ESTABLISHING DIRECT LINKS BETWEEN BASEMENT STRUCTURES AND BASIN EVOLUTION WITH THE LOW TEMPERATURE FLUID-MEDIATED DISSOLUTION-REPRECIPITATION OF MONAZITE AND ZIRCON

The Baker Lake Basin (BLB) is an intracontinental transtensional basin within the western Churchill Province, Nunavut, that formed during retroarc extension between 1840 and 1785 Ma. The first two phases of deposition (1840 -1800 Ma) represent periods when accommodation outpaced deposition, and the final phase (1800-1785 Ma) represents a period of deposition outpacing accommodation, a likely response to abandonment of basin controlling faults. We present monazite and zircon data from brittle structures within the Angikuni Lake region. Outliers of BLB siliciclastic and volcanic deposits drape over many outcrops throughout the region, which suggests that the current exposure is close to the original depositional surface. The Tulemalu fault zone (TFZ) parallels the axis of the BLB and is composed of 100 m wide zone of incohesive protocataclasite with an epidote-bearing matrix. At the core of this breccia zone is a ~1 m wide incohesive ultracataclasite zone containing 83 % SiO₂ and Nd isotopic systematics consistent with derivation from surrounding Neoarchean tectonites. Monazite is internally brecciated, and contains fracture-fillings and rims of apatite, allanite, and thorite. U-Th- Pb results yield ages between 1865–1800 Ma (mean: 1842 ± 12 Ma). Most of the zircon from the same rock are featureless, contain U/Th ratios as high as 228, and yielded a weighted mean of 1851 ± 14 Ma.

A second NW-trending normal shear/fault zone contains cohesive cataclasite approximately 1 km thick in southern Angikuni Lake. Similar to the TFZ, monazite is internally brecciated, and yields a weighted mean of 1820 ± 4.8 Ma. Featureless high U/Th zircon from this sample yielded a weighted mean of 1828 ± 19 Ma. Monazite from both fault zones contain very low Ca + Si and Th concentrations, and are intimately associated with apatite, consistent with relatively low temperature recrystallization. Zircon contains elevated U concentrations. U-Th-Pb results of monazite and zircon are within error from each fault zone, and both correspond to the first two phases of BLB deposition. The monazite and zircon are interpreted to be the product of low temperature fluid mediated dissolution-reprecipitation during the early development of the BLB.