Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of. Technology, Cambridge, MA 02139 (U.S.A.) ...... East Africa and Italy.
Chemical Geology, 43 (1984) 203--221 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
203
GEOCHEMICAL CHARACTERISTICS OF THE SOUTH TUSCANY (ITALY) VOLCANIC PROVINCE: CONSTRAINTS ON LAVA PETROGENESIS
GIAMPIERO POLI"*, FREDERICK A. FREY' and GIORGI F E R R A R A 2
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 (U.S.A.) Laboratorio di Geocronologia e Geochimica Isotopica, C.N.R., Pisa (Italy) (Received March 7, 1983; revised and accepted September 19, 1983)
ABSTRACT
Poli, G., Frey, F.A. and Ferrara, G., 1984. Geochemical characteristics of the south Tuscany (Italy) volcanic province: constraints on lava petrogenesis. Chem. Geol., 43: 203--221. Three suites of volcanic rocks from Radicofani, Mts. Cimini and Mt. Amiata (south Tuscany, Italy) were analyzed for major- and trace-element contents and Sr isotopic composition. All samples are silica saturated, hypersthene normative and some are peraluminous. Samples from Radicofani and the less evolved Mts. Cimini samples have high Mg numbers (74.5--66.3), consistent with the fosteritic-rich nature of their olivine phenocrysts and high bulk-rock abundances of ferromagnesian trace elements. All samples are highly enriched in light REE (150--315 × chondrites) compared to heavy REE (9--15 X chondrites) and have negative Eu anomalies (Eu/Eu* = 0.92--0.44). 87Sr/8+Sr ratios are high and variable: Radicofani = 0.7137--0.7150; Mts. Cimini = 0.7128--0.7138; Mt. Amiata = 0.7119--0.7131. A distinctive feature of the Radicofani samples is that with decreasing age, the rocks increase in K20 content but decrease in La/Ce. These south Tuscany rocks have geochemical characteristics of the calc-alkaline and alkaline series: namely, in addition to the high K, Rb, Cs, Ba and LREE contents, they have high ratios of incompatible elements relative to the high-field-strength ions such as Ti, Zr, Hf, Nb and HREE when compared to an estimated primitive mantle composition. In general, the geochemical characteristics of south Tuscany volcanics are consistent with mixing between crustal-derived material and mantle-derived magma. The mantle-derived component was unusually rich in radiogenic Sr and incompatible elements and had geochemical features similar to basic lavas of the high-K series in the Roman Province.
INTRODUCTION
Widespread magmatism of Late Miocene--Quaternary age occurred in the northern Apennines (Fig. 1), and Barberi et al. (1971) showed that the age of magmatism decreases to the southeast. The Tuscan igneous province *Present address: Istituto di Mineralogia, Petrografia e Geochimica, Universit~ di Firenze, Via La Pira 4, 51021 Florence, Italy.
204
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Fig. 1. Map o f Italian potassic m a g m a t i s m . 1 = central T u s c a n y area; 2 = s o u t h T u s c a n y - n o r t h e r n L a t i u m area; 3 = R o m a n c o m a g m a t i c province.
consists of rhyolites and potassic volcanics and plutonic rocks such as granites, quartzmonzonites and granodiorites. The potassic volcanics developed from Pliocene to Quaternary and several geothermal fields are currently active. Although the Tuscan igneous rocks are generally older than the igneous rocks of the more southerly Roman Province, these igneous provinces overlap in space and time. However, the two provinces are compositionally and petrographically distinct. The Roman volcanics are mainly highly alkaline and silica undersaturated, whereas Tuscan rocks are c o m m o n l y silica saturated and sometimes peraluminous; some Tuscan volcanics resemble K-enriched calc-alkaline rocks. In this region of Italy, three distinct volcanic areas can be recognized
205
(Fig. 1). From north to south they are: (1) Central Tuscany area represented by silicic volcanics and intrusives. (2) South Tuscany--northern Latium area where there is overlap between Tuscan basic--intermediate volcanics (Radicofani, Amiata, Cimini and Tolfa) and Roman volcanics (Sabatini, Vico and Vulsini). (3) Roman Province which is dominated by silica-undersaturated K-rich volcanics. Several major-element, isotopic and trace-element studies of the Roman volcanics (e.g., Hawkesworth and Vollmer, 1979; Taylor et al., 1979; Civetta et al., 1981) have led to a consensus that these volcanics developed from a mantle source unusually rich in incompatible elements, with high 87Sr/86Sr and low 143Nd/144Nd, but that assimilation of crustal rocks was also important in determining their isotopic and trace-element characteristics, especially in the most evolved rocks (Holm et al., 1982). The Tuscan Province has been studied in less detail but geochemical studies (e.g., Coradossi, 1966; Puxeddu, 1972; Ferrara et al., 1976; Taylor and Turi, 1976; Vollmer, 1977; Hawkesworth and Vollmer, 1979) have suggested that assimilation of sediments and crustal anatexis have been major processes in Tuscan magmatism. In this paper we use major- and trace-element abundances and Sr isotopic ratios of representative samples from three volcanic centers (Radicofani, Mt. Amiata, and Mts. Cimini) in the south Tuscan-northern Latium are to constrain the petrogenesis of volcanic rocks from this region. RADICOFANI
The Radicofani Volcano is a small (~ 100 m elevation) neck surrounded by Pliocene clay-rich sediments. A single sample yielded a K--At age of 0.97 Ma (Barberi et al., 1971). On the basis of field relations six units are identified and we studied one sample from each unit. In order of inferred stratigraphic age (Innocenti, 1967) they are: OLDEST
YOUNGEST
Inner conduit (R3) Basal c o n d u i t - - grey ( R 1 ) Basal c o n d u i t - - b l a c k ( R 8 ) Upper conduit (R4) Lava lake ( R A ) Lava flows (RC)
These units are generally homogeneous, but small quartz inclusions rimmed by microlitic clinopyroxenes are common. The upper conduit is a homogeneous vesicular rock and is overlain by a red scoriaceous rock related to a lava lake.
Petrography All samples are porphyritic (16--26%) with phenocrysts of olivine, clinopyroxene and plagioclase. Detailed petrography was given by Innocenti
206 (1967). Olivine content ranges from 9 to 15%, and except for the highly oxidized lava lake sample (RA), olivine cores are fresh (microprobe analyses yielded ~86% Fo; G. Poli, unpublished data, 1980) with inclusions of spinel. Pyroxene phenocrysts are common and plagioclase phenocryst abundance varies inversely with olivine and pyroxene abundance and ranges from 80--84% An cores to rims of 74--76% An. Groundmass consists of glass, plagioclase, K-feldspar, pyroxenes, Fe-oxides and late-stage biotite. K-feldspar comprises ~28% of the youngest rocks and it is a hightemperature phase with 70--73% anorthoclase. Apatite occurs as an accessory phase in all samples. Major elements
Major-element contents are given in Table I. Abundances of SiO2, TiO: and Al~O3 are similar to those in high-A1 basalts, but these Tuscan basalts are distinctive because of their high MgO and K20 contents. All samples except RA are hypersthene normative. Using the definition of Morrison (1980), the Radicofani volcanics belong to the shoshonite association. Mg/(Mg+Fe 2÷) ratios (assuming Fe3+/Fe 2+ = 0.15; Green et al., 1974) range from 67 to 74, which is consistent with their forsteritic olivine phenocrysts. The compositions vary systematically as a function of estimated age. Most notable are decreases in CaO, Na20 and Al203, coupled with increasing SiO2, TiO2, MgO and K~O as the samples become progressively younger (Table I). Trace elements
Trace-element abundance data are given in Table II. Sample R4, the most Mg-rich sample studied, has the highest Cr, Co and Ni contents. The Cr abundances in these Radicofani samples are similar to those in primitive basalts but Ni abundances (100--170 ppm) are lower. All samples are highly enriched in LREE (150--250)< chondrites) compared to HREE (9--12 >( chondrites). Total REE content increases with increasing K20, but a surprising feature of the chondrite-normalized patterns (Fig. 2A) is the variation in (La/Ce)n ratio which decreases systematically with decreasing age and increasing K. Based on extrapolation between Sm and Tb, all samples have small negative Eu anomalies (Table II). For samples R1, R8 and R4, in order of decreasing age, there is a positive correlation between abundances of the incompatible elements K20, Rb, Ba, Zr, Hf and compatible elements such as Mg, Cr, Ni and Co (Fig. 3). The youngest sample, RA, which is less magnesian, also has the highest abundances of Rb, Cs, Zr, Hf and Th. Sr isotopes
87Sr/86Sr ratios are high (Table II) and variable (0.7137--0.7155), com-
207
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208
T A B L E II T r a c e - e l e m e n t c o n c e n t r a t i o n s ( p p m ) a n d Sr i s o t o p i c c o m p o s i t i o n o f Mt. R a d i c o f a n i , Mts. C i m i n i a n d Mt. A m i a t a r o c k s Mt. R a d i c o f a n i R1
R8
Mts. C i m i n i R4
RA
La Ce Nd Sm Eu Tb Yb Lu
46 105 50 7.4 1.81 0.8 1.94 0.36
49 129 60 9.8 1.93 0.S 2.33 0.33
53 146 67 10.9 2.00 0.8 1.76 0.31
52 160 79 10. 2.25 0.8 2.08 0.32
Cs Rb Sr Ba Th Ta Hf Zr Cr Ni V
13 201 335 610 35 1.0 7.2 211 407 97 167
10 210 354 637 32 0.9 7.9 230 515 98 161
14 264 346 669 36 1.1 8.9 269 585 171 177
20 334 378 668 54 1.4 11.4 886 479 100 166
Co
38
36
40
84
8c La/Yb (La/C~n Eu/Eu" REE Sr/Nd 8~Sr/B~Sr
28 28.7 1.23 0.9 240 6.7 0.7137±6
27 21. 1.08 0,8 282 5.9 0.7142±2
25 80.1 1.02 0,76 SlS 5.1 0.7150±2
25 25. 0.91 0,92 340 4.8 0.7185±2
CIM
CIM
CIM
CIM
77-16
77-6
77-9
77-14
94 196 85 11,8 2.40 1. 2.26 0.36
91 196 82 11.8 2.26 1.2 2.81 0.38
92 184 69 10.1 2.03 1.2 2.91 0.44
84 164 63 9.4 2,21 1.1 2.67 0,44
29 353 613 883 67 1.8 10. 862 204 59 117
36 294 547 881 62 1.9 8.7 319 45 19 94
33 287 486 991 54 1.7 7.9 276 33 14 78
13 81.6 1.4 0.72 402 7.9 0.7186±4
11 81.5 1.43 0,84 364 7.7 0,7138±4
27 336 688 1,061 50 1.6 9.7 366 302 108 187 .
.
21 41.6 1.34 0.81 482 8.1 0,7128±3
.
18 82.4 1.3 0,72 480 7.5 0.7134±1
.
R E E , Cs, T h , Ta, H f , Co a n d Sc d e t e r m i n e d 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 . P r e c i s i o n e x p r e s s e d as s t a n d a r d d e v i a t i o n is b e t t e r t h a n 2 0 % f o r T b ; b e t t e r t h a n 1 5 % f o r L u , Y b , T a a n d Cs; b e t t e r than 1 0 % f o r T h , Hf, Ce, N d a n d E u ; b e t t e r t h a n 5% f o r La, S m , Co a n d Sc. Ba, Zr, Cr, Ni a n d V d e t e r m i n e d b y X R F f o l l o w i n g t h e m e t h o d d e s c r i b e d i n L e o n i a n d S a i t t a ( 1 9 7 6 ) . P r e c i s i o n e x p r e s s e d as s t a n d a r d d e v i a t i o n is b e t t e r t h a n 1 0 % f o r Ni, Cr a n d V; b e t t e r t h a n 5% f o r Ba a n d Zr.
parable to a single value, 0.71348, for Radicofani reported by Hawkesworth and Vollmer (1979). With decreasing age 87Sr/8~Sr increases along with abundances of K, Rb and Sr (Fig.3). Discussion
The Radicofani samples have several geochemical features which constrain petrogenetic models. For example: (1) The samples are magnesian and similar to high-alumina basalts in SIO2, TiO~ and A1203, but they have very high K20/Na20 ratios (> 1.5). (2) With increasing K20 there is an increase in the 87Sr/8~Sr ratio, abundance of MgO and several incompatible and compatible trace elements. (3) The samples have high La/Yb (> 21), but there is a trend for La/Ce to decrease with decreasing age and (La/Ce)N is < 1 in the youngest sample.
209
Mt. A m i a t a TR 82
MA 77-14
QLB 86
MA 77-24
MA 77-1
MA 77-19
72 155 69 10.6 1.98 1. 2.54 0,43
73 156 68 11.2 1.83 1.1 2,39 0,48
84 165 73 11.4 1.45 1.1 2.59 0.44
80 164 70 9.9 1,21 1.2 3.10 0,49
75 162 68 10.7 1.38 1.3 2.76 0.47
74 156 66 10.6 1.23 0.9 2.81 0,48
33 320 598 748 89 1.8 7.0 206 94 22 102 17 16 28.8 1.8 0.72 349 8,6 0.7121±2
87 888 581 681 40 1.8 7.0 279 87 24 112 -16 80.5 1.81 0.62 851 8.5 0.7119±8
S4 886 891 518 86 1.5 -244 88 10 64 i0 10 82.4 1.42 0.49 877 5.4 0.7126±2
46 360 884 279 36 1.5 6,5 285 26 12 67 9 11 25.8 1.86 0.44 869 4.8 0,7129±5
48 864 370 887 45 1.6 7.4 248 31 14 67 I0 10 27,2 1.8 0.45 862 5.4 0.7180±S
41 871 856 877 41 1.5 5.4 264 29 11 70 8 9 26.8 1.8 0.4 845 5,4 0.7181±3
R b a n d Sr d e t e r m i n e d b y i s o t o p e dilution. I s o t o p e r a t i o s w e r e d e t e r m i n e d o n a V a r i a n M a t ® T H 5 m a s s s p e c t r o m e t e r . Sr w a s s e p a r a t e d b y s t a n d a r d i o n - e x c h a n g e t e c h n i q u e s a n d t h e i s o t o p e results p r e s e n t e d are t h e m e a n o f m o r e t h a n 2 0 0 m e a s u r e m e n t s . S~Sr/S6Sr p r e s e n t e d h e r e h a v e b e e n n o r m a l i z e d to a value o f 0 . 1 1 9 4 f o r S6Sr/ssSr a n d a r e relative t o E & A = 0 . 7 0 8 0 . U n c e r t a i n t y is 2 o o f last digit.
The variable 8~Sr/S6Sr ratio and the concomitant increase in abundances of incompatible and compatible elements preclude the possibility that these Radicofani rocks are related to a single parental magma by crystal--liquid fractionation. Taylor and Turi (1976) suggested that Tuscan magmas have been derived by large-scale assimilation of argillaceous sediments or metasediments by calc-alkaline magmas or by partial melting of such sedimentary rocks. These alternatives are consistent with the high (+11 to +16%o ) 5180 of the Tuscan magmas and the positive correlation of STSr/S6Sr with K abundance. However, the relatively low SiO2 and high MgO, Cr and Ni contents of the Radicofani rocks are typical of mantle-derived basalts and are inconsistent with a large sedimentary component. Taylor {1980) and De Paolo (1981) emphasized that assimilation is generally accompanied by fractional crystallization and that a complex variety of geochemical trends may develop. Because of their relatively high MgO, Cr
210
and Ni contents these Radicofani rocks do not seem to have segregated large amounts of mafic minerals. However, the decreasing CaO, Na20 and Al203 contents as K20 abundance and 87Sr/8~Sr increase may reflect plagioclase fractionation accompanying assimilation. Nevertheless there is no decrease in Sr or Eu content as expected for plagioclase fractionation. Thus, if plagioclase was an important segregating phase, the assimilated material had high STSr/86Sr and Sr content relative to the mantle-derived end-member. Obviously, it is necessary to know the composition of crustal rocks available for assimilation. Boreholes to 3000 m (Gianelli and Puxeddu, 1979) from the Tuscan geothermal field (Fig. 1 ) p r o v i d e information about the uppermost crust. Based on cores from these drillholes, three Paleozoic rock types are possible contaminants in Radicofani volcanics. In order of generally increasing depth, they are metabasites, phyllites and micaschists. Typical compositions are given in Table III. Although the micaschists and phyllites are rich in Al203, K20 and Rb, they have lower K20, Rb and Sr contents than the youngest Radicofani volcanics. Qualitatively, these upper crustal
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Fig. 2. Chondrite-normalized REE patterns of: (A) Radicofani; (B) Mts. Cimini; and (C) Mt. Amiata rocks.
rocks do n o t seem appropriate as a crustal end-member for Radicofani rocks. Moreover, partial melts of these crustal rocks are n o t appropriate endmembers, because such melts would be even more Sr depleted as a result of abundant feldspar in the residues of partial melting. Although we have not studied sufficient samples to adequately evaluate a fractional crystallization-assimilation process, it is evident that the trends in these Radicofani rocks are not consistent with extensive crystallization of their mafic phenocryst phases accompanied by assimilation of nearby Paleozoic, upper crustal rocks. A geochemical feature not easily explained by crustal processes is the decrease of La/Ce with increasing 87Sr/8~Sr. Crustal rocks do not generally have (La/Ce)n < 1; however, some micaceous kimberlites (Paul et al., 1975) and amphibole-bearing mantle peridotites have this characteristic. Stosch and
212
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213
TABLE III Average major- and trace-element contents of Tuscan Paleozoic basement rocks drilled in Tuscan geothermal field boreholes Metabasic rocks
SiO 2 TiO=
A]203 Fe203 FeO MnO MgO CaO Na20
K20 P~O s LOI Rb Sr Ba Zr La Ce Ni Cr Co V
Phyllites
Garnet micaschists
FIG *~
BOC .1
FIG .1
BUT .1
MCG .1
3*:
5 *2
6 *2
2 *2
1 0 *2
50.21 2.22 15.77 2.92 7.53 0.13 5.20 4.70 3.54 0.52 0.53 5,88 21 241 202 307 33 67 60 205 26 263
43.60 2.66 13.07 4.14 7.61 0.15 5.58 9.75 2.17 1.53 0.45 8,43 57 803 295 227 41 78 54 82 26 306
57.99 66.23 0.98 0.84 19.06 19,34 1.48 3.30 5.46 0.92 0.07 -2.25 1.01 0.90 0.51 2.26 0.53 3.26 4.20 0.24 0.09 4.15 2.85 102
101
53 135 16 152
25 77 10 101
62.91 0.79 18.19 0.71 5.26 0.06 2.31 0.99 1.81 3.21 0.16 3.83 128 180 536 289 40 72 32 90 15 110
Data are from Gianelli and Puxeddu (1979). For more details on Tuscan Paleozoic basement, see Bagnoli et al. (1979). *~Tuscan Paleozoic basement formation: FIG = Filladi Inferiori Group; BOC = Boccheggiano Formation; BUT = Buti Group; MCG = Mica Schist Group. ,2 Number of samples used for average. S e c k ( 1 9 8 0 ) i n t e r p r e t e d l o w L a / C e as a r e s u l t o f a p e r i d o t i t e e q u i l i b r a t i n g in t h e m a n t l e w i t h a m e t a s o m a t i c fluid. Thus, t h e a m p h i b o l e - b e a r i n g i n c o m p a t i b l e e l e m e n t - r i c h p e r i d o t i t e s f r o m D r e i s e r Weiher, F . R . G . ( S t o s c h a n d Seck, 1 9 8 0 ) m a y be a n a l o g s f o r t h e m a n t l e s o u r c e of T u s c a n m a g m a s . I t is p o s s i b l e t h a t t h e m a n t l e s o u r c e f o r R a d i c o f a n i m a g m a s was u n u s u a l l y h i g h in STSr/S6Sr a n d in i n c o m p a t i b l e - e l e m e n t c o n t e n t . F o r e x a m p l e , T a y l o r e t al. ( 1 9 7 9 ) c o n c l u d e d t h a t t h e p a r e n t a l m a g m a f o r t h e R o c c a m o n f i n a V o l c a n o in t h e R o m a n P r o v i n c e c o u l d have b e e n d e r i v e d f r o m a m a n t l e w i t h 8~Sr/S6Sr as high as 0 . 7 0 8 , a n d H o l m e t al. ( 1 9 8 2 ) f o u n d t h a t t h e p a r e n t a l m a g m a at V u l s i n i has 87Sr/S6Sr ~ 0 . 7 1 0 .
214 Mts. CIMINI The Mts. Cimini volcanic center encompasses ~ 400 km 2, but only 60 km 2 are exposed because of extensive cover from the recent volcanoes, Vico and Vulsini, which belong to the Roman Province (Fig. 1). The dominant country rocks are limestones and clay-rich sediments. On the basis of petrography, stratigraphy and K--Ar dating (Evernden and Curtis, 1965; Nicoletti, 1969; Puxeddu, 1972) three major units are identified: (1) Quartz-latite ignimbrites of several generations ranging from 1.18 to 1.36 Ma. (2) Lava domes ranging in composition from latites to quartz latites (rhyodacites) which postdate the ignimbrites. A single sample from Turello dome has a K--Ar age of 1.01 Ma. (3) Lava flows ranging from latites to olivine latites which are the youngest eruptives from the Mts. Cimini Center. A single K--At age of 0.94 Ma was determined on a latite flow.
Petrography Mts. Cimini volcanics are porphyritic ranging from ~47% phenocrysts in ignimbrites to ~24% in olivine latites. Plagioclase, sanidine, biotite and pyroxene phenocrysts occur in all rock types. Olivine (fosterite contents by microprobe analysis range from 84.5% to 92%; G. Poli, unpublished data, 1980) is abundant only in the olivine latites. Clinopyroxene is the principal phenocryst phase in the olivine-latite flows and plagioclase is the major phenocryst in the lava domes and ignimbrites. Accessory phases include apatite, zircon and allanite in the more silicic samples. Also, a wide variety of xenoliths are common. Two generations of phenocryst assemblage occur in the lava domes and latite flows, i.e. large (up to 5 cm) sanidine megacrysts (~76% Or) occur along with an assemblage of smaller sanidine phenocrysts, plagioclase, biotite and pyroxene. In addition, in the more basic olivine latites, one phenocryst assemblage consists of plagioclase, rare clinopyroxene and strongly resorbed olivine whereas a second assemblage has dominant clinopyroxene, olivine and plagioclase, resorbed hypersthene and rare biotite. Taylor and Turi (1976) showed that the sanidine megacrysts are systematically higher in 6180 than the whole rocks, and G. Ferrara (unpublished data, 1980) also found that these sanidines have 8TSr/86Sr differing from the bulk-rock values.
Major elements Extensive major-element data were reported by Puxeddu (1972). We have studied one sample from each unit (Table I). Using Streckeisen's (1967) nomenclature, the samples are mostly latites with the more silicic dome grading into rhyodacites. All samples are hypersthene normative, and norma-
215
tive quartz increases from olivine latites to ignimbrites to the lava domes which are also corundum normative. Mg/(Mg+Fe 2+) ranges from 0.73 to 0.40 (assuming Fe3+/Fe 2+ = 0.15; Green et al., 1974). The higher values in the olivine latites are consistent with their more forsteritic olivine phenocrysts. Major-element contents vary systematically; that is, MgO, TiO2 and PzOs contents decrease with SiO2 content. There is little variation in total alkalis (Na20 + K~O) in the suite, but the lava dome and ignimbrite samples have (CaO + Na20 + K~O)/A1203 (molar basis) near unity, whereas the latite and olivine-latite flows have higher ratios of 1.2--1.3. Trace elements
The Mts. Cimini samples have higher concentration of incompatible elements than samples from Radicofani (Table II). For example, LREE range from 280 to 315 × chondrites and HREE range from 11 to 15 × chondrites (Fig. 2B). All samples have negative Eu anomalies (Eu/Eu* of 0.72--0.84). An important result is the surprising inverse correlation between SiO2 content and abundance of some incompatible elements (LREE, Sr, Zr, Hf) and the La/Yb ratio. Abundances of compatible elements, Cr, Sc, V and Ni, also decrease with increasing SiO2 content. Abundances of these compatible elements in the most magnesian sample, CIM 77-16, are similar to those in basalts. Sr isotopes
STSr/SSSr ratios are high and variable in Mts. Cimini samples, ranging from 0.7128 to 0.7138, comparable to a value of 0.71372 previously reported for a rhyodacite (Hawkesworth and Vollmer, 1979). Fig. 4 is a STSr/S6Sr vs. 1/Sr plot. The Mts. Cimini samples define a rough positive trend which lie on an extrapolation of the trend defined by the high-K series of the Vulsini and Vico volcanic centers (Hawkesworth and Vollmer, 1979; Holm and Munksgaard, 1982). Discussion
Based on 5z80 data, Taylor and Turi (1976) postulated that Mts. Cimini volcanics formed by mixing of Tuscan rhyolitic magmas with K-rich Roman magmas. The positive correlation of 87Sr/86Sr with 1/St (Fig. 4) and isotopic disequilibrium between sanidine megacrysts and groundmass substantiate the importance of mixing in forming the Mrs. Cimini volcanics. Although the mixing process was probably complicated by crystal fractionation, the trends presented in Table II and Fig. 4 imply that the SiO2-poor end-member had STSr/8~Sr < 0.7128 and higher contents of Rb, Sr, Ba, LREE, Zr and Hf than the SiO2-rich end-member. High-K, SiO2-poor lavas from the Roman
216
0.7150 87Sr ~86Sr 0.7140
0.7130
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Province are enriched in incompatible elements relative to typical crustal rocks, e.g., in the range of 47--50% SiO2, silica-undersaturated lavas from the nearby Vulsini Volcano contain more Rb, Sr, Ba and LREE (Holm et al., 1982) than the Mts. Cimini samples. Consequently, the geochemical trends of the Mts. Cimini samples are consistent with K-rich Roman Province lavas as an end-member of the mixing process. Mt. AMIATA
The Mt. Amiata volcanic center (Fig. 1, 85 km 2) formed on Upper Cretaceous--Paleocene limestone and clays. The region has been recently uplifted and contains active geothermal fields. Evernden and Curtis (1965) reported a single sample K/Ar age of 0.43 Ma. Recently, Bigazzi et al. (1981) obtained K/Ar ages on sanidine separates of 0.20--0.29 Ma, while the fissiontrack ages of glasses from the same samples yielded values of 0.20--0.43 Ma. Based on fieldwork, the following magmatic units have been defined (Mazzuoli and Pratesi, 1963): (a) the oldest unit consists of extensive quartz-latite flows which form the basal part of the volcano;
217
(b) an intermediate stage of quartz-latite domes formed on these older flows; (c) a quartz-latite flow extruded in the southern part of the volcano forms another intermediate age unit; (d) the youngest unit consists of two trachytic flows extruded from near the top of the volcano. Pet rograp h y
Mt. Amiata volcanics are highly porphyritic, ranging from ~27% phenocrysts in the trachytes to ~42% in the quartz-latite domes. The main phenocryst phases are plagioclase, sanidine, pyroxene and biotite, with feldspar dominant in the quartz latites and plagioclase plus pyroxene dominant in the trachytes. Groundmasses range from glassy with perlitic texture to glass with feldspar and clinopyroxene microlites. Plagioclase in the quartz latites (mean of 50% An) is oscillatory zoned and often corroded and resorbed by surrounding glass. Biotite, orthopyroxene and clinopyroxene plus zircon are c o m m o n inclusions in plagioclase. Sanidine (Ors2) in the quartz latite is also corroded and resorbed, and as at Mts. Cimini, sanidine megacrysts up to 5 cm are common. Pyroxene is mostly hypersthene but rare clinopyroxene occurs in the trachytes. Inclusions of zircon and apatite are c o m m o n in biotite which is partially corroded in the quartz latites and strongly corroded in the trachytes. Apatite is the most abundant accessory phase and is often included within phenocrysts. Zircon, allanite, rutile and primary magnetite also occur in minor amounts. Major elements
Table I contains major-element abundances for six Mt. Amiata samples: two trachytes, two of the oldest quartz latites and two of the youngest quartz latites. These rocks are hypersthene and quartz normative and the quartz latites are also corundum normative. The peraluminous index (CaO + Na20 + K20/A1203, molecular) is near unity. Relative to the quartz latites, the trachytes are lower in SiO2 and Na20 but richer in MgO and CaO. Trace elements
Relative to the quartz latites, the trachytes are enriched in Sr and Ba but depleted in Rb. Relative to chondrites, all the Mt. Amiata volcanics are strongly enriched in LREE (Fig. 2C). The trachytes have REE contents similar to volcanics from Mts. Cimini, whereas the quartz latites have prominent negative Eu anomalies.
218
Sr isotopes
The 87Sr/86Sr ratios are high, varying from ~0.7120 in the trachytes to ~0.7130 in the quartz latites. For each rock type these ratios are similar to the trachyte and two quartz-latite values reported by Hawkesworth and Vollmer (1979). The 87Sr/86Sr ratios increase with increasing 1/Sr ratio, and define a trend which intersects the trend established by the high-K series from the Vulsini and Vico volcanic centers (Fig. 4). Very limited isotopic data for Nd (Hawkesworth and Vollmer, 1979) and Pb (Vollmer, 1977) indicate that the trachytes and quartz latites are isotopically similar. Discussion
The Mr. Amiata volcanics are broadly similar to Mts. Cimini volcanics in key features such as SiO2 content, high 5180 of +11--12%0 (Taylor and Turi, 1976), K/Rb systematics (Dupuy and All~gre, 1972) and LREE/HREE ratios. In addition, the positive trend of 87Sr/86Sr vs. 1/St suggests that, as for the Cimini rocks, the Mt. Amiata suite was generated by a mixing process between high-K rocks from the Roman Province and crustal material. A distinctive difference between the latites at Mt. Amiata and Mts. Cimini is the lower Eu, St, Ba contents and Sr isotope ratios in the Mt. Amiata samples. These features indicate that feldspar extraction was more important in developing the Mt. Amiata volcanics and/or that the crustal end-member in the Mt. Amiata samples was less enriched in 87Sr/S6Sr and St, Eu and Ba abundances than the crustal end-member in the Mts. Cimini volcanics. CONCLUSIONS
The geochemical data reported in this paper enable the following conclusions to be drawn: (1) Mts. Cimini and Mt. Amiata suites are the product of mixing processes which were probably accompanied by crystal--liquid fractionation. Likely end-members of the mixing process were basic rocks forming the high-K series of the Roman Province, specifically, those at the Vulsini and Vico volcanic centers (Fig. 1), and silicic crustal components which relative to the basic end-members had higher 87Sr/86Sr but lower abundances of incompatible elements. (2) The Radicofani rocks have geochemical characteristics which cannot easily be explained by upper-crustal mixing and fractionation processes. Possibly, many of the geochemical features of Radicofani lavas reflect source characteristics developed by metasomatic processes in the mantle. Relative to estimates for primitive mantle, incompatible-element abundances in Tuscan Province samples from Radicofani and Mts. Cimini and a Roman Province sample from Vulsini exhibit similar structure; specifically, all these samples are relatively deficient in P, Hf, Zr and especially Ta, Nb
219
and Ti whereas they are relatively enriched Cs, Rb, Ba, K, light REE and especially Th (Fig. 5). Therefore, the relative abundances of incompatible elements (Fig. 5) and trends of 87Sr/86Sr vs. 1/St (Fig. 4) provide evidence for some similarities in the genesis of lavas from south Tuscany and the nearby Roman Province lavas at Vulsini. A final point is that the Tuscan and Roman Province lavas have geochemical features characteristic of the calc-alkaline and alkaline series. Namely, in addition to high K, Rb, Cs, Ba and light REE contents, they have high ratios of these elements relative to high-field-strength ions such as Ti, Zr, Hf, Nb and heavy REE (Fig. 5). This combined two-fold geochemical signature can be interpreted as resulting from the superimposition of two contrasting tectonic regimes. Specifically, during the Tertiary, the northern Apennine region was initially affected by a subduction process associated with formation of the Apennines and counterclockwise rotation of the Sardina--Corsica microplate; subsequently, an extensional tectonic regime developed which has led to alkaline volcanism.
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220 ACKNOWLEDGEMENTS
The authors wish to thank Prof. C.A. Ricci, University of Siena, and Prof. F. Innocenti, University of Pisa, for providing the samples analyzed in this work. The manuscript was greatly improved by comments of Professors F. Innocenti, P. Manetti and A. PecceriUo. Neutron activation analyses were performed at the Department of Earth, Atmospheric and Planetary Sciences, M.I.T. (Cambridge, Massachusetts, U.S.A.), while G.P. was a visiting fellow sponsored by a N.A.T.O. fellowship. Nuclear irradiations were made at the M.I.T. reactor. Financial support was partially provided by the National Council of Research of Italy (Centro di Studio per la Mineralogia e Geochimica dei Sedimenti), National Science Foundation Grant EAR 7823423 and M.P.I. of Italy (Grant on Recent and Active Volcanism -- Fondi 40%).
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221 Holm, P.M. and Munksgaard, N.C., 1982. Evidence for mantle metasomatism: an oxygen and strontium isotope study of the Vulsinian District, central Italy. Earth Planet. Sci. Lett., 60: 376--388. Holm, P.M., Lou, S. and Nielsen, A., 1982. The geochemistry and petrogenesis of the lavas of the Vulsinian district, Roman Province, central Italy. Contrib. Mineral. Petrol., 80: 367--378. Innocenti, F., 1967. Studio chimico-petrografico delle vulcaniti di Radicofani. Rend. Soc. Ital. Mineral., 13: 99--128. Leoni, L. and Saitta, M., 1976. X-ray fluorescence analysis of 29 trace elements inrocks and mineral standards. Rend. Soc. Ital. Mineral. Petrol., 32: 497--510. Mazzuoli, R. and Pratesi, M., 1963. Rilevamento e studio chimico--petrografico delle rocce vulcaniche del Monte Amiata. Atti Soc. Toscana Sci. Nat. Pisa, Mere., P.V., Set. A, 70: 355--429. Morrison, G.W., 1980. Characteristics and tectonic setting of the shoshonite rock association. Lithos, 13: 97--108. Nicoletti, M., 1969. Datazione argon potassio di alcune vulcaniti delle regioni vulcaniche Cimina e Vicana. Period. Mineral., 38: 1--20. Paul, D.K., Potts, P.J., Gibson, I.L. and Harris, P.G., 1975. Rare earth abundances in Indian kimberlite. Earth Planet. Sci. Lett., 25: 151--158. Puxeddu, M., 1972. Studio chimico--petrografico delle vulcaniti del Monte Cimino (Viterbo). Atti Soc. Toscana Sci. Nat. Pisa, Mem., P.V., Set. A, 78: 329--394. Stosch, H.G. and Seck, H.A., 1980. Geochemistry and mineralogy of two spinel peridotite suites from Dreiser Weiher, West Germany. Geochim. Cosmochim. Acta, 44: 457--470. Streckeisen, A.L., 1967. Classification and nomenclature of igneous rocks. Neues Jahrb. Mineral. Abh.. 107(2-3): 144--240. Taylor, H.P. Jr., 1980. The effects of assimilation of country rocks by magmas on 180/ 160 and sTSr/86Sr systematics in igneous rocks. Earth Planet. Sci. Lett., 47: 243--254. Taylor, Jr., H.P. and Turi, B., 1976. High lsO/1~O igneous rocks from the Tuscan magmatic province, Italy. Contrib. Mineral. Petrol., 55: 33--54. Taylor, Jr., H.P., Giannetti, B. and Turi, B., 1979. Oxygen isotope geochemistry of the potassic igneous rocks from the Roccamonfina volcano, Roman comagmatic region, Italy. Earth Planet. Sci. Lett., 46: 81--106. Vollmer, R., 1976. Rb--Sr and U--Th--Pb systematics of alkaline rocks: the alkaline rocks from Italy. Geochim. Cosmochim. Acta, 40: 283--295. Vollmer, R., 1977. Isotopic evidence for genetic relations between acid and alkaline rocks in Italy. Contrib. Mineral. Petrol., 60: 109--118. Wood, D.A., 1979. A variably veined suboceanic upper mantle. Genetic significance for mid-ocean ridge basalts from geochemical evidence. Geology, 7: 499--503.
