PERMIAN TO TRIASSIC GEODYNAMIC AND MAGMATIC EVOLUTION OF THE BRESCIAN PREALPS (LOMBARDY, ITALY)

Significant igneous events mark the geodynamic evolution of the Lombardian Southern Alps from post-Variscan to Mesozoic times. During Early Permian, at the end of the Variscan orogeny, a transtensional geodynamic regime developed along the southern Palaeoeuropean border, due to a dextral transform margin between Gondwana and Laurasia. The coeval depositional sequence, represented by calc-alkaline, low- to high- K, intermediate-acidic volcanics and alluvial-lacustrine deposits, infilling fault-bounded, strike-slip or pull-apart basins, is accompanied by a widespread granitoids and minor gabbros from asthenospheric upwelling and crustal contribution. During Late Permian to Anisian, an extensional regime, linked to plate reorganisation and likewise to the opening of Meliata back-arc basin to the East, gave rise to a second sedimentary cycle, following a stratigraphic gap of about 20-25 Ma. The siliciclastic and marine sedimentation, representative of a rapid shallow transgression from East and lacking in volcanic activity, was associated with progressive peneplanation of the pristine irregular topography. The transition to epicontinental arc - back arc conditions, due to the NW-wards subduction of Palaeotethys ocean between Anisian to Carnian times, was defined by the extensive regression of the Bovegno “Carniola”, locally characterized by evaporites and extra-formational clastics deriving from emerged areas. During the Early-Middle Anisian up to Ladinian, the subsidence of the region followed strike-slip to extensional tectonics. In particular, the Ladinian and Carnian interval was characterized by a new igneous cycle, with (base to top): 1) fine-grained tuffites levels (“pietra verde” Auct.) interbedded with limestones; 2) large acidic high K to shoshonitic subvolcanics (231±5Ma; 226±4 Ma, Rb/Sr dating), intermediate rare dikes and abundant extrusives, showing calc-alkaline medium K affinity. 3) high-K calc-alkaline to shoshonitic subaerial volcaniclastics. A regional geodynamic pattern change at Mid-Late Carnian is suggested by transitional basaltic dikes (217±3 Ma 40Ar/39Ar dating) and by lavas, found in the uppermost Val Sabbia continental sandstones. This event preludes to the rifting and to the subsequent opening of the Neothetys ocean in the Alpine region.