Accessory minerals fingerprint post-collisional anatectic metamorphism in continental collision zones

Abstract

Mineral behavior during anatectic metamorphism is a direct factor controlling the chemical and isotopic compositions of melts and residues. However, the mechanism that causes disequilibrium melting remains poorly understood. A combined study of petrological, geochronological and geochemical analyses was carried out on a stromatic migmatite outcrop, the Luoerling migmatite in the North Dabie zone. The in-source leucosomes and leucocratic veins are products of in situ partial melting and later magmatic intrusion, respectively. The finding of Neoproterozoic, Late Triassic and Early Cretaceous zircons suggests that the migmatites have Neoproterozoic protoliths, and experienced eclogite-facies metamorphism during collisional orogeny and anatectic metamorphism in the post-collisional stage. The occurrence of peritectic amphibole suggests that anatectic metamorphism mainly occurs through biotite hydration melting. The occurrence of orthopyroxene and clinopyroxene indicates local dehydration melting under granulite-facies conditions. Leucosomes show considerably higher ⁸⁷Sr/⁸⁶Sr_i, but lower ε_Nd(t) and ε_Hf(t) values than the corresponding melanosomes. Such isotopic differences between leucosomes and melanosomes result mainly from the influx of external fluids derived from the surrounding eclogites and gneisses, indicating a coupled dehydration-hydration mechanism in the continental subduction zone. Peritectic and anatectic zircon, monazite, titanite and apatite that formed during anatectic metamorphism exhibit different petrological and geochemical characteristics. Peritectic and anatectic accessory minerals in the leucosomes and leucocratic veins exhibit smaller variations in Nd-Hf isotopes than those in the melanosomes, suggesting that the anatectic melts were homogenized during transport. Differential dissolution and growth of monazite, titanite and apatite affect the Sm-Nd isotopes of residues and melts during anatectic metamorphism. The dissolution of apatite plays a significant role in the hydration melting of metagranite. The continuous dissolution of relict zircon during cooling/transportation of melt results in decreased ε_Hf(t) values in anatectic zircon, whereas the incomplete dissolution of relict zircon results in whole-rock Hf-Nd decoupling. The difference in Nd-Hf isotopes among whole-rock and different genetic minerals can distinguish between dehydration and hydration melting. Therefore, this study highlights that a combination of multiple isotope analyses on whole-rock and accessory minerals may be conducive to understanding not only the mechanism and process of disequilibrium melting but also the nature of anatectic metamorphism in collisional orogens.