Permian to Triassic geodynamic and magmatic evolution of the Brescian Prealps (eastern Lombardy, Italy)

Significant igneous events mark the geodynamic evolution of the Lombardy Southern Alps from Late Palaeozoic to Mesozoic times. During the Early Permian, a transtensional geodynamic regime developed along the southern Palaeoeurope, due to a dextral transform margin between Gondwana and Laurasia. The calc-alkaline, intermediate to acidic volcanics and alluvial to lacustrine deposits infilling fault-bounded pull-apart basins, are accompanied by widespread granitoids and minor gabbros from asthenospheric upwelling and crustal contributions. During the Late Permian to Anisian, an extensional regime linked to plate reorganization, and likewise to the opening of the Meliata back-arc basin to the east, gave rise to a second sedimentary cycle after a stratigraphic gap of about 20-25 Ma. The siliciclastic and marine sedimentation, representative of a rapid shallow transgression from the east and lacking volcanic activity, was associated with a progressive peneplanation. The transition to epicontinental arc/back-arc conditions, due to the NW-wards subduction of Palaeotethys between Anisian to Carnian times, was defined by the extensive regression of the Bovegno «Carnieule», locally represented by evaporites and extra-formational clastics. In particular, the Ladinian-Carnian interval was characterized by a new magmatic cycle, with (base to top): (1) fine-grained rhyolitic tuffs («pietra verde» Auct.) interbedded with limestones; (2) large volumes of acidic high-K calc-alkaline to shoshonitic subvolcanics, rare intermediate dykes and abundant extrusives; (3) high-K calc-alkaline to shoshonitic subaerial volcaniclastic rocks. A marked geodynamic change probably related to late Carnian is suggested by transitional basaltic dykes (217±3 Ma) and lavas. This rifting event preludes the opening of the Neotethys Ocean.