HR: 0830h
AN: V31C-0953    [PDF]
TI: Fast Orogenic to Anorogenic Magmatic Transition in Central Tyrrhenian Basin at Pontine Islands
AU: * Cadoux, A
EM: cadoux@geol.u-psud.fr
AF: Laboratoire de Geochronologie, FRE 2566 Orsayterre, Bat 504, 1 etage, Universite Paris SUD, Orsay, 91405 France
AU: Pinti, D L
EM: pinti@geol.u-psud.fr
AF: Laboratoire de Geochronologie, FRE 2566 Orsayterre, Bat 504, 1 etage, Universite Paris SUD, Orsay, 91405 France
AU: Gillot, P
EM: gillot@geol.u-psud.fr
AF: Laboratoire de Geochronologie, FRE 2566 Orsayterre, Bat 504, 1 etage, Universite Paris SUD, Orsay, 91405 France
AB: The location of the Pontine Archipelago makes it a key area to describe the initial Neogene volcanism in the Tyrrhenian basin. This archipelago is indeed at the connection between the Northern and the Southern Tyrrhenian basin. A new geochronological and geochemical survey of the two northernmost islands of Ponza and Palmarola permits a detailed examination of the complex tectonic and volcanologic evolution of this area. Previous studies suggested that volcanic activity began before 5 Ma on Ponza. Twenty-five new K-Ar ages, obtained by the Cassignol-Gillot technique, constrain the volcanic activity (mainly rhyolites) of Ponza to the last 4.2 Ma, with two episodes of quiescence between 3.7 and 3.2 Ma and between 2.9 to 1.0 Ma. At 1.0 Ma, a trachytic episode ended the volcanic activity in Ponza. A new volcanic episode dated at 3.2-2.9 Ma has been identified on the central and southern portion of the island, with emplacement of pyroclastic units. Palmarola has rhyolitic products with K-Ar ages from 1.6 to 1.5 Ma. Although only 6-8 km apart, the two islands show significantly different geochemical signatures. Trace element ternary diagrams and Y/Nb and Yb/Ta ratios show that Ponza Pliocene rhyolites are representative of magmas emplaced in a syn-collisional orogenic context, while Pleistocene products (Palmarola rhyolites and Ponza trachytes) are characteristic of a late- to post-collisional context. The source of these Pleistocene products is chemically close to OIB. The transition between orogenic to anorogenic magmatism in this portion of the archipelago has been estimated to last less than 1.3 Ma. Such a fast transition could be related either to a process of slab break-off starting during the Pliocene, or to a slab window in the subducted plate. This window could have channeled plume-type magmas from the lower mantle into Italian volcanic provinces. This would explain the progressive magmatic evolution we observe in Pontine Islands, from a calc-alkaline volcanic-arc type to an alkaline within-plate type volcanism
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting