Highly discrepant Lu-Hf and Sm-Nd dates from crustal xenoliths witness the stability and destruction of a craton

Cratons are ancient and stable fragments of continental lithosphere that have survived over billions-of-years of Earth's geologic evolution. Although many recent studies have focused on improving our understanding of the mechanisms and dynamics of craton formation and destruction, little is known about the thermal history of cratons following their stabilization and preceding their demise (or 'decratonization'). Here, we present a geochronologic study combining zircon U-Pb and coupled garnet Lu-Hf and Sm-Nd dates of garnet-bearing lower-crustal xenoliths hosted in the Mesozoic dioritic porphyry from the eastern North China Craton, which provide unique insight into the thermal and crustal evolution of this craton over its 'stable' period. Four coupled Lu-Hf and Sm-Nd dates were determined from two garnet clinopyroxenite and two garnet amphibolite xenoliths. Lu-Hf dates range from 1737 ± 15 to 1680 ± 13 Ma, while Sm-Nd dates, determined on the same aliquots, span from 703 ± 11 to 661 ± 10 Ma. Lu-Hf dates from each sample are dramatically older than the corresponding Sm-Nd dates by ∼1.0 Ga. The U-Pb dates of metamorphic zircon span a wide range from 1850 to 1691 Ma, overlapping the Lu-Hf garnet dates. This large spread in U-Pb dates is interpreted to reflect protracted zircon growth in the lower cratonic crust. Compositionally homogeneous garnet cores are consistent with long-term residence at high temperatures, allowing for diffusive decoupling of the garnet Sm-Nd and Lu-Hf systems. The unusually large discrepancy between these garnet dates is used to model a thermal history for the lower cratonic crust in the area, which includes prolonged residence (and/or ultra-slow cooling) at mid-crustal depths after the stabilization of the North China Craton followed by Neoproterozoic exhumation. Our data and inferred temperature history illustrate that the coupled use of the Lu-Hf and Sm-Nd chronometers in garnet can provide unique insights into high-temperature thermal histories relevant to the evolution of lower cratonic crust by exploiting the differential effects these two chronometers experience over protracted metamorphic timescales.