END OF THE ARCHEAN: WHIMPER OR BANG?

Densely populated intervals in global U-Pb age-spectra of detrital zircons have been tied to periods of increased crustal growth, while intervals with low age-densities suggest limited, global, crust-forming activity. Over the past several hundred m.y. seafloor spreading, subduction, and arc magmatism have been continuous, which begs the question: What kind of global tectonics could yield periods of hundreds of m.y. in the Precambrian during which modern crustal growth processes ceased? The first, well-defined period of low detrital zircon age-density occurs near the Archean-Proterozoic transition (APT: ~2.4-2.5 Ga). In the Wyoming Province, the APT marks the last stage of cratonization and evidence of tectonism at this time is confined to the margins of the province. The western margin has an extensive record with occurrences of leucogranites and other crustal melts along with high-grade metamorphism from SW Montana (Tendoy Range) to Utah (Farmington Complex). In the buried Great Falls tectonic zone along the northern margin, APT felsic, peraluminous magmatism and metamorphism have been reported from crustal xenoliths. To the east, the most prominent evidence of tectonism in the Black Hills is the 2.48 Ga Blue Draw metagabbro and associated intrusive rocks. Along the southern boundary, passive margin development and fluid metasomatism of Archean crust are the main signals. These observations suggest that after a period of voluminous crustal growth from ~2.6-2.9 Ga related to subduction-driven felsic magmatism, the crust and its lithospheric keel bonded to form the craton. The paucity of Paleoproterozoic magmatism within the craton, however, strongly suggests the APT was characterized by rifting as the craton likely separated from a larger, enveloping continental mass (e.g., Kenorland). The distinctive, high-mu Pb isotopic signature of the craton limits potential conjugates, which include the Slave Province, Minnesota River Valley terrane, and Limpopo Belt. Globally, the APT reduction in zircon production/preservation and implied slow-down in plate production likely reflects an interval when the terrestrial heat flow balance shifted towards plumes and rifting due to subduction of large volumes of oceanic lithosphere associated with the preceding period of voluminous continental crust production.