ZIRCON AND APATITE (U-TH)/HE THERMOCHRONOLOGY AND LATE CENOZOIC EXHUMATION OF THE SHUSWAP METAMORPHIC CORE COMPLEX, BRITISH COLUMBIA

Exhumation and cooling of the Shuswap Metamorphic Core Complex (MCC) occurred in the early Cenozoic. The migmatite dome at the structurally lowest level of the complex crystallized and cooled between 55-47 Ma. Previously reported apatite fission track ages cluster at 45 Ma in the MCC but are younger in the immediate footwall of the detachment system (30-20 Ma). The MCC currently exposes 2 km of topographic relief, primarily in the deeply incised migmatite dome, and the structurally deepest rocks are in the highest peaks. To study the late Cenozoic landscape evolution of the Shuswap MCC we analyzed (U-Th)/He ages of zircon and apatite single grains from a near-horizontal E-W transect from the hanging wall to the footwall units of the Columbia River Detachment. Apatite He ages of the hanging wall range from 34 to 15 Ma. Apatite He ages in detachment zone rocks are highly variable, including within single samples, and range from 27-2 Ma. The footwall units reveal relatively older zircon He ages, between 49-36 Ma, but mostly concentrate between 43-40 Ma (similar to apatite fission track ages), whereas apatite He ages range from 19 to 5 Ma, with a cluster between 7-10 Ma. These results, especially apatite He ages, mark a profound difference in late-stage cooling history between the hanging wall and footwall units. The young apatite He ages correspond to a zone of high angle normal faults that bound the cut the footwall units west of the migmatite dome. Local preservation of high heat flow in these fault zones could be related to focused denudation and/or circulation of hot fluids.