Pb Isotopes Data from the Campanian Volcanic Province : A Model to Generate These Distinctive Pb Isotopic Variations

A broad set of samples from the CVP has been analyzing Pb isotopes. Campanian Volcanic Province (CVP) has been produced by various ranges of mixing between three components: 1) MORB, 2) Hercynian basement and 3) upper crust. Anyhow, the upper crust that has modified the Pb isotopic composition also shows trends towards an Achaean upper crust. This is consistent for all Mediterranean area. This is not in contrast with the past plate tectonic reconstruction. Also the surface sediments (GLOSS) may have been a mixing between the Achaean upper crust and a Hercynian component with an implication worldwide due to the rock cycle processes.


Introduction
The Campanian Volcanic Province (Figure 1) is made of the following volcanic area: Somma-Vesuvius volcano [1], Campi Flegrei Volcanic District [2] [3], Roccamonfina volcano [3] and the Island of Ponza and Ventotene [4].A tectonic scenario has been envisaged on the basis of geophysical, petrologic and geochemical data, where the magma is stored in the deep and hot intrusion zone [4]- [6].The magmas erupted have a sediments signature due to the adjacent Adria slab subducting [4] [7].Tomographic results confirm this scenario ( [8], references therein).In this report, I would like to unravel the features of the Pb isotopes data from the Campanian Volcanic Province [9]- [11].Moreover, a model is built to explain these typical Pb isotopes signatures.

A Geochemical Overview to Summarize the Magmatic Origin and Crustal Effects in Campanian Volcanic Province
The Campanian Volcanic Province is part of the classic western potassic volcanic province of the Italian Peninsula.The Campanian volcanic products show the effects of shallow assimilation and fractional crystallization, and the contribution of regional crustal sources (e.g., Hercynian basement-Calabrian crust).The Roccamonfina, Campi Flegrei, and Ventotene volcanic rocks are characterised by wide isotopic and geochemical variations.Such variations appear to reflect both AFC processes and chemical heterogeneity in the upper mantle that may be linked to subduction processes.Mixing curves (Th/Ce-, Ba/K-and Eu/Eu*-143 Nd/ 144 Nd) linking sediments and mantle end-members account for the variations in the Campanian Province volcanic rocks with a sediment contribution of 2% -10%.The upper mantle sources for the low-and high-K rocks at Roccamonfina have been constrained on the basis of a multi-element normalized diagram.The two sources require different amounts of sediment in the mantle wedge (LK~2% versus HK~10%) and a fluid component probably from altered ocean crust to explain the fluid mobile elements.Low-K Roccamonfina rocks are geochemically similar to those from Campi Flegrei, Ventotene, and Somma-Vesuvius, suggesting a similar proportion of sediment in their upper mantle source regions [3].

Sampling and Analytical Techniques
A representative suite of 300 volcanic rocks of samples was obtained on and around the flanks of Mount Somma-Vesuvius volcano, Roccamonfina volcano, Campi Flegrei Volcanic District and Ventotene Island on the basis of detailed field mapping (Rolandi, unpubl.Data); each rocks weighted 2 Kg, then was crushed to ash size, 2 g for each rocks were analyzed for Pb isotopic determinations.At least half of the samples are pumice and the rest are scoria and lava.Pb isotopes were determined at the US Geological Survey, Reston, VA, on a Finnigan-MAT 262 mass spectrometer.Pb isotopic analyses were done on 50 minerals separates (plagioclase, sanidine, and leucite).And 250 whole rocks (total of ~300 samples, Table 1 and Table 2), using static multi-collection mode.Isotopes were separated using standard anion-exchange methods: Pb isotopic ratios were normalized for mass fractionation by 0.1% per mass unit on the basis of repeated runs of NBS-981; Pb blanks during

Discussion
The Pb isotopic composition is presented from mainly the Somma-Vesuvius volcano and cover the eruption ages from Somma caldera activity [<39 Ka (age of the Campanian Ignimbrite)] until the last vulcanian effusive eruption of 1944 AD (Table 3, [12], references therein).Then, a representative sample collection of the Campi Fle-    [18], all the data worldwide of the shallow sediments and GLOSS (Global Sediment Subducting [19], Archean upper crust [16])].
end-menbers: 1) MORB, 2) Hercynian basement, 3) upper crust.This can be computed as I show with different styles of mixing duo to the large range of the end members.Anyhow, the result and interpretation does not change much.The model that I develop is that these volcanic products are produced by signatures of old arching upper crust and then assimilated with the Hercynian basement.Finally, I can also state that the recent sediments [19] are an average between the Hercynian component and the Archean upper crust as can be suggested by the past plate tectonic and the rock cycles.

Conclusion
Finally, I can say that the Campanian volcanic products are produced by a MORB component that has been contaminated by a series of upper crust component [20].Anyhow, I can assume that the distinctive isotopic trend has also been influenced by an Archean upper crust.The magma has also undergone to storage and melting of a Hercynian component as testified by the presence of this kind of basement below the Campanian Volcanic Province.This model has been called deep and hot intrusion zone by [5].In addition, I state that the GLOSS of [19] are a mixing of the Archean upper crust and Hercynian basement as in rock cycle operating.

Figure 1 .
Figure 1.Aerial photograph of the main volcanic area of the Campanian Volcanic Province.

Figure 2 .
Figure 2. Schematic stratigraphic columns of the Campanian Ignimbrites.Ages are ka and are based on incremental-heating40 Ar/ 39 Ar analyses of pure sanidine mineral separates (modified from[13]).

Table 1 .
Pb isotopic composition of the volcanic rocks and minerals from Mt. Somma-Vesuvius volcano.

Table 3 .
Eruption, age, and explosive tipology of Plinian and interplinian events in the last 25 ky.