Phengite-hosted LILE Enrichment in Eclogite and Related Rocks: Implications for Fluid-Mediated Mass Transfer in Subduction Zones and Arc Magma Genesis

Abstract

Geochemical differences between island arc basalts (IAB) and oceanwt % (0·027 per 11 oxygens). Ba in phengite does not covary strongly with either Na or K. Ba contents of phengite increase from floor basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion lithophile elements (LILE) K, Ba, Rb and Cs are probably some blocks to their transition zones or rinds, or from blocks to mobilized in subduction zone fluids and melts. This study documents their veins. Averaged K/Ba ratios for phengite and host samples LILE enrichment of eclogite, amphibolite, and epidote ± garnet define an array which describes other subsamples of the block and blueschist tectonic blocks and related rocks from melanges of two other analyzed blocks. Phengite carries essentially all of the LILE subduction complexes. The samples are from six localities of the in otherwise mafic eclogite, amphibolite, and garnet blueschist blocks Franciscan Complex, California, and related terranes of Oregon that are enriched in these elements compared with MORB. It and Baja California, and from the Samana Metamorphic Complex, evidently tracks a distinctive type of LILE metasomatism that Samana Peninsula, Dominican Republic. Most Franciscan blocks attends both high-T and retrograde subduction zone metamorphism. are MORB-like in their contents of rare earth elements (REE) and An obvious source for the LILE is a fluid in equilibrium with high field strength elements (HFSE); in contrast, most Samana metasedimentary rocks. High-grade semipelitic schists from subblocks show an IAB signature of these elements. The whole-rock duction complexes and subductable sediment display LILE values K2O contents of both groups range from 1 to 3 wt %; K, Ba, Rb, that resemble those seen in the most LILE-rich blocks. Modeling and Cs are all strongly intercorrelated. Many blocks display K/Ba of Ba and Ti suggests that 1–40 wt % of phengite added to similar to metasomatized transition zones and rinds at their outer MORB can produce their observed LILE enrichment. Thus, the margins. Some transition zones and rinds are enriched in LILE release of LILE from such rocks to fluids or melts in very high-T compared with host blocks; others are relatively depleted in these and -P parts of subduction zones probably depends critically on elements. Some LILE-rich blocks contain ‘early’ coarse-grained the stability and solubility relations of phengite, which is thought muscovite that is aligned in the foliation defined by coarse-grained to be stable at pressures as high as 95–110 kbar at T= omphacite or amphibole grains. Others display ‘late’ muscovite in 750–1050°C. veins and as a partial replacement of garnet; many contain both textural types. The muscovite is phengite that contains ~3·25–3·55 Si per 11 oxygens, and ~0·25–0·50 Mg per 11 oxygens. LowerSi phengite has a significant paragonite component: Na per 11