Silvano Sinigoi

Emplacement dynamics of a large mafic intrusion in the lower crust, Ivrea‐Verbano Zone, northern Italy

The Ivrea-Verbano Zone of northern Italy provides an opportunity to study directly the effects of intrusion of large volumes of mantle-derived melts into the lower continental crust. Alpine uplift has exposed a complex of mafic to intermediate plutonic rocks, which had intruded the lower continental crust during the Permian. Field mapping reveals a gross arcuate structure within the complex that has survived tilting and uplift. In terms of the orientation during intrusion, banding and foliation are subhorizontal near the base of the complex and steepen gradually up section. Outcrop relations indicate that the cumulates were profoundly affected by extensional deformation while they still contained interstitial melts. The intensity of this hypersolidus deformation increases downward in the complex. The above characteristics are explained in terms of a model in which a huge volume of cumulates (8 km thick and >20 km long) crystallized from a small (≤1 km thick and ≤4 km wide), periodically refilled magma chamber which remained at a relatively fixed position through time. Large-scale ductile deformation of the complex and transport of cumulates downward and outward from the magma chamber occurred as a consequence of extensional tectonics. This process is capable of generating large sheets of gabbro in the lower crust. Gabbro complexes formed in this manner would be characterized by (1) widespread evidence of synmagmatic deformation dominated by stretching, (2) downward increasing intensity of deformation, (3) upward steepening layering and foliation that is concave toward the center of spreading, and (4) a scarcity of crosscutting intrusive relationships.

Magmatic plumbing of a large Permian caldera exposed to a depth of 25 km

We report evidence for a >= 13-km-diameter Permian caldera associated with a renowned section through the continental crust comprising the Ivrea-Verbano Zone and Serie dei Laghi of northwest Italy. Correlation of ages of volcanic and middle to deep crustal plutonic rocks suggests that they constitute an unprecedented exposure of a subcaldera magmatic plumbing system to a depth of 25 km, and points to a cause and effect link between intrusion of mantle-derived basalt in the deep crust, and large-scale, silicic volcanism.

Chemical evolution of a large mafic intrusion in the lower crust, Ivrea-Verbano Zone, northern Italy

The Ivrea-Verbano and adjacent Strona-Ceneri zones have been described collectively as a section through the continental crust. While resident in the lower crust, amphibolite to granulite-facies paragneiss of the Ivrea-Verbano Zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafic Complex involved hypersolidus deformation in an extensional environment. Isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle-derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by 87Sr/86Sr > 0.71, δ18O = 10–12.5‰, and a positive Eu anomaly. In the present study, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. Charnockites associated with paragneiss septa in the lower part of the Mafic Complex have the appropriate chemistry to be samples of the contaminating material. These chemical features can be explained by melting of granulite-facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event. The Ba enrichment in the core of the Mafic Complex can be modeled by a replenishment-tapping-fractional-crystallization (RTF) process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by Ba-rich charnockite. Very high Ba/K in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa. In contrast to the Mafic Complex, the chemistry of coeval granites in the adjacent Strona-Ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event. Significantly, the incompatible trace element abundances in the Mafic Complex and Strona-Ceneri granites are similar to model compositions for the lower and upper crust, respectively.

Synmagmatic deformation in the underplated igneous complex of the Ivrea-Verbano zone

The Ivrea-Verbano zone, northern Italy, contains an igneous complex up to 10 km thick that is thought to have been intruded near the interface between the continental crust and mantle during the late Paleozoic. New data indicate that this complex is pervasively deformed and concentrically foliated. Widespread deformation under hypersolidus conditions is indicated by growth of undeformed poikilitic phases across the foliation and segregation of late-stage melts into high-temperature faults and pressure shadows of boudins. The presence of analogous features in ophiolitic gabbros suggests that emplacement of the Ivrea- Verbano zone plutonic rocks involved large-scale flow of crystal mush in a dynamic, and possibly extensional, tectonic environment.

Isotopic constraints on the origin of ultramafic and mafic dikes in the Balmuccia peridotite (Ivrea Zone)

The Balmuccia peridotite massif in the central Ivrea Zone constitutes an upper mantle slice which has been tectonically emplaced into the crust. It represents the residue from partial melting of undepleted mantle material and varies in composition from lherzolite to harzburgite and subordinate dunite. Dikes of websterite and gabbroic pods within the peridotite can be subdivided into an older Crdiopside suite and a younger Al-augite suite. Nd isotopic data on whole rocks of these lithotypes in combination with independent observations suggest that the dikes formed during a Hercynian event about 270 Ma ago. The rocks of the Cr-diopside dikes, in particular, display isotopic signatures similar to those of the lherzolite and represent fractionates from partial melts derived from the lherzolite wall rock. The Sm-Nd data of the pyroxenites and gabbros of the Al-augite suite, in contrast, scatter widely and suggest that partial melting of lherzolite was triggered or at least accompanied by introduction of fluids and/or liquid phases. These fluids or liquids carried exotic isotopic components from elsewhere in the crust-mantle complex, and deposited them within the rocks by metasomatic reactions. Two distinct types of metasomatism must have operated not only within the Balmuccia body, but also in the complex of Finero: The first type of metasomatism introduced mantle-derived volatiles and is responsible for formation of amphibole. The other type has a crustal source and led to formation of phlogopite, which occurs mainly within mantle rocks of Finero, but occasionally, within the Balmuccia body also.

Emplacement of mantle peridotite in the lower continental crust, Ivrea-Verbano zone, northwest Italy

The Ivrea-Verbano zone has been cited repeatedly as an exposed pre-Alpine crust-mantle transition on the basis, in part, of the presence of lenses of mantle peridotite and a possible petrologic Moho. A 10-km-thick mafic complex has been described as an intrusion at the contact between the mantle and a thick crustal sequence of metamorphosed wacke and pelitic rocks. The significance of these features is now challenged by new mapping that demonstrates that mantle peridotites in the southern Ivrea-Verbano zone were lenses tectonically interfingered with metasedimentary rocks prior to intrusion of the gabbroic complex. Although it remains possible that the Ivrea-Verbano section exposes part of a complex crust-mantle interface within which crustal and mantle rocks were interleaved, the association of peridotite and metasedimentary rocks is also consistent with assembly in an accretionary wedge. In either case, present exposures are from an unknown distance above the pre-Alpine contiguous mantle and reference to the section as a complete crust-mantle transition could be misleading.

Differentiation of partial melts in the mantle: Evidence from the Balmuccia peridotite, Italy

The Balmuccia peridotite shows evidence, in the form of a network of dykes, of partial melting and flow crystallization processes. The partial melting processes probably occurred over a fairly long time interval, and seem to have been related to different “melting pulses”. Resultant liquids were broadly picritic. Melting occurred incongruently according to the scheme cpx+opx+(ol+sp)=Mg-richer ol+Cr-richer sp+L.Partial melts tended at first to accumulate in horizontal layers; then, as the critical melting threshold was exceeded, liquids were able to filter slowly towards lower pressure zones. In doing so liquids fractionated initially in situ, via crystallisation of websteritic dykes of the Cr-diopside suite, and later, in the overlying mantle, via crystallisation of transitional dykes and those of the Al-augite suite. This filter-pressing stage, when flow velocities were very low and discontinuous, probably corresponded to the period of maximum deformability of the peridotite.The type of differentiation testified by the dykes of the Balmuccia peridotite, is characterized by a decrease in SiO2, a rapid enrichment in Al2O3 and a mild increase in FeO, and is substantially in accordance with experimental trends from the fo-an-di-SiO2 system in the spinel-peridotite stability field. A close relationship between type of differentiation, flow velocity and mechanical behaviour of the mantle peridotite is a feature of the proposed model.

Intractions of mantle and crustal magmas in the southern part of the Ivrea Zone (Italy)

In the southern part of the Ivrea Zone (Italy), the majority of the Mafic Formation is composed of: 1. amphibole-bearing gabbro; 2. a series of rocks ranging from norites to charnockites; 3. leucocratic charnockites. In the proximity of metasedimentary septa within the Mafic Formation, the igneous lithologies are in many places intimately and chaotically intermingled, giving rise to a marble-cake structure. Whole-rock chemistry, and oxygen and strontium isotopic compositions indicate that the mafic and felsic rocks are dominated by mantle and crustal sources respectively. The norite-charnockite suite may be modelled as the mixing product of basic and acid melts. Abundant plastic deformation structures suggest that mafic and hybrid rocks experienced an important tectonic event during or soon after their crystallization. Melting of crustal country rocks continued after the deformation event and produced the undeformed leucocratic charnockites. The study area exemplifies some of the possible effects of the intrusion of a large volume of basic magma into hot crust.

Evidence of mantle metasomatism and heterogeneity from peridotite inclusions of northeastern Brazil and Paraguay

Abstract Textural-petrographic features, major and trace elements and mineral chemistry are reported for spinel-peridotite xenoliths from northeastern Brazil (NE-suite) and Paraguay (PY-suite). Variation trends defined by bulk rock and mineral chemistry are consistent with the effects of variable degree of high-pressure melting for both suites. The PY-suite strongly differs from the NE-suite by its very high contents of incompatible elements. This different whole-rock chemistry is related to the presence of glassy-microcrystalline blebs which are interpreted as remains of pre-existing hydrous phases of metasomatic origin. The difference between the two suites is also recorded in the chemistry of pyroxenes and spinels, which develop along different variation patterns. Although the suites partially overlap in their ranges of mg opx (Mg/(Fe 2+ + Fe 3+ ) ratio in orthopyroxenes), the PY-suite is, on the average, more mg opx rich (residual) than the NE-suite. Within the PY-suite a rough positive correlation exists between degree of residuality and degree of metasomatic effects. The sharp differences between the NE-suite and the PY-suite imply mantle heterogeneity on regional scale, whereas the variability within each suite is essentially related to different degrees of melting and/or metasomatism and imply mantle heterogeneity on local scale.

Development of a deep-crustal shear zone in response to syntectonic intrusion of mafic magma into the lower crust, Ivrea–Verbano zone, Italy

Abstract A 1 to 1.5 km-thick, high-temperature shear zone is localized in wall rocks subparallel to the eastern intrusive contact of the Permian Mafic Complex of the Ivrea–Verbano zone (IVZ), Italy. The shear zone is characterized by concentrated ductile deformation manifested by a penetrative foliation subparallel to the intrusive contact and a northeast-plunging sillimanite lineation. Evidence of noncoaxial strain and transposition is widespread in the shear zone including such features as rootless isoclinal folds, dismemberment of competent layers, and scattered kinematic indicators. The metasedimentary rocks in the shear zone are migmatitic, and the accumulation of leucosome is variable within the shear zone. Near the intrusive contact with the Mafic Complex leucosome forms ∼20 vol% of the wall rock, whereas leucosome concentrations may locally reach ∼60 vol% of the wall rock near the outer limits of the shear zone. This variation in vol% leucosome suggests melt/magma migration from the inferred site of anatexis along the intrusive contact to lower-strain regions within and near the margins of the shear zone. The leucosome accumulations chiefly occur as layer-parallel concentrations, but are also folded and boudined, and locally are associated with tension gashes and fracture arrays. Networks of granitic dikes and small plutons in the eastern IVZ suggest that some magmas migrated out of the high-temperature shear zone. Some magma apparently migrated laterally along the strike of the shear zone and concentrated in areas of lower strain where the intrusive contact takes a major westward bend. The high-temperature shear zone is interpreted as a “stretching fault” (or stretching shear zone) after Means [W.D. Means, Stretching faults, Geology 17 (1989) 893–896], whereupon the metasedimentary wall rocks and associated leucosome deformed synchronously with the multistage emplacement and deformation flow of the Mafic Complex. The recognition of a high-temperature shear zone associated with the emplacement of mafic igneous rocks into the deep crust is an example of the progressive stratification of the lower crust during magmatic under- or intraplating that has consequences for seismic imaging and its interpretation.