Ultra-high Temperature (UHT) Metamorphism in the Momeik Area, Mogok Metamorphic Belt, Myanmar

Although numerous studies have been conducted along the Mogok Metamorphic Belt (MMB), granulite facies and ultrahigh temperature (UHT) metamorphism have only been inferred in parts of the belt. This study documents typical UHT metamorphism in Momeik area, northeastern MMB, and its associated diagnostic mineral assemblages: (1) the Spl + Qz assemblage in khondalite (medium-grained garnet-sillimanite gneiss) formed by the reaction Grt + Sil = Spl + Qz, in which a partition coefficient between Grt and Spl shows 0.02 as quite good equivalent with the experimental result of Bohlen et al. (1986), and (2) Grt + Opx + Crd + Sil assemblage in sillimanite-garnet-orthopyroxene-cordierite granulite (GOC granulite) from melanosome layer intercalated with heterogeneous stromatic metatexite migmatite. The GOC granulite is very coarse- grained rocks in which garnet formed during prograde metamorphism through reactions involving Sil+Qz+Bt. At peak UHT metamorphism, Opx developed via biotite dehydration melting under strongly anhydrous conditions. During subsequent decompression, garnet became unstable and was replaced by symplectitic coronas composed of Opx, Crd, and Spl. The mineral assemblages and reaction textures surrounding garnet record multiple stages of deformation and metamorphism.

Mineral chemical characteristics, including Fe-rich garnet, Opx-Crd, Crd-Spl and Opx-Spl symplectites, Grt-Qz reaction textures, and high X_Fe values (up to 0.17) in residual F-rich biotite, indicate consistent peak UHT metamorphic conditions of ~6.5 ± 1.5 kbar and 900-1000℃. LA-ICP-MS U-Pb zircon data show detrital age populations of ~60-3100 Ma in khondalite, with metamorphic zircon growth at 26.56 ± 0.76 Ma, whereas GOC granulite record zircon ages of ~30-40 Ma. These results constrain Late Eocene to Early Oligocene prograde burial metamorphism in the Momeik area, subsequently overprinted by localized Oligocene UHT metamorphism during decompression and ductile extension, synchronous with collision-induced extrusion and slab-remnant thermal input during Indian-Asian collision.

Key words: Momeik; ultra-high-temperature metamorphism (UHT); Khondalite (garnet-sillimanite gneiss); sillimanite-garnet-orthopyroxene-cordierite granulite (GOC granulite); reaction textures