The evolution of the “Serie dei Laghi” (Strona-Ceneri and Scisti dei Laghi): the upper component of the Ivrea-Verbano crustal section; Southern Alps, North Italy and Ticino, Switzerland

Abstract

The Serie dei Laghi is the upper-intermediate component of the exposed crustal section of Massiccio dei Laghi in the Southern Alps, the latter including also the Ivrea-Verbano Zone. Its oldest event is a sedimentary cycle at least of Early Paleozoic age. It starts with a pelitic sequence (Scisti dei Laghi) that later suffered a tectono-metamorphic event of rather high pressure. This event is recorded by rare mineral and textural relics detected in the Scisti dei Laghi of the M. Gambarogno area. Later, an arenaceous sequence (Strona-Ceneri) was deposited. It is now represented at the base by basaltic metatuffites resulting from the reworking of an ophiolite belt, then followed by paragneisses with detrital phenoclasts (Ceneri gneiss) and by fine-grained gneisses (“Gneiss minuti”). During the Ordovician, conspicuous tonalitic to granitic masses were emplaced. The induced thermal disturbance, and the circulation of pegmatitic stage magmatic residuals, caused contact metamorphism and granitization in the sedimentary country rocks. During the Hercynian, a main prograde event generated the regional foliation, with isoclinal geometry and transposed character. A syn-kinematic stage produced medium-grade parageneses of relatively high pressure; static recrystallization and sillimanite growth then followed. During the uplift, due to the combined action of hydration and decompression, margarite-bearing coronitic textures grew around kyanite. A retrograde metamorphism was then recorded by the appearance of greenschist facies parageneses, at particular structural sites. The formation of the Cossato-Mergozzo-Brissago (CMB) and Pogallo fault system against the Ivrea-Verbano Zone, as well as the “Schlingen” folds in the northeast, is of late-Hercynian age. Along the CMB fault, calc-alkaline dykes and stocks (appinites) were intruded, probably towards the end of the upheaval, but deep enough to induce partial melting in their country rocks (between Val d'Ossola and Valsesia). The Hercynian evolution is concluded by the Early Permian magmatic cycle that gave rise to both extrusive rhyolitic sheets and to granitic epiplutonic bodies accompanied by mafic dykes. In Mesozoic times the Serie dei Laghi was dissected by many extensional faults. These partly reactivated the Hercynian lineaments, such as the CMB and Pogallo Lines. During the Alpine orogenic cycle, the Serie dei Laghi was affected by S-verging cataclastic overthrusts.