Contrib Mineral Petrol (1981) 78:37 47 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1981 Geochemical Characteristics of Potassic Volcanics from Mts. Ernici (Southern Latium, Italy) L. Civetta 1, F. Innocenti z, P. Manetti 3, A. Peccerillo 3, and G. Poli 3 1 Istituto di GeologJia e Geofisica, University of Naples, Largo S. Marcellino 10, Napoli, Italy z Istituto di Mineralogia e Petrografia, University of Pisa, Via S. Maria 53, Pisa, Italy 3 Istituto di Mineralogia, Petrografia e Geochimica, University of Florence, Via La Pira 4, 1-50121 Firenze, Italy Abstract. Major elements, trace elements and 87Sr/86Sr data are reported for the Quaternary potassic alkaline rocks from the Mts. Ernici volcanic area (Southern Latium - Italy). These rocks are represented by primitive types which display high Mgv, low D.I., variable degrees of silica undersaturation and different K 20 contents which allowed the distinction of a potassium series (KS) and a high potassium series (HKS). All the analyzed sam- ples have high LIL element contents and high S7Sr/86Sr which ranges between 0.707-0.711. They also have fractionated REE patterns. The KS rocks have lower LIL element concentrations and 87Sr/86Sr ratios than the HKS rocks with a large composi- tional gap between the two series. Minor but still significant isotopic and trace element variations are also observed within both KS and HKS. The genesis cannot be completly explained either by crystal liquid fractionation, mixing or assimilation pro- cesses or by different degrees of equilibrium partial melting from a homogeneous source, thus indicating that both the KS and HKS consist of several geochemically and isotopically distinct magma types. The data suggest that the KS and HKS magmas originated by low degrees of melting of a garnet peridotite mantle heterogeneously enriched in LIL elements and radiogenic stron- tium, possibly accompanied by disquilibrium melting of some accessory phases. The occurrence of a geochemical anomaly within the mantle is believed to be due to fluid metasomatism probably generated by dehydration of a lithospheric slab sub- ducted during the Late Tertiary development of the Apennine Chain. Introduction The potassic alkaline volcanism of the so called" Roman Comag- matic Province" of Washington (I906), represents the largest part of the Pliocene-Quaternary magmatism occurring in the central southern part of Italy. It crops out along the Thyrrenian coast from Northern Latium to the Neapolitan area. Petrographical and chemical investigations permitted the rec- ognition of two main rock series (Appleton 1972; Ghiara and Lirer 1977; Civetta et al. 1979; Ghiara et al. 1979): a high potas- sium series composed of rocks ranging from leucititic to leucite- phonolitic composition and a potassium series made up of rocks such as trachytes, latites and alkali basalts. In some areas (e.g. Roccamonfina) both series occur generally displaying a predomi- nance to more evolved rock types. All the volcanic products, Reprint requests to : A. Peccerillo even the most primitive ones, have variable but high contents of many incompatible elements such, as K, Rb, Sr, Ba, LREE, Th as well as high 160/180, 87Sr/86Sr ratios and a low l~3Nd/ 144Nd ratio which make the problem of the origin the K-rich magmatism difficult to understand. The genetic hypotheses so far proposed include deep crustal origin (Marinelli 1967; Capaldi et al. 1972), interaction between crust and upper mantle (Turi and Taylor 1976; Vollmer 1976; 1977) and melting of a LIL element enriched mantle for primitive rock types (Cox et al. 1976; Carter etal. 1978; Hawkesworth and Vollmer 1979; Vollmer and Hawkesworth 1980), and interaction with crust during fractionation (Taylor et al. 1979; Vollmer and Hawkes- worth 1980). A useful way to approach the problem of the genesis of both series is to study the more primitive rock types possibly cropping out in one small area. This reduces the problem of crustal contamination and the chemical variations induced by low-P fractionation processes, a process well documented for the evolved rocks of Roccamonfina volcano (Taylor et al. 1979). For this purpose a particularly suitable zone is that of Mts. Ernici where several small volcanic edifices composed of rather primitive products of both series occur. In this paper chemical and Sr isotopic data on selected sam- ples from Mts. Ernici are presented and their bearing on the problem of the genesis of the Italian potassium-rich rocks is discussed. Geological Setting The Mts. Ernici volcanoes are represented by a series of about twenty small edifices cropping out in a relatively small area (Media Valle Latina) situated at about 70 Km SE of Rome (Fig. 1). Their activity, mainly explosive, produced large quanti- ties of pyroclastics, lava flows being largely subordinate. The volcanic rocks lie on a basement made up of Mesozoic carbon- ates and Upper-Middle Miocene calcareous and arenaceous sedi- ments. Structurally the area is affected by two main tectonic events in the Late Miocene and Pliocene: compressional move- ments followed by distension which caused subsidence of the whole area along NW-SE and NE-SW tectonic trends (Angelucci 1967). The volcanics erupted along these fractures as shown by the alignment of the volcanic centers (Angelucci et al. 1974). The volcanic activity developed between 0.7 and 0.1 m.y. b.p. The most K-rich products were the first to be erupted (0.7- 0.2 m.y.b.p.) followed by the less strongly K-rich volcanics, which range in age approximately between 0.2-0.1 m.y.b.p. (Basilone and Civetta 1975; Civetta unpl. data). 0010-7999/81/0078/0037/$02.20