Rossi, P. L.

Nel Canale di Sicilia, insieme ai processi estensionali che hanno generato le tre principali depressioni tettoniche di Pantelleria, Linosa e Malta, si sono succeduti anche importanti fenomeni vulcanici. A partire da circa 8 milioni di anni fa, in questa regione ha preso posto un vulcanesimo toleiitico e alcalino, che ha creato le due isole vulcaniche di Pantelleria e Linosa ed un numero elevato di apparati sottomarini, molti dei quali ancora poco sconosciuti. Il vulcanesimo è ancora attivo e le eruzioni storiche sono tutte sottomarine; per alcune di esse abbiamo solo indicazioni vaghe, altre sono state segnalate ma mai controllate. Possediamo notizie certe solo delle due attività che hanno portato alla formazione delle isole effimere di Ferdinandea (1831) e Foerstner (1891), quest’ultima localizzata a circa 4 km a N-O delle coste dell’Isola di Pantelleria.

The Miocene-Quaternary eruptive and tectonic history of El Tatio volcanic area (central Andes, northern Chile) is described here on the basis of a new 1:50,000 scale geological map.

We report new petrographic and geochemical data on volcanic rocks erupted over the last 9 Ma ca. within El Tatio volcanic region (Western Cordillera – CVZ). They originated from compound volcanism alternating composite volcano activities, lava domes formation and minor low-mild explosive eruptions, whereas ignimbrite-like deposits outcropping in the region originated from external caldera system (Altiplano Puna Volcanic Complex). The volcanics – mostly erupted in the last 1 Ma - have composition ranging between calcalkaline (CA) to high-K calcalkaline (HKCA) basaltic andesite to rhyolite, but most of them are andesites and dacites. Petrographic features of the studied rocks can be frequently related to strong disequilibrium conditions in the crystallizing system: deeply resorbed and rounded mineral phases, reaction rims, skeletal habits, large ranges of mineral compositions with direct and reverse zoning, oxidations and uralitizations phenomena are observed. Most of these crystal disequilibrium features may be explained by convective self-mixing processes in magma reservoirs cooling from above and characterized by mafic magma batch at the base as a probable heat source, with minor role for magma mixing between compositionally different magmas. On the other hand, the high crystallization degree of the rocks together with evidence provided by geochemical data suggest that FC of recurrent mineral assemblages (plag.±pyrox±hornbl±bt) in closed magma chamber must be considered the most significant differentiation process for several volcanic apparatus forming El Tatio area. Although the FC holds a leading role, the correlation of geochemical and few available isotopic data also point to AFC processes as further important evolutionary mechanism. Isotopic trends and patterns of incompatible elements (i.e LILE enrichment, LILE/HFSE ratios) mirror the interaction of magmas with crustal contaminants occurring both in the mantle wedge (material transported via subduction) and during the ascent/ ponding of the parental magmas along the thick crust beneath the CVZ. In other words, starting from already modified parental magmas, further modification of variable type affected the magmas at higher level. Finally, we suggest that the plumbing systems (high level magma reservoirs) hosting the magmas of El Tatio region were characterized by multistage evolutionary processes consistent both with open- and closed-system regimes.

On 3rd November 2002, at about 3 km off-shore of Panarea Island (Aeolian Islands, Southern Italy), a series of gas vents suddenly and violently opened from the seafloor at the depth of 10-15 m, with an unusually high gas flux and superimposing on the already existing submarine fumarolic field. Starting from the 12th November 2002 a discontinuous geochemical monitoring program was carried out. The emissions consisted in an emulsion whose liquid phase derived from condensation of an uprising vapor phase occurring close to the fluid outlets without significant contamination by seawater. The whole composition of the fluids was basically H2O- and CO2-dominated, with minor amounts of typical «hydrothermal» components (such as H2S, H2, CO and light hydrocarbons), atmospheric-related compounds, and characterized by the occurrence of a significant magmatic gas fraction (mostly represented by SO2, HCl and HF). According to the observed temporal variability of the fluid compositions, between November and December 2002 the hydrothermal feeding system was controlled by oxidizing conditions due to the input of magmatic gases. The magmatic degassing phenomena showed a transient nature, as testified by the almost complete disappearance of the magmatic markers in a couple of months and by the restoration, since January 2003, of the chemical features of the existing hydrothermal system. The most striking feature of the evolution of the «Panarea degassing event» was the relatively rapid restoration of the typical reducing conditions of a stationary hydrothermal system, in which the FeO/Fe1.5O redox pair of the rock mineral phases has turned to be the dominating redox controlling system.