Petrochronology of High-Pressure Veins Reveals the Evolution of Fluid Sources in Subducted Oceanic Crust (Rocciavrè Eclogites, W. Alps)

Abstract

The Rocciavrè massif is a large eclogitized ophiolitic fragment exposed in the Western Alps (Piemonte, Italy) exhibiting an almost complete sequence of the subducted Liguro-Piemont lithosphere. Raman spectroscopy on carbonaceous material in metasediments from Rocciavrè and the juxtaposed Orsiera massif indicates maximum temperatures in the range ~510°C–550°C, whereas thermodynamic modelling in mafic lithologies reveals peak burial metamorphic conditions of 550°C–590°C/2.2–3.0 GPa for both units, suggesting the absence of a metamorphic gap between them. Late Jurassic (ca. 151–158 Ma) zircons extracted from Rocciavrè metagabbros reflect the crystallization age near the seafloor, and no alpine metamorphic rims have been detected. The garnet-omphacite-rutile–dominated Fe-Ti metagabbros are crosscut by a variety of high-pressure vein systems, including garnet-rich, omphacite-rich, omphacite-quartz–rich, glaucophane-quartz–rich and winchite-actinolite-talc veins. Vein textures, mineral assemblages and mineral compositions suggest the formation of garnet-rich and omphacite-rich veins at conditions close to peak burial and the successive formation of omphacite-quartz–rich and glaucophane-quartz–rich types by reopening former omphacite-rich veins at eclogite- to epidote-blueschist-facies conditions along the exhumation path. In contrast, winchite-actinolite-talc veins are interpreted as retrograde greenschist-facies features. In situ U-Pb dating of monazite constrains the age omphacite-quartz–rich veining at 40.4 ± 0.2 Ma. Major and trace element mapping of vein assemblages shows various zoning patterns of omphacite and rutile crystals for a large variety of elements (e.g., Fe, Mg, Mn, Sr, Li, U and Cr). Aqueous primary fluid inclusions trapped in vein-filling and host-rock minerals have intermediate to high salinity values, interpreted to reflect the partial signature of hydrothermal alteration preserved up to eclogite-facies conditions. High fluid inclusion salinity values associated with the presence of N2 (± CO2) suggest the presence of fluids produced by local dehydration reactions at peak burial. In contrast, some inclusions from glaucophane-quartz–rich veins contain a low to intermediate salinity CO2-CH4–bearing fluid interpreted as reflecting a sedimentary contribution and a larger scale of fluid circulation. In addition, the mineralogy of winchite-actinolite-talc veins associated with high-salinity values suggests an ultramafic signature. The successive steps of vein formation are interpreted to record the evolution from a closed to open chemical system during exhumation, with late sedimentary and ultramafic fluid contributions that witness the mobility of fluids within the mafic sequence and transport distances likely reaching the kilometre scale. The Rocciavrè massif, which shares a similar metamorphic history to the Monviso Lago Superiore Unit further south, enables a precise characterization of fluid–rock interaction processes in subduction from eclogite-facies to greenschist-facies conditions.