Multistage crustal reworking in the Dabie orogen: Evidence from zircons in migmatites and retrograded eclogites

Abstract

Crustal rocks are likely to experience multistage reworking processes involving subsolidus metamorphic dehydration and suprasolidus partial melting in collisional orogens. Different types of metamorphic rocks usually exhibit unique geochronological signatures. However, the underlying reasons for these distinctions remain ambiguous. To address this issue, we conducted a combined study of petrology, whole-rock major–trace elements and Sr–Nd isotopes, as well as zircon U–Pb–Hf isotopes and trace elements, on migmatitic granitic gneisses (including melanosomes and leucosomes) and retrograded eclogites exposed in the same outcrop at the Huangwei area in the North Dabie zone, central-east China. Our results reveal contrasting protoliths and metamorphic features between migmatites and retrograded eclogites: (1) The protoliths of the retrograded eclogites are Paleoproterozoic mafic rocks, whereas those of the migmatites are Neoproterozoic granitic rocks. (2) The retrograded eclogites show petrographic features indicative of subsolidus fluid activity rather than partial melting, whereas the migmatites exhibit anatectic textures at macroscopic and microscopic scales. (3) Newly grown zircons in the retrograded eclogite solely record Triassic (221 ± 4 Ma) eclogite-facies metamorphism, whereas those in the migmatites preserve two episodes of high-temperature anatexis in the early Cretaceous (ca. 125 and 100 Ma). (4) Newly grown zircons in the melanosomes and leucosomes differ in their occurrences, U–Pb ages, trace element compositions, and Hf isotope ratios. The Triassic records are related to the continental collision/subduction of the South China Block beneath the North China Block, and the early Cretaceous records are associated with the thinning of the thickened lithospheric mantle and subsequent asthenospheric upwelling in the postcollisional stage. The distinct zircon records in the retrograded eclogites and migmatites stem from the contrasting zircon behavior during dehydration and anatectic metamorphism in different types of rocks. Notably, younger anatectic ages of ca. 100 Ma are newly identified in the Dabie orogen, and the contemporaneous magmatic activity is absent. This indicates that only local areas maintained suprasolidus metamorphic conditions for anatexis at ca. 100 Ma, which is related to the secular cooling of the orogenic lithosphere since the achievement of peak temperature, signifying the late stage of orogenic collapse. Therefore, a comprehensive understanding of the multistage crustal reworking processes needs a thorough examination of the metamorphic and anatectic history of various types of rocks in collisional orogens.