Volcanic and tectonic interaction during the Permian geodynamic event: new insights from the Lugano-Varese district, Southern Alps, (Italy-Switzerland)

The study of ancient volcanic and volcaniclastic sequences provides key insights into geodynamic processes that contributed to their evolution, as well as valuable information on paleoenvironment evolution and basin dynamics.

The Varese area hosts a Permian magmatic complex related to the igneous events that characterized the geodynamic evolution of the Southern Alps during the Late Palaeozoic. While previous research has detailed the petrographic, isotopic, and geochronological aspects of these magmatic rocks, detailed stratigraphic studies on the volcanic and volcaniclastic sequences and their interaction with depositional environments are limited. Compared to more studied areas like the Orobic Basin or the Atesinian District, the Varese area remains underexplored, particularly regarding post-Variscan sedimentary processes.

This study aims to enhance understanding of the Permian volcanic and volcaniclastic sequences in the Varese area through a detailed geological survey and stratigraphic analysis of the lithological units. The investigation focused on two structural blocks separated by the Marzio Fault, a significant tectonic structure in the region potentially linked to syn-magmatic tectonic activity. Stratigraphic sequences on either side of the fault were compared: the Grantola and Boarezzo sequences.

Field and laboratory analyses revealed distinct characteristics of the lithological units in the two sectors of northern Varese area. The Boarezzo 1 section comprises a basal pyroclastic sequence linked to nearby volcanic vents, overlain by thick agglomerate facies with interbedded peperitic layers and pyroclastic deposits. These features suggest a volcanic vent in a subaqueous lacustrine environment. The Boarezzo 2 section features a continental clastic sequence, likely deposited by fluvial systems eroding Variscan rocks and Permian volcanic deposits.

The Grantola section exhibits a thinner volcanic and volcaniclastic sequence. It includes pyroclastic deposits overlain by an acid to intermediate composition lava dome and an olivine-basaltic lava flow with vitrophyric lithofacies. These characteristics indicate a smaller volcanic system compared to the Boarezzo sections.

The Permian geological setting in this area likely consisted of a dome field with multiple medium-to-small effusive centers within a fluvio-lacustrine environment rather than extensive volcanic systems. The sequences comparison suggests that the Marzio Fault bounded two structural blocks, with distinct depositional and volcanic environments. South of the fault, a well-structured basin existed with dynamic sedimentation, while north of the fault, the Grantola section likely represented the basin’s shoulders.

Further research is essential for a comprehensive description of the volcanic and volcaniclastic sequences of the Lugano-Valganna magmatic complex. Additional studies could confirm hypotheses about the Permian geological setting and the interplay between volcanic activity and depositional environments in the Varese area. This research highlights the complexity of the region's geological history and the need for continued exploration to refine our understanding of its Permian evolution.