EXPERIMENTAL CONSTRAINTS ON AMPHIBOLITE/GRANULITE-ECLOGITE TRANSITION IN METABASALTS: IMPLICATIONS FOR SLAB MELTING IN WARM SUBDUCTION ZONES

By definition, eclogites are plagioclase-free rocks dominantly composed of garnet and omphacitic clinopyroxene. Many eclogites are thought to have formed by subduction processes during which oceanic basalts evolve into eclogites through either a low-T blueschist-eclogite transition or a higher-T amphibolite/granulite-eclogite transition depending on the thermal characteristics of the subduction zone. Surprisingly, detailed experimental constraints on the nature of this transition, particularly the high T transition, are few. Here, we report the results of long duration experiments at 800-1050°C investigating the amphibolite/granulite to eclogite transition under fluid-absent conditions. The starting material for our experiments were two high-grade amphibolites, differing somewhat in Fe/Mg but both having bulk compositions similar to mid-ocean ridge basalts. Our results indicate that typical MORB bulk compositions require pressures of at least 17.5 kbar (at 900°C) for the complete disappearance of plagioclase and the development of a true eclogitic mineral assemblage. Plagioclase is consumed through the reaction Hbl+Plag± Qtz=Grt+Cpx+melt. At these temperatures, garnet and plagioclase co-exist over a broad (~7kbar) pressure interval, which is characterized with increasing pressure by an increase in the abundance of garnet and the garnet/cpx ratio, and decreases in the abundance of hornblende and plagioclase. The present experimental results differ from those of Liu et al. (1996) on rocks of similar bulk composition, but under water-rich conditions. Specifically, those experiments indicate plag-out/eclogite-in at a pressure of 12 kbar at 800°C and 10 kbar at 900°C. Our results are more consistent with a down-temperature extrapolation of the anhydrous experimental results of Green and Ringwood (1967) on quartz tholeiite bulk compositions, which indicate pressures of ~15 kbar for plag-out at 900°C. Our results have implications for partial melting of MORB compositions in shallow, hot subduction zones such as may have been prevalent in the Archean. Under these conditions, partial melting of subducted oceanic crust may have occurred in the plagioclase-bearing, high-pressure granulite field rather than in the plagioclase-free, eclogite field.