Geologic Framework
The Sesia magmatic system is composed of coeval and genetically related intrusive and volcanic rocks within the Ivrea-Verbano Zone and Serie dei Laghi, two lithostratigraphic units in the Alps of northwestern Italy that have been historically studied as separate entities (Fig. 1). The Ivrea-Verbano Zone comprises plutonic and high-temperature, high-pressure rocks (Mehnert, 1975; Fountain, 1976) that are juxtaposed against the basement units of the Austro-Alpine Domain by the Insubric Line (Schmid et al., 1987) and bounded to the southeast by amphibolite-facies metamorphic rocks and granites of the Serie dei Laghi (also known as Strona-Ceneri Zone, Boriani et al., 1990b). North of the Sesia Valley, the boundary between the Ivrea-Verbano Zone and Serie dei Laghi corresponds to a high-temperature mylonite mapped as the Cossato-Mergozzo-Brissago Line (Boriani and Sacchi, 1973; Zingg, 1983; Handy, 1987; Boriani et al., 1990), but within the Sesia Valley, a tectonic boundary is not evident (Quick et al., 2003). Most investigators agree that the Ivrea-Verbano Zone together with the Serie dei Laghi are the deep- and the middle- to upper-crustal components, respectively, of a section through the pre-Alpine crust of northwest Italy (e.g. Fountain, 1976; Handy and Zingg, 1991; Henk et al., 1997; Rutter et al., 1999). A dissenting view has been offered by Boriani and Giobbi (2004), who interprete the Ivrea-Verbano Zone and Serie dei Laghi as distinct, unrelated terranes.
Gravity and seismic reflection data suggest that the Ivrea-Verbano Zone dips steeply to the southeast near the surface but flattens into a subhorizontal orientation at a depth of 20 to 30 km beneath the Po Plain (Nicolas et al., 1990; also see Berckhemer, 1968). The emplacement of the Ivrea-Verbano rocks into the upper crust resulted from the combined effects of exumationdue to Mesozoic crustal thinning and subsequent lithospheric wedging related to Alpine collision (Schmid et al., 1987; Nicolas et al., 1990).
Figure 1. Geology of the southern Alps
Geology of the southern Alps in the vicinity of the Ivrea-Verbano Zone based on Zingg (1983). Ivrea-Verbano Zone contains pre-Permian peridotite, pre-Permian paragneiss of the Kinzigite Formation and Permian intrusive rocks of the Mafic Complex. Strona-Ceneri Zone contains pre-Permian gneiss and schist. Abbreviations are CMB Line, Cossato-Mergozzo-Brissago Line; C.L., Cremosina Line. Major mantle peridotites are identified by name. Muscovite-out (musc out) and orthopyroxene-in (opx in) isograds are shown in red.
Rocks of the Ivrea-Verbano Zone have been grouped historically in terms of two major units, the Kinzigite Formation and the Basic Formation (also referred to as the Mafic Formation). The Kinzigite Formation consists of amphibolite- to granulite-facies paragneiss that formed from protoliths dominated by pelitic sedimentary rocks and wackes, but also including limestone and mafic volcanic rocks (Zingg, 1983; Sills and Tarney, 1984; Wedepohl et al., 1989). Amphibolite-facies assemblages dominate in the southeastern Ivrea-Verbano Zone and granulite-facies assemblages are volumetrically more significant in the northwest. The Basic Formation is subdivided into the voluminous Mafic Complex and lenses of mantle peridotite. The Mafic Complex (Rivalenti et al.,1975, 1981, 1984) is a large composite body of mostly gabbroic plutonic rocks and subordinate amounts of dioritic, tonalitic, charnockitic and cumulus ultramafic rocks. These rocks are recrystallized to different degrees and are interleaved with and underlie the Kinzigite Formation in terms of pre-Alpine orientations.
Figure 2. Geology of the Balmuccia area
Geologic map of the Mafic Complex in the Balmuccia area (modified from Quick et al., 1992).
The most detailed investigations (Mulch et al., 2002, Rutter et al., 2007 Siegesmund et al., 2008) agree that the motion on the CMB Line pre-dates the Permian intrusive event at which time the CMB line was invaded by small, bimodal mafic and granitic intrusions (termed “appinites” by Boriani et al. 1974). Therefore, we consider the CMB Line to be one of the several high-T shear zones involved in the pre-Permian mechanical assembly of the Ivrea-Verbano Zone-Serie dei Laghi crustal section. During this assembly, which is poorly contrained in age and may have involved multiple episodes, lenses of mantle peridotite were tectonically emplaced in the crust. In the Ivrea-Verbano Zone, these include the Alpe Morello mantle tectonite, which is bounded by the CMB and metamorphosed in amphibolite facies and the Alpe Piumero, Alpe Francesca, bodies, which occur deeper in the section, along the Rosarolo shear zone (Siegesmund et al., 2008) and were deformed under higher-grade amphibolite facies conditions (Marchesi et al., 1992). The largest of these bodies, the famous Balmuccia peridotite, shows a spinel foliation, consistent with flow at temperatures >1000°C (Boudier et al., 1984), which becomes more intense approaching the borders of the body (porphyroblastic texture, Garuti and Friolo, 1979), suggesting granulite facies conditions during its crustal emplacement. An apparent increase from east to west of the metamorphic assemblages in these mantle lenses is consistent with emplacement at different depths in the crustal section.
The eastern contact of the Balmuccia peridotite (Fig. 2), has been repeatedly interpreted as an exhumed and preserved sub-continental, petrologic Moho (Shervais, 1979; Rivalenti et al., 1981; Voshage et al., 1990; Sinigoi et al., 1983; Boudier et al., 1984, Quick et al., 1992). According to this interpretation, outcrops of the Mafic Complex west of the peridotite are tectonically repeated by a splay of the Insubric Line and the eastern boundary of the peridotite, which is sharp, magmatic and defined by cumulus pyroxenite in contact with mantle tectonite, represent the base of the Permian crustal section. If true, this interpretation would have significant implications for the Sesia magmatic system, as primitive mantle-melts should have reached this level of the section without crustal contamination. However, (Quick et al., 1995) demonstrated that the Balmuccia body is, in fact, a lens of mantle tectonite, enveloped on all sides by a sheath of pyroxenite and gabbro containing belts of crustal paragneiss, termed septa. Similar relationships are present at the Valmala peridotite, about 3 km South of Balmuccia (Fig. 2). Collectively, these observations indicate that the Balmuccia peridotite was a lens that was tectonically incorporated into paragneisses of the Kinzigite Formation prior to the intrusion of the Mafic Complex, whenthe magmatic contacts and pyroxenite sheath were formed. Based on these observations, it must be concluded that the Permian subcontinental Moho of this crustal section was located at some undetermined distance beneath the current level of exposure.
A clear case can be made that the southern Ivrea-Verbano Zone and the southern Serie dei Laghi were heavily impacted by the same early Permian magmatic event. SHRIMP U-Pb zircon ages for volcanic rocks south-east of the Cremosina line and for the spatially-related granite bodies of the Roccapietra- Valle Mosso pluton, which intrude the Serie dei Laghi, indicate that this volcanic/plutonic activity overlapped in time with crystallization and cooling of the Mafic Complex in the southern Ivrea-Verbano Zone (Quick et al., 2009) and that peak magmatic activity was confined to approximately 10 m.y. from 288 to 278 Ma. Quick et al. (2009) also demonstrated that the volcanic field is largely occupied by caldera fill tuffs and megabreccia, concluding that volcanic and middle- to deep-crustal plutonic rocks collectively constitute an unprecedented exposure of the magmatic plumbing system to a depth of 25 km beneath a caldera with a minimum diameter of 13 kilometers. The association of these rocks in space and time points to a cause-and-effect link between intrusion of mantle-derived basalt in the deep crust and large-scale, silicic volcanism.
Historically, the Mafic Complex was considered as a single mafic body extending from Baldissero to the Finero area as reported in Figure 1. However, recent SHRIMP U-Pb zircon ages demonstrated that, while the central Mafic Complex crystallized at the Permo-Carboniferous boundary (Peressini et al., 2007), gabbro constituting the Northern Mafic complex, in the Finero area, is Triassic and can no longer be regarded as part of the same igneous body (Mazzucchelli et al., in progress).