Eclogite Petrochronology of a Megathrust in the Western Gneiss Region, Norway

Abstract

Siluro–Devonian continental collision in the Scandinavian Caledonides is documented by meso‐ and microstructures in eclogite and split‐stream U–Pb petrochronology of zircon from the Western Gneiss Region (WGR) of Norway. A 3–5 km‐wide shear zone (Sandane Shear Zone) spanning the central WGR separates the Baltica autochthon from overlying >5 km thick gneiss sheets that include much of the WGR domain. Eclogite within the shear zone is deformed while eclogite outside it is mostly undeformed. High‐pressure rocks in the hanging wall and footwall record the same peak metamorphism between 420 and 400 Ma. Prograde U–Pb dates in the footwall begin between 460 and 450 Ma, consistent with long‐term underthrusting at dips between 20° and 40° and average vertical rates of ∼1.8 mm yr ⁻¹ . Zircon growth in the hanging wall universally began after c. 435 Ma, implying distinctly later and faster burial (∼2.6 mm yr ⁻¹ ). These different metamorphic paths question the long‐held view that the central WGR is part of the Baltica lower plate. The two structural levels enjoyed the same eclogite‐facies metamorphism, but their journeys leading to this concluding episode of the collision were separate.