The relationship between potassic and calc-alkaline post-orogenic magmatism at Vico volcano, central Italy

Post-orogenic magmatism in Italy took place during the Plio-Pleistocene. Alkaline ultrapotassic magmas and sub-alkaline magmas were erupted from closely-spaced volcanoes throughout the central part of Italy. Therefore, central Italy is one of the best site to study the relationship between calc-alkaline and alkaline magmatism in complex tectonic regimes. Activity at Vico volcano developed in three periods between 0.419 Ma and 0.095 Ma. Vice products overlie leucite-free volcanic rocks of Monte Cimino volcano that have a variable affinity from high-potassium calc-alkaline to ultrapotassic, Most of the Vice rocks are leucite-bearing Roman-type (ultrapotassic). However, during the first period, leucite-free silicic rocks are interbedded with leucite-bearing products. The early-erupted Vice magmas are isotopically distinct from the youngest magmas. Age-corrected Sr (Sr-i = 0.71110-0.71149) and Nd (Nd-i = 0.51210-0.51211) isotope ratios fall within the gap between Roman-type (including younger Vice) and Monte Cimino rocks. Some leucite-free Vice rocks contain orthopyroxene crystals and Al contents in clinopyroxenes are lower than those in clinopyroxenes from Roman-type rocks. These characteristics are, hox ever, also found in Monte Cimino rocks. At the beginning of the first period, leucite-free latites to trachytes and rhyolites with high Sr, (0.71127-0.71149) were erupted. Trachytes and leucite-bearing latites with lower Sr, (0.71110-0.71138) were erupted later. The younger trachytes have higher Rb, and LRE element contents than older leucite-free rocks with similar SiO2 contents. Furthermore, the onset of leucite crystallization is marked by a change in Sr-87/Sr-86 and SiO2. Sr-isotope ratios and their time-related variations, as well as the presence of two different trends in some trace element variation diagrams, define two distinct modes of magma differentiation. Magmas evolved along a stratigraphically "lower trend" at the beginning of the activity whereas they evolved differently along a stratigraphically "upper trend" later in the volcanic history. Modelling of geochemical and Sr-isotope variations indicates that, along the lower trend, fractional crystallisation was the main evolutionary process, whereas along the upper trend mixing between a leucite-free magma and a leucite-bearing magma occurred during crystallisation.