Post-emplacement minerogenetic processes in Upper Tufo Giallo della Via Tiberina (Latium): a mineralogical and volcanological approach

The present research deal with volcanological and mineralogical features of a widespread pyroclastic formation from Sabatini Volcanic district: Upper Tufo Giallo della Via Tiberina (UTGVT). Volcanological and mineralogical aspects allow the reconstruction of minerogenetic processes that affects the three flow sub-units (a, b, c) of this formation. Volcanological data allow to ascribe the UTGVT formation to an emplacement mechanism belonging to pyroclastic flow. Field evidence suggests a water-magma interaction in pre-eruptive conditions. The emplacement temperature and the presence of abundant fluid phase, favoured the crystallization of zeolites. Chabazite is the main zeolite followed by phillipsite, in a and b sub-units, suggesting a calcic-potassic environment inside the system. The Ca-rich environment is confirmed by the occurrence of secondary calcite, spread inside the rock. Wherever deposit preserves high temperature for a long time, zeolites evolves toward a more stable phase (authigenic feldspars, adularia-like). Analcime is the main zeolite in the c sub-unit; its origin is due to the alteration of primary leucite. The lithification process of UTGVT formation is due to the occurrence of a double cement; which is made up of zeolites and secondary calcite. Crystallization of authigenic minerals (zeolites, feldspars) and the occurrence of calcite progressively enhances the physical properties of deposit.