Piochi_corr.qxd Editrice Compositori The attached version of the paper by M. Piochi, L. Pappalardo and G. De Astis, «Geo- chemical and isotopical variations within the Campanian Comagmatic Province: implica- tions on magma source composition», fully replaces the one published in Annals of Geo- physics, 2004, vol. 47, n. 4, pp. 1485-1499. I.R. 1485 ANNALS OF GEOPHYSICS, VOL. 47, N. 4, August 2004 Key words lithosphere – asthenosphere – Campan- ian Province – Tyrrhenian Basin – geochemistry 1. Introduction The Campanian Plain is located between the western margin of the Apennine chain and the eastern border of the Tyrrhenian abyssal plain (fig. 1a,b). Since the Pliocene the Plain has been affected by intense extensive tectonic and mag- matism (e.g., Scandone, 1979; Peccerillo and Manetti, 1985; Di Girolamo, 1987; Doglioni, 1991). At the present time the Campanian Plain is bordered by conjugate NE-SW and NW-SE fault systems formed during the opening of the Tyrrhenian Sea. The volcanic districts of Campi Flegrei, Procida and Ischia, and the strato-vol- cano of Somma-Vesuvius developed on NE-SW structure lines (see Beccaluva et al., 1991, for a review). Campanian volcanoes are among the most studied volcanoes in the world. The many pa- pers published in the last decade (e.g., Civetta et al., 1991a,b, 1997; Civetta and Santacroce, 1992; Belkin et al., 1993; Santacroce et al., 1993; D’Antonio and Di Girolamo, 1994; Cioni et al., 1995; Orsi et al., 1995; Ayuso et al., 1998; D’Antonio et al., 1999a,b; de Vita et al., 1999; Marianelli et al., 1999; Pappalardo et al., 1999, 2002; Piochi et al., 1999; Del Moro et al., 2001; Somma et al., 2001) provide a wide data base of chemical and isotopic analyses. Howev- er the meaning of petrochemical variations Mailing address: Dr. Monica Piochi, Osservatorio Ve- suviano, Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano 328, 80124 Napoli, Italy; e-mail: monky5@ov.ingv.it Geochemical and isotopical variations within the Campanian Comagmatic Province: implications on magma source composition Monica Piochi, Lucia Pappalardo and Gianfilippo De Astis Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Napoli, Italy Abstract A spatial variation in chemical and isotopical composition is observed between the volcanoes belonging to the Campanian Comagmatic Province. At a given MgO content, magmas from volcanic islands (Procida and Ischia) are enriched in Ti, Na, depleted in La, Ba, Rb, Sr, Th, K contents, and shows lower LREE/HFSE (e.g., La/Nb = = 1-2), lower Sr-Pb isotopic ratios and higher Nd isotopic ratios with respect to magmas from volcanoes locat- ed inland (Campi Flegrei and Somma-Vesuvius). The observed compositional variations are explained involving two different mantle sources in the genesis of the magmas erupted in this region: a deeper asthenospheric man- tle source, from which the Tyrrhenian magmas also derived and a lithospheric mantle source enriched by slab- derived fluids. The contribution of the enriched-lithospheric mantle became more pronounced moving from the Tyrrhenian abyssal plain through the Italian Peninsula where it dominates, likely in response to the thickening of the lithosphere observed under the Peninsula. 1486 Monica Piochi, Lucia Pappalardo and Gianfilippo De Astis Fig. 1a,b. a) Southern Italy map showing location of the Campanian volcanic area and Tyrrhenian abissal Plain. Bold line is the cross section in fig. 6; b) geological and tectonic sketch map of the Campanian Plain showing distribution of sedimentary sequences and Campi Flegrei, Ischia, Procida and Somma-Vesuvius volcanic rocks (modified from Orsi et al., 1996). a b 0 km 10 shown by the erupted products is not complete- ly clear, and a debate exists about the genesis and evolution of Campanian magmas and its geodynamic setting (see Turi and Taylor, 1976; Vollmer, 1976; Hawkesworth and Vollmer, 1979; Cortini and Hermes, 1981; Vollmer et al., 1981; Peccerillo and Manetti, 1985; Ellam et al., 1989; Beccaluva et al., 1991; D’Antonio et al., 1996; Ayuso et al., 1998; Peccerillo, 1999, 2001; Somma et al., 2001). An open question about the nature of the most mafic magmas is whether they represent deep asthenospheric mantle magmas or reflects shallower imprinting acquired in the lithospher- ic mantle or in the crust. To shed light on this problem, our paper compares the available com- 1487 Geochemical and isotopical variations within the Campanian Comagmatic Province: implications on magma source composition positional data (major-, trace-elemental, and iso- topes) from rocks erupted from Campanian vol- canoes both situated inland (Somma-Vesuvius and Campi Flegrei) and offshore (Procida and Ischia), with that from magmas erupted on the Thyrrenian abyssal plain. In this way, the re- gional chemical and isotopic variations of mag- mas going from east to west can be investigated in relation to the deepening of the crust - mantle boundary (Ferrucci et al., 1989) and the increase in lithospheric thickness (Cella et al., 1998) ob- served moving from Tyrrhenian Basin towards the Apennine Chain. We assume that if signifi- cant modification of the deep mantle magmas occurs en route to the surface and/or by the crust, then such a process should be revealed by comparing the geochemical and isotopic signa- tures of offshore and inland volcanic complexes. 2. Geological setting The Campanian volcanoes are part of the potassic-rich Italian belt which developed on the eastern side of the Tyrrhenian Sea whose abyssal plain is composed of MORB-like basaltic rocks (Beccaluva et al., 1990). They constitute the Campanian Comagmatic Province that is distin- guished from the Roman one on the basis of some important geochemical differences in the outcropping volcanics (Duschenes et al., 1986; Beccaluva et al., 1991). The two Provinces are separated by a lithospheric discontinuity corre- sponding to the 41° parallel (fig. 1a,b) dividing two different mantle domains (Savelli and Wezel, 1979; Lavecchia, 1988; Spadini and Wezel, 1994; Bruno et al., 2000). The Crust- Mantle boundary beneath the Campi Flegrei and Somma-Vesuvius has been identified at 25-30 km, whereas it rises moving westward, being at less than 25 km beneath Ischia (Corrado and Rapolla, 1981; Ferrucci et al., 1989) and less than 10 km in the Central Tyrrhenian Basin (Duschenes et al., 1986; Gueguen et al., 1997). The nature of the lower crust is not known al- though the correlation with the Southern Apen- nine structures (Finetti and Morelli, 1973; Schutte, 1978; Scandone, 1982) and petrological studies on xenoliths (Pappalardo et al., 2002) suggest that it should be a Hercynian basement. The upper crust consists of a succession of Tri- assic to Cretaceous limestones and dolomites overlain by Miocene arenaceous and/or flysch sediments, or by pyroclastic rocks (D’Argenio et al., 1973; Ippolito et al., 1975) cropping out in the Apennine Chain. This succession, which is displaced down to 4000 m depth in the Campan- ian Plain (D’Argenio et al., 1973; Ippolito et al., 1975; D’Argenio et al., 1987), has been drilled at depths of about 2.4 km beneath the Somma- Vesuvius (Brocchini et al., 2001) and has been identified by seismic profiles at depths of more than 3-4 km in the Gulf of Napoli and Pozzuoli (Finetti and Morelli, 1974; Bruno et al., 1998). The Campi Flegrei, and the Ischia and Proci- da islands are volcanic fields of the Phlegraean Volcanic District characterized by several mono- genic vents that produced both low-energy explo- sive and effusive eruptions. High-volume ign- imbrite eruptions also occurred in the area at least from 200 kyr BP, some of these eruptions gener- ated large caldera structures at Campi Flegrei and Ischia (e.g., Rosi and Sbrana, 1987; Vezzoli, 1988; Orsi et al., 1996; De Vivo et al., 2001). Volcanism is broadly coeval at Campi Flegrei and Ischia where the onset of volcanic activity is dat- ed before 80 kyr and 150 kyr and lasted up to 1532 A.D. and 1302 A.D., respectively (Alessio et al., 1971; Gillot et al., 1982). Volcanic activity occurred from about 80 kyr up to 14 kyr at Proci- da (see Rosi et al., 1988; Beccaluva et al., 1991). Somma-Vesuvius is a strato-volcano that experi- enced lava flows and small-to-high scale energy eruptions; the oldest Somma-Vesuvius volcanic products are dated at 0.4 Myr (Brocchini et al., 2001) and the last eruption occurred in 1944 A.D. At the present time, Campi Flegrei, Ischia and Somma-Vesuvius are the site of widespread fumaroles and thermal springs, as well as seis- mic activity. Furthermore, bradyseismic phe- nomena, characterized by ground movements and seismic activity affected Campi Flegrei in 1972, 1984 and 2000 A.D. 3. Petrological setting The Campanian volcanic rocks belong to the KS series of Appleton (1972) and range in com- position from shoshonites to trachy-phonolites. 1488 Monica Piochi, Lucia Pappalardo and Gianfilippo De Astis The latter are the most widespread products. At Somma-Vesuvius milddly to highly undersatu- rated rocks belonging to the HKS of Appleton (1972), ranging in composition from alkali- basalt to phonolite, also occur. The Campanian rocks have nearly aphyric to strongly porphyrit- ic texture: generally the porphyriticity is higher in Somma-Vesuvius with respect to Phlegraean rocks, and in Ischia with respect to the Campi Flegrei and Procida rocks. In the KS evolved rocks K-feldspar, plagioclase, Fe-rich diopside, magnetite and biotite represent typical minerals. In the least evolved ones, K-feldspar and biotite are absent, olivine and Mg-rich diopside join the other phases reported. Apatite is the most com- mon accessory crystal. The least evolved HKS rocks from Somma-Vesuvius contain olivine, plagioclase and Fe-rich and Mg-poor diopside. Leucite is the most common mineral in the most evolved rocks. Garnet and phlogopite are very common accessory phases. Major and trace element and Sr-Nd-Pb iso- tope variations within the Campanian rocks have been attributed to complex evolutionary processes involving magmas chamber refilling, magmas mixing and crustal assimilation by mantle-derived magmas in a multi-depth mag- matic system (e.g., Cioni et al., 1995; Ayuso et al., 1998; D’Antonio et al., 1999a; Pappalardo et al., 1999, 2002; Piochi et al., 1999; Del Mo- ro et al., 2001). In particular, the existence of both deep and shallower crustal reservoirs has been proposed in several studies of the Cam- panian volcanoes (Belkin et al., 1985; Cortini et al., 1985; Piochi et al., 1999; Marianelli et al., 1999; Pappalardo et al., 2002; Lima et al., 2003; De Astis et al., 2004). Inside the Phle- graean Volcanic District, the deeper reservoir was tapped by a regional fault system during eruptions extruding the least evolved magmas that mingled during ascent with magmas evolving at shallower depth (De Astis et al., 2004; and references therein). Composi- tional differences have been detected in vol- canic rocks from Ischia island with respect to those from Campi Flegrei and Somma-Vesu- vius (e.g., Turi and Taylor, 1976; Cortini and Hermes, 1981; Paterne et al., 1988; Vezzoli, 1988; Pappalardo et al., 1999), although their meaning has not been thoroughly investigated. The source of magmastism has been located in a mantle variably enriched in incompatible elements, radiogenic Sr and unradiogenic Nd (e.g., Hawkesworth and Vollmer, 1979; Pecce- rillo and Manetti, 1985; Beccaluva et al., 1991; D’Antonio et al., 1996; Ayuso et al., 1998; D’Antonio et al., 1999b; Peccerillo, 1999, 2001). Debate exists about the agent of enrich- ment that should be i) mantle-derived fluids in a intra-plate tectonic setting (e.g., Cundari, 1980; Vollmer, 1989) or ii) fluids or melts re- leased by an undergoing oceanic slab (e.g., Di Girolamo, 1978, 1987; Hawkesworth and Vollmer, 1979; Peccerillo and Manetti, 1985; Peccerillo, 1990, 2001; Beccaluva et al., 1991; Serri et al., 1993; D’Antonio et al., 1996) which modify a OIB-type (e.g., Beccaluva et al., 1991) or a MORB-type mantle (e.g., D’An- tonio et al., 1996). Recently, the discovery of xenoliths of crustal origin in the least evolved 87Sr-enriched rocks from the Campi Flegrei strongly con- tributed to highlight both the role of crustal con- tamination in magmas composition and the ex- istence of a homogeneous mantle source be- neath the Phlegraean area (Pappalardo et al., 2002). The nature of xenoliths allows recogni- tion of at least two crustal levels of contamina- tion the deeper of which located in the Hercyn- ian basement. Following Cecchetti et al. (2001), this depth is about 10-15 km and is similar to that proposed by Belkin et al. (1985), Belkin and De Vivo (1993), Marianelli et al. (1999), Li- ma et al. (2003) and by Zollo et al. (1996) for Somma-Vesuvius. Crustal contamination has been proposed also at Somma-Vesuvius on the basis of mineral chemistry, fluid inclusion and isotope data (e.g., Savelli, 1968; Fulignati et al., 1995, 1998; Gilg et al., 1999, 2001; Del Moro et al., 2001; Pappalardo et al., 2004). 4. Data presentation 4.1. Geochemistry Major and trace elements have been pub- lished in previous papers (Civetta et al., 1991a,b, 1997; Civetta and Santacroce, 1992; Orsi et al., 1992, 1995, 1996; Caprarelli et al., 1489 Geochemical and isotopical variations within the Campanian Comagmatic Province: implications on magma source composition 1993; Belkin et al., 1993; Santacroce et al., 1993; D’Antonio and Di Girolamo, 1994; Cioni et al., 1995; Ayuso et al., 1998; D’An- tonio et al., 1999a,b; de Vita et al., 1999; Mar- ianelli et al., 1999; Pappalardo et al., 1999, 2002; Piochi et al., 1999; Del Moro et al., 2001; Somma et al., 2001); the database is available on request from the authors. Figure 2 shows selected major and trace element varia- tion diagrams versus SiO2. These diagrams, although sometimes scattering mainly as con- sequence of different analytical procedures Fig. 2. Selected major and trace elements variation diagrams versus SiO2. Source of data: Civetta et al. (1991a,b, 1997); Civetta and Santacroce (1992); Orsi et al. (1992, 1995, 1996); Caprarelli et al. (1993); Belkin et al. (1993); Santacroce et al. (1993); D’Antonio and Di Girolamo (1994); Cioni et al. (1995); Ayuso et al. (1998); D’Antonio et al. (1999a,b); de Vita et al. (1999); Marianelli et al. (1999); Pappalardo et al. (1999, 2002); Piochi et al. (1999); Del Moro et al. (2001); Somma et al. (2001). Symbols: × = lava ejecta in Solchiaro deposits (Procida); full diamonds: Procida samples; full triangles = Ischia samples; open squares = Campi Flegrei sam- ples; open circles = Vesuvius samples; ∗ = lithic clasts in Campi Flegrei deposits. 1490 Monica Piochi, Lucia Pappalardo and Gianfilippo De Astis Fig. 3a,b. a) REE patterns for selected Campanian magmas. Normalization on the basis of Nakamura (1974); b) trace elements distribution for selected Campanian magmas. Normalization on the basis of Pearce (1983). Symbols as in fig. 2. Dashed area in- dicates the intra-plate component following Thorpe et al. (1984). a b (ICP or XRF) and crystal contents (see previ- ous section), display increasing Na2O content and decreasing P2O5, TiO2, MgO, and Fe2O3 contents. K2O increases up to about 60 wt% of SiO2 and then decreases; MnO decreases up to 60 wt% of SiO2 then became strongly scat- tered. The most mafic compositions (MgO > > 9wt%) are represented just by some rocks from Procida. La, Ce, Rb, Y, Zr, Nb and Th in- crease by factors up to 2 from mafic to silic compositions; Sc, Cr, Co Ni, Eu, V show an opposite trend. Ba is firstly strongly scattered in the range 45-55 wt% of SiO2 content and then decreases, whereas in the same range, Sr is initially constant and then decreases. Some samples from Somma-Vesuvius, related to a single eruption, define a separate array. The most evolved rocks from Campi Flegrei and Is- chia show a steep enrichment in some incompat- ible elements (e.g., Th, REE). Magmas erupted at Somma-Vesuvius, define a trend depleted in Ti, Mg, Fe, P, Ca, V and enriched in Al, Nb, Rb, Th, K, Na with respect to the Cam- panian trends (fig. 2). Procida, Campi Flegrei and Ischia magmas show chemical similarity, al- though, at a given SiO2 content, the Ischia mag- mas are enriched in Ti, Na, Nb, Yb and depleted in Ba, Sr, Th, Eu (fig. 2). However, in the least evolved rocks (MgO > 6 wt%) from each volcanic district display reasonably constant ratios between elements with similar degree of incompatibility. For example (fig. 2), La/Ce, Th/Zr and, Nb/Zr are 0.5 ± 0.1, 0.06 ± 0.04, 0.14 ± 0.06, respectively. Some variations could be related to different ana- lytical procedures and analytical error. REE and other trace element distributions are reported in fig. 3a,b, where Campanian magmas of similar degrees of evolution (3.8 < MgO < 5 wt%) are compared. The only exception in these spiderdiagrams are the Campi Flegrei rocks with MgO contents less than 3.8 wt% that Pappalardo et al. (2002) sug- gest are representative of the least-contaminat- ed magmas. REE and trace element patterns are roughly sub-parallel to each other with no dif- ference between products from each district. Generally, all selected samples show enrich- ment in LREEs and other incompatible trace el- ements (Rb, K, Th, Sr, Ba), and minor enrich- ment in more compatible elements such as HREE and Y. Negative Nb and Eu anomalies characterize the Campanian rocks, although their amplitude decreases toward the less evolved samples. Mafic rocks from Procida de- fine a less enriched pattern, have lower REE and incompatible trace element contents coher- ently with their more primitive nature, and no detectable Eu anomaly. 4.2. Isotope geochemistry 87Sr/86Sr and 143Nd/144Nd ratios are variable and depict a trend from Procida towards the 1491 Geochemical and isotopical variations within the Campanian Comagmatic Province: implications on magma source composition crustal-derived xenoliths recovered in Campi Flegrei rocks (fig. 4a). Magmas erupted at Is- chia are isotopically distinct from that of Som- ma-Vesuvius and Campi Flegrei: Sr-isotope composition ranges from 0.7060 to 0.7068 at Ischia and from 0.7068 to 0.7089 at Somma- Vesuvius and Campi Flegrei. 87Sr/86Sr ratios al- so vary from 0.7051 to 0.7060 in the Procida volcanic rocks. Nd-isotope composition ranges from 0.51245 to 0.51265 in the Ischia rocks and from 0.51230 to 0.51265 at Campi Flegrei and Somma-Vesuvius. 143Nd/144Nd values range from 0.51255 to 0.5127 in rocks from Procida. 5. Discussion The absence of «primary» magmas compo- sitions at Campi Flegrei, Ischia, and Somma- Vesuvius prevents knowing directly the nature of the mantle source. Most of the mafic rocks produced in the Campanian region have much lower Cr, Ni and MgO contents with respect to those indicated by Perfit et al. (1980) (500-600 ppm, 300 ppm and 6 wt%, respectively), to have been in equi- librium with a peridotite magmas source. Some «primitive» compositions have been recognized in the lava ejecta on Procida island and on the basis of their high MgO, Cr and Ni contents (10-11 wt%, 426-610 ppm, 134-233 ppm, re- spectively) they were considered nearly pri- mary magmas (D’Antonio et al., 1996, 1999b). The features of the majority of the Campanian volcanic rocks are characterized by high SiO2 contents and high porphyritic indicis, and the absence of mantle-derived xenoliths. These fea- tures suggest that the primary magmas under- went shallow-level differentiation processes af- ter segregation from their mantle source. The isotopic variations reported above, not associat- ed with Fractional Crystallization (FC) process in closed reservoirs, have to be produced by in- teractions of parental magmas with crustal rocks en route to the surface or by addition of crustal materials to the source. Therefore, as we are interested in identifying the nature of the source, we will discuss the rock features inher- ited by shallow-level processes and then deal with the source related features. 5.1. Shallow level magma evolution Most of the chemical variations in Campan- ian rocks are consistent with a differentiation mechanism involving the fractionation of the observed mineral phases (FC) within closed magmatic systems. The trend defined by Sr and Eu respect to the silica content (fig. 2) and the existence of Eu anomaly in the most evolved rocks (fig. 3a,b) are consistent with the role of feldspar (the main crystal phase) crystallization during magmas evolution. The FC process is supported by the constancy of the Sr-isotopic ratios in rocks from some eruptive events at Is- chia (Civetta et al., 1991b; Piochi et al., 1999), Campi Flegrei (Pappalardo et al., 1999) and Somma-Vesuvius (Ayuso et al., 1998 and refer- Fig. 4a,b. Selected LILE/HFSE diagrams for Cam- panian rocks with MgO > 3.8%. Symbols: full cross- es = Tyrrhenian samples (Beccaluva et al., 1990; Gasperini et al., 2002). Other symbols as in fig. 2. a b 1492 Monica Piochi, Lucia Pappalardo and Gianfilippo De Astis ences therein). On the other hand, variations of the isotopic compositions of magmas with time have generally been related to mixing process- es between distinct parental magmas batches reaching a shallow reservoir (Cioni et al., 1995; D’Antonio et al., 1999b; Pappalardo et al., 1999; Piochi et al., 1999). New studies on Campi Flegrei (Pappalardo et al., 2002), on Somma-Vesuvius (Del Moro et al., 2001; Pap- palardo et al., 2004) and Ischia (Piochi et al., 1999) eruptions have also highlighted that the variability of the Sr-isotope ratio can be ex- plained by crustal contamination in a multi- depth magmatic system. In these cases, mag- mas show an interaction with the wall-rocks because they contain crustal xenoliths (Campi Flegrei) and hornfelsed clasts (Somma-Vesu- vius) with Sr isotope ratios > 0.708 and some- times show chemical and mineralogical dise- quilibria. Moreover, Pappalardo et al. (2002) showed that the Sr-isotopic ratios increase in the younger Campi Flegrei products as their residence time in the magmas chamber increas- es. Furthermore, crustal interaction is indicated by the negative correlation described by Cam- panian volcanics and crustal xenoliths on the classical isotope diagram of Sr and Nd (fig. 5a). Finally, crustal contamination for the Campan- ian magmas is suggested by the La/Nb ratio which is always > 1 (Thompson et al., 1984) and positively correlated to the Sr isotope ratio. Calculation based on the quantitative EC-AFC (Energy Conservation-Assimilation Fractional Crystallisation) approach that accounts for mass and energy conservation (Bohrson and Spera, 2001; Spera and Bohrson, 2001) indi- cates that the isotopic variations relative to the magmas feeding eruptions at Campi Flegrei and Somma-Vesuvius are explained by contamina- tion with less than 40% of crustal rocks (Pap- palardo et al., 2002, 2004). Despite the evidence of crustal interaction, there is a wealth of papers which suggest the ex- istence of a «crucial» Sr-isotopic value below which we can assume to deal with «mafic rocks». A correlation is observed between the Sr-isotopic ratios of rocks and their crystallization depth at Somma-Vesuvius (Mastrolorenzo et al., 2003; Pappalardo et al., 2004). Olivines and pyroxenes from Somma-Vesuvius rocks crystallized at depths >10 km (Belkin et al., 1985; Cortini et al., 1985; Belkin and De Vivo, 1993; Marianelli et al., 1999; Lima et al., 2003) have Sr-isotopic ra- tios lower than 0.7074 (Civetta and Santacroce, 1992; Ayuso et al., 1998, and references therein). These values are significantly different from those of the late-crystallized feldspars (crystal- lization depth < 5 km; Belkin et al., 1985; Cortini et al., 1985; Belkin and De Vivo, 1993; Lima et al., 2003), with 87Sr/86Sr values around 0.7075-7 (Civetta et al., 1991a; Civetta and Santacroce, 1992; Cioni et al., 1995; Ayuso et al., 1998, and references therein). Moreover, clinopyroxene cu- mulitic rocks from Somma-Vesuvius and Campi Flegrei (Ayuso et al., 1998 and references there- in), display 87Sr/86Sr below 0.7065. Pappalardo et al. (2002) exclude that the isotopic variations at Campi Flegrei are related to in situ radioactive growth, because a much longer time than that of the magmatic system (ca. 60 kyr) would be re- quired to produce the observed isotopic variabili- ty, and indicate that petrological variation not re- flect mantle melting processes and/or mantle source(s) heterogeneity because the constancy of ratios between elements with similar degrees of incompatibility of the products. The authors, on the basis of a correlation between Sr-isotopic ra- tios and age, and of Sr-isotopic similarity be- tween the least radiogenic rocks and the mantle xenoliths from Vulture, have linked the least con- taminated magmas erupted at Campi Flegrei with those erupted before the Campanian Ignimbrite eruptions characterized by a Sr-isotopic ratios in the range 0.7067-0.7073. On the basis of these studies, we can choose as the least contaminated Campanian rocks those showing Sr-isotopic ratios lower than 0.70735. We can compare them with the mafic rocks from Procida and Ischia, which have 87Sr/86Sr widely below this limit and geochemi- cal features akin to more primitive rocks. 5.2. Primary magma compositions and the nature of mantle sources The possibility to discriminate the role of the shallow evolutionary processes (in particular crustal contamination) allow some representative mafic or near-mafic rocks to be investigated fur- 1493 Geochemical and isotopical variations within the Campanian Comagmatic Province: implications on magma source composition ther although they have variable MgO contents (3.8-10-11 wt%). These rocks that approach or mirror the «original» geochemical and isotopical composition are those reported in figs. 3a,b, 4a,b and 5a-c. In the following, the Campanian rocks we have chosen will be compared with the tran- sitional MORB-type basalts erupted in the con- tiguous sector of Tyrrhenian Basin (Vavilov Basin, sites 655 and 651 of ODP Leg. 107 and DSDP site 373, Leg. 42; Beccaluva et al., 1990; Gasperini et al., 2002) and with the intraplate basalts erupted on the opposite margin of the sea basin on the Sardinia coast (site 654). The comparison among most mafic rocks from volcanic islands and from volcanoes locat- ed inland, as regards the major, REE and sever- al trace elements pattern (figs. 2 and 3a,b) reveal some significant differences. At a given SiO2 content, Ischia and Procida magmas are en- riched in Ti, Na, depleted in La, Ba, Rb, Sr, Th, K contents, and show lower LREE/HFSE (e.g., La/Nb = 1-2) with respect to the Campi Flegrei and Somma-Vesuvius magmas. Furthermore, LILE/HFSE ratios for rocks characterized by MgO exceeding 3.8% increase moving west to east through the Campanian volcanic area (fig. 4a,b). This trend is highlighted if we consider data (Beccaluva et al., 1990; Gasperini et al., 2002) for MORB-like magmas erupted in the contiguous sector of Tyrrhenian Basin (Vavilov Basin, sites 655 of ODP Leg. 107 and DSDP site 373, Leg. 42) that is also reported in fig. 4a,b. The least contaminated Campanian samples define a unique trend when plotted in the 87Sr/86Sr versus 143Nd/144Nd diagrams (fig. 5a). Procida shoshonitic basalts have the lowest Sr a b c Fig. 5a-c. a) 143Nd/144Nd, b) 208Pb/204Pb, c) 206Pb/204Pb versus 87Sr/86Sr diagrams for selected Campanian magmas. Symbols: open crosses = Etna samples (Tanguy et al., 1997); full crosses = Tyrrhenian samples (Beccaluva et al., 1990; Gasperini et al., 2002); field for the Vulture mantle xenoliths (a) from Downes (2001) and hypothesized (c) on the basis of considerations from Downes (2001) and Dunai and Baur (1995). Other symbols as in fig. 2. 1494 Monica Piochi, Lucia Pappalardo and Gianfilippo De Astis ratios (0.7050) and highest Nd (0.5128) ratios in the Campanian magmas, whereas the Campi Flegrei and Somma-Vesuvius rocks have the highest Sr ratios (0.7073) and the lowest Nd ra- tios (0.5126). Ischia magmas plot between these two compositions (fig. 5a). The Sr-iso- topic differences are highlighted in 87Sr/86Sr versus 206Pb/204Pb diagrams (fig. 5b,c). As in fig. 4a,b, in these isotope-isotope dia- grams (fig. 5a-c) we have also drawn the field for MORB-like magmas erupted in the contigu- ous sector of Tyrrhenian Basin (Vavilov Basin, sites 655 of ODP Leg. 107 and DSDP site 373, Leg. 42) interpreted as derived from an up- welling asthenospheric mantle (Beccaluva et al., 1990) probably having a plume-like imprint (Gasperini et al., 2002). In the Sr-Nd isotope di- agram (fig. 5a), the primitive magmas erupted at Procida plot between the Campi Flegrei and Somma-Vesuvius magmas and the Tyrrhenian magma field. This is supported also by Sr-Pb trends (fig. 5b,c) where the relationship be- tween the Tyrrhenian and the Campanian mag- mas is highlighted. Therefore, we consider the Tyrrhenian basalts as end-members for the iso- topic trend of fig. 5a-c. As we have excluded from the discussion the crustally contaminated rocks, we assume that the other end member is also located in the mantle domain. The trace element patterns of the Campanian magmas (fig. 3a,b) are similar to intraplate-magmas with respect to immobile elements Zr, Hf, Ti, Y, Yb and, at to a lesser ex- tent, Nb but they are enriched in low ionic po- tential elements. Recent studies (Kostoula et al., 1999; Downes, 2001) suggest a lithospher- ic origin for the mantle xenoliths recovered in the volcanic products from Vulture Volcano lo- cated to the east of the Campanian Plain. These xenolith samples are geochemically and iso- topically similar to the Campanian magmas (fig. 5a-c). On this basis we suggest that the other end member could be sub-continental en- riched lithospheric mantle. In particular, the en- richment in the low ionic potential elements, es- pecially Rb and Ba, the small negative anomaly at Nb, and the ratio between incompatible el- ements (e.g., Th/Yb > 2) should be generated by the interaction with subduction related fluid as also suggested for the Vulture xenoliths (Downes, 2001). Following Peccerillo and Manetti (1985) the higher K-enrichment of the most Somma-Vesuvius rocks with respect to the other Campanian rocks should be the conse- quence of melting at greater depths. Trace element distributions and isotope com- positions of the volcanic rocks from the islands (higher Ti and P content and lowest Sr-Pb iso- tope ratios) compared with those from the main- land volcanoes, indicate that the asthenospheric contribution is more evident in the offshore vol- canoes (Procida and Ischia) and it tends to van- ish moving inland (Campi Flegrei and Somma- Fig. 6. Trace elements distribution for Procida maf- ic magmas normalized with respect to sample 654 from western margin of Tyrrhenian Sea (data from Beccaluva et al., 1990), to samples 655 from Vavilov Basin (data from Gasperini et al., 2002) and to sam- ples from Sardinia mafic volcanism (data from Lus- trino et al., 2000). 1495 Geochemical and isotopical variations within the Campanian Comagmatic Province: implications on magma source composition Vesuvius) where the lithospheric contribution becomes dominant. This can be a consequence of the progressive drop of the lithosphere-as- thenosphere boundary moving from the Tyrrhen- ian Basin towards the Appennine Chain. Its depth varies from 25 km under the Tyrrhenian Basin, to 65 km beneath the Thyrrenian coast of Italy and finally up to a depth of 100 km under the Apenninic Chain (Cella et al., 1998). To test this hypothesis we compared the primitive Procida magmas (MgO = 11 wt%) with the anorogenic intraplate tholeiitic magmas erupted on the other border of Tyrrhenian Sea (MgO = 7 wt%), near the Sardinia coast (site 654), in the Vavilov Basin (site 655) and on the Sardinia Island (fig. 6). The similarity between Procida and Sardinia mafic rocks, also in term of Sr, Nd and Pb isotope ratios (see Lustrino et al., 2000), as well as the similarity in immobile elements (i.e. Sr, Nb, Zr, Sm, Yb) contents shown by Procida magmas and Tyrrhenian mag- mas, evidence that moving radially away from the basin toward its eastern (Italian Peninsula) and western (Sardinia Island) borders, the con- tribution of the asthenospheric MORB-type mantle decreases in favour of that of the «in- Fig. 7. Interpretative cross section from Tyrrhenian Sea towards the Campanian margin as indicated in fig. 1a,b. Chemical and isotopical variations moving from the Tyrrhenian Basin to the Campanian Comagmatic Province and to Sardinia are shown. 1496 Monica Piochi, Lucia Pappalardo and Gianfilippo De Astis traplate» lithospheric mantle. Furthermore, moving away from the Tyrrhenian Basin the contribution of subduction increases. 6. Conclusions A spatial variation in chemical composition has been recognised among the volcanoes be- longing to the Campanian Comagmatic Province (fig. 7). When the role of low-P evo- lution processes has been evaluated, the most primitive (not contaminated by crustal assimila- tion) rocks erupted from Procida and Ischia is- lands have: i) lower LILE/HFSE ratios with re- spect to those erupted at Campi Flegrei and Somma-Vesuvius. These ratios are insensitive to partial melting and fractionation processes and can be considered to be representative of the magmas source; ii) lower Sr, Pb isotopic ra- tios and higher Nd isotopic ratios with respect to that of Campi Flegrei and Somma-Vesuvius. These compositional variations can be ex- plained assuming that both asthenospheric and «intra-plate» lithospheric mantle sources are in- volved in the genesis of the «primitive» mag- mas erupted in the Campania Comagmatic Re- gion. The asthenospheric mantle, from which Tyrrhenian magmas were extracted, represents the deeper source. Moving from the Tyrrhenian abyssal plain to the Italian peninsula the contri- bution of enriched-lithospheric mantle became more pronounced in response to the increasing lithospheric thickness that magmas must cross en route to the surface (fig. 7). The involvement of the lithosphere may be explained by geotherms uprising related to the as- thenospheric upwelling. 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