DETERMINING THE EXHUMATION HISTORY OF A GIANT UHP TERRANE USING TITANITE GEOCHRONOLOGY, WESTERN GNEISS REGION, NORWAY

The Western Gneiss Region (WGR), Norway is an archetypal giant ultrahigh-pressure (UHP) terrane that underwent a widespread, post-UHP, amphibolite-facies overprint during exhumation. Titanite is an ideal geochronometer for unraveling the exhumation of the WGR because the closure temperature for Pb diffusion, 650-750°C, lies within amphibolite-facies conditions, and reset titanite ages can be compared with U-Pb zircon ages of eclogites and ⁴⁰Ar/³⁹Ar muscovite cooling ages. Recrystallization/lead loss presumably occurred during exhumation as the UHP minerals broke down, and titanite growth resulted from decompression-related melting. New U-Pb titanite ages from orthogneiss and leucosomes were generated by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and isotope dilution thermal ionization mass spectrometry (ID-TIMS). The titanite U-Pb data show inheritance at ~1600 and ~950 Ma, as well as crystallization and/or lead loss during the Scandian Orogeny, from 398 to 385 Ma. Toward the foreland, in the eastern WGR, although there are ~410 Ma eclogite ages and 400 Ma muscovite ages, titanite was not reset, and retains Precambrian ages. The titanite ages transition abruptly from Precambrian to Scandian across a ~10 km wide zone that correlates roughly with the 390 Ma muscovite age contour. In the hinterland, including the three UHP domains, most of the titanite in orthogneiss was reset to Scandian ages, and the leucosomes contain 400 Ma (east) to 390 Ma (west) titanite; the titanite ages are typically 4-5 Ma older than nearby ⁴⁰Ar/³⁹Ar muscovite ages. Titanite in significant portions of the UHP–HP hinterland was not completely reset during the Scandian, however, documenting the survival of titanite at UHP conditions of 750°C and > 3.2 GPa — where titanite is metastable. These data indicate that the central and western WGR were exhumed from eclogite to amphibolite-facies in ~10 and 15 Myr, respectively.