TIMESCALES AND CONDITIONS OF CRUSTAL MELTING IN THE WESTERN GNEISS UHP TERRANE, NORWAY

We are investigating the conditions, timing, and consequences of partial melting for continental subduction and exhumation in the Western Gneiss Region (WGR), Norway, a large UHP terrane characterized by meter-scale pods of (U)HP eclogite within migmatite. Previous work has established the timing of allochthon emplacement in the WGR at ~480–430 Ma, Scandian collision and subduction to UHP conditions at ~425–400 (with the peak of UHP metamorphism at ~408–404 Ma for some sites), and exhumation from ~400–385 Ma. Migmatization is either Precambrian and unrelated to Scandian UHP metamorphism, or Paleozoic and related to subduction or decompression and exhumation of Scandian eclogites. We used split-stream U-Pb and trace-element laser-ablation ICPMS measurements to analyze zircon extracted from leucosomes and deformed pegmatites found in twelve samples collected from the central and northern UHP domains within the WGR. Only zircons from the northern UHP domains yielded Scandian dates, with the majority of the leucosomes revealing dates from ~400 to 390 Ma. For example, at the Finnøya site in the northern UHP domain, the rims of euhedral zircons from granitic segregations at eclogite pod rims and in boudin necks, as well as from a deformed pegmatite dike, yield dates of 403-391 Ma. In comparison, the results reveal rim dates as old as 417–406 Ma for zircon in layer-parallel (stromatic) leucosomes in migmatite host rocks to UHP eclogite in the same outcrop. Petrologic and textural evidence indicate that some migmatites formed by in situ partial melting of eclogite. Trace-element analyses, obtained simultaneously with zircon U-Pb analysis, indicate that garnet was stable during zircon crystallization from 409-397 Ma, after which zircon crystallized under garnet-unstable conditions. This change may date the transition from (U)HP to low-P conditions during decompression. Results of our study and previously published work document the presence of partially molten crust for at least 20 million years of UHP, HP, and lower-P metamorphism in the WGR. Ongoing work will determine the conditions, regional extent, and timing of migmatization in relation to (U)HP metamorphism and exhumation in other parts of the WGR and its role in the development of this Scandian collisional orogen.