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Rare earth element and other trace element distribution, and the origin of the iblean magmas
Journal o f Volcanology and Geothermal Research, 1 (1976) 331--346 ©Eisevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
331
RARE EARTH ELEMENT AND OTHER TRACE ELEMENT DISTRIBUTION, AND THE ORIGIN OF THE IBLEAN MAGMAS
M. BATTAGLIA', R. CRISTOFOLINI2 , P. DI GIROLAMO 2 and D. STANZIONE 2 ' Istituto di Mineralogia e Petrografia, University o f Catania, Catania (Italy) 2 Istituto di Mineralogia, University o f Naples, Naples (Italy)
(Received March 18, 1976; revised and accepted July 30, 1976)
ABSTRACT
Battaglia, M., Cristofolini, R., Di Girolamo, P. and Stanzione, D., 1976. Rare earth element and other trace element distribution, and the origin of the Iblean magmas. J. Volcanol. Geotherm. Res., 1: 331--346. In the Iblean region, southeast Sicily, a sequence of subaqueous and subaerial volcanics is interlayered in sedimentary levels, Upper Miocene to Lower Pleistocene in age. These rocks range from low-K tholeiites to basanites. Rare earth elements (REE) have been determined by instrumental neutron activation analysis in five samples, and other trace elements (Li, Rb, Sr, Co, Cr, Cu, Ni) by atomic absorption spectrophotometry in thirteen samples, already analyzed also for major elements. The tholeiites differ systematically from rocks of the alkalic suite for elements like Li, Sr and light REE (Sr < 200 ppm, Ce ~ 15 ppm in the former; Sr > 500 up to 2000 ppm, Ce > 150 ppm in the latter), while Ni, Co, Cr and heavy REE ranges overlap in the two rock suites. The results agree in indicating different degrees of partial melting, probably at different levels in a heterogeneous mantle, as responsible for the origin of most of the rocks found in the Iblean region: the tholeiites should have been formed at relatively shallow depth by fusion of large proportions of a depleted mantle, while increasingly undersaturated volcanics of the alkalic suite have been probably generated at greater depth by partial melting of decreasing amounts of mantle material.
INTRODUCTION In r e c e n t y e a r s r e n e w e d a t t e n t i o n h a s b e e n d e v o t e d t o t h e I b l e a n v o l c a n i s m , t h a t has d e v e l o p e d a l o n g t h e n o r t h e r n e d g e o f t h e I b l e a n P l a t e a u , s o u t h e a s t Sicily. H e r e d i f f e r e n t v o l c a n i c e p i s o d e s , e i t h e r s u b m a r i n e o r s u b a e r i a l , t o o k place from the Upper Miocene to Lower Pleistocene, and both tholeiitic and a l k a l i c b a s a l t s h a v e b e e n r e p o r t e d f r o m t h i s z o n e (see C r i s t o f o l i n i a n d B a t t a g l i a , 1 9 7 5 ; H o n n o r e z , 1 9 6 7 ; R o m a n o a n d Villari, 1 9 7 3 ) . T h e t h o r o u g h u n d e r s t a n d i n g o f t h e v o l c a n i s m in t h i s r e g i o n c o u l d be o b t a i n e d o n l y in a c l e a r g e o d y n a m i c f r a m e w o r k o f S i c i l y a n d t h e s u r r o u n d i n g area: u p t o its p r e s e n t t h i s is v e r y c o m p l e x a n d its i n t e r p r e t a t i o n s still a p p e a r q u i t e contradictory.
332
On the whole, the regional structures recognized in Sicily are to be interpreted as a consequence of the collision between the African and the European plates, or among minor sub-plates, derived from the fragmentation of the southern margin of the European plate (Barberi et al., 1973, 1974a; Boccaletti and Guazzone, 1973; Ogniben, 1973; Scandone et al., 1974; Selli, 1973). Some of the structural features, such as the hypothetical transcurrent lines suggested either by Barberi et al. (1973, 1974a) or by Scandone et al. (1974) have not yet gained sufficient support from data from field geology (Atzori et al., 1974; Lentini and Vezzani, 1975); furthermore, the posi.tion and orientation of the supposed Benioff plane dipping toward the Tyrrhenian sea from the CalabroPeloritani Arc is still a matter of debate among different authors, and the distribution of seismic hypocenters does not seem to be unequivocally consistent with the presence of such a feature (Ogniben, 1973; Schick, 1972). Selli (1973) suggests that the continental crust is driven by deep convection currents from the Tyrrhenian Sea toward the southern Apennines and Sicily which should act as relatively immobile crustal fragments. In the light of the still contradictory geologic interpretations, the Iblean volcanism, clearly linked with local tensional phenomena, should be related to
~al .~nts
~Rocks Limest0ne~
~L 1 i~eso tnes Iblean Regi~m 25 km
Fig. 1. Geological sketch map of eastern Sicily,
PLEISTOCENE
L. P L E I STOGENE U. MIOCENE
M. MIOCENE
333
differential horizontal displacements or with torsional movements in the Sicilian region. These occurred over a period of more than 5 m.y., and gave origin to several distinct phases of volcanic activity (Di Grande, 1967, 1969a, b, 1972; Cristofolini et al., 1973; Cristofolini and Battaglia, 1975; R o m a n o and Villari, 1973) in a very narrow belt where fault lines, following different systems, show the importance of the tensional movements (Fig. 1; Di Grande, 1972). It could be concluded that the volcanic effusions at the northern edge of the Iblean Plateau are linked with the uprise of this region, that started during the Upper Miocene (Di Grande, 1972) at the end of the most significant episodes of plate collision between Africa and Europe. This is consistent with recent paleomagnetic data (Barberi et al., 1974b) and with the age of the main phases of the orogenic movements in Calabria and northern Sicily, that are recorded in the structures of the Calabro-Peloritani chain (Vezzani, 1973). PETROLOGY
The most important aspects of the Iblean volcanic petrology are summarized here, with particular regard to the lavas from the Francofonte area, where subaerial lavas, mostly Pleistocene in age, are very widespread; here tholeiitic basalts, quite similar to the oceanic abyssal basalts, and alkali basaltic varieties have been found. Tholeiites (low-K subalkaline) seem to be very c o m m o n and are interlayered with rocks of the alkalic suite and more abundant toward the top of the sequence. Among the latter, basanites have been recognized through petrographic and chemical analysis (Cristofolini and Battaglia, 1975; R o m a n o and Villari, 1 9 7 3 ) . The tholeiites from the Francofonte area are strictly like those found in the Iblean district, and show very few phenocrysts -- bronzite among them -- in a microcrystalline groundmass with an intersertal texture and pigeonite as the mafic phase. Their silica c o n t e n t varies in a narrow range and averages a b o u t 53%, and alkalies, chiefly K20, are very scarce (Table 1, columns 1 to 8, Fig. 2}. On the whole the Iblean tholeiites are dominantly oversaturated, b u t in some cases there are instances of olivine-normative lavas. The general chemistry o f these rocks is consistent with their origin at relatively shallow depth (P ~ 10 kbar, according to the presence of bronzite phenocrysts; Cristofolini and Battaglia, 1975). Moreover, the close similarity inside the group of the Iblean tholeiites indicates an origin from a c o m m o n starting material in similar environmental conditions, and a low degree of fractional crystallization of their parental melt. A wider range of compositions is shown b y the alkalic varieties of the Iblean region: lavas of this t y p e are dark and fine-grained, with olivine and augite as microphenocysts in a microcrystalline intergranular to.hypocrystalline groundmass. Their norms constantly show normative nephelite, which sometimes can also be identified in the modal association. With the exception of a few rocks differentiated toward the hawaiites, most of them are alkali-basalts to basanites with nephelinite affinities (Cristofolini and Battaglia, 1975; R o m a n o and Villari, 1973).
32.8 0.62 -1.51
99.43
100.33
38.4 0.56 -0.27
52.29 14.06 3.81 7.34 0.13 6.80 8.00 2.97 0.19 2.19 0.31 0.92 0.42
47.44 14.43 5.43 3.47 0.08 7.10 9.07 2.89 0.12 1.78 0.46 3.82 4.24
FR14
32.4 0.63 -2.00
99.70
52.29 14.73 4.93 7.01 0.15 6.85 8.65 2.68 0.19 1.73 0.20 0.25 0.04
FR45
32.3 0,62 -2.31
99.78
52.63 14.63 4.32 6.50 0.14 6.55 9.34 2.77 0.15 1.92 0.22 0.15 0.45
Fr56
31.9 0.64 -1.75
100.46
52.47 14.65 5.46 6.41 0.14 6.80 8.42 2.98 0.23 1.68 0.25 0.70 0.27
FR38
S.I. = solidification i n d e x (MgO/MgO + F e O t + NaO + K O ) I.E. = iron e n r i c h m e n t ( F e O t / M g O + F e O t ) M.L.I. = Modified Larsen I n d e x [1/s Si + K - (Ca + Mg)]
S.I. I.E. M.L.I.
SiO 2 AI,O 3 Fe,O 3 FeO Mn0 MgO CaO Na20 K20 TiO~ P205 H,O H20
MI91
Major e l e m e n t d a t a (wt. %)
TABLE 1
30.7 0.64 -1,87
99.83
49.03 13.76 5.58 4.81 0.10 5.73 8.77 2.83 0.27 1.95 0.31 3.41 3.28
LE3
28.7 0.67 -1.25
99.51
52.48 14.35 5.89 5.94 0.13 5.80 8.47 3.04 0.15 1.84 0.18 1.12 0.12
FR2
99.83
100,47 45.6 0.49 -8.60
100.33 26.8 0.69 -1.16
-8.54
44.6 0.50
42.87 11,62 5.85 5.63 0.18 11.40 12.40 2.64 0,63 2.01 1.48 2.30 0.82
44,40 11.98 5.76 5.02 0.16 11,40 12.T0 2.87 0.53 1.86 1.37 1.64 0.78
52.47 14.68 6.91 5.41 0.14 5.45 8.70 3.04 0.20 1.85 0.23 0.89 0.36
FR 1
F R 25
FR13
-11.76
44.1 0.49
99.01
39.30 11.44 6.98 4.24 0.20 11.75 15.95 2.95 0.73 2.02 2.15 0.20 1.]0
FR 5
-5.14
34.5 0.60
100.50
46.47 13.31 6.46 4.99 0.18 7.76 11.70 3.10 0.80 2.25 1.11 1.65 0.72
PE 4
-2.15
28,6 0.66
99.57
45.35 14.71 8.68 3.91 0.12 6.40 9.20 2.78 1.46 3.05 0.88 1.88 1.15
FR 49
c~
c¢
335 T h e clear d i s t i n c t i o n o f t h e tholeiitic and alkali-basaltic suites, s h o w n on p e t r o g r a p h i c and c h e m i c a l g r o u n d s , w i t h o u t a n y d a t a suggesting t h a t o n e c o u l d have been derived f r o m t h e o t h e r , is c o n f i r m e d b y Carter a n d Civetta ( 1 9 7 5 ) : Sr i s o t o p e ratios are significantly l o w e r for t h e tholeiites t h a n f o r t h e alkalic r o c k s o f t h e Iblean region, t h u s s h o w i n g an origin f r o m t w o i n d e p e n d e n t sources, probably under different P--T conditions. T h e p r e s e n t s t u d y is aimed at a b e t t e r u n d e r s t a n d i n g o f t h e origin o f t h e various Iblean r o c k t y p e s and o f the relationships a m o n g t h e m . TRACE ELEMENT COMPOSITION T h i r t e e n r o c k s a n a l y z e d also f o r m a j o r e l e m e n t s (Table 1) have b e e n a n a l y z e d for t h e f o l l o w i n g t r a c e elements: Li, Rb, Sr, Ni, Cr, Co, Cu b y a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y ( A A S ) ~ . Rare earth e l e m e n t s ( R E E ) have b e e n d e t e r m i n e d o n five samples (2, 14, 56, 5, a n d 25) b y i n s t r u m e n t a l n e u t r o n a c t i v a t i o n analysis ( I N A A ) , a c c o r d i n g t o t h e m e t h o d suggested b y Albini et al. ( 1 9 7 6 ) 2. T h e results are s h o w n in Table 2. °1* NQ20 +
KzO 10
&,H D
40 i5 5'0 "1, Si02 Fig. 2. The alkalic rock suite is well discriminated from the tholeiites which are plotted as a well-defined group in the silica-alkalies diagram, o = analyses from this paper; o = analyses reported by Honnorez (1967); • = Romano and Villari's (1973) analyses; ~ = averages for Hawaiian lavas (MacDonald, 1968) -- Th, tholeiites; Oc, oceanites; AB, alkali basalts; H, hawaiites; Bs, basanites; Ne, nephelinites; • = averages for oceanic lavas (Manson, 1967) -- Th', tholeiites; AB', alkali basalts; × = averages for abyssalt basalts -- 1, Cann (1971); 2, Engel et al. (1965).
1Analyses performed at the Institutes of Mineralogy in Naples and Catania, with a Jarrel Ash and an EEL Mark 2 spectrophotometer respectively; Ni has been determined according to the suggestions of Beccaluva and Venturelli (1971 ). 2 Irradiation of the samples has been obtained at the accelerator of the Politecnico in Milan, and radioactivity determinations at CISE, Milan.
336
TABLE2
Trace element data (ppm) MI 91
F R 14
F R 45
F R 56
F R 38
LE 3
FR 2
Li Rb Sr Cu Ni Co Cr La Ce Sm En Tb Dy Yb Lu
5.50 2.70 506 83.5 152 34 190
3,74 2.70 149 121 185 25 175 8.0+_1.0 21.6_+1 4.2_+0.4 2.38-+0.16 0.76-+0.11 5 . 3 0 -+ 0.29 1.30-+0.72 0.50+_0.01
4.29 2.75 149 110 162 35 200
4.18 2.20 182 92 156 35 310 6.6+_0.9 11.9_+0.9 3.6_+0.2 1.61_+0.14 0.53-+0.08 7.10-+ 0 . 2 8 8.00-+0.56 0.37_+0.0
4.18 2.75 145 89 177 30 360
4.73 2.70 200 92 145 38 245
3.41 2.70 136 10J 148 30 165 5.4_+0.7 15.6-+1.5 3.1-+0.2 1.24-+0. 0.63-+0. 4,15_+ 0. 11,70+-0. 0.09±0:
K/Rb Rb]Sr K/Sr Ce/Yb
369 0.005 1.97
584 0.018 10.58 1.66
573 0.018 10.58
566 0.012 6.84 1.49
694 0.019 13.17
830 0.014 ] 1.21
461 0.020 9~15 1.33
Large-ion-lithophile elements. A difference in some alkali and alkaline-earth element concentrations, fairly well correlated with K abundances, is shown between the tholeiites and the rocks from the alkalic suite: in agreement with worldwide average data (Prinz, 1967). As a rule the Sr content is very high in strongly undersaturated basanites (> 1000 ppm), while it is remarkably low in the tholeiites (< 200 ppm); slightly higher values have been reported by Romano and Villari (1973) for three tholeiites out of five, and also our sample MI 91, which is more altered and UpperMiocene in age, with features transitional toward the alkali basalts, exhibits a larger Sr content, The alkali basalts generally show concentrations intermediate between the tholeiites and the basanites (see also Romano and Villari, 1973). A similar behaviour is also shown by Rb, which is on average very scarce (near the detection limit) in all the analyzed lavas, being on the-whole scarcer in the tholeiites than in the alkalic rocks. Our results, particularly for the alkaline lavas, tend to be lower than those of Romano and Villari (1973), and fall in the lower half of the range reported by Prinz (1967). Rb/Sr values are very low, the lowest being found in the basanites;for the tholeiitesthese values conform to what has been reported by Phflpotts and Schnetzler (1970).
337
FR 12
FR 25
FR 1
FR 5
4.29 2.70 162 101 161 34 275
4.68 2.75 1091 111 283 35 410 79.6+-8 160.9-+3 12.6+-0.6 4.29+-0.21 1.84-+0.19 4.55-+0.3 4.90-+0.25 0.75-+0.02
4.95 5.28 1241 141 269 35 475
4.95 5.72 2000 119 246 47 460 141.1-+14.1 249.5-+15 20.0-+0.9 4.20-+0.19 2.27-+0.20 11.50-+0.63 4.40-+0.15 0.9-+0.01
615 0.017 10.25
1600 0.0025 4.03 32.84
991 0.0043 4.21
1059 0.0029 3.03 56.70
PE 4
FR 49
5.50 5.28 841 92.5 201 31 227
4.29 10.12 684 73 112 33 160
1258 0.0062 7.90
1198 0.0148 17.72
Li a b u n d a n c e s ( 3 . 4 1 - - 5 . 5 0 p p m ) d o n o t s h o w significant changes in the d i f f e r e n t suites.
Transitional elements. T h e Ni, Cr, and Co c o n t e n t s in t h e Iblean r o c k s agree with t h e values r e p o r t e d b y Prinz ( 1 9 6 7 ) . Their ranges in the tholeiites and in t h e alkaline r o c k s overlap and d o n o t allow a clear d i s c r i m i n a t i o n b e t w e e n t h e t w o groups; y e t it can be r e c o g n i z e d t h a t t h e Ni and Cr ranges for t h e tholeiites are less wide t h a n in t h e alkaline rocks. A c o m p a r i s o n with the d a t a f r o m R o m a n o and Villari ( 1 9 7 3 ) s h o w s t h a t this is a general f e a t u r e o f t h e Iblean volcanics (Fig. 3). T h e linear c o r r e l a t i o n b e t w e e n Ni and Cr is high in the alkalic lavas (r = 0.90), the l o w e s t c o n c e n t r a t i o n s being f o u n d in an U p p e r C r e t a c e o u s sample (CP 1, R o m a n o and Villari, 1 9 7 3 ) , while these e l e m e n t s are very p o o r l y c o r r e l a t e d in the tholeiites. Cu seems o n average slightly m o r e a b u n d a n t and shows a wider range o f variation in t h e alkaline r o c k s t h a n in t h e tholeiites. Rare earth elements. Results o b t a i n e d t h r o u g h I N A A clearly d i s c r i m i n a t e the tholeiites f r o m t h e alkalic rocks. T h e d i s t r i b u t i o n p a t t e r n s o f t h e R E E
338
Cr
00
p.p,m
0
® ®
o ~®
x
=o(~
"
,
pp.m. Ni
Fig. 3. C r a n d Ni are well correlated in the alkalic rocks, w h i l e in the t h o l e i i t e s Cr range o f variation is m u c h larger than that f o r Ni. Data f r o m the present p a p e r : , = t h o l e i i t e s ; x = a l k a l i c r o c k s . D a t a f r o m R o m a n o and Villari ( 1 9 7 3 ) : o = t h o l e i i t e s ; • = a l k a l i c r o c k s .
normalized to the chondrite composition conform broadly to what is known for tholeiites and alkali basalts: in the Iblean tholeiites, however, the light REE (La, Ce)~ are not as depleted as in common ocean ridge basalts, but are anyhow less concentrated than in ocean island tholeiites (Fig. 4; Gast, I968; Kempe and Schilling, 1974; Masuda et al., 1974; Schilling, 1971; Schilling and Winchester, 1969). Furthermore, starting from Eu the patterns are quite irregular: a positive Eu anomaly is apparent in two patterns out of three; Yb
Lav~,// /Chondr
100
II
10
La Ce
SmEu
Tb Dy
Yb LU
F i g . 4. Chondrite°normalized R E E patterns f o r f i v e I b l e a n r o c k s . For discussion see text.
339
is on the whole more abundant than in other tholeiites and the Lu content is scattered over a wide range (0.09--0.5 ppm). This distribution contrasts with preliminary data from Carter and Bell (1975) for the mildly tholeiitic rocks of Mt. Etna, which are far less depleted in light REE than the Iblean tholeiites, and compares fairly well with the data for the Othris tholeiites (Menzies, 1976). Comparing a Puerto Rico Trench composite sample (Jibiki and Masuda, 1974), taken as representative of abyssal oceanic basalts (low-K subalkaline), with the Iblean tholeiites, the light REE are almost linearly enriched in the latter. The rocks of the alkaline suite show a quite regular decrease in the chondritenormalized abundances, from light to heavy REE, and are on the whole slightly REE-enriched with respect to the average alkali basalt (Hubbard, 1969): this is to be correlated with the basanitic nature of the t w o analyzed samples (Ishikawa, 1974; Graham and Nicholls, 1969), and for this reason the patterns closely resemble some of the lavas from Ascension I (Gast, 1968) and from Cape Verde I (De Paepe et al., 1974, fig. 3). Our results for La and Ce are very close to those obtained by R o m a n o and Villari (1973) for similar rocks from the same area. Owing to the peculiar features of the REE profiles of the Iblean rocks, their Ce/Yb ratios are near b u t outside the ranges of ocean ridge and alkali basalts respectively ( L o u b e t et al., 1975). DISCUSSION
OF THE RESULTS
The major element chemistry and the trace element data show that the Iblean rocks belong to at least t w o different basaltic suites, the tholeiitic and the alkalic one, w i t h o u t any evidence suggesting a parent
340
examining the correlation of other couples of incompatible elements (La-Sm~ La-Tb). Therefore, in the first approximation and assuming the La (or Ce) content to be an indicator of the degree of partial melting, one could get some more information a b o u t the details of the evolution of different magma types by observing the distribution of elements such as Ni and Cr, which are strongly fractionated during the first stages of the crystallization (Fig. 5). Again no evidence appears from the available data that the overall distribution is controlled mainly by fractional crystallization. Only the significant depletion of Ni and Cr in some basanites and one hawaiite ( R o m a n o and Villari, 1973), correlated with a La enrichment could be accounted for by a mechanism of crystal fractionation. The tholeiites are grouped in a clearly defined cluster, showing very little correlation of La with either Ni or Cr. This behaviour and the distribution of the La and Ce values, could indicate for the tholeiites with respect to the alkalic lavas an origin in different levels of the mantle and/or under disequilibrium conditions (Treuil and Varet, 1973, p. 535}. These conclusions are confirmed by examining the distribution of Ni and Cr against Mg: in this case Mg is poorly correlated with both Ni and Cr in all the analyzed lavas, except the high-Mg (MgO ~ 9%) undersaturated rocks. Furthermore the tholeiites show, for similar Mg contents, slightly higher Ni and Cr abundances than the alkali basalts. The available data a b o u t the trace elements, except REE, have been plotted
CF
p.p.m.
300'
~°. lOG
NL 300'
°o
100
Fig. 5. La is plotted as an indicator of the degree o f partial melting against Ni and Cr. The overall point distribution does not c o n f o r m to a h y p o t h e s i s o f d i f f e r e n t i a t i o n by crystal fractionation (the arrow pointing toward a hawaiite indicates possible effects of crystal fractionation). Symbols as in Fig. 2.
341
against a Modified Larsen Index [M.L.I. = l/3 Si + K - (Ca + Mg)] (Fig. 6; Nockolds and Allen, 1953). The tholeiites are plotted as a homogeneous group at M.L.I. ~- - 2 , while the alkalic rocks are dispersed over a wider range of M.L.I. (0 to - 1 2 ) . The correlation of most trace elements with M.L.I. is very poor and again argues against any mechanism of origin involving mainly crystal fractionation. The REE patterns are compatible with the above-stated hypotheses: the alkalic rocks should have been derived by partial melting of very low amounts of mantle material (see Shimizu and Arculus, 1975), while the tholeiites with approximatively linear REE patterns could have originated by melting of large proportions of a depleted low-velocity layer (Schilling, 1971; Masuda et al., 1974), as suggested for the more basic Othris tholeiites (Menzies, 1976). In this case the Eu anomaly and the irregularities in the REE profiles could imply for the tholeiites an origin from mantle rocks with Eu-rich phases (plagio clase), at shallow depth (Masuda et al., 1974). On the other hand, very low Rb concentrations in the Iblean tholeiites are consistent with their origin from a depleted mantle: according to Peterman and Hedge (1971) this in turn agrees
Rb p,pm
Cr I~p.rr
°°
o ©
o
o
2O
• •
o
o
to. %
o O
o o
o
O
i
o OgC~ ~
Cu ppm
(~
Ni Pp~
0
O
..
1oo
o
%
©.
o
ao ki gpm
0
©
Sr
pp m 1
c~:
~
2o
o
o ~
. .. 0
1o
o
0
o M.L.I.
1o
o
M.L.I.
Fig. 6. A Modified Larsen Index (M.L.I.) is plotted against all the determined trace elements (REE excluded). With the exception of St, the trace element distribution is rather poorly correlated with M.L.I., what in turn does not conform with hypotheses of origin by crystal fractionation. Symbols as in Fig. 2.
342
with the very low Sr isotope ratios found for these rocks by Carter and Civetta (1975). This situation differs from the one f o u n d at Mt. Etna (Carter and Belt, 1975) where preliminary data indicate that the subalkaline rocks are much less light-REE-depleted, richer in Sr and with slightly higher Sr isotope ratios. Further information a b o u t the origin of the Iblean volcanics is offered by an examination of the covariance of K, Rb, and St. K / R b ranges from ~ 4 0 0 - - 8 0 0 for the tholeiites up to more than 1000 for the basanites, and is positively correlated with K {Fig. 7). While the former group of rocks falls well in the field of the oceanic ridge basalts (Gast, 1968; Gale, 1975), the alkalic lavas do n o t conform to the general observation (Gast, 1965; Prinz, 1967) that in basalts K / R b is negatively correlated with K. This could be accounted for by the only minor increase of the Rb content against a substantial change of the K (and Sr) proportions from the tholeiites to the alkalic suite rocks, and could be related with an origin of the latter from a mantle with low-Rb, K-bearing phases (Griffin and Murthy, 1969; Hughes and Brown, 1970), but could also be an effect of the change in the solid-liquid partition coefficient for Rb with pressure (Shimizu, 1974). The disagreement with Rb data from R o m a n o and Villari (1973) does not allow a more detailed discussion a b o u t the origin and evolution of the alkalic suite lavas. On the contrary, the good linear correlation of K / R b with K (r = 0.96) in our Iblean tholeiites could either, be the result of partial melting in a homogeneous mantle, if K is to be considered a dispersed element (Gast, 1968) and assuming it has an incompatible behaviour (Treuil and Varet, 1973), o r imply their origin through partial melting of large amounts of a hornblende peridotite
K/Rb
;OOO
1/
500
200
o~' dg i K% Fig. 7. A fairly good correlation is shown between K and K/Rb for the tholeiitea analyzed in this paper. For the alkalic rocks (K > 0.5:) the correlation is much poorer, owing probably to a partially compatible behaviour of K. Symbols as in Fig. 2.
343 with slightly variable phlogopite contents (Griffin and Murthy, 1969), which is in qualitative agreement also with the Rb/Sr and K/Sr distribution against K. Sr is fairly well correlated with Ca in the alkaline lavas and its distribution allows the discrimination of these rocks from the tholeiites, which show a lower Sr c o n t e n t at similar Ca concentrations. This again points toward an independent origin of the two main rock types. CONCLUSIONS Although the present information confirms that the Iblean tholeiites are quite similar to the incompatible-element-depleted ocean-ridge basalts, there is no other indication t h a t the Iblean region ever behaved as an ocean ridge: this is shown by the association of decidedly alkalic varieties with the tholeiites through the whole sequence, the features of the interlayered sediments, and the overall low volume of volcanics. This could then be an instance where the chemical and petrographic features of the volcanic rocks are n o t unequivocally meaningful about the geodynamic evolution of a region, even if t h e y are certainly controlled by the whole set of geodynamic processes (most of which are not known in detail) occurring there. The sequence of volcanic p h e n o m e n a from Upper Miocene to Lower Pleistocene is then to be related to tensional movements that developed at the southern edge of a belt with strongly thickened crust (Ogniben et al., 1973; Selli, 1973) as a late consequence of the collision between the African and the European plates. In this framework some deep fractures should have reached the mantle at different levels in various times, where eruptible magma was available, thus allowing the melts to ascend to the surface. The present data do n o t show any genetic link of the alkalic-suite rocks with the tholeiites: they should have been generated independently in environments differing in one or more factors, like pressure, temperature, mantle composition, degree of partial melting, and rate of formation and extraction of the melt from the mantle. On the basis of the distribution of the incompatible elements and in the first approximation, a different degree of partial melting, possibly at different levels in a heterogeneous mantle, could be suggested as responsible for the origin of the different Iblean magmas also inside the group of the alkalic rocks. Contrary to the assumptions by R o m a n o and Villari (1973) crystal fractionation then appears only as a minor factor of differentiation for few lavas of the alkalic suite.
344
REFERENCES Albini, A., De Gillis, D. and Stanzione, D., 1976. Dati preliminari sull'analisi mediante attivazione neutronica delle terre rare in campioni di rocce dell'USGS. Rend. Acad. Sci~ Fis. Mat., Soc. Naz. Sci. Lett. Arti Napoli (in press). Atzori, P., D'Amico, C. and Pezzino, A., 1974. Relazione geo-petrografica preliminare sul cristallino della catena peloritana (Sicilia), Riv. Min. Sicil., 148-150: 1--8. Barberi, F., Gasparini, P., Innocenti, F. and Villari, L., 1973. Volcanism of the southern Tyrrhenian Sea and its geodynamic implications. J. Geophys. Res., 78: 5221--5232. Barberi, F., Innoce[~ti, F., Ferrara, C., Keller, J. and Villari, L., 1974a. Evolution of eolian arc volcanism (southern Tyrrhenian Sea). Earth Planet. Sci. Lett., 21 : 269--276. Barberi, F., Civetta, L., Gasparini, P., Innocenti, F., Scandone, R. and Villari, L., 1974b. Evolution of a section of the Africa-Europe plate boundary: paleomagnetic and volcanological evidence from Sicily. Earth Planet. Sci. Lett., 22: 123--132. Beccaluva, L. and Venturelli, G., 1971. Nickel, chromium and strontium determinations in some silicate rock standards by atomic absorption spectrophotometry. At. Absorpt~ Newsl., 10: 638--641. Boccaletti, M. and Guazzone, G., 1973. I1 microcontinente sardo-comeo residuo di un sistema arco-fossa miocenico. Rend. Sem. Fac. Sci. Cagliari, Suppl., 43: 56--68. Cann, J.R., 1971. Major element variations in ocean floor basalts. Phil. Trans. R. Soc. London, Ser. A, 268: 495--505. Carter, S.R. and Bell, J.D., 1975. Geological studies of the Bronte and Randazzo areas. In: U.K. Research on Mt. Etna. Royal Society, London, pp. 18--21. Carter, S.R. and Civetta, L., 1975. Sr isotopic composition of volcanics from two different tectonic settings: Afar and eastern Sicily. Presented at Syrup. on Radiogenic Isotopes, IUGG, Grenoble. Cristofolini, R. and Battaglia, M., 1975. Le manifestazioni basaltiche della zona di Francofonte nel quadro del vulcanismo dell'Altopiano Ibleo. Boll. Soc. Geol. Ital., 94: 185--207. Cristofolini, R., Di Girolamo, P. and Stanzione, D., 1973. Caratteri genetici e mineralogici di ialoclastiti dell'Altopiano Ibleo (Sicilia} e dell'Isola di Procida (Campania). Rend. Soc. Ital. Mineral. Petrol., 29: 497--552. De Paepe, P., Klerkx, J., Hertogen, J. and Plinke, P., 1974. Oceanic tholeiites on the Cape Verde Islands: petrochemical and geochemical evidence. Earth Planet. Sci. Lett., 22: 347--354. Di Grande, A., 1967. I sedimenti pleistocenici del margine settentrionale delt'Attopiano Ibleo. Atti Accad. Gioenia Sci. Nat., Catania, Set. 6, 18 (Suppl. Sci. Geol.): 247--263. Di Grande, A., 1969a. Sezione stratigrafica nel Pliocene di Licodia Eubea (Catania). Atti Accad. Gioenia Sci. Nat. Catnnia, Set. 7, 1 (Suppl. Sci. Geol.): 61--90. Di Grande, A., 1969b. L'alternanza neogenico-quaternaria di vulcaniti e di sedimenti al margine nord-occidentale dell'Altipiano Ibleo, Atti Accad. Gioenia Sci. Nat. Catania, Ser. 7, 1 (Suppl. Sci. Geol.): 91--125. Di Grande, A., 1972. Geologia dell'area a Nord di Augusta--Francofonte (Sicilia SE). Atti Accad. Gioenia Sci. Nat. Catania, Set. 7, 4: 1--32. Engel, A.E., Engel, G.C. and Havens, R.G., 1965. Chemical characteristics of oceanic basalts and upper mantle. Geol. Soc. Am. Bull., 76: 719--734. Gale, N.H., 1975. The contribution of trace elements and isotope geochemistry to the petrogenesis of oceanic basalts and the composition of the upper mantle. Rend. Soc. Ital. Mineral. Petrol., 31: 99--123. Gast, P.W., 1965. Terrestrial ratio of potassium to rubidium and the composition of the earth's mantle. Science, 147: 858--860. Gast, P.W., 1968. Trace element fractionation and the origin of the tholeiitic and alkaline magma types. Geochim. Cosmochim. Acta, 32: 1057--1086. Graham, A.L. and Nicholls, G.D., 1969. Mass spectrographic determination of lanthanide element contents in basalts. Geochim. Cosmochim. Acta, 33: 555--568.
345
Green, D.H., 1973. Experimental melting studies on a model upper mantle composition at high pressure under water-saturated and water-undersaturated conditions. Earth Planet. Sci. Lett., 19: 37--46. Griffin, W.L. and Murthy, V.R., 1969. Distribution of K, Rb, Sr, and Ba in some minerals relevant to basalt genesis. Geochim. Cosmochim. Acta, 33: 1398--1414. Honnorez, J., 1967. La palagonitization, un aspect du volcanisme sous marin: l'alteration du verre basique de Palagonia (Sicile). Unpublished Thesis, Univ. Libre Bruxelles. Hubbard, N.J., 1969. A chemical comparison of oceanic ridge. Hawaiian tholeiitic and Hawaiian alkalic basalts. Earth Planet. Sci. Lett., 5 : 346--352. Hughes, D.J. and Brown, G.C., 1972. Basalts from Madeira: a petrochemical contribution to the genesis of oceanic alkali rock series. Contrib. Mineral. Petrol., 37: 91--109. Kempe, D.R.C. and Schilling, J.C., 1974. Discovery tablemountain basalt: petrology and geochemistry. Contrib. Mineral. Petrol., 4 4 : 1 0 1 - - 1 1 6 . Ishikawa, H., 1974. Distribution of rare earths in volcanic rocks. In: L. Civetta et al. (Editors), Physical Volcanology. Elsevier, Amsterdam, pp. 241--253. Jibiki, H. and Masuda, A., 1974. Basalts and serpentinite from the Puerto Rico Trench, 2. Rare earth geochemistry. Mar. Geol., 16: 205--211. Lentini, F. and Vezzani, L., 1975. Le unit~ meso-cenozoiche della copertura sedimentaria del basamento cristallino peloritano (Sicilia nord-orientale). Boll. Soc. Geol. Ital. (in press). Loubet, M., Shimizu, N. and All~gre, C.J., 1975. Rare earth elements in alpine peridotites. Contrib. Mineral. Petrol., 53: 1--12. MacDonald, G.A., 1968. Composition and origin of Hawaiian lavas. Geol. Soc. Am. Mem., 116: 477--522. Manson, V., 1967. Geochemistry of basaltic rocks: major elements. In: H.H. Hess and A. Poldervaart (Editors), Basalts. Wiley Interscience, New York, N.Y., pp. 215--269. Masuda, A., Shimokawa, T., Jibiki, H. and Nagawa, T., 1974. Recognition of conjugate solid-type and liquid-type relationship among the oceanic basalts with respect to REE. Contrib. Mineral. Petrol., 48: 265--274. Menzies, M., 1976. Rifting of a Tethyan continent. Rare earth evidence of an accreting plate margin. Earth Planet. Sci. Lett., 28: 427--438. Nockolds, S.R. and Allen, R., 1953. The geochemistry of some igneous rock series. Geochim. Cosmochim. Acta, 4: 105--142. Ogniben, L., 1973. Conclusioni sulla stato attuale della conoscenze nella geologia dell'Appennino. Accad. Naz. Lincei, Quad., 183:369--445. Ogniben, L., Martinis, B., Rossi, P.M., Fuganti, A., Pasquar~, G., Sturani, C., Nardi, R., Cocozza, T., Praturlon, A., Parotto, M., D'Argenio, B., Pescatore, T., Scandone, P., Vezzani, L., A.G.I.P., Finetti, I., Morelli, C., Caputo, M., Postpischl, D. and Giese, P., 1974. Modello strutturale d'Italia, Map. Scale 1:1,000,000. C.N.R., Rome. Peterman, Z.E. and Hedge, C.E., 1971. Related strontium isotopic and chemical variations in oceanic basalts. Geol. Soc. Am. Bull., 82: 493--499. Philpotts, J.A. and Schnetzler, C.C., 1970. Phenocrysts-matrix partition coefficients for K, Rb, Sr and Ba. Geochim. Cosmochim. Acta, 34: 307--322. Prinz, M., 1967. Geochemistry of basaltic rocks: trace elements. In: H.H. Hess and A. Poldervaart (Editors), Basalts. Wiley-Interscience, New York, N.Y., pp. 271--323. Romano, R. and Villari, L., 1973. Caratteri petrologici e magmatologici del vulcanismo Ibleo. Rend. Soc. Ital. Mineral. Petrol., 29: 453--483. Scandone, P., Giunta,~G.and Liguori, V., 1974. The connection between the Apulia and the Sahara continental margins in the southern Apennines and in Sicily, Mem. Soc. Geol. Ital (preprint). Schick, R., 1972. Die Erdbeben als Ausdruck spontaner Tektonik. Geol. Rundsch., 61: 896--914. Selli, R., 1973. Appunti sulla geologia del Mar Tirreno. Rend. Sem. Fac. Sci. Univ. Cagliari, Suppl., 43: 327--351.
346
Schilling, J.G., 1971. Sea-floor evolution: rare earth evidence. Philos. Trans. R. Soc. Lo~dor}. Ser. A, 268: 663--706. Schilling, J.G. and Winchester, J.W., 1969. Rare earth contribution to the origin of Hawaiian lavas. Contrib. Mineral. Petrol, 23: 27--37. Shimizu, N., 1974. An experimental study of the partitioning of K, Rb, Cs, Sr and Ba between clinopyroxene and liquid at high pressure. Geochim. Cosmochim. Acta, 38: 1789--1798. Shimizu, N. and Arculus, R.J., 1975. Rare earth concentrations in a suite of basinitoids and alkali olivine basalts from Grenada, Lesser Antilles. Contrib. Mineral. Petrol., 50: 231--240. Treuil, M. and Varet, J., 1973. Crit~res volcanologiques, p6trologiques et g6ochimiques de la gen~se et de la differenciation des magmas basaltiques, Example de l'Afar. Bull. Soc. G6ol. Ft., Ser. 7, 15: 506--540. Vezzani, L., 1973. L'Appennino siculo-calabro-lucano. Accad. Naz. Lincei, Quad., 183: 15--37.
331
RARE EARTH ELEMENT AND OTHER TRACE ELEMENT DISTRIBUTION, AND THE ORIGIN OF THE IBLEAN MAGMAS
M. BATTAGLIA', R. CRISTOFOLINI2 , P. DI GIROLAMO 2 and D. STANZIONE 2 ' Istituto di Mineralogia e Petrografia, University o f Catania, Catania (Italy) 2 Istituto di Mineralogia, University o f Naples, Naples (Italy)
(Received March 18, 1976; revised and accepted July 30, 1976)
ABSTRACT
Battaglia, M., Cristofolini, R., Di Girolamo, P. and Stanzione, D., 1976. Rare earth element and other trace element distribution, and the origin of the Iblean magmas. J. Volcanol. Geotherm. Res., 1: 331--346. In the Iblean region, southeast Sicily, a sequence of subaqueous and subaerial volcanics is interlayered in sedimentary levels, Upper Miocene to Lower Pleistocene in age. These rocks range from low-K tholeiites to basanites. Rare earth elements (REE) have been determined by instrumental neutron activation analysis in five samples, and other trace elements (Li, Rb, Sr, Co, Cr, Cu, Ni) by atomic absorption spectrophotometry in thirteen samples, already analyzed also for major elements. The tholeiites differ systematically from rocks of the alkalic suite for elements like Li, Sr and light REE (Sr < 200 ppm, Ce ~ 15 ppm in the former; Sr > 500 up to 2000 ppm, Ce > 150 ppm in the latter), while Ni, Co, Cr and heavy REE ranges overlap in the two rock suites. The results agree in indicating different degrees of partial melting, probably at different levels in a heterogeneous mantle, as responsible for the origin of most of the rocks found in the Iblean region: the tholeiites should have been formed at relatively shallow depth by fusion of large proportions of a depleted mantle, while increasingly undersaturated volcanics of the alkalic suite have been probably generated at greater depth by partial melting of decreasing amounts of mantle material.
INTRODUCTION In r e c e n t y e a r s r e n e w e d a t t e n t i o n h a s b e e n d e v o t e d t o t h e I b l e a n v o l c a n i s m , t h a t has d e v e l o p e d a l o n g t h e n o r t h e r n e d g e o f t h e I b l e a n P l a t e a u , s o u t h e a s t Sicily. H e r e d i f f e r e n t v o l c a n i c e p i s o d e s , e i t h e r s u b m a r i n e o r s u b a e r i a l , t o o k place from the Upper Miocene to Lower Pleistocene, and both tholeiitic and a l k a l i c b a s a l t s h a v e b e e n r e p o r t e d f r o m t h i s z o n e (see C r i s t o f o l i n i a n d B a t t a g l i a , 1 9 7 5 ; H o n n o r e z , 1 9 6 7 ; R o m a n o a n d Villari, 1 9 7 3 ) . T h e t h o r o u g h u n d e r s t a n d i n g o f t h e v o l c a n i s m in t h i s r e g i o n c o u l d be o b t a i n e d o n l y in a c l e a r g e o d y n a m i c f r a m e w o r k o f S i c i l y a n d t h e s u r r o u n d i n g area: u p t o its p r e s e n t t h i s is v e r y c o m p l e x a n d its i n t e r p r e t a t i o n s still a p p e a r q u i t e contradictory.
332
On the whole, the regional structures recognized in Sicily are to be interpreted as a consequence of the collision between the African and the European plates, or among minor sub-plates, derived from the fragmentation of the southern margin of the European plate (Barberi et al., 1973, 1974a; Boccaletti and Guazzone, 1973; Ogniben, 1973; Scandone et al., 1974; Selli, 1973). Some of the structural features, such as the hypothetical transcurrent lines suggested either by Barberi et al. (1973, 1974a) or by Scandone et al. (1974) have not yet gained sufficient support from data from field geology (Atzori et al., 1974; Lentini and Vezzani, 1975); furthermore, the posi.tion and orientation of the supposed Benioff plane dipping toward the Tyrrhenian sea from the CalabroPeloritani Arc is still a matter of debate among different authors, and the distribution of seismic hypocenters does not seem to be unequivocally consistent with the presence of such a feature (Ogniben, 1973; Schick, 1972). Selli (1973) suggests that the continental crust is driven by deep convection currents from the Tyrrhenian Sea toward the southern Apennines and Sicily which should act as relatively immobile crustal fragments. In the light of the still contradictory geologic interpretations, the Iblean volcanism, clearly linked with local tensional phenomena, should be related to
~al .~nts
~Rocks Limest0ne~
~L 1 i~eso tnes Iblean Regi~m 25 km
Fig. 1. Geological sketch map of eastern Sicily,
PLEISTOCENE
L. P L E I STOGENE U. MIOCENE
M. MIOCENE
333
differential horizontal displacements or with torsional movements in the Sicilian region. These occurred over a period of more than 5 m.y., and gave origin to several distinct phases of volcanic activity (Di Grande, 1967, 1969a, b, 1972; Cristofolini et al., 1973; Cristofolini and Battaglia, 1975; R o m a n o and Villari, 1973) in a very narrow belt where fault lines, following different systems, show the importance of the tensional movements (Fig. 1; Di Grande, 1972). It could be concluded that the volcanic effusions at the northern edge of the Iblean Plateau are linked with the uprise of this region, that started during the Upper Miocene (Di Grande, 1972) at the end of the most significant episodes of plate collision between Africa and Europe. This is consistent with recent paleomagnetic data (Barberi et al., 1974b) and with the age of the main phases of the orogenic movements in Calabria and northern Sicily, that are recorded in the structures of the Calabro-Peloritani chain (Vezzani, 1973). PETROLOGY
The most important aspects of the Iblean volcanic petrology are summarized here, with particular regard to the lavas from the Francofonte area, where subaerial lavas, mostly Pleistocene in age, are very widespread; here tholeiitic basalts, quite similar to the oceanic abyssal basalts, and alkali basaltic varieties have been found. Tholeiites (low-K subalkaline) seem to be very c o m m o n and are interlayered with rocks of the alkalic suite and more abundant toward the top of the sequence. Among the latter, basanites have been recognized through petrographic and chemical analysis (Cristofolini and Battaglia, 1975; R o m a n o and Villari, 1 9 7 3 ) . The tholeiites from the Francofonte area are strictly like those found in the Iblean district, and show very few phenocrysts -- bronzite among them -- in a microcrystalline groundmass with an intersertal texture and pigeonite as the mafic phase. Their silica c o n t e n t varies in a narrow range and averages a b o u t 53%, and alkalies, chiefly K20, are very scarce (Table 1, columns 1 to 8, Fig. 2}. On the whole the Iblean tholeiites are dominantly oversaturated, b u t in some cases there are instances of olivine-normative lavas. The general chemistry o f these rocks is consistent with their origin at relatively shallow depth (P ~ 10 kbar, according to the presence of bronzite phenocrysts; Cristofolini and Battaglia, 1975). Moreover, the close similarity inside the group of the Iblean tholeiites indicates an origin from a c o m m o n starting material in similar environmental conditions, and a low degree of fractional crystallization of their parental melt. A wider range of compositions is shown b y the alkalic varieties of the Iblean region: lavas of this t y p e are dark and fine-grained, with olivine and augite as microphenocysts in a microcrystalline intergranular to.hypocrystalline groundmass. Their norms constantly show normative nephelite, which sometimes can also be identified in the modal association. With the exception of a few rocks differentiated toward the hawaiites, most of them are alkali-basalts to basanites with nephelinite affinities (Cristofolini and Battaglia, 1975; R o m a n o and Villari, 1973).
32.8 0.62 -1.51
99.43
100.33
38.4 0.56 -0.27
52.29 14.06 3.81 7.34 0.13 6.80 8.00 2.97 0.19 2.19 0.31 0.92 0.42
47.44 14.43 5.43 3.47 0.08 7.10 9.07 2.89 0.12 1.78 0.46 3.82 4.24
FR14
32.4 0.63 -2.00
99.70
52.29 14.73 4.93 7.01 0.15 6.85 8.65 2.68 0.19 1.73 0.20 0.25 0.04
FR45
32.3 0,62 -2.31
99.78
52.63 14.63 4.32 6.50 0.14 6.55 9.34 2.77 0.15 1.92 0.22 0.15 0.45
Fr56
31.9 0.64 -1.75
100.46
52.47 14.65 5.46 6.41 0.14 6.80 8.42 2.98 0.23 1.68 0.25 0.70 0.27
FR38
S.I. = solidification i n d e x (MgO/MgO + F e O t + NaO + K O ) I.E. = iron e n r i c h m e n t ( F e O t / M g O + F e O t ) M.L.I. = Modified Larsen I n d e x [1/s Si + K - (Ca + Mg)]
S.I. I.E. M.L.I.
SiO 2 AI,O 3 Fe,O 3 FeO Mn0 MgO CaO Na20 K20 TiO~ P205 H,O H20
MI91
Major e l e m e n t d a t a (wt. %)
TABLE 1
30.7 0.64 -1,87
99.83
49.03 13.76 5.58 4.81 0.10 5.73 8.77 2.83 0.27 1.95 0.31 3.41 3.28
LE3
28.7 0.67 -1.25
99.51
52.48 14.35 5.89 5.94 0.13 5.80 8.47 3.04 0.15 1.84 0.18 1.12 0.12
FR2
99.83
100,47 45.6 0.49 -8.60
100.33 26.8 0.69 -1.16
-8.54
44.6 0.50
42.87 11,62 5.85 5.63 0.18 11.40 12.40 2.64 0,63 2.01 1.48 2.30 0.82
44,40 11.98 5.76 5.02 0.16 11,40 12.T0 2.87 0.53 1.86 1.37 1.64 0.78
52.47 14.68 6.91 5.41 0.14 5.45 8.70 3.04 0.20 1.85 0.23 0.89 0.36
FR 1
F R 25
FR13
-11.76
44.1 0.49
99.01
39.30 11.44 6.98 4.24 0.20 11.75 15.95 2.95 0.73 2.02 2.15 0.20 1.]0
FR 5
-5.14
34.5 0.60
100.50
46.47 13.31 6.46 4.99 0.18 7.76 11.70 3.10 0.80 2.25 1.11 1.65 0.72
PE 4
-2.15
28,6 0.66
99.57
45.35 14.71 8.68 3.91 0.12 6.40 9.20 2.78 1.46 3.05 0.88 1.88 1.15
FR 49
c~
c¢
335 T h e clear d i s t i n c t i o n o f t h e tholeiitic and alkali-basaltic suites, s h o w n on p e t r o g r a p h i c and c h e m i c a l g r o u n d s , w i t h o u t a n y d a t a suggesting t h a t o n e c o u l d have been derived f r o m t h e o t h e r , is c o n f i r m e d b y Carter a n d Civetta ( 1 9 7 5 ) : Sr i s o t o p e ratios are significantly l o w e r for t h e tholeiites t h a n f o r t h e alkalic r o c k s o f t h e Iblean region, t h u s s h o w i n g an origin f r o m t w o i n d e p e n d e n t sources, probably under different P--T conditions. T h e p r e s e n t s t u d y is aimed at a b e t t e r u n d e r s t a n d i n g o f t h e origin o f t h e various Iblean r o c k t y p e s and o f the relationships a m o n g t h e m . TRACE ELEMENT COMPOSITION T h i r t e e n r o c k s a n a l y z e d also f o r m a j o r e l e m e n t s (Table 1) have b e e n a n a l y z e d for t h e f o l l o w i n g t r a c e elements: Li, Rb, Sr, Ni, Cr, Co, Cu b y a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y ( A A S ) ~ . Rare earth e l e m e n t s ( R E E ) have b e e n d e t e r m i n e d o n five samples (2, 14, 56, 5, a n d 25) b y i n s t r u m e n t a l n e u t r o n a c t i v a t i o n analysis ( I N A A ) , a c c o r d i n g t o t h e m e t h o d suggested b y Albini et al. ( 1 9 7 6 ) 2. T h e results are s h o w n in Table 2. °1* NQ20 +
KzO 10
&,H D
40 i5 5'0 "1, Si02 Fig. 2. The alkalic rock suite is well discriminated from the tholeiites which are plotted as a well-defined group in the silica-alkalies diagram, o = analyses from this paper; o = analyses reported by Honnorez (1967); • = Romano and Villari's (1973) analyses; ~ = averages for Hawaiian lavas (MacDonald, 1968) -- Th, tholeiites; Oc, oceanites; AB, alkali basalts; H, hawaiites; Bs, basanites; Ne, nephelinites; • = averages for oceanic lavas (Manson, 1967) -- Th', tholeiites; AB', alkali basalts; × = averages for abyssalt basalts -- 1, Cann (1971); 2, Engel et al. (1965).
1Analyses performed at the Institutes of Mineralogy in Naples and Catania, with a Jarrel Ash and an EEL Mark 2 spectrophotometer respectively; Ni has been determined according to the suggestions of Beccaluva and Venturelli (1971 ). 2 Irradiation of the samples has been obtained at the accelerator of the Politecnico in Milan, and radioactivity determinations at CISE, Milan.
336
TABLE2
Trace element data (ppm) MI 91
F R 14
F R 45
F R 56
F R 38
LE 3
FR 2
Li Rb Sr Cu Ni Co Cr La Ce Sm En Tb Dy Yb Lu
5.50 2.70 506 83.5 152 34 190
3,74 2.70 149 121 185 25 175 8.0+_1.0 21.6_+1 4.2_+0.4 2.38-+0.16 0.76-+0.11 5 . 3 0 -+ 0.29 1.30-+0.72 0.50+_0.01
4.29 2.75 149 110 162 35 200
4.18 2.20 182 92 156 35 310 6.6+_0.9 11.9_+0.9 3.6_+0.2 1.61_+0.14 0.53-+0.08 7.10-+ 0 . 2 8 8.00-+0.56 0.37_+0.0
4.18 2.75 145 89 177 30 360
4.73 2.70 200 92 145 38 245
3.41 2.70 136 10J 148 30 165 5.4_+0.7 15.6-+1.5 3.1-+0.2 1.24-+0. 0.63-+0. 4,15_+ 0. 11,70+-0. 0.09±0:
K/Rb Rb]Sr K/Sr Ce/Yb
369 0.005 1.97
584 0.018 10.58 1.66
573 0.018 10.58
566 0.012 6.84 1.49
694 0.019 13.17
830 0.014 ] 1.21
461 0.020 9~15 1.33
Large-ion-lithophile elements. A difference in some alkali and alkaline-earth element concentrations, fairly well correlated with K abundances, is shown between the tholeiites and the rocks from the alkalic suite: in agreement with worldwide average data (Prinz, 1967). As a rule the Sr content is very high in strongly undersaturated basanites (> 1000 ppm), while it is remarkably low in the tholeiites (< 200 ppm); slightly higher values have been reported by Romano and Villari (1973) for three tholeiites out of five, and also our sample MI 91, which is more altered and UpperMiocene in age, with features transitional toward the alkali basalts, exhibits a larger Sr content, The alkali basalts generally show concentrations intermediate between the tholeiites and the basanites (see also Romano and Villari, 1973). A similar behaviour is also shown by Rb, which is on average very scarce (near the detection limit) in all the analyzed lavas, being on the-whole scarcer in the tholeiites than in the alkalic rocks. Our results, particularly for the alkaline lavas, tend to be lower than those of Romano and Villari (1973), and fall in the lower half of the range reported by Prinz (1967). Rb/Sr values are very low, the lowest being found in the basanites;for the tholeiitesthese values conform to what has been reported by Phflpotts and Schnetzler (1970).
337
FR 12
FR 25
FR 1
FR 5
4.29 2.70 162 101 161 34 275
4.68 2.75 1091 111 283 35 410 79.6+-8 160.9-+3 12.6+-0.6 4.29+-0.21 1.84-+0.19 4.55-+0.3 4.90-+0.25 0.75-+0.02
4.95 5.28 1241 141 269 35 475
4.95 5.72 2000 119 246 47 460 141.1-+14.1 249.5-+15 20.0-+0.9 4.20-+0.19 2.27-+0.20 11.50-+0.63 4.40-+0.15 0.9-+0.01
615 0.017 10.25
1600 0.0025 4.03 32.84
991 0.0043 4.21
1059 0.0029 3.03 56.70
PE 4
FR 49
5.50 5.28 841 92.5 201 31 227
4.29 10.12 684 73 112 33 160
1258 0.0062 7.90
1198 0.0148 17.72
Li a b u n d a n c e s ( 3 . 4 1 - - 5 . 5 0 p p m ) d o n o t s h o w significant changes in the d i f f e r e n t suites.
Transitional elements. T h e Ni, Cr, and Co c o n t e n t s in t h e Iblean r o c k s agree with t h e values r e p o r t e d b y Prinz ( 1 9 6 7 ) . Their ranges in the tholeiites and in t h e alkaline r o c k s overlap and d o n o t allow a clear d i s c r i m i n a t i o n b e t w e e n t h e t w o groups; y e t it can be r e c o g n i z e d t h a t t h e Ni and Cr ranges for t h e tholeiites are less wide t h a n in t h e alkaline rocks. A c o m p a r i s o n with the d a t a f r o m R o m a n o and Villari ( 1 9 7 3 ) s h o w s t h a t this is a general f e a t u r e o f t h e Iblean volcanics (Fig. 3). T h e linear c o r r e l a t i o n b e t w e e n Ni and Cr is high in the alkalic lavas (r = 0.90), the l o w e s t c o n c e n t r a t i o n s being f o u n d in an U p p e r C r e t a c e o u s sample (CP 1, R o m a n o and Villari, 1 9 7 3 ) , while these e l e m e n t s are very p o o r l y c o r r e l a t e d in the tholeiites. Cu seems o n average slightly m o r e a b u n d a n t and shows a wider range o f variation in t h e alkaline r o c k s t h a n in t h e tholeiites. Rare earth elements. Results o b t a i n e d t h r o u g h I N A A clearly d i s c r i m i n a t e the tholeiites f r o m t h e alkalic rocks. T h e d i s t r i b u t i o n p a t t e r n s o f t h e R E E
338
Cr
00
p.p,m
0
® ®
o ~®
x
=o(~
"
,
pp.m. Ni
Fig. 3. C r a n d Ni are well correlated in the alkalic rocks, w h i l e in the t h o l e i i t e s Cr range o f variation is m u c h larger than that f o r Ni. Data f r o m the present p a p e r : , = t h o l e i i t e s ; x = a l k a l i c r o c k s . D a t a f r o m R o m a n o and Villari ( 1 9 7 3 ) : o = t h o l e i i t e s ; • = a l k a l i c r o c k s .
normalized to the chondrite composition conform broadly to what is known for tholeiites and alkali basalts: in the Iblean tholeiites, however, the light REE (La, Ce)~ are not as depleted as in common ocean ridge basalts, but are anyhow less concentrated than in ocean island tholeiites (Fig. 4; Gast, I968; Kempe and Schilling, 1974; Masuda et al., 1974; Schilling, 1971; Schilling and Winchester, 1969). Furthermore, starting from Eu the patterns are quite irregular: a positive Eu anomaly is apparent in two patterns out of three; Yb
Lav~,// /Chondr
100
II
10
La Ce
SmEu
Tb Dy
Yb LU
F i g . 4. Chondrite°normalized R E E patterns f o r f i v e I b l e a n r o c k s . For discussion see text.
339
is on the whole more abundant than in other tholeiites and the Lu content is scattered over a wide range (0.09--0.5 ppm). This distribution contrasts with preliminary data from Carter and Bell (1975) for the mildly tholeiitic rocks of Mt. Etna, which are far less depleted in light REE than the Iblean tholeiites, and compares fairly well with the data for the Othris tholeiites (Menzies, 1976). Comparing a Puerto Rico Trench composite sample (Jibiki and Masuda, 1974), taken as representative of abyssal oceanic basalts (low-K subalkaline), with the Iblean tholeiites, the light REE are almost linearly enriched in the latter. The rocks of the alkaline suite show a quite regular decrease in the chondritenormalized abundances, from light to heavy REE, and are on the whole slightly REE-enriched with respect to the average alkali basalt (Hubbard, 1969): this is to be correlated with the basanitic nature of the t w o analyzed samples (Ishikawa, 1974; Graham and Nicholls, 1969), and for this reason the patterns closely resemble some of the lavas from Ascension I (Gast, 1968) and from Cape Verde I (De Paepe et al., 1974, fig. 3). Our results for La and Ce are very close to those obtained by R o m a n o and Villari (1973) for similar rocks from the same area. Owing to the peculiar features of the REE profiles of the Iblean rocks, their Ce/Yb ratios are near b u t outside the ranges of ocean ridge and alkali basalts respectively ( L o u b e t et al., 1975). DISCUSSION
OF THE RESULTS
The major element chemistry and the trace element data show that the Iblean rocks belong to at least t w o different basaltic suites, the tholeiitic and the alkalic one, w i t h o u t any evidence suggesting a parent
340
examining the correlation of other couples of incompatible elements (La-Sm~ La-Tb). Therefore, in the first approximation and assuming the La (or Ce) content to be an indicator of the degree of partial melting, one could get some more information a b o u t the details of the evolution of different magma types by observing the distribution of elements such as Ni and Cr, which are strongly fractionated during the first stages of the crystallization (Fig. 5). Again no evidence appears from the available data that the overall distribution is controlled mainly by fractional crystallization. Only the significant depletion of Ni and Cr in some basanites and one hawaiite ( R o m a n o and Villari, 1973), correlated with a La enrichment could be accounted for by a mechanism of crystal fractionation. The tholeiites are grouped in a clearly defined cluster, showing very little correlation of La with either Ni or Cr. This behaviour and the distribution of the La and Ce values, could indicate for the tholeiites with respect to the alkalic lavas an origin in different levels of the mantle and/or under disequilibrium conditions (Treuil and Varet, 1973, p. 535}. These conclusions are confirmed by examining the distribution of Ni and Cr against Mg: in this case Mg is poorly correlated with both Ni and Cr in all the analyzed lavas, except the high-Mg (MgO ~ 9%) undersaturated rocks. Furthermore the tholeiites show, for similar Mg contents, slightly higher Ni and Cr abundances than the alkali basalts. The available data a b o u t the trace elements, except REE, have been plotted
CF
p.p.m.
300'
~°. lOG
NL 300'
°o
100
Fig. 5. La is plotted as an indicator of the degree o f partial melting against Ni and Cr. The overall point distribution does not c o n f o r m to a h y p o t h e s i s o f d i f f e r e n t i a t i o n by crystal fractionation (the arrow pointing toward a hawaiite indicates possible effects of crystal fractionation). Symbols as in Fig. 2.
341
against a Modified Larsen Index [M.L.I. = l/3 Si + K - (Ca + Mg)] (Fig. 6; Nockolds and Allen, 1953). The tholeiites are plotted as a homogeneous group at M.L.I. ~- - 2 , while the alkalic rocks are dispersed over a wider range of M.L.I. (0 to - 1 2 ) . The correlation of most trace elements with M.L.I. is very poor and again argues against any mechanism of origin involving mainly crystal fractionation. The REE patterns are compatible with the above-stated hypotheses: the alkalic rocks should have been derived by partial melting of very low amounts of mantle material (see Shimizu and Arculus, 1975), while the tholeiites with approximatively linear REE patterns could have originated by melting of large proportions of a depleted low-velocity layer (Schilling, 1971; Masuda et al., 1974), as suggested for the more basic Othris tholeiites (Menzies, 1976). In this case the Eu anomaly and the irregularities in the REE profiles could imply for the tholeiites an origin from mantle rocks with Eu-rich phases (plagio clase), at shallow depth (Masuda et al., 1974). On the other hand, very low Rb concentrations in the Iblean tholeiites are consistent with their origin from a depleted mantle: according to Peterman and Hedge (1971) this in turn agrees
Rb p,pm
Cr I~p.rr
°°
o ©
o
o
2O
• •
o
o
to. %
o O
o o
o
O
i
o OgC~ ~
Cu ppm
(~
Ni Pp~
0
O
..
1oo
o
%
©.
o
ao ki gpm
0
©
Sr
pp m 1
c~:
~
2o
o
o ~
. .. 0
1o
o
0
o M.L.I.
1o
o
M.L.I.
Fig. 6. A Modified Larsen Index (M.L.I.) is plotted against all the determined trace elements (REE excluded). With the exception of St, the trace element distribution is rather poorly correlated with M.L.I., what in turn does not conform with hypotheses of origin by crystal fractionation. Symbols as in Fig. 2.
342
with the very low Sr isotope ratios found for these rocks by Carter and Civetta (1975). This situation differs from the one f o u n d at Mt. Etna (Carter and Belt, 1975) where preliminary data indicate that the subalkaline rocks are much less light-REE-depleted, richer in Sr and with slightly higher Sr isotope ratios. Further information a b o u t the origin of the Iblean volcanics is offered by an examination of the covariance of K, Rb, and St. K / R b ranges from ~ 4 0 0 - - 8 0 0 for the tholeiites up to more than 1000 for the basanites, and is positively correlated with K {Fig. 7). While the former group of rocks falls well in the field of the oceanic ridge basalts (Gast, 1968; Gale, 1975), the alkalic lavas do n o t conform to the general observation (Gast, 1965; Prinz, 1967) that in basalts K / R b is negatively correlated with K. This could be accounted for by the only minor increase of the Rb content against a substantial change of the K (and Sr) proportions from the tholeiites to the alkalic suite rocks, and could be related with an origin of the latter from a mantle with low-Rb, K-bearing phases (Griffin and Murthy, 1969; Hughes and Brown, 1970), but could also be an effect of the change in the solid-liquid partition coefficient for Rb with pressure (Shimizu, 1974). The disagreement with Rb data from R o m a n o and Villari (1973) does not allow a more detailed discussion a b o u t the origin and evolution of the alkalic suite lavas. On the contrary, the good linear correlation of K / R b with K (r = 0.96) in our Iblean tholeiites could either, be the result of partial melting in a homogeneous mantle, if K is to be considered a dispersed element (Gast, 1968) and assuming it has an incompatible behaviour (Treuil and Varet, 1973), o r imply their origin through partial melting of large amounts of a hornblende peridotite
K/Rb
;OOO
1/
500
200
o~' dg i K% Fig. 7. A fairly good correlation is shown between K and K/Rb for the tholeiitea analyzed in this paper. For the alkalic rocks (K > 0.5:) the correlation is much poorer, owing probably to a partially compatible behaviour of K. Symbols as in Fig. 2.
343 with slightly variable phlogopite contents (Griffin and Murthy, 1969), which is in qualitative agreement also with the Rb/Sr and K/Sr distribution against K. Sr is fairly well correlated with Ca in the alkaline lavas and its distribution allows the discrimination of these rocks from the tholeiites, which show a lower Sr c o n t e n t at similar Ca concentrations. This again points toward an independent origin of the two main rock types. CONCLUSIONS Although the present information confirms that the Iblean tholeiites are quite similar to the incompatible-element-depleted ocean-ridge basalts, there is no other indication t h a t the Iblean region ever behaved as an ocean ridge: this is shown by the association of decidedly alkalic varieties with the tholeiites through the whole sequence, the features of the interlayered sediments, and the overall low volume of volcanics. This could then be an instance where the chemical and petrographic features of the volcanic rocks are n o t unequivocally meaningful about the geodynamic evolution of a region, even if t h e y are certainly controlled by the whole set of geodynamic processes (most of which are not known in detail) occurring there. The sequence of volcanic p h e n o m e n a from Upper Miocene to Lower Pleistocene is then to be related to tensional movements that developed at the southern edge of a belt with strongly thickened crust (Ogniben et al., 1973; Selli, 1973) as a late consequence of the collision between the African and the European plates. In this framework some deep fractures should have reached the mantle at different levels in various times, where eruptible magma was available, thus allowing the melts to ascend to the surface. The present data do n o t show any genetic link of the alkalic-suite rocks with the tholeiites: they should have been generated independently in environments differing in one or more factors, like pressure, temperature, mantle composition, degree of partial melting, and rate of formation and extraction of the melt from the mantle. On the basis of the distribution of the incompatible elements and in the first approximation, a different degree of partial melting, possibly at different levels in a heterogeneous mantle, could be suggested as responsible for the origin of the different Iblean magmas also inside the group of the alkalic rocks. Contrary to the assumptions by R o m a n o and Villari (1973) crystal fractionation then appears only as a minor factor of differentiation for few lavas of the alkalic suite.
344
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