Ivrea Zone Research Articles

Imaging lateral heterogeneity in the northern Apennines from time reversal of reflected surface waves

Prominent arrivals in the coda of seismograms from the wider Alpine area can be associated with lateral reflections of Love waves at the northern Apennines mountain chain (Italy), where structural heterogeneity causes an abrupt contrast in phase velocity. We discuss an approach to image lateral heterogeneity from reflected surface waves using intermediate-period, three-component coda waveforms as sources for an adjoint wavefield that propagates the reflections backward in time. We numerically compute three-dimensional sensitivity kernels for the dependence of coda waveforms on P velocity, S velocity and density, based upon correlations between the adjoint and the regular forward wavefields. We consider synthetic coda waveforms for a simplified model of the northern Apennines, as well as real coda observations from five moderate magnitude earthquakes (MW 4.6–5.6) in the southern Alps. Wave propagation is simulated using the spectral-element method, for which a 3-D regional earth model is used in the case of real data. Single and combined event sensitivity kernels provide clear images of the reflectivity associated with the northern Apennines in kernels for density and S-wave speed. The kernels show that surface wave reflections occur near the axial zone of the mountain chain. Apart from the Apennines, the approach is able to image other smaller reflectivity patches from the coda waveforms, like the Ivrea zone in the southern Alps. Our coda misfit kernels can be integrated in a gradient-based waveform tomography, where they could enhance the sharpness of the model at lateral discontinuities.

Exhumation and deformation history of the lower crustal section of the Valstrona di Omegna in the Ivrea Zone, southern Alps

Abstract The Ivrea Zone (southern Alps) is one of the key regions interpreted as exposing a section of the lower continental crust and was the subject of several review-type articles. The Ivrea–Verbano Zone was rotated into an upright position along the Insubric mylonite belt. In the southeast, this unit is in contact with the Strona Ceneri Zone, which is interpreted as upper continental crust crossing the Permian Cossato–Mergozzo–Brissagio Line (CMB Line). The CMB mylonites are locally overprinted by the mylonites and cataclasites of the Pogallo Line, which was active during the Jurassic. In addition, the sinistral, steeply inclined Rosarolo shear zone was active over a long time span from the ductile into the brittle field, i.e. from the Early Permian (high-temperature ultra-mylonites) to the Neo-Alpine basic dykes and pseudotachylites. The high-temperature mylonites accommodated crustal extension and may be related to normal faults generated by magmatic underplating. The reactivation at different crustal levels during exhumation and tilting is documented by strain increments at decreasing P/T conditions. Its present subvertical orientation was attained during the Neo-Alpine deformation. Constraints on its exhumation history are based on new 40 Ar/ 39 Ar hornblende ages, K–Ar biotite ages and zircon fission-track data along the NE–SW trending Valstrona section. A re-interpretation of existing U–Pb monazite ages is included, based on a higher closure temperature for monazite. The oldest monazite ages are observed in proximity to the Pogallo Line ( c . 292 Ma). Heat input by mafic intrusions was sufficient to reset the U–Pb monazite system, as is evidenced by the youngest ages in the vicinity of the Insubric Line. The re-interpretation favours the hypothesis that the oldest monazite ages are the result of complete resetting by a Permian thermal event. The 40 Ar/ 39 Ar hornblende ages and K–Ar biotite ages document the cooling after Permian heating. Roughly parallel age progressions decrease from the Pogallo Line (hornblende: 271 Ma vs. biotite: 227 Ma) towards the Insubric Line (hornblende: 201 Ma vs. biotite: 156 Ma). Zircon fission-track ages run parallel to the biotite ages in the upper part of the profile, whereas towards the Insubric Line a significant deviation from the biotite age progression is attributed to tilting of the basement during the Oligocene. Zircon fission-track ages around 38 Ma are found close to the Insubric Line. No age offset, neither at the CMB nor at the Pogallo Line, is observed. This confirms the hypothesis that the Pogallo Line is an oblique normal fault, and that the CMB Line has accommodated only minor vertical displacement. The capture of the different cooling ages confirms the tilting of the Ivrea–Verbano Zone during the Neo-Alpine deformation and contradicts the tilting of the Ivrea–Verbano Zone during the Permian.

Elastic properties, fabric and seismic anisotropy of amphibolites and their contribution to the lower crust reflectivity

In this paper the elastic properties of amphibolites from the Serie dei Laghi and the Ivrea zone (Southern Alps, N-Italy) were investigated as a function of their mineralogical composition, microfabric and density. Three orthogonal cores were cut parallel and normal to foliation and lineation; from those, bulk and grain density were measured and the interconnected porosity was calculated. Bulk density varies from 2.75 to 3.07 g/cm 3 and calculated porosity ranges from 0.02 to 0.88%. The same cores were also used to measure seismic velocity of ultrasonic waves at room temperature and at increasing confining pressure up to 300 MPa. At high pressure the matrix properties are separated from the crack-induced properties. P-wave velocity varies with respect to the direction of propagation: the slowest direction is always normal to foliation and the fastest parallel to the mineral lineation. The mineral lineation is typically defined by the elongation of amphibole crystals, in which the fastest Vp direction is parallel to the c axis, that is also the elongation axis. The Vp ranges between 6.76 and 7.54 km/s in the direction parallel to lineation and between 6.32 and 7.06 km/s in the direction normal to foliation. This defines a Vp anisotropy of up to 14%, whose shape varies from orthorhombic to axially symmetric (either prolate or flattened). It was observed that both Vp and Vp anisotropy increase with the amount of amphibole and decrease with the amount of plagioclase. Moreover, the c axis distribution of amphiboles is responsible of the Vp anisotropy intensity and shape, in agreement with observations from previous studies. The seismic properties calculated with the approach of Mainprice (1990), using the fabric data, the elastic constants and the modal composition, gave results in good agreement with the measurements. Exposed rocks in the Ivrea and Serie dei Laghi zones show that amphibolites are interlayered with metapelites on a scale from 1 to 100 m. Because of the very large acoustic impedance contrast (20.34 ± 1.75 for amphibolites, 17.16 ± 0.4 for metapelites), they represent a very reflective portion in the middle-lower crust.

The Continental Crust as a Source of Sand (Southern Alps Cross Section, Northern Italy)

All tectonostratigraphic levels of the continental crust, from carbonate platforms in the Dolomites to granulites in the Ivrea Zone, are exposed along strike in the world‐famous Southern Alps cross section. In this extraordinary natural laboratory, we studied the composition of modern stream sands shed by each distinct type of source rock in order to improve on existing models of sediment provenance and specifically to reconstruct the step‐by‐step changes of detrital signatures ideally produced during unroofing of a continental block. Detritus from progressively deeper crustal levels of the Southern Alps changes systematically from lithic sedimentaclastic (unmetamorphosed cover sequences) to lithoquartzose, quartzolithic, and quartzofeldspathic metamorphiclastic (greenschist facies to amphibolite facies basement units) and finally to feldspathoquartzose and feldspathic composition (granulite facies basement units). Six progressively richer types of heavy mineral assemblages (ZTR‐brookite, garnet‐chloritoid, garnet‐staurolite, blue/green hornblende–kyanite, green/brown hornblende–fibrolitic sillimanite, and garnet–brown hornblende–prismatic sillimanite) faithfully reflect the increasing grade of metamorphic source rocks. Hypersthene–brown hornblende and olivine‐enstatite assemblages characterize detritus from lower crustal gabbros and mantle peridotites. In alpine settings undergoing rapid erosion and minor chemical weathering, the mineralogy of detrital sediments faithfully mirrors the mineralogy of their parent rocks. Therefore, our database can be used to integrate direct measurements of physical and chemical properties of exposed bedrock and to reconstruct an ideally complete mineralogy and density profile from the base to the top of the South Alpine crust. This shows a progressive upward increase in quartz and muscovite at the expense of plagioclase and heavy minerals and an effective density stratification from lower crustal (3.10–3.00 g/cm3) to middle crustal (∼2.85 g/cm3) and upper crustal levels (2.75–2.70 g/cm3). Sharp jumps in average rock density observed across major structural discontinuities (e.g., Cossato‐Mergozzo‐Brissago Line) mirror the fact that the South Alpine basement is the extensional stack of several slivers that followed distinct metamorphic and exhumation paths during the multistage tectonic evolution linking the Carboniferous Variscan Orogeny with the Jurassic opening of the Alpine Tethys.

Presentation of the Mineralogical Society of America Award for 2005 to Tiziana Boffa-Ballaran

Mr. President, Ladies and Gentlemen: It was a pleasant surprise when Tiziana asked me to introduce her as the recipient of the MSA award for 2005, as she has worked with many excellent mineralogists and crystallographers who have influenced and guided her career more than I. Indeed, I suspect I have learnt more from Tiziana than she has from me. Like many senior crystallographers, Tiziana comes from a farming background, having grown up in the small village of Sella di Tavigliano on the flanks of the Piemontese Alps in northern Italy. Mineralogists know this area better as the Ivrea zone, and for the eclogites at Monte Mucronne that lie above her home. Despite these mineralogical influences early in her life, Tiziana choose to study chemistry for her undergraduate degree, first at the University of Milan, and then at Pavia. However, when one reads Tiziana’s CV for her undergraduate years one sees the seeds of the two pervasive themes of her subsequent career. First, she won scholarships to spend semesters at other Universities in Europe (Aarhus in Denmark, Aveiro in Portugal, and Wroclaw in Silesia) and so develop scientific collaborations and friendships that cross many international borders. In this sense Tiziana is a representative of the new model of European integration, maintaining her strong regional roots and traditions while developing links that span the continent; her new young family is another product of such collaboration! The other theme that appears in Tiziana’s undergraduate record is thermodynamics of solid solutions, the topic of both her senior thesis and her work in Wroclaw, and it is this theme that formed the basis of the …

Recrystallization or cooling ages: in situ UV-laser 40 Ar/ 39 Ar geochronology of muscovite in mylonitic rocks

The intra-grain 40 Ar/ 39 Ar age distributions of muscovite from greenschist-facies mylonite zones are shown to be sensors of metamorphic history. Textures, compositions and in situ UV-laser ablation 40 Ar/ 39 Ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo- or recrystallization ages. Deformed muscovite porphyroclasts and recrystallized shear band muscovite from the Pogallo Shear Zone (Ivrea Zone, southern Alps) reveal variable intra-grain 40 Ar/ 39 Ar ages with internal age variations of more than 65 Ma. The wide range of 40 Ar/ 39 Ar ages within compositionally homogeneous grains is consistent with diffusion-dominated argon loss controlled by observable intra-grain microstructures. In contrast, intra-grain 40 Ar/ 39 Ar ages of muscovite fish from Proterozoic mylonite of the Porsgrunn–Kristiansand Shear Zone (Southern Norway) display only minor age dispersion, and age variations are correlated with changes in muscovite composition. The sympathetic age–composition correlations in different muscovite microstructures directly relate to sequential neo- and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the Porsgrunn–Kristiansand Shear Zone.

The effect of metasomatism on the Cr-PGE mineralization in the Finero Complex, Ivrea Zone, Southern Alps

The magmatic metasomatism that was responsible for producing chromitite–dunite bodies in the unusual phlogopite peridotite of the Finero Complex in Permian to Triassic times also influenced the Cr-platinum group elements (PGE) mineralization. At least the end stages of this metasomatism are recorded in compositional zoning of chromite grains in the podiform chromitite. Metasomatic melt, with or without vapor, reacted with chromite to produce core-to-rim Cr enrichment of extant chromite grains and was concurrent with pyroxene crystallization. Under conditions of lower melt/rock ratio, metasomatism resulted in core-to-rim Al enrichment in chromite and crystallization of amphibole between chromite and clinopyroxene. This early, high-temperature metasomatism is unrelated to the later and pervasive K-metasomatism that crystallized phlogopite and was associated with the intrusion of clinopyroxenite dikes that cut the peridotite. Much later, serpentinization of olivine locally depleted chromite in Al and enriched it in Fe and formed minor amounts of magnetite. The PGE, which are present mainly as laurite inclusions in chromite, were remobilized by the early metasomatism. This resulted in substantial variation in the PGE contents of chromitites and imposed a characteristic PGE pattern in which chondrite-normalized Os, Ir, Ru and Rh contents are high but Pt and Pd contents are low. The slopes of PGE chondrite-normalized concentration patterns are systematically related to absolute PGE abundance and to rock mode. Chromitites with low modal orthopyroxene, clinopyroxene, and amphibole exhibit negative PGE slopes and contain relatively high PGE concentrations, whereas chromitites rich in these silicate minerals have positive slopes and low PGE contents.

Direct measurement of 3D elastic anisotropy on rocks from the Ivrea zone (Southern Alps, NW Italy)

Lower crustal and upper mantle rocks exposed at the earth's surface present direct possibility to measure their physical properties that must be, in other cases, interpreted using indirect methods. The results of these direct measurements can be then used for the corrections of models based on the indirect data. Elastic properties are among the most important parameters studied in geophysics and employed in many fields of earth sciences. In laboratory, dynamic elastic properties are commonly tested in three mutually perpendicular directions. The spatial distribution of P- and S-wave velocities are then computed using textural data, modal composition, density and elastic constants. During such computation, it is virtually impossible to involve all microfabric parameters like different types of microcracking, micropores, mineral alteration or quality of grain boundaries. In this study, complete 3D ultrasonic transmission of spherical samples in 132 independent directions at several levels of confining pressure up to 400 MPa has been employed for study of selected mafic and ultrabasic rocks sampled in and nearby Balmuccia ultrabasic massif (Ivrea zone, Southern Alps, NW Italy). This method revealed large directional variance of maximum P-wave velocity and different symmetries (orthorhombic vs. transversal isotropic) of elastic waves 3D distribution that has not been recorded on these rocks before. Moreover, one dunite sample exhibits P-wave velocity approaching to that of olivine single crystal being interpreted as influence of CPO.

Nb/Ta, Zr/Hf and REE in the depleted mantle: implications for the differentiation history of the crust–mantle system

High-precision Nb, Ta, Zr, Hf, Sm, Nd and Lu concentration data of depleted mantle rocks from the Balmuccia peridotite complex (Ivrea Zone, Italian Alps) were determined by isotope dilution using multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) and thermal ionisation mass spectrometry (TIMS). The Zr/Hf ratios of all investigated samples from the Balmuccia peridotite complex are significantly lower than the chondritic value of 34.2, and the most depleted samples have Zr/Hf ratios as low as 10. Correlated Zr/Hf ratios and Zr abundances of the lherzolites preserve the trend of a mantle residue that has been depleted by fractional melting. This trend confirms experimental studies that predict Hf to behave more compatibly than Zr during mantle melting. Experimentally determined partition coefficients imply that the major Zr and Hf depletion most likely occurred in the spinel stability field, with ( D Zr/ D Hf) cpx≈0.5, and not in the garnet stability field, where ( D Zr/ D Hf) grt is probably close to one. However, minor amounts of melting must have also occurred in a garnet facies mantle, as indicated by low Sm/Lu ratios in the Balmuccia peridotites. The Nb/Ta ratios of most lherzolites are subchondritic and vary only from 7 to 10, with the exception of three samples that have higher Nb/Ta ratios (18–24). The overall low Nb/Ta ratios of most depleted mantle rocks confirm a higher compatibility of Ta in the mantle. The uniform Nb/Ta ratios in most samples imply that even in ‘depleted’ mantle domains the budget of the highly incompatible Nb and Ta is controlled by enrichment processes. Such a model is supported by the positive correlation of Zr/Nb with the Zr concentration. However, the overall enrichment was weak and did barely affect the moderately incompatible elements Zr and Hf. The new constraints from the partitioning behaviour of Zr–Hf and Nb–Ta provide important insights into processes that formed the Earth’s major silicate reservoirs. The correlation of Zr/Hf and Sm/Nd in depleted MORB can be assigned to previous melting events in the MORB source. However, such trends were unlikely produced during continental crust formation processes, where Sm/Nd and Zr/Hf are decoupled. The different fractionation behaviour of Zr/Hf and Sm/Nd in the depleted mantle (correlated) and the crust (decoupled) indicates that crustal growth by a simple partial melting process in the mantle has little effect on the mass budget of LREE and HFSE between crust and mantle. A more complex source composition, similar to that of modern subduction rocks, is needed to fractionate the LREE, but not Zr/Hf and the HREE.

HT-deformation of garnet: an EBSD study on granulites from Sri Lanka, India and the Ivrea Zone

Garnets from three regional granulite terrains (Sri Lanka, Eastern Ghats, Ivrea Zone) have been analyzed and the results indicate, that garnet can be plastically deformed by dominant dislocation glide/creep with variable dominant slip systems.Garnets from a granulite facies fold hinge in a quartzite of the Highland Complex of Sri Lanka, unequivocally attest to plastic deformation of garnet. Outside the fold hinge the garnets form flat discs elongated parallel to a pronounced stretching lineation and show deformation microstructures like boudinage and pinch and swell structures. In the fold hinge these garnet discs are folded within the quartzitic matrix at temperatures of about 800°C. In spite of the preservation of deformed shapes, intracrystalline deformation microstructures are scarce. The garnets have distinct crystallographic preferred orientations, which can be interpreted by a dominant 1/2〈111〉{110} slip system approaching ‘easy slip’ positions. These are reached rapidly under moderate strains as necessary lattice rotations are small.Similar slip systems were derived for a garnet-bearing quartzite from the Eastern Ghats deformed at temperatures of 750–800°C, but with more influence of initial orientation of garnets. Completely different textures indicative of dominant 〈100〉 [010] slip were found in a restitic garnet–sillimanite granulite from the Ivrea Zone. The variable deformation features and variable slip systems with 1/2〈111〉 and 〈100〉 Burgers vectors prove that garnets bear significant potential for contributing additional information on the HT-deformation of rocks.

Metasomatic coronas around hornblendite xenoliths in granulite facies marble, Ivrea zone, N Italy. II: Oxygen isotope patterns

Up to 20-cm-wide metasomatic reaction bands formed coronas around hornblendite xenoliths in a marble matrix during high grade metamorphism in the Ivrea zone. The coronas are comprised of an innermost monomineralic clinopyroxene layer, a garnet–clinopyroxene layer and an outermost scapolite–clinopyroxene layer. The oxygen isotope composition of the original hornblendite core is 7‰ relative to V-SMOW and the oxygen isotope composition of the marble matrix is 19.7‰. The oxygen isotope transition across the corona is represented by a diffusion front with a step discontinuity at the inner margin of the corona. The systematics of the inter-mineral fractionations indicates preservation of the oxygen isotope compositions from high temperatures and maintenance of grain-scale oxygen isotope equilibrium during corona formation. The oxygen isotope pattern is interpreted in terms of a moving boundary diffusion problem. The growing reaction band and the reactant hornblendite and marble represent a total of five media with different transport properties and moving separation surfaces. Bulk oxygen diffusion was at least three orders of magnitude faster then expected from volume diffusion, suggesting that transport was enhanced by relatively fast diffusion along grain boundaries. Oxygen diffusivities in the individual layers correlate with the oxygen volume diffusivities in the major constituent minerals of the respective layers, suggesting mineralogical control on bulk oxygen diffusion.

Metasomatic coronas around hornblendite xenoliths in granulite facies marble, Ivrea zone, N Italy, I: constraints on component mobility

Centimeter- to decimeter-thick reaction bands occur at hornblendite/marble interfaces in Val Fiorina in the granulite facies metamorphic Ivrea zone. From hornblendite to marble the reaction bands show a consistent succession of sharply bounded mineral layers comprising a monomineralic clinopyroxene layer, a garnet–clinopyroxene layer and a scapolite–clinopyroxene layer. Reaction band formation occurred as a response to gradients in the chemical potentials of calcium and magnesium as defined by the hornblendite assemblage and the marble matrix. The metasomatic corona primarily replaced the hornblendite, and only minor amounts of marble were consumed. The reaction band behaved as an open system with net transfer of calcium from the marble into the reaction band, and a net transfer of iron and magnesium in the opposite direction. Mass balance considerations allow us to constrain a range of feasible mass balance scenarios for which major element fluxes across the boundaries of the reaction band may be quantified. Modeling of layer growth as a steady diffusion process yields ratios of the phenomenological diffusion coefficients for Si, Al, Mg, and Ca of \({{L_{SiSi} } \over {L_{CaCa} }}> 2.5,{\kern 1pt} {\rm }{{L_{AlAl} } \over {L_{CaCa} }} 1.\) . The relative diffusivities are primarily constrained by the sequence of mineral layers of the reaction band and by the relative thickness of the layers. The results of steady-state diffusion modeling are relatively insensitive with respect to variations in the major element boundary fluxes.

Dislocation-mechanism of clinoamphibole formation in deformed augite

Clinoamphibole formation in naturally deformed augite from a pyroxenite dyke of the Balmuccia peridotite massif (Ivrea Zone, NW-Italy) has been studied by conventional and high resolution transmission electron microscopy. The clinoamphibole lamellae are oriented parallel to (010) and have their crystallographic axes parallel to those of the augite. They originate from dissociation of certain lattice dislocations. The shear produced by glide of 1/2 [101] partial dislocations in (010) combines single to double chains, yielding so-called chain multiplicity faults. The ion exchange necessary for amphibole formation most probably is towards and along the core of the partials. Broadening of the lamellae occurs by lateral motion of partial dislocations along the (010) interface two chains apart. The formation of clinoamphibole is discussed with respect to exsolution and alteration, phase stability and reaction kinetics.

Age, Geochemistry and Petrogenesis of the Ultramafic Pipes in the Ivrea Zone, NW Italy

Pipe-like ultramafic bodies, hosting Ni–Cu–PGE sulphide deposits, intrude the Main Gabbro and the roof metasediments of the Ivrea Zone, NW Italy. These bodies were emplaced at 287 ± 3 Ma and represent the last mantle-derived melts associated with an underplating event that largely drove the crustal evolution of this area during the late Carboniferous (∼300–290 Ma). The pipes are composed of volatile-rich ultramafic rocks and gabbros with an alkaline signature simultaneously enriched in both incompatible and the most compatible elements but depleted in elements of intermediate compatibility. The isotope composition of these pipe rocks is ε290 Ma(Nd) ∼3·7 to −1·9 and ε290 Ma(Sr) ∼0·8–26. In a ε290 Ma(Nd) vs ε290 Ma(Sr) diagram they define a linear array between the unmetasomatized and metasomatized peridotites of Finero, but distinctly oblique with respect to the trend defined by Balmuccia peridotites. The δ34S ranges from 0·0 to +0·9‰ and is indicative of a mantle source. We suggest that the pipes represent infiltration of melts derived from a depleted mantle protolith flushed with alkaline metasomatic fluids, probably of juvenile mantle origin, which underwent partial melting as a consequence of the depression of the solidus owing to the increased activity of water and other volatiles. The similarity in age, trace-element, and isotopic signatures indicates that the pipes were probably produced in the course of the same metasomatic event that affected the Finero ultramafic body. The overall geochemical characteristics of the pipes are more consistent with magmatism related to a mantle plume than with a subduction setting.

The Zircon-Bearing Chromitites of the Phlogopite Peridotite of Finero (Ivrea Zone, Southern Alps): Evidence and Geochronology of a Metasomatized Mantle Slab

The phlogopite peridotite unit of the Finero Complex is a restitic largest in the Alps. The chromite is magnesian and locally harzburgite that records two metasomatic events. The first event is contains inclusions of Fe–Ni–Cu sulphide and platinum related to the intrusion of basaltic magma, which reacted with the group element minerals (Ferrario & Garuti, 1990). pyroxene of the host harzburgite to produce chromitite pods with dunite Chromitite forms well-defined bodies with modal chrohaloes. It also produced secondary clinopyroxene and amphibole in mite concentrations as high as 75%. The occurrences the harzburgite and enriched harzburgite in Na and the light rare were described by Roggiani (1948), Friedenreich (1956), earth elements. The second metasomatic event is related to the later Forbes et al. (1978) and Saager et al. (1982). intrusion of clinopyroxenitic dykes. During this event, water-rich The petrology of the Finero Complex is poorly unvapour penetrated the harzburgite along fractures and reacted with derstood. One reason is that the phlogopite peridotite, it to form phlogopite, thus enriching the rock in K. Chromitites host which makes up the core of the complex, exhibits anzircons that yield an age for the first metasomatic event of 207·9 + omalous petrographic and geochemical characteristics 1·7/−1·3 Ma, during which time extensional tectonics prevailed in indicative of metasomatism and of a multistage evolution the Southern Alps. (Coltorti & Siena, 1984; Cumming et al., 1987;Voshage et al., 1987; Hartmann & Wedepohl, 1993; Lu et al., 1997a, 1997b). One aim of this study is to unravel the sequence of events that affected the phlogopite peridotite.

Metamorphism and Anatexis in the Mafic Complex Contact Aureole, Ivrea Zone, Northern Italy

Emplacement of mantle-derived magma (magmatic accretion) is the total thermal budget of the continental lower crust, often presumed or inferred to be an important cause of regional produce regional granulite facies metamorphism, and granulite facies metamorphism and crustal anatexis. The juxgenerate Yand heavy rare earth element (HREE)taposition of mafic cumulates and regionally distributed granulite depleted granitoids (Ellis, 1987). Accretion of mafic facies rocks has led some to consider the Ivrea zone (northern Italy, magma and migration of partial melt will internally Southern Alps) as an important exposure that demonstrates this stratify, chemically differentiate, and deplete the concausal relationship. However, regional PTt paths indicated by tinental lower crust in large ion lithophile elements metamorphic reaction textures and PT conditions inferred from (LILE). Magmatic accretion has been invoked to provide geothermobarometry indicate that the emplacement of mafic plutonic the heat and mass necessary for sustained magmatism in rocks (Mafic Complex) at the Ivrea zone occurred during detectonic settings such as Phanerozoic extensional terranes compression from ambient pressures at the regional thermal maximum. (Lister et al., 1986; Gans, 1987; Fountain, 1989; Mareschal Field and petrographic observations, supported by PT estimates, & Bergantz, 1990; Jarchow et al., 1993) and magmatic indicate that regional retrograde decompression and emplacement of arcs (Hamilton, 1981; Kay & Kay, 1981; Bohlen & the upper parts of the Mafic Complex probably accompanied Lindsley, 1987; Hildreth & Moorbath, 1988). extension during the Late Carboniferous–Early Permian. A spatially However, as few exposed sections of lower continental restricted decompression-melting event accompanied final emcrust show contiguous mafic intrusions and regionally placement, depleting supracrustal rocks enclosed by an >2–3 km distributed granulite facies rocks, estimates of the extent aureole overlying the upper Mafic Complex by 20–30% granite of anatexis and metamorphism accompanying magmatic component. The upper Mafic Complex provided the thermal energy accretion have relied on numerical and analog simto reset mineral assemblages and locally overprint the regional ulations. These models yield disparate results depending prograde metamorphic zonation. The limited extent of the contact on whether heat transfer within the mafic intrusion aureole suggests that magmatic accretion may not inexorably cause and surrounding country rocks is primarily convective regional metamorphism and crustal anatexis. (Campbell & Turner, 1987; Huppert & Sparks, 1988) or conductive (Marsh, 1989; Bergantz & Dawes, 1994; Barboza & Bergantz, 1996). A comparison with field evidence is required to discriminate between the model

The distribution of lithium in peridotitic and pyroxenitic mantle lithologies — an indicator of magmatic and metasomatic processes

Lithium concentrations in orthopyroxene, clinopyroxene, olivine, garnet or spinel from equilibrated spinel peridotite, garnet peridotite and garnet pyroxenite xenoliths and from metasomatised peridotites and pyroxenites were measured using the ion-microprobe (SIMS). With respect to changes in physical and chemical parameters, we present new data on lithium contents in mantle minerals and its partitioning behaviour in (1) equilibrated and (2) metasomatised samples. Lithium is preferentially incorporated into olivine, ranging between 1 and 2 ppm in equilibrated unmetasomatised peridotites. Pyroxenes from peridotitic xenoliths have Li concentrations on the order of several hundred ppb up to 1.3 ppm, while pyroxenes from pyroxenites have somewhat higher abundances (1–3 ppm with a maximum of 19 ppm). The following partitioning relationships have been established: ol>cpx≥opx≫grt or sp for garnet and spinel peridotites, respectively, and cpx≥opx>grt for garnet pyroxenites. The intercrystalline partitioning of Li is independent of T, P and bulk composition (for ultramafic to mafic compositions), making Li a suitable tracer element for chemical processes such as metasomatism. We estimate a bulk Li content of 1.0–1.5 ppm for fertile to moderately depleted lithospheric mantle. Low Li abundances in mantle peridotites and pyroxenites emphasise its incompatibility during partial melting and fractional crystallisation. However, elevated Li concentrations are present in some pyroxenites, presumably due to complete crystallisation of trapped partial melts. Metasomatised samples from peridotite massifs (Pyrenees and Ivrea Zone) and mantle nodules from two Victorian volcanic fields (Australia) clearly show enrichment of Li in both olivine and clinopyroxene, whereby the distribution of Li between olivine and clinopyroxene has generally not achieved equilibrium. Disequilibrium is manifested by preferential Li enrichment in either olivine or clinopyroxene depending on the type of metasomatic agent involved (carbonatite vs. mafic silicate melt). Differences in absolute Li abundances and in its partitioning behaviour allow the identification not only of metasomatic overprints, but also of magmatic processes such as partial melting, crystal fractionation and accumulation. The sensitivity of Li as such a chemical tracer gives an additional criterium for recognising cryptic metasomatism.

Realistic Models of Anisotropic Laminated Lower Crust

—The genesis of the laminated lower crust has been attributed to extensional processes leading to structural and textural ordering. This implies that the lower crust might be anisotropic. Laboratory measurements of lower crustal rock samples and xenolithes show evidence of anisotropy in these rocks due to oriented structure.¶In this paper we investigate the seismic shear-wave response of realistic anisotropic lower crustal models using the anisotropic reflectivity method. Our models are based on representative petrophysical data obtained from exposed lower crustal sections in Calabria (South Italy), Val Strona and Val Sesia (Ivrea Zone, Northern Italy). The models consist of stacks of anisotropic layers characterized by quantified elastic tensors derived from representative rock samples which provide alternating high and low velocity layers.¶The seismic signature of the data is comparable to seismic observations of in situ lower crust. For the models based on the Calabria and Val Strona sequences shear-wave splitting occurs for the Moho reflection at offsets beyond 70 km with travel-time delays up to 300 and 500 ms, respectively. The leading shear wave is predominantly horizontally polarized and followed by a predominantly vertically polarized shear wave. Contrastingly, the Val Sesia model shows no clear evidence of birefringence. Isotropic versus anisotropic modelling demonstrates that the shear-wave splitting is clearly related to the intrinsic anisotropy of the lower crustal rocks for the Val Strona sequence. No evidence of birefringence caused by thin layering is found.

Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)

Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic.

Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone (Southern Alps)

U-Pb analyses of single monazite grains from two granulite facies metapelites in the Ivrea Zone (Southern Alps) reveal the presence, in both samples, of at least three different ages and prove that earlier interpretations of supposedly concordant monazite data as cooling ages are unwarranted. One group of monazite data defines a subconcordant discordia line with an upper intercept age of 293.4 ± 5.8 Ma and a lower intercept age of 210 ± 14 Ma. The upper intercept is interpreted as the real cooling age of the monazites. The lower intercept is interpreted as an episode of fluid-driven Pb-loss, indicated by the presence of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are also rejuvenated at 210 ± 12 Ma. Another group of monazite data lies above the concordia. The presence of excess 206Pb indicates that these crystals have grown below the monazite blocking temperature, thus after the granulite facies metamorphism. The age of growth of the new monazite crystals is approached by their 207Pb/235U ages that range between 273 and 244 Ma. The two groups of post-cooling age (post-293.4 ± 5.8 Ma) monazite data correspond to two distinct late- and post-Variscan geotectonic regimes that affected the Southern Alps, (1) Permian transtension with decompression and anatectic melting; (2) Upper Triassic to Lower Jurassic rifting with geographically dispersed hydrothermal activity and alkaline magmatism.