Miocene calc-alkaline heritage in the pliocene postcollisional volcanism of monte arci (Sardinia, Italy)

Journal o f Volcanology and Geothermal Research, 14 (1982) 133--167 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

133

MIOCENE CALC-ALKALINE H E R I T A G E IN THE PLIOCENE POSTCOLLISIONAL VOLCANISM OF MONTE ARCI (SARDINIA, ITALY)

ROBERTO CIONI l , ROBERT CLOCCHIATTI2, GIOVANNI M. DI PAOLA 1, ROBERTO SANTACROCE 3 and SONIA TONARINI ~

1 Laboratorio Geocronologia e Geochimica Isotopica, CNR, Pisa (Italy) : CNRS, University ofParis-Sud, Orsay (France) 3 Istituto di Mineralogia, University ofPisa, Pisa (Italy) {Received March 4, 1982)

ABSTRACT Cioni, R., Clocchiatti, R., Di Paola, G.M., Santacroce, R. and Tonarini, S., 1982. Miocene calc-alkaline heritage in the Pliocene post-collisional volcanism of Monte Arci (Sardinia, Italy). In: R. Brousse and J. Lameyre (Editors), Magmatology. J. Volcanol. Geotherm. Res., 14: 133--167. At Monte Arci alkaline (hawalites to trachytes), subalkaline with a marked calc-alkaline character (basalts to dacites) and rhyolitic lavas were erupted almost simultaneously in Late Pliocene time. Major- and trace-element chemistry, microprobe mineralogy and isotopic data suggest a partial melting origin for both rhyolites and subalkaline rocks. Different sources are however inferred for two rock series: homogeneous, calc-alkaline in nature for subalkaline rocks; unhomogeneous, richer in 87Sr, for rhyolitic ones. Crystal fractionation differentiation from subcrustal alkali-basalts should have been the main process in the genesis of alkaline rocks. Large-scale contaminations with rhyolitic and/or alkaline rocks are evident in many of these lavas. Such a complicated magmatic association characterizes an area where volcanism related to post-collisional tensional movements in Pliocene time superimposes to Middle Miocene calc-alkaline basic volcanism related to previous subduction processes. The Pliocene volcanic history of Monte Arci emphasizes the influence of the paleogeodynamic environment on the nature of magmas erupted in post-continental collision areas, that are frequently difficult to arrange in the usual schemas connecting magma composition with tectonic setting.

INTRODUCTION

Although there are still many problems to be solved concerning the genesis o f most magma associations and their relationship to tectonic setting, the nature of volcanism has been increasingly used in recent years as an indicator of the geodynamic evolution of a given region. Alkalic and tholeiitic basalt associations of b o t h continental and oceanic environments are c o m m o n l y considered the expression of extensional tectonics, whereas dominantly calcalkaline associations are considered typical of converging plate boundaries.

0377-0273/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

134 Such a scheme may be generally valid; however, there are volcanoes where coheval coexisting magmas cannot be definitely assigned either to "extensional" or "compressional" associations. Monte Arci is an example of this kind. Here alkalic and subalkaline (with a marked calc-alkaline character) lavas were erupted almost simultaneously in Late Pliocene times. The aim of this paper is to investigate the genesis of these rocks and to explain the cause of this complex magmatic association in terms of the geodynamic framework of this part of the Mediterranean area. GEOLOGICAL OUTLINE The western half of Sardinia is largely covered by important volumes of volcanic rocks erupted in Oligo-Miocene and in Late Pliocene times. The Tertiary volcanic activity started in the Upper Oligocene (Coulon et al., 1974; Coulon, 1977). During this first period of activity, which lasted until the Middle--Upper Miocene, the volcanism, both submarine and subaerial, had a typical calc-alkaline nature. Volcanism resumed in Sardinia during the Pliocene and continued up to the Early Pleistocene (Savelli and Pasini, 1973; Coulon et al., 1974; Savelli, 1975; Di Paola et al., 1975). Except for limited volcanic activity along the eastern coast of the island (DorgaliOrosei area, Barisardo, etc.), the majority of Late Pliocene volcanism of Sardinia developed within the graben which affects the pre-Tertiary basement in the western half of the island. This graben possibly began to develop in the Upper Oligocene (Pecorini and Pomesano Cherchi, 1969). Most lavas of this period are fissure basalts that outcrop as small plateaux ("giare") covering both Miocene calc-alkaline products and Tertiary sediments. At the same time some relatively well-defined volcanic complexes like Montiferru and Monte Arci were also formed (Fig. 1). The emplacement of huge volumes of Late Pliocene basaltic lava flows is considered to be related to the present tensional tectonic setting of this part of the Mediterranean area. A marked tensional character has existed since the Upper Miocene, the most spectacular result of which is the formation of the Thyrrenian Sea abyssal plain (Barberi et al., 1978). Monte Arci is located in Western Sardinia about 20 km SE of the town of Oristano. The volcanic complex is elongated in a N--S direction for about 25 km with an average width of about 7 km, covering an area of about 150 km 2. Detailed work on stratigraphy and geochronology (Beccaluva et al., 1974, 1975a; Di Paola et al., 1975; Assorgia et al., 1976b) has shown the existence of rocks belonging to the t w o different periods of volcanic activity of Sardinia: (a) Middle Miocene (14.7--15.8 m.y., Di Paola et al., 1975) calc-alkaline basic rocks erupted under submarine conditions; and (b) subaerial Late Pliocene (2.6--3.7 m.y., Di Paola et al., 1975) silicic, intermediate, Fig. 1. Simplified geological map o f Sardinia (after C o c o z z a et al., 1 9 7 4 , m o d i f i e d ) . 1 = major faults; 2 = Quaternary c o n t i n e n t a l deposits; 3 = Plio-Pleistocene volcanics; 4 = Tertiary s e d i m e n t s ; 5 = O l i g o - M i o c e n e calc-alkaline volcanics; 6 = pre-Tertiary b a s e m e n t .

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137 and basic rocks. Figure 2 is a simplified version o f t h e 1 : 5 0 0 . 0 0 0 geological m a p o f this area (Assorgia et al., 1 9 7 6 a ) . MAJOR-ELEMENT CHEMISTRY AND PETROGRAPHY M a j o r - e l e m e n t analyses m a d e on selected samples o f Pliocene volcanic r o c k s o f M o n t e Arci are r e p o r t e d in Tables I, II and III t o g e t h e r with CIPW n o r m s and p h e n o c r y s t s m i n e r a l o g y . T h e (K20 + Na20) vs SiO~ diagram (Fig. 3) clearly indicates t h e existence o f at least three groups o f r o c k s with strongly d i f f e r e n t alkalinity a m o n g t h e Pliocene lavas o f M o n t e Arci. The m o r e alkalic r o c k s (Table I) include hawaiites, mugearites and t r a c h y t e s . Hawaiites are s u b a p h y r i c t o p o r p h y r i t i c lavas with olivine, labradorite (An61-$2), augitic J

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p

Z~ Z~ •- u t . ~ OO v

~ I

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I

.50

6 Z

~.LCAL N A L I N E A

50 i

I

AND

60 ,

I

ROCKS

MONTE

ARCUENTU

70 i

I

Si02 wt% i

Fig. 3. Alkalies-silica plot of Monte Arci Late Pliocene volcanics. Fields boundaries after Kuno (1960). Circles = subalkaline rocks (full for basaltic rocks); squares = rhyolites; triangles = alkaline rocks. Fields of both Pliocene alkali-basalts of Sardinia (Beccaluva et al., 1975b, 1976) and Miocene calc-alkaline volcanic of Marmilla and M. Arcuentu (Coulon, 1977) are also reported. Montiferru trend inferred from Beccaluva et al. (1976).

Fig. 2. Geological map of Monte Arci volcanic complex (after Assorgia et al., 1976b, simplified). 1 = Quaternary continental deposits; 2 = Pliocene subalkaline and alkaline basic lavas (subalkaline basalts; hawaiites and alkali basalts); 3 = Pliocene intermediate lavas (andesites and dacites); 4 = Pliocene alkalitrachytic lavas; 5 = Pliocene rhyolitic lavas; 6 = Miocene marine sediments ("Marmilla formation", Cherchi, 1971); 7 = Miocene submarine calc-alkaline volcanics; 8 = alluvial fans; 9 = probable volcanic centres; 10 = basaltic dykes and necks; 11 = faults and fractures.

138 TABLE I M a j o r - e l e m e n t a n a l y s e s , CIPW n o r m s a n d p h e n o c r y s t m i n e r a l o g y o f M o n t e A r c i P l i o c e n e s u b a l k a l i n e volcanic rocks Sample

130

82

153

34

182

30

31

28

170

223

Rock type

B

B

B

B

B

B

B

B

B

B

53.01 1.61 15.49 2.12 6.75 0.13 7.78 7.41 3.19 1.36 0.41 0.67

52.79 2.00 16.00 2.76 6.75 0.13 6.30 7.26 3.61 0.99 0.23 1.19

52.54 1.37 16.73 4.33 5.55 0.13 5.66 7.39 3.87 0.53 0.18 1.74

54.28 1.66 15.33 3.01 6.39 0.12 6,50 6.71 3.90 0.86 0.22 1.02

53.79 1.66 16.05 3.37 5.43 0.12 6.23 6.58 3.91 1.11 0.29 1.46

54.10 1.74 15.89 2.99 5.62 0.13 5.48 6.86 3.85 1.20 0.24 1.90

54.86 1.48 15.96 1.63 6.59 0.12 6.13 5.95 4.13 1.10 0.20 1.84

53.50 1.77 15.44 2.87 6.07 0.12 6.56 6.68 3.84 1.88 0.44 0.82

54.07 1.60 15.66 2.36 6.20 0.12 6.54 6.60 3.86 1.92 0.39 0.66

1.4 8.0 27.0 23.9 8.1 23.6 3.1

2.7 5.8 30.5 24.5 8.0 18.9 4.0

4.1 3.1 32.7 26.7 7.0 15.4 6.3

4.0 5.1 33.0 21.8 8.2 19.0 4.4

3.8 6.6 33.1 22.9 6.2 17.2 4.9

4.7 7.1 32.5 22.5 8.0 15.1 4.3

2.8 6.5 34.9 21.8 5.2 21.2 2.4

0.7 11.1 32.5 19.4 8.8 18.2 4.2

0.6 11.3 32.6 19.7 8.4 19.2 3.4

3.1 1,0

3.8 0.6

2.6 0.4

3.2 0.5

3.2 0.7

3.3 0.6

2.8 0.5

3.4 1.0

3.0 0.9

36.4

39.0

39.9

42.1

43.5

44.3

44.3

44.3

44.6

+ + (+)

+ +

(+) (+)

++ ++ ++ +

+ +

(+) (+)

++ ++ ++ +

++ ++ +

(+)

(+)

(+)

M a j o r - e l e m e n t analyses

SiO~ TiO 2 A1203 Fe203 FeO MnO MgO CaO Na~O K20 P2Os L.O,I.

53.21 1.57 16.70 1,15 7.53 0.13 6.78 7.05 3.73 0.87 0.28 1.00

C I P W norms Q 0.5 or 5.2 ab 31.6 an 26.3 di 5.6 hy 24.7 mt 1.7 hm il 3.0 ap 0.7

D.I.

37.3

Phenpcryst mineralogy Olivine Plagioclase Orthopyr. Clinopyr. Opaques Biotite Quartz*

X-ray fluorescence analyses (analysts: M . Menichini and M . Saltta); M g O b y Atomic Absorption analyses (analyst: R . Cioni); F e O titrirnetric(analyst: R . Cioni) B = basalt; A = andesite; D = daeite; * = xenolithic; + + = abundant (up to 1 0 vol.%); + = present (up to 3 vol.%); (+) = scarce (less than 0.5 vol.%).

clinopyroxene and minor opaque phenocrysts or microphenocrysts. The same minerals occur in the microcrystalline groundmass. Mugearites are porphyritic, microcrystaUine, or hypocrystalline lavas with andesine plagioclase, augitic clinopyroxene, opaques and rare olivine phenocrysts; the same mineral, excluding olivine, are found in the groundmass. Trachytes are strongly porphyritic lavas with abundant soda-sanidine commonly with oligoclase core and clinopyroxene, phenocrysts. Opaques, apatite, zircon and rare biotite occur as microphenocrysts. Small corroded quartz crystals,orthopyroxene fragments and resorbed olivine associated with plagioclase and opaque have

139

205

84

26

144

42

81

73

40

25

211

115

197

B

B

B

B

B

B

B

B

B

B

A

A

55.35 1.44 16.41 3.40 5.31 0.12 5.67 6.49 4.18 0.58 0.21 0.83

55.72 1.62 15.43 1.64 6.69 0.11 5.95 6.44 4.10 1.06 0.25 0.99

54.76 1.56 15.75 3.71 5.25 0.12 5.59 6.28 3.88 1.01 0.20 1.89

55.77 1.31 15.99 2.42 5.91 0.12 5.84 6.13 4.16 0.89 0.24 1.22

55.02 1.55 16,13 2,05 6.21 0.12 5.79 6,03 4.02 1.43 0.28 1.37

55.94 1.42 15.67 2.43 6.10 0.11 5.94 6.36 4.33 0.80 0.23 0.61

55.74 1.55 15.62 3.96 5.00 0.11 5.52 6.58 4.13 0.66 0.17 0.96

55.55 1.59 15.24 4.33 4.85 0.10 5.17 6.42 3.97 1.09 0.20 1.49

55.11 1.53 16.09 3.02 5.17 0.12 5.57 6.07 4.04 1.89 0.33 1.05

55.52 1.59 15.12 2.73 5.64 0.10 5.66 6.26 4.01 1.96 0.36 1.02

56.48 1.57 15.77 3.52 5.68 0.10 4.21 6.59 4.27 0.93 0.25 0.63

57.99 1.48 14.52 1.95 5.64 0.10 5.38 5.80 3.66 2.05 0.20 1.23

6.3 3.4 35.4 24.3 5.2 16.4 4.9

4.1 6.3 34.7 20.6 8.0 19.5 2.4

6.8 5.9 32.8 22.6 5.7 15.4 5.4

5.3 5.3 35.2 22.3 5.2 18.9 3.5

3.4 8.5 34.0 21.7 5.2 19.3 3.0

4.6 4.7 36.6 21.0 7.4 18.3 3.5

7.8 3.9 34.9 22.1 7.4 13.7 5.7

8.2 6.4 33.6 20.6 8.0 12.0 6.3

3.5 11.2 34.2 20.2 6.3 15.5 4.4

3.7 11.6 33.9 17.5 9.1 15.4 4.0

8.2 5.5 36.1 21.1 8.1 11.7 5.1

8.3 12,1 31.0 17.1 8.3 15.9 2.8

2.7 0.5

3.1 0.6

3.0 0.5

2.5 0.6

2.9 0,7

2.7 0.6

2.9 0.4

3.0 0.5

2.9 0.6

3.0 0.9

3.0 0.6

2.8 0.5

45.0

45.1

45.5

45.8

45.9

46.0

46.6

48.3

48.9

49.2

49.8

51.3

+ ++ ++

(+) + +

+ (+) +

+ + +

+ + +

(+) (+)

++ + (+)

++ ++

+ + +

(+)

(+)

(+)

(+)

been found in some thin sections. The original hypocristalline or glassy groundmass is generally partly or totally devitrified. The second group has a subalkaline affinity and includes most of the rocks o f Monte Arci (Table II); according to current classification schemes (f.i. Irvine and Baragar, 1971) this group should be considered o f calc-alkaline nature. This is confirmed by the AFM diagram o f Fig. 4 where these rocks show no iron enrichment and overlap the typical calc-alkaline trend of the Cascades Range. However, it will be called the subalkaline group to avoid confusion with the Oligocene--Miocene "true" calc-alkaline association of

140

TABLE I (continued) Sample

168

224

GI0

66

G9

GI2

G2

68

208

179

Rock type

A

A

A

A

A

A

D

D

D

D

analyses 60.49 61.28 1.16 1.12 15.27 15.13 1.28 1.38 5.00 4.66 0.11 0.11 4.36 4.16 4.66 4.52 3.82 3.92 1.93 1.94 0.14 0.15 1.77 1.63

60.41 1.40 14.06 4.37 3.37 0.10 4.48 5.16 4.22 1.71 0.20 0.51

60.61 1.39 14.31 3.47 3.92 0.11 3.89 4.71 3.90 2.01 0.17 1.50

61.81 1.10 13.79 2.88 2.91 0.07 5.94 4.06 3.32 3.02 0.22 0.87

61.72 1.29 14.14 4.86 2.31 0.08 3.85 4.63 3.93 2.12 0.18 0.89

64.39 1.08 13.66 1.56 4.02 0.08 3.80 3.77 3.81 3.20 0.18 0.47

64.82 0.95 13.83 1.27 3.61 0.11 4.04 3.28 3.60 3.38 0.16 0.95

64.92 1.10 13.66 1.55 3.71 0.07 3.29 3.56 3.66 3.66 0.21 0.61

65.42 0.97 14.57 1.30 3.31 0.10 3.04 3.23 3.69 3.57 0.18 0.62

12.6 11.4 32.3 18.8 2.7 15.9 1.9

13.8 11.4 33.1 18.0 2.9 14.8 2.0

14.1 10.1 35.7 14.4 7.7 8.4 6.3

15.3 11.9 33.0 15.6 5.3 9.5 5.0

14.5 17.8 28.1 13.8 3.8 14.3 4.2

16.2 18.9 32.2 10.7 5.5 11.3 2.3

17.3 20.0 30.4 11.6 2.9 12.9 1.8

17.3 21.6 31.0 10.0 5.1 9.6 2.3

18.2 21.1 31.2 12.6 1.8 10.3 1.9

2.2 0.3

2.1 0.3

2.7 0.5

2.6 0.4

2.1 0.5

17.1 12.5 33.2 14.7 5.5 7.0 4.0 2.1 2.4 0.4

2.1 0.4

1.8 0.4

2.1 0.5

1.8 0.4

56.3

58.3

59.9

60.2

60.4

62.9

67.3

67.7

69.9

70.5

+

++

++

++

(+)

C+) +

Major~lement SiO 2 TiO 2 A1203 Fe203 FeO MnO MgO CaO Na:O K:O P2Os L.O.I. CIPW norms Q or ab an di hy mt hm fl ap

D.I.

Phenocryst mineralogy Olivine Plagioclase ++ + Orthopyr. ++ (+) Clinopyr. Opaques Biotite Quartz*

++ ++

Sardinia, which displays some different petrographic and chemical features. The subalkaline group includes basalts, andesites, dacites and rhyolites. Basalts are aphyric to porphyric, microcrystalline or hypocrystalline lavas, cropping out as flows, dikes and necks. Phenocrysts include: olivine (generally slightly altered to iddingsite), andesine-labradorite plagioclase, orthopyroxene (frequently with thin pigeonite rims) and rare augitic clinopyroxene. The same minerals (rare olivine) plus opaque grains constitute the groundmass. Large mottled (small ground mass and glass inclusions) ptagioclase phenocrysts with bytownitic cores commonly occur as well as partially melted quartz xenocrysts with large pyroxene reaction rims. A n d e s i t e s are porphyritic to subaphyric hypocrystalline or glassy lavas with plagioclase {Ans0) and/or orthopyroxene phenocrysts. Plagioclase and orthopyroxene (with pigeonite rims) occur in the groundmass together with calcic clinopyroxene and opaques. Quartz, oligoclase and, more rarely, olivine are common and

141

126

196

G8

80

D

D

D

D

64.52 1.17 14.12 2.78 2.57 0.10 2.24 3.29 3.91 3.68 0.44 1.18

66.76 0.89 14.18 1.06 2.98 0.10 2.71 2.76 3.56 4.13 0.19 0.68

66.87 0.92 13.61 1.56 3.15 0.06 2.60 3.08 3.78 3.75 0.17 0.44

65.24 1.05 13.85 3.23 1.65 0.10 2.21 3.09 3.72 4.19 0.39 1.08

19.0 21.7 33.1 10.1 2.6 5.0 4.0

19.7 24.4 30.1 10.5 1.6 9.3 1.5

20.2 22.2 32.0 9.1 4.0 7.6 2.3

2.2 1.0

1.7 0.4

1.8 0.4

19.6 24.8 31.4 8.7 3.2 4.0 3.3 1.0 2.0 0.9

73.8

74.2

74.4

75.8

++ ++ ++ ++

+ + + (+) (+)

(+) +

++ ++ ++ ++

relatively abundant as partially resorbed xenocrysts. The groundmass is generally devitrified. Dacites are subaphyric to porphyritic hypocrystaUine or glassy lavas with plagioclase, ortho- and clinopyroxene phenocrysts and opaques microphenocrysts. The same minerals occur in the groundmass. Mafic cumulates are abundant. Quartz and, more rarely, feldspar xenocrysts have been found. The most silicic products of Monte Arci (rhyolites, Table III) are generally scarcely porphyritic, with variable proportions of oligoclase, sanidine, hypersthene, clinopyroxene and biotite microphenocrysts. Minor quartz occurs rarely as small rounded grains. Apatite, zircon and opaques are confined to the groundmass which generally consists of a perlitic glass. The most silicic members (SIO2 > 72%) are aphyric glassy lavas, occurring in some phases as perlites or obsidians but otherwise devitrified. They contain microlites of the same minerals of the porphyritic varieties. Although apparently belonging

142

to the subalkaline suite in the alkalies-silica diagram, rhyolites must be considered as an independent group of rocks, most for their different isotopic composition.

T A B L E II Major-element analyses, CIPW n o r m s and p h e n o c r y s t m i n e r a l o g y of M o n t e Arci Pliocene r h y o l i t e s Sample

169

G21

221

177

4 3

158

174

214

6 1

213

172

5 3

Rock type

L

P

P

P

P

L

O

O

O

L

O

O

Major-elementanalyses SiO: TiO~ A1203 F%O~ FeO MnO MgO CaO Na~O K20 P~O5

L.o.I.

68.97 0.54 13.26 1.49 1.80 0.06 0.99 1.65 3.37 4.81 0.15 2.91

70.06 0.48 12.83 1.19 1.59 0.05 0.85 1.42 3.66 4.97 0.16 2.74

68.52 0.43 14.53 1.78 0.58 0.09 0.46 1.13 3.25 5.74 0.14 3.36

70.07 0.39 13.66 1.05 1.10 0.09 0.59 1.04 3.67 4.77 0.10 3.46

71.55 0.28 13.49 0.71 1.05 0.09 0.29 0.80 3.24 5.43 0.10 2.97

71.93 0.42 14.42 0.53 1.59 0.09 0.48 1.17 3.43 5.34 0.12 0.48

72.79 0.34 13.71 1.01 1.38 0.03 0.36 0.94 3.56 5.35 0.12 0.41

73.19 0.34 13.49 1.02 1.22 0.03 0.32 0.92 3.32 5.52 0.12 0.51

73.55 0.21 13.92 0.38 1.29 0.09 0.21 0.75 3.39 5.39 0.10 0.71

73.43 0.32 13.33 1.79 0.46 0.02 0.26 0.87 3.21 5.60 0.12 0.59

74.05 0.26 13.03 1.18 0.94 0.03 0.25 0.79 3.47 5.40 0.09 0.51

75.06 0.18 13.15 0.47 1.02 0.09 0.23 0.64 3.41 5.16 0.06 0.53

25.9 28.4 28.5 6.9

25.8 29.4 31.0 3.9

25.0 33.9 27.5 4.7 1.2

27.3 28.2 31.0 4.5 0.8

29.6 32.1 27.4 3.3 1.1

27.8 31.6 29.0 5.0 1.2

28.9 31.6 30.1 3.9 0.6

30.3 32.6 28.1 3.8 0.7

30.7 31.9 28.7 3.1 1.4

31.4 33.1 27.2 3.5 0.7

31.2 31.9 29.4 3.3 0.3

33.3 30.5 28.8 2.8 0.9

0.4 3.7 2.2

1.7 2.5 1.7

2.2 1.5

1.7 1.0

3.2 0.8

2.1 1.5

1.7 1.5

2.4 0.5

2.0 0.7

0.9 0.4

0.7 0.2

0.5 0.2

0.8 0.3

0.7 0.3

0.7 0.3

0.4 0.2

0.7 0.6 1.4 0.6 0.3

1.0 1.7

1.0 0.4

1.1 0.9 1.1 0.8 0.3

0.5 0.2

0.4 0.1

82.8

86.1

86.4

86.5

89.1

88.4

90.6

91.0

91.3

91.7

92.5

92.6

(+) (+ ) + (+) +

(+)

+

(+)

(+)

(+)

++ ++ ++

++ + +

+

(+)

+

(+)

(+)

+

+

+

+

(+)

+

(+) (+) (+) (+) ?

CIPW norms

Q or

ab an c

di hy mt hm il ap D.I.

Phenocryst mineralogy Orthopyrox Clinopyrox Plagioclase Alkalifeld. Biotite Opaques

++ + ++

++

+

++

+

++

++ ++ ++

+

+

+

Quar tz Analytical m e t h o d s

(+)

(+)

(+)

?

a n d analysts as in Table I. L = devitrified; P = perlite; 0 = obsidian; + + = present;

+ = scarce; (+) = very scarce.

143

T A B L E III Major~lement analyses, CIPW norms and phenocryst mineralogy o f Monte Arci Pliocene alkaline rocks Sample

151

7 7

164

5 9

138

119

G29

194

210

G27

G28

Rock type

H

H

H

M

M

TP

TP

TO

TO

TL

TL

Major ~lement analyses

SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K:O P205 L.o.I.

49.49 51.30 51.33 58.10 59.03 66.75 66.89 69.01 68.52 69.05 68.42 2.07 1.74 2.18 1.89 2.03 0.68 0.64 0.71 0.68 0.64 0.65 17.12 17.33 17.53 15.15 15.10 14.84 14.46 14.20 14.99 14.39 14.91 2.24 2.81 1.38 3.84 3.55 1.62 1.54 3.00 2.52 2.55 2.81 6.52 5.11 6.84 4.07 3.15 1.33 1.54 0.65 0.54 0.65 0.48 0.13 0.12 0.12 0.12 0.11 0.11 0.07 0.07 0.10 0.07 0.04 6.97 6.47 4.81 2.28 2.25 0.60 0.59 0.61 0.41 0.50 0.26 6.90 6.49 5.70 4.67 4.50 1.10 1.15 1.17 0.96 1.00 0.80 4.04 4.17 4.99 4.01 3.85 3.97 4.24 4.42 4.69 4.74 4.80 2.36 2.64 3.08 3.51 3.73 6.07 6.10 5.53 5.77 5.93 6.06 0.63 0.65 0.80 0.95 0.79 0.17 0.17 0.18 0.17 0.17 0.16 1.52 1.18 1.23 1.40 1.91 2.76 2.60 0.45 0.64 0.31 0.60

CIPW norms

Q or

ab an ne c di hy ol mt hm il tn ap D.I.

13.9 30.1 21.6 2.2

15.6 34.9 20.8 0.2

18.2 33.2 16.3 4.9

6.7

5.7

5.5

10.8 20.8 33.9 13.0

12.5 22.0 32.6 12.9

3.2 5.6

3.3 4.1

17.6 35.9 33.6 4.4

16.2 36.0 35.9 2.4

19.7 32.7 37.4 2.6

17.2 34.1 39.7 2.8

17.1 35.0 40.1 0.5

16.0 35.8 40.6 1.2

1.7

1.8 1.3

1.5 0.8

0.6 0.7

2.6 0.1

1.1 0.1

2.3

2.2

1.3

1.2

0.3 2.8 1.3

0.1 2.5 1.3

0.5 2.2 1.2

0.4 87.1

0.4 88.1

0.4 89.8

0.4 90.0

0.4 92.3

2.8 1.1 0.2 0.4 92.4

+ + ++

+ + ++ (+) (+)

(+) (+) ++ (+) (+)

(+) + ÷+ (+) (+)

(+) (+) ++ (+)

(+) (+) ++ (+)

(+) (+)

(+)

(+)

(+)

(+)

0.1

15.1 3.3

12.8 4.1

12.7 2.0

5.6

3.9

3.3

4.1

3.6

4.7 0.3 3.9

1.5 46.2

1.5 50.7

1.9 56.3

2.3 65.5

1.9 67.1

(+) (+)

(+) (+) (+) (+)

(+) (+) (+)

Phenocryst mineralogy Olivine Plagioelase Clinopyrox Opaques A1kalifeld. Biotire Orthopyrox °

Quartz ° Amphibole °

(+)

(+)

(+)

(+)

(+)

Analytical methods and analysts as in Table I. H = hawaiite; M = mugearite; T = alkali-trachyte; P = perlite; O = obsidian; L = devitrified; ++ = very abundant (up to 25 vol.%); + = present (up to 5 vol.%); (+) = scarce or very scarce; ° = probable xenocrystic occurrence.

144 •

Subalkaline Alkaline

basalts Rocks

o

Subalkaline

o

R h y o l Ires

Andesites

-~

Alkalytrachytes

__

Cascades

o" 2~°4~ o° o~

A

o

o

dacites

Trend

MarmiLta-Arcuentu

AA

and

field

000 t

A o

M

Fig. 4. AFM plot of Monte Arci Pliocene volcanics. Same symbols and source of data of Fig. 3.

MICROPROBE DATA Five samples were selected for m i cr opr obe study: {151) hawaiite; (42) subalkaline basalt; (168 ) andesite; (43) rhyolite; (G29) alkali-trachyte. Olivine This mineral is present as slightly altered phenocrysts and microphenocrysts in the hawaiite and subalkaline basalt. In b o t h rocks it is also present in t he groundmass. In some cases phenocrysts in subalkaline basalt are surrounded b y thin c l i n o p y r o x e n e reaction rims. Fe-rich olivine is a rare constituent in the alkali-trachyte where it is generally included in alkali-feldspar phenocrysts, but no data are available for such an occurrence. Olivine composition within the hawaiite evolves regularly from phenocrysts to groundmass decreasing in fosterite c o n t e n t from 90 to 75%. On the co n tr ary no continuous variations are observed in olivine o f t he subalkaline basalt: phenocrysts composition is in fact more or less constant

145 TABLE IV Selected microprobe analyses of olivines Sample

Point no.

151 hawaiite PC

MPC

GM

PC

PC

MPC

GM

46

72

49

1

70

38

71

Major,element analyses SiO 2 40.84 TiO2 Al:O3 FeO ° 9.32 MnO 0.15 MgO 50.71 CaO Na20 K20 0.02 NiO 0.34 Tot

42 subalkaline basalt

101.39

39.15

37.48

0.01 19.33 0.38 42.10 0.09

0.11 22.70 0.37 38.71 0.28 0.02

0.41 101.47

Cations per 4 oxygens Si 0.987 Ti A1 Fe 0.188 Mn 0.003 Mg 1.827 Ca Na K 0.001 Ni 0.007

0.008

3.013 2.026

3.007 2.016

(Y)' 100Mg Mg+Fe

90.7

39.43 0.01 0.07 17.91 0.23 42.49 0.18 0.01

99.67

0.35

0.04 0.31

100.71

99.87

0.991

0.983

0.998

0.409 0.008 1.589 0.002

0.003 0.498 0.008 1.514 0.008

0.002 0.379 0.005 1.603 0.005 0.001

0.001

79.5

3.015 2.032 75.2

38.52 0.10 0.04 19.39 0.07 41.24 0.13

0.991 0.002 0.001 0.417 0.001 1.582 0.003

0.007

0.001 0.006

3.000 2.002

3.004 2.013

80.9

79.1

36.45 0.13 34.55 0.36 28.69 0.20 0.04

35.81 0.06 36.57 0.57 26.03 0.35 0.05 0.96

100.42 1.003 0.003 0.796 0.008 1.177 0.006 0.002

100.40 1.003 0.002 0.857 0.014 1.086 0.010 0.003 0.022

2.995 1.992 59.7

2.997 1.994 55.9

PC = phenocryst; MPC = microphenocryst; GM = groundmass.

a r o u n d Fo80, w h i l e t h e g r o u n d m a s s r a n g e s f r o m F o , 9 t o Fos2. R e a c t i o n c l i n o p y r o x e n e show augitic compositions similar to those rimming o r t h o p y r o x e n e . R e p r e s e n t a t i v e a n a l y s e s o f M o n t e A r c i olivines are r e p o r t e d in T a b l e IV. Pyroxenes C l i n o p y r o x e n e s o c c u r b o t h as p h e n o c r y s t s a n d in t h e g r o u n d m a s s in t h e h a w a i i t e , a l k a l i t r a c h y t e a n d s u b a l k a l i n e b a s a l t . In t h e last s a m p l e c l i n o p y r o x e n e reaction aggregates rim o r t h o p y r o x e n e p h e n o c r y s t s . Very thin

99.39

Mg

opx

PC

opx

PC

R on 5

cpx

29.2 54.4 16.4

0.77

2.02

PC

cpx

36.9 47.0 16.1

0.74

2.02

1.899 0.101 0.014 0.032 0.311 0.003 0.907 0.712 0.023 0.002 0.020

98.54

50.21 1.11 2.59 9.85 0.10 16.09 17.58 0.31 0.04 0.66 .

40

GM

cPx

31.7 47.8 20.5

0.70

2.03

0.014

1.920 0.080 0.002 0.026 0.399 0.008 0.930 0.618 0.029

99.04

0.47 .

50.77 0.93 1.85 12.63 0.25 16.50 15.24 0.40

34

.

R on 8

cpx

38.0 45.5 16.5

0.73

2.02

0.028 0.325 0.006 0.897 0.748 0.026

1.901 0.088

99.41

50.76 1.01 1.99 10.38 0.19 16.07 18.65 0.36

9

R

cpx

32.5 46.9 20.6

0.69

2.01

1.973 0.027 0.002 0.019 0.401 0.011 0.910 0.631 0.021 0.008 0.007

99.43

52.49 0.66 0.65 12.76 0.33 16.25 15.67 0.29 0.09 0.24

25

= g r o u n d m a s s ; R = reaction products.

R on 5

epx

4.7 72.2 23.1

0.76

2,02

0.011 0.454 0.012 1.420 0.093 0.006 0.001 0.023

0.031 0.322 0.010 1.072 0.577 0.020 0,001

.

1.940 0.059

101.71

.

54.32 0.40 1.40 15.19 0.38 26.68 2.42 0.09 0.02 0.81

7

1.897 0.092

99.19

.

51.12 1.10 2.10 10.36 0.31 19.39 14.51 0.27 0.03

6

= microphenocryst; G M

2.5 82.3 15.2

0.84

0.83

3.4 80.2 16.4

2.01

1.918 0.082 0.047 0.005 0.297 0.005 1.609 0.048 0.002

99.09

2.01

P C = phenocryst; M P C

Mg+Fe Wo% En% Fs%

M

Cations p e r 6 o x y g e n s SiIV 1.909 A1VI 0.091 AI 0.044 Ti 0.008 Fe 0.321 Mn 0.003 Mg 1.567 Ca 0.066 Na K Ni 0.001 Cr

Tot.

M a j o r - e l e m e n t analyses SiO2 53.64 53.97 TiO2 0.31 0.18 AI20~ 3.22 3.08 FeO ° 10.79 10.00 MnO 0.11 0.18 MgO 29.56 30.38 CaO 1.72 1.27 Na20 0.03 K20 NiO 0.04 Cr203 . . .

5

4 2 subalkaline basalt

Point no. 8

Sample

Selected m i c r o p r o b e a n a l y s e s of P y r o x e n e s

TABLE V

PC

opx

2.7 81.6 15.7

0.84

2.03

0.003 0,017

1.875 0.125 0.048 0.009 0.305 0.006 1.587 0.052 0.006

100.01

0.09 0.61

52.86 0.33 4.15 10.28 0.20 30.04 1.36 0.08

PC

opx

2.8 81.5 15.7

0.84

2.03

0.090

1.897 0.103 0.032 0.006 0.312 0.005 1.609 0.056 0.010

98.90

0.32

52.90 0.24 3.19 10.42 0.17 30.12 1.45 0.02

86

168 a n d e s i t e 66

PC

opx

3.0 80.3 16.7

0.83

2.02

0.001 0.020

1.888 0.112 0.039 0.004 0.327 0.002 1.569 0.058 0.005

98.95

0.04 0.69

52.61 0.16 3.57 10.89 0,06 29.36 1.51 0.07

65

MPC

opx

2.4 79.4 18.2

0.81

2.03

0.005

1.895 0.105 0.029 0.007 0.360 0.002 1.576 0.048 0.003

98.90

0.17

52.59 0.27 3.15 11.96 0.07 29.37 1.24 0.05 0.01

67

R

cpx

2.1 79.2 18.7

0.82

2.00

1.955 0.045 0.041 0.004 0.363 0.008 1.539 0.040 0.003 0.001

99.56

54.82 0.17 2.04 12.19 0.26 28.97 1.05 0.04 0.02

59

GM

cpx

3.5 76.1 20.4

0.79

2.00

1.923 0.077 0.047 0.013 0.395 0.007 1.474 0.067

99.05

53.25 0.46 2.91 13.09 0.21 27.40 1.73

60

GM

cpx

3.4 71.3 25.3

0.74

2.02

0.002 0.003

0.011 0.508 0.008 1.435 0.069

1.948 0.038

99.62

0.08 0.09

53.23 0.38 0.88 16.62 0.26 26.33 1.75

69

98.52

blg

Mg+Fe Wo% En% Fs%

M

42.8 44.9 12.3

Cpx PC

4.2 69.5 26.3

Cpx GM

0.79

0.73

Cpx GM

46.3 40.3 13.4

0.75

2.02

0.001

2.02

0.004

0.036 0.241 0.008 0.882 0.841 0.027 0.002

98.06

0.12

47.01 2.93 5.12 7.87 0.II 13.26 21.21 0.41 0.02

54

1.794 0.206 0.024 0.084 0.251 0.004 0.754 0.867 0.030 0.001

1.914 0.073

99.50

2.03

Cations p e r 6 o x y g e n s 1.925 SilV 0.075 AIv I A1 0.012 Ti Fe 0.526 Mn 0.008 1.392 Mg 0.085 Ca Na K 0.001 Ni 0.001 CE

Tot.

' SiO~

M c 6 o r - e l e m e n t analyses 51.84 51.24 0.43 1.29 TiO 2 1.72 1.66 A I 2 0~ 16.94 7.73 FeO-0.25 0.26 MnO 25.15 15.84 MgO 2.13 21.01 CaO 0.38 Na20 0.05 K20 0.03 NiO 0.02 0.04 Cr20s

59

151 h a w a i i t e

P o i n t no. 77

Sample

TABLE V (continued)

cpx GM

45.5 40.3 14.2

0.74

2.04

0.006

1.764 0.236 0.015 0.084 0.269 0.002 0.765 0.862 0.039

98.86

0.20

46.41 2.95 5.60 8.46 0.06 13.50 21.17 0.53

51

cpx PC

43.2 48.5 8.3

0.85

2.01

Cpx MPC

41.8 42.2 16.0

0.73

2.01

0.802 0.796 0.052 0.002

0.903 0.803 0.038

0.034

1.968 0.032 0.034 0.018 0.304

99.36

52.57 0.64 1.49 9.70 -14.37 19.84 0.71 0.05 --

59

1.875 0.125 0.056 0.024 0.154

100.30

51.30 0.89 4.20 5.03 -16.58 20.50 0.53 0.01 -1.19

58

G29 alkali-trachyte

cpx GM

39.3 36.2 24.5

0.60

1.99

0.692 0.754 0.044 0.001

0.025 0.004 0.472

2.006

98.78

52.14 0.15 0.84 14.68 -12.06 18.29 0.59 0.02 --

70

~.~ ¢~-

148

clinopyroxene reaction rims also c o m m o n l y surround orthopyroxene phenocrysts in the andesite. In Fig. 5 the variation fields of analyzed Monte Arci pyroxenes are plotted on a CaSiO3-MgSiO3-FeSiO3 molecular diagram. Representative analyses are given in Table V. Ca SiO 3 - ~

~MMR

:,

2o

Mg SiO3

MAUPITIUS OLO SERIES

~,~ nEACI-ION ON OPX

;i

A

A

A

A

A

A

A

A

A

90

80

20

60

50

40

30

20

~0

Fe SiO 3

Fig. 5. CaSiO3-MgSiO~-FeSiO~ plot of Monte Arci pyroxenes. Mauritius trend from Baxter (1975) as an example of alkaline trend. Selected data in Table V.

A decrease in the Mg/(Mg+Fe) ratio from phenocrysts to groundmass is observed in all analyzed samples. The andesite and subalkaline basalt have an identical orthopyroxene trend. No decrease in the Mg/(Mg+Fe) ratio is observed from basalt to andesite. In the basalt Ca-clinopyroxenes occur both as reaction products surrounding orthopyroxene and olivine phenocrysts and as grains in the groundmass, but the two occurrences show different trends. Pigeonite rims orthopyroxene both in basalt and in andesite. The hawaiite has salitic clinopyroxenes showing a short variation trend typical of basic rocks of alkaline suites. Clinopyroxenes of the alkali-trachyte have surprisingly high Mg/(Mg+Fe) ratios; their variation trend in Fig. 9 plots into an intermediate position between alkaline suites (f.i. Mauritius island, Baxter, 1975) and tholeiitic ones (f.i. Skaergaard intrusion).

Feldspars Plagioclase composition was determined by microprobe analyses in all samples but the alkali-trachyte in which oligoclase composition was optically determined in the core of alkali-feldspar phenocrysts. Alkali-feldspar is abundant in the alkali-trachyte and in the rhyolite and it is also present in the hawaiite mesostasis. The variations fields of analyzed feldspars are plotted in the molar An-AbOr triangle {Fig. 6). Selected analyses are reported in Table VI. The plagio-

149 An

g.2 SUBALK,BA SA LT CORE OF PHENOCF

lsl HAWA)iTE

168 ANOESITE /*5 SUBALK, BASALT

lEE ANDESfTE XENOCRYSTS 42 $UI~ALK BASALT XENOCRVSTS 63 RHYOLITE

151 HAWAliT E ~

v

Ab

~

ALKALI-TRACHYTE S V

&3 RHYOLITE

v

Or

Fig. 6. An-Ab-Or molecular plot of Monte Arci feldspars. Selected data in Table VI. Data of alkMi-trachytes feldspars are from Leoni et al. (1976) also.

clase of the hawaiite shows the An content decreasing progressively from 64% (core) to 51% (rim of phenocrysts and microlites). Soda~sanidine compositions (Abs~Or40An04) appears in the mesostasis as end-products of crystallization. Alkali-feldspar phenocrysts of the alkali-trachyte have a practically constant soda-sanidine composition (Ab47OrsoAn03 to Ab46Or49An0s). Slightly different values (Abss-s2Or42-40An6-s) refer to alkali-feldspars mechanically separated from other similar rocks (Leoni et al., 1976). Oligoclase (An~s) and sanidine (Ab31Or67An2) coexist as phenocrysts in the rhyolite. In the andesite and subalkaline basalt plagioclase occurs as phenocrysts, microlites and as large partially resorbed xenocrysts. Stable plagioclase crystals in the andesite range from Anss to An49 while in the subalkaline basalt the phenocrysts show a core with a composition An78-70 abruptly passing to Ans0-48 in the outer parts; microphenocrysts and microlites range from Ans6 to An47. The composition of resorbed xenocrysts ranges in both andesite and subalkaline basalt from An30 to An2~. Reverse zoning (core An2s-20, rim

97.54

Tot.

63.3 34.7 2.0

A n tool% Ab Or

55.3 42.8 1.9

15.99 4.03

52.0 46.7 1.3

15,99 4.00

0.050

0.100 0.017 2.021 1.814 0.051

4.0 56.2 39.8

16.05 3.87

0,152 2.146 1.519

11.776 4.275

96.85

0.77 5.99 6.44

63.71 19.62 0.32

GM

60

9.889 6.101

96.93

53.08 27.78 0.65 0.06 10.12 5.02 0.22

GM

63

2.5 47.2 50.3

15.99 4.08

0.005 0,102 1,924 2.046

11.848 4.101 0.041

101.45

66.52 19.53 0.27 0.02 0.53 5.57 9.01

PC

56

G29

5.8 52.0 42.2

16.01 4.00

0,232 2.077 1,688

11.748 4.190 0.069

99.95

1.20 5.95 7.35

65.25 19.74 0.46

PC

+

110

alkali-trachytes

5.5 54.8 39.7

16.02 3.99

0.219 2.186 1,586

11.737 4.181 0.099

99.93

1.14 6.27 6.91

65.29 19.73 0.59

PC

+

210

77.5 21.5 1.0

15.98 4.08

0.039 3.134 0.868 0.040

8.812 7.074 0.090

98.50

47.20 32.14 0.58 0.14 15.67 2.40 0.17

CPC

48

70.0 28.8 2.2

16.01 4.07

2.847 1.171 0.051

9.057 6.911 0.041

99.57

14.48 3.29 0.22

49.36 31.95 0.27

CPC

49

42 s u b a l k a l i n e basalt

48.2 49.8 2.0

16.01 4.06

0.002 1.955 2.021 0,079

9.952 5.990 0.069

98.73

54.45 27.80 0.45 0.01 9.98 5.70 0.34

RPC

52

4

56.3 42.4 1.3

15.92 4.11

0.041 2.294 1.728 0.054

9.778 6.117 0.028

99.27

53.62 28.45 0.19 0.15 11.74 4.89 0.23

MPC

47.9 49.1 3.0

15.98 4.02

0.027 1.912 1.961 0.119

10.091 5.808 0.085

98.55

55.17 26.93 0.55 0.10 9.76 5.53 0.51

MPC

18

45.0 52.3 2.7

16.00 4.09

0.013 1,835 2.129 0.110

10.168 5.709 0.089

100.26

56.56 26.94 0.59 0.05 9.53 6.11 0.48

GM

2

30,4 65.6 4.0

16.00 4.04

1.229 2,648 0.161

10.739 5.164 0.095

99.67

6.40 7.62 0.70

59.89 24.43 0.63

XC

11

F e " or F e " a c c o r d i a g t o t h e best fit. PC = p h e n o c r y s t ; MPC = m i e r o p h e n o c r y s t : GM = g r o u n d m a s s ; XC = x e n o c r y s t ; C = core: [ = i n t e r m e d i a t e p a r t : R = r i m . + = X-ray f l u o r e s c e n c e a n a l y s e s ( L e o n i et al., 1 9 7 7 ) .

15.99 4.02

Z X

0.019 2.215 1.714 0,078

9.774 6.112 0.102

98.27

53.05 28.14 0.66 0.07 11.22 4.80 0.33

MPC

Cations per 32 o x y g e n s Si 9.458 A1 6.477 Fe'" 0.056 Fe" Mg 0,028 Ca 2.526 Na 1,384 K 0.078

50.77 29.49 0.36 0.10 12.66 3.83 0.33

SiO a AI20, FeO °~ MgO CaO Na~O K20

PC

48

151 h a w a i i t e

Point n o . 53

Sample

Selected m i c r o p r o b e analyses of feldspars

TABLE VI

o

98.20

Tot.

16.00 3.99

54.1 44.5 1.4

Z X

An tool% Ab Or

51.0 46.4 2.6

16.02 3.95

23.0 70.8 6.2

16.00 3.90

0.891 2.739 0.237

0.022 2.006 1.825 0.101

0.032

0.017 2.148 1.769 0.055

11.179 4.821

97.20

4.56 7.74 1.02

61.26 22.41 0.21

Fe"

9.844 6.064 0.094

73

CXC

Mg Ca Na K

Si A1 Fe'"

9.967 5.941 0.109

98.50

54.41 27.51 0.71 0.08 10.22 5.14 0.43

GM

Cations p e r 32 o x y g e n s

53.49 27.95 0.61 0.06 10.89 4.96 0.24

8iO 2 AI20. FeO ~ MgO CaO Na20 K~O

PC

2

168 andesRe

Point no. 68

Sample

T A B L E VI ( c o n t i n u e d )

19

36.5 59.5 4.0

16.00 3.92

0.012 0.005 1.423 2.320 0.158

10.637 5.361

99.24

59.13 25.29 0.08 0.02 7.38 6.65 0.69

IXC

41.2 55.7 3.1

16.01 3.99

0.013 1.636 2.214 0.124

10.306 5.604 0.105

97.21

55.77 25.72 0.68 0.07 8.26 6.18 0.53

RXC

3

25.4 65.8 8.8

16.00 4.00

1.015 2.636 0.353

10.996 4.999

100.18

5.33 7.64 1.55

61.82 23.84

PC

102

43 rhyolite

2.1 30.6 67.3

16.00 4.06

0.085 1.243 2.729

11.865 4.109 0.023

99.77

0.43 3.51 11.70

64.91 19.07 0.15

MPC

99

Cu

152

An4s-40) is common in these crystals. The abundance of these xenocrysts and the identity of their composition with that of rhyolites suggest that subalkaline rocks underwent important contamination with the rhyolitic material. Opaque and other materials Opaque minerals occur in the groundmass of all the rocks analyzed by microprobe and as phenocrysts and/or microphenocrysts in the alkalitrachyte and rhyolite. Unfortunately, microprobe analyses generally close very low, probably due to the small size of minerals, and are consequently not reliable. Only data from phenocrysts (in the alkali-trachyte and rhyolite) are satisfactory and these are reported in Table VII. The presence in the alkali-trachyte mesostatis of the pair ilmenite-magnetite provides information

T A B L E VII Selected microprobe analyses o f various minerals Sample

G 2 9 alkali-trachyte

43 rhyolite

Point no.

67

71

101

100

98

45

47

Phase

Ilmenite

Magnetite

Magnetite

Biotite

Apatite

Spinel

Spinel

0.13 11.43 2.10 78.72

10.40 2.98 81.31

36.59 5.67 14.13 18.74

0.78

11.87

SiO~ TiO: Al:O3 FeO °

0.09 47.65 45.05

151 hawaiite

0.65

Fe:03

MnO MgO CaO Na~O K~O P:O~ NiO Cr:O~ CuO Tot.

5.53

.

2.32 0.07 .

.

0.66 9.19

.

41.96 -

--

0,03 0.15 98.32

94.77

Fe20 ~ FeO

13.28 33.10

52.61 31.37

45.93 39.96

Tot~

99.65

100.03

]00.23

mol% R:O3

12.2

33.0

mol% Usp

fO;

830 10 ~3

29.8

0.53 20.4'2

0.01 20.51 101.52

-

95.65

Recalculated analyses (ilmenite basis)

T(C}

0.24 53.20 0.15

45.94 9.03 6.53 ÷ 0.07 19.80 0.04 0.02

0.06 0.22 45.43 8.64 6.36 ÷ 0.30 19.99

96.85

96.20

102.63

Cations o n 22 O

Cations

Cations on 35 O

8i Al Ti Ti Cr Fe"'

5.492" 2.500 8 . 0 0 0.008 0.632"

Fe' Mn Mg Ca Na

2,353

K Ni P

on 25 O

iI.492~ 0"040t-16 3.425 I

•5.64

2.65E ~ 0.192 .1.95

1.0431

0-0921 0.061~9.91 9.703 I

0.049A

1.603"] 0.013 I 6.262 I 0.009 ~7.99 0.008]

1.760

+ Fe:O~ re-calculated to close 16 the sum Si+Al+Ti+Cr+Fe"'.

0.091 J 6.047

0.013-] I 1.458| 0"036 ~16 3.468[ 1.025] 1.547"] 0.054| 6.374 I

I

o.ool 3

7.98

153 about the T(°C)-fO2 conditions of crystallization. Analyses were recalculated on ilmenite basis according to Carmichael (1967) and plotted on the T(°C)-fO2 experimental plain of Buddington and Lindsley (1964). A temperature of 830°C and an oxygen fugacity of 10 -13 for P = 1 atm were obtained. In Table VII biotite and apatite from the rhyolite and chromiferous spinel from olivines in the hawaiite are also reported. Glasses

Many of the analyzed rocks of Monte Arci are hypocrystalline or glassy lavas. Interstitial glasses were analyzed by microprobe in: andesite (no. 168), subalkaline basalt (no. 42), rhyolite (no. 43) and alkali-trachyte (no. G29). Quartz xenocrysts are abundant in subalkaline rocks and are found in some alkali-trachytes. They generally include glassy drops whose composition indicates their magmatic origin and provides information about the nature of magmatic liquids from which the quartz crystallized (Clocchiatti, 1975). Selected analytical data are reported in Table VIII. A constant and peculiar feature of subalkaline rocks is the presence of an inhomogeneous glassy groundmass made of black spherules that spread out into a light matrix, suggesting glass immiscibility. Such structures have been the object of a recent study by one of us (Clocchiatti, 1979). If they are not simply related to post-eruptive cooling, inhomogeneities at a so large scale as observed in Monte Arci subalkaline rock could have stimulated petrogenetic implications. The light matrix and black spherules both have a similar composition in the andesite and basalts samples. The light residual glasses are rhyolitic in composition with a high K/Na ratio and are generally corundum normative. The black spherules have a composition far from normal silicate melts. They are extremely rich in Fe, Ti and P with very low A1, Si, Na and K content. Figure 7 is a plot of analyzed rocks and glasses in the AFM and KNC triangles. Glass inclusions in quartz xenocrysts have a constant composition perfectly corresponding to the residual glasses of rhyolites. These inclusions can therefore be considered as contaminants of subalkaline rocks and alkalitrachytes. The plot in Fig. 7 of the fields of light residual glasses and black spherules seems to suggest that liquid immiscibility has not played an important role m the genesis of subalkaline rocks, being not hypothizable compositional changes from basalts to andesites and dacites, simply related to a decrease of the volume ratio between Fe-Ti-rich black glass and silica-rich light one. Presently, on the contrary, the importance of such a process in the genesis of rhyolites is out of control.

L

Y

Y

V

V/

K20

~,42]

~

/

V

@42

o.~

V

Y

lighl H g(hl'4L:relidual ~I~d8ui~IA~glasses j '~sses

subatkaUne

V

~rv

Y

Na 2 0

Y

V

(MMIR 168 & 42)

b,. . . . p. . . . . . .

Fig. 7. AFM and KNO plot of microprobe analyzed glasses and rocks. Empty circles: interstitial glasses.

Na20+K20

~

II. . . . . in quart . . . . . liths

I!

~

b°co

(NIMR

btack spher utes t6B

FeO

~t

Y

y

~

\

CaO

155

TABLE

VHI

Selected m i c r o p r o b e analyses o f glasses Sample

G29

43

42

Point no.

65

104

31

61

57

60

21

12

11

16

21

50

R

R

LRG

LRG

BS

BS

QI

LRG

LRG

BS

BS

QI

73.53 0.42 10.32 1.98

24.65 13.02 0.90 29.16 0.41 4.54 10.32 0.51 0.40 6.75 0.40

31.15 12.70 1.84 26.62 0.36 4.40 10.08 0.86 0.48 3.67 0.57

79.12 0.02 10.72 0.66 0.01 0.09 0.45 3.19 4.10 0.01

70.80 73.72 2.98 0.88 9.24 11.15 4.15 2.14 0.03 0.04 0.26 0.01 1.22 0.38 2.27 1.89 4.29 4.55 . . .

SiO2 TiO~ A1203 FeO °

168

CaO Na20 K20 P~Os NiO

66.78 72.98 67.90 0.54 0.04 1.43 15.43 13.46 12.88 2.02 1.04 1.39 . . . . 0.40 0.07 0.15 0.89 0.62 0.64 3.85 2.92 2.87 7.00 5.52 5.76 0.06 0.01 0.45 ---

Tot.

96.96 96.65 93.47 93.57 91.05 92.72 98.37 95.24 94.76 97.68 92.60 98.25

M

n

O

M g O

0.22 0.35 2.27 4.43 0.03

.

.

23.44 38.16 15.41 11.60 0.95 5.09 48.00 25.31 0.33 0.14 7.25 1.15 1.97 7.68 0.20 2.01 0.13 1.46 . .

.

.

77.02 11.63 0.73 0.06 0.65 2.88 5.28

.

CIPW n o r m s

Q or

ab an ne

lc ak C

14.0 41.4 32.5 4.0 . .

. .

.

ol

.

2.8

il ap

.

. 1.1 0.2

.

. 1.5

--

1.0

28.9 33.9 24.1 0.6 . .

.

.

di hy

ns

32.5 32.8 24.6 2.9

.

-2.0 . 0.1 -.

40.9 26.1 19.4 1.7 . .

.

. 1.1

-0.5

-3.6

2.7 1.0

0.8 --

.

-2.8 7.3 -.

.

.

. 1.7

.

-2.2 2.6 -.

.

.

.

. --

9.9 27.4 8.7 24.8 16.0 0.4

43.3 24.2 27.0 2.2 . .

.

. 0.2

--

24.2 21.6 4.2 24.2 '8.7 .

-1.4 --. .

39.6 26.6 20.2 2.5

45.7 28.4 16.9 2.0

2.6

--

3.4 1.8 -5.9 . . . .

-9.3 18.4 0.6 ---

0.6

--

---

--

--

--

-2.7 -1.8 . .

---1.3 0.9 0.6

5.8 -60.7 30.0 . .

35.6 6.3 6.1 23.8

38.7 31.8 24.8 3.3 --

--1.5 --

. .

- - = not detected; R = residual glass; L R G = light residual glass; B S = black spherules; Q I = glass inclusion

in quartz x e n o c r y s t .

svSR/8'SR I S O T O P I C

DATA

STSr/S'Sr ratios have been determined on 21 selected samples of Monte Arci Pliocene volcanic rocks. Results are listed in Table IX and reported as a func, tion of Rb in Fig. 8. InitialSTSr/S'Sr ratios of rhyolites cover a wide range of values (0.7063--0.7115), that are higher than for all the other Pliocene rocks. Subalkaline rocks show 87Sr/8'Sr ratios ranging around 0.7050, nearly constant within the limits of analytical error. The more basic alkaline rocks (nos. 51 and 77, hawaiites) and characterized by the lowest Sr isotopic ratios of the Monte Arci complex: 0.7044. The values of the mugearite (no. 59) and of the alkah-trachyte (no. 119) are higher: 0.7054 and 0.7063 respectively. Isotopic data confirm the genetic link connecting the different subalkaline rocks of Monte Arci and exclude their parentage with both alkaline rocks and rhyolites through equilibrium partial melting of fractional crystallization

156 T A B L E IX 8~Sr/8'Sr ratios o f M o n t e Arc± Pliocene volcanics Sample

Rock type

87Sr/8'Sr ± l o

Rb (ppm)

Sr ( p p m )

(87Sr/8'Sr)i

34 28 30 42 205 31 153 168 66 179 68 80 158 61 177 53 221 77 151 59 119

sub.basalt sub.basalt sub.basalt sub.basalt sub.basalt sub.basalt sub.basalt andesite andesite dacite dacite dacite rhyolite rhyolite rhyolite rhyolite rhyolite hawaiite hawaiite mugearite alkali-trachyte

0.70500 0.70529 0.70560 0.70512 0.70505 0.70540 0.70506 0.70534 0.70470 0.70447 0.70510 0.70530 0.70850 0.71230 0.70670 0.70987 0.70890 0.70441 0.70443 0.70538 0.70679

16 36 38 43 13 37 25 71 80 177 169 170 206 275 237 286 245 63 67 89 185

426 615 587 559 538 502 442 386 417 435 455 365 153 78 125 51 132 738 837 719 44

0.70500 0.70530 0.70560 0.70510 0.70505 0.70540 0.70505 0.70516 0.70470 0.70443 0.70505 0.70524 0.70820 0.71150 0.70630 0.70860 0.70870 0.70440 0.70442 0.70537 0.70630

+- 0 . 0 0 0 7 0 -+ 0 . 0 0 0 6 3 -+ 0 . 0 0 0 4 0 -+ 0 . 0 0 0 3 0 ± 0.00030 ± 0.00060 ± 0.00016 -+ 0 . 0 0 0 3 0 ± 0.00030 ± 0.00060 _+ 0 . 0 0 0 5 0 ± 0.00030 ± 0.00140 +- 0 . 0 0 0 4 0 -+ 0 . 0 0 0 3 0 -+ 0 . 0 0 0 3 3 -+ 0 . 0 0 0 3 0 ± 0.00013 + 0.00025 ± 0.00035 -+ 0 . 0 0 0 2 5

(87Sr/8'Sr)i = (87Sr/S'Sr) - - ( 8 7 R b / s ' S r ) xt. STSr/8'Sr c o r r e c t e d t o E.A. s t a n d a r d = 0.7080. = 1.42 10 -~1 yt-1; t = mineral age = 3.0 m.y.

71 ~,

~S

0

%

@

CK~

Z-

-

704 Rb

50

100

150

200

250

l

i

I

I

ppm

Fig. 8. Initial 87Sr/SSSr ratios o f M o n t e Arc± Pliocene volcanics vs R b c o n t e n t s . S a m e symbols as in Fig. 3. Vertical dash = STSr/S'Sr analytical confiance.

157

processes. The relatively high Sr contents of rhyolites (between 50 and 150 ppm) and the absence in these rocks o f a negative correlation b e t w e e n Sr contents and ~TSr/S6Sr ratios also seem to exclude radiogenic Sr contamination affecting the final products of a crystal fractionation starting from subalkaline basic parental liquids. In the alternative hypothesis that rhyolites are originated b y partial melting, the STSr/S6Sr differences can reflect either different source materials or, alternatively, fractional melting of the same source material operating in isotopic disequilibrium. On the contrary, contamination phenomena with STSr-rich material has to be invoked to explain the higher isotopic ratios of mugearite and alkalitrachyte in respect to hawaiites, if the main process connecting alkaline rocks is, as apparent from chemical and mineralogical data, a fractional crystallization differentiation. TRACE ELEMENTS

On 32 selected samples of Pliocene volcanic rocks of Monte Arci 18 trace elements were determined b y instrumental neutronic activation. Results are listed in Table X and reported in Fig. 9 as a function of the normative (Qz+ or+ab+ne sum). The distinction among alkaline, subalkaline and silicic rocks is also quite evident from these variation diagrams. In the study of Monte Arci Pliocene volcanics, aphyric or subaphyric rock samples, when possible, were selected for trace-elements analyses in order to approximate liquid compositions. Figure 9 shows that among the analyzed trace elements Rb, Th and U are characterized b y the more residual geochemical behaviour in the three rock groups. Plots of these elements against less residual ones, like La, Ce and Ba (Fig. 10), evidentiates some facts: (a) Subalkaline basalts are all genetically connected through process(es) following simple distribution laws as those suggested b y Treuil and Joron (1977); the strong correlation shown b y these rocks in diagrams of Figs. 10 and 11, therefore, indicates different degrees of partial melting in a narrow range of a c o m m o n original material; (b) Silicic rocks appear clearly related among them in all diagrams of Fig. 10. The reliability of the above-mentioned simple distribution laws for silicatic melts, similar in composition to Monte Arci rhyolites, is however limited because of the poor knowledge of the factors (related to liquid composition and structure) affecting partition coefficients in such magmas; (c) Trace~lement data on alkaline and intermediate subalkaline rocks are scattered and t o o scarce to have relevance in understanding the genesis of these rocks on geochemical basis only.

8C Cr Co Ni Rb Sr Zr Cs Ba La Ce Eu Tb Hf Ta Th U Sb

205

13 199 36 120 14 538 113 0.15 289 16 31 1.7 0.52 2.8 0.45 2.2 0.32 0.02

17 211 41 74 21 463 126 0.15 334 20 32 1.8 0.76 3.5 0.85 2.9 0.45 0.02

B

130

Subalkaline

Rock type B

Sample

14 84 11 140 29 586 100 0.16 324 17 30 1.6 0.51 3.0 0.59 2.7 0.55 0.02

B

144

15 204 43 151 16 426 110 0.27 311 19 28 1.5 0.64 2.8 0.57 2.4 0.46 0.06

B

34

16 205 39 73 25 442 132 0.39 363 20 38 1.8 0.66 3.7 0.84 3.1 0.55 0.03

B

153

14 186 10 126 31 583 108 0.47 443 22 39 1.6 0.54 3.1 0.78 3.4 0.68 0.03

B

26 B

13 167 37 118 43 559 201 0.60 528 28 51 1.9 0.50 3.8 1.0 4.5 0.82 0.04

12 222 39 142 36 615 104 0.60 461 22 40 1.6 0.54 3.3 0.68 3.9 0.74 0.04 15 172 37 67 37 502 143 0.60 548 25 46 1.9 0.66 3.8 0.86 4.3 0.86 0.04

B 15 218 38 88 37 587 134 0.57 502 25 44 1.8 0.69 3.8 1.2 4.4 0.78 0.04

42

28 B

31 B

30

T r a c e - e l e m e n t a n a l y s e s o f M o n t e A r c i P l i o c e n e v o l c a n i c r o c k s (values in p p m )

TABLE X

10 146 27 83 71 386 122 1.7 357 26 47 1.5 0.63 3.4 0.81 7.9 1.9 0.11

14 167 11 116 55 594 172 0.85 682 38 63 1.8 0.65 4.1 1.3 6.1 1.3 0.06

168 A

25 B

9 150 27 106 75 412 122 1.7 364 28 49 1.5 0.65 3.6 0.98 8.4 2.0 0.10

A

224

10 141 26 118 80 417 153 1.7 346 37 64 1.7 0.68 4.5 1.0 7.8 1.7 0.11

A

66

9 59 4 38 171 407 202 3.1 749 52 94 2.1 0.89 5.8 2.0 15.0 3.2 0.22

9 59 4 36 170 365 211 2.4 656 50 90 1.7 0.82 5.9 2.1 15.2 3.3 0.17

126 D

D

80

Qo

7 168 21 129 169 455 133 3.9 460 35 65 1.2 0.60 4.3 1.3 13.7 3.1 0.19

7 160 19 94 177 435 146 4.1 463 36 69 1.2 0.57 4.5 1.4 14.4 3.3 0.19

D

179

7 98 15 73 195 340 153 4.5 537 43 83 1.2 0.63 4.9 1.6 16.1 3.6 0.19

D

196

4 21 3 4 206 153 258 2.1 1020 73 136 1.3 0.82 7.6 1.9 27.4 3.6 0.10

R

158

Sflicic

I N A a n a l y s e s ( a n a l y s t s : G . O t t o n e l l o a n d R. Cioni).

Sc Cr Co Ni Rb Sr Zr Cs Ba La Ce Eu Tb Hf Ta Th U Sb

68

Sub~k~ine

Rock type D

Sample

5 9 0.7 1 245 132 309 2.7 857 90 173 1.9 0.96 9.9 2.1 34.2 3.5 0.10

R

221

3 6 1.0 275 78 151 3.8 450 45 87 0.77 0.76 5.2 1.9 22.3 4.8 0.13

R

61

3 18 1.6 8 275 84 188 6.3 571 55 106 0.89 0.71 6.2 2.1 23.5 5.1 0.25

R

43 177

4 22 3 8 237 125 249 6.3 620 53 98 1.1 0.74 7.2 1.9 22.4 4.4 0.34

R

53

3 7 1.2 3 286 51 118 6.9 276 38 75 0.62 0.65 4.4 1.8 21.0 5.4 0.27

R 14 148 38 145 63 738 253 0.56 938 57 95 2.2 0.53 6.7' 2.4 6.4 1.3 0.06

H

77

Alkaline 151

16 155 43 163 67 837 233 0.88 1007 52 90 2.3 0.67 6.1 2.8 6.5 1.4 0.05

H

164

13 72 11 80 91 1083 304 1.4 1206 68 117 2.7 0.70 8.4 4.0 7.4 1.9 0.09

H

59

11 32 18 3 89 719 260 1.1 1687 64 114 3.0 0.94 6.9 1.8 12.5 1.6 0.19

M

138

11 27 20 3 112 723 254 1:3 1789 61 117 2.8 1.0 7.0 1.9 13.8 1.9 0.07

M

189 62 821 2.0 175 70 122 2.1 1.1 21 2.7 15.0 3.3 0.15

7 14 0.5

210 T

119

7 15 1.8 1.4 185 44 740 3.3 186 68 117 1.9 1.1 20 2.3 14.8 3.1 0.21

T

160 30

40

50

i

i

I

60 __

70

8'

90

I )~

Cr 200

",~,

~.0

50

l

~

Ce

ooo :J

160

~ °a

°

a o o

oCo I-~20

I

o •

BO

'~ o

-BC

Q o

• ee

z,0

~L

~q~oo o o°

Rb

3 o

i

8a 2000

o

~2,3c

o

a

~

1600 a

I

I200 a

-800

•

~i

° o

°

o o

6O0 a,o

oo

o

z~ o ~

o

o

• •o

°o

o oO

°°

la

o

o

o oo

U

Sc

-10

° oo o o

o

o o o

o

o 0 o

~%oo

i

L

Oo 4 a

i

Srl

F}O00

;600

Th paO0

I I

PB00 ~,,ill

•

!

Oo

°ee

°°°

o

~o

- 200

Bo

30

0~o

[] o

o

o

o°

2O

La

80 a

a~o o

°°°

60 z.0

~oo A

eA

3O

D.I

-20

a

z, • ooe

o °

o

on

o o o

[ o

o o

eb~

o Dd

I

T - -

80

90

IOC~

Fig. 9. Distribution of trace elements (ppm) as a function of D.I. (Thornton and Tuttle, 1960). Same symbols as in Fig. 3.

DISCUSSION Some petrogenetic considerations arise from the previously discussed data. Silicic rocks A genetic linkage existing among these rocks is strongly suggested by major and trace-element geochemistry. Volume relations, isotopic data and, possibly, trace-element distribution disagree with the possibility that these rocks

161

o o

0

0

o

o

o

G

o

0

o

o

0

o

o

o

o

o

o

o o o

o °o

o

o

.,oo

~

o

°Oo

o

o o

o o

o o

o

0 o

o

o o

o

o

o

o

oo

o

0

o o

oo

~.o

"

o

o

qb

.~

Oo

c)

J

~'~

o °o

o

°

o

~o

o

oo

o

o

o

°o

o

o

2 I o

~

o

o

o

~

0 o

o ,

lu (J

i

o

o

o

o

[ O

162

U La

Rb

I

,,st

,03~

1,0-

,02 i

,01J

0.5-

*b(ppr.) 0

1 20

I 40

I 60

u ( p p r . ) .... I 0,5

1 10

Fig. 11. U vs. U/La, Rb vs Rb/La diagrams o f subalkaline basalts.

be related to fractional crystallization differentiation processes of any Monte Arci basic rock; a crustal partial melting appears the more probable genetic process. No relation is apparent between isotopic ratio and degree of partial melting (which can be well expressed by Qz+or+ab normative sum). 87Sr/~'Sr isotopic ratios range between 0.711 and 0.706, and these differences can reflect different source material or, alternatively, fractional melting of the same source material operating in isotopic disequilibrium. Onto a Qz~r-ab normative triangle (Fig. 12) these rocks should represent, according to the first hypothesis, melting minima at different suggested depths of origin. Subalkaline rocks

Subalkaline rocks range in composition between basalts and dacites. A clear calc-alkaline affinity (which is difficult to understand in the light of tensional geodynamic setting which characterizes this part of the Mediterranean area in the Pliocene) is shown by these rocks. The genetic parentage among all of them is shown by the constancy of strontium isotopic data ( - 0.705) and by the coherency of microprobe mineralogical data. Trace-element geochemistry suggest that basalts were originated by partial melting of a common deep-source material. Available data prevent to discriminate between two possibilities in the origin of andesites and dacites: they could represent lower-degree partial

163 (l

/

/

4 75 ~ / A -b~ -- o o ~

Ab

'

/ An-

.9

-~ O r

Fig. 12. Projection o f the isobaric cotectic lines for 2, 4, 5, 7 and 10 kbar onto the Qz-AbOr plain of the Qz-Ab-Or-An tethraedron. Open circles: experimental data (in Winkler, 1974). Full circles = silicic rocks of Monte Arci.

melts of the same source of the basalts or, alternatively, fractionated liquids of one (or more) of these basalts. In any case, some fractionation certainly occurred; however, the volume proportions of dacites relative to andesites and basalts is the opposite of what is expected in fractionation processes. Bytownitic core of plagioclase phenocrysts and Fo-rich olivine phenocrysts found in basalts possibly are inherited from the source material that must account for the calc~alkaline character shown by these rocks. Contamination with silica-rich material, is suggested by the frequent presence of oligoclase and quartz xenocrysts. Microprobe data (composition of glass inclusions in quartz xenocrysts and An content of plagioclase xenocrysts) allow the recognition of rhyolites as the contaminant material. The trace-element and isotope geochemistry (Figs. 10 and 11), however, indicate that strong chemical modifications have not induced by this phenomena, at least in basaltic rocks. Evidence of liquid immescibility appears in the ground. mass of all the subalkaline hypocrystalline or vitrophyric lavas. Conversely, no traces of such a phenomenon are recognizable in the holocrystalline products (eroded necks and large dykes). This phenomenon is probably related to post-eruptive cooling, but it is not clear when the unmixing actual-

164

ly occurred. Further studies must therefore pay attention to the possible relevance of liquids immiscibility in the petrogenetic history of Monte Arci rocks. Alkaline rocks

Alkaline rocks are relatively scarce and form an heterogeneous group. True alkali-basalts are absent and the most basic members are hawaiites with relatively high D.I. values indicating their evolved nature. Mugearites and alkali-trachytes have a chemistry and mineralogy suggesting a possible parentage with hawaiites through fractional crystallization. The very similar composition of alkali-feldspars found as phenocrysts in trachyte and in the ground. mass of hawaiite also support this hypothesis. Such a simple process probably played an important role but it was surely not the only factor in determining the present composition of alkaline rocks. The increase in STSr/S~Sr ratios from hawaiites to mugearites and to alkali-trachytes corresponds to evidence of contamination (quartz xenocrysts, orthopyroxene fragments). The composition of glass inclusions in the oligoclase of corroded cores of some alkalifeldspar phenocrysts again indicates rhyolites as the contaminant material. Large-scale contamination is, therefore, evident in many of Monte Arci Pliocene rocks. Rhyolites were not the only types responsible for such processes. Petrographic and geochemical evidence are found of contamination both of dacites (samples nos. 80 and 126) and rhyolites (sample no. 221) with alkaline material. Mineralogy of subalkaline basalts (olivine in the groundmass, Ca-clinopyroxene reaction rims on orthopyroxenes, quartz and plagioclase xenocrysts) could even lead one to think these rocks simply as alkali-basalts contaminated by silicic rocks. Trace-element contents, however, appear inconsistent with such a possibility: subalkaline basalts have in fact Rb, Cs, U, Th, etc., contents too low, if compared with both rhyolites and undifferentiated Pliocene alkali basalts of W. Sardinia. CONCLUSIONS

In conclusion, the following course of events can explain the intricate picture of Plio-Quaternary volcanism of Monte Arci: (a) Calc-alkaline volcanism in Miocene time; Monte Arci was a center of activity and erupted basic lavas. The crust beneath the volcano was injected at that time and largely "andesitized" b y large quantities of calc-alkaline material in dykes and intrusions. (b) Starting from the Pliocene, tensional tectonics and a positive thermal anomaly characterized the Tyrrhenian area accompanying the formation of the Tyrrhenian Sea abyssal plain. Fissure eruptions of undifferentiated alkalibasalts were the typical volcanological expression of this geodynamic situation. Only in the Campidano graben area (Monte Arci and Montiferru) did peculiar tectonics (normal faulting with large-scale tilting of faulted blocks)

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allow the formation of magmatic pockets where uprising alkali-basalts stopped and underwent differentiation. (c) As a result of the superposition of the high regional heat flow with the thermal effect related to the cooling of alkali-basalts differentiating within the magmatic pockets, crustal melting temperatures were reached. First-produced anatectic melts had compositions near the minima of the granitic system: large volumes of crust, possibly with different composition, nature and depth, could reach these relatively low melting temperatures, a fact which should explain both coherent geochemical patterns and different isotopic ratios of silicic rocks. The absence of large volumes of anatectic products in Montiferru volcanic complex (where on the contrary differentiated products are very abundant) may be the result of the shallower depth of the Montiferru magma chamber(s). (d) Approaching the alkali-basalt magma chamber, temperature and melting degree increase but, obviously, they affect smaller volumes of deep-crustal material. Subalkaline rocks could represent the products of this high-degree partial melting. A mantle source cannot however be ruled out, most because of the presence of high-temperature magma in the subalkaline group. In any case, their homogeneous mineralogical and geochemical features probably reflect an homogeneous source and the peculiar subalkaline affinity of these rocks suggest that their source, either in the deep crust or in the upper mantle, had been deeply modified by the Miocene calc-alkaline activity. (e) The main alkali-basaltic body should have had several fingerings that evolved and erupted separately, as indicated by the irregular stratigraphic sequence of alkaline products which show a certain "family likeness' but do not display clear trends indicating a common origin through fractional crystallization. (f) In such a complex situation where different petrogenetic processes and different magmas superimpose one upon the other, contamination must have been very common and able in some cases to deeply modify the original composition of magmatic melts. Similar complicated magmatic associations as those found at Monte Arci seem to exist in other areas of the world (e.g. Western USA or Eastern Turkey) where volcanism related to post-collisional tensional movements in Late Tertiary and Quaternary times superimposes to volcanism related to subduction processes in Early--Middle Tertiary times. The evolution of continental margins after plate-collision events is in fact characterized by complex geotectonic processes. These processes are frequently reflected by the production of magmas difficult to arrange in the usual schemes connecting magma composition with tectonic setting, because modified by the influence of the paleogeodynamic environment.

166 ACKNOWLEDGEMENTS This w o r k was financially s u p p o r t e d b y t h e Italian National Research Council ( C N R ) . The m a n u s c r i p t was critically read b y F. Barberi, F. I n n o c e n t i and M. Sheridan w h o s e help and suggestions are highly a p p r e c i a t e d . T h e a u t h o r s are s t r o n g l y i n d e b t e d t o G. O t t o n e l l o w h i c h f r i e n d l y p r o v i d e d I N A A , p e r f o r m e d at t h e Pierre Sue L a b o r a t o r y , Saclay. T h a n k s are d u e to M. Treuil, J.L. J o r o n and t h e G S T o f t h e L a b o r a t o r y f o r their h o s p i t a l i t y .

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