ZIRCON GEOCHRONOLOGY OF MIGMATITES AND PEGMATITES FROM THE WESTERN GNEISS REGION, NORWAY

The Western Gneiss Region of Norway is one of the largest exposed regions of ultrahigh-pressure (UHP)/ high-pressure (HP) metamorphic rocks in the world, making it an excellent location to study the tectonics of UHP metamorphism. During the Caledonian orogeny, nappes of continental crust and ophiolitic material were emplaced onto Baltica basement orthogneiss.

Migmatitic basement gneiss and pegmatites cutting the ophiolitic nappes were collected west of Trondheimsfjord to pinpoint the timing of the high pressure event in that area. Zircons were separated from the samples, mounted in epoxy, imaged by cathodoluminescence (CL), and dated by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). A new technique was used during the dating process, decreasing ablation pit size to ~10 µm in diameter and ~3 µm depth, permitting the use of less material and leading to greater accuracy and precision of ablation. CL images of the zircon grains generally reveal older igneous cores containing oscillatory and/or sector zones and younger, lower uranium rims; partially recrystallized grains contain remnant sector zones. Basement gneisses from the area yield typical Gothian ages of ~1600 Ma with evidence of a Sveconorwegian event at approximately 950 Ma and little sign of Scandian metamorphism. U-Pb ages of zircons from migmatitic basement gneiss, however, indicate that the timing of partial melting was associated with Scandain UHP metamorphism. Pegmatite ages reveal similar Scandian ages with Ordovician protolith ages.