The Western Gneiss Region (WGR) is one of the largest known areas in the world with well exposed ultrahigh-pressure (UHP) domains (including rocks that have been subducted >150 km deep); it is compiled of the Caledonian allochthons overlying Proterozoic Baltica basement (Roberts, 2003). The ultrahigh-pressure event, recorded in basement and allochthons, was Scandian in age (~415–395 Ma). However, many Middle Allochthon rocks record older zircon ages of ~960-920 Ma, indicating involvement in an earlier mountain-building episode called the Sveconorwegian orogeny (Corfu and Andersen, 2002).
In order to develop a more complete tectonic model of the WGR, this study used catholuminescence imaging, radiometric U-Pb dating, and trace element analysis of zircons from the Moldefjord area to determine possible pre-Caledonian tectonic events. Overall, we observed geochronological and trace element data typical of the rocks previously studied in the WGR, with samples correlating to the TIB-2 (~1710-1650 Ma), Trondheim (~493-482 Ma), Taconian (~470-465 Ma) and Caledonian–Scandian (~435–395 Ma) tectonic events. Sample E4716A, the only Middle Allochthon sample, was the only sample that recorded U-Pb ages of ~1150-980 Ma, suggesting that the Sveconorwegian orogeny may have impacted regions farther north than previously recognized. We suggest that the Middle Allochthon was part of the western Baltica basement affected by the Sveconorwegian orogeny before it was thrust to the southeast onto the (para)autochthonous Baltica basement during the Caledonian orogeny. There is currently no evidence that the Lower or Upper allochthons were affected by the Sveconorwegian orogeny.
Stephanie Wheeler, ’14
Cedar Rapids, IA
Sponsor: Emily Walsh