Peridotic mylonite (Balmuccia)

Ivrea-Verbano Zone

The Ivrea Verbano Zone (Fig.1) represents the westernmost sector of the Southern Alps, which was part of the Mesozoic continental margin of the Adriatic plate during the opening of the Alpine Tethys (or Ligurian-Piedmont oceanic basin). The Ivrea-Verbano Zone (IVZ) and Serie dei Laghi (SdL), are two lithostratigraphic units that constitute the deep- and the middle- to upper-crustal components, respectively, of a tilted and exposed section through the pre-Alpine crust of northwest Italy. Both units were significantly affected by a Permian igneous event, leading to underplating of the Mafic Complex in the deep-crustal Ivrea Verbano Zone and intrusion of granitic bodies in the upper-crustal SdL, which is in turn capped by volcanic, predominantly rhyolitic rocks. U-Pb zircon ages of volcanic rocks (288 ± 2 to 282 ± 3 Ma), formation of granitic plutons in the Serie dei Laghi (289 ± 3 to 275 ± 5 Ma), and gabbro in the Ivrea Verbano Zone (289 ± 3 to 286 ± 6 Ma) indicate that the onset of bimodal volcanism and granitic plutonism was coincident with and probably triggered by intrusion of mantle-derived mafic melt in the deep crust, and that volcanic activity and presence of granitic melt at depth persisted after underplating had ceased. Collectively, all these coeval igneous rocks are grouped in the "Sesia Magmatic System".

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Fig.1: Geological sketch map of the the Ivrea-Verbano Zone based on Zingg (1983). 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. From Virtual Explorer.

The Sesia magmatic system is composed of coeval and genetically related intrusive and volcanic rocks within the Ivrea-Verbano Zone and Serie dei Laghi. The Ivrea-Verbano Zone comprises plutonic and high-temperature, high-pressure rocks that are juxtaposed against the basement units of the Austro-Alpine Domain by the Insubric Line and bounded to the southeast by amphibolite-facies metamorphic rocks and granites of the Serie dei Laghi (also known as Strona-Ceneri Zone. 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.

Rocks of the Ivrea-Verbano Zone have been grouped historically in terms of three main units: The Mantle Peridotites, the Mafic Complex and the Kinzigite Formation.

• Mantle peridotites: The largest and more famous Ivrea-Verbano mantle peridotites are, from south to north, the Baldissero, Balmuccia and Finero massifs (Fig.1). These bodies are aligned along the northwestern margin of the Mafic Complex. Tens of minor peridotite bodies, definitely recognized as lithospheric mantle material, occur at various stratigraphic levels throughout the Ivrea-Verbano Zone.

• Mafic Complex: The Mafic Complex is a large composite body of mostly gabbroic plutonic rocks and subordinate amounts of dioritic, tonalitic, charnockitic and cumulus ultramafic rocks. The Mafic Complex has been known since the pioneering work of Artini and Melzi (1900), who described these rocks as mafic granulites. Rivalenti et al. (1975) first recognized it as a huge igneous complex. The Mafic Complex is divided into: Basal Zone, Intermediate Zone, Upper Zone, Main Gabbro and Diorites. The basal and intermediate zones consist of alternating ultramafic and mafic (gabbro) layers, which crystallized under a pressure gradient documented by the coexistence of clinopyroxene and spinel in Basal Zone and olivine and plagioclase in Intermediate Zone. The Upper Zone has only rare ultramafic layers and the lithology is dominantly gabbro-anorthosite. The Main Gabbro unit is constituted by a hornblende-bearing norite. The large-scale internal structure of the Mafic Complex (Fig.2) is dominated by an arcuate structure centered on the village of Varallo and defined by layering, foliation and mappable units. Granitic and dioritic bodies do not crosscut the gabbro. Instead, their concordance with foliation and banding is remarkable and crosscutting relationships are limited to faults, scarce dikes and late-stage melt segregations. Paragneiss septa derived from the Kinzigite Formation and granitic to dioritic bodies are traceable for kilometers around this arcuate structure without major breaks although they are increasingly attenuated with depth in the complex.

• Kinzigite 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. Amphibolite-facies assemblages dominate in the southeastern Ivrea-Verbano Zone and granulite-facies assemblages are volumetrically more significant in the northwest.

The present exposure of the Ivrea-Verbano Zone at the Earth’s surface is today considered a consequence of tilting produced by a series of deformation events that started with the Jurassic continental break-up forming the Alpine Tethys and concluded with the Alpine collision.

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Fig.2: Geologic map of the Mafic Complex in the Sesia Valley area according to Rivalenti et al. (1975). In the inset, foliation patterns simplified from Quick et al. (2003). From Virtual Explorer.

Balmuccia Peridotite

Balmuccia is one of the subcontinental mantle peridotite bodies occurring in the Southern Domain of the Western Italian Alps. It consists of a 4.5km long, 0.8km wide peridotite lens crossing the Val Sesia just east of the Insubric line, between Val Sesia and Val Mastellone (Fig.3). The eastern margin of the massif is in sharp igneous contact with a series of layered websterite, dunite, pyroxenite and minor gabbronorite referred to as the Contact Series. These rocks are part of the Basal zone Of the Ivrea Verbano Mafic Complex (Rivalenti et al.,1975). The western contact of the massif is a mylonite zone several meters wide. Dikes of psuedotachylite ranging in thickness from a few centimeters to several tens of meters are common along this boundary and in the adjacent rocks. Many of these psuedotachylite dikes are characterized by columnar jointing and resemble mafic igneous dikes, but their compositions mimic their host rocks, showing that they formed in situ. The Balmuccia peridotite has a dominant NNE-SSW-trending foliation determined by flattening and elongation of olivine and pyroxenes and by preferential concentrations of minerals (Rivalenti et al., 1975; Shervais, 1979). In places, this foliation evolves to a porphyroclastic texture with a secondary foliation parallel to the peridotite contact at its eastern margin.

Fig.3: Geologic map of the Balmuccia massif. Modified from Shervais (1979c).

Peridotite of the Balmuccia massif consists of two main lithologies: lherzolite and dunite. Lherzolite is by far the dominant lithology, comprising 85% of the massif. Dunite is the next most abundant lithology and occurs throughout the massif but is most common in the southern half.

Lherzolite: Lherzolite typically contains 60-70% olivine, 20-25% orthopyroxene, and 12% clinopyroxene. The lherzolites are characterized by porphyroclastic textures throughout the central and southern portions of the massif, with porphyroclasts of olivine and orthopyroxene up to 1.5 cm across in an equigranular foliate matrix 0.5-2.0 mm grain size of olivine, pyroxene, and spinel.

The lherzolites are discordantly cut by abundant websterite dykes, distinguished on the basis of the clinopyroxene composition into an older Cr-diopside suite (websterites, ol-websterites and orthopyroxenites) and a later Al-augite suite (websterites and late gabbros; Shervais, 1979):

- Cr-diopside suite: Dike rocks of the Cr-diopside suite comprises websterites, olivine-websterites and orthopyroxenites. Websterites form concordant layers parallel to foliation and dikes which crosscut the foliation at moderate to high angles. Layers which are parallel to foliation originated as dikes and were rotated into the foliation. Modal mineralogy of the layers varies from nearly pure Cr-diopside to nearly pure enstatite or bronzite; olivine is a common accessory but spinel, hornblende, and phlogopite are rare. Mineral compositions in the Cr-diopside suite websterites are similar to those in the lherzolites, with olivine Fo_90-91.
- Al-augite suite: Dike rocks of the Al-augite suite are characterized by gray, Cr-poor, Al-rich clinopyroxene and green hercynite spinel. Plagioclase (An_45-50) occurs in the more differentiated members of the suite and in primitive, late-stage dikes which cut the foliation at high angles. Al-augite suite dikes cross-cut dikes of the Cr-diopside suite and are always less deformed. In many places, Al-augite suite pyroxenites intrude Cr-diopside websterite dikes concordantly, either up the center of the older websterite or, more commonly, along one of its margins. When followed along strike, these younger dikelets are seen to enter or leave the older dike at a low angle and continue within the lherzolite wall rock.

Both dyke suites have sharp contacts against the peridotite. The Al-augite dykes may exhibit a thin, pyroxenitic reaction rim, whereas, occasionally, centimetre- to decimetre-wide dunitic bands occur at the contact between the Cr-diopside dykes and the host rock. Both the Cr-diopside and Al-augite suites comprise several dyke generations, distinguished based on their mutual intersections, deformation patterns and composition. These dykes therefore represent a prolonged period of melt intrusion, possibly related to the accretion of the peridotite from the asthenosphere to the lithosphere and, finally, to the Permian intrusion of the mafic-ultramafic Ivrea Verbano complex, when the peridotite body had already been incorporated in the deep crust (Peressini et al.,2007). Dunite: Dunite is common in the southern and central parts of the massif, where it may comprise as much as 10% of the outcrop. Contacts between dunite and adjacent lherzolite may be sharp or gradational, and some tabular dunites with sharp contacts resemble intrusive dikes. Dunite is commonly found between or adjacent to some Cr-diopside websterites. Mineral compositions in the dunites are more refractory than in the lherzolites: olivine ranges from Fo_90-91.

The dunites and associated Cr-spinel layers occurring as tabular bodies within the Balmuccia lherzolite have refractory bulk-rock and mineral phase compositions. The constant association between centimetre-size Cr-diopside websterite lenses, massive spinel layers and dunites that have high modal spinel abundances and the isotopic similarity of the dunites to the websterites of the Cr-diospide suite suggest that Cr-diopside pyroxenites were involved in dunite genesis.

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Cr-diopside (emerald green) in the Balmuccia peridotite.

Al-augite dyke with dark augite crystals in the Balmuccia peridotite.

Large Al-augite dyke (with dark augite crystals) cross-cutting Cr-diopside layers in the Balmuccia peridotite.

Bibliography

• Mazzucchelli, M., Rivalenti, G., Brunelli, D., Zanetti, A., & Boari, E. (2009). Formation of highly refractory dunite by focused percolation of pyroxenite-derived melt in the Balmuccia peridotite massif (Italy). Journal of Petrology, 50(7), 1205-1233.
• Mukasa, S. B., & Shervais, J. W. (1999). Growth of subcontinental lithosphere: evidence from repeated dike injections in the Balmuccia lherzolite massif, Italian Alps. Lithos, 48(1-4), 287-316.
• Shervais, J. W. (1979). Petrology and structure of the alpine lherzolite massif at Balmuccia, Italy (Doctoral dissertation, University of California, Santa Barbara).
• Shervais, J. W., & Mukasa, S. B. (1991). The Balmuccia orogenic lherzolite massif, Italy. Journal of Petrology, (2), 155-174.

Photo
Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 10x (Field of view = 2mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. PPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)
Olivine and spinel porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. PPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	pyroxene porphyroclasts with deformation twin lamellaes in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)
pyroxene porphyroclasts with deformation twin lamellaes in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts with kinkbands in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)
Olivine-fish in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 10x (Field of view = 2mm)	pyroxene porphyroclasts with deformed twinning and green spinel in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. PPL image, 2x (Field of view = 7mm)	pyroxene porphyroclasts with deformed twinning and spinel (isotropic) in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)
Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 10x (Field of view = 2mm)	Porfiroclasti pirosseno e olivina. Si nota una zona di taglio posta a circa 45°. Immagine a NX, 10x (lato lungo = 2mm)
Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)
Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine, pyroxene with deformed twinning and spinel (isotropic) porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene with deformed twinning porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)
Olivine and pyroxene with deformed twinning porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine, pyroxene and green spinel porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene with deformed twinning porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)
Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)
Olivine and pyroxene porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene porphyroclasts in a fine grained matrix. Balmuccia (Ivrea Verbano zone) Italy. XPL image, 2x (Field of view = 7mm)