Fluid–metasedimentary rock interactions in subduction-zone mélange: Implications for the chemical composition of arc magmas
- 1Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109, USA
- 2Institut für Mineralogie, Universität Münster, Corrensstrasse 24, 48149 Münster, Germany
Abstract
Elevated concentrations of certain large ion lithophile elements (LILE; e.g., Ba, K, Rb, Cs, Ca, Sr), U, and Pb in arc magmas relative to high field strength elements (HFSE; e.g., Ti, Th, Hf, Nb, Zr) are considered key indicators of fluid addition to arc magma source regions worldwide, but the fluid sources and processes of mass transfer are controversial. Dehydration of downgoing slabs releases fluids that can flow through and react with metamorphosed ultramafic-mafic rock packages in mélange zones near slab-mantle interfaces. New geochemical data from Syros, Greece, reveal that these fluids preferentially leach LILEs, U, and Pb when they infiltrate and react with subducted metasedimentary rocks. Transfer of these LILE-, U-, and Pb-enriched fluids to the mantle wedge at subarc depths could directly trigger partial melting and generate magmas with elevated Ba/Th, Sr/Th, Pb/Th, and U/Th, as well as radiogenic Sr. Alternatively, if fluid transfer occurs at shallower depths (e.g., Syros), the metasomatized mantle could be carried deeper by wedge corner flow to ultimately undergo partial melting in subarc regions.
Footnotes
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↵*Present address: Breeding—Gemological Institute of America, 5345 Armada Drive, MS33, Carlsbad, California 92008, USA. Ague— jay.agueyale.edu
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↵GSA Data Repository item 2004165, location map (Fig. DR1), photo of an ∼2.5-m-thick alteration zone (Fig. DR2), detailed sample description (Fig. DR3), photomicrographs of the sample (Fig. DR4), brief descriptions of mineral analysis procedures, thermobarometric calculations, and estimated Ba extraction from metasediments (Appendix DR1), and rock chemical data (Table DR1), is available online at www.geosociety.org/pubs/ft2004.htm, or on request from editinggeosociety.org or Documents Secretary, GSA, P.O. Box 9140, CO 80301-9140, USA.
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- Accepted August 31, 2004.
- Received June 9, 2004.
- Revision received August 16, 2004.
- Geological Society of America
