TRACKING LOWER CRUSTAL FOUNDERING, CRUSTAL STRUCTURE, AND THERMAL EVOLUTION OF LAURENTIA WITH PB ISOTOPES

The continental crust is the primary record of Earth history. Pb isotopes are a key geochemical tracer of the origin and evolution of continents (e.g., Blichert-Toft et al., 2016). Here, a newly compiled geospatial database of Pb isotopes is used to investigate the Precambrian evolution of Laurentia. Time integrated model source ²³²Th/²⁰⁴Pb and ²³⁸U/²⁰⁴Pb ratios determined from U-poor and Pb-rich phases (feldspars and sulfides) correlate with major tectonic domains defined by geochronologic data and Sm-Nd isotopes. For example, a low ²³⁸U/²⁰⁴Pb domain in the southern Granite-Rhyolite province supports the Nd line of Bickford et al. (2015) and the interpretation this is a distinct terrane. Pb model ages yield a major peak at ca. 2.7 Ga, a 2.5 to 1.8 Ga minima, and a continuum between 1.8 and 0.8 Ga. Archean model ages are largely confined to the cores of the Slave, Superior, Wyoming, and North Atlantic cratons and may reflect cooling and stabilization of these domains. The range of Proterozoic model ages likely reflect protracted tectonism, terrane accretion, and reworking of older crust. Data from the Archean Slave, Superior, Wyoming, and North Atlantic cratons indicate that 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 USA, Adirondacks, southern 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 lower crust, consistent with petrogenetic models for ferroan granitoids and AMCG-suite plutons that involve lithospheric foundering and later magmatic underplating (e.g., McLelland et al., 2010). The connection between this fractionation and ferroan granitoids and AMCG plutons is reinforced by a lack of fractionation of U and Th from Pb in Proterozoic orogenic belts lacking this style of magmatism. Widespread recycling of the lower crust in the Proterozoic and associated intrusion of mafic magmas and heat advection may have contributed to apparently high mid-Proterozoic geothermal gradients.