LAURENTIAN ISOTOPIC ATLAS CONSTRAINS PRECAMBRIAN CRUSTAL GROWTH, STABILIZATION, AND FOUNDERING

Laurentia, North America's Precambrian core, records the growth, stabilization, and modification of continental crust over the last 4 Ga. Constraining Laurentian crustal evolution, the associated tectonic processes, and spatial distribution of crustal blocks requires continental-scale analysis. We have compiled a new database of radiogenic (Sm-Nd, Lu-Hf, Re-Os, Pb, Rb-Sr) and stable (O) isotopes (n>38,000). The compilation is supported by new in-situ and whole-rock isotopic data and in-situ geochronology. Data are filtered and analyzed with consistent isotopic reference values using Python, R, and ArcMap tools. In this talk we will present new isotopic maps and demonstrate an application of this extensive compilation using Pb isotopes. Model source ²³²Th/²⁰⁴Pb and ²³⁸U/²⁰⁴Pb ratios determined from U-poor and Pb-rich feldspars and sulfides delineate major tectonic domains. Pb model ages yield a major peak at ca. 2.7 Ga, a 2.5 to 1.8 Ga minima, and a broad 1.8 to 0.9 Ga continuum. Archean model ages, largely confined to the cores of the Slave, Superior, Wyoming, and North Atlantic cratons, may reflect cooling and cratonization. The broad span of Proterozoic model ages likely reflects protracted tectonism, terrane accretion, and reworking of older crust. Correlations between ²³²Th/²⁰⁴Pb and ²³⁸U/²⁰⁴Pb in the Slave, Superior, Wyoming, and North Atlantic cratons suggest U and Th have not been significantly fractionated from Pb. In contrast, data from Proterozoic orogenic belts with abundant ferroan and/or AMCG (anorthosite-mangerite-charnockite-granite) suite magmatism (Southwest US, Adirondacks, Appalachian Blue Ridge, and Nain Province) show significant fractionation of both U and Th from Pb. This fractionation is interpreted to reflect the removal of Pb- and sulfide-rich mafic lower crust, consistent with petrogenetic models for ferroan and AMCG-suite plutons that involve lithospheric foundering and later magmatic underplating. The connection with ferroan granitoids and AMCG plutons is reinforced by a lack of fractionation of U and Th from Pb in Proterozoic orogenic belts without this style of magmatism. Recycling of the lower crust in the Proterozoic associated with the intrusion of mafic magmas and heat advection may have contributed to high mid-Proterozoic geothermal gradients.