PELITE THERMOBAROMETRY AND MONAZITE GEOCHRONOLOGY REVEAL PRE-ULTRAHIGH-PRESSURE OROGENY IN THE HORNELEN REGION, NORWAY

The Nordfjord-Sogn Detachment Zone (NSDZ) is widely cited as one of the primary structures responsible for the exhumation of the Norwegian ultrahigh-pressure (UHP) rocks. In Western Norway, the NSDZ is exposed as a series of upper-amphibolite grade allochthons that have been overprinted by lower-amphibolite to greenschist facies shearing, and juxtaposes the UHP rocks of the Western Gneiss Complex in its footwall with lower grade allochthons and the Devonian Basins in its hanging wall. However, before the significance of the NSDZ can be assessed, we must first understand the deformation and geologic setting of the allochthons prior to the onset of extension. Here, as a first step toward this goal, thermobarometry and monazite geochronology are used to establish the peak pressures and temperatures and the timing of deformation that predates the development of the NSDZ in the Hornelen Region.

The tectonostratigraphy of the Hornelen Region is characterized by a basement-cover pair that were isoclinally folded during early contraction, and subsequently sheared by late extension. This study focuses on a coarse muscovite-biotite-garnet-albite schist that can be traced throughout the field area. Depending on the degree of retrogression, the schist varies from coarsely porphyroblastic to pervasively sheared, although primary porphyroblasts with intact inclusion suites and prograde-zoned garnets are found at most localities. THERMOCALC thermobarometry and pseudosections created using Perple_X indicate burial to pressures of 13-17 kb at 560-630°C prior to the onset of late extension. U-Th/Pb geochronology on monazite inclusions within garnet rims was performed using in situ LA-ICPMS. Ages from two separate samples of 465.8 ± 6.5 Ma and 437.5 ± 9.9 Ma are interpreted as maximum ages for the final stages of garnet growth during burial. Previously published 40Ar/39Ar ages indicate that these rocks cooled through muscovite closure much later, at ~400 Ma. These new data suggest that the allochthons in the Hornelen area were involved in a major shortening event and buried to depths of 50-60 km tens of millions of years before the UHP event at 410-405 Ma. Furthermore, they were at or near the base of the crust immediately prior to the onset of extension, the development of the NSDZ, and the exhumation of the UHP rocks.