U-Pb dates and trace-element geochemistry of zircon from migmatite, Western Gneiss Region, Norway: Significance for history of partial melting in continental subduction

摘要

The Western Gneiss Region (WGR), Norway, is dominated by migmatitic gneiss that contains inclusions of edogite, some of which contain evidence for ultrahigh-pressure metamorphism. To evaluate geochemical and age relationships between host migmatite and eclogite, we obtained LA-ICP-MS U-Pb dates and trace-element analyses for zircon from a variety of textural types of leucosome, from layer-parallel to crosscutting. Zircon textures (euhedral, oscillatory- and sector-zone grains) indicate a likely magmatic origin of the leucosomes. Caledonian U-Pb zircon dates from zircon rim and near-rim regions 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 dates indicate crystallization of zircon under garnet-present conditions in the majority of leucosomes. Other zircons, including those from crosscutting pegmatite, yield younger ages (as young as 385 Ma), coinciding with dates determined for amphibolite-facies retrogression of eclogite; trace-element analyses suggest that these zircons grew under plagioclase-present (garnet-absent) conditions. Combined age and trace-element data for leucosome zircons record the transition from high-pressure (garnet-present, plagioclase-absent) crystallization to lower-pressure (plagioclase-present) crystallization. If the euhedral zircons that yield ages coeval with peak or near-peak UHP metamorphism represent crystallization from anatectic leucosomes, these results, combined with field and petrographic 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 strain weakening associated with partial melting may have assisted the rapid ascent of rocks from mantle to crustal depths.