40Ar-39Ar geochronology and evolution of the Cimini volcanic district (Central Italy)

The Cimini volcanic district (CVD) is located in Central Italy and belongs to the Tuscan Magmatic Province (Miocene-Middle Pleistocene). It is characterized by several phases of activity during which lava domes, lava flows and ignimbrites have been erupted (Lardini & Nappi, 1987). Conticelli et al. (2013) indicate that the Cimini volcanic rocks range from ultrapotassic to potassic, where shoshonitic lavas represent the most primitive products and trachytic lava domes are the rocks richest in silica. Although some age determinations were made in the past for the CVD, evidencing a quite long eruptive history (see Fornaseri, 1985, for a review), a great uncertainty persisted on the age, as well as on the stratigraphy of the Cimini volcanic activity. To solve such doubts, a 40Ar-39Ar geochronological study has been performed in the framework of the new sheet No. 345 “Viterbo” of the Geological Map of Italy at 1:50,000 scale. The accurate geological survey at 1:10,000 scale allowed an improvement of the stratigraphy and a detailed and representative sampling of the whole volcanic sequence. Fourteen 40Ar-39Ar datings evidence an interval of activity of about 0.07 Ma, from 1.36 to 1.29 Ma, a period by far shorter than previously thought. Lava domes, which constitute a well preserved dome field, display the widest age dispersion (1.36÷1.29 Ma), while the ignimbritic and lava flows are all in the age interval 1.31÷1.29 Ma. The integration of geochronological and field data opens up to different interpretations about the eruptive history of the CVD than previously thought. The activity started with the growth of trachytic lava domes, followed by explosive eruptions that gave rise to pyroclastic flows which deposited a thick and widespread ignimbritic cover of latitic to trachytic composition. The building of further trachytic lava domes continued after this explosive stage. Latitic, olivinelatitic and shoshonitic lavas, coming from Mt. Cimino dome, are subsequent to the ignimbritic eruption, but both age data and field evidences are unable to fully elucidate their mutual relations and those with some of the last lava domes. However, some evidences question the allocation of olivine-latites and shoshonites as final products. The new datings enlarge the age gap between the end of the CVD and the beginning of the nearby Vico volcanic activity (0.42 Ma, Laurenzi & Villa, 1987).