Depth, geometry and emplacement of sills to laccoliths and their host-rock relationships: Montecampione group, Southern Alps, Italy

This study focused on a set of shallow subvolcanic bodies, mainly laccoliths and sills, that intruded the Upper Permian-Lower Triassic sedimentary sequence of the central Southern Alps, in the area of Montecampione (Val Camonica, Italy). These intrusions represented a shallow magmatic reservoir probably associated with Triassic volcanism. Based on a detailed stratigraphic reconstruction, this paper presents results dealing with the evaluation of the emplacement depth, the estimated volume of the subvolcanic bodies, a description of their geometries, and their relation to the host rock and response of the sedimentary units to the intrusions. The emplacement depth was estimated using the thickness of the sedimentary overburden at the time of emplacement, and by applying simple equations involving laccolith dimensions. The results are comparable, and support an average emplacement depth of about 1300 m. The minimum volume of the intrusions was obtained using a GIS, and is about 1 km(3). Concerning the relationship between the intrusive bodies and the host rock, we observed that sills are mainly emplaced into the Servino Formation, while the laccoliths are emplaced near the contact between the Verrucano Lombardo and the Servino Formation. The two sedimentary units show a different response to the intrusion: the Verrucano Lombardo always appearing fractured and tilted, while the Servino Formation shows a range of deformation patterns, from light ductilization at the contact, to folding, brecciation and foliation. These different responses reflect the mechanics of emplacement and geometry of the intrusions, and local heterogeneities in the host rock. Both units show a local thermal effect close to the contact.