Trace element chemistry and U-Pb dating of zircons from oceanic gabbros and their relationship with whole rock composition (Lanzo, Italian Alps)

The U-Pb ages and the trace element content of zircon U-Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean-continent transition zones. Three Fe-Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161 ± 2, 158 ± 2 and 163 ± 1 Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in ocean-continent transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithosphere

Dissolution-reprecipitation of zircon at low-temperature, high-pressure conditions (Lanzo Massif, Italy)

An eclogite facies meta-plagiogranite from the Lanzo massif (western Alps, Italy) contains crystals of zircon intimately associated with allanite. Zircon displays different microtextures ranging from pristine, euhedral, and magmatic to fractured, porous varieties with mosaic zoning, and pervasive recrystallization into euhedral microcrystals. Fractures and voids in the recrystallized zircon microcrystals are mainly filled by high-pressure Na-rich pyroxene. Electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals, with less than 2° misorientation among neighboring microdomains. The textural change is coupled with chemical and isotopic modifications: recrystallized zircon domains contain significantly less Th and light- to mid-REE, but are richer in Sr than magmatic zircon crystals. Magmatic zircon preserves the protolith U-Pb age of 163.5 ± 1.7 Ma, whereas zircon microcrystals have a mean age of 55 ± 1 Ma. The coexisting allanite also contains inclusions of Na-rich pyroxene and has chemical features (elevated Sr and Ni contents and lack of Eu anomaly) indicating formation at high pressure. Despite being associated texturally with zircon, allanite yields a younger Th-Pb age of 46.5 ± 3.0 Ma, suggesting that the Lanzo unit remained at relatively high pressure conditions for ∼8 m.y. Zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution-reprecipitation process. Recrystallization occurred with minimum transport, in a low-strain environment, and was not significantly enhanced by metamictization. The source of the fluid for zircon recrystallization is most probably related to prograde evolatilization reactions in the surrounding serpentinite

The role of lower crust and continental upper mantle during formation of non-volcanicpassive margins: evidence from the Alps

The remnants of a Mesozoic passive continental margin and of the Tethyan ocean floor are preserved in the Austroalpine and Upper Penninic nappes in eastern Switzerland and northern Italy. Reconstructions of the continent-ocean transition indicate that large areas of subcontinental mantle rocks, but only limited areas of lower-crustal rocks were exposed on the Tethyan sea floor. Microstructures, large shear zones, and the retrograde metamorphic evolution of peridotite and gabbro from Malenco (northern Italy) are investigated to evaluate the role of lower crust and upper mantle during formation of non-volcanic passive continental margins. The combination of petrological constraints and microstructures suggests two contrasting stages: (1) high-temperature (> 650 °C) shearing and annealing of microstructures are attributed to pre-rift tectonics; (2) localized mylonitic shear zones cut the high-temperature structures and developed during nearly isothermal decompression (T <600°C), followed by cooling and hydration of the rocks. These shear zones formed during exhumation of the lower crust and upper mantle and are related to early rifting of the Adriatic passive continental margin. The microstructures of the hydrous mylonites display drastic grain-size reduction, which results from a combination of dynamic recrystallization and metamorphic hydration reactions at temperatures <650 °C. Strain softening facilitated the formation of crustal-scale shear zones along which the lower crust and upper mantle were exhumed to shallow crustal levels of c. 10-15 km. Such large shear zones excised 10-20 km of mostly intermediate and lower crust, and are linked to and contemporaneous with the formation of rift-related basins in the upper crust. Boudinage of the lower crust during early rifting is proposed as a major process to explain the scarcity of exposed lower crust along non-volcanic passive margins. The compilation of pressure-temperature data and rift-related structures in the deep crust and upper continental mantle from the Alps suggests that most peridotites preserve a high-temperature evolution that is not related to Mesozoic rifting. Granulite-facies rocks occur in pre-rift lower and middle continental crust. Exhumed granulites along passive continental margins preserve much of a history that is not related to the exhumation itself, but to tectonic processes predating rifting

The importance of serpentinite mylonites for subduction and exhumation of oceanic crust.

In the ultramatic Erro-Tobbio unit (Voltri-Massif: Western Alps) a set of overprinting structures in serpentinite mylonites is related to Alpine subduction to about 80 km depth and to subsequent exhumation. Antigorite mylonites are cut by en-echelon olivine veins, which in turn are dissected by multiple sets of shear bands containing olivine and titanian clinohumite. The transition from olivine-free to olivine-bearing structures indicates recrystallization during prograde metamorphism. All structures display the same top-to-the-NW kinematics providing evidence for a continuous non-coaxial deformation. The serpentinite mylonites surround km-scale bodies of pre-Alpine peridotite which show only minor Alpine overprint. This indicates that during subduction-related deformation, recrystallization and fluid flow were strongly localized within serpentinite mylonites. Olivine-bearing, discontinuous shear planes with top-to-the-SE sense of movement crosscut the prograde structures. The inversion of shear sense suggests a change in position of the serpentinites relative to the downgoing slab, i.e. from the subducted slab to the upper plate during accretion. Thus, the shear sense inversion marks the change from burial to exhumation of the serpentinites. The low density of antigorite serpentinites (2.75 g/cm3) causes strong bouyancy, thus providing a mechanism for the exhumation of deeply subducted rocks. It is suggested that serpentinites may act as carriers for the uprise of eclogite bodies, which have higher densities than the peridotitic upper mantle

39Ar-40Ar dating of multiply zoned amphibole generations (Malenco, Italian Alps)

Mafic rocks of a Permian crust to mantle section in Val Malenco (Italy) display a multi-stage evolution: pre-Alpine exhumation to the ocean floor, followed by burial and re-exhumation during Alpine convergence. Four prominent generations of amphiboles were formed during these stages. On the basis of microstructural investigations combined with electron microprobe analyses two amphibole generations can be assigned to the pre-Alpine decompression and two to the Alpine metamorphic P-T evolution. The different amphiboles have distinct NaM4, Ca, K and C1 contents according to different P-T conditions and fluid chemistry. Analysing these mixed amphiboles by the 39Ar-40Ar stepwise heating technique yielded very complex age spectra. However, by correlating amphibole compositions directly obtained from the electron microprobe with the components deduced from the release of Ar isotopes during stepwise heating obtained ages were consistent with the geological history deduced from field and petrological studies. The two generations of pre-Alpine amphiboles gave distinguishable Triassic to Late Jurassic/Early Cretaceous ages (-e7225 and 130-140 Ma respectively). High-NaM4 amphiboles have higher isotopic ages than low-NaM4 ones, in agreement with their decompressional evolution. The exhumation of the Permian crust to mantle section is represented by the former age. The latter age concerns C1-dominated amphibole related to an Early Cretaceous oceanic stage. For the early Alpine, pressure-dominated metamorphism we obtained a Late Cretaceous age (83-91 Ma). The later, temperature-dominated overprint is significantly younger, as indicated by 39Ar-40Ar ages of 67-73 Ma. These Late Cretaceous ages favour an Adriatic origin for the Malenco unit. Our data show that 39Ar-40Ar dating combined with detailed microprobe analysis can exploit the potential to relate conditions of amphibole formation to their respective ages

Primary Magmas in Continental Arcs and their Differentiated Products: Petrology of a Post-plutonic Dyke Suite in the Tertiary Adamello Batholith (Alps)

Determining the primary compositions of arc magmas is fundamental in retracing the chemical differentiation processes responsible for the formation of juvenile arc crust and the thermal structure of the mantle wedge. We have investigated a series of post-plutonic dykes that intruded the gab- broic to tonalitic southern part of the Tertiary Adamello Batholith in the Alps. The dyke rocks range in composition from primary, hydrous high-Mg basalts to basalts, basaltic andesites, andesites and dacites. Field relationships and high-precision U–Pb dating of titanite and zircon show that the dyke suite ranges in age from 41.67 ± 0.06 Ma for the high-Mg basalt to 38.62 ± 0.12 Ma for the youngest dacitic dykes, closely associated with plutonic activity from 42.5 to 39.0 Ma. Andesites and dacites have primitive 87Sr/86Sri (0.7032–0.7038) and 143Nd/144Ndi (eNdCHUR +3.5–3.2) isotopic signatures strongly limiting the extent of crustal assimilation, whereas some of the high-Mg basalts have se- lectively assimilated pelitic metasedimentary rocks as shown by high Cs/Rb, Rb/Sr and Rb/Zr ratios, and isotopically more enriched compositions (87Sr/86Sri 0.7039–0.7046; eNdCHUR +1.6–0.0). Primitive high-Mg basaltic dykes that escaped assimilation processes are primary mantle partial melts that were extracted from their source at pressures of 2.7 ± 0.2 GPa and temperatures of 1390 ± 30°C, conditions corresponding to the spinel–garnet transition in mantle peridotite. Major elem- ent modelling constrains the degree of melting to 20 6 2% leaving a harzburgite residue, consistent with the trace element chemistry of the high-Mg basalts, which have moderate [Gd/Yb]N ratios of 1–1.2. Differentiated basaltic andesites and dacites follow experimentally constrained liquid lines of descent for fractional crystallization at mid- to deep crustal levels. The trace element chemistry of amphiboles from basaltic andesite and andesite dykes reveals the coexistence of amphibole with primitive melts, indicating elevated pressures and H2O contents in their parental magmas. Thermobarometric constraints for amphibole phenocrysts result in pressures from 0.65 to 0.78 GPa and temperatures ranging from 930 to > 1000°C. The absence of any significant Eu-anomaly in the rare earth element patterns in these amphiboles indicates the late appearance of plagioclase in the crystallization sequence. The crystallization of amphibole drives the differentiated magmas to slightly peraluminous, corundum-normative compositions that are common for tonalites building the major part of the Adamello Batholith. Fractionation models at mid- to lower crustal conditions result in the cumulative crystallization of 17% olivine, 2% Cr-rich spinel, 18% clinopyroxene, 41% amphibole, 4% plagioclase and 0.1% magnetite to obtain an andesitic composition from a primary, hydrous high-Mg continental arc basalt. Cumulates formed during fractional crystallization atmid- to deep crustal levels are dunites and wehrlites followed by hornblendites and hornblende- gabbros. The trace element signatures of basaltic andesites and dacites display low Rb/Zr and Rb/ Sr, and are consistent with fractionation-dominated processes within the crust in an active contin- ental margin. Significant crustal assimilation is not required to obtain the trace element signatures of the evolved andesitic magmas

The Geological Map of Valmalenco

The 'Carta Geologica della Valmalenco' (Montrasio et al., 2005) is the result of a thirty five-year cooperation between ETH and University of Zürich and the University and CNR-Italia at Milano. The map covers an area of about 350 km2 with the mapping ha

Melt Percolation at the Base of the Pacific Lithosphere: Insights from Petit-spot Mantle Xenoliths

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