THE TRANSITION FROM HADEAN CRUSTAL WORKING TO ARCHEAN CRATON GROWTH: THE EXAMPLE FROM THE SLAVE CRATON (Invited Presentation)

The Slave Craton of the Northwest Territories, Canada preserves an approximately 1.5 Ga record of crust production from the 4.02 Ga rocks of the Acasta region through Neoarchean granites pervasive across the craton. The craton was penetrated by a number of diamond- and mantle/lower crust xenolith-bearing kimberlites in the Mesozoic/Cenozoic that add the deeper record to the story of the growth of this craton. Initial Hf isotopic compositions of zircons and the short-lived ¹⁴⁶Sm-¹⁴²Nd system of whole rocks document that the early stages of Slave craton development, along its current western boundary in the Acasta region, involved melting of Hadean crust to form new felsic rocks until 3.6 Ga. The melting occurred at depths where garnet was not a residual phase and is interpreted as reflecting internal reworking of a thick mafic crustal section produced initially at circa 4.2 Ga. Beginning at 3.4 Ga and extending until 2.85 Ga, crust was added in the Central Slave Basement Complex (CSBC) east of Acasta. The CSBC rocks have mantle-like radiogenic isotopic compositions except when near the Acasta region where more evolved isotope compositions suggest contamination of the parental magmas by older crust. The geographic trend in isotopic composition is reminiscent of those seen in modern convergent margin settings such as the Cordillera. A region of highly melt-depleted lithospheric mantle just east of the CSBC provides evidence for the recycling of crustal materials to diamond formation depths circa 3.5 Ga. Another phase of large-volume crust production in the Neoarchean added substantial volumes of crust to the east of the CSBC. Given the presence of a thick lithospheric mantle keel at 3.5 Ga, the wave of Neoarchean granite magmatism that occurred across most of the craton likely reflects internal crustal differentiation driven by Neoarchean orogenesis (collision, deformation, crustal thickening) rather than arrival of a new mantle heat source, such as a plume, beneath the craton at 2.6 Ga. This internal crustal differentiation moved the majority of heat producing radioactive elements from the lower crust and mantle lithosphere to shallower levels where the heat they provided no longer drove magmatism, resulting in cessation of the long record of magmatism and formation of the long-time stable Slave craton.