Subducted continental crust can attain pressure-temperature conditions that are above the solidus for many crustal lithologies, but the conditions, timing, and consequences of deep crustal melting are largely unexplored. Although there have been experimental investigations of ultrahigh-pressure (UHP) melting, there are very few field-based studies of natural examples; existing work has focused largely on the age and chemical characteristics of granitoid bodies that are associated with UHP terrains and not on migmatites that commonly host UHP rocks. The generation of large amounts of partially molten crust during subduction has implications for mass and heat transfer from the mantle to the crust, orogenic growth by the addition of partially molten material generated at depth, and evolution of orogenic landscapes (e.g. development of plateaux). Moreover, the role of partially molten crust must be considered in understanding the mechanisms by which (U)HP rocks are overprinted during exhumation and subsequent orogenic processes.
This proposed Early Grant for Exploratory Research (EAGER) project is designed to obtain U/Pb zircon ages from magma bodies (leucosomes, pegmatites) spatially associated with eclogite to determine if zircon crystallized and thus melting occurred at UHP conditions. If this exploratory study is successful, it will set up the background canvas for a larger scale project with the ultimate goal of testing the hypothesis that, in some orogens, partial melting of continental crust occurs during continental subduction and is dynamically linked to the thermal-mechanical evolution of orogens and the exhumation of UHP rocks. The pursuit for adequate sample material will take place in the Western Gneiss Region (Norway) because it is an accessible locale with abundant exposure of UHP rocks that are hosted by migmatite; some eclogite bodies are themselves migmatitic.
This project was designed to evaluate the relationship between eclogite and host gneiss (migmatite) in the Western Gneiss Region, Norway -- one of the largest ultrahigh-pressure terranes in the world. We determined the age of crystallization of zircon in leucosomes (crystallized melt) from different textural sites in migmatite+eclogite-bearing outcrops, including layer-parallel leucosomes and leucosomes in eclogite-boudin necks. U-Pb zircon analyses were done by Stacia Gordon at UC-Santa Barbara using a LA-ICP-MS in split-stream mode. Results show that some melt crystallization was coeval with published ages for peak UHP metamorphism (~410-400 Ma). We obtained simultaneous trace element and U-Pb zircon analyses and tracked the timing of zircon crystallization under garnet-stable conditions to zircon crystallization under garnet-unstable (plagioclase-stable) conditions. For example, on the island of Finnoya, in the northern UHP domain, zircon rim crystallization occurred under garnet-stable conditions from ~410-400 million years and reached plagioclase-stable conditions by 397 Ma. These data are complemented by Ti-in-zircon and Ti-in-quartz thermometry for a few key sites. There is abundant petrographic and geochemical (REE) evidence for widespread partial melting of eclogite and involvement of garnet in partial melting. Garnet-bearing mafic gneiss in the WGR is concentrated in regions with eclogite. The gneiss surrounding eclogite pods contains relict pyroxene, garnet, and symplectite textures, indicating derivation of the gneiss from the eclogite. We do not know the pressure conditions of migmatization (other than that some migmatization occurred under garnet-stable conditions), but we detected coesite inclusions in zircon in a layer-parallel leucosome at one site.
