|University of California – Santa Cruz
|Geology; Geochemistry; Archean; Geotherm; Superior Craton; Thermochronology; U-Pb
|Full text PDF:
The Superior craton represents the largest exposure of Archean crust on Earth and can offer insights into the long-term evolution of cratonic geotherms. U-Pb geo- and thermochronology of zircon and rutile from lower and mid-crustal xenoliths record lower crustal crystallization, metamorphism, and long-term cooling prior to surface exhumation within a Jurassic-aged kimberlite. Archean igneous zircon LA-ICPMS ages range from 2751 21 to 2341 41 Ma, indicating a prolonged record of zircon crystallization. Preliminary U-Pb LA-ICPMS rutile data has a far wider range of ages from 2660 140 to 324 17 Ma and are interpreted to record the long-term relaxation of the cratonic geotherm. Nitrogen aggregation data collected from diamonds exhumed within the Jurassic and a nearby older Proterozoic (~1.1 Ga) kimberlite pipe suggest at least two major diamond-forming events after craton formation. These data require that a thermal pulse caused by the Keweenawan Midcontinent Rift at ~1.1 Ga reverted the mostly high N-aggregated IaB diamonds to graphite but did not perturb the deep crust (~50 km depth). As the geotherm relaxed after the thermal pulse, the diamond stability window was reoccupied and a second diamond-forming event took place before later Jurassic eruption of poorly nitrogen-aggregated type IaA diamonds near Attawapiskat, Ontario. Maintenance of Archean rutile U-Pb cooling ages in middle-crustal amphibolite limits the magnitude of reheating.