Brief historical outline and lithologic terminology
The potassic magmatism in the Central Mediterranean area developed through time from Oligocene to present, the last eruption of leucite-bearing magmas occurring in 1944 A.D. at Vesuvius. A pioneering comprehensive study performed by Washington (1906) grouped the several magmatic potassic and ultrapotassic suites in three different magmatic regions on the basis of sole mineralogical and petrographic characteristics: the Tuscan, Roman and Apulian regions. These regions were established without any temporal constraints. Washington (1906) did not describe and group the potassic volcanic rocks from Western Alps, Corsica, Tuscan Archipelago, and intra-Apennine area (Umbria). Washington (1906) also used a local lithologic terminology with rocks terms ranging from Vulsinite, Ciminite, Vicoite, Italite, Sommaite, etc.; rock names not used anymore in the international terminology (Le Maitre et al., 2002).
No further comprehensive studies have been performed until the early sixties when Marinelli (1961, 1967) reviewed the rocks of Tuscan Magmatic Region, including both mantle derived igneous rocks and crustal anatectic ones, volcanic and intrusive, cropping out in the same area from Pliocene to Pleistocene. This study has somewhat complicated the general petrogenetic grid for potassic magmas. Some of the mafic Mg-rich rocks from Tuscan region have been subsequently interpreted as the result of complicate processes of enrichment in MgO, Ni, Cr and other compatible elements by gaseous transfert, starting from a granitic parental magma, of ultimate continental crustal origin (Mazzuoli & Pratesi, 1963; Barberi & Innocenti, 1967; Innocenti, 1967; Barberi et al., 1971). This brought also to perpetuate the hypothesis that leucite-free, Mg-rich, ultrapotassic magmas might have been generated by either direct partial melting of the continental crust or by interaction between leucitic magmas and crustal derived granitic melts (e.g., Taylor & Turi, 1976; Turi & Taylor, 1976; Vollmer, 1976, 1977, 1989; Vollmer & Hawkesworth, 1980; Vollmer et al., 1981; Turi et al., 1991; Gasperini et al., 2002).
Figure 1. Distribution of ultrapotassic and related volcanic and sub-volcanic rocks in Italy and surroundings
The picture has been modified after Conticelli et al., 2007, 2009b; Avanzinelli et al., 2008, 2009.
Avanzinelli et al. (2009) grouped the potassic and associated volcanic rocks of the Central Mediterranean area, by integrating the original division of Washington (1906) with the geochronological and genetic relationships, following the criteria suggested by Turner & Verhoogen (1960). A division in four Magmatic Provinces has been proposed. The Western Tyrrhenian (Corsican) Magmatic Province is the westernmost one, with few magmatic products belonging to volcanic suites ranging from leucite-free ultrapotassic to high-K calc-alkaline of Miocene-Pliocene age (Fig. 1). During Pliocene – Pleistocene, the volcanism migrated eastward to form the Tuscan Magmatic Province (Fig. 1) with emplacement of volcanic rocks belonging to leucite-free ultrapotassic to shoshonitic and calc-alkaline magmatic suites (e.g., Peccerillo et al., 1988; Conticelli & Peccerillo, 1990, 1992; Conticelli et al., 1992, 2001, 2004, 2007, 2009a, 2011a,b; Conticelli, 1998; Peccerillo, 1999, 2005a; Perini et al., 2000, 2003). Coeval intrusive to volcanic silicic rocks of crustal derivation by anatexis have been kept separated because they do not have a common origin, although in some cases hybridization between mantle and crustal derived magmas have been recorded (e.g., Poli et al., 1984; Pinarelli et al., 1989; Pinarelli, 1991; Innocenti et al., 1992; Poli, 1992, 2004). A further southeastward migration of volcanism during Pleistocene brought to the emplacement of the leucite-bearing ultrapotassic rocks of the Roman Magmatic Province, which in some cases are associated with younger shoshonitic to calc-alkaline suites (e.g., Conticelli et al., 1991, 2002, 2009b; Peccerillo, 2005a,b, and references therein; Boari & Conticelli, 2007; Frezzotti et al., 2007; Boari et al., 2009a). The volcanic activity of the Roman Magmatic Province pierced the boundary with the Holocene in its southernmost district, in the Neapolitan area, where a cluster of four volcanoes with historical volcanic activity occurs(i.e., Ischia, Procida, Phlegrean Fields and Vesuvius volcanoes; Peccerillo, 2005a, and references therein). The Lucanian Magmatic Province is the easternmost volcanic area with the association of Pliocenine haüyne- to leucite-bearing ultrapotassic rocks (e.g., Peccerillo, 2005a and references therein; De Astis et al., 2006; Avanzinelli et al., 2008). A carbonatitic lava has also been found in the activity of the Monticchio volcano, during the final stage of the Lucanian Magmatic Province (e.g. D’Orazio et al., 2007, 2008; Stoppa et al., 2008).
The volcanic suites
Ultrapotassic rocks have been defined using chemical parameters after Carmichael (1971) and Foley et al. (1987). A volcanic rock is considered ultrapotassic when it has K2O > 3 wt. % concomitantly to K2O/Na2O = 2 (Le Maitre, 2002). Mineralogical classification of potassic and ultrapotassic rocks is far from being exhaustive and produced a plethora of rock names due also to heteromorphism (Yoder, 1986). Indeed lamproitic rocks on the basis of their mineralogy are made up by a large variety of different rock types; fitzoyite, cedricite, orendite, madupite, wolgidite, cancalite, jumillite, verite, and fortunite are some of the most common lamproitic terms. To avoid this problem and to provide unequivocal classification, Foley et al. (1987) suggested a chemical division in three different clans on the basis of chemical parameters: the lamproite, kamafugite, and Roman (plagioclase-leucititic; Foley, 1992a) clans (Fig. 2). In Italy ultrapotassic rocks are associated in time and space also with calc-alkaline lamprophyres, shoshonitic and high-K calc-alkaline volcanic rocks, and at Monticchio volcanic field, within the Lucanian Magmatic Province, with alvikites.
Figure 2. Chemical classification of Ultrapotassic volcanic rocks
Resemblance classification diagrams after Foley et al. (1987), which should be used only with ultrapotassic rocks. In the transitional field fall also some rocks belonging to the shoshonitic series but that matches with the requirements for being considered ultrapotassic.
The lamproite clan is made up by Mg-rich alkaline ultrapotassic volcanic to hypabyssal rocks (Foley & Venturelli, 1989). According to Foley et al. (1987), the relatively low Al2O3, FeOTot, CaO and Na2O counterbalanced by extremely high MgO, and extremely variable silica contents, the latter ranging from basic to intermediate compositions are distinctive characteristics (Table 1). Lamproites are invariably plagioclase-free ultrapotassic rocks, characterized by highly forsteritic olivine, chromian spinel, Al-poor clinopyroxene, K-richterite, sanidine, picroilmenite, apatite (Fig. 3); leucite might be present in silica-undersaturated lamproites, but they have never been observed among the lithologies found in the Central Mediterranean (i.e., Corsica, and Tuscany).
The lamprophyre (calc-alkaline) clan is an category of sub-volcanic rocks found at convergent plate margins (Rock, 1989). Their names are based on the mineralogy, on the basis of the occurring feldspar and of the possible occurrence of amphibole (Le Maitre, 2002); minette, spessartite, and kersantite are the rock names. They are potassic to ultrapotassic with higher alumina and lime with respect to lamproites (Table 1), but in some cases they are chemically very similar to lamproitic and transitional ultrapotassic rocks (Fig. 2) on the basis of the criteria suggested by Foley et al. (1987).
The kamafugite clan is a group of rare kalsilite-bearing melilitites described for the first time by Holmes & Harwood (1932) and Holmes (1940). Because of the extreme mineralogical variability each rock recorded has taken the name of the locality where it was discovered, but heteromorphism do also occur as in lamproites (Yoder, 1986); Sahama (1974) suggested to keep the old names (i.e., katungite, mafurite, ugandite) for each single rock, but to collect them in a unique clan with a root name from the acronym of the African members: Katungite – Mafurite – Ugandite. Gallo et al. (1984) suggested the inclusion in the kamafugite clan also of the Italian terms, such as the coppaellite and the venanzite. Kamafugitic rocks are ultrapotassic but strongly silica and alumina undersaturated; with this respect they are larnite normative. According to the definition of Foley et al. (1987), kamafugite clan is made up by Mg-rich alkaline ultrapotassic volcanic rocks characterised by relatively low SiO2, Al2O3 and FeOTot but extremely high CaO and Na2O contents (Fig. 2; Table 1). Kamafugites are feldspar-free rocks although dominated by kalsilite, nepheline, and sometimes leucite as felsic phases and olivine, clinopyroxene, phlogopite, and melilite (Fig. 3) among the mafic ones (Sahama, 1952, 1954, 1960; Gragnani, 1972; Yoder 1976, 1979; Gallo et al., 1984; Cundari & Ferguson, 1991; Conticelli & Peccerillo, 1992).
The Roman rocks are leucite-bearing silica undersaturated ultrapotassic rocks and they are known worldwide since the original work by Washington (1906). Originally, a plethora of names has been used also for this group of rocks. These names recall the occurrence locality for the specific rock type (e.g., vulsinite, ciminite, cecilite, vicoite, italite, etc.). Le Maitre (2002) suggested a classification based on mineralogical and chemical parameters (Total Alkali Silica), thus leucite-bearing basanite, tephrite, phonolitic tephrite, tephritic phonolite, and phonolite are presently the basic names beside the term leucitite. All these rocks might be classified in a group name termed leucitite and plagio(clase)-leucitite clan on the base of the frequent occurrence of modal plagioclase (Foley, 1990a). According to the definition of Foley et al. (1987) the leucitite and plagio(clase)-leucitite clan (i.e., Roman rocks) is made up by alkaline ultrapotassic volcanic rocks characterised by widely variable MgO, with relatively low SiO2, and FeOTot, but relatively high CaO, Al2O3, and Na2O contents (Fig. 2; Table 1). This group of rocks was also defined by Appleton (1972) as high potassium series. The relative acronym, HKS, has been widely used in the specific scientific literature about Roman rocks. We prefer the use of the leucitite and plagio-leucitite clan because self explanatory of the main mineralogical characteristics of these rocks, and not locally related to the Italian magmatism. Leucite is ubiquitously present with plagioclase missing only in the most silica undersaturated terms (Fig. 3).
The haüyne-bearing clan is made up by highly silica undersaturated volcanic rocks characterised by high levels of both K2O and Na2O (De Fino et al., 1987; Beccaluva et al., 2002). Haüyne is also found as phenocryst along with leucite and rarely nepheline (Table 1; Fig. 3). These rocks are confined in the Lucanian Magmatic Province and have low silica contents. They are found associated to melilite-bearing rocks and alvikites.
All rocks with approximately 50 % vol. of carbonate minerals can be classified as carbonatite. Calcium-, Magnesium- and Iron carbonatite are divided on the basis of the main carbonate phase in the mode of the rock; namely calcite, dolomite, ankerite. Alvikites is a silica-rich calcium-carbonatite that is distinguished by Sövite on the basis of trace element contents (Le Bas, 1999). A hot debate about the occurrence of primary carbonatite clan in Italy is open since the end of the last century. Stoppa & Woolley (1997) made a review of the possible occurrence of carbonatite-like rocks in Italy, but some of them have been questioned to be real carbonatites. In some cases they have been classified as pyrometamorphic rocks due to either natural or anthropogenic combustion of marly sedimentary rocks (e.g. Melluso et al., 2003, 2005a, 2005b; Capitanio et al. 2004; Stoppa et al., 2005), in other cases some of them have been questioned as the result of carbonate sintexis to provide a wide spectrum of minerals, according to the findings of Tilley (1952), resulting in a dilution of major and trace element contents, but not of CaO (Peccerillo 1998; Peccerillo, 2004, 2005b, Wolley et al., 2005). Unquestioned mantle-derived alvikitic rocks have been documented at Monticchio monogenetic field (e.g., Jones et al., 2000; D’Orazio et al., 2007).
The shoshonitic series is a magmatic sequence of rocks mildly enriched in potassium, with variably silica saturated terms in the most evolved rocks. The rock types range from potassic trachybasalt (shoshonitic basalt) to trachyte, passing through shoshonite s.s. and latite. These rocks have variable enrichment in K2O in the most primitive terms. No definitive boundary between ultrapotassic clans and shoshonitic series has been drawn on the K2O vs. silica diagram (Peccerillo & Taylor, 1976); in some cases shoshonites are ultrapotassic rocks as well (e.g., Conticelli et al., 2011b,c) and they fall within the field of transitional group IV ultrapotassic rocks (Fig. 2) according to the classification scheme of Foley et al. (1987). In Italy this group of rocks has been originally recognised by Appleton (1972) at Roccamonfina volcano, who distinguished them as a low potassium series (LKS) group from the leucite-bearing ultrapotassic series (high potassium series; HKS). Later Civetta et al. (1981) renamed them as potassic series (KS) keeping the Appleton’s (1972) name for the leucite-bearing terms. Conticelli & Peccerillo (1992) interpreted the rocks belonging to this series as transitional (TRANS) from ultrapotassic series to calc-alkaline series. Conticelli et al. (2004) preferred the use of shoshonite the official names provided by IUGS (Le Maitre, 2002).
Figure 3. Microphotographs of thin sections of ultrapotassic and related volcanic and sub-volcanic rocks in Italy and surroundings
Photos have been performed at polarized microscope by Sandro Conticelli and Leone Melluso. White bar is 2 mm long. Legend: apa = apatite; bio = biotite; cpx = clinopyroxene; haü = haüyne; leu = leucite; mel = melilite; ne = nepheline; olv = olivine; phl = phlogopite; plg = plagioclase; sd = sanidine. Localities and Magmatic Provinces: A) Orciatico, Tuscan Magmatic Province; B) Torre Alfina, Tuscan Magmatic Province; C) Torre Alfina, Tuscan Magmatic Province; D) Radicofani, Tuscan Magmatic Province; E) Monte Amiata, hybrid volcano; F) Vesuvius, 1631-1944 A.D., Roman Magmatic Province – Neapolitan District (ND); G) Roccamonfina, Roman Magmatic Province; H) Vulsini, Roman Magmatic Province; I) San Venanzo, Roman Magmatic Province; J) Cupaello, Roman Magmatic Province; K) Pedra della Scimmia - Vulture, Lucanian Magmatic Province; L) Melfi - Vulture, Lucanian Magmatic Province.
The high-K calc-alkaline and calc-alkaline series are defined on the basis of K2O contents with respect to silica (Peccerillo & Taylor, 1976). They also match the chemical and mineralogical parameters provided by Arculus (2003). They are sub-alkaline rock with terms ranging from basalts to rhyolites, passing through basaltic andesite, andesite, and dacite (Le Maitre, 2002). The pre-fix high-K is added when needed according to the Peccerillo & Taylor’s (1976) scheme (Fig. 8b). These rocks are more represented within the ultrapotassic association of the Italian pensula than previously though (Di Girolamo et al., 1976; Perini et al., 2000; Conticelli et al., 2004, 2009a, 2010a,b; Boari & Conticelli, 2007; Frezzotti et al., 2007; Boari et al., 2009a).