Journal of Petrology | Volume 43 | Number 8 | Pages 1415-1434 | 2002
© Oxford University Press 2002
Evidence for Multi-stage Magmatic Evolution during the past 60 kyr at Campi Flegrei (Italy) Deduced from Sr, Nd and Pb Isotope Data
1OSSERVATORIO VESUVIANO, ISTITUTO NAZIONALE DI GEOFISICA E VULCANOLOGIA, VIA DIOCLEZIANO 328, 80124 NAPLES, ITALY
2DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITY OF NAPLES, 80138 NAPLES, ITALY
3DIPARTIMENTO DI SCIENZE FISICHE, UNIVERSITY OF NAPLES, 80126 NAPLES, ITALY
4DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITY OF TRIESTE, 34123 TRIESTE, ITALY
Received May 22, 2001; Revised typescript accepted January 19, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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The Campi Flegrei caldera, an active volcanic field in the Campanian province, Italy, is a nested structure generated by the Campanian Ignimbrite (37 ka BP) and the Neapolitan Yellow Tuff (12 ka BP) eruptions. Since at least 60 ka BP Campi Flegrei has produced magmas with variable chemical and Sr isotopic compositions. 87Sr/86Sr ratios increase through time from 0·7068 to 0·7086, with the highest ratios detected in the least-evolved shoshonitic products. The origin of this progressive Sr isotopic variability has been investigated using new Sr, Nd and Pb isotopic data for volcanic rocks and entrained xenoliths. The data obtained are combined and discussed with previous geochemical and Sr isotope data and used to suggest a multi-stage evolution for the magmatic system, mainly involving deeper and shallower crustal magma storage reservoirs. The deeper reservoir is proposed to be a magma chamber periodically refilled by primitive mafic magmas which subsequently undergo contamination by crustal material. The assimilated crustal material is represented by xenoliths recovered in the shoshonitic pyroclastic products. Magma batches originating from the deeper reservoir migrated towards the surface and fed a shallower complex magmatic system. The deeper chamber was tapped during the eruption of least evolved magmas by regional fault systems. In addition to crystal–liquid fractionation, open-system processes occurred in the shallower system.
KEY WORDS: Campi Flegrei; crustal contamination; xenoliths
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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At any stage of their ascent from source to surface, magmas may become contaminated by assimilation of wall rocks. In particular, crustal contamination is considered a possible mechanism when mantle-derived magmas rise through thick sections of continental crust and pond at different depths before reaching the surface (e.g. Leeman, 1983; Hawkesworth & van Calsteren, 1984).
Crustal contamination of magmas is favoured in high heat flux zones and may occur in a variety of ways including: (1) bulk assimilation of crustal material; (2) assimilation of partial melts of crustal materials; (3) selective exchange of specific elements (e.g. De Paolo, 1981; Leeman, 1983; Huppert & Sparks, 1985; Wilson, 1989). Sr, Nd and Pb isotopes are commonly used to model crustal contamination processes (e.g. Wilson, 1989, and references therein). However, the nature of the contaminant is, in some cases, difficult to identify, in particular when exposures of basement rocks or occurrences of entrained xenoliths in the magmas are lacking.
At Campi Flegrei (CF) (Fig. 1) magmas of different chemical and Sr isotopic composition have been erupted since at least 60 ka BP (Civetta et al., 1991, 1997; Orsi et al., 1995; D’Antonio et al., 1999b; de Vita et al., 1999; Pappalardo et al., 1999, 2002). Although much information has been recently obtained on the temporal evolution of the CF magmatic system (D’Antonio et al., 1999b; Pappalardo et al., 1999; Wohletz et al., 1999) many problems remain unresolved, particularly the meaning of: (1) the large variability of Sr isotopic ratios (0·7068–0·7086); (2) the occurrence of the highest Sr isotopic ratios in the least-evolved, shoshonitic magmas; (3) the progressive increase of Sr isotopic ratios through time.
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The recent discovery of xenoliths of possible crustal origin in the least-evolved shoshonitic rocks erupted in the past 12 kyr offers the opportunity to study the interaction between basic magmas and crustal material.
In this paper we present new Nd and Pb isotope data for the volcanic products erupted in the past 60 kyr at the CF, including geochemical and Sr, Nd and Pb isotope data for xenoliths in shoshonitic scoria and pumice fragments emplaced during the Minopoli 1 (11·05 ka BP) and Pigna St. Nicola (8300 a BP) eruptions (D’Antonio et al., 1999b; Di Vito et al., 1999).
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GEOLOGICAL AND TECTONIC SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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The Campi Flegrei caldera (CF caldera) is an active volcanic field within the Campanian region (Fig. 1), located on the western side of the Cretaceous to Pliocene Apennine mountain chain in central–southern Italy. The Campanian portion of this chain is composed of Triassic to Miocene sedimentary successions (limestones, dolomites and arenaceous rocks) (D’Argenio et al., 1973, 1987; Ippolito et al., 1975; Patacca & Scandone, 1987), whereas low-grade to granulite-facies metamorphic rocks and gabbro to granite plutons of the Hercynian basement crop out in the southernmost portion of the chain. Since the Pliocene, the Campanian margin of the Apennine chain has been affected by extensive tectonism related to the opening of the Tyrrhenian Sea (e.g. Scandone, 1979; Peccerillo & Manetti, 1985; Finetti & Del Ben, 1986; Di Girolamo, 1987; Doglioni, 1991). The structural low of the Campanian Plain, bordered by conjugate NE–SW and NW–SE fault systems, is related to this tectonism. Pliocene to Quaternary volcanism is associated with the conjugate fault system [see Beccaluva et al. (1991) for a review].
Geophysical studies indicate <25 km of crust beneath the CF area (Corrado & Rapolla, 1981; Ferrucci et al., 1989) and a high heat flux (Della Vedova et al., 1984, 1991). The nature of the crust is not directly known because the crustal rocks were depressed up to 4000 m depth during Plio-Pleistocene extension and buried by younger volcanic deposits (D’Argenio et al., 1973, 1987; Ippolito et al., 1975; Ortolani & Aprile, 1985). The only available data concerning the crustal structure beneath the CF are those based on seismic data reported by Finetti & Morelli (1974) and Bruno et al. (1998) suggesting the existence of a carbonate succession at 3·4 km depth in the Gulf of Napoli and Pozzuoli. To the east, this sequence has been reached by drilling at a depth of 
2·4 km beneath the Somma–Vesuvius (Bernasconi et al., 1981) and the succession is likely to be made up of Triassic to Cretaceous limestones and dolomites, overlain by Miocene arenaceous and/or flysch sediments, or pyroclastic rocks (D’Argenio et al., 1973; Ippolito et al., 1975). Calc-alkaline Quaternary volcanic rocks have also been found deeply buried in the area north of the CF caldera (Di Girolamo, 1978; Barbieri et al., 1979; Beccaluva et al., 1991). Comparison with the Southern Apennine structure (Finetti & Morelli, 1973; Schutte, 1978; Scandone, 1982) suggests that this whole succession should be underlain by Hercynian metamorphic basement.
At the CF caldera volcano-tectonic activity was superimposed on the regional tectonic activity to generate a nested caldera structure (Orsi et al., 1996) that formed during the Campanian Ignimbrite (CI, 37 ka BP, Deino et al., 1992, 1994) and Neapolitan Yellow Tuff (NYT, 12 ka BP, Alessio et al., 1971) eruptions. The central part of the caldera has been uplifted over the past 12 kyr through a simple shearing-block resurgent mechanism (Orsi et al., 1996), which provided conditions by which magmas could rise to the surface only in those parts of the caldera floor subjected to extensional stress. As a consequence, the resurgence mechanism has strongly affected the areal distribution of volcanism during the past 12 kyr (Fig. 1; Di Vito et al., 1999). In particular, the least-evolved shoshonitic magmas were extruded only from vents located along a NE–SW regional fault system reactivated during the resurgence, whereas the most-evolved trachytic and phonolitic magmas were emitted from vents located on the caldera rim and on faults bordering the resurgent block.
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SUMMARY OF THE PREVIOUS STUDIES ON THE CAMPI FLEGREI MAGMATIC SYSTEM |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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Volcanism at CF began earlier than 60 ka BP (Pappalardo et al., 1999) and was essentially explosive with subordinate effusive episodes (Rosi & Sbrana, 1987; Orsi et al., 1996; Di Vito et al., 1999). The most recent eruption formed the Monte Nuovo tuff cone in AD 1538.
Geochemical and Sr isotopic studies of the CF products younger than 60 ka (Ghiara et al., 1979; Armienti et al., 1983; Di Girolamo et al., 1984; Villemant, 1988; Ghiara, 1990; Civetta et al., 1991, 1997; Orsi et al., 1995; D’Antonio et al., 1999b; de Vita et al., 1999; Pappalardo et al., 1999, 2002), have suggested the existence of a shallow, large-volume reservoir of mostly trachytic magma that has been periodically refilled by new magma batches, each of which has undergone complex differentiation processes. The variations shown by Sr isotope and major and trace element compositions provide the evidence for new magma input between 60 and 44 ka (87Sr/86Sr = 0·70680–0·70730) and between 15 and 12 ka (87Sr/86Sr = 0·70730–0·70757). In each case, new magma input was associated with an increase of the 87Sr/86Sr ratio. Further input of new magma probably occurred in the last 12 kyr, resulting in the eruption of shoshonites (87Sr/86Sr >0·708), which were extruded only through vents located on a NE–SW regional fault system. It has been argued (D’Antonio et al., 1999b) that these magmas have originated in a deeper reservoir. On the basis of petrological data (D’Antonio et al., 1999b; Pappalardo et al., 1999) and thermal (Wohletz et al., 1999) and magnetic (Orsi et al., 1999) modelling, the existence of a large-volume still active magmatic system, located at shallow depth under the CF caldera, has been proposed. This system is likely to be made up of the residual portions of older, large-volume magma reservoirs that fed the eruptions before 12 ka BP.
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SAMPLE SELECTION, SAMPLE PREPARATION AND ANALYTICAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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Nd and Pb isotope compositions have been determined on 46 whole-rock samples considered representative of the magmas erupted at CF in the past 60 kyr, selected on the basis of their chemical and Sr isotopic compositions and age. These samples have been previously analysed for their major and trace element and Sr isotopic compositions (Orsi et al., 1995; Civetta et al., 1997; D’Antonio et al., 1999b; Pappalardo et al., 1999). These analyses are reported in Electronic Appendix 1, which may be downloaded from the Journal of Petrology Web site at http://www.petrology.oupjournals.org. Each whole-rock sample consists of juvenile fragments (pumice, scoria, or dense glass clasts), collected at the same stratigraphic height, that are texturally similar (in terms of structure of vesicles, glass colour, phenocryst content). Samples were generally fresh and unaltered; in cases where the outer parts of the samples were Fe stained or otherwise altered, these parts were removed before milling. All the samples were washed in distilled water, crushed to lapilli-size particles, then ground and homogenized in an agate mortar. Sr, Nd and Pb isotope ratios have been determined for xenoliths recovered from the Minopoli 1 and Pigna St. Nicola least-evolved products and for their associated host-rocks. The largest xenoliths (<4 cm in size) and their surrounding host-rock were selected and extracted using an electric drill. For each xenolith sample, a number of clasts, homogeneous in colour, texture and mineralogy, were cleaned, washed in distilled water, crushed to lapilli-sized particles, ground and homogenized in an agate mill. It has only been possible to analyse one xenolith for major and trace element compositions, because of the low quantity of material for all samples. Major and trace element analyses of both whole rocks and bulk xenolith were performed by inductively coupled plasma atomic emission spectrometry (ICP-AES) and ICP mass spectrometry (ICP-MS) at the Centre de Recherches Petrographiques et Géochimiques (CRPG, Vandoeuvre Cedex, France; Govindaraju & Mevelle, 1987). Precision is 0·5% for major elements, and variable from 2 to 5% for trace element contents in the range 50–150 ppm, from 2 to 10% for trace element contents in the range 10–50 ppm, and from 5 to 25% for trace element contents in the range 0–10 ppm (J. Morel, personal communication, 1997). The chemical composition of the xenoliths was investigated by microprobe analyses on glass in thin section using a CAMECA SX 50 at Centro Studio per il Quaternario e l’Evoluzione Ambientale at Rome. An accelerating voltage of 15 kV and a beam current of 15 nA was used. Counting time was 120 s, spot size 5 µm. Analytical uncertainties are 1%. Major element composition for the groundmass of the xenoliths is reported in Electronic Appendix 2, which may be downloaded from the Journal of Petrology Web site at http://www.petrology.oupjournals.org.
Sr and Nd isotope analyses of whole-rock samples, xenoliths and their host-rocks were carried out at the Centro Interdipartimentale di Servizio per Analisi Geo-Mineralogiche (University ‘Federico II’ of Naples). The powders were leached with cold 2·5 N HCl for 10 min and with warm 2·5 N HCl for 30 min then rinsed thoroughly in pure sub-boiling distilled water, and finally dissolved with high-purity HF–HNO3–HCl mixtures. Sr and rare earth elements (REE) were separated using standard cation exchange methods. Sr was extracted using chromatographic columns of 0·5 cm width and 15 cm length, filled with 4 cm3 of Dowex 50 W X 8, H+ form, 200–400 mesh resin, conditioned in 2·5 N HCl. REE were extracted from the same aliquot of sample after collection of Sr, using the same chromatographic columns, conditioned in 6·0 N HCl. Nd was successively separated using conventional ion-exchange techniques using chromatographic columns of 0·5 cm width and 12 cm length, filled with 2 cm3 of Eichrom LN Spec SPS resin, conditioned in 0·25 N HCl. Measurements were made using a VG 354 double-collector thermal ionization mass spectrometer. The quoted error is the standard deviation of the mean (2
m) and refers to the last digit. Replicated analyses of NBS 987 and of La Jolla International Reference Standard gave an average value respectively of 87Sr/86Sr = 0·710264 ± 9 (n = 24) and 143Nd/144Nd = 0·511825 ± 4 (n = 50). The total blank was of the order of 6 ng for Sr and 0·4 ng for Nd during the period of measurements.
Pb isotope ratios were determined on whole-rock and xenolith samples at the Department of Geological Sciences, University of California, Santa Barbara. After spiking samples with 205Pb spike, lead was extracted by chromatographic exchange in Dowex 1 anion resin, using standard HBr and HCl elution procedures. Isotopic analyses were performed on a Finnigan MAT 261 multiple collector, thermal ionization mass spectrometer operating in a static mode, in which all five Pb isotopes were collected simultaneously. Replicate analyses of Pb standard NBS 981 indicated that Pb ratios are accurate to within 0·02% ± (2) per mass unit after applying mass discrimination corrections of 0·13 ± 0·01% per mass unit.
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PETROGRAPHY OF THE VOLCANIC ROCKS AND THE XENOLITHS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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Eruptions during the past 60 kyr have mostly resulted in pyroclastic deposits containing pumice and scoria fragments, ranging in size from few millimetres to 20 cm, with variable vesicle contents; only a few lava domes have been recognized during this interval. As already stated in previous publications (Armienti et al., 1983; Rosi & Sbrana, 1987; Civetta et al., 1991; D’Antonio et al., 1999b; Pappalardo et al., 1999), the volcanic rocks range in composition from shoshonite to latite, trachyte and phonolite according to the classification system of Le Bas et al. (1986). The petrographic characteristics of the CF rocks have been described by Armienti et al. (1983), Di Girolamo et al. (1984) and Rosi & Sbrana (1987). The rocks range from porphyritic (
25 vol. % of phenocrysts) to sub-aphyric. Least-evolved shoshonite and latite rocks have clinopyroxene and plagioclase as the most abundant phases, whereas olivine, biotite and opaques are less abundant; evidence for mineralogical disequilibria is found in the least-evolved rocks, which contain xenocrysts of sanidine and, in some cases, quartz. More evolved trachytic to phonolitic rocks contain mainly alkali-feldspar, and subordinate clinopyroxene, biotite, plagioclase and opaques. Accessory apatite is present in almost all the rock types. As reported for the Italian potassic volcanic rocks (Ghiara et al., 1979; Barberi et al., 1981; Giannetti & Luhr, 1983), two varieties of clinopyroxene generally occur: green Fe-rich diopside and pale-coloured Mg-rich diopside crystals (Morimoto, 1988). The least-evolved rocks entrain rare (<5 vol. %) xenoliths ranging in size from <1 mm to 4 cm, the smaller being the commonest. The xenoliths are generally white, green or grey in colour; the largest clasts have a reddish colour. Almost all xenoliths show fairly sharp contacts with their host-rocks, have angular–equant shapes and show, in some cases, reaction coronas characterized by crenulated margins, indicating interaction with the host-rock. The smaller xenoliths form streaks within the host-rocks, with a wavy shape indicating a plastic behaviour inside the hot host-magma. These xenoliths have porphyritic, sometimes glomeroporphyritic textures with low abundances (5 vol. %) of microphenocrysts of quartz, feldspar, biotite and Fe-oxides set in a vesicular glassy groundmass (Fig. 2a). Feldpars are plagioclase or alkali-feldspar, and have prismatic to tabular habits with corroded or, rarely, rounded margins. Pyroxene has tabular habit and has a green colour. Quartz grains are rounded, embayed or cracked and have sharp margins. Biotite and Fe-oxides are sub-euhedral grains. Biotite shows brown to yellow pleochroism.
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Subordinate rounded xenoliths, several centimetres in size, with a holocrystalline texture have also been recognized (Fig. 2b). These latter types are made up of crystals of rounded quartz with infrequent microlites of smaller, sub-euhedral clinopyroxene, surrounded by elongated smaller crystals of tridymite (identified by X-ray diffractometry).
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MAJOR AND TRACE ELEMENT GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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The major and trace element compositions of the CF rocks analysed for Nd and Pb have been given by Orsi et al. (1995), Civetta et al. (1997), D’Antonio et al. (1999b), de Vita et al. (1999) and Pappalardo et al. (1999, 2002). These analyses are reported in Electronic Appendix 1, together with some other analyses considered in the following discussion, published by Di Girolamo et al. (1984) and Civetta et al. (1991). Table 1 provides a whole-rock analysis for a vesicular xenolith that is classified as andesite, according to the classification system of Le Bas et al. (1986). Harker diagrams (Fig. 3) show a single evolutionary trend for the whole suite starting from the less evolved shoshonite. In these diagrams CaO content has been used as differentiation index because it has a large variance compared with silica and a close correlation with major element oxides (Appleton, 1972; Giannetti, 1982); further, it has been commonly used for the CF potassic rocks. For all shoshonites, ratios between elements with similar degree of incompatibility are reasonably constant. For example, volcanic rocks with MgO >3·5 wt % are characterized by Nb/Zr, Th/Yb, Th/Zr, La/Ce and La/Yb ratios of 0·16 ± 0·05, 7·1 ± 1·2, 0·07 ± 0·01, 0·59 ± 0·09 and 23 ± 2, respectively, and these ranges are mostly inside the analytical error. As already pointed out in the literature (e.g. Armienti et al., 1983; Villemant, 1988; Civetta et al., 1991; D’Antonio et al., 1999b; Pappalardo et al., 1999), the evolutionary trend presented in Fig. 3 suggests that fractional crystallization must have played an important role in producing major and trace element variations, although the scattering of some elements such as Rb, V and Sr, and the Sr isotopic variations provide evidence that other processes were superimposed on fractional crystallization. Generally, fractionation of olivine accounts for the decreases in MgO, Ni and Co abundances with decreasing CaO content. Separation of clinopyroxene explains the decreases in MgO, Cr and Sc concentrations. Crystallization of plagioclase accounts for the decreases in Sr, Ba and Eu abundances. Fractionation of alkali-feldspar also explains decreasing Sr and Ba contents and the decrease in K2O abundance in the most evolved rocks. Crystallization of both biotite and Fe–Ti-oxides, starting from latitic magmas, accounts for the strong decrease in Fe2O3tot, TiO2 and V contents, whereas apatite crystallization determines the decrease in P2O5 content. Chondrite-normalized REE patterns are also consistent with fractionation processes. The patterns for all rock types are nearly parallel, with steep light REE (LREE) slopes and gentle heavy REE (HREE) slopes. A negative Eu anomaly is particularly well developed in the more-evolved rocks. Quantitative modelling of crystal fractionation was carried out by Armienti et al. (1983), Villemant (1988) and Civetta et al. (1991), and demonstrates that the whole sequence of rocks can be approximated by fractional crystallization from a single shoshonitic parental magma.
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Sr, Nd AND Pb ISOTOPE GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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Samples of CF volcanic rocks, including xenoliths and their host-rocks (juvenile material closely surrounding the xenoliths) were analysed following the procedures described above.
The Sr isotopic compositions of the analysed whole rocks have been reported by Orsi et al. (1995), Civetta et al. (1997), D’Antonio et al. (1999b), de Vita et al. (1999) and Pappalardo et al. (1999, 2002). 87Sr/86Sr initial ratios range from 0·7068 to 0·7086. Nd–Pb isotope compositions were determined for selected whole-rock samples covering the whole compositional range of magmas erupted in the past 60 kyr at CF. These data are presented in Electronic Appendix 1; however, they are also listed in Table 2, together with ages, 87Sr/86Sr values, and MgO, CaO, Sr, Rb, Nd, Th and Pb contents. The 143Nd/144Nd ratios range from 0·51241 to 0·51279, in agreement with data reported by Civetta et al. (1991) for the CF whole rocks younger than 12 ka. The 206Pb/204Pb ratios vary from 18·850 to 19·246, the 207Pb/204Pb ratios vary from 15·606 to 15·728, and the 208Pb/204Pb ratios from 38·872 to 39·380. The Pb isotope data reported by Vollmer (1976) fall within these ranges.
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The Sr–Nd–Pb isotopic compositions of the xenoliths differ significantly from those of the host-rock immediately surrounding the xenoliths and, in each case, from those of the total whole-rock samples (Table 3). The 87Sr/86Sr composition of the xenoliths ranges between 0·70777 and 0·71133, and is always higher than that of the corresponding host-rock. A 87Sr/86Sr ratio of 0·70832 has been measured in a quartz–tridymite–clinopyroxene holocrystalline xenolith [sample OCF118MIb(gn)], but this value is likely to correspond to that of the pyroxene microphenocrysts, as quartz and tridymite should not contain appreciable amounts of Sr. This particular holocrystalline xenolith was found included in a scoria fragment that is characterized by a Sr isotope ratio of 0·70780.
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The highest 87Sr/86Sr value (0·71133) has been measured in a vesicular xenolith [sample OCF110PNc(gn)] from a scoria characterized by a whole-rock Sr isotopic ratio of 0·70762.
In contrast to the Sr isotopic compositions, the Nd and Pb isotope ratios are lower in the xenoliths relative to the host-rocks. In particular, the 144Nd/143Nd ratio ranges from 0·51242 to 0·51214. 206Pb/204Pb ratios in the xenoliths are roughly constant at 18·85 and the host-rocks have 206Pb/204Pb ratios at 19·00; 207Pb/204Pb ratios in the xenoliths are in the same range as in the whole rock, i.e. between 15·61 and 15·67, whereas 208Pb/204Pb ratios show slightly lower values in the xenoliths relative to the whole rocks (38·9 vs 39·1).
In Sr–Nd–Pb isotope diagrams (Fig. 4), the CF whole-rock and xenolith samples define at least two trends (see also Fig. 8a, below) whose end-members are represented by: (1) the rocks with the lowest radiogenic Sr isotopic compositions and highest Nd and Pb isotopic ratios (87Sr/86Sr 
0·7068; 143Nd/144Nd >0·5125; 206Pb/204Pb >19·2; 207Pb/204Pb >15·7; 208Pb/204Pb >39·3); (2) vesicular xenoliths; (3) holocrystalline xenoliths. Particularly the last trend is more pronounced in Pb–Pb diagrams. It is interested to note that the least-evolved shoshonitic magmas approach the xenoliths in term of their Sr, Nd and Pb isotope values (87Sr/86Sr >0·708; 143Nd/144Nd <0·5124; 206Pb/204Pb <18·9; 207Pb/204Pb <15·6; 208Pb/204Pb <38·9).
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If isotopic data are plotted versus age (Fig. 5) a general progressive decrease in Nd and Pb isotopic ratios, associated with an increase in Sr isotopic ratio, is observed through time, in particular from 60 to 12 ka, whereas the rocks younger than 12 ka are characterized by the largest variation in Sr, Nd and Pb isotope compositions (Fig. 5). In Fig. 5 four main subgroups of magmas can be identified on the basis of their isotopic composition and age: (I) trachytic magmas erupted before the CI eruption (pre-CI type); (II) trachytic magmas erupted during the CI eruption (CI type); (III) latitic to trachytic magmas erupted during the NYT eruption (NYT type); (IV) shoshonitic to latitic magmas and associated xenoliths erupted in the last 12 kyr. In Fig. 5 all the magmas falling outside these four recognized fields have been interpreted as: (1) a residual magma batch from one of the identified magma components; (2) magma batches resulting from mixing between different components (D’Antonio et al., 1999b; Pappalardo et al., 1999). The involvement of three magmatic components in the CF shallow magmatic system in the last 12 kyr, corresponding to the last three subgroups mentioned above, was recognized by D’Antonio et al. (1999b). Those workers discounted the possibility that isotopic variations recognized in the magmatic components were the result of differentiation processes that occurred in the near-surface magmatic system and suggested that the magmas acquired their isotopic imprint elsewhere by deeper processes. This idea is discussed in the following section on the basis of the new Sr–Nd–Pb data for whole rocks and xenoliths.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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Hypotheses on the origin of the isotopic variability of CF magmas
Fractional crystallization has clearly played an important role in producing the major and trace element variations of the magmas erupted at the CF (e.g. Armienti et al., 1983; Villemant, 1988; Civetta et al., 1991; D’Antonio et al., 1999b; Pappalardo et al., 1999). However, the variations of Sr–Nd–Pb isotope ratios require that further processes acted during magma evolution in addition to fractionation. For samples younger than 12 ka (Fig. 5), the Sr isotopic variations indicate (D’Antonio et al., 1999b; Pappalardo et al., 1999) that complex evolutionary mechanisms involving recharge and mixing of distinct magma batches occurred in the shallow system. Volcanological, petrological and geochronological data, however (Orsi et al., 1995; Civetta et al., 1997; D’Antonio et al., 1999b; Di Vito et al., 1999; Pappalardo et al., 1999), suggest that these distinct magma batches acquired their isotopic signature before reaching the low-pressure magma chamber. Moreover, the isotopic variations shown by the CF rocks cannot be related to in situ radioactive growth, because a much longer time than that of the CF magmatic system (
60 kyr) would be required to produce the observed isotopic variability. Therefore, we hypothesize that the isotopic variability could be ascribed principally either to mantle melting processes and/or mantle source(s) heterogeneity or to crustal contamination of the least-evolved parental magmas in a deeper reservoir(s).
Mantle melting processes and mantle heterogeneity
The genesis of the Campanian, and, in general, of the Neogene–Quaternary Italian, potassic magmatism has been the subject of debate for several decades (see Appleton, 1972; Holm et al., 1972; Turi & Taylor, 1976; Vollmer, 1976; Hawkesworth & Vollmer, 1979; Peccerillo & Manetti, 1985; Beccaluva et al., 1991; Serri et al., 1993; De Astis et al., 1997; D’Antonio et al., 1999a). Magmas have been assigned sources in the asthenospheric and/or in the lithospheric mantle. The variable K and incompatible trace element enrichments, as well as the isotopic signatures of the magmas, have been alternatively attributed to different physical–chemical conditions of mantle melting, mantle heterogeneity as a result of recycled components and metasomatism, or to crustal assimilation. In the case of CF, the hypotheses for magma genesis remain mostly speculative because of the absence of primary magmas.
The most radiogenic Sr isotope compositions of shoshonites and vesicular xenoliths (0·7083 and up to 0·711, respectively) approach those of the High K series in Central Italy (e.g. Appleton, 1972; Peccerillo & Manetti, 1985). However, the similar K content (see Electronic Appendix 1) and the constancy of ratios between elements with similar degrees of incompatibility of the CF products, particularly those having MgO >3·5%, seem to exclude the involvement of distinct melting processes or distinct sources in the mantle. Moreover, a variation in the mantle source composition seems unlikely, as a result of both the geologically brief CF history and the limited geographical distribution of eruptive vents. For example, the CI and the NYT magmas have significantly different Sr–Nd–Pb isotopic signatures but were erupted in the relatively narrow time span of 25 kyr. Furthermore, the highest Sr isotopic ratios (>0·708) are not likely to be representative of the lithospheric mantle, which would be characterized by a 87Sr/86Sr ratio <0·706. In fact, mafic, near-primary magmas erupted at the island of Procida in the Gulf of Napoli (Fig. 1), interpreted as partial melts of the mantle beneath the Campanian Region, have Sr isotopic compositions of 0·7051 (D’Antonio et al., 1999a). This is supported also by data for mantle xenoliths derived from 55–75 km depth beneath Vulture volcano to the east of CF and considered to be lithospheric in origin (Kostoula et al., 1999), which are characterized by 87Sr/86Sr ratios in the range 0·70424–0·70580. Similar values characterize mafic magmas generated in the lithospheric mantle beneath Vulcano in the Aeolian Archipelago (De Astis et al., 1997).
Origin of xenoliths and crustal contamination
The studied xenoliths are a very rare type of lithic clast ejected during the CF explosive volcanic activity. The common type of lithic clasts is sub-volcanic, lava, pyroclastic and hydrothermally altered lithic fragments derived from shallow levels beneath the CF caldera (e.g. Di Vito et al., 1999). On the basis of their mineralogy (quartz, tridymite and clinopyroxene), the holocrystalline xenoliths could be derived from the Lower Miocene arenaceous rocks that crop out in the Campanian area at the top of the carbonate platform, as suggested by Di Girolamo et al. (1984). On the other hand, the origin of the vesicular xenoliths is more difficult to determine. In particular, there are doubts regarding whether they are magmatic or sedimentary in origin. Microprobe analyses of glass matrices (Electronic Appendix 2) show high variability from alkaline to sub-alkaline compositions (Fig. 6). The alkaline compositions overlap with the compositions of the host-rocks; the sub-alkaline signature is interpreted as a primary feature as a result of the absence of visible alteration, and this interpretation is supported by the frequent occurrence of quartz and fresh K-feldspar microliths. Therefore, we interpret the glass compositions of the xenoliths to reflect mingling between alkaline CF magmas and a liquid produced by melting of a sub-alkaline igneous protolith. Volcanic rocks of 1–2 Ma age with high-K calc-alkaline affinity (basaltic andesites and andesites) have been drilled in the Campanian Plain, suggesting an episode of calc-alkaline magmatism preceding the CF activity (Di Girolamo, 1978; Barbieri et al., 1979; Beccaluva et al., 1991). However, these calc-alkaline rocks have Sr isotope ratios of 0·7051–0·7081 (Di Girolamo, 1978), which are lower than those of the vesicular xenoliths (up to 0·711). Another possibility is that the sub-alkaline material could be representative of basement rocks similar to those constituting the Calabrian Arc. This is suggested by the similarity of Sr isotopic compositions (0·7113 vs >0·709, Rottura et al., 1991) and trace element patterns (Fig. 7). Therefore, we tentatively hypothesize that mixing processes between CF magmas and partial melts derived from rocks similar to those of the Calabrian Arc were involved in the generation of the glasses found in the vesicular xenoliths. These xenoliths can represent partially melted fragments of rocks that were similar to those of the Calabrian Arc, permeated by CF magmas. Their mineralogy consists of crystals both derived from the rocks of the Calabrian Arc (i.e. quartz) and crystallized from the CF magma. Vesicularity implies decompression of melts probably occurring during the transport within the rapidly rising magma. The rapid ascent of magma can also explain their chemical inhomogeneity as a result of the lack of hybridization between melts.
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, trachyte (CI); 
, latite–trachyte (NYT); 
, shoshonite (Minopoli 1); *, vesicular xenolith (Table
The above data on vesicular xenoliths as well as the high 87Sr/86Sr and low 143Nd/144Nd values of less-evolved rocks lying on a mixing trajectory towards the xenoliths in terms of their Sr–Nd–Pb isotope geochemistry (Figs 4 and 8) indicate that crustal contamination of the CF magmas is possible.
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Crustal interaction: quantitative modelling
The less contaminated magmas are represented by the older, first-emplaced pre-CI rocks that might have resided for a short time span in the deeper crustal reservoir. The isotopic signature (87Sr/86Sr <0·7067; 143Nd/144Nd >0·5125; 206Pb/204Pb >19·2; 207Pb/204Pb >15·7; 208Pb/204Pb >39·3) of these rocks would be closer to that of the most primitive magmas and, possibly, to the mantle source. In particular, their Sr isotopic and trace element signature is similar to that of some CF lithic clasts (Cortini & Don Hermes, 1981; Beccaluva et al., 1991), which have a shoshonitic composition that can be assumed, therefore, to be the parental composition from which the most-evolved pre-CI magmas were derived (composition 1 in Fig. 8a).
The crustal material could be represented by the entrapped xenoliths (compositions 2 and 3 in Fig. 8a), which are assumed to be derived from a lower crust similar to the Hercynian basement (composition 2, vesicular xenolith) or from an upper crust similar to arenaceous sediments (composition 3, holocrystalline xenolith).
Shoshonitic rock and xenolith compositions were used to model a contamination and fractional crystallization process using the EC-AFC (energy constrained assimilation and fractional crystallization) approach of Spera & Bohrson (2001). In Table 4 are listed the thermal and geochemical input parameters used during different simulations. Two scenarios have been simulated: the first scenario consists of a deeper reservoir in which fractionating shoshonitic magma is contaminated by lower-crustal material represented by vesicular xenoliths; the second consists of a shallower reservoir in which shoshonitic magmas are contaminated by upper-crustal material represented by the holocrystalline xenoliths. During the first simulation (Models 1–4 in Table 4) the temperature of the shoshonite is assumed to be a typical anhydrous T of 1200°C (Tlm = Tm0 in Table 4), whereas a T of 700°C (Ta0 in Table 4) is assigned at the Hercynian basement consistent with the high heat flux of the CF area (Della Vedova et al., 1984, 1991). The liquidus T of the crustal rocks is assumed to be 950°C whereas the solidus T of the magmas is 750°C (Tla and Ts, respectively, in Table 4). In the second simulation (Model 5 in Table 4) the temperature of the shoshonite is assumed to be a typical anhydrous T of 1200°C (Tlm = Tm0in Table 4), whereas a T of 450°C (Ta0 in Table 4) is assigned to the holocrystalline rocks, consistent with a depth of 3–5 km and the high heat flux of the area (Della Vedova et al., 1984, 1991). The liquidus T of the crustal rocks is assumed to be 950°C whereas the solidus T of the magmas is 750°C (Tla and Ts, respectively, in Table 4). The model requires the definition of the equilibration temperature, which describes the approach of the system to equilibrium during the irreversible process of heat exchange. The equilibration temperature would be equal to or higher than the eruptive temperature (Spera & Bohrson, 2001). Therefore, an equilibration temperature of 850–950°C is used in the various models. Using a lower temperature (e.g. 1100°C) for the intruding shoshonite does not change significantly the modelling results. In Fig. 8b the calculated EC-AFC curves for Models 1–5 are illustrated, showing that the CF geochemical paths can be simulated by variable degrees of interaction between a CF ‘mafic’ magma with both the lower and the uppermost crust. Distinct partition coefficients during the partial fusion processes (Table 4) can account for the different lines of evolution. Furthermore, successive closed-fractionation processes of the contaminated magmas can explain the compositions of the more evolved CF rocks.
The CF trend of Sr vs 87Sr/86Sr (Fig. 8b and Table 4) is consistent with Sr behaving incompatibly (DSr = 0·1 and 0·2) during melting of Hercynian crust. In particular, an increase of partition coefficient for Sr in the crustal assimilant determines a decrease of Sr exchange between magma and wall-rocks. The isotope–isotope diagrams of Fig. 8 illustrate that the high 87Sr/86Sr ratios of the shoshonites cannot be reached by crustal contamination with the arenaceous sediments (Model 5), whereas they can be achieved by contamination with the Hercynian crust (Models 1 and 2). These data support the hypothesis that important contamination processes occur in a deeper reservoir, particularly for the shoshonitic composition magmas. The mass of the anatectic melt required to simulate the most contaminated magmas is <30% and the ratio between the mass of the anatectic melt and the cumulate mass is 60%. The isotope ratios of the CF latitic magmas can result from contamination with both Hercynian and arenaceous rocks. The mass of each type of anatectic melt is <10% and the ratio between the mass of the anatectic melt and the cumulate mass is 70%. The contamination with the arenaceous crust at shallow depth mostly affects the Pb isotope ratios, as already proposed for other magmatic systems (Wilson, 1989; Esperança et al., 1992).
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC SETTINGSUMMARY OF THE PREVIOUS...SAMPLE SELECTION, SAMPLE...PETROGRAPHY OF THE VOLCANIC...MAJOR AND TRACE ELEMENT...Sr, Nd AND Pb...DISCUSSIONCONCLUSIONREFERENCES |
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The Sr, Nd and Pb isotopic variations detected in CF whole-rock and xenolith samples suggest that crustal contamination had an important role in magma evolution.
On the basis of the Sr–Nd–Pb isotopic data and on the EC-AFC modelling results we hypothesize a multi-stage evolution process for the CF magma system in the past 60 kyr, which occurred in reservoirs localized at different depths (Fig. 9). Figure 9 represents the proposed model, which consists of a deeper magma chamber, periodically refilled by less-evolved magma that must have assimilated crustal material during fractionation through time. This deeper reservoir is inferred to be localized in the lower crust, according to the chemical and isotopic signature of vesicular xenoliths, which is similar to that of Hercynian basement. In this model the less-contaminated magmas had the isotopic imprint of the first-erupted pre-CI magma that resided in the deeper reservoir for a short time span. Batches of evolved magmas recurrently separated from this reservoir, migrated towards the sub-surface, and fed the shallower magmatic system, recording the isotopic signature that the parental magma had at that time. Contamination processes can occur also at this level, as evidenced especially by the Pb isotope data. This reservoir is probably localized at 3–5 km depth according to the chemical and isotopic signature of the holocrystalline xenoliths, which is similar to that of the arenaceous rocks located at the top of the carbonate platform. Moreover, the presence of a magmatic reservoir beneath the CF caldera at this depth is also inferred from seismic data (e.g. Ferrucci et al., 1992) and thermal modelling results (Wohletz et al., 1999).
In this way the CI and NYT magma types were generated, which fed eruptions at 37 and 12 ka, respectively, and whose residual portions fed the system in the last 12 kyr (D’Antonio et al., 1999b).
According to the proposed model, the most recently erupted, least-evolved, magmas would be the most contaminated, probably as a result of the length of parental magma residence time in the deeper chamber.
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ACKNOWLEDGEMENTS |
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Gruppo Nazionale per la Vulcanologia is acknowledged for financial support. Professor G. Tilton is thanked for his support in performing Pb isotope analyses at the University of Santa Barbara (California). We are grateful to M. Serracino (Centro Studio per il Quaternario e l’Evoluzione Ambientale, Rome) for support during microprobe analyses, and to Professor E. Franco (Dip. Scienze della Terra, University of Naples) for performing X-ray diffractometry analyses. We thank also R. Scippa and A. Carandente for helping in xenolith sample preparation and analysis, and G. De Astis for a preliminary review of the manuscript. We are grateful to M. Wilson, J. Davidson, C. J. Hawkesworth and R. Price for critical comments and useful suggestions that helped us to improve the paper.
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FOOTNOTES |
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Extended dataset can be found at http://www.petrology.oupjournals.org
*Corresponding author. Telephone: 39 81 6108446. Fax: 39 81 6100811. E-mail: monky5{at}ov.ingv.it
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