Abstract

Petrographic and cathode luminescence studies of Alpine ophicalcites are combined with stable isotope data of pelagic sediments and associated ophiolite relicts, to document multiple phases of fluid-rock interaction related to sea-floor processes and Alpine orogeny. Internal sediments and primary carbonate microstructures in ophicalcites provide evidence for early sea-floor fragmentation, cementation and fluid activity in the Jurassic Tethys. Ca-carbonate cementation is comparable to cementation in modern transform setting analogues, where alkaline and Ca-enriched fluids are associated with serpentinites. Local mineralization and isotopic compositions of serpentine in ophicalcites are interpreted as signs of hydrothermal activity at temperatures of 100–150 °C.

Differences in the degree of oxygen isotope re-equilibration reflect differences in fluid/rock interaction during Alpine accretionary tectonics and continent-continent collision. Regional-scale homogenization of oxygen isotopes in carbonates and cherts indicate the presence of pervasive metamorphic fluids in the pelagic sediments during early Alpine recrystallization. In contrast, a lack of oxygen isotope re-equilibration in serpentine may reflect lower permeabilities and more limited fluid flow in serpentinites during Alpine metamorphism. Cathode luminescence and preliminary oxygen isotope data of late vein-forming phases suggest that late metamorphic fluids were channelled along discrete brittle fractures in a closed, rock-dominated system.

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