EXHUMATION OF AN ARGON PARTIAL RESETTING ZONE AND A SCANDIAN PROVENANCE FOR TURBIDITES OF THE WINDERMERE SUPERGROUP, ENGLISH LAKE DISTRICT

Individual detrital minerals are being used increasingly as thermochronometers, in attempts to trace the provenance of sedimentary sequences and to quantify the exhumation rates of their source regions. Two techniques that are employed routinely in such studies are 40Ar/39Ar dating of detrital white micas and fission track analysis of detrital zircons. Many 40Ar/39Ar thermochronology studies use single-grain fusion techniques, but the high-spatial-resolution 40Ar/39Ar ultraviolet laser ablation microprobe (UVLAMP) allows the recovery of detailed thermal histories from individual grains, aiding discrimination of potential source regions. We have applied such techniques to Late Silurian turbidites from the foreland basin sedimentary sequence of the Windermere Supergroup, English Lake District. Single-grain 40Ar/39Ar total fusion ages of detrital white micas range from 508 ± 8 to 415 ± 8 Ma (2σ). The gross structure in the data reflects an ‘up stratigraphy' younging trend from ages of ~ 460 to ~ 430 Ma, with scatter in the age range 440 – 450 Ma. Intra-grain apparent age profiles obtained using the UVLAMP reveal complex thermal histories and apparent diffusion profiles within individual grains, indicating the partial resetting of Ordovician ages during a Late Silurian tectonothermal event. Such partial resetting is partly responsible for the scatter in the 40Ar/39Ar single-grain ages. Detrital zircons from the same samples form single fission track age populations of ~ 430 Ma, indicating their complete resetting at temperatures sufficient to only partially open the argon system in the accompanying micas and suggesting a source with a common thermal history. Mica lag times and zircon track length distributions indicate a source in a rapidly exhuming terrane. A source for these sediments is likely to be in the allochthons of the Scandinavian Caledonides, emplaced during Laurentia - Baltica collision in the Scandian Orogeny (435 – 425 Ma). A complex pre-Scandian polyorogenic history is recorded within these allochthons which experienced variable Scandian overprinting. The data reflect erosion of the highest structural levels of these nappes and the unroofing of an argon ‘partial resetting zone'.