Geological and Petrographical study of Montecampione Triassic subvolcanic bodies (Southern Alps, Italy): preliminary geodynamic results.

The research began with a geological and structural survey of the Triassic subvolcanic bodies cropping out in the area of Montecampione (Val Camonica-BS, Italy). This study allowed the determination of the main phases of intrusion of these bodies, and new and significant sampling of the magmatic rocks in order to reconstruct the geodynamic framework based on high quality geochemical analyses. These intrusive bodies, belonging to the «Montecampione group», show a minimum volume of about 1 km3 and could represent a shallow (about 1000 m) magmatic reservoir producing a volcanic area, since we recognised some structures related to probable feeding conduits, and the presence of volcanic sandstones coeval with or little younger than the radiometric age of these intrusions (231±5 Ma). The subvolcanic bodies can be classified following the terminology of volcanic rocks because of the presence of abundant devitrified groundmass and a vacuolar texture. On the basis of the chemical composition, these subvolcanic rocks can be classified in the alkaline series as trachyandesites to trachytes, although it is not possible to determine the sodic or potassic affinity because of the high fluid circulation. The study of trace elements, and particularly the pronounced enrichment in LREEs and LILEs, allows us to relate the genesis of these magmas to a back-arc setting associated with the great depths achieved by a subducting slab. Based on the radiometric dating and on the paleogeographical reconstructions available, this geodynamic framework could be related to the subduction of Palaeotethyan oceanic crust during the early phases of the Cimmerian orogeny. The formation of the magmatic bodies constituting the «Montecampione group» emplaced at shallow level can thus be related to magmatic upwelling in the framework of a back-arc with limited extension