Carboniferous, not Paleoproterozoic, eclogite-facies metamorphism in the northern North China Craton as revealed by in situ garnet Lu-Hf and U-Pb geochronology

Abstract

Eclogites exposed within orogenic belts or cratonic margins provide critical records of oceanic subduction, continental collision, and supercontinent assembly. Understanding the timing and processes of Precambrian eclogite facies metamorphism is essential for elucidating the onset of modern-style plate tectonics on Earth. The eclogite in the northern margin of the North China Craton (NMNCC) has been regarded by some as key petrological evidence supporting the operation of modern-style plate tectonics during the Paleoproterozoic. However, there is significant controversy regarding the timing (Paleoproterozoic versus Carboniferous) of the peak eclogite-facies metamorphism in the NMNCC, primarily due to the presence of at least two generations of metamorphic zircons within the eclogites and their associated paragneisses. Here we conduct a comprehensive in situ Lu-Hf and U-Pb dating analysis of garnet, the principal mineral for recovering peak eclogite-facies P-T conditions, combined with zircon U-Pb isotopic and mineral inclusion analyses. This approach provides direct and precise geochronological constraints on the peak eclogite-facies metamorphism in the NMNCC. One paragneiss sample yields many Paleoproterozoic (ca. 1.85 Ga) zircon grains with metamorphic characteristics, while metamorphic zircons from other studied eclogite and paragneiss samples yield SIMS U-Pb ages ranging from 325.5 ± 2.0 Ma to 308.9 ± 3.4 Ma. In contrast, the garnets from all eclogite and paragneiss samples do not indicate Paleoproterozoic age and yield consistent in situ Lu-Hf ages of 354 ± 91 Ma to 369 ± 89 Ma and in situ U-Pb ages of 245 ± 180 Ma to 335 ± 27 Ma. The diffusivity of Lu, Hf, U, and Pb ions is substantially slower than that of major elemental cations (i.e., Ca²⁺, Mg²⁺, and Fe²⁺) in garnet. Therefore, the consistent Carboniferous garnet Lu-Hf and U-Pb ages cannot be ascribed to retrograde metamorphism or post-eclogite facies metamorphic overprint. Despite large analytical uncertainties, these ages provide definitive evidence that the eclogite facies metamorphism occurred during the Carboniferous, rather than the Paleoproterozoic. Considering the presence of garnet, rutile, and phengite inclusions within Carboniferous zircons, the obtained zircon U-Pb ages of 326–309 Ma are regarded as the best estimate for the timing of the eclogite facies metamorphism in the NMNCC. The Paleoproterozoic zircons in the paragneiss sample are likely of detrital origin, despite showing some metamorphic characteristics in CL images. The eclogite facies metamorphism likely recorded a collision event between the NMNCC and an island arc within the Paleo-Asian Ocean during the Carboniferous, rather than the onset of modern-style plate tectonics during the Paleoproterozoic. This study underscores the ability of in situ Lu-Hf and U-Pb dating of garnet in distinguishing various potential metamorphic episodes within individual samples and advocates for the reexamination of reported Precambrian eclogites worldwide using the in situ garnet U-Pb and Lu-Hf dating techniques, which are crucial for constraining the timing of the onset of modern-style plate tectonics.