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Journal of Petrology | Volume 45 | Number 1 | Pages 139-182 | 2004
© Oxford University Press 2004; all rights reserved
Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas
1 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DEGLI STUDI DI FIRENZE, VIA LA PIRA 4, I-50121, FIRENZE, ITALY
2 ISTITUTO DI GEOSCIENZE E GEORISORSE, C.N.R., SEZIONE DI FIRENZE, VIA G. LA PIRA, 4, I-50121, FIRENZE, ITALY
3 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DHI 3LE, UK
* Corresponding author: Telephone: +390552756224. Fax: +39055218628. E-mail: gperini{at}steno.geo.unifi.it
RECEIVED APRIL 24, 2002; ACCEPTED JULY 21, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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Vico volcano has erupted potassic and ultrapotassic magmas, ranging from silica-saturated to silica-undersaturated types, in three distinct volcanic periods over the past 0·5 Myr. During Period I magma compositions changed from latite to trachyte and rhyolite, with minor phono-tephrite; during Periods II and III the erupted magmas were primarly phono-tephrite to tephri-phonolite and phonolite; however, magmatic episodes involving leucite-free eruptives with latitic, trachytic and olivine latitic compositions also occurred. In Period II, leucite-bearing magmas (87Sr/86Srinitial = 0·71037–0·71115) were derived from a primitive tephrite parental magma. Modelling of phonolites with different modal plagioclase and Sr contents indicates that low-Sr phonolitic lavas differentiated from tephri-phonolite by fractional crystallization of 7% olivine + 27% clinopyroxene + 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at low pressure, whereas high-Sr phonolitic lavas were generated by fractional crystallization at higher pressure. More differentiated phonolites were generated from the parental magma of the high-Sr phonolitic tephra by fractional crystallization of 10–29% clinopyroxene + 12–15% plagioclase + 44–67% sanidine + 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Ti oxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial = 0·70812–0·70948) were derived from a potassic trachybasalt by assimilation–fractional crystallization with 20–40% of solid removed and r = 0·4–0·5 (where r is assimilation rate/crystallization rate) at different pressures. Silica-saturated magmas of Period II (87Sr/86Srinitial = 0·71044–0·71052) appear to have been generated from an olivine latite similar to some of the youngest erupted products. A primitive tephrite, a potassic trachybasalt and an olivine latite are inferred to be the parental magmas at Vico. These magmas were generated by partial melting of a veined lithospheric mantle sources with different vein–peridotite/wall-rock proportions, amount of residual apatite and distinct isolation times for the veins.
KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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Alkaline potassium-rich magmatism occurs in both extensional and convergent tectonic settings (e.g. Peccerillo, 1985
; Beccaluva et al., 1991; Edwards et al., 1991; Gibson et al., 1995; O'Brien et al., 1995; Carmichael et al., 1996; Rogers et al., 1998). In Italy, during Plio-Pleistocene times, potassic and ultrapotassic magmas were produced along with calc-alkaline and crustal derived magmas (Peccerillo, 1999, and references therein). This magmatism occurred in an area characterized by post-subduction tectonic regime, which post-dated the continental collision between the Adria and European plates (Patacca et al., 1990; Doglioni, 1991; Boccaletti et al., 1997; Doglioni et al., 1999, and references therein). The genesis and geodynamic implications of this magmatism are still a matter of debate, but large-scale mantle heterogeneity in its petrogenesis is widely accepted; north to south variations in the geochemical characteristics of most primitive potassic magmas have been related to lateral geochemical heterogeneity within a metasomatized mantle source (Conticelli et al., 2002, and references therein).
Vico volcano is located at the northern edge of the Roman Volcanic Province (Fig. 1) where it overlaps in space and time the volcanic products of the Tuscan Province to the north. Vico is one of a number of volcanoes in Italy, which erupted mainly ultrapotassic (K2O/Na2O > 2; MgO 
3 wt %; Foley et al., 1987) magmas during Neogene–Quaternary times. In the Vico area, an abrupt change in the petrology and chemistry of the erupted magmas is observed, passing from prevalently alkaline leucite-free magmas (lamproites, transitional olivine latites and shoshonites) in the north to mainly alkaline leucite-bearing magmas (high-K series, hereafter HKS) in the south (Conticelli & Peccerillo, 1992; Innocenti et al., 1992; Conticelli et al., 2002, and references therein). Among the potassic volcanoes in central Italy (Vico, Vulsini, Sabatini, Colli Albani), Vico is the smallest edifice with <100 km3 of volcanic products (Bertagnini & Sbrana, 1986) dispersed over an area of 784 km2.
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The volcanic sequence at Vico comprises mainly leucite-bearing rocks, although leucite-free magmas have been erupted during its history (Perini et al., 1997
). The observed rock types are mainly differentiated (phono-tephrite to phonolite and latite to high-K rhyolite) with primitive magmas (potassic trachybasalt and olivine latite) erupted only in the final stage of the volcanic activity. The different types of leucite-bearing and leucite-free magmas at Vico may be unrelated to each other, or may be derived from a common silica-undersaturated leucite-bearing parental magma through variable open-system differentiation processes (e.g. combined assimilation and crystal fractionation; AFC). The Vico volcano rocks partially overlie an older volcanic system, the Monte Cimino volcanic complex to the NE, which erupted primitive silica-saturated, leucite-free potassic and ultrapotassic rocks with shoshonitic to olivine latitic compositions. The occurrence of potassic and ultrapotassic magmas with very different petrological characteristics, which were erupted through spatially overlapping magma plumbing systems, but at different times, argues for the presence of a heterogeneous mantle source beneath Vico. Studying the petrogenesis and evolution of Vico magmas will help, therefore, to understand the nature of the upper mantle beneath this area. Major element, trace element and Sr, Nd and Pb isotope data are presented, which document a complex petrogenetic history for the ultrapotassic magmatism of Vico, and which suggest vertical upper-mantle heterogeneity beneath central Italy. |
GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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Vico volcano is located in central Italy, west of the Apennine thrust front, within the Siena–Radicofani–Cimino basin, one of a number of NW–SE-trending Messinian–Pleistocene extensional basins (Fig. 1; Barberi et al., 1994
). The Apennine chain is composed of east-vergent nappe structures produced during NW- to west-dipping subduction of the Adria lithosphere below the European lithosphere (Principi & Treves, 1984; Doglioni, 1991; Boccaletti et al., 1997; Doglioni et al., 1999). A continuous west-dipping subduction model is envisaged for the Apennine systems starting from Eocene–late Oligocene time up to the present (Jolivet et al., 1998; Doglioni et al., 1999). An increase in the angle of dip of the subducted slab is inferred with evolution of the Apennine subduction system, with eastward migration of the foredeep and thrust front (Doglioni et al., 1999). The geometry of the present-day subducted slab shows a steeply dipping Benioff Zone both in the northern Apennines (Selvaggi & Amato, 1992) and the southern Tyrrhenian Sea (Selvaggi & Chiarabba, 1995) sectors of the orogen, which is confirmed by mantle tomography studies (Spakman, 1991; Wortel & Spakman, 1992). The west-directed Apennine subduction system defines a mantle wedge, comprising both lithospheric and asthenospheric mantle domains, which, in the northern sector of the Apennines, may have been intersected by a previous eastward-dipping Alpine subduction zone (Doglioni et al., 1999). Contemporaneously with the eastward migration of the foredeep, a back-arc basin, the Tyrrhenian Sea, opened progressively from the Eocene–late Oligocene to the present (Faccenna et al., 1996, 1997). Subsequently, several Messinian–Pleistocene sedimentary basins, at present oriented north–south to NW–SE, developed along the Tyrrhenian Sea border of the Apennine chain (Jolivet et al., 1998, and references therein).
The substratum of Vico volcano and of the other Plio-Pleistocene volcanic edifices within the Roman Province includes Cretaceous–Oligocene flysch sequences and Mesozoic–Cenozoic carbonate–siliciclastic formations, which are intersected in several boreholes (Sollevanti, 1983; Buonasorte et al., 1987). At Torre Alfina, north of Vico, Mesozoic–Cenozoic carbonate formations including limestone and abundant marlstones, have been found in a borehole down to 3700 m (Buonasorte et al., 1991). Messinian–Middle Pliocene clays and sandstone filled the Siena–Radicofani–Cimino basin (Barberi et al., 1994). The Mesozoic–Cenozoic sedimentary cover lies on a Palaeozoic metamorphic basement of phyllites and quartz micaschists, which crop out 50 km to the NW (Monti Romani) of Vico (Sollevanti, 1983). Garnet micaschists, gneisses and granulites are found as xenoliths enclosed in lamproitic magma at Torre Alfina, north of Vico (Orlando et al., 1994).
Vico comprises a single major volcanic edifice from which most of the magmas were erupted, and a minor intracaldera cone, Monte Venere; three small monogenetic cones are present on the northern caldera rim and to the NE (Fig. 1). In contrast, most of the other potassic volcanoes in central Italy are composed of multiple volcanic edifices scattered over a large area (De Rita & Sposato, 1986; Nappi et al., 1987; Vezzoli et al., 1987; De Rita et al., 1995). The Vico volcanic sequence is composed of leucite-bearing and leucite-free lava flows and pyroclastic rocks erupted episodically between 419 and 95 ka (Perini et al., 1997, and references therein). The eruptive history of the volcano has been subdivided into three periods (Fig. 2; Perini et al., 1997). A schematic description of the eruptive history of Vico is presented in Fig. 2, based on previously published age determinations. Most of the available age data on Vico rocks have been determined by 40Ar/39Ar techniques on separated sanidine crystals (Laurenzi & Villa 1987; Barberi et al., 1994) or by K–Ar techniques on sanidine crystals and whole rocks (Sollevanti, 1983; Laurenzi & Villa, 1985). The ages of the main pyroclastic formations are well-constrained 40Ar/39Ar ages with relatively small errors (Laurenzi & Villa 1987; Barberi et al., 1994); additionally, the available K–Ar ages on lava flows are also precise (Sollevanti, 1983; Laurenzi & Villa, 1985). On the basis of the analytical errors provided along with the age data in the original papers we consider that we can determine the time elapsed between eruptions with some confidence.
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The volcanic succession of Period I activity (419–400 ka) is composed mainly of pyroclastic fall deposits (Rio Ferriera formation and S. Angelo tephra), with a minor lava flow episode (Petrignano lava), outcrops of which occur mainly to the north and east of the central crater (Fig. 1). At the beginning of Period II (305–138 ka), after about 100 kyr of quiescence, eruptive activity was dominated by leucite-bearing lava flows (
50 km3; Bertagnini & Sbrana, 1986), which built up the main cone (305–258 ka). At the end of Period II, voluminous leucite-bearing pyroclastic flows and minor fall deposits (in chronological order, from the oldest to the youngest: Farine formation, Sutri formation, Carbognano formation; Fig. 2) were erupted from the main Vico vent. Leucite-free pyroclastic flows, minor falls and lava flows (Ronciglione formation; 157 ka) lie between the Farine and Sutri formations. The largest volume eruption is that of the Sutri formation (10 km3; ‘Ignimbrite C’ of Bertagnini & Sbrana, 1986); the smallest those of the Ronciglione and Carbognano formations (0·5–1 km3; ‘Ignimbrite B’ and ‘Ignimbrite D’ of Bertagnini & Sbrana, 1986). Each pyroclastic flow eruption was accompanied by collapse of parts of the roof of the magmatic reservoir, resulting in the formation of a summit caldera. Post-caldera activity (Period III <138 ka) led to the emplacement of minor leucite-free lava flows (Poggio Nibbio lavas) followed by leucite-bearing scoriae falls (Poggio Nibbio tephra), erupted from monogenetic cinder cones. Subsequently, phreatomagmatic eruptions generated mainly surge deposits (Caprarola formation), which occur both inside and outside the caldera. These post-caldera deposits dominate the northern and eastern parts of the Vico edifice. A small intra-caldera cone of leucite-bearing lava flows formed the last activity of Vico (Monte Venere lavas; 95 ka). A detailed description of the individual formations of Vico volcano has been presented elsewhere (Perini et al., 1997); this forms the stratigraphic basis for the discussion of the geochemical and petrological data presented here. |
ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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From more than 200 rock samples collected from Vico volcano, a subset of 117 samples was selected for the present study. The selected samples are the best-preserved rocks and have been chosen to represent all the eruptions that occurred at Vico (Fig. 2). In most cases, more than one sample has been collected from the same eruption to show the potential intra-eruption compositional variation.
Samples were prepared by crushing in an agate mortar and powdered in an agate mill. The powder was subsequently pressed with a boric acid support to form a pellet for X-ray fluorescence (XRF) analysis. Clinopyroxene minerals were handpicked after magnetic separation for Sr isotope analysis.
Mineral composition data were obtained using a JEOL JXA-8600 electron microprobe at the Istituto di Geoscienze e Georisorse of CNR in Florence equipped with wavelength- and energy-dispersive spectrometers. The accelerating voltage was 15 kV and beam current 10 nA. The data were corrected using the Bence & Albee (1968) method. Accuracy was evaluated using international mineral reference samples as unknowns (Vaggelli et al., 1999). The relative error was <5% for major and minor elements.
Major elements and some trace elements (Rb, Sr, Ba, Pb, Zr, Nb, Y, Ni, Co, Cr) were determined on powder pellets by XRF at the Dipartimento di Scienze della Terra of the University of Florence. Matrix effects for major elements were corrected using the method of Franzini et al. (1972); the method of Jenkins & De Vries (1971) was used for the trace elements. Wet chemical techniques were used to determine Na, Mg and Fe2+. Loss on ignition (LOI) was determined after heating to 950°C. The concentrations of other trace elements [Th, Hf, Ta, Sc and rare earth elements (REE)] were obtained by instrumental neutron activation analysis (INAA) at the Dipartimento di Scienze della Terra of the University of Florence following the method of Poli et al. (1977). International and internal standards were used to evaluate accuracy and precision, which is better than 5% relative for Rb, Sr, Ba, Zr, Hf, Ta, Th, La, Ce, Eu, Sc, Cr, Co and Ni; better than 10% for Nb, Y, Yb, Nd and Sm; and better than 15% for Pb, Tb and Lu.
Sr and Nd isotope compositions of whole-rock and bulk-mineral separates were measured using dynamic mode on a seven-collector VG 54-30 sector mass spectrometer at the University of California, Los Angeles. The data were normalized to 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219, respectively. Repeated analyses of NBS 987 and La Jolla standards yielded values of 87Sr/86Sr = 0·710240 ± 30 (n = 11), and 143Nd/144Nd = 0·511862 ± 22 (n = 4), respectively. Total process blanks measured were <370 pg for Sr and <120 pg for Nd. Blank corrections were therefore negligible relative to the 25–73 µg of Sr and 2–5 µg of Nd analysed for the samples. Rb and Sr concentrations of bulk-mineral separates were determined by isotope dilution, adding to the sample a mixed Rb–Sr spike.
Pb was analysed on VG 54-30 multi-collectors in static mode using standard technique. Pb isotope data were corrected by 0·05
per a.m.u for mass fractiona-tion, as determined from measured values of NBS981 (208Pb/204Pb = 36·538, 207Pb/204Pb = 15·496, 206Pb/204Pb = 16·897; n = 2) relative to the accepted values of 208Pb/204Pb = 36·722, 207Pb/204Pb = 15·496, 206Pb/204Pb = 16·937. Although blanks were not run specifically in this study, Pb blanks in the UCLA laboratory are typically <500 pg (Bohrson & Reid, 1997).
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CLASSIFICATION OF ROCK TYPES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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Vico volcano erupted a range of magmas of variable composition from leucite-bearing, nepheline-normative melts to leucite-free, quartz- and orthopyroxene-normative melts (Barbieri et al., 1988
; Perini et al., 1997). According to the Le Bas et al. (1992) classification, the leucite-bearing rocks include potassic trachybasalts, phono-tephrites, tephri-phonolites and phonolites, whereas the leucite-free rocks are latites, trachytes and rare high-K rhyolites (Fig. 3). Among the phonolites, a few samples contain haüyne microphenocrysts instead of leucite; for simplicity, these have been included in the leucite-bearing group. Potassic trachybasalts, with groundmass leucite, have been erupted at Vico and they are described in subsequent sections as leucite-bearing rocks. On the basis of the presence of highly forsteritic equilibrium olivine phenocrysts, we refer to latites of Period III as olivine latites to highlight their primitive character (Perini & Conticelli, 2002).
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PETROGRAPHY AND MINERAL COMPOSITIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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Detailed petrographic descriptions and mineral composition data have been reported by Perini et al. (1997)
and Perini & Conticelli (2002). The petrographic characteristics of representative samples are summarized in Table 1.
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Most of the Vico rocks are holocrystalline; however, glassy groundmasses are present in the pyroclastic rocks. Phenocryst contents vary from 9 to 60 vol. %. Clinopyroxene is ubiquitous in all the studied samples. Minor olivine phenocrysts are generally present in the most mafic leucite-bearing and leucite-free rocks (potassic trachybasalt, phono-tephrite, tephri-phonolite and olivine latite). Plagioclase phenocrysts are present in all the samples with the exception of the potassic trachybasalts. The modal plagioclase content has been used to discriminate between two groups of phonolitic lavas in Period II. One group of phonolites has a plagioclase content <10 vol. % whereas a second group has plagioclase content >10 vol. %. Generally, leucite phenocrysts are present in all leucite-bearing rocks. Sanidine phenocrysts are abundant in the leucite-free rocks but are rare in the phonolites. Mica phenocrysts are present in the leucite-free rocks, excluding the olivine latites and some phonolites. Magnesiohastingsitic amphibole is restricted to some of the trachytes. Apatite and Fe–Ti oxides are the most common accessory minerals in all the rocks as microphenocrysts or inclusions in clinopyroxene and leucite. In some of the most differentiated leucite-bearing and leucite-free rocks titanite is found as an accessory phase. Zircon is also present in some trachytes and rhyolites. The groundmass is generally composed of the same mineral phases as observed as phenocrysts. In the potassic trachybasalts leucite and plagioclase are present in the groundmass and sanidine is present in the groundmass of the olivine latites.
Mg-chromite inclusions are present in olivine crystals in the latites and trachytes of Period II and in the olivine latites of Period III. Xenocrysts of olivine are present in the latites and trachytes of Period II. Xenocrysts of analcite occur in the latites of Period II. Sanidine megacrysts occur in phono-tephrites, tephri-phonolites and olivine latites of Periods II and III.
Representative analyses of the major mineral phases in the Vico rocks are presented in Tables 2 and 3; additional data have been published elsewhere (Perini & Conticelli, 2002). Olivine has a wide range of compositions (Fo91–24). Olivines in latites and potassic trachybasalts have forsterite-rich cores (olivine latites Fo84–91; potassic trachybasalts Fo84–88) in apparent chemical equilibrium with their host rocks. Fe/MgKD min/liq values for olivine cores in latites and trachytes of Period II, calculated using bulk-rock compositions, are significantly lower (0·10–0·22) than that expected for equilibrium liquidus olivine (0·25–0·38; Roeder & Emslie, 1970; Roeder, 1974; Ford et al., 1983; Takahashi & Kushiro, 1983). Clinopyroxene is diopside and rarely augite with Mg number [Mg/(Mg + Fe2+)] ranging from 0·93 to 0·56. Normal, reverse and oscillatory zoning is common. Plagioclase has a wide range of composition varying from An36 to An93. In the latites and trachytes of Period II, phenocrysts have detectable normal zoning (latite cores An82–72 and rims An39–57; trachyte cores An86–79 and rims An50–47) whereas the groundmass crystals have compositions between those of the phenocryst cores and rims. Sanidine has variable compositions (Or98–53) and phenocrysts are generally not zoned. Mica is phlogopite with Mg number varying from 0·95 to 0·62. Fe–Ti oxides are generally Ti-magnetite with ulvöspinel contents in the range 12–54 mol %, but Mg-chromite is present as olivine-hosted inclusions in the latites and trachytes of Period II and in the olivine latites of Period III. The Cr-number values [Cr number = (Cr/(Cr + Al)] of the Mg-chromite are between 0·53 and 0·61.
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MAJOR AND TRACE ELEMENT DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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Representative major and trace element analyses of Vico rocks are reported in Tables 4 and 5. The leucite-bearing samples, from potassic trachybasalt, phono-tephrite and tephri-phonolite to phonolite, are typically mildly to strongly silica-undersaturated, with CIPW normative ne (nepheline) between 4 and 14% (Table 4; Fig. 4). Latites, trachytes and rhyolites of Period I are mildly silica-saturated to silica-oversaturated and have CIPW normative hy (hypersthene; 1–7%) and qz (quartz; 2–22%). The olivine latites are silica-saturated with high CIPW normative hy (7–10%) whereas the latites and trachytes of Period II are, generally, mildly silica-saturated (hy 1–6%) to undersaturated (ne 1–7%) (Table 4; Fig. 4).
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Q [The most primitive rocks at Vico are olivine latites, which have the highest Ni and Cr contents (Ni 90–120 ppm; Cr 200–260 ppm), and potassic trachybasalts, which have similar or slightly lower contents (Ni 70–90 ppm; Cr 200–250 ppm) (Table 4). Both olivine latites and potassic trachybasalts have fosteritic olivine phenocrysts apparently in equilibrium with their host rocks. Following the classification scheme of Foley et al. (1987)
, the potassic trachybasalts are clearly ultrapotassic (MgO > 3 wt %, K2O > 3 wt %, K2O/Na2O weight ratio > 2), but the olivine latites have lower K2O/Na2O ratios close to that of potassic rocks (K2O/Na2O weight ratio 
2 at MgO > 3 wt % and K2O > 3 wt %) (Table 4; Fig. 4).
Figure 5 illustrates the variation of some major elements and Mg number [Mg number = 100Mg/(Mg + 0·85Fetot); Frey et al., 1978] against SiO2, which has been chosen as a differentiation index. The leucite-bearing and leucite-free rocks of Periods II and III define distinct coherent trends. In both cases, as SiO2 increases Al2O3, Na2O and K2O increase and CaO, TiO2, P2O5 and Mg number decrease. The trend of the leucite-free rocks of Periods II and III is displaced towards higher Mg number and lower K2O and Al2O3 contents than the trend for the leucite-bearing rocks (Fig. 5). Latites, trachytes and rhyolites of Period I define distinct trends on the Al2O3 and K2O vs SiO2 plots and have lower Na2O and Al2O3 contents than the younger rocks (Fig. 5).
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Trace element contents are also correlated with SiO2 content (Figs 6 and 7); different levels of enrichment are, however, evident at the same degree of differentiation (i.e. SiO2 content) (Figs 6 and 7). The two groups of phonolitic lavas of Period II, distinguishable on the basis of their modal plagioclase content (<10 vol. % and >10 vol. %; see ‘Petrography and mineral compositions’), are also distinct in terms of their trace element characteristics. They have similar SiO2 contents (56–58 wt %), but different Sr contents. Phonolites with >10 vol. % plagioclase have Sr <1330 ppm and are referred to subsequently as low-Sr phonolitic lavas. Those with <10 vol. % plagioclase have Sr >1600 ppm and are referred to as high-Sr phonolitic lavas (Figs 6 and 8; Table 4). On the other trace element diagrams (e.g. Ce, Zr, Th Nb, Ba and Rb) low-Sr and high-Sr phonolitic lavas are indistinguishable. In Fig. 6 is possible to distinguish a group of tephri-phonolites that has light REE (LREE; e.g. Ce) and high field strength elements (HFSE; e.g. Zr, Th and Nb) contents distinctly higher than those of the other tephri-phonolites. This group has Sr contents similar to those of the low-Sr phonolitic lavas and is referred to subsequently as low-Sr tephri-phonolitic tephra (LSrt; Fig. 6). The latter are exclusively represented by the tephra of the Farine formation (Fig. 2).
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In all the leucite-bearing rocks of Periods II and III, transition metals (Ni, Cr, Co) correlate negatively with SiO2 whereas LREE and HFSE correlate positively with SiO2 (Figs 6 and 7). Ba and Sr contents increase or remain constant and then decrease with increasing SiO2 content, but Rb contents tend to increase with SiO2 (Fig. 6). LREE (e.g. Ce), HFSE (e.g. Zr) and large ion lithophile elements (LILE; e.g. Ba, Th) variation trends within the leucite-bearing rocks of Period III are displaced toward lower values than those for the older leucite-bearing rocks (Fig. 6). Generally, similar variation trends are observed for the leucite-free rocks but latites and olivine latites of Periods II and III are displaced towards lower Ba and Sr contents than those of leucite-bearing rocks at similar SiO2 contents (Fig. 6).
The primordial mantle-normalized trace element patterns of the potassic trachybasalts (VCO 3 and VCO 20) and olivine latites (VCO 38, VCO 76 and VCO 155) are characterized by negative Ta, Nb and Ti anomalies (Fig. 9) similar to primitive potassic trachybasalts from the Vulsini volcanic district (‘leucite-basanites’ of Rogers et al., 1985; Conticelli & Peccerillo, 1992), primitive typical HKS rocks (tephrite and phono-tephrite) from the Sabatini volcanic district and Alban Hills (Conticelli & Peccerillo, 1992; Conticelli et al., 1997) and primitive leucite-free rocks from central Italy (Conticelli & Peccerillo, 1992), respectively (Fig. 9). Incompatible element enrichments in the Vico potassic trachybasalts are similar to those of potassic trachybasalts from Vulsini (Rogers et al., 1985; Conticelli & Peccerillo, 1992) but lower than those of typical primitive HKS rocks from the Sabatini volcanic district and Alban Hills (Conticelli & Peccerillo, 1992; Conticelli et al., 1997) (Fig. 9). Moderate differences exist in the shape of the trace element patterns of the potassic trachybasalts with respect to that of the primitive potassic trachybasalts from Vulsini and primitive HKS rocks. The potassic trachybasalts have a negative P anomaly, no Sr anomaly and lower LREE/HFSE ratios than the potassic trachybasalts from Vulsini and primitive HKS rocks (Fig. 9). For comparison, the normalized data for more differentiated leucite-bearing and leucite-free rocks of Vico are also shown in Fig. 9.
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Sr, Nd AND Pb ISOTOPE DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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The Sr, Nd and Pb isotopic compositions of the analysed samples are given in Table 6. The Vico rocks have highly radiogenic age-corrected 87Sr/86Sr (87Sr/86Srinitial = 0·70812–0·71169) and unradiogenic age-corrected 143Nd/144Nd (143Nd/144Ndinitial = 0·51210–0·51223) ratios. They plot in the ‘enriched’ quadrant of the conventional Sr–Nd isotope diagram and overlap the fields of other primitive potassic and ultrapotassic rocks from Italy (Fig. 10). A southward decrease of 87Sr/86Sr and contemporaneous increase of 143Nd/144Nd has been observed in Italian Plio-Pleistocene rocks (Conticelli & Peccerillo, 1992
; Conticelli et al., 2002). It is worth noting that the youngest Vico rocks (Period III) have the lowest 87Sr/86Sr values yet documented in leucite-bearing rocks from the central part of Italy (Fig. 10). 87Sr/86Srinitial continuously decreases as 143Nd/144Ndinitial increases from the oldest to the youngest rocks (Fig. 10). The leucite-free rocks of Periods II and III have similar 87Sr/86Srinitial (87Sr/86Srinitial = 0·71013–0·71052; Fig. 10, Table 6).
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87Sr/86Srinitial of clinopyroxene from phono-tephrites of Period I (87Sr/86Srinitial = 0·71138) and latite and trachyte of Period II (87Sr/86Srinitial in latite = 0·71051; 87Sr/86Srinitial in trachyte = 0·71055) are slightly lower and higher, respectively, than those of the host rocks (87Sr/86Srinitial phono-tephrite = 0·71168; 87Sr/86Srinitial latite = 0·71044; 87Sr/86Srinitial trachyte = 0·71048) (Table 6).
Considering all the Vico rocks, 87Sr/86Srinitial does not form a coherent trend with incompatible and compatible element contents (Fig. 11). In the leucite-bearing rocks of Period III it increases as Ni decreases and SiO2 and Th increase; the opposite trend seems to occur among the leucite-bearing rocks of Period II (Fig. 11).
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The Pb isotope compositions of Vico rocks are limited in their variability: 206Pb/204Pb ranges from 18·699 to 18·809, 207Pb/204Pb from 15·619 to 15·680, and 208Pb/204Pb from 38·910 to 39·047 (Table 6; Fig. 12). Pb isotope ratios overlap the field of Roman potassic rocks from Italy (Fig. 12) (D'Antonio et al., 1996
; Conticelli et al., 2002).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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At Vico, magmas with different chemical characteristics were periodically erupted and leucite-bearing magmas are intercalated with leucite-free magmas. This peculiarity is unique among the volcanoes of the Roman Province. This alternation may be either a primary characteristic, caused by different magma sources, or acquired during magma evolution processes (differentiation, AFC). Potassic silica-saturated and silica-undersaturated magmas have been produced experimentally by melting appropriate mantle compositions at low and high pressure, respectively (Melzer & Foley, 2000
). On the other hand, the dominance of differentiated magmas at Vico provides evidence that magmatic differentiation processes at low pressure have had an important role. 87Sr/86Srinitial and 143Nd/144Ndinitial isotope compositions suggest that the more mafic and differentiated Vico rocks are not genetically related by simple fractional crystallization (Fig. 10). The variation of trace element concentrations with 87Sr/86Srinitial reveals that more complex magmatic differentiation processes have been involved in the petrogenesis of the Vico magmas (Fig. 11). Assimilation of local crustal materials cannot genetically link the leucite-bearing potassic trachybasalts to the primitive leucite-free olivine latites. The process of assimilation of sialic crustal rocks by basaltic magmas typically leads to an increase in 87Sr/86Sr and a concomitant decrease in compatible elements such as Ni, removed in crystallizing phases such as olivine. In fact, 87Sr/86Srinitial and Ni contents both increase from potassic trachybasalt to olivine latite (Fig. 11). On the other hand, the geochemical similarity of the leucite-free rocks of Periods II and III could suggest that they are genetically linked (Figs 9 and 10).
Period I
Magma evolution during Period I of Vico activity has been considered by Perini et al. (2000). Those workers demonstrated that the first erupted leucite-free magmas from Vico differentiated primarily by fractional crystallization dominated by 18–43 vol. % plagioclase + 0–64 vol. % sanidine + 8–29 vol. % phlogopite + 5–16 vol. % clinopyroxene + 4–9 vol. % Fe–Ti oxides + 1–3 vol. % apatite from latites to trachytes and by 65 vol. % sanidine + 21 vol. % plagioclase + 7 vol. % phlogopite + 4 vol. % Fe–Ti oxides + 1 vol. % apatite ± 1 vol. % titanite ± 0·1 vol. % zircon from trachytes to high-K rhyolites. Toward the end of this period a leucite-bearing mafic magma refilled the shallow magma reservoir occupied by residual high-K rhyolitic magmas. This resulted in continuous mixing of mafic leucite-bearing and residual differentiated leucite-free magmas and subsequent fractional crystallization (MFC) of 25% phlogopite + 20% clinopyroxene + 13% plagioclase + 3% sanidine + 2% Fe–Ti oxides + 1% apatite. MFC processes have produced the youngest trachyte of Period I and the oldest leucite-bearing latite of Vico.
Period II
Period II began with the eruptions of leucite-bearing lavas (Lago di Vico lava formation), which were piled up to form the original Vico cone. Leucite-free magmas were erupted as pyroclastic rocks and lavas during the caldera-forming phase and they are intercalated between leucite-bearing lavas and tephra. Most of the leucite-bearing rocks of Period II are phonolites. Mafic rocks are rare and represented by phono-tephrites and tephri-phonolites (Fig. 3). 87Sr/86Srinitial of tephri-phonolites are variable (0·71115–0·71057) and in some case higher than those of the phonolites (0·71086–0·71049) (Table 6; Fig. 11). This suggests that evolution from a parental, mafic, leucite-bearing magma occurred via one or more open-system differentiation processes.
Differentiation processes in leucite-bearing magmas
From Fig. 11 is evident that for a similar degree of differentiation leucite-bearing rocks with different Sr isotope compositions are present. In particular, there is a group of rocks with high 87Sr/86Srinitial (>0·7108) and a second one with lower 87Sr/86Srinitial (<0·7106). To understand the genetic link between these two leucite-bearing groups with different 87Sr/86Srinitial it is important to know the chronology of the magmatic events. The group with high 87Sr/86Srinitial are all lava flows (Lago di Vico lava formation), which represent an early event of Period II, whereas the group with low 87Sr/86Srinitial are pyroclastic rocks (Farine, Sutri and Carbognano formations) and rare lavas (see sample VCO 67) intercalated between the pyroclastic deposits, and they all represent a late stage of Period II (Table 6 and Fig. 2 for stratigraphic locations).
The high-Sr phonolitic lavas represent the oldest lava flows erupted during Period II (Fig. 2; Table 4). These phonolites are characterized by a paucity of modal plagioclase but high Sr contents (Fig. 8). In addition, they have Sr isotope ratios (87Sr/86Srinitial = 0·71086) significantly lower than those of tephri-phonolite (87Sr/86Srinitial = 0·71115) (Table 6), precluding a genetic link with the latter by simple fractional crystallization.
Subsequent to the high-Sr phonolitic lavas, phonotephrites (VCO 123), less differentiated tephri-phonolites (e.g. VCO 99), more differentiated tephri-phonolites (e.g. VCO 119) and low-Sr phonolitic lavas (e.g. VCO 118) were erupted (Fig. 2). These rocks have high and nearly constant 87Sr/86Srinitial (Table 6) and in an Sr vs Mg number diagram they form a path of constant and then decreasing Sr content with decreasing Mg number (see differentiation path 1 in Fig. 13). These data suggest that at this stage of Vico magmatic history phono-tephrite, tephri-phonolites and low-Sr phonolitic lavas were related by fractional crystallization, possibly of olivine, clinopyroxene and plagioclase phenocrysts, which are present in all these rocks (Table 1).
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As already mentioned, low-Sr phonolitic lavas have higher plagioclase and lower Sr contents than high-Sr phonolitic lavas (Fig. 8). These characteristics suggest that during the early Period II (Lago di Vico lava formation) evolution of magmas in the magma chamber/s occurred at different total pressure conditions. High total pressure in basaltic systems lowers plagioclase stability and promotes clinopyroxene crystallization (Thy, 1991
; Sisson & Grove, 1993, and references therein). The inversely correlated Ca and Fe trends with decreasing Mg number in clinopyroxene from the low-Sr phonolitic lavas have been interpreted as evidence of protracted plagioclase crystallization and lower-pressure crystallization conditions than the conditions of differentiation of the high-Sr phonolitic lavas (Perini & Conticelli, 2002). Therefore, in the early Period II, we consider that mafic leucite-bearing magma differentiated to produce high-Sr phonolitic lavas in a high total pressure regime, in which plagioclase crystallization was depressed. On the other hand, fractionation of plagioclase may easily occur at low pressure, driving the composition of mafic magmas toward the low-Sr phonolitic lavas. After the stage of piling up of lava flows (cone-building phase; Lago di Vico Lava formation), the volcanic system began a phase of highly explosive activity with the emplacement of several pyroclastic units, each related to a caldera-forming event. Minor lava flows are, however, still present, intercalated with the main pyroclastic units. This phase is still characterized by leucite-bearing rocks, but a temporarily restricted intercalation of leucite-free rocks (pyroclastic rocks and lavas) is also found (Fig. 2).
The first leucite-bearing pyroclastic event (Farine formation) is represented by a low-Sr tephri-phonolitic tephra (e.g. VCO 88), followed by a phonolitic lava flow (e.g. VCO 67; Casale della Montagna lavas) (Fig. 2), whereas the last leucite-bearing eruptions are represented by the phonolitic tephra of the Sutri formation (e.g. VCO 167, VCO 169) and Carbognano formation (e.g. VCO 114) (Fig. 2). Among these formations the Sutri formation is the only one showing a significant chemical zonation trend in terms of trace element contents. In particular, Sr (and also Nb, Zr, Ce) shows a clear bimodal distribution (Fig. 13; Table 4), with less silica-rich tephra (SiO2 on water-free basis <58·6%) having much higher Sr contents (high-Sr phonolitic tephra) than more silica-rich tephra (silica on water-free basis >58·6%). In Fig. 13 the high-Sr phonolitic tephra of the Sutri formation (e.g. VCO 169) have an Sr content higher than that of the low-Sr tephri-phonolitic tephra of the Farine formation. Despite the temporal compositional differences between the Farine formation and the Carbognano formation, and those within the Sutri formation, 87Sr/86Srinitial values do not show significant variations. These values are lower than the 87Sr/86Srinitial values of the lavas of Lago di Vico lava formation (Table 6). This suggests closed-system evolution of the magmas generating the leucite-bearing tephra and intercalated lavas. However, the strong enrichment in several incompatible trace elements, such as Ce, Zr, Th, Nb, etc. (Table 4), in the low-Sr tephri-phonolitic tephra (Farine formation) with respect to that in the high-Sr phonolitic tephra (Sutri formation), cannot be accounted for simply by fractional crystallization of the main mineral phases observed in these rocks (Table 1). In addition, the presence of leucite-free rocks intercalated with leucite-bearing rocks of this phase, which have different compositional characteristics (Table 4), argues against a simple process of protracted crystal fractionation. On the other hand, considering the sequence of magmatic events and the correlation between Sr contents and Mg number, we can argue that fractional crystallization has been responsible for generating the phonolitic Casale della Montagna lavas from low-Sr tephri-phonolitic magmas (Farine formation) (Fig. 2, and differentiation path 2 in Fig. 13). The same consideration suggests that the differentiated phonolites of the Sutri and Carbognano formations might have differentiated from the parent magma of the high-Sr phonolitic tephra (Fig. 2, and see differentiation path 3 in Fig. 13). In both cases the main fractionating mineral assemblage might be clinopyroxene, plagioclase, sanidine and phlogopite, which represent the main phenocrysts in both the low-Sr tephri-phonolitic tephra and high-Sr phonolitic tephra (Table 1).
Fractional crystallization modelling in leucite-bearing magmas
To test fractional crystallization in those cases suggested above, mass-balance calculations (Stormer & Nicholls, 1978) were performed (Table 7) using as parent magmas a less differentiated tephri-phonolite (VCO 99) for differentiation path 1, a low-Sr tephri-phonolitic tephra (VCO 88) for differentiation path 2, and a high-Sr phonolitic tephra (VCO 169) for differentiation path 3 (Table 4; Fig. 13).
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Differentiation path 1 reflects the fractional crystallization process that occurred during the cone-building phase (Lago di Vico lava formation). This has been modelled in two steps: the first from less differentiated tephri-phonolite (VCO 99) to a more differentiated tephri-phonolite (VCO 119) and the second from the latter to a low-Sr phonolite (VCO 118) (Table 4).
Differentiation path 2 reflects the fractional crystallization process that occurred during the progression from the Farine formation to the Casale della Montagna lavas. This has been modelled from a parent low-Sr tephri-phonolite (VCO 88) to a daughter phonolite (VCO 67) (Table 4).
Differentiation path 3 reflects the fractional crystallization process during the late stage of volcanic activity between the Sutri and Carbognano formations. This has been reproduced using, as parent, the high-Sr phonolitic tephra (VCO 169) and, as daughters, two more differentiated phonolites (VCO 167 and VCO 114) (Table 4).
The compositions of mineral phases from the different parents and daughters have been used to construct the models. The results of these calculations are reported in Table 7. All models yield low residuals (
R2 
0·8) (Table 7) suggesting that fractional crystallization is, in principle, a viable process of magmatic evolution. The mass of solid phases that have fractionated from the parental magmas to yield the derivative magmas varies from 10 to 34%, and the extract is generally composed of clinopyroxene, plagioclase, apatite, and, in some cases, Fe–Ti oxides, ± olivine, ± sanidine, ± phlogopite (Table 7). Olivine crystallized only along differentiation path 1 (Fig. 13) from less differentiated tephri-phonolite to more differentiated tephri-phonolite (23% of total solid) and low-Sr phonolitic lavas (7% of total solid). Sanidine (44–67%) and phlogopite (1–4%) fractionated along differentiation paths 2 and 3 (Fig. 13).
These crystal fractionation processes for paths 1, 2 and 3 (Fig. 13) have also been tested for the trace element data using the Rayleigh crystal fractionation equation [CL/CO = F(D - 1)] (Table 8), with the liquid fraction (F) value and proportions of minerals taken from the least-squares major element models of Table 7, and using the lowest and highest solid–liquid partition coefficients (KD) for similar whole-rock compositions from the literature (i.e. Higuchi & Nagasawa, 1969; Schnetzler & Philpotts, 1970; Goodman, 1972; Kyle & Rankin, 1976; Sun & Hanson, 1976; Leeman et al., 1978; Larsen, 1979; Le Roex, 1980; Villemant et al., 1980; Wörner et al., 1983; Fujimaki, 1986; Francalanci et al., 1987; Lemarchand et al., 1987; Foley et al., 1996). In all models under consideration the depletion or enrichment factors (CL/CO) for different trace elements in the derivative magmas (CL) with respect to the parental magmas (CO) are typically within the range of those calculated (Table 8). In some cases, along differentiation path 1 the measured enrichment of Rb (step from VCO 119 to VCO 118) is higher than that calculated (Table 8). The low-Sr phonolitic lavas (VCO 118) have wide range of Rb contents (Rb 688–909 ppm) associated with a high modal content of leucite (modal leucite is 74–84 vol. % of phenocrysts). Leucite has never been observed as a fractionating mineral in the leucite-bearing rocks at Vico; indeed Rb, which is preferentially incorporated into leucite (Francalanci et al., 1987), always increases during differentiation (Fig. 6). In the low-Sr phonolitic lavas accumulation of leucite phenocrysts might have occurred as a result of gravitational settling of more dense mineral phases causing the Rb content to be higher than predicted. Thus, the high Rb enrichment factor from VCO 119 to VCO 118 might be related to the accumulation of leucite.
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Along differentiation path 2 (from VCO 88 to VCO 67) the calculated enrichment factors generally agree with those measured (Table 8). However, lower measured enrichment factors for Rb and Th than those calculated have been observed (Table 8). Low-Sr tephri-phonolitic tephra (VCO 88) contain phenocrysts of leucite altered to analcite. During weathering of leucite to analcite an increase in Rb has been observed (Giannetti & Masi, 1989
). Thus it might be that weathering of leucite in the low-Sr tephri-phonolitic tephra has concentrated Rb, thus imparting an anomalous high Rb content to the parental magma (CO) and a lower measured enrichment factor than the real one. Th has a high KD for apatite and titanite (Wörner et al., 1983); thus either fractionation of titanite or of a higher abundance of apatite than that obtained by least-squares calculation might produce a lower calculated enrichment factor for Th. In spite of abundant sanidine fractionation (Table 7), the negative Eu anomaly (Eu/Eu*) does not increase from low-Sr tephri-phonolitic tephra to phonolite (Table 5); Eu typically has a high KD in sanidine relative to the other REE (Francalanci et al., 1987), thus Eu/Eu* might be expected to increase from VCO 88 to VCO 67. However, oxygen fugacity influences the oxidation state of Eu and thus the tendency for Eu to partition into feldspar minerals (Cox et al., 1979). High oxygen fugacity can elevate Eu to the 3+ oxidation state, thus preventing preferential partitioning of Eu from the other REE in feldspar. Conditions of high oxygen fugacity might have prevailed during magma crystallization from the low-Sr tephri-phonolite to the phonolite VCO 67.
Even though fractional crystallization was certainly the main differentiation process along differentiation paths 1, 2 and 3, 87Sr/86Sr ratios decreased slightly in the last erupted and more differentiated phonolites of each differentiation path (Table 7). These Sr isotope variations can be explained by small amounts of assimilation of carbonate rocks, which are present in the Vico shallow substratum (Sollevanti, 1983; Buonasorte et al., 1987; Barberi et al., 1994). Italian Mesozoic–Cenozoic carbonate rocks have mean 87Sr/86Sr and Sr contents of 0·7075–0·7077 and 645–563 ppm, respectively (Cortini & Don Hermes, 1981; Conticelli, 1989; Conticelli et al., 2001, 2002). A small amount of carbonate assimilation associated with fractional crystallization with high DSr (>1) and low r values (where r is assimilation rate/crystallization rate) is able to reproduce the decrease of 87Sr/86Sr and Sr concentrations along the most differentiated part of the differentiation paths. Alternatively, these small 87Sr/86Sr decreases could be caused by mixing with a low 87Sr/86Sr magma intruded into the base of the shallow magma chamber (MFC processes).
Leucite-free magmas
Leucite-free magmas occur rarely in the late stage of Period II, intercalated between leucite-bearing rocks (Fig. 2). On the basis of the Melzer & Foley (2000) experimental study, it is possible that silica-saturated differentiates (e.g. trachytes) could be derived at high pressure (1·8 GPa) from silica-undersaturated parental magmas through simple fractional crystallization of phlogopite and clinopyroxene. During Period II, leucite-free rocks are represented by trachytes and latites, which have chemical and mineralogical characteristics very similar to the olivine latites of Period III. Both latites and trachytes (Period II) and olivine latites (Period III), have distinctive Mg-chromite inclusions in the olivine phenocrysts, which none of the other leucite-bearing rocks (potassic trachybasalts, phono-tephrite, tephri-phonolite) have. Latites and trachytes (Period II) have high Mg number, Ni and Cr contents (Figs 5 and 7); latites have Sr and Ba contents lower than those of other rocks at similar SiO2 contents (Fig. 6); 87Sr/86Srinitial of latites and trachytes are broadly similar (Table 6; latite, 87Sr/86Srinitial = 0·71044; trachytes, 87Sr/86Srinitial = 0·71048–0·71052). Characteristics similar to those of the latites of Period II are observed in Period III olivine latites, which have slightly lower Sr contents and 87Sr/86Srinitial than those of Period II latites (87Sr/86Srinitial = 0·71013–0·71021; Fig. 11). However, preliminary modelling to produce trachyte and latite starting from a leucite-bearing magma failed to give reasonable results. Thus the leucite-free magmas of Period II are more likely to be genetically related to the leucite-free magmas of Period III rather than any other leucite-bearing magma. Therefore, the genesis and differentiation of the leucite-free rocks will be discussed separately in a subsequent section.
Period III
Differentiation processes in leucite-bearing magmas
The leucite-bearing rocks of Period III have the lowest 87Sr/86Srinitial of all of the Vico leucite-bearing rocks (Fig. 11) and are less enriched in LREE (e.g. Ce), HFSE (e.g. Zr) and LILE (e.g. Ba, Rb, Sr) than the older leucite-bearing rocks (Fig. 6; Tables 4 and 5). The most primitive leucite-bearing magmas were erupted from Vico during the youngest phase of activity. The range of major and trace element variation within the leucite-bearing rocks erupted within this period suggests that the magmas differentiated from a potassic trachybasaltic parent magma (Figs 5–7). Increasing 87Sr/86Srinitial is coupled with increasing incompatible element contents (e.g. Th, Zr), decreasing Mg number and compatible element contents (e.g. Ni), and variable Sr contents (Figs 11 and 14). These characteristics suggest that assimilation of continental crustal material associated with fractional crystallization (AFC) was involved in the differentiation of the leucite-bearing magmas of Period III.
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The oldest leucite-bearing magma erupted during Period III is represented by the potassic trachybasalts of the Poggio Nibbio tephra (Fig. 2); the latites and tephri-phonolites of the Caprarola formation followed this eruption, whereas the phonolites and, subsequently, tephri-phonolites of the Monte Venere lavas were the last erupted products of Vico volcano (Fig. 2).
From the trachybasalts of Poggio Nibbio to the tephri-phonolites of the Caprarola formation Sr contents remain constant with 87Sr/86Srinitial increasing (Fig. 14). From trachybasalt to tephri-phonolite of the Monte Venere lavas Sr content increases with increasing 87Sr/86Srinitial (Fig. 14). The phonolites of Monte Venere lavas have the highest 87Sr/86Srinitial among the leucite-bearing rocks of Period III (Fig. 14). Both tephri-phonolites (Caprarola formation and Monte Venere lavas) have similar 87Sr/86Srinitial ratios (Caprarola formation, 87Sr/86Srinitial = 0·70907; Monte Venere lavas, 87Sr/86Srinitial = 0·70898; Table 6) and have plagioclase phenocrysts in the equilibrium mineral assemblage (Table 1). The Eu/Eu* of the Caprarola formation tephri-phonolites (Eu/Eu* = 0·72) is lower than that of the Monte Venere lavas tephri-phonolite (Eu/Eu* = 0·86) (Table 5). Fractional crystallization processes cannot generate Monte Venere lavas tephri-phonolite from a parental Caprarola formation tephri-phonolite because plagioclase should have fractionally crystallized producing an Sr decrease. Plagioclase-bearing phonolites of the Monte Venere lavas (Table 1) cannot differentiate from a tephri-phonolite of the Caprarola formation by AFC processes because from the tephri-phonolite to the phonolites the 87Sr/86Srinitial increase is coupled with Sr increase (Fig. 14). On the contrary, AFC processes from a plagioclase-bearing tephri-phonolite, such as that of the Caprarola formation, would have decreased the Sr content in the derivative magma.
On the basis of the petrography, geochemical composition and chronology of the erupted magmas, it seems likely that the tephri-phonolites of the Caprarola formation are produced from the potassic trachybasalts by assimilation of continental crust associated with fractional crystallization of mafic phases and plagioclase. Subsequently, the tephri-phonolite of the Monte Venere lavas seems to have been formed by an AFC process starting from a potassic trachybasalt; however, in this case, fractional crystallization of plagioclase did not occur. The correlation of Sr content and compatible and incompatible trace element contents with 87Sr/86Srinitial from the tephri-phonolite of the Monte Venere lavas to the phonolite of the same formation might be explained by assimilation of continental crust associated with fractional crystallization of mafic mineral phases and plagioclase. To test this hypothesis, AFC modelling using DePaolo's (1981) equation is illustrated in Fig. 14. The average composition of the metamorphic basement beneath central Italy from the literature has been chosen as contaminant (Conticelli, 1989, 1998; Conticelli et al., 2002).
Magmatic evolution from the potassic trachybasalts to the tephri-phonolites of the Caprarola formation can be modelled using a high bulk partition coefficient for Sr (DSr) of 0·7 and a low r value of 0·4 (Fig. 14; LP, low-pressure regime). The high DSr of 0·7 is based on fractionation of plagioclase associated with mafic minerals (30% olivine + 40% clinopyroxene + 25% plagioclase + 4% Fe–Ti oxides + 2% apatite). Using a low DSr of 0·2, based on crystallization of mainly mafic mineral phases (12% olivine + 85% clinopyroxene + 3% Fe–Ti oxides), a high r value of 0·8 would be necessary to produce the tephri-phonolite of the Caprarola formation from the potassic trachybasalts (Fig. 14; HP, high-pressure regime). A value of r of 0·8 is extremely high, especially considering that the tephri-phonolites of the Caprarola formation are silica-undersaturated magmas. This suggests that the variations of Sr and other incompatible trace elements and of 87Sr/86Srinitial observed from the potassic trachybasalts to the tephri-phonolites of the Caprarola formation are more likely to be explained by an AFC process with DSr of 0·7, an r value of 0·4 and a moderate degree of fractionation (solid fraction removed 40%).
Evolution of the tephri-phonolite of the Monte Venere lavas from parental trachybasalt has been modelled with DSr of 0·2, an r value of 0·5 and a low amount of fractionation (S = 20–30%) (high-pressure regime in Fig. 14). The differentiation of tephri-phonolite to phonolite of the Monte Venere lavas has been modelled by AFC using a high DSr of 1·0 (11% olivine + 35% clinopyroxene + 40% plagioclase + 10% Fe–Ti oxides + 4% apatite) and low r value (r = 0·4) and amounts of fractionated solid (S = 20–30%) (low-pressure regime in Fig. 14).
The different DSr values (0·7 and 0·2) used for deriving the tephri-phonolites of the Caprarola formation and Monte Venere lavas from the parental potassic trachybasalts may reflect AFC at different pressures. Conditions of high total pressure regime in the magma chamber during the evolution from trachybasalt to Monte Venere lavas tephri-phonolite might have reduced plagioclase stability (low DSr), whereas low total pressure might have prevailed during the evolution from trachybasalt to the Caprarola formation tephri-phonolite, producing the high DSr as a result of significant plagioclase fractionation. The presence of shallow-derived lithic fragments (lavas and Pliocene sediments) in the post-caldera tephra of the Caprarola formation (`Tufi Finali' of Bertagnini & Sbrana, 1986) suggests the hypothesis of shallow magma storage.
Low-pressure AFC processes have been also evaluated considering as contaminant local Mesozoic–Cenozoic carbonate and marl rocks. Values of 87Sr/86Sr and Sr content of carbonate formations in central Italy are in the range 0·7075–0·7077 and 645–563 ppm, respectively, and of marl formations around 0·7112 and in the range 983–339 ppm, respectively (Cortini & Don Hermes, 1981; Conticelli, 1989; Conticelli et al., 2001, 2002). Neither assimilation of carbonate nor marl rocks are able to reproduce the magmatic differentiation trend from tephri-phonolite to phonolite in the Monte Venere lavas because they are unable to generate the required increase in 87Sr/86Srinitial. In fact, the tephri-phonolites of the Monte Venere lavas, because of their Sr content, which is higher than those of carbonate or marl (Table 4), should be resistant to the effects of contamination. This suggests that carbonate or marl rocks do not represent a significant contaminant in the low-pressure AFC processes of the leucite-bearing rocks of Period III.
In summary, the observed trace element and isotopic variations within the leucite-bearing magmas of Period III can be explained by assimilation plus polybaric fractional crystallization, high-pressure AFC (HP AFC) and low-pressure AFC (LP AFC).
Differentiation processes in leucite-free magmas
The petrography and mineral chemistry of the leucite-free latites and trachytes of Period II clearly suggest that these magmas are hybrids (Perini & Conticelli, 2002). Forsteritic olivine cores in the latites and trachytes are not in equilibrium with their hosts because Fe/MgKD min/liq values are lower (Fe/MgKD min/liq = 0·25–0·38; Roeder & Emslie, 1970; Roeder, 1974; Ford et al., 1983; Takahashi & Kushiro, 1983) than those found in equilibrium phenocrysts (latite: 0·14–0·22, trachyte: 0·06–0·1; Perini & Conticelli, 2002). In the latites and trachytes, clinopyroxene and plagioclase phenocrysts show reverse and direct abrupt (An variation >10%) zoning (latite An82–39, trachyte An86–45; Perini & Conticelli, 2002). Abrupt anorthite variation has been interpreted as the result of reaction between an An-rich plagioclase, crystallized from a mafic magma with high CaO/Na2O ratio, and a felsic melt with lower CaO/Na2O ratio, forming An-poor rims. Furthermore, in the latites xenocrysts of analcite are present, possibly derived from the alteration of leucite crystals, as observed in other potassic rocks from Italy (Giannetti & Masi, 1989). All these petrographic and mineral chemistry characteristics indicate an interaction between mafic melts and more differentiated magmas.
The petrography and mineral chemistry data suggest that the latites and trachytes of Period II are produced by magma mixing or mingling events. These data, together with the geochemical data, suggest that in the mixing or mingling processes a mafic (high Mg number, Ni and Cr content) magma containing olivine phenocrysts with Mg-chromite inclusions, possibly represented by a leucite-free magma such as the olivine latite magma of Period III, mixed with a more differentiated magma. The degree of silica saturation in the latites and trachytes varies from mildly silica-saturated (hy 1–6%) to silica-undersaturated (ne 1–7%) (Table 4), suggesting that one of the two components of the mixing or mingling processes, the differentiated end-member, is represented by a leucite-bearing magma. On the basis of the presence of analcite and the major and trace element chemistry, the differentiated liquid might be a differentiated leucite-bearing phonolite with a low Sr content.
Modelling mixing process
To model the mixing process forming the trachytes we chose a phonolite, VCO 67 (Casale della Montagna lavas, Fig. 2), which was erupted slightly before the trachytic magma (Table 4) and, as the mafic end-member, an olivine latite (VCO 38; Poggio Nibbio lavas, Fig. 2) of Period III (Fig. 15). Mass balance calculations were performed to test such a two-end-member mixing model; the results for major and trace elements are presented in Fig. 16. The model illustrates that, excluding some incompatible and compatible trace elements (Zr, Rb, Th and Sr), which plot outside the mixing trend, mass balance calculations support the hypothesis that the trachytes have been produced by magma mixing (Fig. 16). Bulk mixing of about 20% of an olivine latite component and 80% of a phonolitic component is able to generate the trachytes (Fig. 16). The higher Zr, Rb and Th, and lower Sr contents of the trachyte than those predicted by mass balance calculation suggest that the chosen phonolite might not be the correct end-member, and that a more differentiated phonolite might be more appropriate. We back-calculated the Zr, Rb, Th and Sr contents of the hypothetical phonolite by mass balance using the value of the slope (x) in Fig. 16. This hypothetical phonolite has higher Zr, Rb and Th contents (Zr 1072 ppm, Rb 831 ppm, Th 310 ppm) and lower Sr content (433 ppm) than VCO 67 (Table 4). None of the phonolites analysed at Vico volcano have these precise characteristics; however, the Zr, Rb, Th and Sr contents of the hypothetical phonolite are very close to those of the more differentiated Vico phonolites. On the other hand, the lack of agreement only for Zr, Rb, Th and Sr contents with mass balance calculations using VCO 67 is because these trace elements in trachytes are the most incompatible and compatible elements, respectively. Therefore, only a small amount of fractional crystallization would be required to change their contents slightly in the product of mixing.
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The latites of Period II may also be hybrid magmas because their olivine phenocrysts are actually xenocrysts from a more mafic magma (Perini & Conticelli, 2002
), and analcite xenocrysts are also present. We propose that the latites are generated by the same mixing process as the trachytes, where the mafic end-member was mafic and leucite-free, having olivine with Mg-chromite inclusions, and the differentiated end-member was a phonolite. The olivine-bearing magma might be a melt similar in composition to the olivine latites of Period III. Nevertheless, such a simple mixing model cannot explain the increase of Sr content from the olivine latites to the latites (Fig. 15). Thus, it is suggested that the latites originated by mixing plus fractional crystallization starting from an olivine-bearing, mafic, end-member.
Mafic magmas at Vico
Distinct differentiation paths from different parental magmas involving fractional crystallization, crustal assimilation plus polybaric crystallization and magma mixing have been identified and described above. The possible genetic relationships between the parental magmas of these differentiation paths are more difficult to constrain. Sr and Nd isotope ratios cannot uniquely reveal the primitive magmas at Vico, because the K-rich magmas in Italy were probably generated from a heterogeneous metasomatized lithospheric mantle source with variably high 87Sr/86Sr and low 143Nd/144Nd (Hawkesworth & Vollmer, 1979; Conticelli & Peccerillo, 1992; Di Battistini et al., 1998; Conticelli et al., 2002).
Petrographic and geochemical data indicate that the most primitive magmas at Vico are the potassic trachybasalts and olivine latites of Period III (Tables 2 and 4; Perini & Conticelli, 2002). The potassic trachybasalts have generated the leucite-bearing magmas of Period III by AFC processes. AFC processes starting from potassic trachybasalts might also be responsible for the genesis of parental leucite-bearing magmas such as those of Periods I and II. Using a low DSr value (0·2), necessary to account for the higher Sr concentration of the parental leucite-bearing magmas of Periods I and II than that of the potassic trachybasalt (Table 4), high r values (0·6–0·8) are required to explain the Sr isotope compositions of the older leucite-bearing magmas. Starting with a potassic trachybasaltic melt, a low degree of fractionation (S = 20–30%; r = 0·8) is required to reach the tephri-phonolites of Period II, but a higher degree (S = 50–60%; r = 0·6) to reach the high-Sr phonolitic lavas (crust composition Sr 190 ppm and 87Sr/86Sr = 0·7244; Conticelli, 1989, 1998; Conticelli et al., 2002). However, the Vico leucite-bearing magmas are silica-undersaturated magmas with broadly similar degrees of silica undersaturation. This suggests a moderate rate of assimilation and therefore low r values. For this reason, we prefer the hypothesis that the leucite-bearing magmas of Period II were generated from a silica-undersaturated parent magma, which had a higher 87Sr/86Sr ratio than that of the potassic trachybasalts of the Vico Period III. The higher incompatible element contents (LILE and LREE) of the leucite-bearing rocks of Period II with respect to those of Period III (Fig. 6) also suggest that the parental melt of the leucite-bearing magmas of Period II was more enriched in incompatible elements than the potassic trachybasalts. In the neighbouring volcanoes of central Italy (e.g. Vulsini, Sabatini Volcanic Districts, Alban Hills) leucite-bearing primitive magmas have also been erupted (Rogers et al., 1985; Conticelli & Peccerillo, 1992; Conticelli et al., 1997). Some of these, with tephritic compositions and typical HKS characteristics, from Sabatini and the Alban Hills have LREE and LILE contents (e.g. Ba, Th) higher than those of the potassic trachybasalts of Period III (Fig. 9). The 87Sr/86Srinitial ratios of these primitive HKS rocks are in the range 0·71054–0·71062 (Conticelli & Peccerillo, 1992; Conticelli et al., 2002), which is close to that of the composition of the leucite-bearing rocks of Period II (87Sr/86Srinitial = 0·71115–0·71053; Table 6). A tephritic magma with geochemical characteristics similar to those described at Sabatini and the Alban Hills might be the magma from which most of the parental leucite-bearing magmas of Period II were generated. Indeed, this magma might generate, by differentiation processes at different pressures, the variably enriched phonolites (high-Sr and low-Sr phonolitic lavas) of Period II. In this scenario, moderate assimilation of crustal material might be responsible, in some cases, for the higher 87Sr/86Srinitial of the high-Sr phonolitic lavas and of the tephri-phonolites than those measured in the primitive tephrites from Sabatini and the Alban Hills (Tables 6; Conticelli & Peccerillo, 1992; Conticelli et al., 2002).
The most primitive leucite-free magma of Period III is not primary and has experienced a moderate degree of differentiation (Perini, 2000). In the olivine latites of Period III, the Sr isotope compositions of clinopyroxene phenocrysts and core–rim Sr isotope variation of sanidine megacrysts reveal that mixing with mafic magma and limited crustal contamination occurred in these magmas before they were erupted. However, highly forsteritic olivine in equilibrium with the host magma and high compatible element contents (Tables 2 and 4) reveal that the olivine latites have not been strongly modified from the original primary magma composition. The incompatible element patterns of the olivine latites resemble those of alkaline leucite-free primitive magmas from central Italy (Fig. 9), as do the presence of subcalcic clinopyroxene and the tetrahedral site deficency of the clinopyroxene phenocrysts (Perini & Conticelli, 2000).
On the basis of the above discussion, at Vico volcano, it is suggested that three different parental magmas were responsible for generating the different leucite-bearing and leucite-free magmas. These are thought to be (1) a primitive tephrite, similar to that erupted at Sabatini and the Alban Hills (Conticelli & Peccerillo, 1992; Conticelli et al., 1997, 2002); (2) a potassic trachy-basalt; (3) an olivine latite, the last two being similar to those erupted during Period III of Vico activity.
Despite the similarity in the incompatible trace element patterns of the potassic trachybasalts and the tephrites (Fig. 9) some differences do exist. Potassic trachybasalts from Vico, in contrast to tephrites and all other leucite-bearing primitive rocks from central Italy, have a small negative P anomaly and no negative Sr anomaly in the mantle-normalized patterns. In addition, they have lower LREE/HFSE ratios (e.g. La/Nb, La/Zr), but no significant difference in LILE/HFSE ratios (e.g. Sr/Zr, Sr/Nb) and higher Rb/Ba ratios (Fig. 17). Vico potassic trachybasalts have significantly lower 87Sr/86Srinitial than the primitive tephrites from Sabatini and the Alban Hills (Table 6; Conticelli & Peccerillo, 1992; Conticelli et al., 2002).
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Olivine latites of Period III have distinctly lower LILE/HFSE ratios (e.g. Sr/Zr, Sr/Nb) than all the primitive leucite-bearing rocks at Vico, but they have low LREE/HFSE ratios and a negative P anomaly in mantle-normalized trace element patterns similar to the potassic trachybasalts (e.g. La/Nb, La/Zr; Fig. 17).
Genesis of the parental magmas
In Italy, partial melting of a heterogeneous upper-mantle source has been called on to produce a wide spectrum of alkaline magmas ranging from potassic to ultrapotassic, and from silica-saturated to silica-undersaturated (e.g. Rogers et al., 1985; Civetta et al., 1989; Beccaluva et al., 1991; Conticelli & Peccerillo, 1992; Peccerillo, 1993, 1999; Conticelli et al., 2002). A phlogopite-bearing peridotitic lithospheric mantle, metasomatized by subduction-related fluids or melts, is thought to be the source of these magmas (Beccaluva et al., 1991; Conticelli & Peccerillo, 1992; Peccerillo, 1999). Small-scale heterogeneity is commonly considered to be due to the presence of a vein network permeating the upper-mantle peridotite (Foley, 1992); the veins are inferred to be rich in clinopyroxene together with accessory phases, such as phlogopite, apatite, alkali-titanate and oxides, uncommon in normal peridotitic mantle assemblages. Hybridization mechanisms between partial melt of the vein and wall-rock peridotite components are supposed to be responsible for the generation of a variety of alkaline magmas (Foley, 1992, 1993).
Experimental melting studies of phlogopite-bearing harzburgite or lherzolite indicate that under F-rich conditions increasing pressure (>12 kbar for harzburgite or >18 kbar for lherzolite) changes melt compositions from silica-saturated to silica-undersaturated (Melzer & Foley, 2000, and references therein). A decrease in the degree of silica saturation of potassic melts has also been observed experimentally under F-poor, H2O-rich conditions with increasing pressure (Foley, 1993). The degree of silica saturation of the primary potassic melts, however, is also controlled by the fluid composition during partial melting. The predominance of CO2 over H2O during magma generation will depress the field of olivine, favouring the formation of silica-undersaturated melts (Wendlandt & Eggler, 1980). In Italy, large-scale mantle source heterogeneity is thought to be due to the occurrence of different metasomatic agents characterized by particular fluid or melt proportions, and by the different nature of the source of the metasomatic agent (e.g. silicate vs carbonate sediments; Peccerillo, 1985; Conticelli et al., 2002).
The range of variation in the trace element (LREE/HFSE, negative P anomaly, Rb/Ba ratio, Fig. 17) and isotope characteristics of the primitive parental magmas at Vico might be attributed to small-scale mantle heterogeneity. In particular, it might be attributed to variable roles in the residuum for accessory phases (e.g. phlogopite, apatite) or difference in the proportion of veins and peridotitic wall-rock components, which contribute to the melt. Accessory phases, such as apatite, possibly present in the veins, could produce fractionation of LREE/HREE ratios but not LILE/HFSE ratios (Sun & Hanson, 1976; Larsen, 1979; Pearce & Norry, 1979; Wörner et al., 1983; Ionov et al., 1997). The partition coefficients for LREE in apatite dominate those of LREE in the other mineral phases in the veins (e.g. clinopyroxene; Ionov et al., 1997); thus apatite controls the LREE characteristics of the partial melts. The occurrence of apatite in the residuum of partial melting of the veins could produce the negative P anomaly and the low LREE/HFSE ratios characteristic of the potassic trachybasalt (and olivine latite) parental magmas (Fig. 17). The potassic trachybasalts, in contrast to the primitive tephrites, have a distinct negative P anomaly suggesting that apatite was stable and not completely melted out from the vein component in the mantle source.
The lower (Tb/Yb)N in the potassic trachybasalts compared with that in the primitive tephrites from central Italy suggests that garnet was still present in the source residuum of the primitive tephrites but was probably starting to melt out from that of the potassic trachybasalt. The partition coefficient of Yb in garnet is higher than that of Tb (Zack et al., 1997); thus the (Tb/Yb)N ratio of a partial melt is high when garnet is present in the mantle source.
The genesis of alkaline potassic and ultrapotassic magmas requires the presence of phlogopite in the source. Assuming a veined mantle, with phlogopite accommodated in the veins, the variations in K2O content and in particular of the K2O/Na2O ratio of the primary magmas can be explained in terms of different proportions of the vein with respect to the wall-rock component entering the partial melt. The K2O/Na2O ratio of the potassic trachybasalts is lower than those of the primitive tephrites (Fig. 4), suggesting a higher wall-rock component/vein (phlogopite) component ratio in the genesis of the potassic trachybasalts. In addition, the potassic trachybasalts have lower 87Sr/86Srinitial than the primitive tephrites (Table 6; Conticelli & Peccerillo, 1992; Conticelli et al., 2002). Production of magmas with highly radiogenic Sr isotopic compositions requires the presence in the source of appreciable amount of minerals with a high Rb/Sr ratio such as mica, which will cause evolution of radiogenic Sr over time. The lower 87Sr/86Srinitial of the potassic trachybasaltic magmas with respect to the primitive tephrites might also be the result of a high wall-rock/vein component ratio during partial melting; the effect of the wall-rock is to dilute the radiogenic Sr isotope signature of the vein component. The greater importance of the wall-rock component in the genesis of the potassic trachybasalts than in that of the primitive tephrites is also reflected in the lower incompatible element contents of the potassic trachybasalts than those of the primitive tephrites (Fig. 9). Slight differences in Rb/Ba ratios exist between the potassic trachybasalts and the primitive tephrites; the trachybasalts have higher Rb/Ba ratios than the primitive tephrites (Fig. 17). Ba is partitioned into phlogopite to a greater extent than Rb (Ionov et al., 1997); thus, partial melts, which have phlogopite in the residuum, will acquire a high Rb/Ba ratio, whereas significant removal of phlogopite from the residuum by partial melting of the veins will tend to decrease the Rb/Ba ratios. The difference in Rb/Ba ratios between the potassic trachybasalts and the primitive tephrites might be related to significant melting of phlogopite from the vein component in the petrogenesis of the primitive tephrites. However, the partition coefficient of Ba in phlogopite is positively correlated with temperature (Guo & Green, 1990). Under upper-mantle conditions, rising temperature increases the Ba partition coefficient between phlogopite and melt as a result of the compositional effect of TiO2 in the phlogopite structure (Guo & Green, 1990). Difference in the Rb/Ba ratio between potassic trachybasalts and primitive tephrites can be related to temperature-dependent variation of the Ba partition coefficient for phlogopite in the veins. A higher wall-rock/vein ratio, as inferred for the potassic trachybasalts, is consistent with a higher degree of partial melting and thus higher temperature. In this partial melting scenario the resulting primary melt (potassic trachybasalt) could have acquired a higher Rb/Ba and lower K2O/Na2O ratio than that of the primitive tephrites.
Differences in the degree of silica saturation of the primitive melts at Vico (potassic trachybasalts and tephrites vs olivine latites) cannot be ascribed solely to partial melting of a heterogeneous veined mantle source; they are, additionally, strongly controlled by the depth of melting and possibly by the fluid composition of the mantle, more enriched in H2O and CH4 than in CO2 (Brey & Green, 1976; Wendlandt & Eggler, 1980; Melzer & Foley, 2000). Vico leucite-free magmas could have been generated from a mantle source at shallower depths or characterized by a lower XCO2 than that of the leucite-bearing magmas. At Vico, leucite-free olivine latite parental magmas were possibly generated in a sector of the lithospheric mantle in which the metasomatic agent (silicate instead of carbonate-rich) was different from that of the source of the leucite-bearing magmas, as suggested for other volcanic systems in Italy (Peccerillo, 1985; Conticelli et al., 2002, and references therein). This could also explain the LILE/HFSE ratios, which are distinctly lower in the olivine latite parental magmas than in the potassic trachybasalts and primitive tephrites (Fig. 17).
The presence of apatite in the vein residuum of the partial melts is likely in the petrogenesis of the olivine latite parental magmas as evidenced by the presence of a negative P anomaly and low LREE/HFSE ratios (Fig. 17). The low K2O/Na2O ratios of the olivine latites (Fig. 4; Table 4) suggest a high wall-rock/vein ratio; the high 87Sr/86Srinitial of the olivine latites, similar to that of the primitive tephrites (low wall-rock/vein component ratio), might be related to a higher isolation time of the veined mantle. This might suggest a two-stage history of mantle metasomatism, in which the veins producing the leucite-bearing magmas are related to a younger metasomatic event than the veins producing the leucite-free silica-saturated ultrapotassic magmas.
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SUMMARY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESCLASSIFICATION OF ROCK TYPESPETROGRAPHY AND MINERAL...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONSUMMARYREFERENCES |
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Complex processes of magma differentiation occurred at Vico volcano in a period of a few hundred thousand years (about 324 kyr; Barberi et al., 1994
). These involve a number of different primitive magmas, derived from a heterogeneous mantle source, polybaric fractional crystallization combined with wall-rock assimilation and magma mixing.
The petrogenesis of the magmas erupted during Period I of Vico activity, older than 400 ka, has been discussed in a previous paper (Perini et al., 2000). The oldest Vico magmas, which are leucite-free, differentiated by fractional crystallization of plagioclase + phlogopite + Fe–Ti oxides + apatite ± clinopyroxene ± sanidine ± titanite ± zircon. Subsequently, the arrival of mafic, leucite-bearing magmas into the magma chamber system has generated, by MFC (mixing plus fractional crystallization) processes, the earliest leucite-bearing magmas. The mafic leucite-bearing magma, which pervaded the Vico magma system at this stage, might be a tephrite, similar to that erupted at Sabatini and the Alban Hills (Conticelli & Peccerillo, 1992; Conticelli et al., 1997, 2002).
The history of Vico magmas younger than 300 ka (Period II and III) and the main volcanological events that occurred after about 100 kyr of quiescence, are summarized in Figs 2 and 18. The beginning of Period II was characterized by a cone-building phase, lasting about 50 kyr (from 300–250 ka), which resulted in the outpouring of a large volume (about 50 km3) of lava flows. In this period, the Vico system is inferred to have been fed frequently by primitive tephrite magmas, similar in composition to those erupted from the neighbouring Sabatini or Alban Hills systems. The primitive tephrite differentiated by AFC processes with crystallization of prevalently ferromagnesian mineral phases giving rise first to high-Sr phonolitic lavas and later to phono-tephrite and tephri-phonolitic lavas (Figs 2 and 18). The phono-tephrite magmas differentiated subsequently in a low-pressure magma chamber by fractional crystallization of olivine + clinopyroxene + plagioclase + apatite ± Fe–Ti oxides producing low-Sr phonolitic lavas of differentiation path 1 (Fig. 18).
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Primitive tephrite magmas probably continued to feed the Vico magma chambers producing the magma of the low-Sr tephri-phonolitic tephra of the Farine formation (Figs 2 and 18), which continued to differentiate by fractional crystallization of clinopyroxene + plagioclase + sanidine + phlogopite + apatite + Fe–Ti oxides (differentiation path 2), generating the phonolites of the Casale della Montagna lavas (Figs 2 and 18). An olivine latite parental magma entered the magma reservoir at this time in which a residual differentiated phonolite was present. Consequent magma mixing or mingling between the differentiated phonolite and the olivine latite produced the leucite-free trachytes and latites of Period II.
The primitive tephrite magmas were probably still refilling the deep magma chamber, where they were differentiating by fractional crystallization to produce the magmas of the high-Sr phonolitic tephra (Fig. 18). The magma of the high-Sr phonolitic tephra (Sutri formation) continued to differentiate by fractional crystallization of clinopyroxene + plagioclase + sanidine + phlogopite + apatite + Fe–Ti oxides (differentiation path 3), forming the more differentiated phonolite of the Sutri formation (Fig. 18). Finally, the differentiated phonolites of the Carbognano formation, generated from the high-Sr phonolitic magmas remaining in the magma reservoir, fed the latest caldera-forming eruption (Carbognano formation; Fig. 18).
The post-caldera period would have started with the emplacement of primitive, leucite-free, olivine latite magmas. The lack of age constraints for these lavas does not allow us to place them precisely in the post-caldera history of the volcano. Stratigraphic observations suggest that the olivine latites could have been emplaced directly after the final phase of caldera collapse (Figs 2 and 18).
Subsequently, eruptions of leucite-bearing potassic trachybasalt occurred from three scoria cones near the north rim of the caldera (Figs 1 and 18). Differentiation of potassic trachybasalts occurred by AFC at low pressure, with fractional crystallization of olivine + clinopyroxene + plagioclase + Fe–Ti oxides + apatite generating the tephri-phonolites of the Caprarola formation (Figs 2 and 18) at higher pressure and fractional crystallization of olivine + clinopyroxene + Fe–Ti oxides generating the tephri-phonolites of the Monte Venere lavas (Figs 2 and 18). Finally, the latter tephri-phonolite magmas differentiated by AFC at low pressure, with fractional crystallization of olivine + clinopyroxene + plagioclase + Fe–Ti oxides + apatite, forming phonolitic lavas of the Monte Venere lavas (Figs 2 and 18).
The cone-building phase in this model was dominated by frequent and voluminous arrivals of magma from the source, which ponded initially at depth and only later migrated up to the shallower reservoir. This prevented extreme magmatic differentiation and high enrichment in volatile elements. In contrast, the cone-destroying phase seems to have occurred when, after the formation of a shallow reservoir, the magma flux from depth to the surface was significantly reduced. This allowed both the generation of highly differentiated and volatile-rich magmas, capable of triggering explosive eruptions, and a reduction in the size of magma chamber as a result of abundant crystal fractionation and roof collapses.
Continental crustal assimilation, associated with fractional crystallization, seems to have been more active during periods of magma chamber expansion, as occurred in the cone-building and in the post-caldera phase, instead of magma chamber reduction, as in the cone-destroying phase (Fig. 18). This suggests that crustal assimilation is favoured during periods of magma chamber stability.
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ACKNOWLEDGEMENTS |
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The authors wish to thank F. Ramos, K. Knesel and F. Tepley for skilful assistance and suggestions during isotope analyses; M. L. Todaro for assistance during wet chemical analyses; the late G. Gambassi for producing thin and polished sections; G. Vaggelli and F. Olmi for assistance and suggestions during microprobe analyses; S. Trifogli for field assistance; and P. Manetti, A. Sbrana, S. Tommasini and O. Vaselli for stirring and focusing discussions and suggestions. Constructive reviews by S. Foley, M. Wilson and an anonymous reviewer of an earlier version of the manuscript are gratefully acknowledged. Financial support was provided by funds issued to S.C. by the C.N.R (bilateral Italy–USA grant), the MIUR (Cofin2002, grant no. 2002048873_003), and Università degli Studi di Firenze (grant ex 60%_2002).
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