Geochronology and time of magmatism
The first geochronological data related to the potassic magmatism in the Central Mediterranean area date back to many decades ago (Evernden & Curtis, 1965). Several data were accumulated with time, with variable geological confidence, with different accuracy and with an uneven distribution on the various volcanic centers (e.g., Marra et al., 2004 and Laurenzi, 2005 and references therein). K/Ar and 40Ar-39Ar data constitute almost the bulk of the age data on the Italian ultrapotassic and associated magmatic rocks, being data obtained with all other methods, as Rb/Sr, U-Th disequilibrium, Fission Tracks subordinated. 14C dating is quite widespread on products younger than 50 kyr, obviously limited to organic material found in and within volcanic products.
During this long period of time the technological progress led to instrumental improvements which enabled precise analyses on very small samples, at least for conventional K/Ar method and, above all, for 40Ar-39Ar method. It is sufficient to compare the weights of mineral separates used for the Ar analyses listed in Table 7 of Evernden & Curtis (1965) with the single crystal laser total fusion age data appeared in literature at the end of last century (Alvarez et al., 1996; Karner & Renne, 1998). Quite often published age data related to the same volcanic units disagree. This is particularly true for several old K/Ar data, and there are many reasons to explain the observed discrepancies. K/Ar ages are in fact model ages, because an atmospheric initial isotopic ratio is assumed in their calculation, and if this is not the case the obtained “age” value is older than the true one. Other “wrong” ages likely derive from the use of altered mineral phases, as revealed by their non-stoichiometric K contents. A further possibility of biased younger K/Ar ages is limited to sanidines, which have difficulties to melt completely (McDowell, 1983), with consequent underestimate of 40Ar concentration. It is clear from the above consideration that 40Ar-39Ar data, when available, will be preferred to K/Ar data.
Sometimes only age data obtained several years ago with the K/Ar method are available for the geochronological reconstruction. And rarely the whole set of products of a volcanic district has been dated and/or ages were inferred from stratigraphic reconstructions. The geochronological discussions on the various volcanoes and volcanic districts have different levels of deepening. Those areas having wide and recent geochronological data have on average short descriptions, whereas areas having few and/or old data have often much longer discussions, and sometimes new age calculations. There are many critical points and open questions on the majority of volcanic districts, and several data are needed to constrain these volcanic activities. Within the limits of published data, an effort was done at least to confine the beginning and the end of the known volcanic activity in each area. Pyroclastic products are widespread and likely overwhelming by volume among products of the potassic volcanism. These products are often found as distal tephra within marine and continental sedimentary succession. Their use and their ages are beyond the scopus of this paper, due also to the difficulty to identify precisely their provenance. Hence only pyroclastic deposits of consistent width and clearly assigned to a volcanic district will be considered. Within the limits of published data, we’ll try at least to confine the beginning and the end of the known volcanic activity in each area. Mentioned age data published before 1977 have been recalculated with Steiger & Jäger (1977) constants. 40Ar-39Ar ages, which are relative to monitor ages that might not be strictly comparable, have not been normalised to a unique monitor and monitor age. Consequent incidental age biases are well below the degree of detail of the following discussion, which aims to give a chronological framework for the ultrapotassic volcanism. For the same reason, on average age errors are not shown.
In the following paragraphs we will report a description of the different magmatic provinces occurring in Italy. This scheme follows the geographic location of Magmatic Provinces and then of Volcanic Districts from Northwest to Southeast, which broadly correspond to the onset of ultrapotassic magmatism (Fig. 6 - data used for drawing figure are from the following selected list: Evernden & Curtiss, 1965; Krummenacher & Evernden, 1965; Barberi et al., 1967; Borsi et al.,1967; Carraro & Ferrara, 1968; Nicoletti, 1969; Hunziker, 1974; Lombardi et al., 1974; Basilone & Civetta, 1975; Civetta et al., 1978; Bigazzi et al., 1981; Radicati et al., 1981; Cassignol & Gillot, 1982; Gillot et al., 1982; Metzeltin & Vezzoli, 1983; Pasquarè et al., 1983; Sollevanti, 1983; Fornaseri, 1985a, b; Laurenzi & Villa, 1985, 1987; Poli et al., 1987; Metrich et al., 1988; Savelli, 1983, 1988; Ballini et al., 1989a; Villa et al., 1989; D’Orazio et al., 1991; Turbeville, 1992; Cioni et al., 1993; Barberi et al., 1994; Laurenzi et al., 1994; Nappi et al., 1995; Laurenzi & Deino, 1996; Bellucci et al., 1999; Pappalardo et al., 1999; Villa et al., 1999; Brocchini et al., 2000, 2001; Giannetti & De Casa, 2000; Conticelli et al., 2001; De Vivo et al., 2001; Giannetti, 2001; Karner et al., 2001a, b, c; Mascle et al., 2001; Altaner et al., 2003; Marra et al., 2003, 2009; Rolandi et al., 2003; Deino et al., 2004; Cadoux et al., 2005; Freda et al., 2006; Florindo et al., 2007; Rouchon et al., 2008; Scaillet et al., 2008; Boari et al., 2009b; Cadoux & Pinti, 2009; Gasparon et al., 2009; Giaccio et al., 2009; Sottili et al., 2010). They are: i) the Oligocene Magmatism in the Western Alps; ii) the Miocene Magmatic events of the Western Tyrrhenian Magmatic Province (Corsican); iii) the Plio-Pleistocene Magmatic events of the Tuscan Magmatic Province; iv) the Monte Amiata, a Middle Pleistocene “hybrid” volcano; v) the Pleistocene Magmatic events I: the Roman Magmatic Province including the Latian districts (e.g., Vulsini, Vico, Sabatini, Colli Albani, Middle Latin Valley, and Roccamonfina), the intramontane Umbrian district (found within the Apennine chain), and the Neapolitan district (i.e., Ischia, Procida, Campi Flegrei, and Somma-Vesuvius volcanoes); vi) the Pleistocene Magmatic events II: the Lucanian Magmatic Province including the Monte Vulture volcano and the nested Monticchio lakes monogenetic field.
Figure 6. Age distribution of ultrapotassic and related volcanic and sub-volcanic rocks in Italy and surroundings.
Age distribution of ultrapotassic and related volcanic and sub-volcanic rocks in Italy and surroundings. For data source of geochronological data see text.
In each of these sections the ages reported are from a thoughtful evaluation of the original geochronologic data.