Thermal evolution of the North China Craton revealed by microsampling garnet geochronology on single xenoliths

• Published in: Chemical geology Vol. 663; p. 122264
• Main Authors: Du, Kai-Yang, Cheng, Hao, Yang, Ruyi
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20-09-2024
• Subjects: Lu–Hf; Microsampling; Sm–Nd; Xenolith; Microsampling; Lu–Hf; Sm–Nd; Xenolith
• ISSN: 0009-2541
• DOI: 10.1016/j.chemgeo.2024.122264

Granulite xenoliths, sourced from the lower cratonic crust and brought to the Earth's surface within volcanic rocks, offer unique insights into the formation and destruction of cratons. Age determinations are essential for constructing the thermal history of cratons. However, accurate dating is complicated by the xenoliths' extended exposure to high temperatures and their complex thermal histories. Specifically, the effects of extended lower crust residency and entrainment heating on coupled Sm–Nd and Lu–Hf systematics remain to be explored. Six xenoliths hosted in the Mesozoic dioritic porphyry from the eastern North China Craton were sampled from center to edge using micro-drilling/sawing techniques. From each xenolith, two to four concentric zones were successfully extracted for coupled Lu–Hf and Sm–Nd geochronological analysis. These xenoliths predominantly exhibit a granoblastic texture, with minerals that are mostly unzoned. Textural and chemical analyses of minerals indicate a decompression process following the granulite-facies metamorphism, with subsequent multiple reheating events. Thermobarometric calculations suggest granulite-facies equilibration at temperatures of ∼750 °C and pressures of 1.2–2.0 GPa. Consistent Lu–Hf and U–Pb ages of ca. 1.7 Ga, obtained from garnet, zircon, and titanite in chemical and textural equilibrium, are attributed to granulite-facies metamorphism. The Paleoproterozoic Lu–Hf ages and the Neoproterozoic Sm–Nd dates are indistinguishable across all concentric zones within each xenolith, whereas Sm–Nd ages are consistently ∼1.0 Ga younger than the corresponding Lu–Hf ages. This discrepancy suggests a complex history for this section of the lower cratonic crust, marked by a granulite-facies metamorphic event around 1.7 Ga, followed by prolonged cooling. A significant heating event during the Tonian, likely due to regional mantle plume activity, is inferred to have partially or completely reset the Sm–Nd isochrons. This thermal event and subsequent thermal relaxation on the samples at different residence depths thereby decoupled the Sm–Nd system from the Lu–Hf system, signifying a thermal pulse at the southern margin of North China Craton. The Mesozoic entrainment heating event appears to have had minimal impact on the Lu–Hf/Sm–Nd systematics of garnet and U–Pb of zircon/titanite. Our data highlight the potential of coupled microsampling Lu–Hf and Sm–Nd geochronology on individual xenoliths to reveal tectonic episodes within the lower crust that might be undetected by accessory mineral U–Pb geochronology.