Constraints on craton stability from thermochronologic and geochronologic studies of the Slave and Wyoming cratons

摘要

New thermochronological and geochronological data from the Archean Slave and Wyoming cratons provide key constraints on the interaction of surface, deep crustal, and mantle processes. An extensive apatite (U-Th)/He (AHe) thermochronometry dataset from the Slave craton better resolves the Phanerozoic history of burial, unroofing, and elevation change. AHe dates across the northwestern Canadian shield vary from ~210 to ~360 Ma, and define NNE-SSW domains with older dates in eastern Slave and younger dates westward. Thermal history simulations reveal Paleozoic heating and burial to depths u3e4.4 km, a subsequent westward wave of unroofing to near surface conditions by earliest Cretaceous, and a lesser burial and unroofing in Cretaceous-Early Tertiary time. These data and additional observations are consistent with western Canadian shield-wide subsidence in Paleozoic time followed by surface uplift, and require u3e300 m of post-100 Ma elevation gain of the Slave craton. The transition from Paleozoic-Mesozoic subsidence to uplift may reflect a shift from northern to western plate boundary controls, with the Cretaceous-Early Tertiary history controlled by Cordilleran evolution. Dynamic topography and proximity to sediment sources likely influenced the burial, erosion, and vertical motion history of the craton. Slave kimberlites emplaced during the Paleozoic-Mesozoic burial phase would have been denuded during unroofing, potentially explaining the ~250 myr gap in kimberlite ages. The AHe method has been little applied in cratons, but new advances enable its successful application to these settings. The large Slave craton AHe dataset is used to more comprehensively evaluate potential causes of dispersion, such as apatite U-Th zonation, on AHe results. U-Th maps for 70 apatites reveal that all grains are zoned. However, thermal history simulations show that, except in unusual circumstances, the conventional unzoned apatite assumption will not cause misinterpretation of AHe datasets. In the Wyoming craton, automated mineralogical analysis and refined SIMS U-Pb geochronology were used to locate and date in situ mafic dike zircons as small as 5 microns, a grain size undatable by conventional techniques. Results demonstrate that zircons housed in quartz were shielded from thermal resetting, and expand the documented extent of Paleoproterozoic granulite facies metamorphism in the northwestern Wyoming craton.