Lead Isotopes Constrain Precambrian Crustal Architecture, Thermal History and Lithospheric Foundering in Laurentia

ABSTRACT Laurentia (ancestral North America) records nearly 4 billion years of crustal evolution. Here, a newly compiled continental‐scale Pb isotopic database is used to evaluate the Precambrian crustal evolution of Laurentia. Pb model ages yield a 2.7 Ga peak, a 2.5–1.8 Ga minimum and 1.8–0.9 Ga continuum. Pb model ages yield thermochronometric data and track crustal growth via arc‐related magmatism and accretionary orogenesis. Model 232 Th/ 204 Pb and 238 U/ 204 Pb broadly correlate with mapped crustal domains. More homogeneous and less radiogenic 238 U/ 204 Pb and 232 Th/ 238 U after 2.7 Ga suggests a shift to more juvenile sources, loss of early isotopic reservoirs and greater crustal reworking. U and Th are fractionated from Pb in Proterozoic orogens with abundant ferroan and anorthosite–mangerite–charnockite–granite(AMCG)‐suite magmatism. This fractionation suggests the removal of Pb‐rich lower crust, supporting petrogenetic models involving lithospheric foundering and magmatic underplating. Lithospheric thinning and associated magmatism may have contributed to high middle Proterozoic geothermal gradients.