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.
The Western Gneiss Region (WGR), Norway, contains abundant mafic rock that reached ultrahigh-pressure (36 kbar pressures, depths of 100 km below the surface) conditions (eclogite) as inclusions in gneiss that underwent partial melting (migmatite). To evaluate geochemical and age relationships between eclogite and migmatite, we obtained LA-ICP-MS U-Pb dates and trace-element analyses for the mineral zircon from a variety of textural types of crystallized partial melt layers, from those that are layer-parallel to the fabric in the gneiss to those that crosscut the fabric. Caledonian U-Pb zircon dates from the rims of the zircon crystals are as old as 410–406 Ma, coeval with previously determined ages of high- and ultrahigh-pressure metamorphism of WGR eclogite. Trace-element analyses obtained simultaneously with U-Pb ages indicate crystallization of zircon under garnet-stable conditions (high pressure/high temperature) in the majority of leucosomes. Other zircons, including those from crosscutting pegmatite, yield younger ages (as young as 385 Ma), coinciding with ages determined for amphibolite-facies retrogression (15–5 kbar, depths of 50–15 km) of eclogite; trace-element analyses reveal that these zircons grew under plagioclase-stable (garnet-unstable) conditions. Combined age and trace-element data for leucosome zircons record the transition from high-pressure (garnet-stable, plagioclase-unstable) crystallization to lower-pressure (plagioclase-stable) crystallization. These results, combined with field observations of eclogite–migmatite relationships, are consistent with the presence of partially molten crust in at least part of the WGR during continental subduction. The decreased viscosity and increased buoyancy and weakening associated with partial melting may have assisted the rapid ascent of rocks from mantle to crustal depths.
