Primary fluids in low-temperature eclogites: evidence from two subduction complexes (Dominican Republic, and California, USA)

Abstract

Eclogites occur as isolated blocks in melanges of both the Samana Peninsula, Dominican Republic, and the Franciscan Complex, California, USA. In some of these eclogites, fluid inclusions were found in omphacite and sodic-calcic amphibole grains. Textures show that non-planar populations of fluid inclusions formed during growth of clinopyroxene and amphibole. In addition, planar arrays of secondary fluid inclusions are found along healed cracks. Homogenization temperatures to liquid were used to calculate isochores for the fluid inclusions. These data were compared with petrologic geothermobarometry. Temperature conditions of 500–700° C were estimated from garnetclinopyroxene geothermometry. The jadeite contents of omphacite indicate minimum pressures of 8–11 kbar in this temperature range. The P-T estimates agree well with calculated isochores for primary fluid inclusions from the Samana Peninsula, and show some overlap for both primary and secondary fluid inclusions from the Franciscan Complex. Salinities of 1.2–5.3 wt% NaCl equiv. were estimated for both primary and secondary fluid inclusions from Samana and Franciscan eclogites. These data suggest that low-salinity aqueous fluids attended eclogite-facies metamorphism and perhaps retrograde metamorphism in both subduction complexes. The salinities and densities of fluid inclusions in eclogites from the Samana Peninsula and the Franciscan Complex resemble those of counterparts from garnet amphibolites of the Catalina Schist, southern California. An external source for such fluids is suggested by their homogeneous populations coupled with their low salinities. Geologic evidence suggests that the Samana and Franciscan eclogites may have been derived from a Catalina-like source terrane. The Catalina rocks are inferred to have interacted with large volumes of sediment-derived fluid during subduction zone metamorphism at similar P but higher T conditions than those determined for Samana and Franciscan eclogite blocks. These results contrast with data for fluid inclusions from eclogites of the Monviso area, western Alps. The Monviso eclogites yield similar estimates for metamorphic P-T to those obtained in this study, but contain fluid inclusions of brine and of other saline aqueous fluids, all of which are less dense than expected for incorporation at the reported eclogite-facies conditions. The differences between the properties of fluid inclusions from the ecologites and garnet amphibolites of the Samana-Franciscan-Catalina subduction complexes and those of Monviso probably reflect differences between fluid-flow regimes during metamorphism.