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

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

MIOCENE CALC-ALKALINE HERITAGE IN THE PLIOCENE POST- COLLISIONAL VOLCANISM OF MONTE ARCI (SARDINIA, ITALY)

ROBERTO CIONI l , ROBERT CLOCCHIATTI 2, 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 composi- t ion 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 both continental and oceanic environments are commonly considered the expression of extensional tectonics, whereas dominantly calc- alkaline 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 sub- aerial, 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 (Dorgali- Orosei 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 two 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) sub- aerial Late Pliocene (2.6--3.7 m.y., Di Paola et al., 1975) silicic, intermediate,

Fig. 1. S impl i f ied geological map o f Sardinia (after C o c o z z a et al., 1 9 7 4 , modi f i ed) . 1 = major faults; 2 = Quaternary cont inenta l depos i t s ; 3 = Pl io -Ple i s tocene volcanics; 4 = Ter- t iary sed iments ; 5 = Ol igo-Miocene calc-alkaline volcanics; 6 = pre-Tertiary basement .

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and basic rocks. Figure 2 is a simplified version o f the 1 : 5 0 0 . 0 0 0 geological ma p o f this area (Assorgia et al., 1976a) .

MAJOR-ELEMENT CHEMISTRY AND PETROGRAPHY

Major-element analyses made on selected samples o f Pliocene volcanic rocks o f Monte Arci are repor ted in Tables I, II and III toge the r with CIPW norms and phenocrys t s minera logy. The (K20 + Na20) vs SiO~ diagram (Fig. 3) clearly indicates the existence o f at least three groups of rocks with s trongly d i f ferent alkalini ty among the Pliocene lavas o f Monte Arci. The more alkalic rocks (Table I) include hawaiites, mugeari tes and t rachytes . Hawaiites are subaphyr ic to po rphyr i t i c lavas with olivine, labradori te (An61-$2), augitic

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50 60 70 Si02 wt% i I , I i I 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

T A B L E I

M a j o r - e l e m e n t ana lyses , 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 o f M o n t e Arci P l i o c e n e subalkal ine vo lcan ic rocks

S a m p l e 130 82 153 34 182 30 31 28 170 223

R o c k t y p e B B B B B B B B B B

Major -e l emen t analyses

SiO~ 53 .21 53.01 52.79 52 .54 54 .28 53.79 54 .10 54 .86 53 .50 54.07 TiO 2 1.57 1.61 2 .00 1.37 1.66 1.66 1.74 1.48 1.77 1.60 A1203 16 .70 15.49 16 .00 16.73 15 .33 16.05 15.89 15.96 15 .44 15.66 Fe203 1,15 2.12 2 .76 4.33 3.01 3.37 2.99 1.63 2.87 2 .36 FeO 7.53 6.75 6.75 5.55 6.39 5.43 5.62 6.59 6.07 6 .20 MnO 0.13 0 .13 0.13 0 .13 0 .12 0 .12 0 .13 0 .12 0.12 0 .12 MgO 6.78 7.78 6 .30 5.66 6 ,50 6.23 5.48 6.13 6.56 6 .54 CaO 7.05 7.41 7.26 7.39 6.71 6.58 6 .86 5.95 6.68 6 .60 Na~O 3.73 3.19 3.61 3.87 3 .90 3.91 3.85 4.13 3.84 3.86 K20 0 .87 1.36 0.99 0.53 0.86 1.11 1.20 1.10 1.88 1.92 P2Os 0.28 0.41 0.23 0.18 0.22 0.29 0 .24 0 .20 0 .44 0.39 L .O, I . 1.00 0.67 1.19 1.74 1.02 1.46 1.90 1.84 0.82 0 .66

C I P W norms Q 0.5 1.4 2.7 4.1 4.0 3.8 4.7 2.8 0.7 0.6 or 5.2 8.0 5.8 3.1 5.1 6.6 7.1 6.5 11.1 11.3 ab 31.6 27 .0 30.5 32.7 33.0 33.1 32.5 34.9 32 .5 32.6 an 26.3 23.9 24.5 26.7 21.8 22.9 22.5 21.8 19.4 19.7 di 5.6 8.1 8.0 7.0 8.2 6.2 8.0 5.2 8.8 8.4 h y 24.7 23.6 18.9 15.4 19.0 17.2 15.1 21.2 18.2 19.2 m t 1.7 3.1 4.0 6.3 4.4 4.9 4.3 2.4 4.2 3.4 h m il 3.0 3.1 3.8 2.6 3.2 3.2 3.3 2.8 3.4 3.0 ap 0.7 1,0 0.6 0.4 0.5 0.7 0.6 0.5 1.0 0.9

D.I . 37.3 36.4 39 .0 39.9 42.1 43 .5 44.3 44.3 44.3 44 .6

Phenpcryst mineralogy Olivine + + (+) ++ ++ ++ Plagioc lase + + (+) ++ + (+) ++ ++ O r t h o p y r . (+) ++ + (+) ++ + C l i n o p y r . + + O p a q u e s B i o t i t e Q u a r t z * (+) (+) (+)

X-ray fluorescence analyses (analysts: M. Menichini and M. Saltta); MgO by Atomic Absorption analyses (analyst: R. Cioni); FeO titrirnetric (analyst: R. Cioni) B = basalt; A = andesite; D = daeite; * = xenolithic; ++ = abundant (up to 10 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

1 3 9

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 55 .72 54.76 55.77 55 .02 55 .94 55 .74 55.55 55.11 55 .52 56.48 57.99 1 .44 1.62 1.56 1.31 1.55 1.42 1.55 1.59 1.53 1.59 1.57 1.48

16.41 15.43 15.75 15.99 16,13 15.67 15 .62 15 .24 16.09 15.12 15.77 14.52 3.40 1 .64 3.71 2.42 2,05 2.43 3.96 4.33 3.02 2.73 3.52 1.95 5.31 6.69 5.25 5.91 6.21 6 .10 5.00 4.85 5.17 5.64 5.68 5.64 0 .12 0.11 0 .12 0 .12 0 .12 0.11 0.11 0 .10 0 .12 0.10 0 .10 0 .10 5.67 5.95 5.59 5 .84 5.79 5.94 5.52 5.17 5.57 5.66 4.21 5.38 6.49 6 .44 6.28 6.13 6,03 6.36 6.58 6.42 6.07 6.26 6.59 5.80 4.18 4 .10 3.88 4.16 4.02 4.33 4.13 3.97 4 .04 4.01 4.27 3.66 0 .58 1.06 1.01 0.89 1.43 0 .80 0.66 1.09 1.89 1.96 0.93 2.05 0.21 0 .25 0 .20 0 .24 0.28 0 .23 0.17 0 .20 0.33 0.36 0.25 0 .20 0 .83 0.99 1.89 1.22 1.37 0.61 0 .96 1.49 1.05 1.02 0.63 1.23

6.3 4.1 6.8 5.3 3.4 4.6 7.8 8.2 3.5 3.7 8.2 8.3 3.4 6.3 5.9 5.3 8.5 4.7 3.9 6.4 11.2 11.6 5.5 12,1

35 .4 34.7 32.8 35.2 34 .0 36.6 34.9 33.6 34.2 33.9 36.1 31 .0 24 .3 20.6 22.6 22.3 21.7 21.0 22.1 20.6 20.2 17.5 21.1 17.1

5.2 8.0 5.7 5.2 5.2 7.4 7.4 8.0 6.3 9.1 8.1 8.3 16.4 19.5 15 .4 18.9 19.3 18.3 13.7 12.0 15.5 15.4 11.7 15.9

4.9 2.4 5.4 3.5 3.0 3.5 5.7 6.3 4.4 4.0 5.1 2.8

2.7 3.1 3.0 2.5 2.9 2.7 2.9 3.0 2.9 3.0 3.0 2.8 0.5 0.6 0.5 0.6 0,7 0.6 0.4 0.5 0.6 0.9 0.6 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 of 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

1 4 0

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

S a m p l e 168 224 G I 0 66 G9 G I 2 G2 68 208 179

R o c k t y p e A A A A A A D D D D

M a j o r ~ l e m e n t ana lyses SiO 2 60 .49 61 .28 60.41 60.61 61.81 61 .72 64 .39 64 .82 64 .92 65 .42 TiO 2 1.16 1.12 1.40 1.39 1.10 1.29 1.08 0.95 1.10 0.97 A1203 15 .27 15 .13 14 .06 14.31 13.79 14.14 13.66 13 .83 13.66 14.57 Fe203 1.28 1.38 4.37 3.47 2.88 4.86 1.56 1.27 1.55 1.30 FeO 5.00 4.66 3.37 3.92 2.91 2.31 4 .02 3.61 3.71 3.31 MnO 0.11 0.11 0 .10 0.11 0.07 0 .08 0.08 0.11 0.07 0 .10 MgO 4.36 4 .16 4.48 3.89 5.94 3.85 3 .80 4.04 3.29 3 .04 CaO 4.66 4 .52 5.16 4.71 4.06 4 .63 3.77 3.28 3.56 3.23 Na:O 3 .82 3.92 4.22 3.90 3.32 3 .93 3.81 3.60 3.66 3.69 K:O 1.93 1 .94 1.71 2.01 3.02 2.12 3 .20 3.38 3.66 3.57 P2Os 0 .14 0.15 0 .20 0.17 0 .22 0.18 0.18 0 .16 0.21 0.18 L.O.I . 1.77 1.63 0.51 1.50 0.87 0.89 0.47 0.95 0.61 0.62

C I P W n o r m s Q 12.6 13.8 14.1 15.3 14.5 17.1 16.2 17.3 17.3 18.2 or 11 .4 11.4 10.1 11.9 17.8 12.5 18.9 20.0 21.6 21.1 ab 32.3 33.1 35.7 33.0 28.1 33.2 32.2 30.4 31.0 31.2 an 18.8 18.0 14.4 15.6 13.8 14.7 10.7 11.6 10.0 12.6 di 2.7 2.9 7.7 5.3 3.8 5.5 5.5 2.9 5.1 1.8 hy 15.9 14.8 8.4 9.5 14.3 7.0 11.3 12.9 9.6 10.3 m t 1.9 2.0 6.3 5.0 4.2 4.0 2.3 1.8 2.3 1.9 hm 2.1 fl 2.2 2.1 2.7 2.6 2.1 2.4 2.1 1.8 2.1 1.8 ap 0.3 0.3 0.5 0.4 0.5 0.4 0.4 0.4 0.5 0.4

D.I. 56.3 58.3 59.9 60.2 60 .4 62.9 67.3 67.7 69.9 70.5

P h e n o c r y s t m i n e r a l o g y Olivine Plagioclase ++ + Orthopyr . ++ (+) Cl inopyr . Opaques Biot i te Quartz*

C+) + ++ ++ ++ (+) +

++ ++

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. Andes 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

1 4 1

126 196 G8 80

D D D D

64 .52 66 .76 66.87 65 . 24 1.17 0.89 0 .92 1.05

14 .12 14 .18 13.61 13 .85 2.78 1.06 1.56 3 .23 2.57 2.98 3.15 1.65 0 .10 0 .10 0 .06 0 .10 2 .24 2.71 2 .60 2.21 3.29 2.76 3.08 3.09 3.91 3.56 3.78 3.72 3.68 4 .13 3.75 4 .19 0 .44 0.19 0 .17 0 .39 1.18 0.68 0 .44 1.08

19.0 19.7 20 .2 19.6 21.7 24 .4 22 .2 24.8 33.1 30.1 32 .0 31.4 10.1 10.5 9.1 8.7

2.6 1.6 4.0 3.2 5.0 9.3 7.6 4.0 4.0 1.5 2.3 3.3

1.0 2.2 1.7 1.8 2.0 1.0 0 .4 0.4 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, hyper- sthene, 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 composit ion.

TABLE II

Major-element analyses, CIPW norms and phenocrys t minera logy of Monte Arci Pliocene rhyol i tes

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 70.06 68.52 70.07 71.55 71.93 72.79 73.19 73.55 73.43 74.05 75.06 0.54 0.48 0.43 0.39 0.28 0.42 0.34 0.34 0.21 0.32 0.26 0.18

13.26 12.83 14.53 13.66 13.49 14.42 13.71 13.49 13.92 13.33 13.03 13.15 1.49 1.19 1.78 1.05 0.71 0.53 1.01 1.02 0.38 1.79 1.18 0.47 1.80 1.59 0.58 1.10 1.05 1.59 1.38 1.22 1.29 0.46 0.94 1.02 0.06 0.05 0.09 0.09 0.09 0.09 0.03 0.03 0.09 0.02 0.03 0.09 0.99 0.85 0.46 0.59 0.29 0.48 0.36 0.32 0.21 0.26 0.25 0.23 1.65 1.42 1.13 1.04 0.80 1.17 0.94 0.92 0.75 0.87 0.79 0.64 3.37 3.66 3.25 3.67 3.24 3.43 3.56 3.32 3.39 3.21 3.47 3.41 4.81 4.97 5.74 4.77 5.43 5.34 5.35 5.52 5.39 5.60 5.40 5.16 0.15 0.16 0.14 0.10 0.10 0.12 0.12 0.12 0.10 0.12 0.09 0.06 2.91 2.74 3.36 3.46 2.97 0.48 0.41 0.51 0.71 0.59 0.51 0.53

CIPW norms Q o r

ab a n

c

di hy mt hm il ap

D.I.

25.9 25.8 25.0 27.3 29.6 27.8 28.9 30.3 30.7 31.4 31.2 33.3 28.4 29.4 33.9 28.2 32.1 31.6 31.6 32.6 31.9 33.1 31.9 30.5 28.5 31.0 27.5 31.0 27.4 29.0 30.1 28.1 28.7 27.2 29.4 28.8

4.5 3.3 5.0 3.9 3.8 3.1 3.5 3.3 2.8 1.2 0.8 1.1 1.2 0.6 0.7 1.4 0.7 0.3 0.9

0.4 1.7 3.7 2.5 1.1 2.2 1.7 3.2 2.1 1.7 2.4 0.7 1.0 2.0 2.2 1.7 0.9 1.5 1.0 0.8 1.5 1.5 0.5 0.6 1.7 0.7

1.1 1.4 1.0 0.9 0.8 0.7 0.5 0.8 0.7 0.7 0.4 0.6 0.5 0.4 0.4 0.4 0.3 0.2 0.2 0.3 0.3 0.3 0.2 0.3 0.2 0.1

6.9 3.9 4.7

82.8 86.1 86.4 86.5 89.1 88.4 90.6 91.0 91.3 91.7 92.5 92.6

Phenocryst mineralogy O r t h o p y r o x ++ ++ + (+) (+) + (+) (+) (+) C l inopyrox + ( + ) Plagioclase ++ ++ ++ + ++ ++ + (+) + (+) Alkal i fe ld . ++ (+) ++ + (+) Biot i te + ++ ++ + ++ + (+) (+) + (+) O p a q u e s + + + ( + ) + + + ( + ) + (+) Quartz (+) (+) ? ?

Analytical methods and analysts as in Table I. L = devitrified; P = perlite; 0 = obsidian; ++ = present;

+ = s c a r c e ; ( + ) = v e r y s c a r c e .

TABLE III

Major~lement analyses, CIPW norms and phenocryst mineralogy of Monte Arci Pliocene alkaline rocks

143

Sample 151 7 7 164 5 9

Rock type H H H M

138 119 G29 194 210 G27 G28

M T P T P T O T O T L T L

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 o r

ab an 21.6 20.8 16.3 13.0 12.9 ne 2.2 0.2 4.9 c di 6.7 5.7 5.5 3.2 3.3 hy 5.6 4.1 ol 15.1 12.8 12.7 mt 3.3 4.1 2.0 5.6 4.7 hm 0.3 il 3.9 3.3 4.1 3.6 3.9 tn ap 1.5 1.5 1.9 2.3 1.9 D.I.

10.8 12.5 17.6 16.2 19.7 17.2 17.1 16.0 13.9 15.6 18.2 20.8 22.0 35.9 36.0 32.7 34.1 35.0 35.8 30.1 34.9 33.2 33.9 32.6 33.6 35.9 37.4 39.7 40.1 40.6

4.4 2.4 2.6 2.8 0.5 1.2

0.1 1.8 1.5 0.6 2.6 1.1

1.7 1.3 0.8 0.7 0.1 0.1

2.3 2.2 0.3 0.1 0.5 2.8 2.5 2.2 2.8

1.3 1.2 1.3 1.3 1.2 1.1 0.2

0.4 0.4 0.4 0.4 0.4 0.4 46.2 50.7 56.3 65.5 67.1 87.1 88.1 89.8 90.0 92.3 92.4

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

o" - 000 A A 2 ~ ° 4 ~ t

o ° A

o ~ o o o

A

• S u b a l k a l i n e b a s a l t s

A l k a l i n e R o c k s

o S u b a l k a l i n e A n d e s i t e s and d a c i t e s

o Rhyol Ires

-~ A l k a l y t r a c h y t e s

_ _ C a s c a d e s T r e n d

M a r m i L t a - A r c u e n t u f i e l d

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 microprobe 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 both rocks it is also present in the groundmass. In some cases phenocrysts in subalkaline basalt are surrounded by thin c l inopyroxene reaction rims. Fe-rich olivine is a rare const i tuent in the alkali-trachyte where it is generally included in alkali-feldspar phenocrysts , but no data are available for such an occurrence.

Olivine composit ion within the hawaiite evolves regularly from phenocrysts to groundmass decreasing in fosterite content f rom 90 to 75%.

On the contrary no continuous variations are observed in olivine of the subalkaline basalt: phenocrysts composit ion is in fact more or less constant

TABLE IV

Selected microprobe analyses of olivines

145

Sample 151 hawaiite 42 subalkaline basalt

PC MPC GM PC PC MPC GM

Point no. 46 72 49 1 70 38 71

Major,element analyses SiO 2 40.84 39.15 37.48 39.43 38.52 36.45 35.81 TiO2 0.01 0.10 0.13 Al:O3 0.01 0.11 0.07 0.04 0.06 FeO ° 9.32 19.33 22.70 17.91 19.39 34.55 36.57 MnO 0.15 0.38 0.37 0.23 0.07 0.36 0.57 MgO 50.71 42.10 38.71 42.49 41.24 28.69 26.03 CaO 0.09 0.28 0.18 0.13 0.20 0.35 Na20 0.01 0.04 0.05 K20 0.02 0.02 0.04 NiO 0.34 0.41 0.35 0.31 0.96

Tot 101.39 101.47 99.67 100.71 99.87 100.42 100.40

Cations per 4 oxygens Si 0.987 0.991 0.983 0.998 0.991 1.003 1.003 Ti 0.002 0.003 A1 0.003 0.002 0.001 0.002 Fe 0.188 0.409 0.498 0.379 0.417 0.796 0.857 Mn 0.003 0.008 0.008 0.005 0.001 0.008 0.014 Mg 1.827 1.589 1.514 1.603 1.582 1.177 1.086 Ca 0.002 0.008 0.005 0.003 0.006 0.010 Na 0.001 0.002 0.003 K 0.001 0.001 0.001 Ni 0.007 0.008 0.007 0.006 0.022

3.013 3.007 3.015 3.000 3.004 2.995 2.997 (Y)' 2.026 2.016 2.032 2.002 2.013 1.992 1.994

100Mg 90.7 79.5 75.2 80.9 79.1 59.7 55.9

Mg+Fe

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

a r o u n d Fo80, whi le t h e g r o u n d m a s s ranges f r o m Fo ,9 to Fos2. R e a c t i o n cl ino- p y r o x e n e show augi t ic c o m p o s i t i o n s s imi lar to t h o s e r i m m i n g 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 ana lyses o f M o n t e Arc i ol ivines are r e p o r t e d in Table IV.

P y r o x e n e s

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 and in t he g r o u n d m a s s in t h e hawa i i t e , a l k a l i t r a c h y t e and suba lka l ine basa l t . In t h e last s ample cl ino- p y r o x e n e r e a c t i o n aggregates r im o r t h o p y r o x e n e p h e n o c r y s t s . Ve ry t h in

TA

BL

E V

Sel

ecte

d m

icro

pro

be

an

aly

ses

of

Py

rox

en

es

Sam

ple

4

2 s

ub

alk

alin

e b

asal

t 1

68

an

des

ite

Poin

t no.

8

5

6

7

40

34

9

2

5

66

8

6

65

6

7

59

6

0

69

Ma

jor-

ele

me

nt

an

aly

ses

SiO

2

TiO

2

AI2

0 ~

F

eO °

MnO

M

gO

C

aO

Na20

K20

NiO

Cr203

Tot.

Ca

tio

ns

pe

r

SiI

V

A1

VI

AI

Ti

Fe

Mn

M

g C

a N

a K

N

i C

r

M M

g

Mg

+F

e

Wo

%

En

%

Fs%

53

.64

5

3.9

7

51

.12

5

4.3

2

50

.21

5

0.7

7

50

.76

5

2.4

9

52

.86

5

2.9

0

52

.61

5

2.5

9

54

.82

5

3.2

5

53

.23

0

.31

0

.18

1

.10

0

.40

1

.11

0

.93

1

.01

0

.66

0

.33

0

.24

0

.16

0

.27

0

.17

0

.46

0

.38

3

.22

3

.08

2

.10

1

.40

2

.59

1

.85

1

.99

0

.65

4

.15

3

.19

3

.57

3

.15

2

.04

2

.91

0

.88

1

0.7

9

10

.00

1

0.3

6

15

.19

9

.85

1

2.6

3

10

.38

1

2.7

6

10

.28

1

0.4

2

10

.89

1

1.9

6

12

.19

1

3.0

9

16

.62

0

.11

0

.18

0

.31

0

.38

0

.10

0

.25

0

.19

0

.33

0

.20

0

.17

0

,06

0

.07

0

.26

0

.21

0

.26

2

9.5

6

30

.38

1

9.3

9

26

.68

1

6.0

9

16

.50

1

6.0

7

16

.25

3

0.0

4

30

.12

2

9.3

6

29

.37

2

8.9

7

27

.40

2

6.3

3

1.7

2

1.2

7

14

.51

2

.42

1

7.5

8

15

.24

1

8.6

5

15

.67

1

.36

1

.45

1

.51

1

.24

1

.05

1

.73

1

.75

0

.03

0

.27

0

.09

0

.31

0

.40

0

.36

0

.29

0

.08

0

.02

0

.07

0

.05

0

.04

0

.03

0

.02

0

.04

0

.09

0

.01

0

.02

0

.04

0

.81

0

.66

0

.47

0

.24

0

.09

0

.32

0

.04

0

.08

.

..

..

..

..

0

.61

0

.69

0

.17

0

.09

99

.39

9

9.0

9

99

.19

1

01

.71

9

8.5

4

99

.04

9

9.4

1

99

.43

1

00

.01

9

8.9

0

98

.95

9

8.9

0

99

.56

9

9.0

5

99

.62

6 o

xy

ge

ns

1.9

09

1

.91

8

1.8

97

1

.94

0

1.8

99

1

.92

0

1.9

01

1

.97

3

1.8

75

1

.89

7

1.8

88

1

.89

5

1.9

55

1

.92

3

1.9

48

0

.09

1

0.0

82

0

.09

2

0.0

59

0

.10

1

0.0

80

0

.08

8

0.0

27

0

.12

5

0.1

03

0

.11

2

0.1

05

0

.04

5

0.0

77

0

.03

8

0.0

44

0

.04

7

0.0

14

0

.00

2

0.0

02

0

.04

8

0.0

32

0

.03

9

0.0

29

0

.04

1

0.0

47

0

.00

8

0.0

05

0

.03

1

0.0

11

0

.03

2

0.0

26

0

.02

8

0.0

19

0

.00

9

0.0

06

0

.00

4

0.0

07

0

.00

4

0.0

13

0

.01

1

0.3

21

0

.29

7

0.3

22

0

.45

4

0.3

11

0

.39

9

0.3

25

0

.40

1

0.3

05

0

.31

2

0.3

27

0

.36

0

0.3

63

0

.39

5

0.5

08

0

.00

3

0.0

05

0

.01

0

0.0

12

0

.00

3

0.0

08

0

.00

6

0.0

11

0

.00

6

0.0

05

0

.00

2

0.0

02

0

.00

8

0.0

07

0

.00

8

1.5

67

1

.60

9

1.0

72

1

.42

0

0.9

07

0

.93

0

0.8

97

0

.91

0

1.5

87

1

.60

9

1.5

69

1

.57

6

1.5

39

1

.47

4

1.4

35

0

.06

6

0.0

48

0

.57

7

0.0

93

0

.71

2

0.6

18

0

.74

8

0.6

31

0

.05

2

0.0

56

0

.05

8

0.0

48

0

.04

0

0.0

67

0

.06

9

0.0

02

0

.02

0

0.0

06

0

.02

3

0.0

29

0

.02

6

0.0

21

0

.00

6

0.0

10

0

.00

5

0.0

03

0

.00

3

0,0

01

0

.00

1

0.0

02

0

.00

8

0.0

01

0

.00

1

0.0

23

0

.02

0

0.0

14

0

.00

7

0.0

03

0

.09

0

0.0

01

0

.00

2

0,0

17

0

.02

0

0.0

05

0

.00

3

2.0

1

2.0

1

2.0

2

2,0

2

2.0

2

2.0

3

2.0

2

2.0

1

2.0

3

2.0

3

2.0

2

2.0

3

2.0

0

2.0

0

2.0

2

0.8

3

0.8

4

0.7

7

0.7

6

0.7

4

0.7

0

0.7

3

0.6

9

0.8

4

0.8

4

0.8

3

0.8

1

0.8

2

0.7

9

0.7

4

3.4

2

.5

29

.2

4.7

3

6.9

3

1.7

3

8.0

3

2.5

2

.7

2.8

3

.0

2.4

2

.1

3.5

3

.4

80

.2

82

.3

54

.4

72

.2

47

.0

47

.8

45

.5

46

.9

81

.6

81

.5

80

.3

79

.4

79

.2

76

.1

71

.3

16

.4

15

.2

16

.4

23

.1

16

.1

20

.5

16

.5

20

.6

15

.7

15

.7

16

.7

18

.2

18

.7

20

.4

25

.3

op

x

op

x

cpx

ep

x

cp

x

cPx

c

px

c

px

o

px

o

px

o

px

o

px

c

px

c

px

c

px

PC

PC

R on 5

R on 5

PC

GM

R on 8

R PC

PC

PC

MPC

R GM

GM

PC = phenocryst; MPC

= microphenocryst; GM

= groundmass; R

= reaction p

roducts.

TA

BL

E V

(c

on

tin

ue

d)

Sa

mp

le

15

1 h

aw

aii

te

G2

9 a

lka

li-t

rac

hy

te

Po

int

no

. 7

7

59

5

4

51

5

8

59

7

0

Mc

6o

r-e

lem

en

t a

na

lyse

s

' SiO

~

TiO

2

AI2

0 ~

FeO

--

Mn

O

Mg

O

CaO

N

a2

0

K2

0

NiO

C

r20

s

To

t.

Ca

tio

ns

pe

r 6

ox

yg

en

s

Sil

V

AIv

I A

1 T

i F

e M

n

Mg

C

a N

a K

N

i C

E

M b

lg

Mg

+F

e

Wo

%

En

%

Fs%

51

.84

5

1.2

4

47

.01

4

6.4

1

51

.30

5

2.5

7

52

.14

0

.43

1

.29

2

.93

2

.95

0

.89

0

.64

0

.15

1

.72

1

.66

5

.12

5

.60

4

.20

1

.49

0

.84

1

6.9

4

7.7

3

7.8

7

8.4

6

5.0

3

9.7

0

14

.68

0.25

0.26

0.II

0.06

--

--

--

25.15

15.84

13.26

13.50

16.58

14.37

12.06

2.13

21.01

21.21

21.17

20.50

19.84

18.29

0.3

8

0.4

1

0.5

3

0.5

3

0.7

1

0.5

9

0.0

5

0.0

2

0.0

1

0.0

5

0.0

2

0.03

0.20

--

--

--

0.02

0.04

0.12

1.19

98.52

99.50

98.06

98.86

100.30

99.36

98.78

1.9

25

1

.91

4

1.7

94

1

.76

4

0.0

75

0

.07

3

0.2

06

0

.23

6

0.0

24

0

.01

5

0.0

12

0

.03

6

0.0

84

0

.08

4

0.5

26

0

.24

1

0.2

51

0

.26

9

0.008

0.008

0.004

0.002

1.392

0.882

0.754

0.765

0.085

0.841

0.867

0.862

0.027

0.030

0.039

0.002

0.001

0.001

0.006

0.0

01

0

.00

1

0.0

04

2.0

3

2.0

2

2.0

2

2.0

4

1.875

1.968

2.006

0.125

0.032

0.056

0.034

0.025

0.024

0.018

0.004

0.154

0.304

0.472

0.903

0.802

0.692

0.803

0.796

0.754

0.0

38

0

.05

2

0.0

44

0

.00

2

0.0

01

0.0

34

2.0

1

2.0

1

1.9

9

0.7

3

0.7

9

0.7

5

0.7

4

0.8

5

0.7

3

0.6

0

4.2

4

2.8

4

6.3

4

5.5

4

3.2

4

1.8

3

9.3

6

9.5

4

4.9

4

0.3

4

0.3

4

8.5

4

2.2

3

6.2

2

6.3

1

2.3

1

3.4

1

4.2

8

.3

16

.0

24

.5

Cpx

C

px

Cpx

c

px

c

px

C

px

cp

x

~.~

GM

P

C

GM

G

M

PC

M

PC

G

M

¢~-

148

clinopyroxene reaction rims also commonly surround or thopyroxene phenocrysts in the andesite. In Fig. 5 the variation fields of analyzed Monte Arci pyroxenes are plotted on a CaSiO3-MgSiO3-FeSiO3 molecular diagram. Re- presentative analyses are given in Table V.

Ca SiO 3

- ~ MAUPITIUS OLO SERIES

~ M M R ~,~ nEACI-ION ON OPX

2o :, ; i

A A A A A A A A A

Mg SiO3 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 or thopyroxene 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 or thopyroxene and olivine phenocrysts and as grains in the groundmass, but the two occurrences show different trends. Pigeonite rims or thopyroxene 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 surprising- ly 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 composit ion was determined by microprobe analyses in all samples but the alkali-trachyte in which oligoclase composit ion 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-Ab- Or triangle {Fig. 6). Selected analyses are reported in Table VI. The plagio-

An

149

g.2 SUBAL K, BA SA LT CORE OF PHENOCF

lsl HAWA)iTE

168 ANOESIT E

/*5 SUBALK, BASALT

lEE ANDESfTE XENOCRYSTS

42 $UI~ALK BASALT XENOCRVSTS

63 RHYOLITE

151 HAWA liT E ALKALI-TRACHYTE S

~ ~ V &3 RHYOLITE

v v Ab 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

TA

BL

E V

I

Sel

ecte

d m

icro

pro

be

an

aly

ses

of

feld

spar

s

o

Sam

ple

Po

int

no

.

15

1 h

awai

ite

alk

ali-

trac

hy

tes

42

su

bal

kal

ine

bas

alt

G2

9

11

0

21

0

53

4

8

63

6

0

56

+

+ 4

8

49

5

2

4 1

8

2 11

PC

M

PC

G

M

GM

P

C

PC

P

C

CP

C

CP

C

RP

C

MP

C

MP

C

GM

X

C

SiO

a

50

.77

5

3.0

5

53

.08

6

3.7

1

66

.52

6

5.2

5

65

.29

4

7.2

0

49

.36

5

4.4

5

53

.62

5

5.1

7

56

.56

5

9.8

9

AI2

0,

29

.49

2

8.1

4

27

.78

1

9.6

2

19

.53

1

9.7

4

19

.73

3

2.1

4

31

.95

2

7.8

0

28

.45

2

6.9

3

26

.94

2

4.4

3

FeO

°~

0.3

6

0.6

6

0.6

5

0.3

2

0.2

7

0.4

6

0.5

9

0.5

8

0.2

7

0.4

5

0.1

9

0.5

5

0.5

9

0.6

3

Mg

O

0.1

0

0.0

7

0.0

6

0.0

2

0.1

4

0.0

1

0.1

5

0.1

0

0.0

5

CaO

1

2.6

6

11

.22

1

0.1

2

0.7

7

0.5

3

1.2

0

1.1

4

15

.67

1

4.4

8

9.9

8

11

.74

9

.76

9

.53

6

.40

N

a~O

3

.83

4

.80

5

.02

5

.99

5

.57

5

.95

6

.27

2

.40

3

.29

5

.70

4

.89

5

.53

6

.11

7

.62

K

20

0

.33

0

.33

0

.22

6

.44

9

.01

7

.35

6

.91

0

.17

0

.22

0

.34

0

.23

0

.51

0

.48

0

.70

To

t.

97

.54

9

8.2

7

96

.93

9

6.8

5

10

1.4

5

99

.95

9

9.9

3

98

.50

9

9.5

7

98

.73

9

9.2

7

98

.55

1

00

.26

9

9.6

7

Ca

tio

ns

per

32

ox

yg

en

s S

i 9

.45

8

9.7

74

9

.88

9

11

.77

6

11

.84

8

11

.74

8

11

.73

7

8.8

12

9

.05

7

9.9

52

9

.77

8

10

.09

1

10

.16

8

10

.73

9

A1

6.4

77

6

.11

2

6.1

01

4

.27

5

4.1

01

4

.19

0

4.1

81

7

.07

4

6.9

11

5

.99

0

6.1

17

5

.80

8

5.7

09

5

.16

4

Fe

'"

0.0

56

0

.10

2

0.0

41

0

.06

9

0.0

99

0

.09

0

0.0

41

0

.06

9

0.0

28

0

.08

5

0.0

89

0

.09

5

Fe

" 0

.10

0

0.0

50

M

g 0

,02

8

0.0

19

0

.01

7

0.0

05

0

.03

9

0.0

02

0

.04

1

0.0

27

0

.01

3

Ca

2.5

26

2

.21

5

2.0

21

0

,15

2

0,1

02

0

,23

2

0.2

19

3

.13

4

2.8

47

1

.95

5

2.2

94

1

.91

2

1,8

35

1

.22

9

Na

1,3

84

1

.71

4

1.8

14

2

.14

6

1,9

24

2

.07

7

2.1

86

0

.86

8

1.1

71

2

.02

1

1.7

28

1

.96

1

2.1

29

2

,64

8

K

0.0

78

0

,07

8

0.0

51

1

.51

9

2.0

46

1

,68

8

1,5

86

0

.04

0

0.0

51

0

,07

9

0.0

54

0

.11

9

0.1

10

0

.16

1

Z

15

.99

1

5.9

9

15

,99

1

6.0

5

15

.99

1

6.0

1

16

.02

1

5.9

8

16

.01

1

6.0

1

15

.92

1

5.9

8

16

.00

1

6.0

0

X

4.0

2

4.0

3

4.0

0

3.8

7

4.0

8

4.0

0

3.9

9

4.0

8

4.0

7

4.0

6

4.1

1

4.0

2

4.0

9

4.0

4

An

to

ol%

6

3.3

5

5.3

5

2.0

4

.0

2.5

5

.8

5.5

7

7.5

7

0.0

4

8.2

5

6.3

4

7.9

4

5.0

3

0,4

A

b

34

.7

42

.8

46

.7

56

.2

47

.2

52

.0

54

.8

21

.5

28

.8

49

.8

42

.4

49

.1

52

.3

65

.6

Or

2.0

1

.9

1.3

3

9.8

5

0.3

4

2.2

3

9.7

1

.0

2.2

2

.0

1.3

3

.0

2.7

4

.0

Fe

" o

r F

e"

ac

co

rdia

g t

o t

he

bes

t fi

t. P

C =

ph

en

oc

ryst

; M

PC

= m

iero

ph

en

oc

ryst

: G

M =

gro

un

dm

ass

; X

C =

xen

ocr

yst

; C

= c

ore

: [

= i

nte

rme

dia

te p

art:

R

= r

im.

+ =

X-r

ay f

luo

resc

ence

an

aly

ses

(Le

on

i et

al.

, 1

97

7).

TA

BL

E

VI

(co

nti

nu

ed)

Sam

ple

Po

int

no

.

16

8 a

nd

esR

e 4

3 r

hy

oli

te

68

2

73

1

9

3 1

02

9

9

PC

G

M

CX

C

IXC

R

XC

P

C

MP

C

8iO

2

53

.49

5

4.4

1

AI2

0.

27

.95

2

7.5

1

FeO

~

0.6

1

0.7

1

Mg

O

0.0

6

0.0

8

CaO

1

0.8

9

10

.22

N

a20

4

.96

5

.14

K

~O

0

.24

0

.43

To

t.

98

.20

9

8.5

0

Ca

tio

ns

pe

r 3

2 o

xy

ge

ns

Si

9.8

44

9

.96

7

A1

6.0

64

5

.94

1

Fe'

" 0

.09

4

0.1

09

F

e"

M

g 0

.01

7

0.0

22

C

a 2

.14

8

2.0

06

N

a 1

.76

9

1.8

25

K

0

.05

5

0.1

01

Z

16

.00

1

6.0

2

X

3.9

9

3.9

5

An

to

ol%

5

4.1

5

1.0

A

b

44

.5

46

.4

Or

1.4

2.

6

61

.26

5

9.1

3

55

.77

6

1.8

2

64

.91

2

2.4

1

25

.29

2

5.7

2

23

.84

1

9.0

7

0.2

1

0.0

8

0.6

8

0.1

5

0.0

2

0.0

7

4.5

6

7.3

8

8.2

6

5.3

3

0.4

3

7.7

4

6.6

5

6.1

8

7.6

4

3.5

1

1.0

2

0.6

9

0.5

3

1.5

5

11

.70

97

.20

9

9.2

4

97

.21

1

00

.18

9

9.7

7

11

.17

9

10

.63

7

10

.30

6

10

.99

6

11

.86

5

4.8

21

5

.36

1

5.6

04

4

.99

9

4.1

09

0.105

0.023

0.032

0.012

0.005

0.013

0.8

91

1

.42

3

1.6

36

2

.73

9

2.3

20

2

.21

4

0.2

37

0

.15

8

0.1

24

16

.00

1

6.0

0

16

.01

3

.90

3

.92

3

.99

23

.0

36

.5

41

.2

70

.8

59

.5

55

.7

6.2

4

.0

3.1

1.0

15

2

.63

6

0.3

53

16

.00

4

.00

25

.4

65

.8

8.8

0.0

85

1

.24

3

2.7

29

16

.00

4

.06

2.1

30

.6

67

.3

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 alkali- trachyte 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

TABLE VII

Selected microprobe analyses o f various minerals

Sample

Point no.

Phase

G29 alkali- trachyte 43 rhyol i te 151 hawaiite

67 71 101 100 98 45 47

Ilmenite Magnetite Magnetite Biotite Apatite Spinel Spinel

SiO~ 0.09 0.13 36.59 0.06 TiO: 47.65 11.43 10.40 5.67 0.22 Al:O3 2.10 2.98 14.13 45.94 45.43 FeO ° 45.05 78.72 81.31 18.74 0.65 9.03 8.64

6.53 ÷ 6.36 ÷ Fe:03 0.07 0.30 MnO

MgO 5.53 2.32 0.78 11.87 0.24 19.80 19.99 CaO 0.07 53.20 0.04

0.66 0.15 0.02 Na~O K~O . . . . 9.19 P:O~ -- 41.96 NiO - 0.53 0.01 Cr:O~ 0,03 20.4'2 20.51 CuO 0.15 -

Tot . 98.32 94.77 95.65 96.85 96.20 102.63 101.52

Recalculated analyses (i lmenite Cations Cations Cations on 35 O basis) on 22 O on 25 O

8i 5.492" Al 2.500 8 .00 Ti 0.008 Ti 0.632" Cr

mol% R:O3 12.2 F e " ' Fe' 2,353

mol% Usp 33.0 29.8 Mn T(C} 830 Mg 2.65E

Ca ~ fO; 10 ~3 Na 0.192

K 1.760 Ni P

Fe20 ~ 13.28 52.61 45.93 FeO 33.10 31.37 39.96

T o t ~ 99.65 100.03 ]00 .23

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

• 5.64

.1.95

0 - 0 9 2 1

0 .061~9 .91 9.703 I 0.049A

6.047

iI.492~

0"040t-16

3.425 I 1.0431 1.603"] 0.013 I 6.262 I 0.009 ~7.99 0 . 0 0 8 ]

0.091 J

0.013-] I 1.458|

0"036 ~16

3.468[ 1.025] 1.547"] 0 . 0 5 4 | 6.374 I

I 7.98

o .oo l 3

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 ob- tained.

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 alkali- trachytes. 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 com- positional 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.

Fe

O

Na

20

L

Y

Na

20

+K

20

bta

ck

sp

he

r u

tes

~ rv

(NIM

R t6B

~,4

2]

b°co

su

ba

tka

Un

e

o .~

~ @42

! lig

hl

relid

ual

glas

ses

t I

~

/ H g

(hl'4

L : ~I~

d8 u i~

I A ~ j

'~ss

es

b, ..

..

p ...

....

~

~

II ...

..

in qu

art .

....

lith

s ~

(MM

IR 16

8 &

42)

Y

V

V/

V

V

Y

V

Y

Y

V

Y

y \

K20

C

aO

Fig

. 7.

AF

M a

nd K

NO

plo

t of

mic

ropr

obe

anal

yzed

gla

sses

and

roc

ks.

Em

pty

cir

cles

: in

ters

titi

al g

lass

es.

1 5 5

T A B L E V H I

Selected microprobe analyses o f glasses

Sample G 2 9 4 3 4 2 1 6 8

Point no. 6 5 1 0 4 31 61 57 60 21 12 11 16 21 5 0

R R L R G L R G BS BS Q I L R G L R G BS BS Q I

SiO2 TiO~ A1203 FeO ° M n O

M g O

CaO Na20 K20 P~Os NiO

Tot.

CIPW n o r m s Q o r

ab a n

n e

lc ak C

di hy ol il ap n s

6 6 . 7 8 7 2 . 9 8 6 7 . 9 0 7 3 . 5 3 2 4 . 6 5 3 1 . 1 5 7 9 . 1 2 7 0 . 8 0 7 3 . 7 2 2 3 . 4 4 3 8 . 1 6 7 7 . 0 2 0 . 5 4 0 . 0 4 1 . 4 3 0 . 4 2 1 3 . 0 2 1 2 . 7 0 0 . 0 2 2 . 9 8 0 . 8 8 1 5 . 4 1 1 1 . 6 0

1 5 . 4 3 1 3 . 4 6 1 2 . 8 8 1 0 . 3 2 0 . 9 0 1 . 8 4 1 0 . 7 2 9 . 2 4 1 1 . 1 5 0 . 9 5 5 . 0 9 1 1 . 6 3 2 . 0 2 1 . 0 4 1 . 3 9 1 . 9 8 2 9 . 1 6 2 6 . 6 2 0 . 6 6 4 . 1 5 2 . 1 4 4 8 . 0 0 2 5 . 3 1 0 . 7 3 . . . . 0 . 4 1 0 . 3 6 0 . 0 1 0 . 0 3 0 . 0 4 0 . 3 3 0 . 1 4 0 . 4 0 0 . 0 7 0 . 1 5 0 . 2 2 4 . 5 4 4 . 4 0 0 . 0 9 0 . 2 6 0 . 0 1 7 . 2 5 1 . 1 5 0 . 0 6 0 . 8 9 0 . 6 2 0 . 6 4 0 . 3 5 1 0 . 3 2 1 0 . 0 8 0 . 4 5 1 . 2 2 0 . 3 8 1 .97 7 . 6 8 0 . 6 5 3 . 8 5 2 . 9 2 2 . 8 7 2 . 2 7 0 . 5 1 0 . 8 6 3 . 1 9 2 .27 1 . 8 9 0 . 2 0 2 . 0 1 2 . 8 8 7 . 0 0 5 . 5 2 5 . 7 6 4 . 4 3 0 . 4 0 0 . 4 8 4 . 1 0 4 . 2 9 4 . 5 5 0 . 1 3 1 . 4 6 5 . 2 8 0 . 0 6 0 . 0 1 0 . 4 5 0 . 0 3 6 . 7 5 3 .67 0 . 0 1 . . . . .

- - -- 0 . 4 0 0 . 5 7 . . . . .

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

1 4 . 0 3 2 . 5 2 8 . 9 4 0 . 9 - - - - 4 3 . 3 3 9 . 6 4 5 . 7 - - - - 3 8 . 7 4 1 . 4 3 2 . 8 3 3 . 9 2 6 . 1 2 .2 2 .8 2 4 . 2 2 6 . 6 2 8 . 4 - - 9 . 3 3 1 . 8 3 2 . 5 2 4 . 6 2 4 . 1 1 9 . 4 2 .6 7 .3 2 7 . 0 2 0 . 2 1 6 . 9 - - 1 8 . 4 2 4 . 8

4 . 0 2 .9 0 . 6 1 .7 - - - - 2 .2 2 .5 2 .0 1 .3 0 .6 3 .3

. . . . . . . . . 0 .9 - - - -

. . . . . . . . . . 0 . 6 - - - -

. . . . . . . . . 0.6 - - - - -- 1.5 1.7 1.1 -- -- 0.2 -- 2.6 -- -- --

1.0 - - - - - - 9 . 9 2 4 . 2 - - 3 .4 - - 5 .8 3 5 . 6 - - 2 .8 2 .0 0 .5 3 .6 2 7 . 4 2 1 . 6 1 .4 1 .8 2.7 - - 6 . 3 - -

. . . . 8 .7 4 . 2 - - - - - - 6 0 . 7 6 .1 1 .5 1 .1 0 .1 2 .7 0 .8 2 4 . 8 2 4 . 2 - - 5 .9 1 .8 3 0 . 0 2 3 . 8 - - 0 . 2 - - 1 .0 - - 1 6 . 0 ' 8 . 7 . . . . . .

. . . . 0 .4 . . . . . . .

- - = not detected; R = residual glass; L R G = light residual glass; BS = black spherules; Q I = glass inclusion in quartz xenocryst .

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

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. Initial STSr/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

TABLE IX

8~Sr/8'Sr ratios of Monte Arc± Pliocene volcanics

Sample Rock type 87Sr/8'Sr ± l o Rb (ppm) Sr (ppm) (87Sr/8'Sr)i

34 sub.basal t 0 .70500 +- 0 .00070 16 426 0.70500 28 sub.basalt 0 .70529 -+ 0 .00063 36 615 0 .70530 30 sub.basal t 0 .70560 -+ 0 .00040 38 587 0 .70560 42 sub.basalt 0 .70512 -+ 0 .00030 43 559 0 .70510 205 sub.basalt 0 .70505 ± 0 .00030 13 538 0.70505 31 sub.basalt 0 .70540 ± 0 .00060 37 502 0 .70540 153 sub.basal t 0 .70506 ± 0 .00016 25 442 0.70505 168 andesi te 0 .70534 -+ 0 .00030 71 386 0.70516 66 andesi te 0 .70470 ± 0 .00030 80 417 0 .70470 179 daci te 0.70447 ± 0 .00060 177 435 0 .70443 68 dacite 0 .70510 _+ 0.00050 169 455 0.70505 80 dacite 0 .70530 ± 0 .00030 170 365 0.70524 158 rhyol i te 0 .70850 ± 0 .00140 206 153 0.70820 61 rhyol i te 0 .71230 +- 0 .00040 275 78 0 .71150 177 rhyol i te 0 .70670 -+ 0 .00030 237 125 0.70630 53 rhyol i te 0.70987 -+ 0 .00033 286 51 0 .70860 221 rhyol i te 0 .70890 -+ 0 .00030 245 132 0.70870 77 hawaiite 0.70441 ± 0 .00013 63 738 0.70440 151 hawaii te 0 .70443 + 0.00025 67 837 0.70442 59 mugeari te 0 .70538 ± 0.00035 89 719 0.70537 119 alkali- trachyte 0.70679 -+ 0 .00025 185 44 0 .70630

(87Sr/8'Sr)i = (87Sr/S'Sr) - - (87Rb/s 'S r ) xt . STSr/8'Sr cor rec ted to E.A. s tandard = 0.7080. = 1.42 10 -~1 yt-1; t = mineral age = 3.0 m.y.

~S

% CK~

Z-

71 ~,

- 704

0 @

Rb ppm

50 100 150 200 250 l i I I

Fig. 8. Initial 87Sr/SSSr ratios o f Monte Arc± Pl iocene volcanics vs Rb contents . Same symbols as in Fig. 3. Vertical dash = STSr/S'Sr analytical conf iance.

157

processes. The relatively high Sr contents of rhyolites (between 50 and 150 ppm) and the absence in these rocks of a negative correlation be tween 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 by 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 alkali- t rachyte 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 by 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 s tudy 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 by 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 by Treuil and Joron (1977); the strong correlation shown by these rocks in diagrams of Figs. 10 and 11, therefore, indicates different degrees of partial melting in a narrow range of a common 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 composi t ion 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 too scarce to have relevance in understanding the genesis of these rocks on geochemical basis only.

Qo

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66

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126

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A

D

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Cr

Co

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La

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13

17

14

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1

4

19

9

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1

84

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36

41

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74

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51

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32

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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

1 6 1

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* b ( p p r . ) 1 I I

0 20 40 60

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,02

i

,01J

U La

u (pp r . ) .... I 1

0,5 10

Fig. 11. U vs. U/La, Rb vs Rb/La diagrams of 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 Mediter- ranean 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

(l

163

/ /

4

75 ~ /

/ A n - .9

Ab -oo ~ -~ -

A b ' -~ O r

Fig. 12. Projection of the isobaric cotectic lines for 2, 4, 5, 7 and 10 kbar onto the Qz-Ab- Or 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 at tention 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 t rachyte 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 composit ion 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, or thopyroxene fragments). The composition of glass inclusions in the oligoclase of corroded cores of some alkali- feldspar 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 or thopyroxenes, 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 pic- ture 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 "andesit ized" by 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 alkali- basalts 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)

165

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 Ter- tiary 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 work was f inancial ly suppor ted by the Italian Nat ional Research Council (CNR). The manuscr ip t was critically read by F. Barberi, F. Innocen t i and M. Sheridan whose help and suggestions are highly appreciated.

The authors are s t rongly indebted to G. Ot tone l lo which f r iendly provided INAA, pe r fo rmed at the Pierre Sue Labora to ry , Saclay. Thanks are due to M. Treuil, J .L. J o r o n and the GST of the L a b o r a t o r y for their hospi ta l i ty .

REFERENCES

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

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