Journal of Volcanology and Geothermal Research, 6 (1979) 375--389 375 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

GEOCHEMISTRY AND ORIGIN OF ANDESITIC ROCKS FROM NORTH- WESTERN SARDINIA

C. DUPUY ~ , J. DOSTAL 2 and C. COULON ~

1 Centre G~ologique et G~ophysique, U.S.T.L., 34060 MontpeUier Cedex (France) 2 Department o f Geology, Saint Mary's University, Halifax, N.S. (Canada) 3 Laboratoire de P$trologie, Universit~ Saint-Jer5me, 13397 Marseille Cedex (France)

(Received October 18, 1978; revised and accepted May 16, 1979)

ABSTRACT

Dupuy, C., Dostal, J. and Coulon, C., 1979. Geochemistry and origin of andesitic rocks from northwestern Sardinia. J. Volcanol. Geotherm. Res., 6: 375--389.

The chemical variations in the continental-margin andesitic suite from northwestern Sardinia are compatible with a mechanism involving both fractional crystallization and contamination. Contamination is probably related to interaction with ignimbrites formed by crustal anatexis, which are spatially and temporally associated with andesitic rocks. A similar process may have also affected other calc-alkaline andesitic suites such as those of New Zealand and southern Peru where the contents of lithophile elements and the Sr isotope ratios appear to be related to the composition of associated ignimbrites.

INTRODUCTION

Among the various hypotheses which have been proposed for the origin of parental magmas of andesitic series of orogenic areas, the most widely accepted models involve partial melting of either the subducted oceanic lithosphere or upper mantle peridotite. Although these processes are in agreement with the data of experimental petrology (Mysen et al., 1974; Ringwood, 1974), numer- ous geochemical studies (Nicholls, 1974; Noble et al., 1975; Thorpe et al., 1976) have shown that the abundances of lithophile elements in many ande- sitic suites are not readily consistent with simple models of anatexis of the subducted ocean-floor tholeiites or of "normal" upper mantle peridotite.

In order to explain the relatively high content of these elements in the andesitic rocks, several processes leading to their enrichment either in the paren- tal source or directly in the andesitic magmas have been evoked. Alteration of ocean-ridge basalts (Gill, 1974}, addition of oceanic sediments to the sub- ducted basalts (Armstrong and Cooper, 1971; Church, 1976) and enrichment of the upper mantle overlying the Benioff zone (Nicholls, 1974; Best, 1975) were suggested as possible processes for the enrichment of the source material. On the other hand, the high contents of lithophile elements in the andesites of continental margins (Jake~ and White, 1972) have been attributed to various

376

processes of crustal contaminat ion (Dupuy and Lef~vre, 1974; Klerkx et al., 1977) and even to a direct mixing of basaltic and rhyolit ic magmas as sug- gested by Fenner (1926) and Eichelberger (1975). Although there are data, such as the Sr isotopes (Klerkx et al., 1977), indicating that crustal contamina- t ion played a role in the genesis of andesitic suites, only a limited amount of geochemical information is available on the extent and significance of this process in andesite petrogenesis.

The purpose of this paper is to present some geochemical data on calc-al- kaline volcanic rocks of nor thwestern Sardinia and to evaluate the influence of crustal contaminat ion on the distribution of incompatible elements in ande- sitic rocks. More specifically, the interaction of andesitic and ignimbritic mag- mas, which are closely associated in cont inental orogenic zones, suggests a mechanism which probably can, to a large extent , account for the high and varied contents of lithophile elements in many andesitic suites.

GEOLOGICAL NOTES

The calc-alkaline volcanic series studied outcrops in the Logudoro-Bosano area, nor thwestern Sardinia (Fig.l) . The series has been geologically and geochemi- cally described by Coulon et al. (1973) and Coulon (1977). The Cenozoic calc- alkaline volcanism consists of an andesitic suite (basalts, andesites, dacites and rhyolites) and ignimbrites which are closely associated in t ime and space. In the area, three successive episodes of andesitic volcanism have been recognized

~ ~,~:%~.~'~..~. '::i~: ~: :i:i :

Fig. 1. Generalized geological map of northwestern Sardinia; 1 = andesitic series SA-1, 2 = andesitic series SA-2, 3 = andesitic series SA-3, 4 = ignimbrites SI-1, 5 -- ignimbrites SI-2, 6 = rhyolitic lava domes associated with ignimbrites, 7 = Plio-Quaternary alkali basalt series, 8 = Miocene sediments, 9 = Quaternary sediments.

377

on the basis of detailed geological mapping (Coulon et al., 1973, 1974). The first episode (SA-1), with an age ranging between 21 and 24 m.y., is composed of basalts, basic andesites (SiO2 < 56%) and subordinate andesites. The second cycle (SA-2), about 17 m.y. old, includes basalts, basic andesites and dacites. A volumetrically minor third episode of volcanism (SA-3) dated at 13.3--14.3 m.y. consists of basic andesites, dacites and rhyolites. The rocks of all three episodes are intercalated with the contemporaneous calc-alkaline ignimbrites whose surface exposure volume is about four times that of the other rocks. The ignimbrites have dacitic to rhyolitic composit ions and belong to two erup- tive episodes. The older episode comprises the " lower" ignimbritic series (SI-1) which is about 400 m thick and was formed by ash and pumice flows with variable degrees of welding. The second episode includes the "upper" ignim- britic series (SI-2) up to 200 m thick.

The rocks of the andesitic series are porphyritic, with a groundmass normal- ly containing both microphenocrysts and glass or cryptocrystalline material. Phenocrysts of strongly zoned plagioclase and cl inopyroxene occur together with olivine in the basalts and basic andesites and with or thophyroxene in the andesites and dacites. Phenocrysts of amphibole and bioti te are sporadic and those of quartz are rather scarce.

The andesitic lavas experienced extensive fractional crystallization at a maximum depth of 20--30 km (Coulon, 1977). The temperatures obtained from the or thopyroxene-cl inopyroxene pairs of these rocks are 1000--1030°C for phenocrysts and 930--1000°C for groundmass. The close spatial and temporal association, together with the similarities in the mineralogical and chemical compositions, indicate that the andesitic rocks of all three episodes were derived from the same source region (Coulon, 1977), probably an upper mantle peridotite overlying the Benioff zone (Dostal et al., 1976).

There is some evidence that the Sardinian ignimbrites are not differentia- tion products of andesitic magma. This evidence includes the observed volu- metric relationship between andesitic rocks and ignimbrites, and the relative- ly small differences in lithophile elements between andesites and ignimbrites. Coulon et al. (1978) have argued that ignimbrites were generated at depths of about 20 km by partial melting of crust, probably of tonalitic rocks. The equilibrium temperature deduced for the ignimbrites by several geothermom- eters (Verni~res et al., 1977; Coulon et al., 1978) are consistently high, in the range of 910--1070°C. The high estimated temperatures are incompatible with the normal geothermal gradients and imply a local increase of tempera- ture which is probably related to the ascending andesitic magma, the liquidus of which is about 1050°C at a depth of 20--30 km (Green, 1972). It may be noted that the depth of the extensive fractionation of andesites is comparable to that of the generation of ignimbrites.

ANALYTICAL METHODS

The major elements and Li, Rb, Sr and Ba were determined by atomic ab- sorption and La, Ce, Zr and Nb were analyzed by X-ray fluorescence. The

378

rare earth elements (REE) of ignimbrites were determined by instrumental neutron activation (Gordon et al., 1968) while those of andesitic rocks were determined by a radiochemical neutron activation technique adapted from Denechaud et al. (1970). The precision and accuracy of the trace element data can be evaluated from replicate analyses of the standard rock W-1 (Tables 1 and 3).

R E S U L T S AND D I S C U S S I O N

The average chemical compositions of the individual rock-types of several andesitic volcanic complexes from the Logudoro-Bosano area*, grouped ac-

T A B L E 1

The average chemica l c o m p o s i t i o n s of t he individual rock- types of several andes i t ic sui tes and ign imbr i t es f rom n o r t h w e s t e r n Sardin ia

Andes i t i c series

Bosano area F r o m m a Mt. Seda Oro

SA-1 SA-1 SA-2

n=3 3 3 2 3 4 10 6

Majorelemen~(wt.%)

SiO~ 48.8 51.1 54.2 50.3 48 .6 54.1 59.5 64.4 TiO 2 1.0 0.9 0.8 0.9 1.0 0.8 0.7 0.6 A1203 18.9 19.1 19.8 18.4 18.4 17.2 16.9 15.9 F e : O 3 * 10.6 9.7 7.8 10.2 11.0 8.7 6.3 4 .6 MnO 0.2 0.2 0.1 0.2 0.2 0.1 0.1 0.1 MgO 4.8 4.0 3.3 5.2 4.4 3.8 2.1 1.1 CaO 10.2 9.0 8.2 9.6 10.0 7.6 5.4 3.7 Na20 2.5 2.8 2.8 2.4 2.6 2.7 3.3 3.7 K20 1.0 1.2 1.5 1.0 1.1 2.8 3.3 3.2 P20s 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 H20 1.8 1.4 1.0 1.2 2.3 1.7 1.8 2.1

100 .0 99 .6 99.7 99 .6 99.8 99.7 99.6 99.6

~aceelemen~(ppm)

Li 10 14 16 12 11 24 19 17 R b 21 30 37 19 23 89 109 131 Ba 188 265 346 196 194 291 395 504 La 16 23 27 32 Ce 39 54 60 67 Zr 112 169 191 205 Nb 6 10 11 12

n = n u m b e r o f samples ; * -- t o t a l Fe as Fe203 ; ( ) = s t anda rd devia t ion .

• The c o m p l e t e chemica l analyses can be o b t a i n e d o n reques t f r o m the au thors .

379

cord ing to their age, are given in Table 1. I t shows tha t equivalent rocks f r o m the d i f fe ren t volcanoes have ra ther similar major e lement compos i t ions bu t di f fer in the i r abundances o f K and o the r l i thophi le elements. These differ- ences also appear when two ne ighbour ing volcanoes o f the same cycle are com- pared. The low con ten t s o f l i thophi le e lements in the volcanic comp lex o f Mt. Ozzas t ru resemble those o f the island arc calc-alkaline rocks o f Jake~ and White (1972) while the compos i t i ons o f the volcanic rocks o f Mt. Seda Oro are similar to those o f con t inen ta l margin rocks. The SiO2 c o n t e n t o f the vol- canic rocks o f Mt. Ozzas t ru ranges f r o m 48.9% to 57.2% and in Mt. Seda Oro it varies f rom 47.5% to 65.8%. As the rocks o f b o t h these volcanoes were prob- ably derived f rom the same or a similar parental m a g m a (Coulon, 1977) , the

Mt. Ozzastru Mt. Larenta

SA-2 SA-3

2 2 3 3 5 2

Ignimbrites W-1

SI-1 SI-2

14 13

49.0 54.0 57.1 53.1 64.4 71.0 64.5 65.1 1.0 0.7 0.7 0.7 0.4 0.2 0.5 0.5

19.0 18.7 17.2 17.6 15.9 14.7 14.2 14.9 11.1 8.1 7.7 8.5 4.7 2.6 4.0 4.1

0.2 0.2 0.2 0.2 0.1 0.0 0.1 0.1 4.6 3.4 3.4 4.7 1.3 0.3 0.8 1.4

10.1 8.7 7.2 8.6 4.5 2.4 2.4 3.4 2.1 2.7 2.7 2.7 3.5 3.4 3.1 3.4 1.1 1.3 1.6 1.4 2.8 4.1 3.9 3.7 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 1.5 2.0 1.8 2.0 1.9 0.9 6.2 3.2

99.9 100.0 99.8 99.7 99.7 99.7 99.8 99.9

11 14 12 13 15 25 7 10 13(1) 29 39 54 37 75 137 141 155 21(1)

170 245 267 298 472 614 580 678 177(7) 34 35 35 31 34 10(1) 70 72 73 65 74 23(1)

166 164 236 230 111(6) 11 14 16 13 8(0.9)

3 8 0

differences in the contents of lithophile elements are probably due to subse- quent processes. The trace element data may put constraints on the character of such mechanisms.

There is mineralogical and chemical evidence that the andesitic magma under- went fractional crystallization (Dupuy et al., 1975; Coulon, 1977). The be- haviour of two trace elements with bulk partition coef f ic ien ts (DS°lidfliq uid ) "~ 1 during magmatogenic processes can be graphically expressed, in a simplified manner, on a rectangular diagram with a normal scale. According to the Rayleigh fractionation law, the variations of two such trace elements during fractional crystallization fall on the straight line passing through the origin. The variations of Rb vs. Ba and of Ba vs. La in the rocks of three volcanic complexes with a large range of SiO2 contents are plotted in Fig.2. The rocks from individual volcanoes do not fall on straight lines going through the origin and their trends of variation extend towards the position of ignimbrites, indi- cating that the process of fractional crystallization alone cannot explain the abundances of lithophile elements in the andesitic rocks. A convergence of the chemical composit ions of the more evolved rocks from the andesitic series on those of the ignimbrites is further suggested by the data in Table 1, giving the average composi t ion of these rocks.

In order to ascertain whether similar variation trends are present in other volcanic series, the abundances of Ba and Rb in the rocks of several volcanic complexes are plot ted in Fig.3, together with pairs of trace elements from the andesitic rocks of northwestern Sardinia and from associated ignimbrites. The volcanic rocks of Tonga were emplaced in an area devoid of continental crust, while the other series were extruded on the continent and are closely associated with voluminous ignimbrites. Fig. 3 shows that the trends of vol-

800 800

600

400

200

~Sl - 2 p . . . . .

i

Y" / "'" • 400

200 x 3

sM - 1 T ~ I - z

I I I 0 I I I I

50 100 150 0 10 20 30 40

Rb (ppm) La (ppm)

Fig. 2. T h e r e l a t i o n s b e t w e e n t h e a v e r a g e c o n t e n t s o f Ba a n d R b a n d o f Ba a n d La in t h r e e a n d e s i t i c se r i e s o f n o r t h w e s t e r n S a r d i n i a ; 1 --- v o l c a n i c c o m p l e x o f Mr. S e d a Oro , 2 = v o l c a n i c c o m p l e x o f Mt . L a r e n t a , 3 = v o l c a n i c c o m p l e x o f Mt . O z z a s t r u . × = ba sa l t ; a = bas i c a n d e - s i te , a = a n d e s i t e , v = dac i t e , o = r h y o l i t e , o = i g n i m b r i t e . T h e b a r s f o r i g n i m b r i t e s co r r e - s p o n d t o 1 s t a n d a r d d e v i a t i o n .

381

1400

1200

I000

800

000

400

200

i

2

2

1 "4

• 1 /

• 3 - " ...--'Ib ."

• /

~ . //] a

4, // i

0 5o RI3 ~ ppm )

Fig. 3. T h e re la t ions b e t w e e n Ba and R b in andes i t i c series a n d as soc ia ted ignimbri tes ; 1 = n o r t h w e s t e r n Sardin ia (a = Mt. S e d a Oro, b = Mt. Larenta; C o u l o n et al., 1 9 7 8 ) , 2 = A r i q u i p a s o u t h e r n Peru ( D u p u y and Lef'$vre, 1 9 7 4 ; our u n p u b l i s h e d data) , 3 = Chris t iana Is lands , A e g e a n Sea ( P u c h e l t et al., 1 9 7 7 ) ; 4 = N e w Ze a l a nd ( E w a r t e t al., 1 9 6 8 ) , 5 = T o n g a - K e r m a d e c ( E w a r t et al., 1 9 7 7 ) . ~ = basalt , x = basic andes i t e , = - andes i t e , v = g r o u n d m a s s o f ande- s ite , • = dac i te , 0 = rhyo l i t e , o = ignimbri te . T h e bars for ign imbr i te s c o r r e s p o n d to 1 s tandard dev ia t ion .

canic rocks of Tonga, which have the lowest abundances of lithophile ele- ments, are consistent with the fractional crystallization model. In fact, this process has been documented for the rocks of Tonga by Ewart et al. (1973, 1977). On the other hand, the andesitic lavas of other suites display trends towards the ignimbrites and the contents of some lithophile elements in the andesitic series appear to be related to their abundances in associated ignim- brites. The high contents of Ba in andesites from southern Peru are accom- panied by high concentrations in ignimbrites, while in New Zealand, the Aegean Sea and Sardinia, both andesites and ignimbrites have lower abun- dances. Some other lithophile element pairs such as Ba-La, Zr-Nb show similar trends. Available Sr isotope data on closely associated andesitic and ignim- britic rocks are plotted in Fig.4 and their variations also suggest the relation- ships between the Sr isotope ratios of these two types of rocks. This indicates that, in addition to fractional crystallization, the rocks emplaced on the con- tinental crust were also affected by crustal contamination, more specifically by interaction either with silicic melts produced by crustal anatexis or direct- ly with the crust from which the ignimbrites were derived.

The role of these two processes (fractional crystallization and crustal con- tamination) in the evolution o f the two Sardinian volcanoes with different

382

2.00

1.00

0.50

0.10

0 0 5

J

4 I 3 / I z /

, / 1 /

/ /

/ /

/ /

/ /

0.02 '7 ' ~ ' ' ' 0.703 o. o4 o.7o5 0 .06 o.7o7 o.7os o.7og

Sr 87 / Sr 86

Fig. 4. The relations between sTSr/8~Sr and Rb/Sr ratios in andesitic series and associated ignimbrites; 1, 2 = southern Peru: Ariquipa and Barroso, respectively (James et al., 1976), 3 = New Zealand (Ewart and Stipp, 1968), 4 = Central America (Pushkar et al., 1972). A = basalt, × = basic andesite, D = andesite, * = dacite, o = ignimbrite. The bars for ignim- brites correspond to 1 standard deviation.

contents of lithophile elements, is further assessed by a series of least-squares linear mixing calculations (Wright and Doherty, 1970), which have been ap- plied to various combinations of whole rock and phenocryst major element compositions. The major element analyses have been recalculated on a water- free basis to 100% with total Fe as Fe203. For the calculations of crustal con- tamination, it was assumed that the process represents a simple mixing of ig- nimbritic and andesitic magma. The major element compositions of the whole rocks used were those given in Table 1 while the compositions of phenocrysts obtained by microprobe have been reported by Coulon (1977). For the sake of simplicity, only the weight fractions of variables and residuals are given in Table 2.

The results indicate that the major element compositions of the sequence of basalt--basic andesite--andesite from the volcano Mt. Ozzastru can be reproduced by simple crystal fractionation. Furthermore, the mineral propor- tions required are also consistent with the observed modal data (Coulon, 1977). In the Mt. Seda Oro sequence, the fractional crystallization model seems to be compatible with the derivation of the basic andesites from basalts. However, the compositions of dacites show a better fit with the mixing model. K produces the main discrepancy of the fractionation model between ob- served and calculated compositions of andesite and dacite. These calculations confirm tha t fractional crystallization alone cannot explain the evolutionary

TA

BL

E 2

Le

ast

-sq

ua

res

ca

lcu

lati

on

s o

f m

ixin

g a

nd

fra

ctio

nal

cry

sta

lliz

ati

on

Mix

ing

F

rac

tio

na

l c

ryst

all

iza

tio

n

Mt.

Sed

a O

ro

Mt.

Oz

za

stru

M

t. S

eda

Oro

M

t. O

zz

ast

ru

1 2

3 4

5 6

7 8

9 1

0

SiO

2 --

0.3

7

--0

.14

+

0.1

0

+0

.22

+

0.0

2

+0

.13

--

0.3

1

--0

.33

--

0.0

2

+0

.02

T

iO2

--0

.06

+

0.0

1

+0

.02

--

0.1

1

0.0

0

--0

.02

0

.00

--

0.1

0

---0

.05

--

'0.0

2

A12

03

--0

.24

+

0.7

3

+0

.33

+

1.0

1

+0

.79

0

.00

+

0.0

3

--0

.03

+

0.0

1

+0

.05

F

e20

3 (

tota

l)

--0

.20

--

0.3

1

--0

.15

--

0.8

9

+0

.54

+

0.0

8

---0

.17

--

0.1

7

0.0

0

+0

.02

M

nO

--

-0.0

3

--0

.01

+

0.0

1

+0

.02

+

0.0

2

+0

.06

+

0.0

1

--0

.03

+

0.0

2

+0

.06

M

gO

+

0.3

9

--0

.46

--

0.2

3

--0

.61

+

0.9

4

---0

.03

+

0.1

2

+0

.07

+

0.0

1

+0

.02

C

aO

--0

.23

--

0.1

1

+0

.03

+

0.6

8

--0

.17

0

.00

--

0.0

8

--0

.10

+

0.0

1

+0

.02

N

a20

--

0.0

8

+0

.32

+

0.4

2

+0

.28

--

0.2

1

+0

.28

--

0.3

0

--0

.20

--

0.1

8

--0

.18

K

20

+

0.8

7

+0

.02

--

0.5

0

--0

.55

--

0.3

0

--0

.45

+

0.7

6

+0

.92

+

0.2

8

+0

.05

~R

2

1.2

07

0

.97

5

0.6

22

3

.08

9

1.9

63

0

.30

9

0.8

14

0

.93

0

0.1

14

0

.04

3

2:R

2 =

su

m o

f sq

ua

res

resi

du

als.

C

alcu

late

d w

eig

ht

fra

cti

on

s o

f c

om

po

ne

nts

:

A.

Mix

ing

1 =

bas

ic a

nd

esi

te =

0.6

9 b

asal

t +

0.3

1 i

gn

imb

rite

*

2 =

an

de

site

= 0

.56

bas

ic a

nd

esi

te +

0.4

4 i

gn

imb

rite

3

= d

acit

e =

0.3

3 a

nd

esi

te +

0.6

7 i

gn

imb

rite

4

= b

asic

an

de

site

= 0

.72

bas

alt

+ 0

.28

ig

nim

bri

te

5 =

an

de

site

= 0

.77

bas

ic a

nd

esi

te +

0.2

3 i

gn

imb

rite

B.

Fra

cti

on

al

cry

stal

liza

tio

n

6 =

bas

alt

= 0

.52

bas

ic a

nd

esi

te +

0.0

45

ol

+ 0

.08

3 c

px

+ 0

.03

9 o

pq

+ 0

.31

plg

7

= b

asic

an

de

site

= 0

.57

an

de

site

+ 0

.08

5 c

px

+ 0

.03

4 o

pq

+ 0

.05

5 o

px

+ +

0.2

5 p

lg

8 =

an

de

site

= 0

.72

dac

ite

+ 0

.02

3 c

px

+ 0

.03

5 o

pq

+ 0

.03

5 o

px

+ +

0.1

9 p

lg

9 =

bas

alt

= 0

.54

bas

ic a

nd

esi

te +

0.0

69

ol

+ 0

.06

7 c

px

+ 0

.04

0 o

pq

+ +

0.2

8 p

ig

10

= b

asic

an

de

site

= 0

.69

an

de

site

+ 0

.05

5 c

px

+ 0

.02

6 o

pq

+ 0

.23

plg

.

*A

ve

rag

e o

f ig

nim

bri

tes

of

SI-

1 a

nd

SI-

2.

(30

384

TABLE 3

REE abundances (in ppm) in andesitic and ignimbritic rocks from Mt. Seda Oro

Basalt Basic Andesite Ignimbrite W-1 andesite

1532 1570 1569 1789 1738 2226

La 15.0 23.9 28.9 28.5 34.9 34.6 10.5(0.9) Ce 34.5 53.5 59.7 56.2 79.1 67.1 22.3(1.6) Sm 4.8 5.6 6.0 8.5 7.1 6.9 3.2(0.14) Eu 1.49 1.39 1.44 0.96 1.64 1.12 1.0(0.11) Tb 0.63 0.69 0.72 0.63 0.97 1.05 0.59(0.08) Yb 1.99 2.13 2.47 2.7 3.8 3.6 2.3(0.23) Lu 0.32 0.33 0.38 0.41 0.56 0.53 0.31(0.02)

( ) = standard deviation.

trend of Mt. Seda Oro and it is probable that in the later stages of differenti- ation, this process was complemented by interaction with the ignimbrites. The contamination of the rocks of Mt. Seda Oro is also indicated by the Sr iso- tope data (Dupuy et al., 1974).

The abundances of REE in basalt, basic andesite and andesite from Mt. Seda Oro {Table 3) normalized to chondrites (Frey et al., 1968) are shown in Fig.5. The rocks have REE patterns typical of calc-alkaline rocks (Jakes and Gill, 1970) with an enrichment of light REE and fractionation of heavy REE. The REE contents and the La/Yb ratio of the rocks from Mt. Seda Oro slightly increase with the increase of SIO2. However, the model calculations of frac- tional crystallization for REE using the degrees of solidification and mineral proportions obtained from the least-squares calculations for major elements (Table 2) show that the relatively small differences in the total REE among the analyzed rocks are not consistent with this process. The calculated patterns (Fig.5) have significantly higher absolute abundances of REE than those ob- served in the corresponding rocks. The chondrite-normalized REE patterns of associated ignimbrites are enriched in light REE, have only limited heavy REE fractionation, and lack a distinct Eu anomaly; they are similar to the patterns of ignimbrites from New Zealand (Ewart et al., 1968) and some granitic rocks (Herrmann, 1970). Coulon et al. {1978) have argued that the REE abundances are compatible with the origin of ignimbrites by anatexis of tonalitic rocks. A comparison of the REE patterns of the ignimbrites with those of the andesitic rocks (Fig.5) shows that a simple mixing of basic and ignimbritic magmas in proportions obtained from the least-squares calculations for major elements {Table 2) cannot alone account for the observed REE abundances in basic andesite and andesite. This is also illustrated in Fig. 6 where the calcu- lated abundances of Ba, Rb and La in various products of mixing and frac- tional crystallization are plotted. However, the position of the observed

385

I00

5 0

0 10

_z ~ 100[, -I o 50

i I I I I J

B

1 0

I I I La Ce Srn Elu T~b Yb Lu

Fig. 5. A. The observed REE abundances in basalt (e) and basic andesite (D) from Mt. Seda Oro compared to the calculated REE distribution in basic andesite (x) derived by fractional crystallization from basalt using the degree of solidification (F) and mineral proportions given in Table 2. The partition coefficients of Arth and Hanson (1975) and the equation of total equilibrium fractional crystallization were employed. The shaded area represents the range of the REE abundances in ignimbrites SI-2 (average +- 1 standard deviation). B. The observed REE abundances in basic andesite (e) and andesite (o) from Mt. Seda Oro compared to the calculated REE distribution in andesite (x) derived by fractional crystalli- zation from basic andesite using F and mineral proportions given in Table 2. The others are the same as in Fig.5A.

a b u n d a n c e s o f these e l emen t s b e t w e e n the two ca lcula ted t rends , par t icular - ly in m o r e evolved rocks , is cons i s t en t wi th the s imu l t aneous pa r t i c ipa t i on o f b o t h these processes . The obse rved var ia t ion t rends for Ba vs. R b (Figs. 2 and 6) are n o t l inear. Changes o f d i r ec t ion b e t w e e n basal ts - -bas ic andes i tes and basic andes i tes - -dac i tes suggest a d i f f e rence in the process o f evo lu t ion . Since the leas t -squares ca lcu la t ions show t h a t basic andesi tes can be der ived f r o m basal ts by f rac t iona l c rys ta l l iza t ion , pa r t i cu la r ly if K is exc luded , it is p r o b a b l e t h a t the basal ts were on ly enr iched in K and re la ted e lements . The process of select ive e l e m e n t exchange m a y also expla in the a b u n d a n c e s o f l i thophi le e l emen t s in Mr. Ozzas t ru , where the c o m p o s i t i o n s o f the m a j o r e l emen t s are c o m p a t i b l e wi th f rac t iona l c rys ta l l iza t ion .

386

600

400

200

600

20O

0 I00 150 I0

Rb (pprn)

2'0 30 4'0

La (ppm)

Fig. 6. A comparison of the calculated (open symbols) and observed (filled symbols) variations of Ba vs. Rb and Ba vs. La in andesitic series of Mt. Seda Oro. The fractional crystallization model (dashed lines) assumes a parent-daughter relationship between successively more differentiated rocks. The element abundances in the rock are generated from those of less differentiated rocks via removal of phenocryst phases in proportions obtained by least-squares calculations (Table 2) using the partition coefficients of Arth and Hanson (1975) and the equation of total equilibrium fractional crystallization. The mixing model (solid lines) assumes that the successively more differentiated rocks were produced by the addition of the ignimbrites magma (* -- average of SI-1 and SI-2) in pro- portions obtained by least-squares calculations (Table 2). The composition of andesitic rocks used are those given in Table 1. The symbols are the same as in Fig.2.

CONCLUSION

Two neighbouring, genetically related volcanoes in northwestern Sardinia have calc-alkaline andesitic suites of different geochemical characteristics. One volcanic complex, Mt. Ozzastru, is composed of rocks with abundances of lithophile elements similar to island arc calc-alkaline rocks while the second, Mt. Seda Oro, consists of rocks of the continental margin type. The compo- sition of the sequence of Mt. Ozzastru is compatible with an evolution process dominated by fractional crystallization. On the other hand, a plausible ex- planation for the chemistry of the volcano Mt. Seda Oro is a mechanism in- volving both fractional crystallization and contamination. Contamination is probably related to ignimbrites which are spatially and temporally associated with andesitic rocks. It seems that basic magma of upper mantle origin pro- vided the heat for the partial melting in the crust. The resulting anatectic dacitic to rhyolitic liquid then interacted with the basic magma.

The actual mechanism of contamination is not clear and may vary from place to place. It appears that among the least evolved rocks of northwestern Sardinia, the process was limited mainly to a selective element exchange, al- though Eichelberger (1975) has documented direct mixing between basaltic and rhyolitic magmas in Glass Mountain, California (cf. also Pichler and Zefl, 1972). The process of exchange probably also affected the island arc calc- alkaline rocks (Mr. Ozzastru), as indicated by the abundances of some litho-

387

phile e lements . At the la ter stages o f d i f f e ren t i a t ion o f the con t inen ta l margin series, mix ing is p r o b a b l y the d o m i n a n t process o f con t amina t ion . Contamina- t ion o f basalt ic or andesi t ic magma p r o b a b l y s tar ted deepe r in the crust , con- t inued dur ing its ascent th rough the crust and was acco m p an ied by f rac t ional crys ta l l iza t ion. The degrees o f c o n t a m i n a t i o n and h o m o g e n i z a t i o n are p rob- ab ly a f f ec t ed by the crustal res idence t ime o f the magma.

Processes involving b o t h f rac t ional crys ta l l iza t ion and crustal con tamina- t ion p r o b a b l y p layed an i m p o r t a n t role in the genesis o f calc-alkaline ande- site suites no t on ly f rom n o r t h w e s t e r n Sardinia b u t also f ro m o t h e r areas such as New Zealand and sou the rn Peru. It appears tha t the co n t en t s o f l i thophi le e lements and the Sr i so tope rat ios in individual andesi t ic suites are re la ted to the compos i t i ons of associated ignimbri tes and t h a t the high co n t en t s o f these e lements in m a n y andesi t ic series, which p en e t r a t ed con t inen ta l crust e i ther in island arc or con t inen ta l margin env i ronments , do n o t requ i re a source enr iched in l i thophi le e lements . This mechan i sm m a y also explain the diffi- cult ies in quan t i t a t ive model l ing o f the f rac t ional crys ta l l iza t ion process in calc-alkaline andesi t ic suites (Gill, 1978) as c o m p a r e d to tholei i t ic series (Ewar t e t al., 1973) and the compos i t iona l similarities be tween ignimbri tes and acid rocks o f andesi t ic suites which have been descr ibed f ro m several areas (Fe rnandez et al., 1973; Kussmaul e t al., 1977) . I t is also p robab le tha t the d i f fe rences be twe en Andean (con t inen ta l margin) and island-arc types o f calc- alkaline andesi t ic rocks are, to a large degree, re la ted to the crustal inf luence.

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