The Western Gneiss Region (WGR), Norway, is an ultrahigh-pressure (UHP) terrane containing coesite. Coesite forms at >29 kbar and >700oC indicating the WGR, which consists of Baltica basement and the overlying Lower, Middle, Upper, and Uppermost allochthons, was subducted to depths of ~125 km or greater (Smith, 1984). Zircon U–Pb ages from the different units record multiple orogenic events, with all but the Uppermost Allochthon recording the Scandian (~415–395 Ma) ultrahigh-pressure event (Roberts, 2003). We analyzed zircon geochronology and trace element geochemistry from the Moldefjord area to better understand the pre-Caledonian events and reconstruct the units involved in the Caledonian orogeny and UHP metamorphism.
Our zircon ages ranged from 2697 ± 73 Ma to 394 ± 8 Ma, but generally clustered around dates that correspond to known events. Of the main age clusters, the oldest corresponds to the Transcandinavian Igneous Belt (1832 ± 11 Ma), the next to the Sveconorwegian orogeny (1060 ± 5 Ma), the northeast Høybakken extensional detachment (463 ± 5 Ma), and lastly the Caledonian orogeny (436 ± 9 Ma). One sample from the Middle Allochthon, E4721G, contains Sveconorwegian ages but is located at the northeastern edge of the WGR. The Sveconorwegian orogeny was thought to have affected only the southwestern WGR. Our data suggest the Middle Allochthon was affected by the Sveconorwegian orogeny in its original position as the westernmost portion of the Baltica basement. These rocks were then thrust to the SE, indicating that the Sveconorwegian orogeny affected a larger area than previously recognized.
Maggie Savage, ’14
Majors: Geology, History
Sponsor: Emily Walsh