When did the Chicheng eclogite form? Revisiting the Paleoproterozoic-late Paleozoic controversy via zircon inclusion and U-Pb dating

Abstract

Since its discovery in 2002, the tectonic significance of the Chicheng eclogite along the northern margin of the North China Craton (NCC) has been debated, with conflicting interpretations regarding its peak metamorphic age (Paleoproterozoic vs. Late Paleozoic). This controversy arises from unclear correlations between isotopic ages and metamorphic mineral assemblages. Here, we resolve this issue by integrating mineral inclusion analysis with zircon textural characteristics to constrain metamorphic generations of zircons. Systematic investigations of two eclogite samples (CC26 and CC28) reveal three distinct indicator mineral assemblages preserved in zircon domains: (1) eclogite-facies indicators (omphacite, kyanite and rutile); (2) non-eclogite-facies indicators (plagioclase, diopside, orthopyroxene, amphibole and ilmenite); and (3) persistent phases (garnet and quartz). LA-ICPMS U-Pb geochronology delineates three metamorphic stages: (1) peak eclogite-facies metamorphism at 334–339 Ma recorded in eclogite-facies indicators bearing zircon domains or metamorphic cores; (2) prograde metamorphism at 357–361 Ma preserved in a few zircon cores or the domains with non-eclogite-facies indicators; and (3) retrograde stage at 320–316 Ma recorded in zircon rims, respectively. Notably, scattered ages of 283–304 Ma likely reflect post-collisional magmatic thermal events. The Hongqiyingzi Complex (hosting the Chicheng eclogite) exhibits contrasting lithological and tectono-thermal records compared to the NCC's Dantazi Complex. These findings support a Late Paleozoic origin for the Chicheng eclogite, related to orogenic processes of the Central Asian Orogenic Belt (CAOB), rather than Paleoproterozoic NCC amalgamation events.