STABLE-ISOTOPE SYSTEMATICS OF THE VENTINA OPHICARBONATE ZONE, BERGELLCONTACT AUREOLE

This study documents systematic oxygen, hydrogen and carbon isotope va riations in the Ventina Ophicarbonate Zone (VOZ), a discrete sheet of ophicarbonate rocks located in the NW part of the Malenco ultramafic b ody and exposed across the contact aureole of the Oligocene Bergell pl uton. The Ventina ophicarbonates are brecciated rocks consisting of fr agments of schistose serpentinite embedded in a matrix of predominantl y calcite composition. Towards the Bergell pluton, the mesoscale brecc iated textures are preserved, but the regional Alpine mineral assembla ges and metamorphic microfabrics become progressively overprinted by c ontact metamorphism. Relatively constant carbon isotope compositions o f the carbonate matrix in the Ventina ophicarbonates are comparable to marine carbonate signatures. Together with field observations, the ca rbon isotope ratios provide evidence that these rocks formed through f aulting and brecciation during emplacement of the Adria subcontinental mantle on the seafloor. In contrast, the oxygen and hydrogen isotope signatures in the VOZ ophicarbonates are highly variable and reflect p rocesses of fluid-rock interaction during regional and contact metamor phism Within the contact aureole, systematic shifts in isotopic ratios are observed towards the lithological boundaries to the surrounding m assive serpentinites in profiles sampled subparallel to the contact me tamorphic isograds. These variations provide strong evidence for infil tration and exchange with external, water-rich fluids, probably derive d from dehydration reactions in the surrounding serpentinites during c ontact metamorphism. A lack of isotopic homogenization even on a hand specimen scale reflects varying degrees of overprinting and isotopic d isequilibrium. As a consequence, field-based geothermometric estimates from oxygen isotope fractionations between calcite and silicate phase s along the VOZ were not possible within the Bergell contact aureole. The observed isotopic patterns can be explained by kinetically control led mineral-fluid exchange during fluid-rock interaction.