Phengite-hosted LILE enrichment in eclogite and related rocks: Implications for fluid-mediated mass transfer in subduction zones and arc magma genesis

Geochemical differences between island arc basalts (IAB) and ocean-floor basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion lithophile elements (LILE) K, Ba, Rb and Cs are probably mobilized in subduction zone fluids and melts. This study documents LILE enrichment of eclogite, amphibolite, and epidote +/- garnet blueschist tectonic blocks and related rocks from melanges of two subduction complexes. The samples are from six localities of the Francisan Complex, California, and related terranes of Oregon and Baja California, and from the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Most Franciscan blocks are MORB-like in their contents of rare earth elements (REE) and high field strength elements (HFSE); in contrast, most Samana blocks show an IAB signature of these elements. The whole-rock K2O contents of both groups range from 1 to 3 wt %; K, Ba, Rb and Cs are all strongly intercorrelated. Many blocks display K/Ba similar to metasomatized transitions zones and rinds at their outer margins. Some transition zones and rinds are enriched in LILE compared with host blocks; others are relatively depleted in these elements. Some LILE-rich blocks contain 'early' coarse-grained muscovite that is aligned in the foliation defined by coarse-grained omphacite or amphibole grains. Others display 'late' muscovite in veins and as a partial replacement of garnet; many contain both textural types. The muscovite is phengite that contains similar to 3.25-3.55 Si per 11 oxygens, and similar to 0.25-0.50 Mg per 11 oxygens. Lower-Si phengite has a significant paragonite component: Na per 11 oxygens ranges to similar to 0.12. Ba contents of phengite range to over 1 wt % (0.027 per 11 oxygens). Ba in phengite does not covary strongly with either Na or K. Ba contents phengite increase from some blocks to their transition zones or rinds, or from blocks to their veins. Averaged K/Ba ratios for phengite and host samples define an array which describes other subsamples of the block and other analyzed block. Phengite carries essentially all of the LILE in otherwise mafic eclogite, amphibolite, and garnet blueschist blocks that are enriched in these elements compared with MORB. It evidently tracks a distinctive type of LILE metasomatism that attends both high-land retrograde subduction zone metamorphism. An obvious source for the LILE is a fluid in equilibrium with metasedimentary rocks. High-grade semipelitic schists from subduction complexes and subductable sediment display LILE values that resemble those seen in the most LILE-rich blocks. Modeling of Ba and Ti suggests that 1-40 wt % of phengite added to MORB can produce their observed LILE enrichment. Thus, the release of LILE from such rocks to fluids or melts in very high-T and -P parts of subduction zones probably depends critically on the stability and solubility relations of phengite, which is thought to be stable at pressures as high as 95-110 kbar at T = 750-1050 degrees C.