Petrographic characteristics and several other related problems of the layered basic-ultrabasic intrusives in the Upper Yangtze Valley

VoL 1 5To. 1 ® E O C 14 E M I S T R Y Jan.-l~far. 1982

Petrographic Characteristics and Several Other Related Problems of the Layered Basic-ultrabasic Intrusives

in the Upper Yangtze Valley

L~u Ruox~s ( ~ ] ~ , ~ ) , XzE GU^NGHONG ( f N f ' ~ ) , aND N~ J~ZHONG ( N ~ )

(Institute of Geochemistry, Academia Siniva)

Abstract

Three iron, titanium-rich layered basie-ultrabasie intrusives have been described. They ~)ceur in an approximately south-north striking belt of tectonie-magmatie complex within an anteklise. Iron and titanium were concentrated in the lower or middle-lower parts of these intrusives at the early and the middle stages of differentiation, during which F e e may have :played an important role. Intimate spacial and temporal association as well as common petrochemical features (Le., high iron and titanium contents) have been found between these

layered intrusives and the syenite and alkali-syenite plutens. All of these rocks axe considered as hypogene products of the differentiation of the Omeishan basaltic magma. In other words, • he hypogene differentiation of the iron (titanlum)-rich, sub-alkali Omeishan basltie magma under the same structural-geological conditions resulted in the formation of the rock series of layered intrusives (peridotite >iron, tltanlum-rich ultramafie reeks • iron-, titanium- rich gabbro, plagioclasite) • syenite, alkali-syenite.

Fe r ro - t i t ano~ox ide m i n e r a l s f rom some l a y e r e d bas i c -u l t r ahas i c i n t r u s i v e s in the

U p p e r Yang tze R i v e r V a l l e y were r e p o r t e d in the p rev ious a r t i c l e " . Th is p a p e r is

c o n c e r n e d wi th pe t ro log i ca l d iscuss ion on these in t rus ives .

Brief Description of Geology

Three l a y e r e d i n t ru s ive s a re loca ted on the wes te rn side of an o ld antekl i se on a

b a s e m e n t of the P r e c a m b r i a n sys tem o c c u r r i n g in a south- or n o r t h - n o r t h e a s t s t r i k i n g

b e l t of t ec ton ic -magmat i c complex. These i n t r u s i v e s h a p p e n to be c losely associa ted

w i t h syen i t e in space as i n d i c a t e d in the p r e v i o u s ar t ic le , a n d also t h e y m a y be the

i n t r u s i v e facies of the O m e i s h a n basal t . F r o m F i g . 1 we can see t h a t these in t rus ives

~ a n be r e g a r d e d as one rock complex ,on a c c o u n t of the close a s soc ia t ion of syen i t e s

w i t h them. I n genera l , the l a y e r e d i n t r u s i v e s a n d syen i tes ( i n c l u d i n g quar tz - syen i te ,

b io t i t e - syen i t e , ho rnb lende - syen i t e , a r f v e d s o n i t e - p e g m a t i t e , etc.) axe f o u n d a t the west

:r im of the basal t . A c c o r d i n g to W a n g Qian e t a l ~. t he Omei sha~ b a s a l t is m a i n l y

c o m p o s e d of e r u p t i v e s be tween u p p e r a n d lowe~ P e r m i a n , w i th m i n o r d i f f e rences in

t h e i r p e t r o g r a p h i ~ cha rac te r i s t i c s , c o n s t i t u e n t s a n d t ime of e r u p t i o n w i th r e spec t to

d i f f e r e n t areas. Cong B o l i n g et al EI~. t h o u g h t t h a t the basa l t e r u p t i o n in the g iven

1) Liu Ruoxin et al.: 1974, Iron-titanium oxides from some layered basic-ultrabasie intrusives in the Upper Yangtze Valley, ' ' GF, OCHIM~C~' ~, No. 1.

2) Wang Qian et al.: 1963, A preliminary study of the Omeishan Basalt.

52 GEOCEEMISTRY Vol. I

~ 2 ~ 3

~ 6 ~ 7 ~ 8 ~ 9

~13 ~---714

IT i

' . .: : . , ' . ' . , . ? . . . : . . . ° .

.: . : : , . . . .

i ' .

: ' : : " :'...- ;

: "2 :"

. . . . . .

i:i:i:i . . . . .

+

÷

+

i

, . ' . " , ' , . . . . : ." : . ' . . .

L

L

L

L

L

L

L L

Fig. 1. GeneraLized regional geological map of intruslves "P", "L', "B" and "T'. 1. Quaternary; 2. Tertiary; 3. Mesozoic; 4. Upper Palaeozoic; 5. Lower Palaeozoic; 6. Proterozoic; 7.syenite; 8. granite; 9. granodiorite; 10. diorite;

11. basalt; 12. gabbro; 13. gneissic granite; and migmatite; 14. fault.

No. 1 GEOCHEMISTRY 53

L

L/ L L L ~, L L L L / L L L L L L L L /

L L L L L L L L . L L L L L L L L

L L L L L ~L L L L L L

L L L L L ,, v

L L L L L " ~'-5o. L L L L v v 0 %

L L L L v ,, v P-51 ,

L L L ,, v v " , / L L L "", v v p_4?vP 4

L L ,, , , , , ,, ,2 " t v

, / A A A ~ V V" V V

A ~. ~ ~ p V o 4 0 V V

A A A ° A A A

P - 3 9 A ^ ~" "" P-38 i o ^

P-37

L L

V

L L L L

L pYs5 L L

L L

L L

v

pO_ 52

v

v v

" v

v y

V V

Fig. 2.

~ 9 ~10[~11~12~13 "~]14~15~16 Generalized geological map of the middle section of Rock Body "P" (modified

aceording to the original map by Geological Survey Team).

1. Triadic; 2. marble; 3. sehistoze amphibollte; 4. olivine-gabbro; 5. tita- nomagnefit~te; 6. tltano-rieh magnetite gabbro; 7. gabbro; 8. gabbro-plaglo- elaslte; 9. fitanomagnefite pyroxenite; 10. hornblende-gabbro; 11. eoar- se.grained gabbro; 12. fault; 13. surveyed and ex-pected geological boundary;

14. layer strike and dip; 15. profile fine; 16. sample locality.

~4~ GEOCHEMISTRY Vol. I

area started at early Permian. The biotite K-Ar age of the syenite associated vith l~ock Body " L " was measured to be 263 m.y. 2~ Geological investigations revealed that the layered intrusives intruded into the basalt and were in turn intruded by syenites. The three were geochronologically close to one another, that is, between 260 and 280 minion years. Their sequence is: basalts--layered intrusives--syenites. In addition, these kinds of rock possess very similar petrochemical characteristics. We are sure that they belong to an identical magmatic or magmatic series of layered intrusive~ syenite complex derived from basaltic magma and formed under the same structural- geological conditions.

Petrographical Outline of the Layered lntrusives

In the previous article, the petrography and mineral constituents of Rock Body " P " were briefly mentioned together with its geological map and mineral constituent variation diagram. The geological map of the middle portion of Rock Body " P " is illustrated in Fig. 2. From this figure wecan see that this rock body shows a distinct layered distribution pattern and thus can be divided into several belts from top to bottom.

The bottom fringe belt

10--200 metr~ thick, mainly composed of fine:grainec[ or medium fine-grained olivine-gabbro, 60% of which consists of mafic minerals, such as olivine and monoclinie pyroxenes. The rock has a slightly layered structure which' becomes more apparent toward the inner part. But even the fringe rock nearest to the contact rim of the rock body does not show a trace of evidence of rapid quenching,. Schistose amphibolite of small thickness may be the product of reaction between magma and surrounding rocks. Sinian marbles along the contact zone have been subjected to serpentinization, within which serpentinized pseudomorphs of olivine are occasionally recognized. This indicates that the temperature at which the contact zone was formed must have been quite high, and the f~inge rock belt of that rock body couldn't have crystallized by rapid quenching.

The tita~omag'netite rock belt

It is mainly made up of titanomagnetitite containing over 50% ferro-titano-oxide minerals with randomly intercalated thin layers of gabbro, displaying remarkably layered structure and small scale rhythm. Below this, there is a 3 m thick layer consisting of peridotite anorthosite~eridotite or olivine-pyroxenite. According to the con- cept of accumulated rocks c~J, two main accumulated rock layers, i.e., olivine pyroxenite accumulated rock and titanomagnetite-titanoaugite-olivine accumulated rock can be d/stinguished.

Besides, a later-stage coarse-grained gabbro, parallel to the main layering structural strike (Fig. 2), but with clear intrusive relationship with fine-grained olivine- gabbro and titanamaWnetitite, occurs in both bottom fringe belt and titanomagnetitite belt. Such coarse-grained gabbro has never been found in the middle and upper

1) Determined by Isotope Geochronology Lab Institute of Geochemistry, Academia Sinica.


parts of the rock body.

The titanium-rich magnetite-olivine-gabbro belt

Among its components, ferro~titanimn oxides account for 30 percent; part of titanoaugites is automorphous in addition to those being nearly eutectic with titano- magnetites. Olivines in (2) and in this rock belt are usually inset within titanoaugite or on titanomagnetite, occurring as automorphous or speroidal crystals. Olivines are also found as "reaction r ims" between titanomagnetite and titanoaugite in these rocks as well as in Rock Body " L " . Thin alternating layers of gabbro have also been

found.

The titanomagnetite-gabbro belt

Ferro-titanium oxide content is unually between 15---25%; titan,omagnetite is xenomorphic with respect not only to titanoaugite but also to plagioclase. There is often titanoamphibole "reac,~ion r~ns ''~ between titanomagnetite and titanoaugite or between titanomagnetite and plagioclase. Ferro-titanium oxides, titanoaugites and pla- gioclases may be relatively concentrated to form small rhythmdc layers.

The ptagioc~ase-gabbro belt

Plagioclase ranges from 60 to 80% in average, and olivine and augite are generally within the range from 10 to 15%. Apatite increases from 1 to 7% with decreasing titanohornblende.

The layers mentioned above belong to the first accumulating rhythmic cycle.

The titanomagnetite-pyroxenite belt

With a thickness of several metres to several dozen metres, the striking character of this rock belt is that olivines are commonly xenomorphous relative to titanoangites. Ferro-t~tanium oxide minerals fill in the interstice among silicate minerals. Apatite grains in small size occur mainly as mosaic members in ferro-$itanium minerals. This rock belt is quite stable in the upper part of Rock Body " P " . Its petrographic characteristics and structure are evidently different from those beneath it. I t is, therefore, considered as the base of another accumulating cycle.

The upper hornblende-gabbro belt

In this belt the amount of t~tanohornblende increases to 10--15%, but apatite and olivine could hardly be found. In the lower part of this belt a layer-structured rock can be found, which usually graduates to medium-grained massive rock in the upper part. Sample P55 was collected from a layer relatively rich in dark minerals in the uppermost part of the rock body. Although the upper part of the rock body is in fault contact with Mesozoic strata, Sample P55 is probably a representative of the rock near the upper fringe, as evidenced by its petrographic characteristics and its occurrence as a medium-grained massive rock.

Petrographic data on RoekBody " L " have been reported by Mei Houjun ~), who

1) Mei Houjun: 1972, The rhythmic layers observed in the southern part of intrusion " L " .

56 GEOCI~EMISTRY Vol. 1

dindded this rock body into three major m~gmatie accumulating cycles. Cycles I and I I and the lower part of Cycle I I I are all ultramafie rocks (including peridotite, titanium- rich, magnetite-peridotite, titanomagnetitite, titanomagnetite-pyroxenite, olivine- pyroxenite and pyroxenite, etc.). Rocks at the base of each cycle are considered as olivine-titanomagnetite accumulated rocks, exhibiting olivine inclusive texture with titanomagnetite occurring as automorphons crystals setting in titanoaugite crystals. They give way to a more equigranular titanoaugite rock in the upper part, which has a hypidiomorphic texture, consisting of closely packed crystals of automorphism of various degrees. The middle and upper parts of Cycle I I I are composed of titanomagnetite- gabbro, gabbro and a small amount of anorthosite, with apatite-rich layers. Coarse- grained or pegmatoida3 gabbro and gabbro pyroxen:ite of later stages are commonly observed with some remnants of medium-grained rock formed at the earlier stage.

Rock Body " B " is a monoclinical layered intrusive mass striking NS and dip- ping westward. It is cut by a later intruded syenite into two parts, i.e., the east part and the west part. The east part (lower) is mainly made up of titanomagnetite- olivine-gabbro and gabbro. The olivine-gabbro in the lowest part is intruded and mixed with later formed irregular pegmatoidal gabbro. Like the two rock bodies mentioned above, such kind of later formed coarse-grained or pegmatoidal rock can be found throughout the lower part of this rock body. In the higher position dark-colored olivine-gabbro, plagioclase-perid~>tite and a small amount of olivine-pyroxenite are recognized, with alternating titanomagnetite-rich layers and plagioclase-rich layers overlain by a banded (or thin layered) titanomagnetSte olivine-gabb,ro. The olivine content declines gradually further upwards and the banded titanomagnetite-gabbro or banded gabbr0 becomes predominant,

The west part of the rock body, which is composed of gabbro, diabase-gabbro and diorite, has not been investigated yet.

The northernmost Rock Body " T " in Fig. 1 is also an intrusive of iron-, titanium- rich layered gabbro, but the majority of it is covered by Quaternary deposits. In the thin layered titanium-rich magnetite-gabbro near its lower part, later intrusives of coarse-grained pegmatoidal gabbro are recognized as well.

Major Rock-forming Minerals

Principal rock-forming minerals in these three layered intrusive rock bodies are very similar in mineral species, physical property and chemical c~mposition. With the exception of olivine which ranges from Fo 85 to 63 in composition, mafic silicate minerals such as monoclinic pyroxene, hornblende and biotite are characterized by their titanium-rich varieties, i.e., titanoangite, titanohornblende, titanobiotite, etc.

Olivine

It is common in the three intrusives, except for their upper parts. In the lower parts of the rock bodies or in the base parts of each of the accumulating cycles olivine is the dominating rock-forming mineral in peridotite or olivine-rich gabbro. It varies in composition from the base part upwards. In Rock B o d y " P " , the compositional variation of olivine is notieed from Fo 82 (Nm = 1.692) at the bottom to Fo 63 (Nm =-

No. 1 GEOCHEMISTI~¥ 57

1.733) at the upper part, while in Rock Body " L " it varies from Fo 85 (Nm= 1.684) to Fo 67 (Nm = 1.722). In the lawer part of Rock Body " B " , the composition of olivine varies within a limited range from Fo 78 (Nm =- 1.696) to F~) 70 (Nm -- 1.715). Chemical analyses (wt%) and optical data for one olivine sample are listed as follows :

SiO2 38.68

TiO2 0.04

Ah03 0.10

Fe203 2.06

Cr~O3 trace

Fe0 12.58

MnO 0.42

NiO trace

CoO trace

MgO 36.02

Ca0 0.28

tLO + 0.31

tLO- 0.07

99.56

Ng 1 .728

N m 1.711

specific gravity 3.51

Fo 75%

As pointed out previously, in apatite-rich titanomagnetite pyroxenite in the upper part of Rody Body " P " , olivine of Fo 63 is found more xenomorphous in comparison with titanoaugite, indicating a later precipitation than that of augite. In addition, in Rock Body " P " , olivine "reaction r im" is more commonly observed between titanomagnetite and titanoaugite. This "reaction r im" is not remarkably different in composition from the automorphous olivine setting in titanoaugite in the same thin section. Refractive indices of olivines of these two types are:

automorphous olivine "reaction r im" olivine

M 28 2ira 1.698 (Fo 78) 1.698 (Fo 78)

M 33 hrm 1.693 (Fo 80) 1.698 (Fo 78)

No satisfactory explanation can be made for the above facts by referring to available experimental data. However, it is quite evident from our observation that "reaction r im" olivine must trove resv_lted from reaction rather than crystallization from the original magma. Moreover, it seems unlikely that such a reaction could take place in a dry system. I t may be effective between the pre-existing pyroxene and a fluid which is rich in iron and titanium and contains "~olatile components. Zhao Bing 1) has demonstrated experimentally that crysolite could be formed by reaction between MgCl~+ FeCl~ and hedenbergite or diopside under 500--1,000 arm and 600--800°C (or higher). At the same pressure and even lower temperatures, fayalite was fromed through reaction between FeCl~ and hedenbergite. This experimental result can be used to explain the above mentioned "reaction r i m " olivine as well as the fact that olivine was formed later than augite in apatite-rich titanomagnetite pyroxenite with relatively high concentration of F and C1. Ferro-titanium oxides were formed at 700--970°C, which is high enough for the completion of reaction mentioned above.

Titanoaugite

0rthopyroxene has never been found in the three layered intrusives. Pyroxene

1) According to e~xpexlmental da ta o n p y r o x e n e e

58 GEOCHEMISTRY Vol. 1

1500-

1000-

500

Plagioclase Titano-augite MgTiOs Titanohorn- (Mg:Ca:Fe) molecular blende

percentage in ilmenlte(%) Apatite

7

I _J

3

Z

(An%) Ol ivine (Fo%)

-57

-57 -53 ~63 40 -63 l

-40 -63

-49 -68

• 55 -72

• 55 -76

.56 -76

59 59 -77

59 -78

68 -78

-78 59 -80 68 8 2 o

(m)

:24.6 .9

-42.o'42.1" -6 .5 15.9

-42.9:42.1:-5.9 15.0

13 4

42.9:42.5: -14.6 .12 6 "42.8:42.2: 1 4 ' 15.o -12.2

41.8:44.0: 14.2 23.9

44.2:42.6:!25 3 13.2 26'.8

-26.8

Fig. 3. The composition variation diagram for minerals in Rock Body "P' .

I. Cycle I, II. Cycle II

1. fine-grained ol]vine-gabbro; 2. coarse-grained ollvine-gabbro; 3. titanomagnetitite; 4.tita- ninm-rie~ magnetite-gabbro; 5. titanomagnetite.gabbro; 6. plagioclase.gabbro; 7. titanomag-

nefite-pyroxenite; 8. hornblende-gabbro.

is p redominant ly Ti-bearing augite of calcium-rich type, characterized by slight violet-

brown tint. Chemically it is relatively uni form, especially in its CaSi03 content, bu t slight variat ion in iron content can be seen f rom Figs. 3 and 4. Trends of Composition- al var ia t ion of t i tanoaugi te in these three rock bodies are s imilar to one another and

are consistent with tha t . of augite f rom the Skaergaard intrusive. However, ferro-

augite of the late different ia t ion stage is absent in this area. B y comparing monoclinic pyroxenes f rom the Omeishan basalt wi th augites f rom these intrusives, we have

found tha t the rap id ly cooled augite is calcium-poor and iron-rich, but is more similar

N o . 1 GEOCHEMISTRY 59

10 20

50 " ~ 50

0

o °

l o 2 o 3 ~ 4 o 5 e 6 ''-~"

70

Fig. 4. The composition variation diagram for pyroxene.

1. Rock Body "P~; 2. Rock Body ~L~; 3. Rock Body ~B~; 4. Omelshan (Hongl~angshan) basalt; 5. basic vein rock of Omeishan (Hongliangshan) basalt; 6. Skaergaard rock body.

(4 and 5 after Wang Qian et al., 1963; 6 after L. Wager et al., 1968rs~).

to the augite from its basic vein rock. Because the compositions of augites from the Omeishan basalt and its basic vein rock are plotted according to their optical properties, there should be a systematic deviation in Fig. 4 between the projection based on optical properties and that on chemical analyses. IS such a deviation could be eli- minated, more close compositional resemblance would be expected between the augites from intrusives P, L and B, and those from the basic vein r~eck of the Omeishan basalt.

Usually the Alz of titanoaugite (the number of A1 atoms in Si-O tetrahedra) is greater than 7. In these intrusives the contents of AltOs and TiO~ increase from bottom upwards. The Na content of titanoaugites from Rock Body " P " and "B", 0.50--0.48%, is higer than that of ordinary augites. Just like Al~03 and TiO~, the Na

content also increases with decreasing depth. Titanoaugite from the ultramafic rock body " L " c~ontaLus lower Na. Chemical analyses for 8 titanoaugites from Rock

Body " P " are shown in Table 1.

Titanohornblende

This mineral is common but never in large amount. Being formed at the later stage, it occurs as independent xenomorphic grains or as "reaction r i m " between tita-

nomagnetite and titanoaugite or plagiocla~e. In gabbros at the upper par t of Rock Body " P " and in the first cycle of Rock Body " L " , the concentrations of titanohornblen-

de are relatively high, up to 12% and 15%, respectively. Chemical analyses of two titanohornblendes from Rock Body " P " are presented in Table 2. The refractive index, optic angle and chemical composition of titanohornblende are very similar to that of basaltic hornblende. However, the extinction angle of titanohornblende is usually larger; the TiO~ content and the ratio of Fe~+/Fe ~+ are generally lower than that of basaltic hornblende.

Plagioclase and apatite

The composition of plagioclase in Rock Body " P " ranges from An 70--68 at the base to An 40 at the upper part (Fig. 3). Plagiocla~e in the ultramafic rock of Rock

Tab

le

1:

Che

mic

al

com

posi

tion

(w

t.

%)

and

phys

ical

pro

per

ties

of

tita

no

aug

ite

in !

~oc

k B

ody

gP

"

Seq

uent

ial

No.

~a

mpl

e N

o.

M27

M33

M35

M38

M45

M46

P 47

P 52

Si0

, [r

io,

AL

O,

Cr,

O3

Fe,O

3 F

eO

MnO

M

gO

CaO

N

a20

K20

H

,O +

lq

,0-.

V,0

, P~

O~

50.5

0 1.

64

4.72

tr

. 3.

54

4.74

0.

20

13.5

1 19

.79

0.78

tr

.

50.1

0 1.

76

4.52

tr

. 2.

53

5.43

0.

20

14.8

0 19

.79

0.70

tr

.

51.6

9 1.

44

4.43

tr

. 3.

63

4.43

0.

20

13.7

0 19

.51

0.84

tr

.

52.0

9 1.

40

3.89

0.

04

3.21

5.

67

0.24

13

.96

19.1

0 0.

59

tr.

51.3

7 1.

40

3.58

0.

04

2.81

5.

98

0.20

14

.26

19.7

1 0.

52

tr.

51.2

1 1.

16

3.40

0.

04

3.01

5.

98

0.24

14

.04

19.6

9 0.

53

tr.

0.76

0.

25

0.08

0.

08

0.37

0.

16

0.08

0.

08

0.50

0.

19

0.08

0.

08

0.24

0.

06

0.08

0.

10

0.4

2

0.0

6

0.08

0.

04

0.34

0.

16

0.08

0.

06

50.9

0 0.

80

2.11

0.

04

3.60

5.

67

0.40

14

.99

20.4

2 0.

50

0.10

0.

08

0.21

0.

08

0.08

51.9

1 0.

87

2.48

tr

. 3.

15

6.45

0.

36

14.2

8 19

.93

0.52

0.

10

0.51

0

.16

0.

08

0.08

Tot

al

100.

59

100.

52

100.

72

100.

67

100.

47

99.9

4

99.9

8

100.

88

Mg

Ca

Fe

41.8

44

.0

14.2

44.2

4

2.6

13

.2

42

.6 4

3.6

13.8

42.8

42

.2

15.0

42.9

42

.5

14.6

"42.

3 42

.6

15.1

42.9

42

.1

15.0

42.0

42

.1

15.9

S.G

. +

2V

N~

3.33

43.

5 °

1.69

8

3.33

1.

701

3.33

43.

5 °

1.69

9

3.33

45.

5 °

1.70

0

3.33

45.

5 °

1.70

4

3.34

i6

° 1.

702

Ana

lyst

: C

entr

al A

naly

tica

l L

ubor

ator

y o

f th

e In

stit

ute

of

Geo

chem

istr

y.

Tab

le 2

. C

hem

ical

co

mpo

slti

on (

wt.

%

) an

d p

hysi

cal

pro

per

ties

of

tita

noho

rnbl

ende

in

Roc

k B

ody

"P"

Seq

uent

ial

No.

S

ampl

e N

o.

M27

M35

M25

SiO

, T

iO2

A

L03

C

r203

1%

203

FeO

M

nO

MgO

C

aO

Na2

0 K

~0

H~O

+ H

20

- V

20~

P20

,

43.9

9 3.

77

12.8

1 tr

. 4.

35

7.07

0.

16

12.0

1 11

.39

2.6

6

0.6

5

1.11

0

.57

0.

04

0.08

43.2

4 4

.24

12

.93

tr.

2.9

6

7.07

0.

12

12.9

4 10

.96

2.96

0

.76

0.

73

0.38

0.

04

0.12

43.8

5 4.

26

12.8

0 /

2.44

7.

94

0.05

12

.52

11.7

9 2.

60

0.80

0.

90

0.34

/

/

Tot

al

100.

66

99.4

5

100.

29

--2V

Nm

C

AN

g

80 °

1.68

1 17

°

84 °

25 °

86 °

12 °

/ n

ot

anal

yzed

, th

e sa

me

bell

ow;

anal

yst':

dlt

to

Tab

le 3

. C

hem

ical

co

mpo

siti

on (

wt.

%)

of

apat

ite

in R

ock

Bod

y "P

"

i~lu

enti

al

No.

S

ampl

e N

o.

P 52

M61

SiO

2 A

I20~

Fe2

03 M

nO

MgO

C

a0

P20

, H

alO

K

~0

H20

+

H2

0-

F C

] C

O2

0.0

5

0.52

0.

48

0.04

0.

91

54.0

1 40

.98

0.40

0.

13

0.41

0.

14

2.72

0.

06

0.24

0.04

0.

32

0.13

0.

05

0.74

54

.90

41.0

1 0.

39

0.12

0.

24

--

3.09

0.

04

0.15

ISum

101.

09

101.

22

--1

.14

--1

.30

Tot

al

99.9

5

99.9

2

-- n

ot

dete

cted

, th

e sa

me

bell

ow;

anal

yst:

dit

to

O

5G

b~


Body " L " is generally bytownite of An 75--70 and laboradorite of An 60--52 in lesser amount. In rare cases, individual plagioclase included in olivine may be bytownite of An 80. Plagioclase in Cycle I I I gabbro may range from laboradorite to an- desite. Plagioelase from the lower section of Rock Body " B " exhibits a restricted range of composition, varying between An 64 54.

In most rocks of the three rock bodies, apatites are present only as accessory minerals. But in certain layers near their upper parts, for example, in the upper titanomagnetite-gabbro and in the lower section of plagioclase-gabbro of Rock Body " P " and in the upper gabbro and plagioelase-gabbro of Rock Body " L " , the apatite contents run as high as 4 10%. Close coexistence is observed between apatite and titanomagnetite, the former automorphically setting in the latter. Chemical analysis indicates that it belongs to the variety of fluoro-apatite carrying low concentration of rare-earths.

As has already been discussed in the previous article, titanomagnetite and ilmenite, like other major rock-forming minerals, can reflect some regular changes during differentiation with their composith)nal variation. So, all this will not be reiterated here.

Chemical Composition of Rocks and Chemical Evolution of Magma

Systematic petrochemical analyses wer~ conducted on. each layer of Rock Body " P " . The nineteen petrochemical analyses for the layers from bottom to top are shown in Tab. 4. Among them, P29 and P43 represent rocks from the bottom fringe belt, the composition of which is thought to be very similar to that of P55 representing the upper fringe belt. In F (Fe s+ + Fe 2+ ~-Mn)-M(Mg)-A(Na + K) diagram, each of them is restricted in an extremely narrow area, but differs widely as compared with the bulk composition of the whole rock body. Therefore, neither of them should be taken to represent the composition of original magma responsible for Rock Body " P " .

Being rich in iron and titanium is one of the mast remarkable petrochemical characteristics of Rock Body " P " as well as " L " and " B " , and other related layered intrusives. Besides, being unsaturated with respect to SiO~ and considerably high concentration of alkali are the other two important feartures. These iron and titanium constituents not only contributed to the massive accumulation in the lower part of the intrusives, but also exerted a profound influence on the process of magmatic differentiation of the layered intrusives. The variation of FeO +Fe~O~ (Fig. 5) in the profile of the intrnsives can be used as an indicator of the composition variattion of intrusive magma as a whole. Taking Rock Body " P " for example, during early crystallization stage of the mamga, except in the fringe belt, olivine-rich rocks were first precipitated as a not very thick layer of peridotite. Afterwards olivine would not be further enriched during the subsequent massive precipitation of iron and titanium oxides. Gravitational differentiation may have played an important role during this course. Although the Fe component increases gradually from the bottom upwards, yet the olivine concentration remains in a rather stable range of ?,---10% with respect to the entire profile. Moreover, a significant proportion of olivine seems not to have resulted directly from magmatic crystallization, but the product of reaction between

~uen

tial

'~_

. _

' N

o.

~am

pie

~o

. S

iO~

8 9 10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

Tab

le 4

. C

hem

ical

com

posi

tion

s (w

t,

96)

of

intr

usi

ve

rock

"P

" an

d a

ssoc

iate

d sy

enit

oida

l ro

cks

TiO

, A

hO~

Crl

O3

F

e20

3

FeO

M

nO

MgO

C

aO

Na2

0 K

20

P2

0~

V

20,

H~O

÷

HiO

-CO

, S

Tot

al

43.8

1 3.

12

15.7

0 0.

02

7.22

7.

72

0.1

6

6.60

11

.22

2.7

6

0.50

0.

28

0.07

0

.74

0.

30

--

100.

17

29

P 42

M28

M

29

M33

M

27

M35

M37

M

38

M45

P 37

P

39

P 47

P

50

P 51

P

52

P 53

M

61

P 55

39.8

8 0.

22

4.97

/

14.7

2 4.

73

/ 29

.45

3.00

14

.27

4.63

--

32

.01

37.1

4 0.

29

5.43

1.

93

14.7

3 3

.76

0.

29

30.5

4 39

.55

0.29

5

.24

7.

53

16.9

2 6.

07

0.15

24

.81

36.2

6 0.

29

3.90

7.

27

15.4

1 5.

50

0.20

21

.93

38.9

8 0.

31

5.80

12

.40

12.5

1 7.

09

0.16

30

.69

24.6

3 0.

20

5.70

4.20

0.

20

0.05

/

/ /

/ 0.

54

0.07

0.

01

0.08

0.

50

1.09

0.

05

0.76

0.

08

0.01

0.

10

0.56

1.

49

0.10

1.

34

0.36

0.

07

0.07

0.

48

1.33

0.

09

1.53

0.

24

0.05

0.

10

0.49

0.

92

0.07

3.

06

0.45

0.

05

0.05

0.

48

1.20

0.

38

/ / 0.

54

/ o.

68

/ o.

71

/ 0.

78

/ o.

53

3~.8

0 5.

50

14.4

6 0.

10

8.38

18

.92

0.1

6

26.9

5 8.

95

11.4

2 0.

13

11.3

0 25

.78

0.20

34

.20

11.4

3 11

.70

0.19

13

.49

11.0

0 0.

11

4.54

8.

58

1.88

0.

23

0.07

0.

20

1.47

0.

10

4.9

5

6.90

1.

40

0.17

0.

07

0.29

0.

92

0.04

5.

41

7.78

1.

42

0.18

0.

06

0.18

1.

75

0.52

/ 0.

39

/ 0.

72

/ O

.Ol

40.7

0 8.

00

11.6

1 tr

. 10

.02

9.40

0

.16

43

.81

4.80

18

.48

tr.

7.2

4

6.68

0.

12

42.8

5 4

.24

19

.66

tr.

6.64

4.

66

0.1

6

53.7

6 1.

84

19.6

9 tr

. 4.

80

3.98

0.

06

43.5

5 5

.44

13

.52

--

6.18

7.

49

0.26

41

.02

6.00

14

.27

tr.

9.68

8.

46

0.26

6.30

9.

48

2.1

5

0.24

0.

02

0.1

6

1.31

0.

51

/ --

4.

48

8.00

4.

15

0.48

0.

13

0.1

6

0.92

0.

38

/ --

5.

48

10.5

1 3.

75

0.24

1.

40

0.04

0.

76

0.08

/

--

2.0

6

6.11

5

.81

0

.41

0.

50

0.04

0.

61

0.16

/

--

5.9

5

8.99

4.

05

0.4

4

2.40

/

0.67

0.

54

0.20

6.

12

8.03

3.

20

0.2

4

1.81

0.

08

0.9

4

0.3

6

/ __

24.5

3 9.

01

1.23

0

.41

17

.76

25.3

5 0.

99

30.3

3 7.

77

1.88

0

.34

19

.21

20.4

5 1.

34

9.03

9.

42

0.55

0.

06

0.12

0.

13

0.73

0.

35

/ 0.

01

4.87

10

.45

0.5

6

0.25

0.

23

0.03

1.

22

0.28

/

0.03

98.4

2 99

.65

100.

11

100.

38

100.

03

99.5

8

99.7

8 10

0.19

99

.43

100.

06

99.8

3 10

0.47

99

.83

99.6

8 10

0.47

99.6

8 99

.24

43.7

2 4.

72

12.8

3 /

6.62

8.

75

0.18

7.

24

10.8

1 2

.50

0.

50

--

0.12

1.

71

0.57

/

--

100.

27

38.8

7 6.

25

13.4

3 0.

05

10.0

0 11

.35

0.20

5.

83

8.62

2.

72

0.32

0.

48

0.1

6

1.19

0.

36

tr.

0.10

99

.93

1.11

1.

17

7.25

5

.89

0.

17

0.45

--

0.

26*

1.65

2

.11

6.

91

4.75

0.

32

0.97

0.

02

0.29

* 0.

91

0.71

6.

70

4.66

0.

31

0.53

0.

28 0

.04*

0.3

3**

0.8

6

0.5

6

6.32

4.

70

1.37

"*

0.5

0

1.27

7.

15

3.47

0.

79**

0

.94

0

.31

7.

48

3.23

0.

10

0.77

0.

57

0.07

* 0.

80

0.77

8.

19

3.08

0

.06

0.

48

0.39

0.

28*

0.5

1

0.4

1

7.97

4.

77

--

0.29

0.

03

0.37

**

61.2

5 0.

72

16.9

9 2

.18

2.

34

0.09

59

.56

1.49

17

.05

2.4

6

3.15

0.

13

63.6

0 0.

70

15.9

5 3.

47

2.2

6

0.1

9

64.2

0 0.

67

16.0

8 4.

51

0.92

0

.14

63

.84

0.59

16

.33

8.55

1.

98

0.2

6

61.1

6 0.

54

14.4

8 6.

68

2.08

0.

13

61.9

0 0

.46

15

.07

5.20

2

.84

0.

16

64.5

4 0.

04

17.2

7 2

.45

0.

49

0.09

4.13

2.

12

0.1

6

0.8

4

0.8

4

7.41

4.

00

0.13

62

.53

0.58

15

.83

0.58

0

.27

0.2

1"

0.7

7*

*

100.

87

100.

86

100.

64

99.2

0 99

.73

98.6

4 99

.21

99.2

3

100.

40

* vo

lati

le

** l

oss

on i

gn

itio

n.

1. t

he a

vera

ge c

ompo

siti

on o

f P

29 (

fin

e.g

rain

ed o

livi

ne-g

abbr

o) a

nd

P42

(o

livi

ne-g

abbr

o) (

rep

rese

nti

ng

the

roc

k at

the

bo

tto

m o

f th

e fr

ing

e b

elt)

; 2.

per

ido

tite

(a

fter

Sic

huan

Geo

logi

cal

Bu

reau

);

3--

7.

tita

no

-mag

net

itlt

e; 8

--1

0.

tita

niu

m-r

ich

mag

neti

te-g

abbr

o; 1

1--1

2.

gabb

ro;

13

--1

4.

plag

iocl

ase.

gabb

ro;

15

--1

6.

gabb

ro;

17--

18.

tlta

no

mag

net

lte

pyro

xeni

te;

19.

horn

blen

de-g

abbr

o (a

t th

e u

pp

er p

art

of

the

frln

ge

bel

t);

10.

wei

ghte

d av

erag

e co

mpo

siti

on o

f R

ock

Bod

y "P

';

21.

horn

blen

de-s

yeni

te (

the

aver

age

of

2 sa

mpl

es);

22.

bi

otit

e sy

enit

e (t

he

aver

age

of

2 sa

mpl

es);

28.

arf

veds

onit

e-ae

giri

naug

ite-

syen

lte

(th

e av

erag

e o

f 7

sam

ple

s);

24.

quar

tz-s

yeni

to (

the

aver

age

of

2 sa

mpl

e~);

25

syen

ite

(th

e av

erag

e o

f 2

sam

plee

); 2

6.

llgh

t-co

lore

d sy

enit

e po

gmat

ite

(th

e av

erag

e o

f 2

sam

les)

; 27

. ar

fved

e-

soni

te s

yeni

te p

egm

atlt

e (t

he

aver

age

of

10 s

ampl

es);

28.

alk

ali-

syen

lte-

pogn

mti

te (

the

aver

age

of 2

sam

ples

);

29.

the

tota

l av

erag

e o

f 2

1--

28

. 2

1--

28

ana

lyze

d b

y S

ichu

an G

eolo

gica

l B

urea

u, I

nst

itu

te o

f G

eolo

gy,

Aca

dem

ia S

iaic

a an

d I

nst

itu

te o

f G

eoch

emis

try,

Aca

dem

ia S

inic

a;

the

rest

an

alyz

ed b

y C

entr

al A

nal

yti

cal

Lab

ora

tory

, In

stit

ute

of

Geo

chem

istr

y, A

cade

mia

Sin

ica,

unl

ess

stat

~l

othe

rwis

e.


iron-titanium oxide fluid and titanoaugite. It seems likely that the increase in iron content in olivine is chiefly related to temperature variation rather than to the concentration of MgO and FeO. In the diagram with 100 × (Fe0 ~- Fe~Os)/(FeO -~Fe~Os ~- MgO + Na~O + K~O) as abscissa, it can be seen that all the oxides of iron group, namely, TiO~, V~05, 1VfnO, and rock-forming oxides, such as SiO~, AI~O,, Ca0, Na~O, t(~O show a fluctuation with the FeO + Fe~03 concentration in magma. The oxides of iron group will be enriched with the enrichment of FeO + Fe,0a, forming prevailing- ly iron-~itanium-oxide mieral~. I~n contrast, however, the opposite trend is observed for rock-forming oxides. During the entire process of ma~o~matic differentiation, MgO shows no significant variation (Fig. 6), except in the earliest stage and in the fringe belt rocks. It can be justified therefore that because of the gravitational differentiation and the high concentration of iron and titanium in magma, the role played by MgO must haste been obscured in the presence of FeO during the course of magmatic differentiation. But this doesn't deny the fact that MgO tends to crystallize earlier than FeO in magma. Even in the melt of iron-titanium oxide, titanomagnetite and i].me~aite crystallizing at the earlier stage contain more Mg0 than those in the rocks at the upper parts of intrusives.

~10o

s0

X

+

A

I

Fig. 5. The evolution trend of (FeO + Fe203) along the profile A-B in Rock Body u p . .

In F-M-A triangular diagram we can notice the tendency mentioned above, and the three intrusives follow very similar tracks of magmatic evolution, resulting in massive iron-titanium oxide accumulation at the early and middle stages of magmatic differentiation. Afterwards, alkali (mainly Na) components in the magma increase gradually, and F value decreases abruptly with no significant variation in M value (Fig. 7). In view of abrupt changes in the M / F ratios in peridotites and gabbroidal rocks, some authors have argued that the two intrusives are the products of different periods and of different o~gins, that the former is the broken block of the upper mantle squeezed ,out of the earth's crust by tectonic movement, and that the gabbro is a differentiated ab:~ssal mass from basaltic magma. It should be pointed out, however, that no evidence has been found to support this idea because the peculiar behavior of FeO and MgO during the magmatic evolutionary process of these intrusives is neglected.


1.5- 1. O-

MnO 0.3- 0.2 0.I-

O-

20~

Ti 0., 10

0

10 l Na20+K20 5

0

30. 25.

MgO i0. 5. 0

Si02

~o

o

o ,Oo

~o o o

o

oo

oo o

0 o

~ o ~0

c~cX~ o Q)O

0 O 0

60-

40- o o

Q 20

o O. ~o

l oo 9'0 s'o

0 0 0 o o 0 o o o

o

O 0 ~ O0

0 0 ~ 0 0

o

O 0 ~ 0 0 0

O O (~Oo O O

O o O~Ooo O

-0.6

0.4 V2Os -0.2 0

o

75 -65 55 -45 "Fe20~ +FeO

"35 "25 o "15 .5

t 0 5 CaO 0

o

~20

- I0 AI20 o

-0

OO ~O O

o O0 0 o

o ° O

O o

O0 000 0

70 go 5o ,~o

(Fe~O~FeO t )<l O0/(Fe203 +Fe O+Mg O+Na~O+Kz O)

= l o 2

Fig. 6. The main constituents in the roek~ of Rock Body ~P" and their relationship with the variation of (Fe20~-b~eO) X 100/

(Fe,0, + :FeO + MgO + Na,O + K20). 1. Cycle I; 2. Cycle II. Note: oxides all in wt. %.

E v o l u t i o n a r y R e l a t i o n s h i p a m o n g t h e L a y e r e d [ n t r u s i v e s , S y e n i t e a n d O m e i s h a n B a s a l t

I n Tab le 2 a r e also s h o w n the c h e m i c a l c o m p o s i t i o n s o f s y e n i t e s a s soc i a t ed w i t h


F F F

03

h M M M

Fig. 7. F (Fe ~+ + Fe 2+ + Mn), M(Mg) and A(Na + K) compositions of intrusives"P ", "L" and "B'.

1. gabbro and ultramafic rocks; 2. syenite; 3. olivine.

the layered intrusives " P " , " L " and " B " . Compared with ord inary syenites or alkali syenitoidal rocks, they are characterized by relatively high contents of iron and ti tanium. ~ This is demonstrated by the great amounts of arfvedsonite, aegirineaugite, biotite, sphene and chevkinite, along with t i tanium-rich pyrochlore. In Fig. 7 the continuation of composition is obvious between syenitoidal rocks and layered intrusi- yes. Moreover, the composition of a contact mixed rock complex between a syenite and

a t i tanomagnetite gabbro in Rock Body " L " overlaps that of Rock Body " P " or " L " .

Thus, syenitoidal rocks not only exhibit an intimate spatial and temporal cor- relation with the layered intrusives, but also share co~mmon petrochemical characters

with them. I t is therefore reasonable to believe tha t : syenite alkali syenite plutons and the layered intrusives " P " , " L " and " B " must have originated from the same magma.

Chemical analyses of th Omeishan basalt are not referred to in Table 2, but in Fig. 8. the compositional range of 13 analyses by Wang Qian et al. is given. The average composition of 12 phenocryst-free basalt samples from this area is given ( ~ . % ) :

SIO2 47.30 TIO2 3.40 A1203 13.28 Fe~O~ 6.18 ~O 9.80

MnO 0.12 CaO 7.74 MgO 4.45 Na,O 2.67 K,O 1.59

PzO, 0.30 H20 + 2.94 H20- 0.52

100.29

As can be seen, the high contents of iron and t i tanium ( F e O + F e 2 0 s ~ 1 5 % ) are

the outstanding feature of the basalt which is consistent with the p r imary petrochemical characters of syenites and layered intrusives in this area. Therefore, we have come to some deeper unders tanding that the syenite plutons and layered intrusives " P " , " L " and " B " are the products of hypogene differentiat ion from a magma


equivalent to the Omeishan basalt in composition. Part of the magma, which was rich in Fe,Ti,Mg and Ca, gave rise to the layered intrusives " P " ) " L " and "B"; then the residual magma, being rich in Si and alkali resulted in syenite and alkali-syenite. From the basaltic magma reservoir and under the same structural and geological conditions, ~ series of igneous rocks were derived with variable compositions but extremely similar characteristics. I f this idea holds true, it would be possible to obtain a ~om- position that will be very close to the average composition of the Omeishan basalt by mixing, in certain proportion, the average composition of the syenite alkali-syenite with that of the layered basic-ultrabasic intrusives. On account of the respective layer sequence and thickness of the rocks in Rock Body " P " , their average composition is first calculated (Tab. 2), then if four portions of such an average rock is mixed with one portion of syenite---alkali-syenite, the final composition (wt.%) is expected to be:

SiO2 48.6

TiO, 5.11

A1,O3 13.91

Fe~O3 8.83

FeO 9.50

MnO 0.19

CaO 7.06

MgO 4 .83

Na~O 3.66

K20 1.06

H20 + 1.07

H,O- 0.82

P,05 0.41

Total

99.55

The finally calculated composition is very close to the average composition of the Omeishan basalt, and the two compositional points almost coincide with each other. Of course, this approach is quite arbitrary, but what is concerned with here is just to de- monstrate that this is one of the possible mechanisms.

Experiments by J. R. Holloway et al. TM on the melting relations of tholeiites from Kilauea, Hawaii (PH~o~ PT) indicated that at 5 kb'and 8 kb, and at 1,000°C the liquid phase coexisting with ~nonoclinic pyroxene, olivine, hornblende and magnetite is of quartz-diorltic composition near its solidus, thus supporting the model in which the partial melting of a water-rich basalt could lead to the production of a magma equivalent to calc-alkaline series in composition. H. Kuno m (1968) pointed out that crystallization differentiation from a tholeiite magma may produce a rock series of gabbro--~ ferrogabbro, traehybasalticJ pegmatite -~ granophyre, while from an alkali-basalt magma, a rock series of trachybasalt--~traehybasaltie pegmatite--~syenite is expected. Per- haps, this is compatible with the rock series of peridotite) iron-rich ultramafic rock --~ iron-rich gabbro, anorthosite--~syenite derived from the Omeishan basaltic magma. So, the series of iron-rich ultramafic rock--~iron-rich gabbro, anorthosite --*syenite found in this area seems mostlikely to have resulted either from hypogene differentiation or from partial melting of alkali- or sub-alkali-Omeishan basalt magma.

Here, some further discussion is needed on the type of Omeishan basaltic magma.

As is evidenced in Fig. 8, the Omeishan basalt is different from the Hawaiian tholeiite (6~ and from the original magma composition of the Stillwater layered intrusive t6~ which was considered as tholeiitie magma , as well as from the Cenozoic alkali- basalt of eastern China. E,J By plotting on a ~Si-alkali diagram aceording either to G. A. l~IacD,onald Ee~ or to H. Kuno ~'~, we have found that, with the exception of some individual points, all the points fall on the side of al~rA]i-basalt. We can thus conclude that the Omeishan basalt in this region is a unique type of iron (titanium)-rich, sub- alkali basalt. I t is also significantly different from the iron-rich, high-aluminium


A/ v v v - ~ M

Fig. 8. Comparison of magmati~ evolution trends for the layered intrusives up, , "L" and "B ' , the Skaergaard intrusive and their genetic relationship with basaltic magma.

I. the Omeishan basalt (after Wang Qian et al., 1963); IL Cenozoic bas~lts in eastern China (the analyses on 40 samples from 11 regions are averaged according to corresponding regions) (after Zhao Zhongpu v~, 1956); HI. Rock Body "P" of the fringe belt; IV. tholei-

ires from 7 regions of Hawaii (after G.A. MacDonald et al. E~j, 1964).

1. average composition of Rock Body " P ' ; 2. average composition of syenite; 3. average composition of syenite and Rock Body "P'; 4. average composition of Omeishan basalt (after Wang Qian et al.); 5. composition of mixed rocks of syenite and titanomagnetite pyroxenite; 6. composition of the cooling fringe rock of the Sfillwater intrusive (after H. H. Hess ts~, 1956); 7. average composition of 181 tholeiltes from Hawaii (after G. A. Mac- Donald et al. ~6~, 1964); 8. composition of the cooling fringe rock of the Skaergaard intru-

sive(after L. Wager and G. Brown l*~, 1968).

(1) petrochemical eomposltion area of Rock Body "P'; (2) petrochemical composition area of Rock Body ~L'; (3) petrochemical composition area of Rock Body ~B'; (4) magmatic

evolution curve of the Skaergaard intrusive (after IJ. Wager and G. Brown t*~, 1968).

Skaergaard intrusive, despite their high iron enrichment. As demonstrated in Fig. 8, the magmatic differentiation track and the composition of the original magma of the Skaergaard intrusive tS~ are not the same as the magmatie differentiation track of Rock Body " P " and the average composition of the Omeishan basalt.

H. Kuno m (1968) divided tholeiites and high-a]iminium basalts into high-iron- concentration and medium-iron-concentration sub-series (taking 15% FeO-bFe,0s as the dividing point), and suggested that there should be no iron enrichment or only mild degree of iron enrichment in alkali-basalts. However, in view of the existence of highly iron-rich rock series derived from the iron-rich, sub-alkali 0meishan basalt, it is advisable to make some modification on the classification proposed by H. Kuno.

C o n c l u s i o n s

1. Rock bodies " P " , " L " and "B" are iron-, titanium-rich basic-ultrabasic layered intrusives. Iron and t itanium were enriched in the lower and middle-lower parts


of the intrusives (or the lower and middle-lower parts of each accumulating rhythmic cycle) during the early or middle stages of magmatic cyrstallization-gravitational differentiation processes. " F e O " component played an important role during the magmatie differentiation process.

2. The three layered intrusives and the syenite alkali-syenite plutons which are intimately associated with the former in space and time and share with the former the common petrochemical characteristics of high iron and high titanium enrichment, have originated from the same magma source.

3. Hypogene differentiation of the Omeishan basaltic magma probably resulted in the rock series of layered basic-ultrabasic intrusives (peridotite ---> iron-titanium- rich ultramafic rocks--* iron-titanium-rich gabbro, anorthosite) -* syenite, alkali-syenite.

The Omeishan basalt in this region is an iron-titanium-rie~ sub-alkali basalt.

A c k n o w l e d g e m e n t s

Thanks to our colleagues Duan Yucheng ( ~ ) , Lin Xuenon ( ~ ) , Zhang Yuxue (~=~,_~), Yang Jiwu ( ~ ) , Zhan Xingzhi (~ , .=~ , ) , Wang Kuereng (~E ~ : : : ) , Mei Houjun ( ~ ) , Yuang Qiling ( ~ ) , Tan Jizu ( ~ ) , Zhu Shou- hua ( :~ j~-~) , Xie Yinwen ( ~ ) ~ ) , Chen J inyu (~J~s~I), Chen Nansheng ( ~ ~b), Bei Zhenghua (~]]~-~) , Chen Damei (~j~j-,:~:~), Tang Chunjin ( ~ J ~ ) , Shi Jixi ( ~ j } ) , Xu Xueyen ( ~ j ~ , ~ ) , and Gao J iyuan ( ~ ' : ~ ) for their efforts in this work.

R e f e r e n c e s

[ 1] ~ : 1973, ] ~ / ~ l g - ~ f ~ ~ : ~ r ~ l ~ , , / ~ , ~ 8 ~ , ~ 175-191 ~o [ 2 ] Jackson E. D. : 1967, U l t ramaf ie Cumulates in the Still~vater, Great Dyke and Bushveld In -

trusions. In: Wyl]ie P. J.: '=Ultramafic and Related Rocks '~, p. 21. [3 ] Holloway J. R. Burnham G. W.: 1972, l~[elting Relations of Basalt with ]~quillbrium Water

Pressure Less than Total Pressure. J. Petrology, V. 13, No. 1, pp. 1--29. [ 4 ] Kuno H.: 1968, Differontation of Basalt Magmas. In: H. H. : "Basalts", V. 2, p. 623---688. [ 5 ] Hess H. H.: 1956, Stillwater Igneous Complex, Montana. Geol. Soei. Amer. Menoir. 80. p 152. [ 6 ] MacDonald G. A. and Katsura T.: 194, Ohemical Composition of Hawaiian Lavas. J. Petrology,

Y. 5, No. 1, pp. 82--133. [ 7 ] ~ ; , ~ : 1956, ~ m ~ j { ~ : ~ a : { ~ : ~ - ~ { ~ , ¢ : [ ~ [ ~ : , . , ~ 36 ~ ,~g 3 ~], pp. 315--367. [ 8 ] Wager L. and Brown G.: I968, Layered Igneous Rocks.

Documents

Petrographic characteristics and several other related problems of the layered basic-ultrabasic intrusives in the Upper Yangtze Valley