Breccias of the Sovana Eruptive Unit, Latera caldera, Vulsini Volcanic District, Italy.

摘要

Pyroclastic density currents (PDCs) are hot, swiftly-moving flows of volcanic gas, entrained air, juvenile particles, and accidental particles. Some types of PDCs, especially those from large caldera-forming eruptions, form relatively massive, poorly sorted, pumiceous deposits referred to as ignimbrites. Lithic breccia is the facies of ignimbrite which is lithic-rich, coarse-grained, rounded to angular, and often polymictic. This study documents the properties of the lithic breccia in the 205 +- 5 ka Sovana Eruptive Unit (SEU) which formed as a result of the first major phase of caldera collapse of the piecemeal Latera caldera (Vulsini Volcanic District, central Italy). The characteristics of SEU breccias, including morphologies, particle types and abundances, and relationship with the surrounding pumiceous ignimbrite, evidence a complicated history at the vent region and during flow. While there are many complications and limitations inherent in interpreting source and flow histories from deposit features, this study investigates the origin of lithic breccia within ignimbrite based on deposit features. The relative roles of both large-scale effects such as vent processes and caldera collapse as well as small-scale effects such as emplacement processes in forming these bodies were studied. In studying the trends of particle populations in Sovana Eruptive Unit deposits, the timing of caldera collapse and the propagation of the fragmentation surface during eruption relative to the supply of lithic particles in the vent region was determined. The variations in the characteristics of the individual lithic horizons resulting in unique stratigraphies at each outcrop location are the result of local variations in flow properties from various interacting phenomena.