Prograde Sulfide Metamorphism in Blueschist and Eclogite, New Caledonia

Abstract

In New Caledonia, blueschist and eclogite preserve, as inclusions in porphyroblastic minerals, a record of sulfide present during prograde subduction processes. Sulfide inclusions in prograde garnet and lawsonite became chemically isolated from the matrix whereas sulfide minerals in the matrix continued to equilibrate with matrix fluids, or grew later, during retrogression. Cu–Fe sulfide mineral inclusions have been found across metamorphic grade within silicate-defined metamorphic mineral zones spanning a crustal profile of ∼30 km. Bulk area scans of sulfide inclusions provide compositions that represent mixtures of the solid sulfide that were included as the host silicate minerals grew. In general, single sulfide inclusion compositions and aggregate sulfide assemblages are distinct from those of matrix phases. High Cu contents in sulfide inclusions are interpreted to be a consequence of Fe lost from sulfide to growing garnet, rather than the result of intrinsically high Cu in the bulk-rock. The distribution of sulfide inclusion compositions across metamorphic grade, considered together with the available thermodynamic data, suggests that covellite/nukundamite-bearing inclusions in lawsonite, high in both Cu and S, disappear at higher grades as these sulfide minerals are no longer stable. Similarly, clustering near the ratio Fe:Cu = 1:1 may cease with increasing grade owing to the replacement of chalcopyrite/intermediate solid solution (iss) by the denser assemblage pyrite + pyrrhotite + bornite/digenite.