Geochronology and geochemistry of zircon from the northern Western Gneiss Region: Insights into the Caledonian tectonic history of western Norway

The Western Gneiss Region (WGR) of Norway is divided by the Møre-Trøndelag shear zone (MTSZ) into a southern region that contains domains of Caledonian ultrahigh-pressure (UHP) metamorphic rocks (>. 2.5 GPa) and a northern area of similar Caledonian-aged rocks that record a maximum pressure reported thus far of ~1.5 GPa. Although both regions contain similar lithologies (primarily migmatitic quartzofeldspathic gneiss containing mafic lenses) and structural relationship of basement rocks to infolded nappes, this difference in maximum pressure implies a difference in tectonic history (continental subduction south of the shear zone, none to the north) and raises questions about the role of the MTSZ in the metamorphic history (including exhumation) of the WGR. Previous geochronology results indicated a difference in timing of peak metamorphism (older in north, younger in south). In order to better understand the tectonic history of the northern WGR and the MTSZ, and in particular the late- to post-Caledonian tectonic history, U-Pb zircon geochronology and trace-element abundances were obtained using the split-stream, laser-ablation ICPMS technique from metabasaltic lenses and migmatitic quartzofeldspathic host rocks from the structurally lowest exposed region of the northern WGR (Roan Peninsula basement), as well as leucosomes from an intercalated portion of the Seve Nappe Complex and a pegmatite in the MTSZ. Zircon from Roan gneiss and metabasite yield metamorphic ages of ca. 410-406 Ma, and zircon from a variety of migmatite samples (foliation-parallel leucosome to dikes) indicate melt crystallization at ca. 410 to 405 Ma. The Seve Nappe leucosomes yield only early Caledonian dates that cluster at ca. 437 Ma and ca. 465 Ma, suggesting that the allochthons in this region did not experience (or record) the same Scandian tectonic history as the basement rocks. Zircon from a weakly deformed pegmatite dike within the MTSZ crystallized at ca. 404 Ma, indicating that this shear zone was active during the end-stages of high-grade metamorphism in both the southern and northern WGR domains. Results of this study show that the northern and southern WGR experienced a coeval Scandian metamorphic, magmatic and deformation history, despite a possible difference in maximum P-T conditions.