Ph

ase

a+se

mb

lag

e d

uri

ng

h

eati

ng

H°

nm°g

ta~i

u~sa

t

.-

., G

=~.0~

0' G

=IS~

0 G

=10%

G

=0./0

, G = ',~°o

G =15%

G=B%

g = 30 ",~

G..

...

A

~ 7~

'C

~ ~,

,

G =

35

%

G ~

30 %

G

=20

~i

G=

10N

O

G~0

".0

G=4

0%

TE

G

=45

%

l M

G

= 2.

.5 *,,~

G=I0%

G=O °4

G

2s~;

T

E

G=

30%

6

=2

0%

G

=15"

%

G =1

2~,;

G=5

%

G=O

%

CF'

=70°

Z I

M

CM

cF

.-C

si C

M

CF~

= 70g

CS,

=30

%

CS

i=3

0%

c~l~

c~,

~'---L

c

.~'c

~F¢=

65%

Cs,

=35%

I

M

CM

C =

15

~

C=

5%

~ G

=2

5g

70

0°c

1200

"C

C=

40%

G

=30%

C

=~

%

C=:

t5%

A

=75"

~ IM

c,

=.2

0g

G=2

o¢

G=1

5%

G=5

%

,M

C3

-C) =-i

G=4

5%

G-=

10~

G=5%

G=4

0%

M

G=

35

%

G=3.%

G=2

0%

G=

8%

Fig

. 3.

P

hase

-ass

embl

age

var

iati

on

o

f he

ated

m

eh

incl

usi

on

s in

ba

salt

s fr

om

eas

tern

C

hin

a.

IM--

init

ial

mel

tin

g;

TE

--th

erm

al

exp

ansi

on

; C

M -

--co

mp

lete

m

ixin

g.

CF

t-C

si

CM

CF

e-C

si

C M

C5

G=

0'g

0 G=

0%

©

G=0

%

G =O

%

to

o c~

116 GEOCHEMISTRY Yol. 5

2. Amorpbom mel't inclusions are fairly common in olivine, pyroxene and feldspar in the studied basalts. They are composed o,f amorphous solid phase+gas or solely of so.lid ph.ases. Three types are distinguished on the .basis ,of tLheir ph~aseasse~blage fe.atu,res (Photos 3--8).

(1) One-phase melt inclusions: Composed only of silicate phase. They are usually small (n-10 ~t), ronnded or oval-shaped, and felund in all the basatts studied. Possibly owing to the relatively l~arge &ensity of ~he trapped manna and its rapid cooling, g~ses were retained in the melt before consolidation.

(2) Two or more-phase amorphous melt inclusions: C*o~z~mo~nly seen in basalt sam- ples from various localities, and m~de up mainly of amorphous silicate+gas, or a~orph- ous silicate+ferrugi~aus phase+gas. The gas phase consists of ,a single regular rounded or me.re irregular b~Lbble (s), sometimes exhibiting u "sieve texture" (Pho~os 3--6). The slmpe and ratio in the gas ph,ase may reflect the viscosity and eooli~g rate of the trapped b~saltie mag]na. Ch, amcteristies of the melt inclusions i:n ~basalts are hi,ghly varia/ble from place t~o place. For example, sas phase, comprising 20 40% of the ?otal volume of the inclusion, occurs mostly as single or multiple rounded 'bubbles with a small part taking ,a sieve-like appearance in inclusions from the Wud~lianchi basatts of tteilongjiang (Photo 3). ~as phase in melt inclusions from the Jia~gamAnhui area, occurring as o~al or irre,g~lar bu~bles (Photo 4), ,acco~unts fc~" 15---25 volume percent. ~as phase in melt ineluisons from the Zhejiang-Fujian area, occurring mostly as irregular or sieve-like bubbles, generally amount to 10 to 20% (r~rely as high as 40% in Mingxi, Fujian) of tl~e total volume of the inclusion. The melt inolusions ,are u~u~al~y rounded, ¢~,~1. square or irregular in shape (Photos 5 and 6). Slight ,difference can also be observed in ,abundance a~d size o,f amorphous melt inclusions for ~basalts f r e ~ different localities. For e~a~nple, more (m~aking up about 60% of the ,tota~ volume of host miner,als) and larger (18×5 to 120×40 ~t, usually 20 to 40 ~) inclusions are found in the Wudali,anc.hi basalts. In c()ntruet. ,basalts in the Jiao_gs~-Anhui a~rea contain sparsely scattered inclusions which comprise only 1~5% (lo,ca~ly 20%) o.f the host mineral. Melt inclusions from this area are all small in size, varying between 10×10--40×20 ~, generally 15 t(~ 20 ~. Basa~lts fr(~n the Zhejia'ng-Fujian area contain ~as many me~t inclusions as those from the Jiangsu-Anhui area, but the size of indi~Ldu'al inclusions varies £rom 10×10 to 40×20 ~.

From the above discussion, it is indicated that the abundance, size a~d ~as ~atio of amorphot~s melt inel,usions in basalts te~d t~t decrease from Wudalia~ehi towards east China. In other wo.rds, the Wudal ia~hi basalts may co~ntain higher amounts 0£ water and other volatiles and were derived from a ,magm,a of lower viscosity than .~salts from Nns,han. F,angshan, Zl~u~i a~,d ]VIingxi. Additionally, a wide div, evsity of inclusion ph,ase assemblages in basalts from different .areas pr.o,~ides evidence of inhomogeneous ~mg~nas and boiling (luring their formation.

(3) Mixed-phose me~t inclusions: Principally found in the ]Vu~dalianchi basalts (in feldsp,ar and olivine), very rarely seen in basalts from other localities, a~ad composed of ferruginou~ t~nd silicate phases. Irregular in shape, dark brown or light yellow in co~or, and approxim~te]y .~0×40--200×60 ix in size. The fermginons phase, accounting for 50--70~, o~'~'urs ~s p~)wd~r in the glass. Melt inclusions of this kind ~a~r,e considered to

No. 2 G E O C H E M I S T R Y 117

be fo~zned from a ma~o~a of high viscosity trader rapid cooling conditions (Photos 7 a~d 8).

Rock-forming Temperature Measurements

Homogenization temperatures ha,~e been measured with a LGHS-1 Type he~ting stage. The results are presented in T~ble 3 and Fig. 3, together with some data obtain- ed with the quenching method. The uncertainties invc~lved in the measuren~er~s are in the range ~f -----5°C.

1. Wudali,anchi basalts For .amorphous melt inclusions in olivine, initial melting takes place at 460°--700°C when ferrt~ginous silicate starts to be oxidized, with the color becoming dark brown. At 892°--11500C the inclusions homogenize, being filled by sili- entre phase.

2. Nushan hasalts Thermarl exp~ansic,n is observed at 600°C in a.morphous melt inclusions present in pyro~e~e xenoerysts as shown by the increase in size of the gas bubbles in mi.crofr.~ctures in the solid phase. At 700°C, the solid phase begins to melt accompanied by ,an increase in volun~e at the expense ~)f ~o~.s. The ho~n:ogeni~ation tem- per'atare is 1200°C.

Crystalline melt inc]lusio~s c o~tained in pyroxene ~enaerysts fro,m the Nus]~a~ basalt,s arc composed of crystalline silicate and ferruginous phases., which have different initial melting temperatures ,as well ,as melting ~emperatures. The former begins melting at 980°C and completes ,at 1160°--1040°C. Initial mel'~ing of the ferraginous phase occurs at 1100°--1150°C, which l~cmogenizes with the silicate phase at 1200°--1300°C. It is generally believed tha~ the pyro~ene xenc~l~-sts are of deep (mantle) origin. So, their homogeafizatio.n temperature may represent the 'temperature under which the pyroxene crystallized in the mant'le. The discovery o.f crystalline melt inclusions in the pyroxene xenocrysts provides a direct ,approach for the study of physicoehemical conditions and phase equilibria pertaining to~ the formation of mantle-derived rocks.

3. Fangshan basalts ~as phase in amorphous melt inclusions in olivine generally oe.elu~'s ,as m~ltipl~ gas bubbles (in rare c~ses only a sino'le bubble). The bubbles are irregalar']y dispersed in the glass or preferentially distributed in both ends ,olf the inclusi(m. Initial melting ~akes place at 720°--850°C. Homogenization temperatures are within the t~ange of 1200°--1260°C.

4. Zhuji basaltts Initial melting of the amorphous melt inclusions in olivine occurs ,at 800°C, and their homo~enizati~o~ temperatares ar,c estin~atcd at 1200°--1280°C.

The crystal please starts to ~e l t at 580°C in c~stalline melt inclusion% while the silicw~e phase does rmt show any obvious sign of initial melting till 800°--1150°C. Ho- magenizati,o.n temperatures are 1220°--1360°C, but a few inclusions homogenize even at 1450°C. The olivine hosting crystalline melt inclusions had probably crystallized in the deep crust or u~p,per mantle before it was tr~pped by ascending basaltic ma.g~a. The relattively high ~emperatures measured on such olivine should therefore represent the roak-f~rming temperatures in the deep cru~t or upper mantle environments. On the other hand, crystalline r~elt inclusions in other o~livine may have crystallized from the

Tab

le

3.

Hom

ogen

izat

ion

te

mp

era

ture

s of

m

elt

incl

usi

ons

in b

asal

ts

from

ea

ster

n

Ch

ina

Init

ial

mel

ting

H

omog

eniz

atio

, S

ampl

e N

o.

Roc

k ty

pe

Loc

alit

y H

ost

min

eral

T

ype

of

mel

t in

clus

ion

tem

pera

ture

(°C

) te

mpe

ratu

re

(0(3

]

Pyr

oxen

e A

mor

phou

s 46

0 89

0 V

-l-6

V-l

-a

V-2

V-3

LS

-I

LS

-PY

I

LS

-PY

II

LS

-PY

III

Alk

ali

basa

lt

Bas

anit

e

Pyr

oxen

e xe

nocr

yst

in b

asal

t

Wud

alia

nch

i, H

eilo

ng] i

an~

Nus

han,

A

nhui

Oli

vine

Oli

vine

Pyr

oxen

e

Oli

vine

Pyr

oxen

e

Pyr

oxen

e

Am

orph

ous

Am

orph

ous

Am

orph

ous

Am

orph

ous

Cry

stal

line

Cry

stal

line

500

700

900

700

800

900

i000

1100

1150

1200

1200

--13

00

1200

--12

50

Pyr

oxen

e C

ryst

alli

ne

900

1200

JSX

-1

Alk

ali

basa

lt

Liu

he,

Jian

gsu

Oli

vine

A

mor

phou

s 75

0--8

20

1260

--12

00

ZJX

-1

Oli

vine

A

mor

phou

s 80

0 12

20

Oli

vine

ba

salt

Z

hu

ji,

Zhe

jian

g Z

JX-2

O

livi

ne

Cry

stal

line

11

00

1360

--14

50

FJX

-1

Oli

vine

bas

alt

Min

gxi,

Fu

jian

O

livi

ne

Am

orph

ous

800

] 12

00--

1120

Sam

ple

Ana

lysi

s N

o.

No.

V-1

1

V-1

2

V-1

3

V-1

4

Ave

rage

Ave

rage

of

the

Wu

dal

ian

chi

basa

lts

Tab

le

4.

'Hos

t m

iner

al

Na,

O

0.68

1.56

Oli

vine

1.

10

1.32

i. 1

7

3.89

Ele

ctro

n

mic

rop

rob

e an

alys

es o

f am

orp

hou

s m

elt

incl

usi

ons

in

the

Wu

dal

iau

ehi

bas

alts

KzO

3.51

3.02

3.65

3.62

3.45

5.25

CaO

9.70

8.02

8.97

7.66

8.59

6.29

MgO

2.70

1.28

1.84

1.38

1.68

6.45

A12

03

16.6

3

17.4

17.3

1

17.4

8

17.2

1

13.7

7

SiO

2

52.1

3

53.6

7

52.6

8

52.7

5

52.8

Tie

2

3.15

3.09

3.29

2.80

3.08

2.32

Crz

O~

0.57

0.03

0.01

0.09

0.17

5

MnO

0.01

0.10

0.01

0.07

0.04

7

0.14

Fee

5.46

4.41

4.53

4.71

4.71

5.67

FezO

3

3.00

P2O

s

1.04

Los

s on

ig

niti

on

0.64

Rem

ark

u

tm

•

Ana

lyst

: In

stit

ute

of

Geo

logy

, th

e C

hine

se A

cade

my

of

Geo

logi

cal

Sci

ence

s,

© N

N

No. 2 GEOCHEMISTRY 119

basaltic magma deeply buried ur~der the thick-layered larvas in the prolonged course of basalt .eruption. Hemogenisation tempev~t~ures for such olivine ~re thot~ght to reflect the temperatures that prevailed in the early history of t~he basalts.

5. Mingxi b,asaJts Initial melting of the ~morphous melt inclusions o,ccurs at ~00°--1140°C. and their homogenization temperatures are within the range of 1120 ° - 1220°C.

In summary, the ,b:aSalts from different a~eas in eastern Chin~a were formed at different temperatures. Generally speak-ing, the Wud, alianchi basalts were formed at slight- ly ]ow(~r t,~mperatures (980°--1150°C) than those from the J ian~u-Anhui a~d Zhe- jia~ag-Fujiar~ areas. B asalts ef the 1,atter two ar.eas show almost ~o,, difference i~n rock- formin~ temperature, being 1200°--1260°C and 1120°--1280°C, respe:ctive]y.

Chemical Data for Melt Inclusions

Melt inclusions from the ~basalts w,e~'e analyzed by electron microprobe, scanning clectrr)n microscopy, laser spe ctro~,owaphy and gas chromato,~oTaphy. Electron microprobe analyses are ~ven in Table 4 of melt inclusions in olivine from the Wud.alianchi basalts.

Ir~ co~np,~ristm to the .average ch, eanieal composition of the host basa3ts, it can .be seen that t,hc contents of Si0~, FeO and Mn0 in melt inclusions are similar to, while the contents of AI~O~, C~O, TiO~ and Cr~O, are higher than, those of the original basa]ts. The CroP3 content is very close to th~at of the Jil[n meteo~ite (0.61---0.28%) t~°-~2~ and of the g~ass phase in melt inclusions of the metev~te (0.07--0.71). Na~O, K20 a~d ]Y[gO are lower than the average ,contents of the host basalts. All this shows that the .melt trapped is chemically .~onsi~tent with the origin,a~ basaltic m~gma at its early stage o.f evolution. Relatively low 1Y[gO contents may be accounted ~ 'by tl~e preferential con- eentr~tion oil Mg in host olivine. Studies of the chemistry of melt inclusions will shed light on the compo~sition of the original magma and c~he~nica] variatio~ns during magTna- tie differenti~ation.

The same ,Conclusion can be drawn ~rom SEM ana~,ys~s of melt inch~sio:ns.

The results o';f gas chromatographic and ma~ spectrometric an,alyses of mett inclusions show that CO~ is the major gas component (169.3--12.5.8 ml/100 g) in the magma. The content of CO is 15 47.6 ml/]00 g and CH~ 22 m]/100 g. In other wo~,ds, CO~> CO>CH~ in the m,agma.

Conclusions

1. As indicated by the preliminary results in this p alaer, the Cen.ozoic basalts in eastern China ~and the W~d+al~.a~+hi basa.lts in tteilongjiang are quite diffel, enlt from those i+l the Ji,mngsu-Anhmi and Zhejiang-Fujian areas elf east C,hina; the former basalts are characterized by higher amounts of alkali a'ad greater depth a~,d 1,ower tempe+a~ure (+f formation.

'2. The, ~;haraeteristies of melt inclusions in basalts ha,~-e .a ~beaving on the physico- ehemic~d ~:,nditions of their formation. For example, melt inclusions in the Wudalian-

12o GEOCHEMISTRY Vol. 5

ehi basalts, which were t rapped at great depths .as deser[bed above, are characterized by higher gas ratio, larger size, greater abundance, and lower tempera.tare of for~ati'~n. These indicate that water and other volatiles must have increased as the m~gma was ascending from depths, resulting in a decrease in magma viscosity and melting point. How- ever, the case is quite different with alkali bas~lts formed at sha~llow depths, such as those from Nushan, A~a. hui. No i,~clicati~on can be fomad of strong influence of differen- t iation conso.li,dlation and sialic contamination on the magzrm as it ferupted onto the sur- face along fault zo~aes. Sa melt inc~usioaas in .alkali basalts would have crystallized at r~latively high temperatures because the magma remained to be very viscous.

3. Melt inclusions in basalts d i f fe r from one areu to an.other in phase assemblage, phase ratio, composition and hom, ogenizatio.n temperature. Even within the same area,

sc~ne differences caaa still be recognized in phase assemblage and homagenization tempe- r ature probably due to different depths of ~a.gana sources and possible boiling. Varia- tions in phase assembl, age and formation temperature of melt inclusions are indicative of the enviromnen~ in which l~cjst basa~ts were formed, and thus can provide clues to

the mech~nis~n ,of formation of basa~ts.

4. The discoveries o~ pyroxene xenocrysts in basalts and of melt in cluzions formed in the upper nmntle have opened a new way for the study of r~antle-derived r o~eks. As viewed fro.m tLhe fact that the protmrtio~ of fevrl~girm.us p,hase to silicate plmse is approxim,a~ety 3:1 in melt in elusi~ons in pyroxene xenocrysts, the iron content o$ ma.ntIe- derived rocks may be higher than the estimated values b7¢ some .other authors. As sam- ples of an~ntle-deviwd rocks, crystalline melt inclusions in xen~,liths qare expected to provide m~ck~ more critical infor~$t ion .~n the geochemistry of the deep interior.

5. The distribution o,f Cerrozoic basalts in eastern Chin'a is apparenbly controlled by deep faults. I t seems tl~at collision between the Pacific plate and the Eurasian plate is oaa,e of the hnpc~oant factors governing the evolution of the Cenozoic f~ult system and hence the activities of basaltic magmas. Wit,h the .&evelop,men~t of the ~ault system on the coaatinent,n] si~e of e~llis[on, eruptions of tho.leiite o ceuvred in shallow depths in various provinces of svutheust China, followed by the W~dalianc,hi alkali basalts from m~ore deep-se,ate~d so,urces. This has been wel,l eo~firmed by the studies ~f melt inclusions in the bas~llts.

Acknowledgements

We are g~ateful to Professors Xu Keqin and Wang Dezi for their advice and to

Zhon Xinmin and Cb2n Tuhua far their help in this work. We a/so would like to~ thank Engineer WaY~g Bixi,ang of the Insti tute of Ge~ology, t'he Chinese Academy of Geologi- cal Sciences for her kind assi~staxtce.

References

1 ] Zhou Xinmin et al., Bultetin o£ Nanjing University, 3(1978), 92--116. [ 2 ] Zhou Xinmin et al., Treatises on Oceanography, 1(1981), 99--110. [ 3 ] Department of Geology, Nanjing University, Petrology of Vo!canics, Geological Publishing House,

Beijing, 1980, 266--273. [ 4 ] Wu Liren et al., Acta Geologica Sinica, 3(1982), 223--234.

No. fi GEOCHEMISTRY 121

[ 5 ] Xia Linqi, Clocchiatti R., f~exue Tongbao, 9(1984), 551--556. [ 6 ] Li Zhaolin, Studies of Inclusions in Minerals (I), Science Press, Beijing, 1978, 48--55. [ 7 ] Roedder, E., Bull. Mineral, 102(1979), 487--510. [ 8 ] Yaimacov, H. P., Dolgov, U. A., Thermobarogeochemistry, Moscow, ~Hegra~, 1979, 165--178 (in

Russian). [ 9 ] Dolgov, U. A., Shugutova, H. A., Geothermometry and Geobarometry of Minerals, NAUKA USSR,

V. 1(1965), 118--122 (in Russian). [10] Zhang Peishan et al., Treatises on JiIin Meteorite Shower, Science Press, Beijing, 1979, 68--80. [11] Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences, Treatises

on Jilin Meteorite Shower, Science Press, Beijing, 1979, 81--99. [12] Wang Xiyue et at., Treatises on Jilin Meteorite Shower, Science Press, Beijing, 1979, 122--127.

F'hotos 1 - 8 : 1. Crystalline melt inclusions in pyroxene xenocrysts from Nushan basalts. 2. Crystalline melt inclusions in olivine from Zhuj i basalts (Csl + G); black gas and l i g h t - c o l o r e d - - c r y s t a l l i n e silicate (X360) 3. Amorphous melt inclusions in olivine from Wudal ianchi basalts (Asi + G); l ight g r ey - - - - amorphous glass and dark gas ( X 3 6 0 ) • ~. Amorphous melt inclusions in olivine from Fangshan basalrs(Asi + nG); l igh t grey-glass and d a r k - - - - g a s (X360) 5. Amorphous melt inclusions in olivine from Zhuji basalts

(Asi + CF~ + G); dark b l ack gas, dark s t r i p e s - - f e r r u g i n e o u s minerals , and l ight- colored ...... glass 6. Amorphous melt inclusions in olivine from Mingxi basalts (Asi + nG); dark black - gas and light-colored amorphous glass 7. Amorphous melt inclusions of ~fixed phase in olivine from Wudal ianchi basalt; dark b l ack - - - - f e r rug ineous p h a s e + a m o - rphousglass and l i g h t - c o l o r e d - - - g l a s s ( X 3 6 0 ) 8. Amorphous melt inclusions in olivine of

mixed phase from Nushan basalts (CFe + Asi); d a r k - - g l a s s + ferrugineous phase (X360) .

Studies of melt inclusions in some basalts from eastern China

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