Age of the Sesia Magmatic System
The geochronologic framework of the Mafic Complex is reviewed in detail by Peressini et al. (2007). More than 30 years of geochronologic investigations of the Mafic Complex resulte in published ages ranging from 600 Ma (Hunziker, 1974; Hunziker & Zingg, 1980, Pin & Sills, 1986, Voshage et al., 1987; Gebauer et al., 1992) to 30 Ma (Gebauer et al., 1992). However, ages older than 400 Ma were generated by Rb-Sr and Sm-Nd whole-rock methods and can be interpreted as mixing lines with no age significance (Voshage et al., 1990). Ages in the range of 30 Ma date Alpine events that clearly postdate the igneous crystallization age of the Mafic Complex (Mayer et al., 2000).
Peressini et al. (2007) provided age determinations based on SHRIMP U-Pb analyses on zircons from 11 samples of the Mafic Complex. Five samples from the upper Mafic Complex range in age from 289±3 to 286±6 Ma, in agreement with a conventional U-Pb zircon age measured by Pin (1986) and three Pb-Pb evaporation ages measured by Garuti, et al. (2001) and providing the best age constraint on the injection of mantle-derived melt into the deep crust to form the Upper Mafic Complex. Individual SHRIMP spot ages at deeper levels of the Mafic Complex range from >310 to <250 Ma, reflecting inheritance and continuous recrystallization of zircons during a prolonged period of intrusion, slow cooling and deformation in the deep crust punctuated by repeated intrusive events.
Quick et al., (2009) determined SHRIMP U-Pb ages on zircons from granites intruded in both the IVZ and Serie dei Laghi above the Mafic Complex and from volcanic rocks exposed in Val Sesia across the Cremosina line. As described by Quick et al. (2009), the exposed volcanic field is dominated by caldera fill tuffs and megabreccia, the distribution of which indicate a caldera ≥13 km across. SHRIMP ages are reported on the geologic map of the Sesia magmatic system in Figure 17. The oldest single-age for the volcanic rocks is of 288±2 Ma, determined on zircons from andesitic basalt sample, R6. This age matches very well, within errors, that of a granodiorite collected at deep levels in the Roccapietra granite and that of the upper Mafic Complex determined by Peressini et al., (2007). Zircons from samples of the predominant rhyolitic tuffs provided bi-modal ages, with a cluster of zircons at about 289±3 Ma which were interpreted as “antecrysts” produced in early phases of related magmatism (e.g. Charlier et al., 2004; Bryan et al., 2008) and younger zircon ages spreading towards 282 Ma. The youngest ages of 278±5 or 275±4 were measured for the upper Valle Mosso aplitic granite, which constitutes the highest part of the Valle Mosso body and intrudes the volcanic rocks. An intermediate age of 282 Ma is reported by Schaltegger and Brack (2007) on the Montorfano Granite, 12 km north of Figure 17. A palinspastic restoration of the Alpine effects, considering the displacement of Alpine faults and the differential tilting, places the caldera and the Roccapietra-Valle Mosso pluton above the Mafic Complex (Fig. 18)
These data indicate that, in the Sesia Valley, bimodal volcanism and incremental growth of granitic plutons occurred within about 5 to 10 million years, during a time interval similar to early Permian volcanic activity elsewhere in the Alps (Schaltegger and Brack, 2007; Marocchi et al., 2008) and well within the time frame for volcanic activity in silicic large igneous provinces (e.g. Bryan et al., 2008) and the growth of zoned granitic plutons (Coleman et al., 2004). The coincidence of ages in the upper Mafic Comples indicates that onset of bimodal volcanism and granitic plutonism was most likely triggered by intrusion of mantle-derived mafic melt in the deep crust.
Figure 17. Geologic map of the Sesia magmatic system
Figure 18. Palinspastic restoration of the Sesia magmatic system