THE CLUMPED-ISOTOPE GEOCHEMISTRY OF EXHUMED SHEAR ZONE IN CARBONATE ENVIRONMENT: EXAMPLES FROM THE TORNGAT OROGEN (CANADA)

Clumped isotope thermometry is a technique that ideally recovers the temperature of carbonate formation without the need to assume the composition of precipitating water and/or oxygen composition of another mineral phase, as is required for conventional stable isotope thermometry. However, applications of this technique to high-grade metamorphic conditions are sparse. Here, we present new single- (O and C) and clumped- (∆47) isotopes compositions measured across sheared marbles from the Torngat orogen, Canada, to explore how retrograde metamorphic reactions /deformation may affect ∆47 compositions. Clumped isotope compositions of the host dolomite marbles record apparent peak metamorphic equilibrium temperatures of 300-500˚C. Although these temperatures are significantly lower compared to peak temperature derived from C-isotope equilibrium between carbonate and graphite (~725˚C), and from metamorphic reactions (~750˚C), they are higher than other apparent ∆47 temperatures of dolomitic marbles elsewhere (e.g. metamorphic core-complex of Naxos, Notch Peak aureole). Surprisingly, ∆47 temperatures recorded along the shear zone are virtually very low, giving temperatures as low as 0˚C. Our petrographic investigations show that shear zones are associated with local dissolution of carbonate grains and the apparition of amphiboles. Both metamorphic assemblage and C-isotope equilibrium between carbonate and graphite suggest that shear zones formed during retrograde conditions at temperatures in the range 550-625˚C. Our results suggest that ∆47 doesn’t record the absolute peak temperature experienced by marbles. However, the abrupt change in ∆47 values across the shear zone may suggest 1) a possible kinetic exchange of C and O between carbonate and newly-formed amphiboles, 2) strain or dynamic recrystallization, or 3) fluid-rock interaction.