Geochemistry of Alpine serpentinites from rifting to subduction: A view across paleogeographic domains and metamorphic grade

Abstract

Serpentinites from several tectono-metamorphic units of the Western Alps were studied to constrain their origin and tectonic setting of serpentinization. Study areas were selected to cover the whole width of the orogen and a wide range of metamorphic grades from anchizone (Canavese Zone) to greenschist facies (St. Barthelemy, Piemonte Zone) to blueschist facies (Rocca Canavese unit and Punta Rossa unit). Bulk rock serpentinite samples have high REE concentrations, compared to typical mid-ocean ridge serpentinites, with nearly flat REE patterns. Relict spinels from the Rocca Canavese unit have extremely low Cr#s (average=0.087) and high Mg#s (average=0.798) suggesting very low degrees of melt depletion. Both of these observations are consistent with an abyssal origin in a hyper-extended rifted margin with minimal melt depletion, or refertilization. Seafloor hydration between 150 and 200°C is indicated by oxygen isotope data (δ18O values=+5.2 to+9.4‰), supporting lithostratigraphic evidence of exhumation to the floor of the Alpine Tethys already available for the Canavese, St. Barthelemy and Punta Rossa serpentinites. Subsequent interaction with the metasediments during Alpine metamorphism resulted in variations in trace element concentrations and stable isotope compositions with decreasing distance to the interface between the sediment and serpentinite. The chemical gradient between the ultramafic rocks and the neighboring metasediments is best seen in the Punta Rossa unit, where Pb, Ba, Cs, U, and Rb concentrations increase, δ18O values increase, δ37Cl values decrease within the serpentinite with decreasing distance to the contact and a “blackwall” of pure chlorite is found at the contact. As these contacts between ultramafic rocks, continental basement and meta-sediments are analogous to the slab–mantle interface, our results support the mobility of Pb, Ba, Cs, U, Rb, Cl, and water at the scale of <10m across the interface during Alpine metamorphism. However, the preservation of geochemical gradients within the Punta Rossa serpentinite indicates a limited role for externally derived fluid flux.