Basic Dykes Crosscutting the Crystalline Basement of Valsugana (Italy): New Evidence of Early Triassic Volcanism in the Southern Alps

Basic dykes crosscutting the crystalline basement in Valsugana (Southern Alps, Italy) have been investigated for the first time in the framework of the known tectonomagmatic cycles. Petrographic observations and bulk rock analyses suggest a serial affinity variable between calc-alkaline (subordinate) and shoshonitic (prevalent), which are generally ascribed to a convergent plate setting. This is confirmed by Sr-Nd-Pb isotopic analyses that display extreme values that are often observed in postcollisional settings. A bulk rock Rb-Sr pseudoisochron suggests an age of the magmatism around 260 Ma, whereas K-Ar datings yield ages between 227 and 251 Ma, suggesting that these dykes plausibly represent a transition between the Permian and the Triassic volcanic episodes that are known in neighboring sectors of the Southern Alps. Considering that Permo-Triassic active subduction beneath the South Alpine is scarcely constrained, we ascribe the metasomatism of the related mantle sources to the Variscan cycle, proposing that magma genesis was delayed with respect to the time of the active subduction(s). According to recent reconstructions, parts of southeastern Europe, including the South Alpine domain, were formed by the breakup of the northern Gondwana margin from the Late Cambrian, in connection with important transtensional movements, leaving rifted continental basins or narrow oceanic seaways. In our view, the subduction processes that induced metasomatism in mantle sources of the South Alpine region occurred in the connection with the subsequent (Carboniferous?) consumption of lithosphere of these basins, a framework that is compatible with pervasive recycling of continental crust components within the mantle wedge. Then, calc-alkaline/shoshonite magmatism was triggered in the Early Triassic by postcollisional extensional tectonics that followed the Variscan orogenic cycle.