Protolith Signatures and Element Mobility of the Maksyutov Complex Subducted Slab, Southern Ural Mountains, Russia

摘要

Geochemical data from the Maksyutov Complex in the southern Ural Mountains provide evidence for the nature of the protoliths of eclogite boudins and sedimentary gneiss from the ultra-high-pressure metamorphosed (UHPM) subducted slab, and for an evaluation of fluid-mediated mass transfer in the Devonian to Carboniferous subduction zone. Eclogites have major- and trace-element compositions of tholeiitic basalts with enriched mid-ocean ridge basalt (E-MORB) characteristics, whereas sedimentary gneiss resembles continent-derived shales. The geochemical data combined with field relations and Proterozoic protolith ages suggest the eclogite boudins formed as dikes or sills of basalt in sedimentary continental material of the East European platform during the initial opening of the paleo-Uralian ocean. Portions of the mafic boudins show large-ion lithophile-element (LILE) and light-rare-earth element (LREE) enrichment. Fluid transport of the LILE and LREE in the subduction zone seems to have been channelized (or otherwised spatially organized) on a scale of meters, rather than resulting from pervasive fluid flow. The metasomatizing fluids were most likely dehydration fluids produced during prograde metamorphism of sediments or K-altered basalts that were preserved through peak metamorphism and may have transported additional elements during retrograde metasomatism.