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Phengite-hosted LILE Enrichment in Eclogite and Related Rocks: Implications for Fluid-Mediated Mass Transfer in Subduction Zones and Arc Magma Genesis |
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| Authors: | Sorensen Sorena S; Grossman Jeffrey N; Perfit Michael R |
| Institution: | 1 Department of Mineral Sciences NHB-119, National Museum of Natural History, Washington,DC 20560, USA 2 US Geological Survey MS-923, Reston, VA 22094, USA 3 Department of Geology, University of Florida Gainesville, FL 32611, USA |
| Abstract: | Geochemical differences between island arc basalts (IAB) andocean-floor basalts (mid-ocean ridge basalts; MORB) suggestthat the large-ion lithophile elements (LILE) K, Ba, Rb andCs are probably mobilized in subduction zone fluids and melts.This study documents LILE enrichment of eclogite, amphibolite,and epidote ± garnet blueschist tectonic blocks and relatedrocks from melanges of two subduction complexes. The samplesare from six localities of the Franciscan Complex, California,and related terranes of Oregon and Baja California, and fromthe Samana Metamorphic Complex, Samana Peninsula, DominicanRepublic. Most Franciscan blocks are MORB-like in their contentsof rare earth elements (REE) and high field strength elements(HFSE); in contrast, most Samana blocks show an IAB signatureof these elements. The whole-rock K2O contents of both groupsrange from 1 to 3 wt %; K, Ba, Rb, and Cs are all strongly intercorrelated.Many blocks display K/Ba similar to metasomatized transitionzones and rinds at their outer margins. Some transition zonesand rinds are enriched in LILE compared with host blocks; othersare relatively depleted in these elements. Some LILE-rich blockscontain ‘early’ coarse-grained muscovite that isaligned in the foliation defined by coarse-grained omphaciteor amphibole grains. Others display ‘late’ muscovitein veins and as a partial replacement of garnet; many containboth textural types. The muscovite is phengite that contains 3.25–3.55 Si per 11 oxygens, and 0.25–0.50 Mg per11 oxygens. Lower-Si phengite has a significant paragonite component:Na per 11 oxygens ranges to 0.12. Ba contents of phengite rangeto over 1 wt % (0.027 per 11 oxygens). Ba in phengite does notcovary strongly with either Na or K. Ba contents of phengiteincrease from some blocks to their transition zones or rinds,or from blocks to their veins. Averaged K/Ba ratios for phengiteand host samples define an array which describes other subsamplesof the block and other analyzed blocks. Phengite carries essentiallyall of the LILE in otherwise mafic eclogite, amphibolite, andgarnet blueschist blocks that are enriched in these elementscompared with MORB. It evidently tracks a distinctive type ofLILE metasomatism that attends both high-T and retrograde subductionzone metamorphism. An obvious source for the LILE is a fluidin equilibrium with metasedimentary rocks. High-grade semipeliticschists from subduction complexes and subductable sediment displayLILE values that resemble those seen in the most LILE-rich blocks.Modeling of Ba and Ti suggests that 1–40 wt % of phengiteadded to MORB can produce their observed LILE enrichment. Thus,the release of LILE from such rocks to fluids or melts in veryhigh-T and -P parts of subduction zones probably depends criticallyon the stability and solubility relations of phengite, whichis thought to be stable at pressures as high as 95–110kbar at T=750–1050°C. KEY WORDS: geochemistry; LT eclogite; mineral chemistry; metasomatism; phengite |
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