The Cotoncello Shear Zone (Elba Island, Italy): The deep root of a fossil oceanic detachment fault in the Ligurian ophiolites

The ophiolite sequences in the western Elba Island are classically interpreted as a well-exposed ocean-floor section emplaced during the Apennines orogeny at the top of the tectonic nappe-stack. Stratigraphic, petrological and geochemical features indicate that these ophiolite sequences are remnants of slow-ultraslow spreading oceanic lithosphere analogous to the present-day Mid-Atlantic Ridge and Southwest Indian Ridge. Within the oceanward section of Tethyan lithosphere exposed in the Elba Island,we investigated for the first time a 10s of meters-thick structure, the Cotoncello Shear Zone (CSZ), that records high-temperature ductile deformation. We used a multidisciplinary approach to document the tectono-metamorphic evolution of the shear zone and its role during spreading of the western Tethys. In addition, we used zircon U–Pb ages to date formation of the gabbroic lower crust in this sector of the Apennines. Our results indicate that the CSZ rooted below the brittle–ductile transition at temperature above 800 °C. A high-temperature ductile fabric was overprinted by fabrics recorded during progressive exhumation up to shallower levers under temperature b500 °C.Wesuggest that the CSZ may represent the deep root of a detachment fault that accomplished exhumation of an ancient oceanic core complex (OCC) in between two stages of magmatic accretion.We suggest that the CSZ represents an excellent on-land example enabling to assess relationships between magmatism and deformation when extensional oceanic detachments are at work

Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the influence of subduction-related metamorphism

Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc-associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid-rock interactions during metamorphism and give insights on sulfur mobilization. A detailed microtextural and geochemical study was performed on mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert) in high-pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of microtextural investigations with d34S values from in situ ion probe analyses within individual pyrite and chalcopyrite grains allowed evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth-Ghinivert, where limited metamorphic recrystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the submillimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic reequilibration or records evidence of intense fluid-rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies

The formation of eclogitic metatroctolites and a general petrogenetic grid in the Na2O - CaO - FeO - MgO - Al2O3 - SiO2 - H2O (NCFMASH) system.

Eclogite facies metatroctolites from a variety of Western Alps localities (Voltri, Monviso, Lanzo, Allalin, Zermat-Saas, etc.) that preserve textural evidence of their original form as bimineralic olivine-plagioclase rocks are considered in terms of calculated mineral equilibria in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH). Pseudosections, based on a new petrogenetic grid for NCFMASH presented here, are used to unravel the metamorphic history of the metatroctolites, considering the rocks to consist of different composition microdomains corresponding to the original olivine and plagioclase grains. On the basis that the preservation of the mineral assemblage in each microdomain will tend to be from where on a rock's P-T path the metamorphic fluid phase is used up via rehydration reactions, P-T pseudosections contoured for water content, and P-T path-M-H2O (amount of water) pseudosections, are used to examine fluid behaviour in each microdomain. We show that the different microdomains are likely to preserve their mineral assemblages from different places on the P-T path. For the olivine microdomain, the diagnostic mineral assemblage is chloritoid + talc (+ garnet + omphacite). The preservation of this assemblage, in the light of the closed system P-T path-MH2O relationships, implies that the microdomain loses its metamorphic fluid as it starts to decompress, and, in the absence of subsequent hydration, the high pressure mineral assemblage is then preserved. In the plagioclase microdomain, the diagnostic assemblage is epidote (or zoisite) + kyanite + quartz suggesting a lower pressure (of about 2 GPa) than for the olivine microdomain. In the light of P-T path-M-H2O relationships, development of this assemblage implies breakdown of lawsonite across the lawsonite breakdown reaction, regardless of the maximum pressure reached. It is likely that the plagioclase microdomain was mainly fluid-absent prior to lawsonite breakdown, only becoming fluid-present across the reaction, then immediately becoming fluid-absent again

Modifications of volume, shape and structure during metamorphism: Unearthing some ignored strain paths

A deformational process - that is the joint transformation of volume, shape and structure of a continuum playing the role of an actual material - can be described as a transformation of a vector space. Using such description one may notice that there are some cases of homogeneous deformation the very existence of which is usually ignored. It is suggested here that the recognition of these additional cases is a prerequisite for a fully comprehensive consideration and interpretation of tectonic fabrics, and a constructive reassessment of basic concepts of continuum mechanics

H2O in metamorphism and unexpected behaviour in the preservation of metamorphic assemblages.

The preservation of mineral assemblages that were fluid-present during their prograde history is primarily related to the consumption of the fluid by growth of more hydrous minerals as the retrograde history begins. The range of behaviour relating to the preservation of mineral assemblages is examined using calculated phase diagrams for fluid-saturated conditions, contoured for the H2O content of the mineral assemblage. At equilibrium, as a mineral assemblage crosses contours of decreasing H2O content along a pressure-temperature path, it dehydrates, the fluid being lost from the rock. If the assemblage crosses contours of increasing H2O content, the mineral assemblage starts to rehydrate using any fluid on its grain boundaries. When the rock has consumed its fluid, the resulting mineral assemblage is that preserved in the rock. Conditions relating to the preservation of mineral assemblages are discussed, and examples of the consequences of different pressure-temperature paths on preservation in a metapelitic and a metabasic rock composition are considered on phase diagrams calculated with THERMOCALC