PALEOPROTEROZOIC EROSION OF MAFIC BODIES AS REVEALED BY LA-MC-ICP-MS DETRITAL ZIRCON GEOCHRONOLOGY, HURWITZ BASIN, WESTERN CHURCHILL PROVINCE, NUNAVUT, CANADA

Previous reconnaissance detrital zircon geochronology documented a time break of ~ 200 Ma in Paleoproterozoic intracratonic basin deposits of the Hurwitz Group, which are preserved as remnants across the Hearne domain of the western Churchill Province. Continental siliciclastic rocks in sequences 1 and 2 represent regional sagging due to > 2.11 Ga stretching; mixed siliciclastic-carbonate ramp and fluvial deposits in sequences 3 and 4 record < 1.91 Ga basin rejuvenation by crustal flexure. Despite the length of the hiatus, it lacks an obvious geologic expression. At the one site where the unconformity has been observed, dolostones of the Watterson Formation (sequence 3) concordantly drape wave-rippled surfaces of the Whiterock Member (sequence 1). Lacking signs of pre-Watterson tilting, weathering, or erosion, it remains unclear what combination of uplift, erosion, sedimentation, and sedimentary bypass occurred during the hiatus. To better understand the timing and nature of the hiatus we sampled two sites immediately above the unconformity. Although siliciclastic-contaminated dolostones are poor targets for detrital zircons, LA-MC-ICP-MS enables rapid, low-cost acquisition of large datasets. This allowed us to determine ~ 100 ages from each sample, despite poor zircon yields comprising mostly angular, low-U, silt-sized grains. Two samples from Mountain Lake display bimodal U-Pb age distributions, with peaks at 2.60 – 2.56 Ga and 2.14 – 2.12 Ga. A sample from Griffin Lake displays one peak (~ 2.72 Ga), and contains the youngest zircon from this study (~ 1.94 Ga). The restricted zircon populations imply local sources, in contrast to previously studied units higher in the section, which record pan-continental sediment dispersal. The ~ 2.1 Ga grains may have been derived from the Griffin gabbros, a regional sill swarm injected mainly into sequence 2 pelites (Ameto Formation), which would imply erosional stripping of 0.5 – 1.0 km of section before sequence 3. Local older basaltic flows in the Ameto Formation (Happotiyic Member) are another possible source. Considering that Precambrian continental flood basalts may only be represented by relatively deep level dyke/sill swarms, the ~2.1 Ga detrital zircons could conceivably be the indirect record of now-eroded volcanic rocks equivalent to the Griffin sills.