A new Geochronological and Geochemical Study of the Mt. Amiata Volcano (Tuscany Province, Italy)
Abstract
Mt. Amiata (1738 m a.s.l.) is the least known of the major volcanoes of Central Italy. It is the youngest (Late Pleistocene) and the southernmost volcanic complex of the Tuscan Magmatic Province (TMP). Built between 300ka and 180ka over a horst bounded to the west by the Siena-Radicofani graben, Mt. Amiata consists essentially of felsic effusive products erupted from a set of ENE-WSW fractures and covering an area of 85km2. Mt. Amiata products are grouped in 3 main units: the Basal Trachydacitic Complex (BTC), the Domes and Lava Flows Complex (DLC), and the final Olivine Latitic Lava flows (OLL). The origin of BTC is controversial: either a lava flow-unit or a pyroclastic flow (rheoignimbrite or ignimbrite) has been suggested. The chaotic aspect of the flow, highly fragmented crystals and flow-like texture of the glassy groundmass, suggest an ignimbrite flow; however, the massive and homogeneous aspect of this unit, and the lack of welded blocks, rather suggests a lava flow. Whatever the origin, observed flow banding structures indicates that the BTC emplaced at high temperature. We performed a geochronological (K-Ar; "Cassignol-Gillot" technique) and geochemical study to trace the magmatic evolution of Mt. Amiata, and discuss it in the context of the TMP. Preliminary K-Ar ages show that the BTC emplaced quickly, in less than 10,000 yrs, between 294ka+/-4ka and 283ka+/-5ka, covering more than 90km2 and reaching a volume of 18km2. After 15,000 yrs of apparent quiescence, DLC emplaced between 268+/-5ka and 200+/-11ka, differing from previous results that restrained this episode at 200+/-10ka. The final latitic unit erupted at 209 ka+/-4ka. The erupted products of Mt. Amiata define a shoshonitic sub-alkaline serie, the rocks of the BTC and DLC being trachydacites, while the final lava flows are trachyandesites (latites). They all display Nb-Ta negative anomalies reflecting metasomatic events affecting their sources and/or involvement of a crustal component. Their high Th/Ta (25-27) and La/Nb ratios (> 3) indicate an IAB-type orogenic affinity. Nb/Ta ratios (9-10) are similar to crustal values. Y/Nb ratios >1.2 reflect a source chemically similar to island arc or continental margin basalts. For all these ratios, the final latites display higher values. When plotted in Rb-Yb-Ta-Y-Nb geotectonic discriminating binary diagrams, Mt. Amiata products fall in the field of syn-collision setting, with the nearby Pliocene (5.4 to 2.3 Ma) rhyolites of Roccastrada and San Vincenzo
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.V32D1054P
- Keywords:
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- 1035 Geochronology;
- 1040 Isotopic composition/chemistry;
- 1065 Trace elements (3670)