HT-UHT metamorphism in the Dabie orogenic belt: A petrological study of garnet-plagioclaseamphibolite

Abstract

The Dabie orogenic belt is pivotal for exploring ultrahigh-pressure (UHP) metamorphism. However, the occurrence of ultrahigh-temperature (UHT) metamorphism and its association with UHP metamorphism within this belt remain poorly constrained. Therefore, we conducted a detailed petrological investigation of garnet-plagioclase amphibolite from the Northern Dabie metamorphic belt, revealing five distinct metamorphic stages (M1, M2, M3-a, M3-b, and M4) with corresponding P-T conditions of 0.99 GPa/774 ℃, 0.75 GPa/766 ℃, 0.65 GPa/970–996 ℃, 0.65 GPa/824–854 ℃, and 0.27 GPa/737 ℃, respectively. Notably, the M3-a stage is identified as UHT metamorphism, supported by rare earth elements-based thermometry and the presence of amphibole symplectites (Amp = Cpx + Opx + Pl ± melt). Zircon U-Pb dating has yielded three age groups: Zrn-I (inherited magmatic zircons, 668–263 Ma), and the Zrn-II and Zrn-III groups (231 ± 3 Ma and 207 ± 3 Ma), with the latter two corresponding to the M1 and M2 metamorphic events. Furthermore, the protolith age of the granitic mylonite in structural contact with the garnet-plagioclase amphibolite was dated to 122 ± 1 Ma, indicating that the M4 event is contemporaneous or slightly older. The constructed P-T path reflects a gradual uplift process within the crustal level in the study area, involving isothermal decompression, isobaric heating, isobaric cooling, and decompression cooling. The transition from M1 to M3-b suggests initial uplift followed by termination, mirroring post-Triassic exhumation of the Dabie orogen, while the transition from M3-b to M4 may relate to Cretaceous extension.