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Detailed field mapping reveals that the Sesia Zone is subdivided into two complexes with the Barmet Shear Zone (BSZ) outlining the tectonic contact between them. This greenschist-facies contact reflects a metamorphic gap between the Internal Complex (eclogite facies, eclogitic micaschists dominant) and the External Complex (epidote blueschist facies). The BSZ comprises a wedge shape area in which fragments and slices of orthogneiss and paragneiss are wrapped by siliceous dolomite marbles displaying a mylonitic foliation. Conspicuous cornieules and high pressure breccias occur along this contact. We propose that the eclogite facies Internal Complex is subdivided into three basement units, called sheets, delimited by discontinuous metasedimentary trails of probable Mesozoic age. Thin monocyclic bands thus separate kilometre scale polycyclic sheets. The External Complex comprises three epidote blueschist facies sheets of comparable size, which are separated by lenses retaining a pre-Alpine high temperature imprint. These weakly overprinted fragments (parts of the classically termed 2DK zone) are aligned along greenschist facies shear zones that separate the gneissic sheets. The BSZ, with a wedge rich in meta-sediments, chiefly siliceous dolomite marbles, is a key element in which fragmentation and reworking of materials from the internal and external complexes are evident. A carbonate breccia occurs in this shear zone, with clasts displaying a HP foliation randomly oriented in a ductile carbonate matrix. Siliceous dolomite marbles appear to have acted as lubricants to accommodate deformation related to the juxtaposition of the two basement complexes during exhumation. We propose a model of the Sesia Zone, with the BSZ as the thrust responsible for the juxtaposition of eclogite facies rocks of the Internal Complex on top of epidote blueschist facies rocks of the External Complex. The two complexes were already assembled when this shear zone became active. The entire stack was finally rotated (40-60°) during the Vanzone Phase. [ABSTRACT FROM AUTHOR]

In the Sesia Zone (Italian Western Alps), slivers of continental crust characterised by an Alpine high-pressure imprint are intermingled with abundant mafic rocks and Mesozoic metasediments. An extensive study of the central Sesia Zone was undertaken to identify and reconstruct the lithological setting of the mono-cyclic sediments of the Scalaro Unit. A new geological map (1:5000) and schematic cross sections across the Scalaro Unit and the adjoining Eclogitic Micaschist Complex are presented here. In order to delimit the size and shape of the mono-metamorphic unit and understand its internal geometry with respect to the poly-metamorphic basement, an integrated approach was used. Linking observations and data across a range of scales, from kilometres in the field down to petrological and chronological data obtained at micrometre scale, we define for the first time the real size and internal geometry of the Scalaro Unit, as well as its large-scale structural context. [ABSTRACT FROM AUTHOR]

Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Studies of high-pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to mid-crustal levels, but the details of such dynamics are controversial. To understand the dynamics of a subduction channel we report the results of a petrochronological study from the central Sesia Zone, a key element of the internal Western Alps. This comprises two polymetamorphic basement complexes (Eclogitic Micaschist Complex and Gneiss Minuti Complex) and a thin, dismembered cover sequence (Scalaro Unit) associated with pre-Alpine metagabbros and metasediments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro–Bonze Units) yield unequivocal petrological and geochronological evidence of two distinct high-pressure stages. Ages (U–Th–Pb) of growth zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples. Two independent tectono-metamorphic ‘slices’ showing a coherent metamorphic evolution during a given time interval have been recognized: the Fondo slice (which includes Scalaro and Bonze rocks) and the Druer slice (belonging to the Eclogitic Micaschist Complex). The new data indicate separate stages of deformation at eclogite-facies conditions for each recognized independent kilometer-sized tectono-metamorphic slice, between ∼85 and 60 Ma, with evidence of intermittent decompression (ΔP ∼ 0·5 GPa) within only the Fondo slice. The evolution path of the Druer slice indicates a different P–T–time evolution with prolonged eclogite-facies metamorphism between ∼85 and 75 Ma. Our approach, combining structural, petrological and geochronological techniques, yields field-based constraints on the duration and rates of dynamics within a subduction channel. [ABSTRACT FROM AUTHOR]

Mushroom- and atoll-shaped garnet crystals were found in high-pressure quartz-rich pelitic layers from the Monte Mucrone area (Western Alps, Italy). These garnet crystals are characterized by a peninsula-shaped core surrounded by a partially crystallized, dodecahedral external rim. Textural observations and thermodynamic modelling point towards growth of the atoll garnet from the Monte Mucrone area during two distinct orogenic cycles. The core region and the inner part of the ring forming the edge of the atoll grew under Barrovian metamorphic conditions of likely Hercynian age, while the outer rim of the atoll structure developed under Alpine high-pressure conditions. Electron backscatter diffraction analyses indicate that the atoll-shaped structure has one single crystallographic orientation, despite its complex compositional zoning. Thermodynamic modelling reveals marked changes in equilibrium assemblage leading to changes in stoichiometry of the garnet-forming reaction, which in turn explain the characteristic garnet morphology. Small amounts of quartz are consumed during the earlier stage of garnet growth history, whereas the production of garnet requires a much larger amount of quartz during the final stage of its growth. This leads to a change from initial poikiloblastic to non-poikiloblastic textures. This change is responsible for the formation of atoll-shaped garnet. Finally, garnet in intercalated mica-rich layers forms idiomorphic crystals, continuous from the centre to rim. This study highlights the importance of the difference between the local matrix composition and the aggregate composition of the reactants needed for the garnet-forming reaction. Finally, we show that interaction between matrix and reaction stoichiometry can lead to porphyroblast precipitation inside the already grown porphyroblast. [ABSTRACT FROM AUTHOR]

Abstract: This study addresses the mechanical and chemical behavior of allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in allanite during geological processes is essential for robust petrochronology. Dating of allanite in meta-granodiorite showing variable amounts of strain (from an undeformed protolith to mylonite) at Monte Mucrone (Sesia Zone, NW Italy) gave surprising results. Based on structural and petrographic observations the shear zones at Mt Mucrone are Alpine, yet allanite located within an eclogite facies mylonite yielded Permian ages (208Pb/232Th average age: 287±7Ma). These mm-sized allanite grains are rimmed by an aggregate of coarse-grained garnet+phengite, thought to derive from former epidote. These aggregates were immersed in a weak matrix that experienced granular flow, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permian monazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th age: 291±5Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian allanite. These (re)crystallized grains are Sr-rich and show mosaic zoning. These results indicate that allanite crystals retained their chemical and isotopic characteristics, and thus their Permian age, as a result of strong strain partitioning between the epidote group porphyroclasts and the eclogite facies matrix in HP-mylonites. The observed partial mobilization of Pb isotopes, which lead to the Cretaceous-aged rims or grains in undeformed samples was facilitated by (re)crystallization of allanite and not by mere Pb diffusion alone under the HP conditions. [Copyright &y& Elsevier]

The Western Alpine Sesia–Lanzo Zone (SLZ) is a sliver of eclogite-facies continental crust exhumed from mantle depths in the hanging wall of a subducted oceanic slab. Eclogite-facies felsic and basic rocks sampled across the internal SLZ show different degrees of retrograde metamorphic overprint associated with fluid influx. The weakly deformed samples preserve relict eclogite-facies mineral assemblages that show partial fluid-induced compositional re-equilibration along grain boundaries, brittle fractures and other fluid pathways. Multiple fluid influx stages are indicated by replacement of primary omphacite by phengite, albitic plagioclase and epidote as well as partial re-equilibration and/or overgrowths in phengite and sodic amphibole, producing characteristic step-like compositional zoning patterns. The observed textures, together with the map-scale distribution of the samples, suggest open-system, pervasive and reactive fluid flux across large rock volumes above the subducted slab. Thermodynamic modelling indicates a minimum amount of fluid of 0·1–0·5 wt % interacting with the wall-rocks. Phase relations and reaction textures indicate mobility of K, Ca, Fe and Mg, whereas Al is relatively immobile in these medium-temperature–high-pressure fluids. Furthermore, the thermodynamic models show that recycling of previously fractionated material, such as in the cores of garnet porphyroblasts, largely controls the compositional re-equilibration of the exhumed rock body. [ABSTRACT FROM AUTHOR]

Garnets in continentally derived high-pressure (HP) rocks of the Sesia Zone (Western Alps) exhibit three different chemical zonation patterns, depending on sample locality. Comparison of observed garnet zonation patterns with thermodynamically modelled patterns shows that the different patterns are caused by differences in the water content of the subducted protoliths during prograde metamorphism. Zonation patterns of garnets in water-saturated host rocks show typical prograde chemical zonations with steadily increasing pyrope content and increasing XMg, together with bell-shaped spessartine patterns. In contrast, garnets in water-undersaturated rocks have more complex zonation patterns with a characteristic decrease in pyrope and XMg between core and inner rim. In some cases, garnets show an abrupt compositional change in core-to-rim profiles, possibly due to water-undersaturation prior to HP metamorphism. Garnets from both water-saturated and water-undersaturated rocks show signs of intervening growth interruptions and core resorption. This growth interruption results from bulk-rock depletion caused by fractional garnet crystallization. [ABSTRACT FROM PUBLISHER]