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 232Th/204Pb and 238U/204Pb broadly correlate with mapped crustal domains. More homogeneous and less radiogenic 238U/204Pb and 232Th/238U 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.